DESIGN FOR MANUFACTURING AND ASSEMBLY

Design for manufacturing and assembly is mainly a combination of two other

branches of manufacturing technology, namely:

(i) Design for manufacturing: It is mainly based on the production of single product

such as gear, shafts, etc.

(ii) Design for assembly: It is related to the assembly of all the single products

manufactured individually, in order to get the final product (e.g. a motor car).

This branch of manufacturing is mainly applied in manufacturing industries. The job

of manufacturing and assembly is done by a team of engineers, manufacturing

managers, sales professionals, market professionals and cost accountants.

The main two aspects of an industry is time and economy. The products are to be

manufactured in such a way that the product is cost effective and needs less time of

production.

# How to control economy in production?

- The economy can be controlled in production by taking into consideration of

the following factors:

By reducing number of single products to form a final product

By reducing product handling

By reducing production time

By reducing inventory

By reducing inspection operations

By reducing development time, etc.

Design for manufacturing guidelines (DFM guidelines)

1. To reduce number of parts

Design the manufacturing process for improved efficiency, ease of

manufacturing and cost effectiveness.

This leads to reduction of handling time due to which the price of the product

comes down.

The purchase of raw material is less.

Due to less number of parts their assembly time is also reduced to a large

extent.

Testing time is also less.

Number of items to be inspected is less.

Less number of equipment.

2. Design for minimized handling

Due to less number of parts, the handling time will be reduced due to which the

price of the product comes down.

3. Design for maximum compliance

Maximum compliance should be provided for guiding the parts properly during

assembly.

4. Select standard components

Standardization of product was introduced by Colonel Charles Renard.

Standardization of product yields very less variety which is very much necessary to

control economy in production process.

5. Modularize the design

It refers that the design/manufacturing process should be simplified. Simplicity

will help in controlling the economy in the manufacturing process.

6. To design following concurrent engineering principle

To develop the design as well as the manufacturing process simultaneously.

This refers to product design development and process development.

7. Avoid use of fasteners

It is related to manufacturing efficiency

Less number of fasteners raises the manufacturing efficiency

Manufacturing of fasteners will result in increase of production cost

Use snap fit instead of fasteners

8. Minimize assembly design

The gravity force must help the components in their assembly

The components should be designed in such a manner that all the components

can be assembled from a single direction.

9. Design for multiple functional components

The components to be manufactured should act as a support and also as heat

dissipating unit.

10. Design for ease of manufacturing

In this guideline two factors are considered-

(a) Material type

The material should have better machinability. If not selected

properly, the material may be difficult to machine.

But some components require materials having some specific

properties. In that case, we may have to select a material having

lower machinability.

When we choose a material which is difficult to machine on, we

should try to improve the machinability of the material. Thus, we

are not deviating from the guideline directly.

(b) Less post processing operation

Design the component such that it requires less post processing

operations like polishing, painting, etc.

Guidelines for DFM & DFA

1. Design for single item product i.e. multifunctional product.

2. Standardization of product.

3. Dont use multi parts.

4. Design to distinguish similar parts by non-geometric means (e.g. Color coding).

5. Design for using open space for assembly of items.

6. Design so that no special orienting element is incorporated.

7. Design for proper alignment.

8. Design to avoid tangling.

9. Design for providing adequate space between fasteners.

10.

11.

Guidelines for design for Machining

1. Standardization

a) Use standard components.

b) Design pre-shaped components by casting, rolling, forging etc.

c) You should use standard pre-shaped components.

d) Use standard profiles.

2. General

a) Design for use of single machine tool so that all the components can be

machined by single machine tool.

b) To design the component so that unexposed surface does not require

machining.

c) Design the component so that maximum rigidity is restored.

d) Design so that the work, tool holder, work holder should not foul each other.

e) Design so that bent holes are absent.

3. Rotational component

a) To design the component so that cylindrical surfaces are concentric.

b) To design the component so that exposed surfaces are parallel to the axis as

well as some exposed surfaces can be perpendicular to the axis of the

component also.

c) To design the component so that diameter of internal bore decreases from

exposed surface.

d) Not to design long component with internal hole.

e) Not to design a component having small L/D ratio.

f) Not to design a component having larger L/D ratio.

g) To design a component having hole such that the corner radius is larger.

h) To design the component so that corner radius of the internal feature is

inconformity with standard tool corner radius.

i) To design the component so that unexposed surface does not require any

machining work.

