# Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege - PowerPoint PPT Presentation

1 / 28

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

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

#### Presentation Transcript

Engineering 22

GD&TBonus Tolerance

Bruce Mayer, PE

### 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

### 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

### 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

### GD&T Tolerances

• From ASME Y14.5M-1994

• Some ACAD Features based on Y14.5-1982

### 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.

### MMC Bonus Tolerance Example

• Condsider theGD&T Spec’dPart At Right

• In particularnote 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

PositionSymbol

### MMC Bonus Tolerance cont

• Then the ToleranceZone at MMCShown 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

### 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 .497. If the hole is bigger, we get a bonus tolerance equal to the difference between the MMC size and the actual size.

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

### Gage Fixture Design

• Made from Hardened Tool Steel

• Axis Walls Shifted by the Pythagorean XY distance equal to half the Tolerance Zone

Put Small-Hole Part in Gage Block

### Min-Hole Part Checked by Gage

• The Perfect Part Just Fits over the Pin, and Clears the Walls by ½ the Positional Tol.

Zoom In

• Expand Origin Area

Put Large-Hole Part in Gage Block; move it to Up & Rt Till it hits the Gage Pin

### Max-Hole Part Checked by Gage

• 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

This means that the tolerance is .010 IF the hole size is at the MMC, or 0.497. If the hole is BIGGER, we get a BONUS tolerance equal to the difference between the MMC size and the actual size.

### MMC Bonus Tolerance

NOT to scale

Read Carefully §10-17 thru §10-23 in Text Book

REJECTED – Out of Spec

### 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…

The Gage we Designed is Necessary but NOT sufficient

### Go, NoGo, and Go-NoGo Gages

• However if part DOES Fit, it COULD Still be BAD

• e.g., If The hole is LARGER than 0.503 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

To Complete the Gage Set, We need to design a Precision Gage-PIN to Test for OverSize Holes

### Complete Go-NoGo Gaging

• 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)

Now that We Understand how to USE GD&T let’s APPLY it with ACAD

• Width now 4.00 ±0.01

• Height now 1.50 +0.00/-0.02

Time For

Live Demo

• Let Dim & Tol Our Spacer Plate with Changes

### All Done for Today

The AMEandBonus Tol.

If a hole, for instance, has the following size and geometric control, and the hole measures .502. It would be incorrect to use a bonus tolerance of .003 (.502 - .499(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.

Engr/Math/Physics 25

Appendix

Time For

Live Demo

Bruce Mayer, PE

Open File

GDT_Demo_Lec-19.dwg_Start.dwg

Set LtScale to 0.5

Set Snap to 0.0625”

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

### GDT Demo - 1

Change to DimLayer and Dim Thickness at 0.5 offset xline

Use Line to Extend the 0.625 Dim-Ext_ln up to 1.0 offset

Open Geo Tol Dialog Box

Use Dialog Box to Spec Flatness & Back Surface as the Datum

### GDT Demo - 2

Make DimStyle half_2dec_1side

Dim the 1.50 +0.00/-0.02 Hgt

Make DimStyle half_3dec_Basic

Apply Basic (boxed) Dims

2.000

0.750

### GDT Demo - 3

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