Comparison of Dimensional Repeatability and Accuracy for

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

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Comparison of Dimensional Repeatability and Accuracy for Deformation

Machining Stretching Mode with Sheet Metal Components

Arshpreet Singh1, Anupam Agrawal2*

School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology

Ropar, Rupnagar-140001, India. 1 [email protected],

2*[email protected]

Abstract

In the present work a comparative study of dimensional repeatability and accuracy for deformation machining

stretching mode and sheet metal components has been performed. Deformation machining enables the creation of

complex structures and geometries, which would rather be difficult or sometimes impossible to manufacture. This

process allows the creation of monolithic components with novel geometries which were earlier

assembled.Experimental studies have been performed for parts created by the DM ‘stretching mode’ process, in

which a thin horizontal floor is machined on the part through high speed machining, and then incrementally formed

into a conical frustum with a single point forming tool.Ten similar components were fabricated by DM stretching

mode, single point increment forming and conventional stretch forming. These components were measured at

various forming depths using a coordinate measuring machine (CMM) and the dimensional repeatability of these

processes was compared.The dimensional repeatability of the DM stretching mode components largely depends

upon the accuracy of the machined floor. Other factors influencing the repeatability of the process are residual

stresses generated during machining, elastic deformation, spring back and highly localised yielding. Keywords: Deformation machining, single point increment forming, thin structure machining.

1 Introduction Deformation machining (DM) is a combination of

two processes-thin structure machining and single point

incremental forming (SPIF). This hybrid process

enables the creation of lighter weight components with

novel and complex geometries which earlier required

complex tooling and equipments. It allows the creation

of monolithic parts which were earlier assembled[Smith

et al. (2007)]. Therefore, enabling cost reduction in

equipment, fabrication and weight of the components.

Thin structure machining is different from

conventional machining due to the lack of stiffness of

machined structure. Therefore, it requires different

machining techniques like use of long slender end mills

[Tlusty et al. (1996)] along with high speed

machining[Smith and Dvorak (1998)]. Single point

incremental forming (SPIF) is a die less forming process

where a hemispherical shaped single point solid tool is

used to deform the thin structure to a desired shape

incrementallyusing computer numeric control.[Jeswiet

et al. (2005)].In this process the thin structure or sheet

metal is deformed locally into plastic stage, enabling

creation of complex shapes according to the tool path

generated by a CNC machining centre.[Malhotra et

al.(2010)]. SPIF hasenabled flexibility in creation of

symmetric, asymmetric and random shapes with

sufficient amount of accuracy.The potential application

of such monolithic parts with complex geometries is in

aerospace industry (e.g. mold lines of fuselage, avionic

shelf, impellers, pressurized bulk heads), biomedical

engineering (cranial plate, bone and joint support,

prosthetics) [Ambrogio et al. (2006)], heat transfer

(irregular, curved fins).

This process can be broadly classified into two –

Deformation machining stretching mode; where the

deformation along the axis of tool resulting in stretching

of the machined thin horizontal structure (thin floor)

(Figure 1) and Deformation machining bending mode;

where the deformation perpendicular to axis of tool

resulting in the bending of thin vertical structure (thin

wall) which was prior machined (Figure 2).

Figure 1 Components manufactured by DM

Stretching Mode process [Smith et al. (2007)]

Figure 2 Components manufactured by DM Bending

Mode process [Smith et al. (2007)]

Comparison of Dimensional Repeatability and Accuracy for Deformation Machining Stretching Mode with Sheet Metal Components

Comparative studies on dimensional repeatability

and fatigue life of DM components

bent components have shown that the DM components

have shown better repeatability than SPIF components

but worse than conventionally bent components

better fatigue life than conventionally bent components

[Agrawal et al. (2012)].

accuracy of the process is mainly attributed to the

influence of residual stresses induced during forming

and spring back effect[Duflou et al

happens when the forming load is removed and the

formed component tries to regain its preformed shape,

thus reducing the overall accuracy of the component.

Geometric accuracy of the formed components can be

improved by providing necessary tool

compensation taking the elas

[Wei et al.(2011)].Bending effect at the beginning of

forming also plays an important role in reducing the

accuracy of formed components.

2 Methodology In the present work ten components each

stretching mode, SPIF and conventional stretch

were fabricated on a 3 axis CNC vertical milling

machine (Make: BFW, Model: VF 30 CNC VS)

inspected for dimensional accuracy and repeatability

using a coordinate measuring machine (CMM) (Ma

Accurate). A fixture holding DM stretching mode

components (Figure 3a) was designed and fabricated

consists of a holding plate and clamps for holding thick

components, a backing plate for supporting the

components during high speed machining, and a

plate for overall stability of the fixture. During forming,

the backing plate (Figure 3b)

fixture and the components were formed through a

circular orifice in the holding plate.

components and conventionally formed compo

same fixture was used by replacing the above mentioned

plates with a set of normal plates with a circular

(Figure 3c).

