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

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. Autumn Quarter

Direct limits and tolerance values Can be: Limits: Upper limit – 3.53 Lower limit – 3.49 Unilateral – vary in only one direction 3.49 0 -.0X +.0X - 0 Bilateral – vary larger or smaller (may or may not be same amount) 3.50 +.05 -.01, +.10 -.20 +/- 0.05 +.04 +.03 -.01 Autumn Quarter

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) Autumn Quarter

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) Autumn Quarter

Review – Fitting Two Parts Part A Tolerance of A Part B Tolerance of B Tolerance: Clearance or Interference Autumn Quarter

Review – Shaft and Hole Fits Clearance Interference CLEARANCE FIT Allowance = + .003 Autumn Quarter

Review – Shaft and Hole Fits Transition CLEARANCE FIT + .003 Autumn Quarter

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

Review – Basic Hole System 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 and Cmin < 0 --- Transition fit Allowance = Hmin - Smax (i.e., Cmin) GOOD TO KNOW Autumn Quarter

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 Autumn Quarter

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. Autumn Quarter

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. Autumn Quarter

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. Autumn Quarter

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. Autumn Quarter

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. Autumn Quarter

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