Upload
rohitgupta
View
36
Download
6
Tags:
Embed Size (px)
DESCRIPTION
lab manual
Citation preview
Department of Mechanical Engineering,RTU
EXPERIMENT NO.1
Object: - Study of general requirement of machine tool design.
Introduction: - Any machine should satisfy the following requirements.
1. High productivity
2. Ability to provide the required accuracy of shape and size and also necessary surface
finish
3. Simplicity of design
4. Safety and convenience of control
5. Good appearance
6. Low cost of manufacturing and operation
1. Productivity: - Productivity of a metal cutting machine tool is given by the expression
Q= (1/tc+tn0).n
tc = machine time
tn0 = non-productivity time that include job handling time.
a. Cutting down machining time: - This is possible if high cutting speeds and feed rates
are available on the machine tool in accordance with the latest development in cutting
tool material and design.
b. Machining with more than one tool simultaneously: - This principle employed in
multiple-spindle lathes, drilling machine etc.
c. Improving the reliability of the machine tool to avoid break down and adopt proper
maintenance policy to prevent unscheduled stoppages and delays.
2. Accuracy: - The accuracy of a machine tool depends upon its geometrical and kinematic
accuracy and its ability to retain this accuracy during operation. Accordingly the ability of a
machine tool to consistency machine parts with a specified accuracy with in permissible
tolerance limits can be improved by the following method.
a. Improving the geometrical accuracy of the machine tool: This is mainly determined by
the accuracy of guideways, power screw etc.
b. Improving the kinematic accuracy of the machine tool: This is determined the
relationship between velocities of two or more forming motion and it depends upon the length
of kinematic accuracy of machine tool can be improved.
c. Increasing the static and dynamic stiffness of machine tool structure. The greater in the static
stiffness of the machine tool structure the smaller will be its deformation due to cutting forces
and will be the accuracy of machining.
d. Providing accurate devices for measuring distance of travel.
e. Arranging the machine tools units in such a manner that the thermal deformation during the
machining operation result in the least possible change in the relative position between the tool
and the workpiece.
2
Department of Mechanical Engineering, RTU
3. Simplicity of design: - Simplicity of design of machine tool determines the ease of its
manufacture and operation. The design of machine tool can be simplified by using standard
parts and sub assembly as far as possible. The complexity of design of a machine tool depends
to a large extend upon the degree of its university. Thus a general purpose machine tool is a
rule more complex than a special purpose machine tool design doing similar operation.
4. Safety and convenience of control: - A machine tool cannot be deemed fit for use
unless it machine tools the requirement of safety and convenience of operation.
a. Shielding the rotating and moving parts of the machine tool with hoods.
b. Protecting the worker from chips, abrasive dust and coolant by means of screws shield etc.
c. Providing reliable clamping for the tool and workpiece.
d. Providing reliable earthing of the machine, providing device for safe handling of heavy
workpiece.
5. Appearance:- Good appearance of the machine tool influence the mood of the worker
favourably and thus facilities better operations it is generally conceded that a machine tool that
is simple in design and safe in operation and also good in appearance although factors, such as
external finish colour.
Nowadays, painting of machines in different colours according to the production purpose is
becoming popular.
6. Cost of manufacturing and operation: - The cost of manufacturing a machine tool
is determined by the complexity of its design. Therefore factors that help in simplifying the
machine tool design also contribute towards lowering its manufacturing cost.
The cost can also be brought down by reducing the amount of metal required in manufacturing
the machine tool. This is achieved by using stronger materials and more precise design
calculation pertaining to the strength and rigidly of parts to keep the safety margins as low as
possible.
3
Department of Mechanical Engineering, RTU
EXPERIMENT NO.2
Object: - Study of working and auxiliary motion of machine tool.
Introduction: - Obtaining the required shape on the workpiece, it is necessary that the
cutting edge of the cutting tool should move in a particular manner with respect to the
workpiece the relative movement between the workpiece and cutting edge can be obtained
either by the motion of the workpiece the cutting tool or by a combination of the motion of the
workpiece and cutting tool.
These motion which are essential are working to impart the required shape to the workpiece
are known as working motion. Working motions are further classified into two categories:
1. Drive motion or primary cutting motion
2. Feed motion
Working motion in machine tools generally of two types:
1. Rotary
2. Translatory
Fig: lathe fig: drilling
Fig: shaping fig: grinding
1 .For lathes and boring machines
Drive motion: Rotary motion of workpiece
Feed motion: Translatory motion of cutting tool in the axial or radial direction
2 .For drilling machines
Drive motion: Rotary motion of workpiece
4
Department of Mechanical Engineering, RTU
Feed motion: Translatory motion of drill
3 .For milling machines
Drive motion: Rotary motion of the cutter
Drive motion: Translatory motion of workpiece
4 .For shaping, planning and slotting machines
Drive motion: Reciprocating motion of cutting tool
Feed motion: Intermitted translatory motion of the workpiece
5 .For grinding machines
Drive motion: Rotary motion of grinding wheel
Feed motion: Rotary as well as translatory of the workpiece
Besides the working motion a machine tool also has provision for auxiliary motions. In
machine tool, the working motions are powered by sources of energy. The auxiliary motion
may be carried out manually or may also be power operated depending upon the degree of
automation of the machine tool. In general purpose machine tools, most of the auxiliary
motions are executed manually.
Parameters defining working motions of a machine tool
The working motions of the machine tool are numerically defined by their velocity, the velocity
of the primary cutting motion or drive motion is known as cutting speed while the velocity of
feed motion is known feed.
The cutting speed is denoted by ‘v’ and measured in the units m/min. Feed is denoted by‘s’
and measured in the following units.
1. mm/rev. in machine tool with rotary drive motion e.g. lathes, boring machine etc.
2. mm/tooth, in machine tool using multiple-tooth cutters e.g. milling machines.
3. mm/stroke, in machine tools with reciprocating drive motion e.g. shaping and planning
machine.
4. mm/min, in machine tools which have a separate power source for feed machines.
In machine tools with rotary primary cutting motion, the cutting speed is determined by the
relationship
v = 𝜋𝑑𝑛
1000 m/min
d= diameter of workpiece or cutter
n= revolution per minute (rpm) of the workpiece or cutter
In machine tools with reciprocating primary cutting motion, the cutting speed is determined by
u = 𝐿
1000Tc m/min
L= length of stroke mm
5
Department of Mechanical Engineering, RTU
Tc = time of cutting stroke min
If the time of the idle stroke in minutes is denoted by Ti, the number of strokes per minute can
be determined as
n = 1
Tc +Ti
Generally, the time of idle stroke Ti, is less than the time of cutting stroke, if the ratio Tc/Ti is
denoted by K, the expression for number of strokes per minute may be written as
n = 1
𝑇𝑐(1+𝑇𝑖
𝑇𝑐) =
𝐾
𝑇𝑐(1+𝐾)
Now combining equations the relationship between cutting speed and number of strokes per
minute may be written as follows
v = 𝑛𝐿(𝐾+1)
1000𝐾
The feed per revolution and feed per stroke are related to the feed per minute by the relationship
Sm = s.n
Where, Sm = feed per minute
s = feed per revolution
n = number of revolution
The feed per tooth in multiple tooth cutter is related to the feed per revolutions as follows:
S = Sz.z
Where, S = feed per revolution
Sz = feed per tooth of cutter
z = number of tooth on the cutter
The matching time of any operation can be determined from the following basic expression
Tm = 𝑙
Sm min
Where, Tm = matching time, min
l = length of machined surface, mm
Sm = feed per minute
6
Department of Mechanical Engineering, RTU
Experiment No.-3 Objective: Design criterion for machine tool structure, Static & dynamic stiffness.