(a)


tudies on dimensional repeatability

and fatigue life of DM components with conventionally

have shown that the DM components

have shown better repeatability than SPIF components

but worse than conventionally bent components and

onventionally bent components

]. Relatively poor geometric

of the process is mainly attributed to the

influence of residual stresses induced during forming

Duflou et al.(2007)]. Spring back

happens when the forming load is removed and the

formed component tries to regain its preformed shape,

thus reducing the overall accuracy of the component.

Geometric accuracy of the formed components can be

improved by providing necessary tool path

taking the elastic spring back into account

Bending effect at the beginning of

forming also plays an important role in reducing the

accuracy of formed components.

In the present work ten components each from DM

stretching mode, SPIF and conventional stretch forming

3 axis CNC vertical milling

machine (Make: BFW, Model: VF 30 CNC VS) and

inspected for dimensional accuracy and repeatability

using a coordinate measuring machine (CMM) (Make:

A fixture holding DM stretching mode

) was designed and fabricated. It

consists of a holding plate and clamps for holding thick

components, a backing plate for supporting the

components during high speed machining, and a base

plate for overall stability of the fixture. During forming,

(Figure 3b) was removed from the

fixture and the components were formed through a

circular orifice in the holding plate. For SPIF

and conventionally formed components

same fixture was used by replacing the above mentioned

plates with a set of normal plates with a circular orifice

(a)

Figure 3 Fixture for holding the components

The workpiece

6101-T6, a commonly used alloy in aerospace, aviation

and marine industry. Table 1 depicts the me

properties of Al 6101

material billet was firstly machined to component size

of 90×90×12 mm to be held in the fixture. Then, floor

of 1.0 mm thickness of diameter 70

by high speed plunge milling technique

components, five were machined within a machining

tolerance of ± 10 µm and the rest five within ±

Thereafter, the

incrementally formed into a conical frustum using a

single point tool with

geometry of the

figure 4.

Table 1

Properties

Density

Melting Point

Poisson’s ratio

Modulus of elasticity

Tensile strength

Yield strength

Figure 4 Geometry

For SPIF and conventionally formed

sheet metal of the same alloy was prepare

150×150 mm. The sheet was

formed using a single point tool and a mandrel of the

size of the conical frustrum to be formed

Contour tool path was employed f

forming. Table 2 show

machining,


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(b) (c)

Figure 3 Fixture for holding the components

workpiece material used in the present study is Al

T6, a commonly used alloy in aerospace, aviation

and marine industry. Table 1 depicts the mechanical

properties of Al 6101-T6. For the DM components raw


f 90×90×12 mm to be held in the fixture. Then, floor

mm thickness of diameter 70mm were machined

by high speed plunge milling technique.Out of the ten


tolerance of ± 10 µm and the rest five within ± 25 µm.

Thereafter, the machined floor of desired thickness was


single point tool with a hemispherical end. The

of the formed conical frustum is shown in

Table 1 Mechanical properties of 6101-T6

Properties Magnitude

Density 2.7 gm/cc

Melting Point 600°C

Poisson’s ratio 0.33

Modulus of elasticity 70 GPa

Tensile strength 97 MPa

Yield strength 76 MPa

Figure 4 Geometry of conical frustum

For SPIF and conventionally formed components,

sheet metal of the same alloy was prepared of size

150×150 mm. The sheet was held in the fixture and


size of the conical frustrum to be formed, respectively.

Contour tool path was employed for incremental

forming. Table 2 show various parameters of

incremental and conventional forming.

2

material used in the present study is Al

T6, a commonly used alloy in aerospace, aviation

chanical

. For the DM components raw


f 90×90×12 mm to be held in the fixture. Then, floor

mm were machined

Out of the ten


25 µm.

was


emispherical end. The

in

components,

d of size

held in the fixture and


.

cremental

ious parameters of

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12


Table 2. Parameters for high speed m

and conventional forming

High

Speed

Machining

Tool material Tungsten

Carbide

Tool

Diameter 16mm

Spindle speed 1200 rpm

Transverse

feed (x,y)

400

mm/min

Axial feed (z) 20 mm/min

Cooling/Lubr

ication

Flood

cooling

Figure 5 (a) shows the schematic; (b)

component formed by DM stretching mode.

