Design For Manufacturing and Assembly

Design for Manufacturing and Assembly (DFMA) is the method of design of the product for ease of assembly. The goal is to design parts that will transition to productions at a minimum cost, focusing on the number of parts, handling and ease of assembly.

    Goals of DFA:

  1. Minimize the number of components in the assembly
    • Minimum number of assemblers, fixtures & tools
    • Minimum number of stacked tolerances
    • Fewer documents, inventories, suppliers & rejects
  2. Ensure that parts are easy to assemble
    • Design parts with self-locating and self-fastening features
    • Minimize use of threaded fasteners
    • Minimize reorientation of parts during assembly
    • Design parts for ease of retrieval, handling, & insertion
    • Encourage modular design
    • Design for a base part to locate other components
    • Emphasize Top-Down assemblies
  3. Increase use of standard parts
  4. Design with widest possible tolerances
  5. Optimize material selection for manufacturing
Improving Design Efficiency

The Boothroyd Dewhurst method provides a quantitative measure called the design efficiency based on analysis of an existing product. The efficiency compares the total assembly time for a product with the total assembly time for an ideal product. The efficiency can be used to compare various designs in terms of their relative efficiencies.

The design improvement is brought about by two considerations: (1) A decision is made as to whether the part can be considered a candidate for elimination, or combination with other parts of the assembly (2) An estimation of the time taken to grasp, manipulate, and insert the part.

The procedure:

  1. Obtain Engineering drawings, or Exploded 3-D views, or Existing product, or Prototype
  2. Take assembly apart (or imagine doing so) -- assigning identification to each part as it is removed. Consider sub-assemblies as parts, and analyse them separately (recursively)
  3. Begin re-assembly of the product. Start with the part with the highest identification number, going all the way up to the part 1. Fill up the assembly worksheet as you go along.
  4. Compute the design efficiency, given as: E = 3N/T
pic missing
Figure 1. - Pneumatic Piston Design Improvement.

Use side A of figure 1. The computation is done by systematically completing the data in the following table:

Assembly Name: Pneumatic Piston A
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
6 1 30 1.95 00 1.5 3.45 $1.38 1 Main Block
5 1 10 1.5 10 4.0 5.50 $2.2 1 Piston
4 1 10 1.5 00 1.5 3.00 $1.2 1 Piston Stop
3 1 05 1.84 00 1.5 3.34 $1.34 1 Spring
2 1 23 2.36 08 6.5 8.86 $3.54 0 Cover
1 2 11 1.8 16 8.0 16.6 $6.64 0 Screw
Total: 40.75 $16.3 4 Design Efficiency (E) = 3N/T = 0.29
T C N

C1: Part ID
C2: No. of times the operation is carried out consecutively
C3: Manual Handling code
C4: Manual Handling time per part
C5: Manual Insertion code: 1st digit comes from the 3rd Column, 2nd digit comes from the 4th Row (see table below)
C6: Manual Insertion Time Per Part (see table below)
C7: Operation Time C2(C4+C6)
C8: Operation Cost: $0.4 C7
C9: Estimation for theoretical minimum part, if the part can be eliminated enter "0"
C10: Part Description

The data requires several estimates for assembly efficiency of different components based on their characteristics. This data was compiled empirically by a large number of time-motion studies conducted over may years. That data is presented in charts put together by Boothroyd and Dewhurst shown below.

As you can see the efficiency number (E) for design A is 0.29, which qualify the assembly as one that is costly and time-consuming. After a Design revision the new BOM shows fewer parts, as seen in side B of figure 1. Now entering the data for this new design in the table:

Assembly Name: Pneumatic Piston B
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
4 1 30 1.95 00 1.5 3.45 $1.38 1 Main Block
3 1 10 1.5 00 1.5 3.00 $1.20 1 Piston
2 1 05 1.84 00 1.5 3.34 $1.34 1 Spring
1 1 10 1.5 30 2.0 3.50 $1.40 1 Cover and Stop
Total: 13.29 $5.32 4 Design Efficiency (E) = 3N/T = 0.90
T C N

For assembly B the efficiency number (E) is 0.90, which denotes a design that is well throughout, simple, economical and easy to assemble. This type of organized study is an effective way to document the design improvement process. Each report should contain at least some basic cost and time estimates.

MANUAL HANDLING-ESTIMATED TIMES (seconds)

ONE HAND Parts are easy to grasp and manipulate Parts present handling difficulties (1)
thickness > 2 mm thickness ≤ 2 mm thickness > 2 mm thickness ≤ 2 mm
size > 15 6 ≤ size ≤ 15 size < 6 size > 6 size ≤ 6 size > 15 6 ≤ size ≤ 15 size < 6 size > 6 size ≤ 6
0 1 2 3 4 5 6 7 8 9
Parts can be grasped and manipulated by one hand without the aid of grasping tools (a+b) < 360° 0 1.13 1.43 1.88 1.69 2.18 1.84 2.17 2.65 2.45 2.98
360° ≤ (a+b) < 540° 1 1.50 1.80 2.25 2.06 2.55 2.25 2.57 3.06 3.00 3.38
540° ≤ (a+b) < 720° 2 1.80 2.10 2.55 2.36 2.85 2.57 2.90 3.38 3.18 3.70
(a+b) = 720° 3 1.95 2.25 2.70 2.51 3.00 2.73 3.06 3.55 3.34 4.00