Introduction:- Machine tool consists of machine tool structure, bed, column, housings. These are the base of machine tool on which the guideways, spindle, carriage, etc. are mounted. These elements must able to withstand at higher permissible load. These elements are discussed in detail in the following section. Objectives is to understand
Functions of machine tool structure and the design criteria for selection of material for
sideways,
The design of bed,
The design of column, and
The design of housing.
FUNCTIONS OF MACHINE TOOL STRUCTURE:
Machine tool structure consists of bed, base, columns, box type housings, overarms,
carriages, tables etc.
The structures are divided into three categories according to their functions :
Category 1 An element, upon which various subassemblies are mounted, falls under this
category. Example: bed and base.
Category 2 Elements consist of box type housings in which individual parts are assembled
fall under this category. Example: Speed box housing, spindle head, etc.
Category 3 Elements consist of parts that are used for supporting and moving the workpiece
and cutting tool fall under this category. Example: Table, carriage, knee, tailstock etc.
Machine tool structure must satisfy the following requirement :
(a) The initial geometrical accuracy of the structure should be maintained for the whole life
of the machine tool.
(b) All mating surfaces of the structure should be machined with a high degree of accuracy to
provide the desired geometrical accuracy.
(c) The shape and size of structure should not only provide safe operation and maintenance of
the machine tool but also ensure that working stresses and deformation should not exceed
specific limits.
(d) The selection of material and high static and dynamic stiffness are the fundamental
requirement to fulfill above-mentioned requirement.
7
Department of Mechanical Engineering, RTU
DESIGN CRITERIA FOR MACHINE TOOL STRUCTURE:
The simple machine tool bed with two-side wall is represented as a simply supported beam. Figure 3.1 depicts a simply supported beam. Point load F acts at its center. The maximum normal stress acting on the beam is given by
Figure 3.1 : Simply Supported Beam
where Bmax = Maximum bending moment = FL/4 ,
In = Moment of inertia of the beam section about the neutral axis=bh3/12 On substituting these values in Eq. σnmax changes to
The permissible normal stress under tension for the beam material is given by
Or minimum volume of material (Vmin) required to make sure that beam has sufficient strength is given by
The maximum deflection of simply supported beam is given by the following expression :
8
Department of Mechanical Engineering, RTU
Where E is young modulus of beam material. If the deflection of the beam dper is not to exceed a permissible value, then
Where Vmin = minimum volume of metal required to make sure that deflection of the beam under load does not exceed the permissible value.
The condition of optimum design is given by
Hence Eq. indicates that for every structure, there exists an optimum ratio l/h and the ratio
l/h depends upon :
(a) Operation constraint i.e. dper.
(b) The material of the structure i.e. E and σper.
Materials for Machine Tool Structure
The commonly used material for machine tool structures are cast iron and steel. Earlier cast
iron structures were widely used but due to advances in welding technology, welded steels
are widely used now days.
The selection of material for machine tool structure depends upon following factors:
Material properties
a) Cast iron has higher damping properties than steel. Welded steel also shows good
damping properties.
b) Cast iron has better sliding properties.
c) Steel has higher strength under static and dynamic loading.
d) The unit rigidity of steel under tensile, torsional and bending loads is higher than cast
iron.
Manufacturing Problems
Welded structures of steel have much thinner wall thickness as compared to cast structure.
Walls of different thickness can be welded more easily than casting it. Machining allowances
9
Department of Mechanical Engineering, RTU
for cast structures are generally greater than for weld steel structures. Machining allowance is
necessary in casting to remove defects such as inclusions, scales, etc. Welded structure can be
easily repaired as compared to cast structure.
Economy
The selection of material for structure will also depend upon its cost. The weight of steel is
lesser and but actual metal consumption is higher than that of cast iron. Hence in such cases
the cost increases. Holes are obtained with the help of core in the casting structure but holes
are made in welded steel structure by machining. These will not only increase the material
cost but also increases labour cost. Cost of patterns, welding fixtures, and cost of machining
are considered while selecting material for structure.
On considering above factors, the cast iron and steel may be used for following application:
a) Cast iron should be used for complex structure subjected to normal loading which are
to be produced in large number.
b) Steel should be used for simple and heavy loaded structures which are to be produced
in small number.
c) Combined welded steel and cast iron should be used where steel structure is
economically suitable. Example: Cast bearing housings that are welded into the feed
box.
Machine Dynamics: The machining and machine dynamics within the machine system should be well understood, optimized and controlled, because they have the following direct effects:
They may degrade machining accuracy and the machined surface texture and
integrity.
They may lead to chatter and unstable cutting conditions.
They may cause accelerated tool wear and breakage.
They may result in accelerated machine tool wear and damage to the machine and
part.
They may create unpleasant noises and sounds on the shopfloor because of the chatter
and vibrations.
Loop Stiffness within the Machine-tool-work piece System: Stiffness: stiffness normally can be defined as the capability of the structure to resist
deformation or to hold a position under the applied loads. Static stiffness in machine tools
refers to the performance of structures under the static or quasi-static loads. Static loads in
machine tools normally come from gravity and cutting force etc. apart from the static loads,
machine tools are subjected to constantly changing dynamic forces and the machine tool
structure will deform according to the amplitude and frequency of the dynamic excitation
loads, which is termed dynamic stiffness
Machine-tool-workpiece Loop Concept
From the machining point of view, the main function of a machine tool is to accurately and
repeatedly control the contact point between the cutting tool and the uncut material - the
‘machining interface’. Figure 3.2shows a typical machine tool-work piece loop. The
machine-tool-work piece loop is a sophisticated system which includes the cutting tool, the
tool holder, the slideways and stages used to move the tool and/or the workpiece, the spindle
holding the workpiece or the tool, the chuck/collet, and fixtures, etc. If the machine tool is
10
Department of Mechanical Engineering, RTU
being taken as a dynamic loop, the internal and external vibrations, and machining processes
should be also integrated into this loop as shown in Figure 3.3.
Stiffness can normally be defined as the capability of the structure to resist deformation or
hold position under the applied loads. Whilst the stiffness of individual components such as
spindle and slideway is important, it is the loop stiffness in the machine-tool system that
determines machining performance and dimensional and forming accuracy of the surface
being machined, i.e., the relative position between the workpiece and the cutting tool directly
contributes to the precision of a machine tool and correspondingly leads to the machining
errors.
Fig. 3.2 A typical machine tool loop
Fig.3.3 The machine-tool-workpiece loop taking account of machining processes and
dynamic effects
Static Loop Stiffness
Static loop stiffness in machine tools refers to the performance of the whole machine-tool
loop under the static or quasi-static loads which normally come from gravity and cutting
forces in machine tools.
A simplified analogous approach to obtaining the static loop stiffness is to regard the machine
tool individual elements as a number of springs connected to each other in series or in
parallel, so that the static loop stiffness can be derived based on the stiffness of each
individual element:
11
Department of Mechanical Engineering, RTU
Typically, a well-designed machine-tool-workpiece system may have a static loop stiffness of
around 50N/µm; a figure of 500 N/µm is well desired for heavy cutting machine tools in
particular. While a loop stiffness of about 10N/µm seems not rigid enough, it is quite
common in precision machines. Static loop stiffness can be predicted at the early design stage
by analytical or numerical methods and thus design optimization and improvement are
essential; also, a continuous process because of the increasing demands from the various
applications.