4. Non-rotational component

a) To design the component so that it has base compatible for proper mounting.

b) To design the component so that it has parallel as well as perpendicular faces

with respect to base.

c) Design the component so that diameter of the cylindrical hole reduces from

exposed surface.

d) Design a component so that the cross-section is not in excess considering

machining.

e) Design so that standard internal contour can be obtained.

f) Design for comparatively larger radius of internal contour.

g) Avoid design for longer component.

h) Avoid design of thin component.

i) Avoid design of longer hole in longer component.

5. Assembly

a) Design so that assembly is easier.

b) Surfaces of the parts are to be machined.

6. Surface Roughness

a) Use wider surface roughness value (according to requirement).

# On design concepts

a) For chucking with better design

b) Less number of operations required with better design

c) Easy access of cutting tool for machining with better design.

d) Clamping work will be easier with better design

# Overview

(a) Typical machined part

1. A machined part requires high surface finish.

2. Components meant for motion should be finally formed by machining process only.

3. Machining is required for interchangeable parts.

4. Machined part may be of various sizes.

(b) Design recommendation

1. If possible select component with other processes. E.g. casting, forging, rolling, etc.

because of economic reasons.

2. Design for better fixturing.

3. Design for proper surface finish specification.

4. Design for stock specification in order to reduce.

5. Avoid high wall.

6. Reduce number of solders.

7. Use same diameter component to reduce number of operations.

8. Avoid machining of difficult to cut material.

9. Design for rigid component for easy clamping.

10. For component requiring better surface finish provide adequate machining

allowance.

Material Selection

To select a suitable material for designed part-

(a) Study the functional mode of the part

(b) Study the service environment

(c) Study the effect of part on environment

(d) Study the effect of process on environment

Properties of material

(a) Strength

(b) Toughness

(c) Hardness

(d) Roughness

(e) Thermal property, etc.

Selection of processes

(a) Machining

(b) Welding

(c) Casting

(d) Forging

(e) Rolling, etc.

Materials

(a) Steel

(b) Copper

(c) Cast Iron

(d) Aluminium

(e) Ceramics

(f) Composites

(g) Polymers

# Post processing is possible with steel to improve fracture toughness properties.

# Cost of material depends on the compatibility factor of the material.

Steps of material selection

1. Translation: Function, constant, objective

2. Screening: Eliminate the materials which do not do the job.

3. Ranking: To select the material which do the best job.

4. Information: referring to handbooks, websites to verify whether ranking is correct

or not.

Casting process

Appraisal of various casting processes:

1. Sand casting

2. Hot chamber pressure die casting

3. Cold chamber pressure die casting

4. Centrifugal casting

5. Investment casting

6. Shell molding

7. Gravity die casting

8. Low pressure die casting

Design guidelines for casting

1. Parting line should be planar.

2. Proper draft angle should be assigned.

3. Shrinkage cavity should be eliminated by proper alteration in the shape of

casting.

4. Incorporate feeders for parts having flats to avoid shrinkage cavity.

5. Use required number of flasks. Try to use minimum no. of flasks. If possible

use of core is permissible.

6. Calculate (V/A) ratio: - Design so that the V/A ratio decreases from higher to

larger values.

7. Runner passage way should be properly dimensioned considering hot tear

problem. Wastage of metal volume should be reduced.

8. For design of junction, specify relevant fixture so that any hot tear or crack

does not form. Use of chill is permissible. Use of core if possible is

permissible. Reduction of section thickness if possible is permissible. Fillet

radius can be incorporated on requirement for sharp corners.

9. Any boss element below the flange can be extended up to bottom of the flange

to avoid removal of the boss pattern from the mold.

10. Reduce number of cores.

11. If possible incorporate number of fillet radii for various sharp corners.

12. Reduce metal concentration (if loading can be sustained).

13. Provide tapering wherever applicable to promote fast solidification.

14. Provide taper feature extending from cold region to hot region to promote

direction of solidification.

15. Do not use intersecting ribs.

16. Provide elastic design wherever possible.

17. Use padding in the area where sectional changes occurs.

Forging

It is caused by deforming a material (material flow).