ONE HAND
with
GRASPING AIDS		 Parts need tweezers for grasping and manipulation Parts need standard tools other than tweezers Parts need special tools for grasping and manipulation
Parts can be manipulated without optical magnification Parts require optical magnification for manipulation
Parts are easy to grasp and manipulate Parts present handling difficulties (1) Parts are easy to grasp and manipulate Parts present handling difficulties (1)
thk > 0.25 thk ≤ 0.25 thk > 0.25 thk ≤ 0.25 thk > 0.25 thk ≤ 0.25 thk > 0.25 thk ≤ 0.25
0 1 2 3 4 5 6 7 8 9
Parts can be grasped and manipulated by one hand without the aid of grasping tools a ≤ 180° 0 ≤ b ≤ 180° 4 3.60 6.85 4.35 7.60 5.60 8.35 6.35 8.60 7.00 7.00
b = 360° 5 4.00 7.25 4.75 8.00 6.00 8.75 6.75 9.00 8.00 8.00
a = 360° 0 ≤ b ≤ 180° 6 4.80 8.05 5.55 8.80 6.80 9.55 7.55 9.80 8.00 9.00
b = 360° 7 5.10 8.35 5.85 9.10 7.10 9.55 7.85 10.10 9.00 10.00

TWO HANDS
for
MANIPULATION		 Parts present no additional handling difficulties Parts present additional handling difficulties (e.g. sticky, delicate, slippery, etc.) (1)
a ≤ 180° a = 360° a ≤ 180° a = 360°
size > 15 6 ≤ size ≤ 15 size < 6 size > 6 size ≤ 6 size > 15 6 ≤ size ≤ 15 size < 6 size > 6 size ≤ 6
0 1 2 3 4 5 6 7 8 9
Parts severely nest or tangle or are flexible but can be grasped and lifted by one hand (with the use of grasping tools if necessary) (2) 8 4.10 4.50 5.10 5.60 6.75 5.00 5.25 5.85 6.35 7.00

TWO HANDS
required for
LARGE SIZE		 Parts can be handled by one person without mechanical assistance Two persons or mechanical assistance required for parts manipulation
Parts do not severely nest or tangle and are not flexible part severely nest or tangle or are flexible (2)
parts are easy to grasp and manipulate parts present other handling difficulties (1) parts are easy to grasp and manipulate parts present other handling difficulties (1)
Part weight < 10 lb Parts are heavy (>10 lb)
a ≤ 180° a = 360° a ≤ 180° a = 360° a ≤ 180° a = 360° a ≤ 180° a = 360°
0 1 2 3 4 5 6 7 8 9
Two hands, two persons or mechanical assistance required for grasping and transporting parts 9 2.00 3.00 2.00 3.00 3.00 4.00 4.00 5.00 7.00 9.00

MANUAL INSERTION-ESTIMATED TIMES (seconds)

PART ADDED
but
NOT SECURED		 After assembly no holding down required to maintain orientation and location (3) Holding down required during subsequent process to maintain orientation and location (3)
easy to align and position during assembly (4) not easy to align and position during assembly easy to align and position during assembly not easy to align and position during assembly
no resistance to insertion resistance to insertion (5) no resistance to insertion resistance to insertion (5) no resistance to insertion resistance to insertion (5) no resistance to insertion resistance to insertion (5)
0 1 2 3 6 7 8 9
Addition of any part (1) where neither the part itself nor any other part is finally secured immediately part and associated tool (including hands) can easily reach the desired location 0 1.50 2.50 2.50 3.50 5.50 6.50 6.50 7.50
part and associated tool (including hands) cannot easily reach the desired location due to obstructed access or restricted vision (2) 1 4.00 5.00 5.00 6.00 8.00 9.00 9.00 10.00
due to obstructed access and restricted vision (2) 2 5.50 6.50 6.50 7.50 9.50 10.50 10.50 11.50

PART SECURED
IMMEDIATELY		 no screwing operation or plastic deformation immediately after insertion (snap/press fits, circlips, spire nuts, etc.) plastic deformation immediately after insertion screw tightening immediately after insertion (6)
plastic bending or torsion rivetting or similar operation
easy to align and position during assembly (4) not easy to align or position during assembly easy to align and position during assembly (4) not easy to align or position during assembly easy to align and position with no torsional resistance (4) not easy to align or position and/or torsional resistance (5)
easy to align and position with no resistance to insertion (4) not easy to align or position during assembly and/or resistance to insertion (5) no resistance to insertion resistance to insertion no resistance to insertion resistance to insertion (5)
0 1 2 3 4 5 6 7 8 9
Addition of any part (1) where the part itself and/or other parts are being finally secured immediately part and associated tool (including hands) can easily reach the desired location and the tool can be operated easily 3 2.00 5.00 4.00 5.00 6.00 7.00 8.00 9.00 6.00 8.00
part and associated tool (including hands) cannot easily reach the desired location or tool cannot be operated easily due to obstructed access or restricted vision (2) 4 4.50 7.50 6.50 7.50 8.50 9.50 10.50 11.50 8.50 10.5
due to obstructed access and restricted vision (2) 5 6.00 9.00 8.00 9.00 10.00 11.00 12.00 13.00 10.00 12.00

SEPARATE
OPERATION		 mechanical fastening processes [part(s) already in place but not secured immediately after insertion] non-mechanical fastening processes [part(s) already in place but not secured immediately after insertion] non-fastening processes
none or localized plastic deformation bulk plastic deformation (large proportion of part is plastically deformed during fastening) metallurgical processes chemical processes (e.g. adhesive bonding, etc.) manipulation of parts or sub-assembly [e.g. orienting, fitting of part(s), etc.] other processes (e.g. liquid ingestion, etc.)
bending or similar processes riveting or similar processes screw tightening (8) or other processes no additional material required (e.g. resistance, friction welding, etc.) additional material required
soldering processes weld/braze processes
0 1 2 3 4 5 6 7 8 9
assembly processes where all solid parts are in place 9 4.00 7.00 5.00 3.50 7.00 8.00 12.00 12.00 9.00 12.00

DESIGN FOR MANUFACTURING TIPS