Dynamic Loop Stiffness and Deformation
Apart from the static loads, machine tools are subjected to constantly changing dynamic
forces and the machine tool structure will deform according to the amplitude and frequency
of the dynamic excitation loads, which is termed dynamic stiffness. Dynamic stiffness of the
system can be measured using an excitation load with a frequency equal to the damped
natural frequency of the structure.
Following Equations provide a rough approximation of dynamic stiffness kdyn and
deformation xdyn:
where F is the dynamic load applied to the machine tool, kstatic is the static stiffness of the
machine tool, and Q is the amplification factor which can be calculated from:
where M and c is the mass and damping:
Therefore,
12
Department of Mechanical Engineering, RTU
In order to accurately predict and calculate dynamic loop stiffness or the behaviour of a
whole machine-tool system, a dynamic model including all elements in the machine-tool loop
needs to be developed. The finite element method has been widely used to establish the
machine tool dynamics model and provide the solution with reasonable accuracy.
13
Department of Mechanical Engineering, RTU
Experiment No: 4
Objective: - Function & important requirements of spindle unit.
Functions:
The spindle unit of machine tool performas the following functions:.
1. Centering the workpiece, e.g., in lathes, turrets, boring machines, etc., or the cutting
tool as in drilling and milling machines, 2. Clamping the workpiece or cutting tool, as the case may be, such that the workpiece
or cutting tool is reliably held in position during the machining operations, and
3. Imparting rotary motion( in lathes) or rotary cum translatory motion (drilling
machines) to the cutting tool or workpiece.
The operational capabilities of a machine tool in terms of productivity as well as accuracy
and finish of machined parts largely depends upon the extent to which these functions are
qualitatively satisfied.
Important design requirements of spindle units are listed below:
The spindle should rotate with a high degree of accuracy. The accuracy of rotation is
determined by the radial and axial runout of the spindle nose, and these must not
exceed certain permissible values which are specified depending upon the required
machining accuracy. The rotational accuracy is influenced maximum by the stiffness
and accuracy of spindle bearings, particularly the one located at the front end.
The spindle unit must have a high static stiffness. The stiffness of the unit is made up
of the stiffness of the spindle unit proper and the spindle bearings. Machining
accuracy is influenced by bending axial as well as torsional stiffness.
The spindle unit must have high dynamic stiffness and damping. Poor dynamic
stability of the spindle unit adversely affects the dynamic behaviour of the machine
tool as a whole, resulting in poor surface finish and loss of productivity due to
restriction on the limiting unreformed width of cut.
The mating surfaces that are liable to wear restrict the life of the spindle unit. These
surfaces, such as journals, quills(in drilling machine) etc., must be hardened to
improve their wear resistance. The spindle bearing must also be selected or designed
to retain the initial accuracy during the service life of the machine tool.
The deformation of the spindle due to heat transmitted to it by the bearings, cutting
tool, workpiece, etc. should not be large as this has an advers effect on the machining
accuracy.
The spindle unit must have a fixture which provides quick and reliable cantering and
clamping of the cutting tool or workpiece. The centering is achieved by means of an
external or internal taper at the front end of the spindle.
14
Department of Mechanical Engineering, RTU
Table: Spindle Ends
DESIGN OF SPINDLE
Figure 4.1 shows schematic diagram of spindle. A spindle represents a shaft with
(a) length ‘a’ which is acted upon by driving force F2, and
(b) Cantilever of length ‘m’ acted upon by external force F1.
d max
≤ d
per
F2 F1
a m
15
Department of Mechanical Engineering, RTU
Figure 4.1: Principle of Working of Spindle
The total deflection of spindle nose consists of deflection d1 of the spindle axis due to bending forces
F1 and F2 and deflection d2 of the spindle axis due to compliance of the spindle supports. When the spindle has tapered hole in which a center or cutting tool is mounted, the total deflection of the center
or cutting tool consists of deflections d1, d 2 and d3 of the center or cutting tool due to compliance of the tapered joint.
Deflection of Spindle Axis due to Bending To calculate the deflection of the spindle nose due to bending, one must establish a proper
design diagram. The following guidelines may be used in this regard.
(a) If the spindle is supported on a single anti-friction bearing at each end, it may be
represented as a simply supported beam, and
(b) If the spindle is supported in a sleeve bearing, the supported journal is analyzed as a beam on an elastic foundation; for the purpose of the design diagram the sleeve
bearing is replaced by a simple hinged support and a reactive moment Mr acting at the
middle of the sleeve bearing. The reactive moment is given as :
Mr = C . M where M = bending moment at the support, and
C = constant = 0 for small loads and 0.3 to 0.35 for heavy load.
F2 F1
k b M
(a) Mr
F2 F1
(b) Mr
(c) d1
Figure 4.2 : Effect of Various Force on Spindle
Figure 4.2 (a) shows schematic diagram of spindle. Figure 4.2 (b) depicts the design diagram of
the spindle and figure 4.2 (c) illustrates deflected axis of the spindle. Consider the spindle shown in Figure 4.2 (a). By replacing the rear ball bearing by a hinge and the
16
Department of Mechanical Engineering, RTU
front sleeve bearing by a hinge and reactive moment Mr, the spindle can be reduced to the design
diagram as shown in Figure 4.2 (b). The deflection at the free end of the beam (spindle nose) can be determined by Macaulay’s method and is found out to be
Where E is Young’s modulus of the spindle material. Ia is average moment of inertia of the spindle section. The
deflection of the beam is shown in Figure 4.2 (c).
Materials of Spindles
The blank for a machine tool spindle may be:
1. Rolled stock in the case of spindles having diameter < 150 mm, and
2. Casting in case of spindles having diameter >150 mm.
In machine tool spindle design the critical design parameter is not strength but stiffness.
If we compare the mechanical properties of various steels, we found that modulus of
elasticity is more or less equal, although the strength of alloy steels can be considerably
greater than that of mild steel. Since stiffness is primarily determined by the modulus of
elasticity of the material, it may be concluded that no particular benefits accrues from using
costly alloyed steels for making spindles.
Recommendations for selecting spindle materials:
1. For normal accuracy spindles, plain carban steels C45 and C59, hardened and
tempered to RC =30.
2. For above normal accuracy spindles- low alloy steel 40Cr1Mn60Si27Ni25 induction
hardened to Rc= 50-56.
3. For spindles of precision machine tools, particularly those with
17
Department of Mechanical Engineering, RTU
Experiment No. 5
Objective: Importance of machine tool compliance with respect to machine tool.
18
Department of Mechanical Engineering, RTU
Consider a uniform shaft being machined between centres on a lathe (Fig. 5.1). Let KA be the
stiffness of centre A and Kg that of centre B. Due to radial component Py of the cutting force,
centre A will be displaced by a distance
YA = PA /KA
and centre B by
YB= PB/KB
19
Department of Mechanical Engineering, RTU
Fig. 5.1 Schematic diagram of a simple turning operation
Here PA and PB are the forces of reaction at ends A and B, respectively. They can be
determined from the following equations of static equilibrium:
1. Moment of Forces about Point B = 0, i.e.,
PAl= Py (l— x)
Substituting the values of yA and yB from Eqs. And yields
If it is assumed that KA/KB = α, Eq.