Different types of forging:

a) Cold working (below recrystallization temperature)

b) Hot working (above recrystallization temperature)

c) Open die forging process

d) Closed die forging process also known as impression die forging

Guidelines for forging

1. Avoid sharp corners.

2. Rib should be perpendicular to parting line.

3. Land should be sufficiently long and not too narrow (otherwise frictional resistance

affects).

4. These should be proper taper angle for easy metal removal.

5. Exterior part taper angle should be less than interior part taper angle.

6. Necessary allowances should be provided during die design to consider shrinkage

during tapering.

7. Internal cavitation should not be made unnecessarily complex because complexity

within the cavitation may cause infilling of die.

8. Thinner section should be designed properly so that the minimum thickness exceeds

the critical thickness resist metal flow.

9. Lubricant is necessary to apply within the die cavity to reduce friction.

10. Depending upon shape of the part, proper orientation of the part is recommended

so that mould cavity shape doesnt increase the height in vertical direction.

11. Parting line should be located in the forging die so that disruption of the grain flow

doesnt occur.

Guidelines for forging process design

Extrusion

Features:

a) Forming process

b) Material flow

c) Material flow through die moulds

d) Metal flow takes place almost without changing force

e) Based on thermo mechanical deformation.

#Non-uniform flow of material in the extrusion die

Types of Extrusion:

a) Direct Extrusion

b) Indirect Extrusion

#Due to non-uniform flow of material in the extrusion process causes centrally located

crack.

# Extruded part undergoes cooling action following shrinkage.

Guidelines for Extrusion

1. Symmetric cross section is preferred if not functionally effective

2. Round profile can be used instead of sharp corners can be accepted.

3. Sharp tips are avoided.

4. Symmetrical profile are accepted

.

5. A friendly (extrusion) profile will be accepted instead of complex profile.

6. For enveloping, a small cut out may be selected at the internal corner instead of

sharp corner as illustrated in figure no 6(a) and 6(b).

7. To select uniform wall thickness for extruded part. Standard wall thickness can also

be selected for the extruded parts.

8. Whenever uniform wall thickness cannot be incorporated, in such a case design

should be made with concentrated mass away from centre of gravity for higher

strength.

9. For deep channel, (width/height) ratio should be 1/3. But by incorporating round

profile the ratio can be changed (higher radius of curvature).

10. If possible, the hollow part can be modified with rigid part.

11. Extruded part can be made opened and proper closing can be done by rolling

process.

12. No cavity should be reduced to reduce extrusion.

Sheet working processes

Sheet metal working process consist of following processes-

a) Punching

b) Blanking

c) Deep drawing

d) Bending

In case of punching following design consideration is taken

a) Diameter of punch should be less than diameter of the die.

b) Distance between hole from the edge is known as web

c) For small hole web 1.5t (where t= thickness).

d) For large hole web =2t.

e) For large hole d 10t (where d= punch diameter)

f) For small hole d 5t.

g) Punch hole diameter 1.2t

In case of a slotted hole:

a) Web for large slot is equal to 4t (t =thickness of sheet).

b) Web for small slot is 2t.

c) Length of large slot (l)> 10t

d) Length of small slot(l)

Depth of punch should be greater than the die diameter.

Design constrain:-

i. Clearance (10.7% to11.5%)t ,where t =thickness of sheet

ii. Drawing ratio (Db/Dp), which is denoted by R 2

(where Db= blank diameter, Dp= punch diameter)

iii. Reduction (r)=

100% 50 %

iv. If thickness is so less than thickness ratio (

1% )

v. Radius of the curvature of die( ) =4t , radius of curvature of

punch( ) ,part radius of irregular parts ( ) 6t. (where t

=thickness of sheet )

vi. Reduction friction reduces by using lubricant, oil, wax etc. (during

the drawing process the wall thickness reduce, it is known as

thinning )

vii. Allowable thing up to 25% reduction.

Design for Manual handling

Design guideline for handling of parts-

1. Design Modification according to the illustration cited in the figure 1(a) and 1(b)

will be permissible if functional performance of the part remain unaffected (This may

reduce the jamming problem of the part).

