Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu 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

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 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

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

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 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 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 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 Tolerance 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. NOT to scale Actual Hole Size Bonus Tolerance Ø of Tol. Zone Ø .497 (MMC) .010 Ø .499 (.499 - .497 = .002) .002 (.010 + .002 = .012) .012 Ø .500 (.500 - .497 = .003) .003 (.010 + .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

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 sufficient 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 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 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 1.50 +0.00/-0.02 Time For Live Demo Let Dim & Tol Our Spacer Plate with Changes

All Done for Today The AME and Bonus Tol. http://www.tec-ease.com/tips/august-97.htm 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 .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.

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

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 - 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 - 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 - 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

GD&T Result w/ xLines

GD&T Demo Final Result