20
Department of Mechanical Engineering, RTU
EXPERIMENT NO.4
Object: Draw a neat schematic diagram of herring bone gear and explain
Introduction
A herringbone gear, a specific type of double helical gear, is a special type of gear that is a side
to side (not face to face) combination of two helical gears of opposite hands. From the top,
each helical groove of this gear looks like the letter V, and many together form a herring bone
pattern (resembling the bones of a fish such as a herring). Unlike helical gears, herringbone
gears do not produce an additional axial load.
Like helical gears, they have the advantage of transferring power smoothly because more than
two teeth will be in mesh at any moment in time. Their advantage over the helical gears is that
the side-thrust of one half is balanced by that of the other half. This means that herringbone
gears can be used in torque gearboxes without requiring a substantial thrust bearing. Because
21
Department of Mechanical Engineering, RTU
of this herringbone gears were an important step in the introduction of the steam turbine to
marine propulsion.
Precision herringbone gears are more difficult to manufacture than equivalent spur or helical
gears and consequently are more expensive. They are used in heavy machinery.
Where the oppositely angled teeth meet in the middle of a herringbone gear, the alignment may
be such that tooth tip meets tooth tip, or the alignment may be staggered, so that tooth tip meets
tooth trough. The latter alignment is the unique defining characteristic of a Wuest type
herringbone gear, named after its inventor.
Benefits
Since a herringbone gear is non-linear in the teeth the gears won't slip out from grabbing one
another if the axle or another force moves the gears up and down. This is also a benefit with
machinery that needs very straight movement, because a herringbone gear is designed to 'self
center' and is much less likely to skip a tooth or fall out of place. With some gears sets that use
herringbone gears; an axle can be lost and the gear will stay in place, a herringbone planetary
gear system.
Manufacture
A disadvantage of the herringbone gear is that it cannot be cut by simple gear
hobbing machines, as the cutter would run into the other half of the gear. Solutions to this have
included assembling small gears by stacking two helical gears together, cutting the gears with
a central groove to provide clearance, and (particularly in the early days) by casting the gears
to an accurate pattern and without further machining. With the older method of fabrication,
herringbone gears had a central channel separating the two oppositely-angled courses of teeth.
This was necessary to permit the shaving tool to run out of the groove. The development of the
Sykes gear shaper made it possible to have continuous teeth with no central gap. Sunderland,
also in England, also produced a herringbone cutting machine. The Sykes uses cylindrical
guides and round cutters; the Sunderland uses straight guides and rack-type cutters. The W. E.
Sykes Co. dissolved in 1983–84. Since then it has been common practice to obtain an older
machine and rebuild it if necessary to create this unique type of gear. Recently, the Bourn and
Koch Company has developed a CNC-controlled derivation of the W. E. Sykes design called
the HDS1600-300. This machine, like the Sykes gear shaper, has the ability to generate a true
22
Department of Mechanical Engineering, RTU
apex without the need for a clearance groove cut around the gear. This allows the gears to be
used in positive displacement pumping applications, as well as power transmission. Helical
gears with low weight, accuracy and strength may be 3D printed.
The herring bone gear is essentially a pair of helical gear in which the helix angel is oppositely
direct.
In a gear transmission, the rpm of the drives shapes is determined as
𝑛2 = 𝑛1 .𝑧1
𝑧2
Where 𝑛1=rpm of the driven shaft
𝑛2=rpm of the driving shaft
𝑧1=no. of teeth of the drawing gear
𝑧2=no. of teeth of the driven gear
The ratio 𝑧1/ 𝑧2 is known as the transmission ratio of the gear driven and is constant for a
particular gear pair.
EXPERIMENT NO.5
Object – Study of different mechanism used for transforming rotary motion into
translator Slider crank mechanism
1. Cam mechanism
2. Rack and pinion mechanism
3. Nut and screw mechanism
Theory – These elementary transmissions are employed in feed mechanism of most of the
machine tools and also in the drives of machine tools have a reciprocating primary cutting
motion.
The Important elementary transmissions that are used in machine tools for transforming rotary
motion into translatory motion are:
23
Department of Mechanical Engineering, RTU
1. Slider crank mechanism –
Fig: Slider crank mechanism
The machine consists of a crank, connecting rod and slider. The forward and reverse stroke
each take place during a revolution of crank therefore the need speed of forward and reverse
speed in slider crank mechanism since metal removal occur during one stroke. It is desirable
from the point of view of productivity to have a higher speed of the other stroke. Due to this
property of slider crank mechanism is used only in an appreciable increase of productivity e.g.
in the driving of primary cutting motion of gear shaping machine the length of stroke may be
change by adjusting the crank radius and is equal to
L = 2R, where R is the crank radius
2. Crank and Rocker mechanism –
The crank and rocker mechanism consist of a rotating crank which makes the rocker arm
oscillate by means of a block sliding along the groove in the rocker arm the clockwise rotation.
The forward cutting stroke takes place during the clockwise rotation of the crank through angle
‘𝛼’ and the reverse stroke during rotation of the crank through angle ‘𝛽’ since “𝛼 > 𝛽” and the
crank rotation with uniform speed. The ideal stroke completes transfer than the cutting stroke.
The length of stroke can be varied by adjusting the crank radius with a decrease in crank radius.
The ratio of angle 𝛼 𝛽⁄ decrease and the speed of cutting and reverse stroke tend to become
equal preferred in machine tool with large stroke (up to 1000 mm) where it can be effectively
employed e.g. in drive of the primary cutting motion of shaping and slotting machine.
Length of stoke can be calculated,
L = 2(𝐿
𝑒) R mm
L = length of rocker
e = offset distance
R = radius of crank
24
Department of Mechanical Engineering, RTU
3. CAM Mechanism –
The cam mechanism consists of a cam and a follower the cam mechanism provides the desired
translatory motion is a suitable profile is selected. The profile may be provided.
a. On the periphery of a disc-disc type mechanism.
b. On the face of a disc-face type cam mechanism.
c. On a cylindrical surface-drum type cam mechanism.
The main advantage of cam mechanical is that the velocity of the operative element is
independent of the design of driving mechanism and is controlled by the cam profile.
In a disc type cam if the radius change from R1 to R2 along an spiral while the cam rotate
through angle 𝛼, the velocity of the follower can be determined from the expression.
v = 𝑅2−𝑅1
𝛼. 360.
𝑛
1000 m/min
n = rpm of the cam
R1,R2 = radius mm
In face or drum type cam mechanism the speed of the follower depends upon the steepness of
the grove consider for instance. The profile development of drum cam segment a deplict the
steep rise of follower corresponding to the rapid advanced segment deplict the slow rise
corresponding to the steep full corresponding to the rapid withdraw of cutting tool.
v = ℎ
𝑏 .
𝜋.𝐷.𝑛
1000 m/min
h = rise during the working stroke
b = length of the working stroke
D = diameter of the drum in mm
s = rpm of drum
4 .Nut and Screw transmission –
25
Department of Mechanical Engineering, RTU
Schematic diagram of anti-friction nut and screw transmission
A nut and screw mechanism is schematically depicted the screw and nut have a trapezoidal
thread. The direction movement can reverse by reversing rotation of the screw. The nut and
screw transmission is compact but has a high load carrying case capacity its other advantage
are simplicity case of manufacturing the possibility of achieving slow and uniform movement
of the operating member.
The speed of operating member can be found from relationship
Sm = t.k.n mm/min
t = pitch of thread
k = number of thread
n = rpm of the screw
5. Rack and pinion transmission
Fig: rack and pinion transmission
26
Department of Mechanical Engineering, RTU
When the rotating gear meshes with a stationary rack, the centre of the gear moves in
straight line on the other hand if the gear axis is stationary then the rack executes
translatory motion. The direction of motion can be reversed by reversing the rotation of the
pinion.
