Drafting a sprocket with CAD is pretty simple. A quick tutorial in steps.

You will need the pitch, roller diameter, number of teeth, and width of the rollers for the chain you need a sprocket for. I am using the dimensions for a 420 chain as follows: Pitch = .5", Roller diameter = 0.306", Width = 0.312", Mean radius of the intended 37 tooth sprocket = ((37 * .5) / 3.14159265358) / 2 This is to make sure the lines you draw are long enough.

**1: ** Start with a single vertical line longer than the radius starting at the origin (0, 0).

**2: **Determine half of the angle for the number of teeth you are shooting for: 90-((360/37) / 2) Draft a second line to that angle. I do all of the calculations using the calculator in the computer: it makes importing numbers of considerably greater decimal places than a CAD program normally uses, and insures there are no mistakes in transcribing. That is if you did the calculations correctly!

**3: **Mirror the line.

**4: **Translate the vertical line to half of the pitch (0.25")

**5: **Draft a horizontal line from the intersection of the vertical line and the right angled line to the left angled line. It will be equal to the pitch of the chain.

**6: **Draw two circles equal to the roller diameter at each end of the horizontal line. You can erase or sequester the lines.

**7: **Zoom in on the two circles.

**8: **Draft a circle centered on either of the circles to the quadrant of the other circle. Repeat for the other circle. This describes the motion of one link of the chain.

**9: **Run an arc from the quadrant of the circles to the intersection of the two larger circles. This is the tooth profile.

**10: **Truncate the tip of the tooth with a horizontal line.

**11: **Polar duplicate the tooth profile.

**12: **Draft an arc from one tooth profile to the next tooth profile. Erase both circles.

**13: **Polar duplicate the arc. The sprocket teeth are ready to extrude.

**14: **Using dimensions from whatever the sprocket is going to be attached to, draft a central circle for the register, and the bolt circle.

**15: **Extrude to the width of the chain roller minus ~5% for running clearance.

There are some finishing touches to do, particularly chamfering the sides of the teeth from the tip to about 30% of the height of the teeth, and if you are so inclined scroll work to lighten the sprocket.

For the math of sprockets go here: http://www.gearseds.com/files/design_draw_sprocket_5.pdf

Rob