Chapter 7 EFFECT OF MANUFACTURING PROCESSES ON DESIGNfaculty1.aucegypt.edu/farag/presentations/Chapter7.pdf · Materials and Process Selection for Engineering Design: Mahmoud Farag

Materials and Process Selection for Engineering Design: Mahmoud Farag 1

Chapter 7

EFFECT OF MANUFACTURING

PROCESSES ON DESIGN


Chapter 7: Goal and objectives

The goal of this chapter is to illustrate how the manufacturing

processes influence the design of components, with emphasis on

the following topics:

1. Types of available manufacturing process and their selection

2. Design for manufacture and assembly

3. Design considerations for cast components

4. Design considerations for molded plastic components

5. Design considerations for forged components

6. Design considerations for powder metallurgy parts

7. Designs involving welding processes

8. Designs involving machining processes

Classification of manufacturing processes

Many processes form a natural sequence for shape generation.

For example, casting and forging are normally followed by

machining then surface finishing if needed, see Figure 7.1.

Processes can be grouped as follows:

a) Primary processes:

casting, bulk forming (forging, rolling, extrusion), etc.

b) Primary/secondary processes:

joining and welding, sheet-metal work, heat treatment,

metal cutting, etc.

c) Tertiary processes or finishing processes:

surface treatment, grinding, coating, etc.



Selection of manufacturing processes

Not all processes are suitable for all materials.

For example,

• cast iron cannot be forged

• powder metallurgy is uneconomical for a limited production

run.

Table 7.1 outlines the compatibility between some widely used

metallic materials and processes


Design for manufacture and assembly

Design example 7.1 - Application of DFMA principles to the

design of a motor-drive assembly I (based on Boothroyd)

It is required to design a motor-drive assembly,

The motor must have a removable cover, a rigid base that supports

both the motor and sensor in addition to sliding up and down the

guide rails.

Figure 7.2 shows a proposed design, which requires two

subassemblies for the motor and sensor in addition to 8

additional parts and 9 screws making a total of 19 items to be

assembled.



Table 7.1 Compatibility between some widely used metallic materials and processes

Carbon

steel

Stainless

steel

Cast iron Al alloys Cu and

alloys

Mg and

alloys

Zn and

alloys

Ti and

alloys

Super-

alloys

Sand casting

XX

XX

XX

XX

XX

XX

X

nr

XX

Investment casting XX XX nr XX XX X nr X XX

Die casting nr nr nr XX X XX XX nr nr

Powder metallurgy XX XX nr XX XX nr nr X XX

Forging XX XX nr XX XX XX nr X X

Rolling XX XX nr XX XX XX X X XX

Extrusion X X nr XX XX XX X X X

Sheet-metal work XX XX nr XX XX X X X X

Cold heading XX XX nr XX XX nr nr nr X

Metal cutting XX XX XX XX XX XX XX X X

Fusion welding XX XX X XX XX XX nr XX XX

XX Common practice, X less common or performed with difficulty, nr not recommended


Design example 7.1 - Application of DFMA principles to

the design of a motor-drive assembly II

Analysis

It is possible to eliminate some of the parts, see Figure 7.3.

• The number of items is reduced to 6

• The assembly time is reduced from 160 seconds to 46 seconds

• The cost reduced from $1.33 to $0.38.

The base is machined out of nylon instead of aluminum to eliminate

the bushings and reduce the cost of from $2.34 to $0.49.

The new base has less tapped holes, with further reduction of cost.

The total cost is reduced from $35.08 to $22.00 .



Factors to be considered when selecting

casting as a manufacturing process I

1. Casting is particularly suited for parts that contain inaccessible

internal cavities, complex, or large.

2. It is better to cast complex parts when required in large numbers,

especially if they are to be made of aluminum or zinc alloys.

3. Casting techniques can be used to produce a part which is one of a

kind, especially when it is not feasible to make it by machining.

4. Precious metals are usually shaped by casting, as there is little loss

of material.

5. Parts produced by casting have isotropic properties.


Factors to be considered when selecting

casting as a manufacturing process II

6. Casting is not competitive when the parts can be produced

by punching from sheet or by deep drawing.

7. Extrusion can be preferable to casting in some cases,

especially in the case of lower - melting nonferrous alloys.

8. Casting is not usually a viable solution when the material

is not easily melted, as in the case of metals with very high

melting points such as tungsten.