Sm = 𝜋.z.n mm/min
Sm = feed per minute of the operative member
m = module of the pinion
z = number of teeth of the pinion
n = rpm of the pinion
Rack and pinion transmission is the simplest and cheapest among all types of transmission used
in reversible driven. It also has high efficiency and provides a large transmission ratio which
makes it possible to use it in the feed as well as main drive mode.
EXPERIMENT NO.6
Objective: Draw the diagram of following mechanism.
1. Ratchet gear mechanisms
2. Geneva mechanism
3. Reversing mechanism
4. Differential mechanism
Introduction –
Devices for intermittent motion –
27
Department of Mechanical Engineering, RTU
In some machine tools, it is required that the relative position between the cutting tool and
workpiece should change periodically.
a .Machine tools with a reciprocating primary cutting motion e.g. shaping machine in which
the workpiece must be intermittently upon completion of one full stroke of the cutting tool.
b .machine tools with reciprocating feed motion.
1. Ratchet gear mechanism –
The Ratchet gear mechanism is generally consists of a pawl mounted on an oscillating pin.
During each oscillation in the anticlockwise direction, the pawl turns the ratchet wheel through
a particular angle. During the clockwise oscillating in the opposite direction, the pawl simply
slides over the ratchet teeth and the latter remain stationary. The ratchet wheel is linked to the
machine tool table through a nut and screw transmission. Therefore the periodic rotation of the
ratchet wheel is transformed into the intermittent translator motion of the table for a particular
nut and screw pair of some constant transmission ratio. The feed of the table during each
oscillation depends upon the swing of the oscillating pawl. The rotation of the ratchet wheel in
one stroke of the pawl should not exceed 45’. The ratchet gear mechanism is most suitable in
case when the periodic displacement must be completed in a short time.
2. Geneva mechanism -
Fig: Geneva mechanism
28
Department of Mechanical Engineering, RTU
Geneva mechanism consists of a driving disc which rotates continuously and a wheel a wheel
with four radial slots. The arc on the driving disc and wheel provide a locking effect against
rotation of the slotted wheel e.g. position of the wheel cannot rotate. As the disc continuous to
rotate, point A of the disc comes out of contact with the arc and immediately thereafter pin ‘p’
mounted at the end of the driving arm enters the radial slot.
The wheel now begins to rotate when it has turned an angle 90° the pin comes out of the radial
slot and immediately thereafter point ‘B’ comes in contact with the next arc of the wheel
preventing its further rotation. In the Geneva mechanism the angle of rotation of the wheel
cannot varied.
Application –
(i) Mainly used in torrents.
(ii) Single spindle automatic machine for indexing cutting tools.
(iii) Multi spindle automatic machines for indexing spindle through a constant angle.
3. Reversing mechanism –
These mechanisms are used for changing the direction of motion of the operative member.
Reversing is accomplished generally through spur and helical gears. A few reversing
arrangements using spur and helical gear. In this arrangement the gear on the driving shaft are
mounted rigidly. While the idle gear and gears on the driven shaft are mounted freely. The jaw
clutch is mounted on a key, rotation may be transmitted to the driven shaft either through gear
(A/B), (B/C) or through D/E depending upon whether the jaw clutch is shifted to the left to
mesh with gear C or to the right to mesh with gear E.
In the second arrangement, the gears on the driving shaft are again rigidly mounted and the idle
gear is free. On the driven shaft a double cluster gear is mounted on a spline. By sliding the
cluster gear transmission to the driven shaft may again be achieved either through gear (A/B),
(B/C) or through gear pair D/E.
In the third arrangement gear A on the driving shaft and gear D on the driven shaft are both
rigidly mounted. A quadrant with constantly meshing gear B and C can be swivelling about the
axis of the driven shaft. By swivelling the quadrant with the help of a lever transmission to the
driven shaft may be achieved through (A/C), (C/D) or through (A/B) (B/C)(C/D).
4. Differential mechanism –
29
Department of Mechanical Engineering, RTU
Differential mechanisms are used for summing two motions in machine tools in which
operative member gets input from two separate kinematics trains. They are generally employed
in thread and gear cutting machines where the machined surface is obtained as a result of the
summation of two or more forming motions.
A simple differential mechanism using spur or helical gears is shown. The mechanism is
essentially a planetary gear mechanism consisting of sun gear A, planetary gear B and arm C.
The planetary gear is mounted on the arm which can rotate about axis of gear A. suppose gear
A makes nA and arm C, nC revolutions per minute in the clockwise direction. The relative
motion between the elements of the mechanism will remain unaffected if the whole mechanism
is rotated in the anticlockwise direction with nC revolution per minute.
The transmission ratio of the mechanism may be written as
nA – nC/nB – nC =- zB/zA
Where zA and zB are the number of teeth of gear A and B, respectively. The above expression
may be written as follows.
nB = nC(1+zA/zB) – nA(zA/zB)
Differential mechanisms are using a double cluster planetary gear. The mechanism consists of
gear A, cluster gear block B-B’ mounted on arm C and gear D. If nA, nB, nC are the rpm’s of
gear A, arm C and gear D, respectively then the transmission ratio of the kinematic train
between gear A and D may be expressed as
nD – nC/nA – nC = zA/zB . zB’/zD
The mechanism consist of bevel gears A and D and planetary level gears B and C. Planetary
gear can be rotated about the common axes of gear A and D.
1. By means of a ring gear – this differential is used in automobiles.
2. By means of a T- shaped shaft – this differential is used in machine tools.
3. if gear A,B and D make nA, nB, nD revolutions per minute, respectively, then the transmission
ratio of the kinematic train between gear A and D can be written as
nA – nB/nD – nB = - zA/zB . zB/zD
Where zA, zB, zD are the number of teeth of gears A, B and D respectively.
30
Department of Mechanical Engineering, RTU
EXPERIMENT NO.7
Object: Which Speed Series are used in machine tool gear box. Justify the insure
with process reason.
Gear boxes
-Ap & Gp for steeping speeds of gears.
-Structural Formula & Structural diagrams.
Gear boxes
Machine tool characterized by their large number of spindle speeds and feeds of cape with
the requirements of machine parts of different materials and dimension using different types
31
Department of Mechanical Engineering, RTU
of cutting tool materials and geometries. The cutting speed is determined on the bases of the
cutting ability of the tool used. Surfaces finish required and economical consideration.
Speed Range for different Machine Tools
Machine Range
Numerically Controlled lathes 250
Boring 100
Milling 50
Drilling 10
Surface Finish 4
Stepping of Speed According to Arithmetic Progression (AP)-
Let 𝑛1,𝑛2,……..,𝑛𝑛 be arranged according to arithmetic progression.
Then 𝑛1-𝑛2 = 𝑛3-𝑛2 = Constant
The saw tooth diagram in such a case is show in fig. Accordingly, for an economical cutting
Speed 𝑣0, the lowest speed 𝑣1 is not constant, it decrees with increasing dia. Therefore , the
arithmetic progression does not permit economical machine at large diameter ranges. The main
disadvantage of such an arrangement is that the percentage drop from step to step decrees as
the speed increase. Thus the speed are not evenly distribution and more concentrated and
closely stepped , in the small diameter range than in the large one. Stepping speeds according
to arithmetic progression are used in Norton gear box with a sliding key when the number of
shaft is only two.
