6.1 Introduction to Design for Assembly (DFA)
6.1.1 Background
DFA is perhaps one of the most effective techniques to reduce production cost and certainly
the most established methodology in concurrent engineering. The basic concepts are mostly a
collection of common sense rules that are often neglected in the design process. DFA applies
to any product that requires assembly: automotive parts and systems, electronics, toys,
appliances, etc.
DFA attracted strong industry interest when G. Boothroyd and P. Dewhurst (University of
Rhode Island) systematized the methodology in the late 70's. Other researchers include R.
Sturges at Westinghouse (now at Virginia Tech.) and a group at Hitachi's Production
Engineering Research Laboratory (PERL). By the mid 80's, DFA received overwhelming
industry response. Boothroyd and Dewhurst along with others developed a PC-based
program and a handbook to review an assembly design. Whereas these quantification
methods are effective in trade analysis of different designs, the basic concepts and design
guidelines remain very important at the layout design stage.
The fundamental concept is to design the product so that it is easy to assemble. The
important thing to remember: apply DFA design rules to early layout designs! Also, keep in
mind that we should look at the compatibility between assembly methods and designs.
Boothroyd and Dewhurst have published design rules that apply to manual and automatic
assembly. The method calculates normalized assembly time. Their programs are effective
but tend to be tedious and time consuming. Many companies consider DFA to be most
effective in training designers to incorporate assembly concerns. In ME317, we introduce a
simplified method derived from R. Sturges’ Westinghouse methodology.
6.1.2 Pertinent Factors in DFA:
•
•
•
•
Number of Parts
Handling of Parts (Reach distance, orientation, part size and weight)
Part Insertion (Direction, fixture)
Part tie-down (fasteners, fastening process)
Note that there are other concerns such as assembly costs related to product variation. Simply
put, if designers can qualitatively simulate or “walk through” the assembly process of their
designs, they will come out ahead significantly in assembly cost and quality (reduced defect
rate).
6.1.3 DFA101: The Basic DFA Rules
CAUTION--DFA is not always perfect!!!: Interpret GOOD as "desirable for assembly" and
BAD as "undesirable for assembly." You must also consider other life-cycle issues such as
tooling costs, component complexity, service needs, and product variety that are explored in
later lectures.
6.1.3.1. Number of parts for each sub-assembly
If the following does not apply and the parts are separate => combine parts.
1) The part moves relative to all other parts already assembled?
2) The part must be of a different material.
3) The part must be isolated from all other parts already assembled.
4) The part must be separate from all other parts already assembled, because
otherwise, necessary assembly or disassembly of other parts would be impossible.
Three Pieces
Bolt
Washer
One Piece
Spacer
Cold Headed Fastener
Figure 6.1.1 Minimize number of parts
6.1.3.2 Ease of handling.
Manual assembly
1) If your hand can enclose the part, it is too small to handle easily -- BAD
2) If two hands are needed to handle the part, it will delay assembly and present
handling difficulties -- BAD
3) If the part has sharp corners, it presents a hazard to the worker -- BAD
Robotic assembly
1) If the part is slippery, we may need a special gripper -- POOR
2) If the part is fragile, we may need force sensing to prevent the gripper from
damaging the part -- BAD
Avoid flexible parts.
Figure 6.1.2 Avoid flexible parts
Avoid tangly parts
Figure 6.1.3 Avoid tangly parts
Provide tool access
Figure 6.1.4 Provide tool access
6.1.3.3 Ease of Insertion
1) Direction of insertion from:
Above (straight line) -- EXCELLENT
Side (straight line) -- GOOD
At an angle (not straight) -- BAD
Bottom -- VERY BAD
Figure 6.1.5 Minimize number of insertion angles
2) If the part is self-aligning (chamfers, holes, etc.) -- EXCELLENT
Figure 6.1.6 Use recesses or bosses to locate components
3) If alignment is by arrows, labels, or matching colors -- GOOD
4) If there are no features to align the part properly -- BAD
6.1.3.4 Fastening Methods
Velcro -- EXCELLENT
Snap-on -- VERY GOOD
Tab -- GOOD
Screw -- VERY BAD (higher penalty depending on quantity and variation)
Permanent - BAD (application of glue/cement/weld/solder slows down
assembly)
Figure 6.1.7 Use integral fasteners
6.1.4 Assembly Sequence Diagram
One of the most effective ways to enhance product design for ease of assembly is to plan the
assembly process in advance. To facilitate this advance planning, we encourage designers to
use the following diagram to qualitatively "walk through" the assembly process. This
procedure in itself forces the designers to identify cost-driving assembly tasks and steps that
may lead to defects. We view this diagram as an essential document to the evaluation of
assembly difficulties.
