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

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

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  1. Engineering 22 GD&TBonus Tolerance Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

  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 Y14.5-1982

  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 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

  8. 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

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

  10. 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

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

  12. 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

  13. 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

  14. 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

  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. 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

  17. 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)

  18. 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

  19. 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.

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

  21. 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

  22. 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

  23. 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

  24. 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

  25. GD&T Result w/ xLines

  26. GD&T Demo Final Result

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