2. Engineering H191 - Drafting / CAD
Tolerancing – Control of Variability
• Goals
– Understand the description and control of
variability through tolerancing.
– Use standard tables for tolerancing and
control of fit
• Reference (BTG)
– P. 312-317 – Dimensioning for Interchangeable
Parts
– P. 349-354 – Standard Tables for Fits
– P. 358-369 – Geometric Tolerancing
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 2
3. Engineering H191 - Drafting / CAD
Definition of Tolerance
• Tolerance is the total amount a dimension may
vary. It is the difference between the maximum
and minimum limits.
• There is no such thing as an "exact size".
• Tolerance is key to interchangeable parts.
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 3
4. Engineering H191 - Drafting / CAD
Ways to Express Tolerance
• Direct limits or as tolerance limits applied to a
dimension
• Geometric tolerances
• Notes referring to specific conditions
• A general tolerance note in title block
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 4
5. Engineering H191 - Drafting / CAD
Direct Limits and Tolerance Values
Can be:
Limits: Upper limit – 3.53
Lower limit – 3.49
Unilateral – vary in only one
direction +.04
0
3.49
0 -.0X
+.0X - 0
Bilateral – vary larger or smaller
(may or may not be+.03
same amount)
-.01
3.50
Autumn Quarter +.05 -.01, +.10 -.20
Gateway Engineering Education Coalition +/- 0.05
Lect 20 P. 5
6. Engineering H191 - Drafting / CAD
Geometric Tolerance System
• Geometric dimensioning
and tolerancing (GDT) is a Feature Control Frame
method of defining parts
Concentricity Symbol
based on how they
function, using standard
ANSI symbols.
• (More about this in a
couple of weeks.)
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 6
7. Engineering H191 - Drafting / CAD
Notes and Title Block
ALL DECIMAL
DIMENSIONS THAT
ARE THREE PLACE
ACCUARCY (.XXX)
TO BE HELD TO
+/-.005"
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 7
8. Engineering H191 - Drafting / CAD
Important Terms – Single Part
• Nominal Size – a general size, usually expressed
as a common fraction (1/2”)
• Basic Size – theoretical size used as starting
point (.500”)
• Actual Size – measured size (.501”)
• Limits – maximum and minimum sizes shown by
tolerances
• Tolerance – total allowable variance in
dimensions (upper limit – lower limit)
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 8
9. Engineering H191 - Drafting / CAD
Important Terms – Multiple Parts
• Allowance – the minimum clearance or maximum
interference between parts
• Fit – degree of tightness between two parts
– Clearance Fit – tolerance of mating parts
always leave a space
– Interference Fit – tolerance of mating parts
always interfere
– Transition Fit – sometimes interfere,
sometimes clear
• Tolerance – total allowable variance in
dimensions (upper limit – lower limit)
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 9
10. Engineering H191 - Drafting / CAD
Fitting Two Parts
Tolerance of B Tolerance: Clearance
or Interference
Part B
Tolerance of A
Part A
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 10
12. Engineering H191 - Drafting / CAD
Shaft and Hole Fits
Transition
CLEARANCE
FIT
+ .003
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 12
13. Engineering H191 - Drafting / CAD
Standard Precision Fits: English Units
• Running and sliding fits (RC)
• Clearance locational fits (LC)
• Transition locational fits (LT)
• Interference locational fits (LN)
• Force and shrink fits (FN)
See Tables in the Appendix (pp. A11-A23)
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 13
14. Engineering H191 - Drafting / CAD
Basic Hole System or Hole Basis
• Definition of the "Basic Hole System":
– The "minimum size" of the hole is equal to the
"basic size" of the fit
• Example: If the nominal size of a fit is 1/2", then
the minimum size of the hole in the system will be
0.500"
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 14
15. Engineering H191 - Drafting / CAD
Fit Calculations
• Clearance = Hole – Shaft
• Cmax = Hmax – Smin
• Cmin = Hmin – Smax
Both Cmax and Cmin >0 – Clearance fit
Both Cmax and Cmin <0 – Interference fit
Cmax > 0, Cmin < 0 – Transition fit
• Allowance = Hmin - Smax (i.e., Cmin)
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 15
16. Engineering H191 - Drafting / CAD
Fit Calculations
• System Tolerance = Cmax - Cmin (Sometimes
called Clearance Tolerance)
• Also, System Tolerance = Σ Ti
• So, System Tolerance, or Ts , can be written as:
Ts = Cmax - Cmin = Σ Ti
• Thus, you always have a check value
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 16
17. Engineering H191 - Drafting / CAD
Example
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 17
18. Engineering H191 - Drafting / CAD
Metric Limits and Fits
• Based on Standard Basic Sizes – ISO Standard,
see the Appendix material (Appendices 8 - 12)
• Note that in the Metric system:
Nominal Size = Basic Size
• Example: If the nominal size is 8, then the basic
size is 8
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 18
19. Engineering H191 - Drafting / CAD
Metric Preferred Hole Basis System of Fits
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 19
21. Engineering H191 - Drafting / CAD
Good Review Material
• BTG Chapter 7
– Dimensions and Tolerances
– Pages 290-335
• BTG Chapter 8
– Dimensions For Production
– Pages 340-375
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 21
22. Engineering H191 - Drafting / CAD
Assignments
• Dwg 39 – G27 – Tolerances – Single Fits
– Calculate the missing values for each
situation.