A fishbone style diagram for describing the assembly sequence, such as the one shown in
Figure 6.1.8, is extremely effective in promoting the advanced planning of the assembly
process. The diagram also promotes DFA by forcing the engineers to identify assembly
difficulties and to come up with remedies. In fact, the identification of the assembly sequence
is the first step in any DFA methodology. The modified Westinghouse method presented in
the next section will also assume that you have completed the assembly sequence diagram.
One can characterize each assembly step by indicating fixturing needs (the symbol “F”),
reorientation (circular arrow), and insertion directions (straight and rotational arrows). This
information feeds directly into the DFA worksheet for computing assembly ratings. The
diagram can include other symbols indicating time penalty factors addressed on the next page.
We recommend engineers to include at least the fixturing, reorientation, and insertion angle
information. These symbols will facilitate the rapid evaluation of assembly difficulty.
Handle
Core
Cap
Cap
F
Tip
F
Handle
Core
Tip
Assembly
Figure 6.1.8 Assembly sequence diagram for a mechanical pencil
6.1.4.1 Guide to Creating an Assembly Sequence Diagram
1) Start with the part that other parts attach to (usually fixtured).
2) Parts that attach directly are shown with a slanted arrow.
3) Denote special operations with icons next to the arrows.
F
Fixture
Rotation (flip assembly or screwing action)
Straight-down attachment
Attachment at an angle
Attachment from below
Attachment from the side
Other icons may be defined if necessary.
4) A sub-assembly consists of a separate tree that attaches to the main assembly.
Le ft Carriage
Base
Drive Gear 1
F
PC Board
Drive Gear 2
F
Power S u pply
Motor
Pape r Tray
Le ft Carriage
S ubassembly
Drive Roller
Right Carriage Frame
IBM Proprinter
Assembly
Sequence
Diagram
Platten
Pape r Guide
Te nsion S haft
Drive S crew and Motor
Eccen tric S h aft and Print He ad
Tail S haft
Ribbon Drive Gear 1
Ribbon Drive
Gear 2
Roller S haft
Roller
Tractor Drive Ass'y
Roller Guide
S ubassembly
Paper Width Adjuster (2)
Keypad
Ribbon
F Fixture
Rotation (flip assembly or
screwing action)
Straight-down attachment
Hood
Attachment at an angle
Knob
Attachment from below
Cove r
Attachment from the side
Printe r Asse mbly
Figure 6.1.9 Assembly sequence diagram for the IBM Proprinter
6.1.5 Assembly Evaluation: The Modified Westinghouse Method
While there are several DFA methods available, we feel that many are unnecessarily complex
and require too much time to complete. For the Stanford DFM class, we have developed a
simplified version of the Westinghouse methodology. We sincerely appreciate the help of Dr.
Martin Hinckley of Sandia National Labs in proposing the simplification. Professor Bob
Sturges, one of the original developers of the Westinghouse Methodology and now at
Virginia Tech., also gave us advice. Table 6.1.1 shows the worksheet used to compute the
assembly times. A step by step guide to the Westinghouse method is shown below the chart.
One can determine the assembly time penalty for each column using Table 6.1.2. One can
easily create a spreadsheet to perform the computation in Table 6.1.1, and further combine it
with Table 6.1.2. The DFA data came from:
Sturges, R.H. and Kilani, M. 1992. Towards an Integrated Design for an Assembly
Evaluation and Reasoning System. Computer Aided Design. Vol. 24, No.2. pp. 6779.
Westinghouse Design for Assembly Calculator.