Speed stepping according to arithmetic progression
Stepping of Speed According to Geometric Progression (GP)-
32
Department of Mechanical Engineering, RTU
As show in Figure, the percentage drop from one step to the other is constant, and the absolute
loss of economically expedient cutting speed ∆v is constant all over the whole diagram range.
The relative loss of cutting speed ∆𝑉𝑚𝑖𝑛/𝑉0 is also constant Geometric progression. Therefore,
allow machining to take place between limits 𝑉0 and 𝑉𝑢 independent of the WP diagram, where
𝑉0 is the economical cutting speed and 𝑉𝑢 is the allowable minimum cutting speed. Now
suppose that 𝑛1,𝑛2,……..,𝑛𝑧 are the spindle speeds. According to the geometric progression
𝑛2
𝑛1=
𝑛2
𝑛1= ∅
Where Ø is the progression ratio. The spindle speed can be expressed in term of the minimal
speed n1 and progression ratio Ø
Hence, the maximum spindle speed nz is given by
𝑛𝑧 = 𝑛1∅𝑧−1
Where z is the number of spindle speed, therefore
∅ = √𝑛2
𝑛1
𝑧−1
= √𝑅𝑛𝑧−1
𝑧 =log 𝑅𝑛
log ∅+ 1
ISO Standard values of progression ratio Ø
n1 n2 n3 n4 nz
n1 n1 Ø n1 Ø2 n1 Ø
3 n1 Øz-1
33
Department of Mechanical Engineering, RTU
(1.06, 1.12, 1.26, 1.4, 1.6, 1.78, 2.0)
Justify ensuring with reason
1. Transmission ratio imax =2, imax =1/4, ig = imax/ imin=8
2. Minimum total shaft size
The torque transmitted by a shaft is given by
𝑇 ∝1
𝑁
From the strength consideration: (𝑑1
𝑑2⁄ ) = (
𝑁2𝑁1
⁄ )1/3
3. For last radial dimensions of gear box imax* imin = 1
4. No of gears on last shaft should be minimum.
5. No of gears on any shaft should be limited to 3
EXPERIMENT NO.8
Object: Design Procedure of machine tool gear box design
Gear Box
Machine tools are characterized by their large number of spindle speeds and feeds to cope with
the requirements of machining parts of different materials and dimensions using different types
of cutting tool materials and geometries. The cutting speed is determined on the bases of the
cutting ability of the tool used, surface finish required, and economical considerations. A wide
variety of gearboxes utilize sliding gears or friction or jaw coupling. The selection of a
particular mechanism depends on the purpose of the machine tool, the frequency of speed
change, and the duration of the working movement. The advantage of a sliding gear
transmission is that it is capable of transmitting higher torque and is small in radial dimensions.
Among the disadvantages of these gearboxes is the impossibility of changing speeds during
running. Clutch-type gearboxes require small axial displacement needed for speed changing,
less engagement force compared with sliding gear mechanisms, and therefore can employ
helical gears. The extreme spindle speeds of a machine tool main gearbox nmax and nmin can be
determined by
𝑛𝑚𝑎𝑥 =1000 𝑉𝑚𝑎𝑥
𝜋𝑑𝑚𝑖𝑛
𝑛𝑚𝑖𝑛 =1000𝑉𝑚𝑖𝑛
𝜋𝑑𝑚𝑎𝑥
where Vmax = maximum cutting speed (m/min) used for machining the most soft and
machinable material with a cutting tool of the best cutting property Vmin = minimum cutting
speed (m/min) used for machining the hardest material using a cutting tool of the lowest cutting
34
Department of Mechanical Engineering, RTU
property or the necessary speed for thread cutting dmax, dmin = maximum and minimum
diameters (mm) of WP to be machined
The speed range Rn becomes
𝑅𝑛 =𝑛𝑚𝑎𝑥
𝑛𝑚𝑖𝑛=
𝑉𝑚𝑎𝑥
𝑉𝑚𝑖𝑛 .
𝑑𝑚𝑎𝑥
𝑑𝑚𝑖𝑛= 𝑅𝑣𝑅𝑑
Rv = cutting speed range Rd = diameter range In case of machine tools having rectilinear main
motion (planers and shapers), the speed range Rn is dependent only on Rv. For other machine
tools, Rn is a function of Rv and Rd, large cutting speeds and diameter ranges are required.
Generally, when selecting a machine tool, the speed range Rn is increased by 25% for future
developments in the cutting tool materials.
Design procedure for gear box
1. Determine the maximum and minimum speed of the output shaft. Then calculate the
number of steps or speeds reduction stages for this range. This depends on the
application as well as space optimization. Higher reduction stages require more space
because of more number of gears and shafts requirements.
2. Select types of speed reduction or gear box based on the power transmission
requirements, gear ratio, and position of axis space available for speed reducer.
Also make sure that for low gear ratio requires single speed reduction. Select worm
gear for silent operation and level gear for interesting axis.
3. Determine the progression ratio which is ratio maximum speed and minimum speed of
output shaft of the gear box the nearest progression ratio should be a standard one and
it taken either from R20 or R40 series.
4. Draw the structural diagram and kinematic arrangement indicating various arrangement
possibilities during speed reduction or increment.
5. Select materials for gears so that gear should sustain the operating condition and
operating load. Normally cast iron is chosen for housing and cast steel or other all can
be selected as per the load requirements.
6. Note down the maximum power output in horse power (H.P) or transmission power
and revolution per minute of shaft i.e. rpm of each shaft.
7. Determine the centre distance between the driven and driver shaft based on the surface
compressive stress.
8. Determine the module of gear by beam strength as well as fix the number of teeth
required.
35
Department of Mechanical Engineering, RTU
9. Calculate the diameter of the shaft by torque requirements and bending moment
consideration.
10. Calculate the key size, shape or type of transmission key for each gears.
11. Select appropriate fit and tolerance for matting parts like shaft and gear.
12. Select bearing types or the loading and operating conditions. Also make sure to include
consideration of maximum speed and expected life of gear and gear box.
13. Make the shaft stepped or provide collar to prevent axial displacement of bearing and
gear.
14. Provide suitable clearance between gear and walls of the housing of gear box and based
on this considerations design the casing/housing of gear box.
15. Complete the design of casing in drawing by providing fires if necessary to have
increased heat transfer by convection and conduction. Put inspection hole/man hole as
well as drain hole to drain lubricating oil. Also provide oil level indicator to have proper
amount of oil during operation, if not out, this will lead to failure of gear and shaft due
to over heating or due to friction failure.
16. Draw neat a clean working drawing in suitable software like auto cad, pro engineer etc.
indicating required details during manufacturing or assembly.
17. One can also perform finite element analysis of the complete gear box after it
completely designed.
36
Department of Mechanical Engineering, RTU
EXPERIMENT NO.9
Object: Description of stick-slip and sliding friction in machine tool design.
Stick-slip Friction
Stick-slip can be described as surface alternate between sticking to each other and sliding
over each other with a corresponding change in the force of friction coefficient between two
surfaces is larger than the reduction of the friction to the kinetic friction can cause a sudden
jump in the velocity of the movements. The attached picture shows symbolically an example
of stick-slip.
V is the drive system, R is the elasticity in the system and M is the load i.e. lying on floor and
is being pushed horizontally. When the drive is started, the spring R is loaded and its pushing
force against load M increases until the static friction coefficient between load M and floor is
not able to hold the load anymore. The load start sliding and the friction coefficient decreases
from its static value to its dynamic value. At this moment, the spring can give more power
and accelerate M.
