Using Datums for Economic
Process Planning
Dr. R. A. Wysk
IE550
Fall 2008

Process Planning
• Single datum planning
• Multiple datum plans

Process Tolerance Chart
Values in Process Tolerance Charts
typically represent the BEST attainable
values. They also represent single-feature
relationships. We refer to these intra-
feature process planning.
•Process Boundary Matrices

A 2” piece or bar stock needs to be “faced” so
that the required length and surface finish can be
obtained.
4.0
+ .005
- .0
2.0 + .005
- .005
A
Example #1 - The simplest case; single datum, single feature

Solution:
In checking the work piece, datum -A-
becomes the reference plan for the
length, 4.0 . The OD accuracy is
obtained at the rolling mill, and no OD
turning is required. The length needs
to be faced to final dimension.
+ 0.005
- 0.0

Oper. Description Machine Tool
10 Retrieve 2’’ Bar Warehouse --
20 Cut to 4.25’’ length Cut-off saw --
30 Face backside (remove
1/8 ‘’ stock)
Lathe Facing tool
40 Flip and face front-side Lathe Facing tool
50 Remove and inspect -- --
Process Plan-Example #1

1.0  .005
4 + .005
- 0
2.0 + 0.01
- 0
A
5  .005
Sort of like Example #1 but with a 2nd feature related
to the same datum -A-.
Example #2 -- Single datum; 2 features.

Solution:
- 4” segment is the same as in Example #1
-Addition segment requires that:
-OD is reduced to 1”
-Length needs to be reduced to 5  .005

Process Plan for Example #2
OP# Description Machine Tool Time
10 Retrieve 2” bar Warehouse
20 Cut to 5.25” Cut-off saw
30 Face backside and
invert
Lathe Facing
40 Turn 1” Dia. @ .25 in
depth
Lathe Turning
(2 passes)
50 Face to 4” Lathe Facing
60 Face to 5” Lathe Facing
70 Remove and Inspect

A
C12
C23
C4
M12
M13
Cij is part specification or
Constraints
Mij is Manufacturing method were
i is the datum feature, and
j is the surface produced
The General Case and Notation.

From the part, you can see that
C12 @ M12
This reads, “C12 comes directly from process
M12 (our facing operation).”
Also from the drawing, one can see that
TC23 = TM12 + TM13
This reads, “the tolerance for feature C23 can be
as large as the sum of the tolerance for
producing M12 and the tolerance for producing
M13”  Tolerance Stacking

Notation: subscript m implies minimum
M implies maximum
C23m = -M12M + M13m
Let’s suppose
0
005
.
4 

12
C 005
.
005
.
1

23
C
Then
TM12 = .005
TC23 = TM12 + TM13
.010 = .005 + TM13
TM13 = .005

If a negative value results then the process
specification is unfeasible
Since C23m = - M12M + M13m
.995 = -4.005 + M13m
5.000 = M13m
 Set the process specifications for M13 at
5.000 - 5.005

.750  .010
All hole features are specified with respect to
datums A-B-C and can be treated as intra-feature
entities.
 Ø .01  C A B
M
Example #4
+ +
+
4 .008
2 holes
.250  .010
 Ø .008 C A B
M
2.0  .01
A
1 1 1
B
1
.5  .01
C
Raw Material 4’’ x 2’’ x .5’’

Process Plan for Example #4
OP# Description Machine Tool
10 Load part in vise Fadal CNC
20 Drill 1st
small hole Fadal CNC .25  drill
30 Drill 2nd
small hole Fadal CNC .25  drill
40 Drill large hole Fadal CNC .25  drill
50 Unload and inspect

Example #5
 

B
.5 1 1
.750  .010
  .01  C D E
M
+
.25  MAX
.75
D
.25± .01 A
.5 ±
.01
E
2 holes
.250  .010
  .008 C D E
M
.50 ±.01
C
M12
M13
C23
M14 M15
.25±.01
Raw Material 4’’ x 2’’ x
.5’’

C12 @ M12
TC12 = ± .01
TC23 = TM12 + TM13
C23m = -M12M + M13m
.008 = -.51 + M13m
From 
.518 = M13m
TC23 = TM12 + TM13
.008 = .01 + TM13
TM13 < 0  infeasible
We need to position w.r.t -E-