– Use the tables for preferred limits and fits for
cylindrical parts.
• Dwg 40 – TOL–1A – Metric Tolerances
– Using the given nominal sizes and fit
specifications, calculate remaining values.
Autumn Quarter Gateway Engineering Education Coalition Lect 20 P. 22
Editor's Notes
Instructor: Some students have problems with tolerances and it is worth taking this lesson slowly and making sure that all of the students are with you. If you are not using the Boyer Technical Graphics book, you will need to find these sections in your text book.
Instructor: Emphasize that we are working on getting the maximum and minimum sizes for a particular part first. Then we will worry about parts fitting together. Also remind them that the closer you hold a tolerance the higher the cost of an item.
Instructor: There are a variety of ways to specify part sizes. This set of notes is going to use upper and lower limits most of the time. Just as a drawing must have the scale listed in the title block it must also have the overall tolerance of parts and part features. Typically it might be something like: All parts are +/-.01 unless otherwise specified.
Instructor: Here are some of the ways to specify the limits on parts.
Instructor: There is large body of knowledge that deals with Geometric Dimensioning and Tolerancing. We are introducing it here so that the students have an idea of what to look for in drawings that have GDT. The A in the sign is the base dimension – in this case the diameter. The rectangular box specifies that the cylinder on the right end must be concentric with the cylinder labeled A within 0.01 inch
Instructor: Point out to your students that the overall tolerance on this drawing depends on the number of decimal places in the given dimension. Note that they specify the tolerance on angles (+/- 1 degree) as well as the other dimensions. In this case, there is also an overall surface finish specified – the symbol that looks like a check mark.
Instructor: Take your time going through this slide. Make sure that they understand each definition.
Instructor: Note that the definitions on the previous page dealt with one part and how it varies. On this page the definitions deal with how parts fit together.
Instructor: The blue area represents the fit between A and B where the dark area represent the variance in the size of A and B
Instructor: Here is another way to look at how parts fit together. In this illustration, the beige area represents the variation in size of one part and it is easy to see that on the left side we have a clearance fit where the smallest hole is larger than the largest shaft. This is called a clearance fit. The opposite is true on the other end. This is a force fit, shrink fit or an interference fit. Note the small print at the bottom: Allowance always equals the smallest hole minus the largest shaft. When you have an interference fit the allowance is negative.
Instructor: This figure is slightly different than the previous one. Here they have put the smallest shaft on the left end and the largest shaft on the right side. The shaft on the left clears by .003 inch while the one on the right interferes by .002 inch.
Instructor: Here are the types of fits specified by ANSI. The tables are found in the Appendices in most graphics books.
Instructor: There needs to be some order to doing tolerances and in this case the choice was to make the smallest hole the basic size. If the nominal size is given as a common fraction (1/2) then the basic size is .500 or .5000 depending on the type of fit.
Instructor: Go through this table of equations with your class. These are the equations that they will use to complete today’s exercises.
Instructor: Here are the equations to check your work by looking at the system tolerance – the sum of the tolerances of the parts.
Instructor: Here is an example of looking at the tolerance of each part and the fit tolerance.
Instructor: In the metric system the nominal size is equal to the basic size given with the correct number of decimal places.
Instructor: When working in the metric system they use a series of letters and numbers where the capital letters represent the hole and the lower case letters represent the shaft sizes.
Instructor: Here is an example done in the metric system.
Instructor: Make sure the students are reading the text book. You might want to put your text under the document camera (if you use one) to show them figures from the chapter and to show them examples from the Appendices.
Instructor: If you do not use a document camera make a copy of one of the pages from the Appendix at a large enough scale to be easily read when projected and show them how the table works.