During M’s movements, the force of the spring decreases, until it is insufficient to the
overcome the dynamic friction. From this point M de-accelerate to a stop. The drive system
however, continues and the spring is loaded again etc.
37
Department of Mechanical Engineering, RTU
Fig.- Stick-Slip Phenomenon
Sliding (motion) Friction
Sliding is a type of friction motion between two surfaces in contact. This can be constructed
to rolling friction. Both types of motion may occur in bearing.
Friction may damage or wear the surface in contact. However, it can be reduced by lubrication.
The science and technology of friction, lubrication, and wear is known as tribology.
Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the
friction force associated with the rotational movement of a somewhat dislike or other circular
object along the surface.
In engg sliding friction occur in numerous types of sliding components such as journal bearing,
cams, linkage, and pistons in cylinders. Static friction is the friction required to move two
surfaces that are not in relative motion.
38
Department of Mechanical Engineering, RTU
EXPERIMENT NO.10
Object: Free body diagram of following machines:
(1) Lathe
(2) Drilling
(3) Shaping
(4) Milling
Introduction
Lathe
The lathe is a machine tool used principally for shaping articles of metal (and sometimes wood
or other materials) by causing the workpiece to be held and rotated by the lathe while a tool bit
is advanced into the work causing the cutting action. The basic lathe that was designed to cut
cylindrical metal stock has been developed further to produce screw threads, tapered work,
Drilled holes, knurled surfaces, and crankshafts. The typical lathe provides a variety of rotating
speeds and a means to manually and automatically move the cutting tool into the workpiece.
Machinists and maintenance shop personnel must be thoroughly familiar with the lathe and its
operations to accomplish the repair and fabrication of needed parts.
39
Department of Mechanical Engineering, RTU
Types of lathe
Lathes can be divided into three types for easy identification: engine lathes, turret lathes, and
special purpose lathes. Small lathes can be bench mounted, are lightweight, and can be
transported in wheeled vehicles easily. The larger lathes are floor mounted and may require
special transportation if they must be moved. Field and maintenance shops generally use a lathe
that can be adapted to many operations and that is not too large to be moved from one work
site to another. The engine lathe is ideally suited for this purpose. A trained operator can
accomplish more machining jobs with the engine lathe than with any other machine tool. Turret
lathes and special purpose lathes are usually used in production or job shops for mass
production or specialized parts. While basic engine lathes are usually used for any type of lathe
work. Further reference to lathes in this chapter will be about the various engine lathes.
Drilling Machine
A drilling machine comes in many shapes and sizes, from small hand-held power drills to bench
mounted and finally floor-mounted models. They can perform operations other than drilling,
such as counter sinking; counter boring, reaming, and tapping large or small holes. Because
the drilling machines can perform all of these operations, this chapter will also cover the types
of drill bits, took, and shop formulas for setting up each operation. Safety plays a critical part
in any operation involving power equipment. This chapter will cover procedures for servicing,
maintaining, and setting up the work, proper methods of selecting tools, and work holding
devices to get the job done safely without causing damage to the equipment, yourself, or
someone nearby. A drilling machine, called a drill press, is used to cut holes into or through
metal, wood, or other materials. Drilling machines use a drilling tool that has cutting edges at
its point. This cutting tool is held in the drill press by a chuck or Morse taper and is rotated and
fed into the work at variable speeds. Drilling machines may be used to perform other
operations. They can perform countersinking, boring, counter-boring, spot facing, reaming, and
tapping.
Drill press operators must know how to set up the work, set speed and feed, and provide for
coolant to get an acceptable finished product. The size or capacity of the drilling machine is
usually determined by the largest piece of stock that can be center-drilled. For instance, a 15-
inch drilling machine cans center-drill a 30-inch-diameter piece of stock. Other ways to
determine the size of the drill press are by the largest hole that can be drilled, the distance
between the spindle and column, and the vertical distance between the worktable and spindle.
40
Department of Mechanical Engineering, RTU
All drilling machines have the following construction characteristics: a spindle, sleeve or quill,
column, head, worktable, and base.
1. The spindle holds the drill or cutting tools and revolves in a fixed position in a sleeve. In
most drilling machines, the spindle is vertical and the work is supported on a horizontal table.
2. The sleeve or quill assembly does not revolve but may slide in its bearing in a direction
parallel to its axis. When the sleeve carrying the spindle with a cutting tool is lowered, the
cutting tool is fed into the work: and when it is moved upward, the cutting tool is withdrawn
from the work. Feed pressure applied to the sleeve by hand or power causes the revolving drill
to cut its way into the work a few thousandths of an inch per revolution.
3. The column of most drill presses is circular and built rugged and solid. The column supports
the head and the sleeve or quill assembly.
4. The head of the drill press is composed of the sleeve, spindle, electric motor, and feed
mechanism. The head is bolted to the column.
5. The worktable is supported on an arm mounted to the column. The worktable can be adjusted
vertically to accommodate different heights of work. or it may be swung completely out of the
way. It may be tilted up to 90° in either direction, to allow for long pieces to be end or angled
drilled.
6. The base of the drilling machine supports the entire machine and when bolted to the floor,
provides for vibration-free operation and best machining accuracy.
7. The top of the base is similar to a worktable and maybe equipped with T-slots for mounting
work too large for the table.
41
Department of Mechanical Engineering, RTU
FBD of Drilling Machine
Shaping Machine
The main functions of shaping machines are to produce flat surfaces in different planes. The
cutting motion provided by the linear forward motion of the reciprocating tool and the
intermittent feed motion provided by the slow transverse motion of the job along with the bed
result in producing a flat surface by gradual removal of excess material layer by layer in the
form of chips. The vertical infeed is given either by descending the tool holder or raising the
42
Department of Mechanical Engineering, RTU
bed or both. Straight grooves of various curved sections are also made in shaping machines by
using specific form tools. The single point straight or form tool is clamped in the vertical slide
which is mounted at the front face of the reciprocating ram whereas the workpiece is directly
or indirectly through a vice is mounted on the bed.
Fig. Shapping Machine
Milling Machine
Milling is the process of machining flat, curved, or irregular surfaces by feeding the workpiece
against a rotating cutter containing a number of cutting edges. The milling machine consists
basically of a motor driven spindle, which mounts and revolves the milling cutter, and a
reciprocating adjustable worktable, which mounts and feeds the workpiece. Milling machines
are basically classified as vertical or horizontal. These machines are also classified as knee-
type, ram-type, manufacturing or bed type, and planer-type. Most milling machines have self-
contained electric drive motors, coolant systems, variable spindle speeds, and power-operated
table feeds. Free body diagram of the milling machine shown in figure below
44
Department of Mechanical Engineering, RTU
EXPERIMENT NO.11
Object: Application of slideways profiles and their combinations
FUNCTIONS OF GUIDEWAYS
The Guideway is one of the important elements of machine tool. The main function of the guideway
is to make sure that the cutting tool or machine tool operative element moves along predetermined
path. The machine tool operative element carries workpiece along with it. The motion is generally
circular for boring mills, vertical lathe, etc. while it is straight line for lathe, drilling, boring machines,
etc.
Requirements of guideways are:
(a) Guideway should have high rigidity.
(b) The surface of guideways must have greater accuracy and surface finish.
(c) Guideways should have high accuracy of travel. It is possible only when the deviation of the
actual path of travel of the operative element from the predetermined normal path is minimum.