Table 7.2 Approximate values of surface roughness and tolerance that are normally obtained

with different manufacturing processes

Process Typical tolerance (±) Typical surface roughness (Ra)

(mm) (in x 103) (μm) (μin)

Sand casting 0.5-2.0 20-80 12.5-2.5 500-1000

Investment casting 0.2-0.8 8-30 1.6-3.2 63-125

Die casting 0.1-0.5 4-20 0.4-1.6 16-63

Powder metallurgy 0.2-0.4 8-16 0.8-3.2 32-125

Forging 0.2-1.0 8-40 3.2-12.5 125-500

Hot rolling 0.2-0.8 8-30 6.3-25 250-1000

Hot extrusion 0.2-0.8 8-30 6.3-25 250-1000

Cold rolling 0.05-0.2 2-8 0.4-1.6 16-32

Cold drawing 0.05-0.2 2-8 0.4-1.6 16-32

Cold extrusion 0.05-0.2 2-8 0.8-3.2 32-125

Flame cutting 1.0-5.0 40-200 12.5-25 500-1000

Sawing 0.4-0.8 15-30 3.2-25 125-1000

Turning and boring 0.025-0.05 1-2 0.4-6.3 16-250

Drilling 0.05-0.25 2-10 1.6-6.3 63-250

Shaping and planning 0.025-0.125 1-5 1.6-12.5 63-500

Milling 0.01-0.02 0.5-1 0.8-6.3 32-250

Chemical machining 0.02-0.10 0.8-4 1.6-6.3 63-250

EDM and ECM 0.02-0.10 0.8-4 1.6-6.3 63-250

Reaming 0.02-0.05 0.4-2 0.8-3.2 32-125

Broaching 0.01-0.05 0.4-2 0.8-3.2 32-125

Grinding 0.01-0.02 0.4-0.8 0.1-1.6 4-63

Honing 0.005-0.01 0.2-0.4 0.1-0.8 4-32

Polishing 0.005-0.01 0.2-0.4 0.1-0.4 4-16

Lapping and surface finishing 0.004-0.01 0.16-0.4 0.05-0.4 2-16


Table 7.3 Characteristics of different casting processes and powder metallurgy

Process Alloy Weight

kg

Surface finish

μm

Tolerance

m/m

Minimum

thickness

mm

Porosity

rating

Least

econ.

quantity

Relative

production

rate

Sand casting

Most 0.2 and up 12.5-25 0.03-0.2 3-5 Fair 1 1

Shell

molding

Most 0.2-10

1.6-12.5

0.01-0.03 2-5

Good 500 4

Gravity die

casting

Non-ferrous 0.2-10

1.6-12.5

0.02-0.05 3-5

Very good 500 4-5

Pressure die

casting

Al, Zn, Mg,

Cu alloys

0.2-10

0.4-1.6

0.001-0.05 1-2

Excellent 10,000 10

Investment

casting

Most 0.1-10

1.6-3.2

0.002-0.005 0.5-1

Very good 50 6

Powder

metallurgy

Most 0.01-5

0.8-3.2

0.002-0.005 0.8 Variable 5000 8








Guidelines for designing weldments I

1. Welded structures should be designed to have sufficient

flexibility.

Structures that are too rigid do not allow shrinkage of the weld

metal and are subject to distortions and failure.

2. Accessibility of the joint for welding, welding position, and

component match up are important elements of the design.

3. Thin sections are easier to weld than thick ones.

4. Welded sections should be about the same thickness.

5. It is better to locate welded joints symmetrically around the axis

of an assembly in order to reduce distortion.


Guidelines for designing weldments II

6. If possible, welded joints should be placed away from the

surfaces to be machined.

7. An inaccessible enclosure in a weldment, or the mating surfaces

of a lap joint, should be completely sealed to avoid corrosion.

8. Where strength requirements are not critical, short intermittent

welds are preferable to long continuous ones.

9. Help shrinkage forces to work in the desired direction by

presetting the welded parts out of position before welding so that

shrinkage forces will bring them into alignment.

10. Use weld fixtures and clamps to reduce distortion.


Guidelines for designing weldments III

11. Whenever possible, meeting of several welds should be

avoided.

12. Balance shrinkage forces in a butt joint by welding

alternately on each side.

13. Remove shrinkage forces by heat treatment or by shot

peening.

14. Tolerances in the order of +1.5 mm (ca. +1/16 in) are

possible in welded joints.

Surfaces that need closer tolerances should be finished by

machining after welding and heat treatment.




Load carrying capacity of two fillet welds, P = 2 x 0.3 S x 0.707 t x L

Where: L = Length of weld

t = leg of weld, in this case same as plate thickness


Advantages and limitations of using

adhesives I

The main advantages of adhesives include:

1. Thin sheets and parts of dissimilar thicknesses can be easily bonded.

2. Adhesive bonding is ideal for joining polymer matrix composites.

3. Dissimilar or incompatible materials can be bonded.

4. Adhesives are electrical insulators and can prevent galvanic action

in joints between dissimilar metals.

