1. Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engineering 22
GD&T Bonus Tolerance
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer

2. Skill-Development Goal
Learn How to Use the GD&T MMC to Avoid Inspection-Rejection of Entirely Functional Parts
Use AutoCAD to Construct GD&T Feature Control Frames

3. Recall Matl Condition Modifiers
Maximum Material Condition (MMC)
the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size.
Least Material Condition (LMC)
the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size
Regardless of Feature Size (RFS)
indicates a geometric tolerance applies at whatever size the part is produced M L S

4. Material Condition Usage
MMC
usually when geometric tolerances are applied at MMC the function is assembly
LMC
function is usually to insure that a minimum distance on a part is maintained
RFS – Default Condition
often specified to insure symmetrical relationships
No “Bonus Tolerance” allowed

5. GD&T Tolerances From ASME Y14.5M-1994
Some ACAD Features based on Y

6. MMC → Bonus Tolerance
Maximum material condition for a HOLE feature is the SMALLEST sized hole.
Maximum material condition for a SHAFT feature is the LARGEST shaft.
The maximum material condition modifier in a feature control frame means that the tolerance applies at max. material condition.
If the actual hole is LARGER than the smallest allowable hole, a BONUS tolerance can be added to the stated tolerance value.

7. MMC Bonus Tolerance Example
Condsider the GD&T Spec’d Part At Right
In particular note the SIZE, and POSITION TOLERANCE on the Hole
Ø = 0.50 ±0.003
Ø-POSITION Tolerance = 0.010
Relative to BASIC Dims 2.00 & .75
MMC Modifier
Position Symbol

8. MMC Bonus Tolerance cont
Then the Tolerance Zone at MMC Shown at Right
The actual center of the hole (axis) must lie in the round tolerance zone. The same tolerance is applied, regardless of the lateral direction.
i.e., The Tolerance Zone is Circular, NOT Rectangular
Same tolerance at 0°, 45°, 90° Measurement angles

9. MMC Bonus Tolerance cont.2
Recall The GD&T Spec
This means that the tolerance is .010 if the hole size is the MMC size, or If the hole is bigger, we get a bonus tolerance equal to the difference between the MMC size and the actual size.

10. Gage Fixture Design
As the Part Design Engineer We find that Parts that FIT & WORK in the Assembly are Being REJECTED by RI (Receiving Inspection)
Discover that RI does Not Understand “Bonus” Tolerances
To Avoid Scraping Good Parts, Work with a High precision Machine shop to make a Gage Fixture
Gage Pin Precisely Located at (2, 0.75)
Gage Pin Precisely Machined to Ø0.4965
A bit Less than Lo-Lim on Hole-Size; i.e., at the MMC

11. Gage Fixture Design Made from Hardened Tool Steel
Axis Walls Shifted by the Pythagorean XY distance equal to half the Tolerance Zone

12. Min-Hole Part Checked by Gage
Put Small-Hole Part in Gage Block
The Perfect Part Just Fits over the Pin, and Clears the Walls by ½ the Positional Tol.
Zoom In
Expand Origin Area

13. Max-Hole Part Checked by Gage
Put Large-Hole Part in Gage Block; move it to Up & Rt Till it hits the Gage Pin
Note How the Axis Wall Clearance has Expanded by 0.003
Thus the Hole-Ctr for this LMC Part can be Misplaced by 0.008
Zoom In
Expand Origin Area

14. MMC Bonus Tolerance
This means that the tolerance is .010 IF the hole size is at the MMC, or If the hole is BIGGER, we get a BONUS tolerance equal to the difference between the MMC size and the actual size.
NOT to scale
Actual Hole Size
Bonus Tolerance
Ø of Tol. Zone
Ø .497 (MMC)
.010
Ø ( = .002)
( = .012)
.012
Ø .500 ( = .003)
( = .013)
.013
Ø .502
.005
.015
Ø .503 (LMC)
.006
.016
Ø .504 ?
Read Carefully §10-17 thru §10-23 in Text Book
REJECTED – Out of Spec

15. GD&T Caveat → Use with Care
GDT is VERY Powerful, BUT…It can be Quite CONFUSING and ESOTERIC
Many Degreed Engineers, as well as Most Drafters/Designers (and Some Machinists) have only a Vague Notion About Meaning of GDT Symbols
MisApplication and Confusion-Induced Delays are COMMON
e.g. Try asking what MMC or RFS means…

16. Go, NoGo, and Go-NoGo Gages
The Gage we Designed is Necessary but NOT sufficient
However if part DOES Fit, it COULD Still be BAD
e.g., If The hole is LARGER than it could easily fit the gage
Thus our Gage is a NoGo Instrument
i.e., if part does NOT fit the Gage, Then REJECT it

17. Complete Go-NoGo Gaging
To Complete the Gage Set, We need to design a Precision Gage-PIN to Test for OverSize Holes
This is Also a NoGo Gage
If the Pin FITS in the Hole, Then it is REJECTED
Thus we have a GO When the Part
Fits in the Gage Block (Position & Small Hole)
Does NOT Fit the Pin (Large Hole)

19. GD&T In AutoCAD Time For Live Demo
Now that We Understand how to USE GD&T let’s APPLY it with ACAD
Width now 4.00 ±0.01
Height now /-0.02
Time For
Live Demo
Let Dim & Tol Our Spacer Plate with Changes

20. All Done for Today The AME and Bonus Tol.
Calculating Additional Tolerance Permitted when a Position Tolerance is Modified at MMC.  An additional tolerance, often called Bonus tolerance, is permitted when a position tolerance is modified at maximum material condition (MMC) and the actual mating envelope of the feature has departed from the MMC.  In these illustrations, the part is black and the actual mating envelope is red.
If a hole, for instance, has the following size and geometric control, and the hole measures It would be incorrect to use a bonus tolerance of .003 ( (MMC)) if the hole is not perfectly oriented to the Datums. If the hole is out of perpendicular to datum A by .002, for instance, the bonus that may be used is reduced by that amount. The bonus would be merely .001 and the allowable position tolerance = .016.

21. Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engr/Math/Physics 25
Appendix 
Time For
Live Demo
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer

22. GDT Demo - 1
Open File
GDT_Demo_Lec-19.dwg_Start.dwg
Set LtScale to 0.5
Set Snap to ”
Xline at Top Edge
Offset TWICE by 0.5
xLine at Lt Edge
Offset by 0.5
Make Test-Dim to check Text-Height Relative to Part
Text & Arrows at Bit Big
Make DimStyle half_3dec
Scale = 0.5
Precision = 3-decimals

23. GDT Demo - 2 Change to DimLayer and Dim Thickness at 0.5 offset xline
Use Line to Extend the Dim-Ext_ln up to 1.0 offset
Open Geo Tol Dialog Box
Dimension → Tolerance Menus
Use Dialog Box to Spec Flatness & Back Surface as the Datum

24. GDT Demo - 3 Make DimStyle half_2dec_1side
Dim the /-0.02 Hgt
Make DimStyle half_3dec_Basic
Apply Basic (boxed) Dims
2.000
0.750

25. GDT Demo - 4 Make DimStyle half_2dec_2sided Dim 4.00 ±0.01
Make DimStyle half_3dec_2sided_ManualDim Hole Dia.
0.75 Above Top Edge by Offset
Make Feature Control Frame (FCF) using PullDowns: Dimension → Tolerance
Manually position the FCF Below the Ø Symbol

26. GD&T Result w/ xLines

27. GD&T Demo Final Result