The two major evaluation measures: assembly rating (AR) and part efficiency (PE):
AR (Assembly Rating) is a normalized measure that compares the total assembly time with a
reference time of 2.35 seconds per part. Hence AR of 1 indicates that the design is quite good
from ease of assembly viewpoint. As penalty time increases, AR decreases. AR = 2.35 x
Number of Parts / Total assembly time.
PE (Part Efficiency) compares the total number of parts with the theoretical minimum
number of parts. The theoretical minimum is calculated based on the designer’s response to
the factors cited in 6.1.3.1. PE=1 is good, less than one encourages elimination of parts. PE
= Theoretical Minimum / Number of Parts.
Table 6.1.1 Assembly Evaluation Worksheet
A
hand.
cond.
Part/Operation
Description
1 Handle
2 Core
3 Cap
4 Flip sub-assy
5 Tip
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
B
C
D
E
F
G
H
I
J
K
L
siz e thick. insert. end to inset. insert. insert. fastn. fastn. time/ no. of
align.
end direc. cond. clear.
proc oper. reps.
align.
(T op) (Nrep)
0.1
0.25
1
0.6
2.25
4.2
1
0.4
0.25
1
0.6
0.25
1
3.5
1
0.4
0.25
1
0.6
0.9
1
4.15
1
2.25
2.25
1
0.1
0.25
1
0.6
0.25
4
6.2
1
5
TOP
Step
Step
Step
Step
1:
2:
3:
4:
Draw the Assembly Sequence Diagram
List Parts & operations in order (left column)
Enter times from Estimated DFA Time Chart
Sum time p er part/op er. in column K
Enter no. of repetitions for each operation in col. L
Enter K*L in col. M
Step 5: Enter a 1 in col. N if a part was inserted during operation
Enter a 1 in col. O if part or operation can be eliminated
Step 6: Calculate Summary Statistics
M
repet.
time
K*L
4.2
3.5
4.15
2.25
6.2
20.3
TAT
N
O
insert eliminate
part?
part?
0 = no, 1 = y es
1
0
1
0
1
0
0
0
1
0
4
NUP
Summary Statistics
NUP 4
= number of unique parts
TOP 5
= total no. of operations
TAT 20.3 = total assembly time
NP
4
= no. of parts = sump rod.(L,N)
Tavg 4.06 = avg time/op eration = TAT/(sumRep)
Pmin 4.0
= min # parts = NP - sumprod.(L,N,O)
AR 0.463 = Assembly rating = 2.35 * NP /TAT
PE
1
= Part Effciency = Pmin/NP
Table 6.1.2 Assembly Time Chart (Modified Westinghouse method)
Use this table to select operation time (in seconds) for each part or assembly operation. Enter the time into the
Modified Westinghouse Assembly Evaluation Worksheet. If the condition does not apply, leave the entry blank
(default is zero seconds). If special conditions apply, enter the corresponding time penalty into the worksheet.
A.
B.
HANDLING
Handling Condition
Time
Condition
0.5
heavy
1.0
delicate
1.5
nests/tangles
2.0
tweezers required
3.5
other tools required
6.0
severe nest/tangle
G.
Size
Time
0.6
0.4
0.1
0.0
note:
H.
Part Size
< 2mm
2 - 6mm
6 - 12mm
> 12mm
time x 2 if insert. orientations < 4
C.
Thickness
Part Thickness
< 0.5mm
0.5 - 2mm
> 2mm
time x 2 if insert. orientations < 4
Time
0.5
0.2
0.0
note:
D.
Orientation about Insert. Axis
Time
Insertion Orientations
1.50
1 (subtle feature)
1.00
1 (obvious feature)
0.25
2 or more
E.
End to End Alignment
Time
End Insertion Orientations
1.5
1 (subtle feature)
1.0
1 (obvious feature)
0.5
2 or more
1
F.
2
3
INSERTION
Insertion Direction
Time
Condition
0.6
down
1.4
from the side
1.7
diagonally or twist/turn/tilt
2.0
up
Insertion Conditions
Time
Insertion Condition
1.25
constrained motion
1.35
temporary hold down
1.50
two hands
2.25
rotate or fixture
6.00
flexible
Time
0.25
0.90
1.60
Insertion Clearance
Insertion Clearance
large
small
very small
Time
0.0
1.0
2.5
4.0
5.0
6.0
Fastener Time
Fastener Type
washer
pin
retaining ring
screw
nut
rivet
I.
