Bonus - Engineering Design Graphics - Lecture Slides, Slides of Computer-Aided Design for Engineers

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GD&T

Bonus

Tolerance

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

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-

• Some ACAD Features based on Y14.5-

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. - Ø-POSITION Tolerance = 0. - Relative to BASIC Dims 2.00 &. - 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

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. - 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 ∆ x = ∆ y = ( 0. 01 2 ) 2 = 0. 0035355

Max-Hole Part Checked by Gage

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

 Expand Origin Area

 Note How the Axis Wall Clearance has Expanded by 0.

• Thus the Hole-Ctr for this LMC Part can be Misplaced by 0.

Zoom In

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.

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

Actual Hole Size Bonus Tolerance Ø of Tol. Zone Ø .497 (MMC) 0. Ø .499 (.499 - .497 = .002) .002 (.010 + .002 = .012). Ø .500 (.500 - .497 = .003) .003 (.010 + .003 = .013). Ø .502 .005. Ø .503 (LMC) .006. Ø .504? REJECTED^ –^ Out of Spec?

NOT to scale

Go, NoGo, and Go-NoGo Gages

• The Gage we Designed is Necessary but NOT sufficient

 i.e., if part does NOT fit the Gage, Then REJECT it

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

• e.g., If The hole is LARGER than 0. it could easily fit the gage  Thus our Gage is a NoGo Instrument

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

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

 Let Dim & Tol Our Spacer Plate with Changes

• Width now 4. ±0.
• Height now 1. +0.00/-0.

All Done for Today

The AME and Bonus Tol.

If a hole, for instance, has the following size and geometric control, and the hole measures .502. It wouldbe 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 isreduced by that amount. The bonus would be merely .001 and the allowable position tolerance = .016.