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Engineering 22. GD&T Bonus Tolerance. Bruce Mayer, PE Licensed Electrical & Mechanical Engineer [email protected] 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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Presentation Transcript
slide1

Engineering 22

GD&TBonus Tolerance

Bruce Mayer, PE

Licensed Electrical & Mechanical [email protected]

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
  • 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 BlockMin-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 PinMax-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
  • 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 sufficientGo, 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 HolesComplete 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 ACADGD&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

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.

slide21

Engr/Math/Physics 25

Appendix

Time For

Live Demo

Bruce Mayer, PE

Licensed Electrical & Mechanical [email protected]

gdt demo 1
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
gdt demo 2
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

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

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

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
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