J.
Fastening Process Time
Time
Fastening Process
1
snap or press fit
3
bending or crimping
4
screwing
5
polymer weld or stake
7
solder
9
weld or braze
11
adhesive
=
°
end-end sw it ch OK
swit ch not OK
4 or more
Number of Insertion Alignments
(insertion axis perpendicular to page)
End to End Insertion Alignments
Two allowable orientations compared to only one
6.1.5.1 Guidelines for Assembly Time Chart: Modified Westinghouse Method
HANDLING
A. Handling Condition
Handling conditions apply to special cases, the default is 0 seconds. These times can
be added when more than one condition applies.
0.5
1.0
heavy
delicate
1.5
nests/tangles
2.0
3.5
tweezers required
other tool required
6.0
severe nest/tangle
More than 4.5 kg (10 lbs.)
Parts prone to break, scratch, bend, crinkle, parts with
sharp edges for example: shims, foils, brittle
materials.
Parts supplied to operator in bulk and requires simple
manipulation to separate it from other parts in the
container, for example: retaining ring, sticky parts.
Part cannot be inserted without the use of other tools,
for example: pliers, wrench.
Parts supplied to operator in bulk and requires
considerable manipulation to separate it from other
parts in the container, for example: cable, coil springs.
B. Size
Size is the largest dimension of the rectangular solid that can enclose the part, default
is 0 seconds. (see diagram)
0.6
0.4
0.1
time x2
< 2 mm
2 - 6 mm
6 - 12 mm
if insert. orientat. <4
(< .08 in.)
(.08 - .25 in.)
(.25 - .5 in)
If less than four possible orientations about axis
of insertion then multiply time penalty by two.
C. Thickness
Thickness is the smallest dimension of the rectangular solid that can enclose the part,
default is 0 seconds. (see diagram)
0.5
0.2
time x2
< 0.5 mm
(< 0.02 in.)
0.5 – 2 mm
(0.02 – 0.08 in.)
if insert. orientat. <4 (see Size)
thickness
size
D. Orientation About Insertion Axis
The number of orientations (about the axis of insertion) a part may have where it can
still be properly inserted. (see diagram below)
1.50
1 (subtle feature)
1.00
0.25
1 (obvious feature)
2 or more
One correct orientation where asymmetric
feature is not very noticeable
One easily recognizable orientation
More than one correct insertion orientation.
(screws, pins, rectangular pieces)
E. End to End Alignment
The number of end to end orientations a part has where it can still be properly
inserted. A part with two end-to-end orientations can be inserted upside down (see
diagram below).
1.50
1 (subtle feature)
1.00
0.5
1 (obvious feature)
2 or more
One correct orientation where asymmetric
feature is not very noticeable
One easily recognizable orientation
More than one correct insertion orientation.
in serti on a xi s
en d to end
ori enta tion
orientations about
insertion axis
1
>4
end to end
orientations
2
1
INSERTION
F. Insertion Direction
The insertion direction refers to the direction the part will be inserted. If more than
one direction applies, the times may be added.
0.6
1.4
1.7
2.0
down
from the side
diagonally or twist/turn/tilt
up
G. Insertion Conditions
1.25
constrained motion
1.35
temporary hold down
1.50
2.25
two hands
rotate or fixture
6.00
flexible
Operator’s access limited by fixturing or other
parts.
Part will stay in place only if supported until
another subsequent part is added.
fixture-fixturing operation precedes insertion of
part
rotate- re-orientation precedes insertion of part
Requires extra manipulation during insertion, or
does not stay in place when released.
H. Insertion Clearance
0.25
0.90
1.60
large
small
very small
10-50 %
1-10 %
<1%
I. Fastener Time
0.0
1.0
2.5
4.0
5.0
6.0
washer
pin
retaining ring
screw
nut
rivet
% = 100% (A-B) / B
B
A
J. Fastening Process Time
When inserting a part, if these fastening processes apply, use the corresponding time.
1.0
3.0
4.0
5.0
7.0
9.0
11.0
snap or press fit
bending or crimping
screwing
polymer weld or polymer stake
solder
weld or braze
adhesive