1. Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engineering 22
Standard Tolerancing
Bruce Mayer, PE
Registered Electrical & Mechanical Engineer

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.
The SMALLEST-hole = SMALLEST-shaft

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. Surface Roughness → RA
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
Make “k” Rouhness Height, HR, Measurements Relative to the Mean Surface Height
HR can be Positive or Negative
Find Ra as

20. Surface Finish Specification
The Basic Surface Finish “Check Mark” Can have a Number of
Metrics
Modifiers
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
Show Bubbler Tank ElectroPolished to about 5 Ra

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

24. Tolerance Demo Start Pt
Time For
Live Demo

25. Demo Result

26. What’s your Tolerancing Tolerance?
All Done for Today
What’s your Tolerancing Tolerance?

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

28. Tolerance Demo - 1 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
Make CL using MText->Symbol

29. Tolerance Demo - 2 Apply Bilateral (Symmetrical) Tolerances to 5 dims
Apply Deviation Tolerance
Slot Width =
Base Length =
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”

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

31. Demo - 2
Apply Symmetrical Tolerance with Style Tol_BiLat

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

33. Demo - 4
Apply Nominal Tolerances with STD