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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.

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Bruce mayer pe licensed electrical mechanical engineer bmayer chabotcollege

Engineering 22

GD&TBonus Tolerance

Bruce Mayer, PE

Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu


Skill development goal
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
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
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
GD&T Tolerances

  • From ASME Y14.5M-1994

    • Some ACAD Features based on Y14.5-1982


Mmc bonus tolerance
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
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
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
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.


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

Gage Fixture Design


Gage fixture design1
Gage Fixture Design WORK in the Assembly are Being REJECTED by RI (Receiving Inspection)

  • Made from Hardened Tool Steel

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


Min hole part checked by gage

Put Small-Hole Part in Gage Block WORK in the Assembly are Being REJECTED by RI (Receiving Inspection)

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


Max hole part checked by gage

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


Mmc bonus tolerance1

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
GD&T Caveat → Use with Care the MMC, or 0.497. If the hole is BIGGER, we get a BONUS tolerance equal to the difference

  • 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…


Go nogo and go nogo gages

The Gage we Designed is Necessary but NOT sufficient the MMC, or 0.497. If the hole is BIGGER, we get a BONUS tolerance equal to the difference

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


Complete go nogo gaging

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)


Gd t in autocad

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

GD&T In AutoCAD

  • 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
All Done for Today ACAD

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.


Bruce mayer pe licensed electrical mechanical engineer bmayer chabotcollege

Engr/Math/Physics 25 ACAD

Appendix

Time For

Live Demo

Bruce Mayer, PE

Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu


Gdt demo 1

Open File ACAD

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


Gdt demo 2

Change to DimLayer and Dim Thickness at 0.5 offset xline ACAD

Use Line to Extend the 0.625 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

GDT Demo - 2


Gdt demo 3

Make DimStyle half_2dec_1side ACAD

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


Gdt demo 4

Make DimStyle half_2dec_2sided ACAD

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

GDT Demo - 4