2. Entanglement of the part could be avoided by proper redesign of the part according

to the cited illustration in figure 2(a) and 2(b)

3. Consider symmetric part for better handling, figure 3(b)

4. Under certain unavoidable circumstances some asymmetry in the part may be

accepted

5. Avoid vary small part, Figure 5(a); Avoid slippery part ,Figure 5(b); Avoid needle

like part, Figure5(c); Avoid flexible part, Figure 5(d).

Design Guide line for insertion and fastening

1. Sufficient clearance should not be provided between mating components which

may cause jamming. Design alteration according to the illustration provided in the

figure 1(b) may be permissible if functional performance of the mating component

remain unaffected.

2. Entrapped air with in the part hole during assembly can be expelled out by proper

design modification keeping a small hole according to the illustration provided in the

figure 2(a), 2(b), and 2(c). For shearing pin application, centrally located hole of a flat

according to the illustration provided in the figure 2(c) can be recommended.

3. A part has to be located before releasing during assembly as illustrated in figure

3(b).

4. Holding down for proper orientation and alignment of the part can be avoided by

proper design modification, figure 4(b)

5. The part can be located after the release by proper design modification, Figure 5(b)

6. A curve profile in the part end can be recommended for easy insertion, figure 6(a)

Classification system

symmetry : 0 180 180 360 90 360 (values are in degree)

symmetry : 0 0 90 0 360 360 (values are in degree)

To control the economy in the production process,

Efficiency is given as = (Nm t1) / tes

Where Nm = Number of parts

tes = Estimated time

Effect of thickness on handling time

Effect of size on handling time

Effect of weight on handling time

tpw= 0.0125w + 0.011 th.w

th=1.13

tpw= 0.025w

Where tpw= penalty time

th= handling time

w= weight

Effect of symmetry of parts which can tangle and which may require use of

tweezers for manipulation

We need tweezers for handling when:

i. Part is very small and sharp

ii. When vision is obscured

iii. When the part is hot

iv. When finger cannot be accessible.

Chamfer design

Clearance= (D-d)/D

Formula for insertion time

Insertion time, t= -70 lnC + f(chamfer) + 3.7L +0.75d (in milliseconds)

Or t1= 1.4L + 15 (in milliseconds)

Where f(chamfer) = -100 (no chamfer)

-200 (chamfer on hole)

-250 (chamfer on peg)

-370 (chamfer on peg and hole)

Automatic assembly system

Advantages:

1. Higher production rate

2. Easy economic control

3. Benefit to the unskilled worker

Types of Automatic assembly system

1. Inline assembly system

In this system the base part moves along a line through different stations. The base

part first moves under the station 1 where the 1st component falls and gets assembled.

Then the base part moves to the station 2 where the 2nd

component falls and gets

assembled. Thus the base part keep on moving to different stations until all the

components are assembled and finally the completely assembled part comes out of the

system.

The component flow system consists of the following:

(i) Hopper

(ii) Part feeder

(iii) Selector/orientor

(iv) Feed track

(v) Escapement/placement

(vi) Pick and place arm

2. Dial type assembly system

In this system the base part moves in a circular path and so it is called as dial type or

rotary type assembly system. Here the base part enters the nest and moves to the

station 1 where the first component falls and then gets assembled. Then the base part

moves to the station 2 where the 2nd

component falls and gets assembled. Thus the

base part keep on moving to different stations until all the components are assembled

and finally the completely assembled part comes out of the nest.

The time required for the base part to enter the nest and move to the station 1 is

equal to the time required by the 1st component to fall on the nest. This is known as

the synchronous movement of the base part and the component.

3. Carausel configuration system

In this system the base part moves in a oval path and it is called as Carausel type

assembly system. Here also the base part enters the nest and moves to the station 1

where the first component falls and then gets assembled. Then the base part moves to

the station 2 where the 2nd

component falls and gets assembled. Thus the base part

keep on moving to different stations until all the components are assembled and

finally the completely assembled part comes out of the nest.

This system also uses synchronous movement of the base part and the

component.

4. Pace free or non-synchronous

In this system buffer stock is implemented. If any station is not working properly, the

activities of the system is not stopped. The components are provided from the buffer

stock and the station is been repaired side by side. Thus the performance of the system

is not affected due to malfunction of a station.

5. Single station

Placement/ Escapement

1. Horizontal type

2. Vertical type

3(i). Pick and place placement

3 (ii). Pick and placement

Geneva indexing mechanism