DESIGN OF PARTS ,REGARDING EASE OF ASSEMBLY

1. A presentation
0n
Design for assembly
Under the guidance of
Dr. R. Prabhu Sekhar
By
ARUN KUMAR
2018PD15
M.TECH(PDD)
MNNIT ALLAHABAD

2. INTRODUCTION
DFA is the method of design of the product for ease
of assembly.
The purpose statement of Design for assembly is to
minimize product cost through design and process
improvements.

3. DFMA Design for Manufacturing and Assembly
Design for Assembly (DFA)
concerned only with reducing product assembly cost
– minimizes number of assembly operations
– individual parts tend to be more complex in design
Design for Manufacturing (DFM)
concerned with reducing overall part production cost
– minimizes complexity of manufacturing operations
– uses common datum features and primary axes

4. SEQUENCES OF ANNALYSIS
Concept Design
Design for Assembly
Design for Manufacturing
Detailed Design

5. Two Phases of Assembly
• Handling
• Insertion
can be done
• Manually
• Automatically

6. Design for assembly rules
Minimize part count
Design parts with self-locating features
Design parts with self-fastening features Minimize
reorientation of parts during assembly
Design parts for retrieval, handling, & insertion
 Emphasize ‘Top-Down’ assemblies
 Standardize parts…minimum use of fasteners.
 Encourage modular design
 Design for a base part to locate other components
 Design for component symmetry for insertion

7. Determine Theoretical Min. No. of Parts

8. Can Parts Be Combined?
• Since there is no relative
motion between the parts .
• So these parts can be
made of the same material

9. Theoretical Part Count Efficiency
Theoretical Part Count Efficiency
Theoretical Min. No. Parts
Total Number of Parts

10. Guidelines for part assembly
try to design parts having the maximum
possible symmetry
 Provide features that prevent jamming of
parts
Avoid features that allow tangling of parts
when stored in bulk
 Avoid parts that stick together or are
slippery, delicate, flexible, very small or very
large, or that are hazardous to the handle

11. Do not nest or tangle

12. Design Guidelines for Insertion
. Design so that there is little or no
resistance to insertion
provide chamfers to guide the insertion of
two mating parts
Use pyramid assembly
Provision of self-locating features to avoid
holding down and alignment.
Design so that a part is located before it is
released.

15. Design Guidelines for fastening
There must be self fastening feature
Minimum use of fasteners
Consider the least expensive fastening
method that meets the requirements
Avoid the need to reposition the
partially completed assembly in the
fixture

16. Fasteners
There must be self fastening feature

17. Consider the least expensive fastening method
that meets the requirements

18. Minimum use of fasteners.

19. Top down assembly
• Insert vertically from above (i.e. don’t fight
gravity)

20. Effect of Part Thickness and Size on Handling Time
 The thickness of a “cylindrical” part is defined as its radius
 for non cylindrical parts the thickness is defined as the maximum height of the
part with its smallest dimension extending from a flat surface
Cylindrical parts are defined as parts having cylindrical or other regular cross-
sections with five or more sides.

21. Effects of Obstructed Access and Restricted Vision on
Insertion of Threaded Fasteners of Various Designs.
When the distance from the obstructing surface to the hole center is
greater than 16 mm, the surface has no effect on the manipulations and
the restriction of vision is the only factor

22. Assembly Efficiency
An essential ingredient of the DFA method is the use
of a measure of the DFA index or “assembly efficiency
• Assembly Efficiency
N min= the theoretical minimum number of parts

23. Effect of Part Symmetry on Handling Time
One of the principal geometrical design features that affects the time
required to grasp and orient a part is its symmetry
Hera we can define two kinds of symmetry for a part:
Alpha Symmetry: Symmetry about an axis perpendicular to the axis of
insertion
Beta Symmetry: symmetry about the axis of insertion to repeat its
orientation
Total angle of symmetry = alpha+ beta

24. Parts Requiring Two Hands for Manipulation
A part may require two hands for manipulation when:
 • The part is heavy
. • Very precise or careful handling is required.
 • The part is large or flexible.
 • The part does not possess holding features, thus making
one-hand grasp difficult.

25. Parts may Require Tweezers for Grasping and
Manipulation
A part may require tweezers when
• Its thickness is so small that finger-grasp is
difficult.
 • Vision is obscured and prepositioning is difficult
because of its small size.
 • Touching it is undesirable, for example, because
of high temperature.
• Fingers cannot access the desired location.

26. Worksheet analysis for the controller assembly
Item
name
Number
of items
Manual
handling
cod
Handling
time per
item, s
Manual
insertion
cod
Insertion
time per
item, s
Total
operation
time, s
Figures
for min.
parts
1
2
3
4
5
6
7

27. Handling Difficulty
•Size
•Thickness
• Weight
•Fragility
•Flexibility
•Slipperiness
• Stickiness
•Necessity for using 1) two hands, 2) optical
magnification, or 3) mechanical assistance

28. Large Assemblies
In the original DFA method, estimates of average handling and
insertion times were established.
DFA method in practice has shown that assembly time estimates are
reasonably accurate for small assemblies in low-volume production
where all the parts are within the easy arm reach of the assembly
worker
Clearly, with large assemblies, the acquisition of the individual parts
from their storage locations in the assembly area involves significant
additional time.
 . However, in high-volume production situations, the screws are
often automatically fed, and so the time is reduced
The DFA method was extended to allow these possibilities, and more
accurate estimates of assembly times are obtained.

29. Identify opportunities to reduce secondary
operations
 Re-orientation (assemble in Z axis)
 Screwing, drilling, twisting, riveting, bending,
crimping.
 Welding, soldering, gluing.
Painting, lubricating, applying liquid or gas
Testing, measuring, adjusting

30. Types of Manual Assembly Methods
Bench assembly For small parts placed within an easy reach of the
assembly worker
Multi station assembly handling times are adequate if multi
station assembly is employed.
Modular assembly center several parts that weigh more than
about 5 lb or that are over 12 in and not to easy arm’s reach of the
assembly worker
Custom assembly layout Here, the product is assembled on a
worktable or on the floor and the various storage shelves and
auxiliary equipment are arranged around the periphery of the
assembly area
Flexible assembly layout for large products a more flexible
arrangement can be used; this is called the flexible assembly layout