1. Review of Fits and Tolerances AutoCAD 3D Multiview
Lecture 21
Autumn Quarter

2. Review – 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:
This is a review of the previous day’s lecture in the first part and then a bit about using sold models in AutoCAD to orthographic drawings.
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3. Direct limits and tolerance values
Can be:
Limits: Upper limit – 3.53
Lower limit – 3.49
Unilateral – vary in only one direction X
+.0X
Bilateral – vary larger or smaller (may or may not be same amount)
3.50
, /- 0.05
+.04
+.03
-.01
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4. Important Terms – Single Part
Nominal Size – general size (1/2”)
Basic Size – theoretical size used as starting point (.500”)
Actual Size – measured (.501”)
Limits – maximum and minimum sizes shown by tolerances
Tolerance – total allowable variance in dimensions (upper – lower limit)
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5. 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 – lower limit)
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6. Review – Fitting Two Parts
Part A
Tolerance of A
Part B
Tolerance of B
Tolerance: Clearance or Interference
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7. Review – Shaft and Hole Fits
Clearance Interference
CLEARANCE FIT Allowance =
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8. Review – Shaft and Hole Fits
Transition
CLEARANCE FIT
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9. 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)
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10. Review – Basic Hole System
Clearance = Hole – Shaft
Cmax = Hmax – Smin
Cmin = Hmin – Smax
Both Cmax and Cmin > Clearance fit
Both Cmax and Cmin < Interference fit
Cmax > 0 and Cmin < Transition fit
Allowance = Hmin - Smax (i.e., Cmin)
GOOD TO KNOW
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11. Review – Basic Hole System
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
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12. Multiview Drawings From 3D Models
A major advantage of creating a 3D model of a part is that the separate views do not need to be created in 2D.
The views can be created in Paper Space from the 3D Model
Instructor:
In prior AutoCAD exercises we created 3D models. Now we want to extract the appropriate views to make a good engineering drawings. The next AutoCAD lessons take the students through that process.
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13. Multiview Layout of V-Block
What views would be necessary to describe this part?
What Dimensions will you need in these views?
Instructor:
Have the students work in pairs. They probably won’t thing about having an auxiliary view (not one of the principal views) to show the true shape of the inclined plane.
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14. Auxiliary Views
An Auxiliary view of this surface will be needed… would this be easy to draw in 2D?
Once modeled in 3D, creating the view is quick and easy
Instructor:
Point out to the students that you can request the CAD system to give you the view of the block where the inclined plane is true size.
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15. Notes on Lesson 8
Pay close attention to the use of the Properties button
This is a powerful tool that can be used to modify all objects created
Note the method shown for setting the scale and aligning the views. It is more complex than what we have done in the past, but a good method when dealing with many views.
Instructor:
If you go through Lesson 8 from the AutoCAD 3D tutorial before you give this lecture, you can provide some guidelines about where they might go wrong in the process.
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16. Notes on Lesson 8 Another command used in this lesson is ‘SOLPROF’
This command creates a 2D profile from the 3D model – with options for showing hidden lines and object lines correctly.
Be sure to closely follow the steps to set the correct options
Instructor:
Here is another reason to do Lesson 8 before giving your lecture. SOLPROF converts the solid model into views with visible and hidden lines.
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17. Assignments Dwg 41 – TOL–3B – Fits & Tolerances
Calculate the missing tolerances for the given sets of parts.
Use the tables for preferred limits and fits for cylindrical parts.
Dwg 42 – AC3D Lesson 8
Multiview Drawings From 3D Models
Instructor:
Have the students work on the AutoCAD Lesson 8 in class and do the Fits and Tolerances problem at home.
Autumn Quarter