5. Flexible adhesives spread bonding stresses over wide areas and

accommodate differential thermal expansion.

6. Flexible adhesives can absorb shocks and vibrations, which

increases fatigue life.



adhesives II

7. Preparation of bonded joins requires no fastener holes which

gives better structural integrity and allows thinner gage materials

to be used.

8. Adhesives provide sealing action in addition to bonding.

9. The absence of screw heads, rivet heads, or weld beads in

adhesive bonded joints is advantageous where interruption of

fluid flow cannot be tolerated or where appearance is important.

10. Adhesive bonding can also be used in conjunction with other

mechanical fastening methods to improve strength of the joint.



adhesives III

The main limitations of adhesives include:

1. Bonded joints are weaker under cleavage and peel loading

than under tension or shear.

2. Most adhesives cannot be used at temperatures above 300oC.

3. Solvents and UV light can attack adhesive bonded joints.

4. The designer should also be aware of the adhesive's impact

resistance and creep, or cold flow, strength.




Table7.5 Machinability index of some common metallic materials

Material Hardness(BHN) Machinability index

Steels

AISI 1015 121 50

1020 131 65

1030 149 65

1040 170 60

1050 217 50

1112 120 100

1118 143 80

1340 248 65

3140 262 55

4130 197 65

4340 363 45

18-8 stainless steel 150-160 25

Cast irons

Gray cast iron: soft 160-193 80

medium 193-220 65

hard 220-240 50

Malleable iron 110-145 120

Nonferrous alloys

Aluminum alloys 35-150 300-2000

Bronze 55-210 150-500

Magnesium alloys 50-75 500-2000

Zinc alloys 80-90 200






Design example 7.3–Redesign of a part for easy machining I

Figure 7.17 shows the initial design of the shaft support bracket,

which is bolted to a housing to support a rotating shaft. Accurate

machining is needed for the bore with high tolerance in locating

the bore relative to the dowel holes.

Analysis

Initial design had the following features that are difficult to machine:

• Different diameters for the dowels and bolt holes, which requires

tool change and loss of time

• The bore and oil hole are long relative to their diameter, which

require long processing steps.

• The is no obvious features on the outer surface to fix the part and

prevent rotation during machining.



Design example 7.3–Redesign of a part for easy machining II

Solution

For easy machining, the part was redesigned as shown in Fig. 7.18:

• The dowels and bolt holes have the same diameter.

• The center of the bore has a larger diameter than the ends to reduce

length to be machined.

• The length of the oil hole is reduced.

• Flat surfaces were cast on outer surfaces for ease of location while

machining.

Conclusion

These changes reduced the machining time from 173 to 119 seconds,

(33%). Quality is also better and higher tolerances are possible.



Chapter 7: Summary I

• As the design progresses from concept to configuration and the material choices get narrower, manufacturing processes, which have initially been broadly defined, also need to be better identified. The compatibility between materials and processes is used to narrow down the available alternatives.

• DFMA seeks to minimize the cost by designing components that are easier to manufacture (DFM) and designing components that are easier to assemble (DFA).

• Casting is particularly suited for parts which contain internal cavities that are inaccessible, too complex, or too large to be easily produced by machining. Cast parts can contain shrinkage cavities if not designed well.


Chapter 7: Summary II

• Compression, transfer, and injection molding processes are commonly used for molding plastic parts.

• Forged parts have wrought structures which are usually stronger and more ductile than cast products. Rapid changes in thickness of forged components could result in cracks and surface laps.

• Powder metallurgy techniques can be used to produce a large number of small parts to the final shape with no machining, and at high rates. Many metallic alloys, ceramic materials, and composites can be processed by powder metallurgy techniques.

• Welding has replaced riveting in many applications including steel structures, boilers, and motorcar chassis. Welded joints represent areas of discontinuities and should be located away from highly stressed regions.


Chapter 7: Summary III

• Adhesives are an attractive method of joining and are increasingly used for thin sheets, polymer composites, and dissimilar or incompatible materials.

• Adhesives are electrically insulting, which can prevent galvanic corrosion in joints between dissimilar metals. However, they are relatively weaker and can be attacked by organic solvents.

• Many alloys can be heat treated to achieve certain desired properties. Heat treatment can make the material hard and brittle or it can make it soft and ductile.

• Machining operations are the most versatile and most common manufacturing processes. Machining could be the only operation involved in the manufacture of a component or it could be used as a finishing process.

Documents

Chapter 7 EFFECT OF MANUFACTURING PROCESSES ON DESIGNfaculty1.aucegypt.edu/farag/presentations/Chapter7.pdf · Materials and Process Selection for Engineering Design: Mahmoud Farag