Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

1. Engineering 22 StandardTolerancing Bruce Mayer, PE Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

2. Skill-Development Goal • To learn how to effectively tolerance parts such that • The Parts Function Correctly • Fabrication Cost Is Kept To A Minimum • Read & Create Limit Dimensions • Range & Deviation Forms • Symmetrical • BiLateral • UniLateral

3. Tolerance Introduction • The total amount a specific dimension can vary. (ANSI/ASME Y14.5M-1994) • Tolerances are assigned so that any two mating parts will fit together. • Highly accurate parts are extremely expensive, so tolerances should be as generous as possible while still maintaining proper function for the part.

4. Tolerancing • Definition: Allowance for specific variation in the size and geometry of a part • Need for Tolerancing • It is IMPOSSIBLE to manufacture a part to an EXACT size or geometry • Since variation from the drawing is inevitable we must specify the acceptable degree of variation • Large variation may affect part functionality • Small allowed variation affects the part cost • requires precise manufacturing • requires inspection and potential False-Rejection of otherwise Functional parts

5. Tolerance Follows Function • Assemblies: • Parts will not fit together if their dimensions do not fall with in a certain range of values • Interchangeable Parts: • If a replacement part is used it must duplicate the original part within certain limits of deviation • The relationship between functionality and size/shape of an object varies with the part • Automobile Transmission is Very Sensitive to the Size & Shape of the Gears • A Bicycle is NOT Too Sensitive to the Size & Shape of the Gears (sprockets)

6. Two Forms of Physical Tolerance • Size • Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing • Geometry • Geometric Dimensioning & Tolerancing (GD&T) • Allows for specification of tolerance for the geometry of a part separate from its size • GD&T uses special symbols to control different geometric features of a part • Will Discuss GD&T Next Time

7. Cost Sensitivity • Cost generally increases with “tighter” tolerances • There is generally a ceiling to this relationship where larger tolerances do not affect cost • e.g.; If the Fabricator ROUTINELY Holds to ±0.5 mm, Then a ±3 mm Specification will NOT reduce Cost • Tolerances at the Limits of the Fabricator’s Capability cause an exponential increase in cost • Parts with small tolerances often require special methods of manufacturing • Parts with small tolerances often require greater inspection, and higher part-rejection rates • Do NOT specify a SMALLER Tolerance than is NEEDED

8. Tolerance Spec Hierarchy • Generally Three Levels of Tolerances • DEFAULT: Placed in the Drawing Title-Block by The Engineering Firm • Typically Conforms to Routine Tolerance Levels • GENERAL: Placed on the Drawing By the Design-Engineer as a NOTE • Applies to the Entire Drawing • Supercedes the DEFAULT Tolerance • SPECIFIC: Associated with a SINGLE Dimension or Geometric Feature

9. Default Tolerance Example

10. Range Limit Dimensions • Range Limit dimensions specify the upper and lower value. • An acceptable part may be at the upper limit, lower limit, or any value in between • Advantage: Fabricators Measure Total Distance, and Limit Dims show these

11. Allowance & Fit • The minimum clearance space or maximum interference is the ALLOWANCE. • Fit is the range of tightness or looseness between parts for their function.

12. Types of Fit • CLEARANCE Fit: internal member always has space or clearance. • INTERFERENCE Fit: internal member is always larger and has to be forced inside external member. • TRANSITION Fit: either clearance or interference. • LINE Fit: clearance or surface contact results.

13. Basic Hole System • Hole will be machined with a standard sized tool. • Determine type of fit necessary. Use fit table or otherwise determine allowance. • Apply tolerances using hole size as the base (nominal) dimension. • Generally Easier to “Turn Down” a Shaft, Than to Make a Non-Standard Sized Hole

14. Basic Shaft System • Less commonly used than basic hole system. • Most commonly used when many parts will fit on a standard shaft. • Shaft size is the base dimension.

15. Deviation Limit Dimensions • Basic size of Dimension is Given With Tolerances Noted As A Plus/Minus (±) Range.

16. Symmetrical (Equal) Bi-lateral Tolerance Forms • Most Easily Fabricated • Most Fabrication Processes Vary Randomly • “Target” Value Given by BiLat Dims • Advantage: Target Value stated – Fabricator can set Machine to Target

17. Baseline vs. Continued • Baseline tolerances don’t “stack-up” • Continued (chained) tolerances CAN “stack-up”

18. Surface Finishes • Surface Finish Refers, Primarily, to the “Roughness” of a Surface • Use of “Finish Marks” • MicroScopic Definition of Finish Elements

19. In the “Old” days the Surface Roughness was Stated in µ-inches “RMS” Root of the Mean Square (see Engr43) Now we use µm or µin with the “Arithmetic Average” (AA or Ra) as the Roughness Metric Surface Roughness → RA • Make “k” Rouhness Height, HR, Measurements Relative to the Mean Surface Height • HR can be Positive or Negative • Find Ra as

20. The Basic Surface Finish “Check Mark” Can have a Number of Metrics Modifiers Surface Finish Specification • The Modifer List and how to Draw the Check Mark

21. Tool-Mark Lay • The “Lay” of the marks left by the Surface Finishing Tool Refers to the Mark Orientation Relative to the Surface or Some Reference Direction

22. Manufacturing Process Determines Ra

23. Say we have this part 3mm 36 +0.4/-0.0 13 Tolerancing with AutoCAD • Default Tol. (±0.5 mm) is OK except for mating features that need Tol. of ±0.2 • Top Groove Width = nominal 58-22 = 36 • Position Relative to CL • Top Groove Hgt = 27 • 105mm Base Width • Position Relative to CL • Edge Notch Height = nominal 13 36 +0.4/-0.0 105 +0.0/-0.4

24. Tolerance Demo Start Pt Time For Live Demo

25. Demo Result

26. All Done for Today What’s yourTolerancingTolerance?

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

28. Open File Tolerance_Demo_Lec-17_Start_0508.dwg Contains F, T, B, RS views Make and Label w/ CL extended Ctr-Lines in F & RS views Modify STD Dim Style for overall scale of 19 Adjust LTSCALE to 12.7 Make 3 more Dim Styles Tol_BiLat with Tolerance of 0.2 Tol_LowLim with +0.4/-0.0 Tol_HiLim with +0.0/-0.4 Tolerance Demo - 1

29. Apply Bilateral (Symmetrical) Tolerances to 5 dims Apply Deviation Tolerance Slot Width = 36.0 +0.4.-0.0 Base Length = 105.0 +0.0.-0.4 Apply std dims to balance of part Note that Slot Ctrs located relative to MACHINED Surface Slots shown on CL ( -.5mm Tol) NOTE for Rounded edges For R29 may need to “Place Text Manually” Tolerance Demo - 2

30. Extend CL’s 10mm beyond part envelope To make CL symbol ROMANS style KeyBd Input = \U+2104 Demo - 1

31. Apply Symmetrical Tolerance with Style Tol_BiLat Demo - 2

32. Apply ASymmetrical Tolerance with Styles Tol_HiLIm Tol_LowLim Demo - 3

33. Apply Nominal Tolerances with STD Demo - 4