(d) Guideways should be durable. The durability depends upon the ability of guideways to retain the
initial accuracy of manufacturing and travel.
(e) The frictional forces acting on the guideway surface must be low to avoid wear.
(f) There should be minimum possible variation of coefficient of friction.
(g) Guideways should have good damping properties.
Guideways can be classified as:
(a) Guideways with sliding friction
(b) Guideways with rolling friction
Guideways with Sliding Friction
The friction between the sliding surfaces is called as guideways with sliding friction. These
guideways are also called as slideways. The slideways are further classified according to the
lubrication at the interface of contacting surfaces. The friction between the sliding surfaces
may be dry, semi-liquid, and liquid. When the lubrication is absent in between contacting
surfaces, it is called as dry friction. Dry friction is rarely occurred in machine tools.
When two bodies slide with respect to each other having lubrication between them, the
sliding body tends to rise or float due to hydrodynamic action of the lubricant film. The
principle of slider is shown in Figure 1.
45
Department of Mechanical Engineering, RTU
Figure1. Principle of a Slider
The hydrodynamic force,
𝐹ℎ = 𝐶 ∗ 𝑣𝑠 (1)
Where C is constant and depends upon wedge angle θ, the geometry of sliding surfaces,
viscosity of the lubricant and parameter of lubricant film.
vs is sliding velocity.
W is weight of the sliding body.
The resultant normal force acting on sliding body,
R = Fh
– W
From Eq. (1), it is clear that the hydrodynamic force increases with increase in sliding velocity.
The sliding body rests on the stationary body when hydrodynamic force is less than the weight
of the sliding body. Here, there are semi-liquid type friction conditions and under these
conditions the two bodies are partially separated by the lubricant film and partially have metal
to metal contact. The resultant normal force on sliding body starts to act upwards and the body
floats as hydrodynamic force is greater than the sliding weight of the body. The sliding surfaces
are completely separated by the lubricant film and liquid friction occurs at their interface. The
slideways in which the sliding surfaces are separated by the permanent lubricant layer are
known as hydrodynamic slideways. This permanent lubrication layer is due to hydrodynamic
action. A permanent lubricant layer between the sliding surfaces can be obtained by pumping
the liquid into the interface under pressure at low sliding speed. The sliding body is lifted by
this permanent lubricant layer. Such slideways are called as hydrostatic slideways.
46
Department of Mechanical Engineering, RTU
Guideways with Rolling Friction
These are also called as anti friction ways. The anti friction slideways may be classified
according to the shape of the rolling element as:
(a) Roller type anti friction ways using cylindrical rollers.
(b) Ball type anti friction ways using spherical balls.
DESIGN OF SLIDEWAYS
Slideways are designed for wear resistance and stiffness.
1. Design of Slideways for Wear Resistance
The wear resistance of slideways is mainly dependent upon maximum pressure acting on the
mating surfaces. This condition may be given as
pmax ≤ pmp (2)
where
pm = maximum pressure acting on the mating surface, and
pmp = permissible value of the maximum pressure.
It is seen during the subsequent analysis that slideway designed for maximum pressure is
quite complicated. Sometimes, this design is replaced by a simple procedure based upon the
average pressure acting on the mating surfaces. The condition is that:
Pa ≤ Pap (3)
Where Pa = average pressure acting on the mating surface, and
Pap = permissible value of the average pressure.
Hence from Eqs. (2) and (3), the design of slideways for wear resistance requires that.
(a) pm and pa to be known,
(b) pmp and pap to be known, and
(c) The values of pmp and pap are given for different operating conditions of slideways on the
basis of experience. For determining pm and pa, the first and foremost task is to determine the
forces acting on the mating surfaces.
Forces acting on the mating surfaces in combination of V and flat slideways.
The combination of V and flat slideways is commonly used in lathe machines. The schematic diagram
of slideways and the forces acting on the system for the case of orthogonal cutting are illustrated in
Figure 2.
47
Department of Mechanical Engineering, RTU
Fig 2 Forces Acting on Combination of V and Flat Sideways
The forces acting on V and flat slideways are :
(a) Cutting force component Fz (in the direction of the velocity vector) and Fy (radial),
(b) Weight of carriage W, and
(c) Unknown forces F1, F2 and F3 acting on the mating surfaces.
The unknown forces are calculated from following equilibrium conditions:
Sum of components of forces acting along Y-axis = 0
Substituting value of F3 in Eq., we get
48
Department of Mechanical Engineering, RTU
If the apex angle of the V is 90o , and assume that present angle γ may change to γ = 90 – λ, the
solution of simultaneous algebraic Eqs. gives :
Substituting the values of F1 and F2 in Eq., we get,
Above eq. represents the forces acting on the mating surfaces in combination of two flat sideways.
The schematic diagram of the slideways and the forces acting on the system under orthogonal
cutting conditions are shown in Figure (3).
Fig. 3 Forces Acting on Combination of Two Flat Slideways
The forces acting on combination of two flat slideways are :
(a) Cutting force components, i.e. axial Fx, radial Fy, and Fz in the direction of velocity vector.
(b) Weight of carriage, W.
(c) Unknown forces F1, F2 and F3 acting on the mating surfaces.
(d) Frictional forces μF1, μF2, μF3, where μ is the coefficient of friction between the sliding surfaces.
49
Department of Mechanical Engineering, RTU
The unknown forces F1, F2 and F3 are calculated from following equilibrium conditions :
From above eq.
On substituting the value of F3 in Eq.
50
Department of Mechanical Engineering, RTU
Slideway
Profile and
Combinatio
n for Bads
Sketch Application
Open V +
Open V
Planning
Machines
Closed V +
Closed V
Precision lathes
and turret lathes
Open flat +
Open V
Surface-grinding
machines
Closed flat +
Closed V
Genral-purpose
lathes & heavy
duty boring
machine
For vertical columns
Closed flat +
Closed flat
Most
Commonly used
for all types of
vertical columne
Closed flat +
Closed flat
Knee types
milling machine
small vertical
drilling machine
and traverses of
radial drilling
machine
51
Department of Mechanical Engineering, RTU
Closed flat +
heavy-closed
dovetail
Same us above
For cross slides and compound rests
Closed
devotail
Cross slides &
compound rests
Closed flat +
Closed flat
Cross slides of
heavy duty
machine tools
For Rotary Blades
Flat
Surface-grinding
machine and
small hobbing
machine
W
Precission gear-
hobby machine
52
Department of Mechanical Engineering, RTU
EXPERIMENT NO.12
Object: Functions and types of Guide ways
Shapes of slideways
(a) (b)
(c)
(d) (e)
Slideways profiles: (a) Flat; (b) Symmetrical V; (c) Asymmetrical V; (d) dovetail
(e) Cylindrical
53
Department of Mechanical Engineering, RTU
EXPERIMENT NO.13
Object: Commonly used bed section and wall arrangement and their applications.
Wall Arrangement Applications
(a) (b)
1.) Beds on logs or sheas
a.) without stiffening, diagonal wall, used
in lathe, turrets, etc.
b.) without stiffening diagonal wall has 30-
40% height stiffness than arrangement (a);
used in multiple tool and height production
lathes.
(c) (d)
c.) with stiffening wall and provision of
chip disposal through opening in rear wall,
used in large sized lathes & turret with
stiffing wall, also used in large-sized laths.
(d) With stiffening wall also used in large
size lathe and turret
2.) Covered top closed profile bed, used in
plan milling, clothing & boring machines.