1. Fits and Tolerances
Lecture 20
Autumn Quarter

2. 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 – Dimensioning for Interchangeable Parts
P – Standard Tables for Fits
P – Geometric Tolerancing
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.
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3. 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.
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.
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4. 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
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.
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5. Direct Limits and Tolerance Values
Can be:
Limits: Upper limit – 3.53
Lower limit – 3.49
Unilateral – vary in only one direction
3.49 X
+.0X
Bilateral – vary larger or smaller (may or may not be same amount)
3.50
, /- 0.05
+.04
Instructor:
Here are some of the ways to specify the limits on parts.
+.03
-.01
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6. Geometric Tolerance System
Geometric dimensioning and tolerancing (GDT) is a method of defining parts based on how they function, using standard ANSI symbols.
(More about this in a couple of weeks.)
Feature Control Frame
Concentricity Symbol
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
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7. Notes and Title Block
ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005"
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.
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8. 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)
Instructor:
Take your time going through this slide. Make sure that they understand each definition.
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9. 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)
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.
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10. Fitting Two Parts Part A Tolerance of A Part B Tolerance of B
Tolerance: Clearance or Interference
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
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11. Shaft and Hole Fits Clearance Interference 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.
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12. Shaft and Hole Fits Transition CLEARANCE FIT + .003 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.
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13. 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)
Instructor:
Here are the types of fits specified by ANSI. The tables are found in the Appendices in most graphics books.
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14. 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"
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 depending on the type of fit.
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15. 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)
Instructor:
Go through this table of equations with your class. These are the equations that they will use to complete today’s exercises.
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16. 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
Instructor:
Here are the equations to check your work by looking at the system tolerance – the sum of the tolerances of the parts.
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17. Example
Instructor:
Here is an example of looking at the tolerance of each part and the fit tolerance.
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18. Metric Limits and Fits
Based on Standard Basic Sizes – ISO Standard, see the Appendix material (Appendices )
Note that in the Metric system:
Nominal Size = Basic Size
Example: If the nominal size is 8, then the basic size is 8
Instructor:
In the metric system the nominal size is equal to the basic size given with the correct number of decimal places.
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19. Metric Preferred Hole Basis System of Fits
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.
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20. Metric Tolerance Homework – Example TOL-1B
Instructor:
Here is an example done in the metric system.
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21. Good Review Material BTG Chapter 7 Dimensions and Tolerances
Pages
BTG Chapter 8
Dimensions For Production
Pages
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.
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22. 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.
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.
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