Figure 7 Graph of standard

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

Table 2. Parameters for high speed machining, SPIF

and conventional forming

High

Speed

Machining

SPIF

Conventio

nal

Forming

ten

Carbide SS 304 SS 304

16mm 10mm Conical

Mandrel

1200 rpm 100 rpm 50 rpm

mm/min

200

mm/min N.A.

20 mm/min 10

mm/min 10 mm/min

Flood

cooling

Mobil

oil-40

Mobil oil-

40

(a) shows the schematic; (b) actual

component formed by DM stretching mode.

Thereafter, all the components fabricated by the

three processes were inspected on the coordinate

measuring machine (CMM). Diameters of the formed

cone at 0 mm

2mm were measured. Figure 6 shows the schematic of

one of the components measured on the CMM showing

diameters at different depths.

Figure 6 Measured component on the CMM

Figure 7 Graph of standard deviation in diameter of ten components of respective processes v/s the forming

depth.

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

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cone at 0 mm up to 22 mm depth with an interval of



diameters at different depths.

Figure 6 Measured component on the CMM

deviation in diameter of ten components of respective processes v/s the forming

, 2014, IIT

3




up to 22 mm depth with an interval of




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3 Results and discussion 3.1 Dimensional Repeatability

Variation in diameters at different points across the

depth of conical frustum for all the similar components

made by three different processes was recorded.

Standard deviation in the diameter of the ten

components of the respective processes has been plotted

in the figure 7 across the forming depths. From the

graph it is clear that the dimensional repeatability of the

conventionally formed components is the best among all

the processes studied. The standard deviation of

measured diameters for the ten components across the

forming depth varies between 0.033 to 0.053mm with

an average of 0.045 mm. Dimensional repeatability of

the SPIF components is found to be the least among the

three processes. The standard deviation of measured

diameters for the ten components formed by SPIF

across the forming depth varies between 0.229 to

0.259mm with an average of 0.244 mm. Dimensional

repeatability of the DM components has been found

dependant on the machining tolerance of the thin floor

to be formed. The standard deviationof measured

diameters for the five components formed by DM with

machining tolerance of ±10µm varies between 0.074 to

0.097mm with an average of 0.088 and rest five

components with machining tolerance of ±25µm varies

between 0.195 to 0.217mm with an average of 0.204

mm. The results reveal that the dimensional

repeatability of the DM components is comparable with

the conventionally formed components subject to the

machining accuracy of the thin floor to be formed.

The variation in the dimensions of SPIF and DM

components could be further attributed, though

unconclusively, to uneven redistribution of residual

stresses during the incremental forming, compared to

conventional forming. But, redistribution of residual

stresses during high speed machining of the thin floor

for DM components might have a positive influence as

their dimensional repeatability is better than the SPIF

components and comparable to the conventionally

formed components.

3.2 Dimensional Accuracy

Average of the ten diameters at different points

across the depth for all the components formed by the

three processes have been plotted and compared with

the actual required dimensions in figure 8. From the

graph it is evident that the dimensional accuracy of the

conventionally formed components is the best with the

average variation of 1.875mm from the actual required

dimensions. Average variation of the measured diameter

from the required diameter across forming depths for

DM components and SPIF components is comparable

with the magnitude of 6.430mm and 5.982mm

respectively.

Poor dimensional accuracy for DM and SPIF

components is attributed mainly to the bending effect at

the start of the forming (figure 9) and elastic spring back

effect prominent in incremental forming.

Figure 9 Bending effect in DM components

Figure 8. Graph average diameter of the ten components of respective processes v/s the forming depth.

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT


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4 Conclusion Comparative study on dimensional repeatability

and accuracy for deformation machining stretching

mode components with sheet metal components has

been performed. Dimensional repeatability of

conventionally formed sheet metal components is the

better than that of DM components and SPIF sheet

metal components. The poor repeatability of the DM

and SPIF components could be attributed the uneven

redistribution of residual stresses, however this could

not be confirmed conclusively. The role of residual

stresses in incremental forming could be seen as a new

research scope.

Dimensional accuracy ofthe DM components and

SPIF componentsis poorer than the conventionally

formed sheet metal components. This is attributed to the

prominent bending effect and the elastic spring back

effect. Future work goes into developing of a good

strategy to counter these twin problems so as to improve

the overall process accuracy.

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Ambrogio, G., Napoli, L.,Filice, L.,Gagliardi, F. and

Muzzupappa, M. (2006), Application of incremental

forming process for high customized medical product

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Comparison of Dimensional Repeatability and Accuracy for