1. Computer Aided Design (CAD)
Week 9 :: Involute Gears and Dimensioning

2. Gears - Introduction Single Gear Gear Train
Driver and Driven Gear Wheels

3. Types of Gears Spur Gear Rack & Pinion Helical Gear
Worm Gear and Wheel
Note: Not an exhaustive list.

4. Some Gear Terminology Pitch Circle Diameter Circular Pitch
Fillet Radius
Circular Pitch

5. Gear Data and Equations
Pressure Angle: Normally 20º
Pitch Circle Diameter (PCD): Module x No. Teeth
A circle representing the two gears as tangent cylinders.
Addendum = Module
The part of the tooth above the Pitch Circle.
Clearance = 0.25 x Module (1/4 of Module)
The gap between the tip and the root of two meshing teeth.
Dedendum = Module + Clearance
The part of the tooth below the Pitch Circle.

6. Gear Data and Equations
Circular Pitch = Module x 3.14 (Pi)
The distance from a point on one tooth to a similar on the next.
Tooth Thickness = Circular Pitch / 2
Fillet Radius (approx) = Circular Pitch / 7

7. Spur Gear Calculation & Drawing
The method used here to draw a typical spur gear is called “Unwin’s Construction”. It is also called the “Approximate Method” because it is only produces an Approximate “Involute Curve”.
Tip: When drawing a Gear in AutoCAD – make full use of Layers and Colours.

8. Spur Gear Calculation & Drawing #1
diameter.

9. Spur Gear Calculation & Drawing #2
a circle

10. Spur Gear Calculation & Drawing
Now its your Turn ->
Draw a Spur gear with the following details:
Module: 2mm
Number of Teeth: 25

11. Dimensioning to BS308 Standards
Each dimension required should only appear once. There should be no more dimensions than necessary.
Linear Dimensions are normally in millimeters (mm). The symbol may be ommitted provided the drawing carries a statement of the unit used.
Angular Dimensions should be expressed in degrees and minutes, e.g.:
20° 30’

12. Dimensioning to BS308 Standards
Projection lines and dimension lines should normally be placed outside the outline of the view.
Crossing of projection and dimension lines should be avoided.
Projection lines should start just clear of the outline of the feature and should extend a little beyond the dimension line.
Projection lines should normally be drawn perpendicular to the dimension required.

13. Dimensioning Overview

14. Dimensioning Overview
Dimensions should be placed near the middle, and above the dimension line.
Larger Dimensions should be placed outside smaller dimensions.

15. Dimensioning Methods Parallel Dimensioning:
Consists of a number of dimension lines originating from a datum feature.
Chain Dimensioning:
Should only be used where the accumulation of tolerances does not endanger the function of the part.

16. Diameter Dimensioning
Diameter dimensions of a circle or cylinder is shown by the symbol Ø in front of the No.

17. Radius Dimensioning
Radii should be dimensioned by a dimension line that passes through, or is in line with, the center of the arc.
The dimension line should have one arrowhead only, that which touches the arc.
The symbol R is placed in front of the No.

18. Dimensioning Features
Dimensioning of equally-spaced features may be simplified by using methids similar to below:

19. Dimensioning Holes etc.
Notes:
- The term “spotface” implies that the depth is the minimum necessary to provide a machined surface.
- Chamfers at 45° should be dimensioned as below and not described by a note.

20. Dimensioning Holes etc.

21. Dimensioning Tapered Features

22. Dimensioning Screw Threads
The letter M, standing for ISO metric thread, is followed by, the nominal diameter and the pitch, both in millimeters. E.g. M8 x 1.
The absence of a pitch (e.g. M8), means that a course pitch is specified. (e.g. The course pitch for an M8 thread is 1.25mm. (See Page 7 in AutoCAD Notes part II)
Thread tolerance 6H is suitable for INTERNAL thread tolerance. 6g is suitable for EXTERNAL thread tolerance. E.g. M10 x 1 – 6g

23. Dimensioning External Threads

24. Dimensioning Internal Threads
Note the hatch lines and line weights.

25. Dimensioning Threads

26. Typical Dimensioned Drawing