Step 1: Accepting the Inevitable
Necessity is truly the mother of invention, and if there were ever a case-in-point, today is that day:
It is a Sunday, and I have the bare minimum required for this transformation.
Step 2: First Things First
As you can see, the bolt has a center-punch dimple so I can use dividers to scribe circles reasonably accurately.
Step 3: Now for the Geometry
Now that we know the pitch, we can determine how many teeth we can get away-with.
Bear in mind that the maximum radius will end-up being the tips of the teeth, and the minimum radius will be the valleys that the rollers engage.
As you can see from the second photo, the maximum radius for me is somewhat "variable", but it averaged about 9.175".
Since the roller diameter is .750", I'll be using a 3/4" drill-bit to form the valleys, so I'll need to have at least 1/2" from the center of the 3/4" hole to the maximum radius, because I'm using a drill-press, and I can't risk the bit tearing-out of the hole.
I settled on 20 teeth by using this formula: pitch X number of teeth / pi.
that gave me 7.957 as my minimum radius.
Step 4: Laying It All Out
what I should have done is add 3/8" to the dividers when I scribed the radius, because this would be the center of the roller hole, and not the minimum radius! Oh well: extra work!
Step 5: Pilot Holes
In the end, all of the holes were good! (do not spare the cutting-oil!)
Step 6: The Big Holes
Step 7: Figuring-out the Teeth
Since the outside diameter is rough, I clean it up whith a grinder, and blacken the area with a permanent-marker. (I don't have any layout-fluid)!
Now I scribe the maximum radius line all around the perimeter, and use a straight-edge to draw lines from the center point of the sprocket through the marks between the holes.
Now. all that remains is scribing lines from the edges of the holes, to the points at the maximum radius scribe lines.
Step 8: Sawing the Teeth
Step 9: Now to Clean-up the Teeth, and Bevel the Top and Bottom.
Step 10: Conclusion
Since I reduced the number of teeth, the torque went up, and the speed went down, (it is a drive-sprocket), but I was able to adjust the chain tension, and the speed isn't critical in this application.
This is my first instructible, and it was composed on my GALAXY NOTE 3, so I apologize if it's not up to the usual standards, but I've often finished one of these repairs and said: "I should have done an instructible!"
Sooooooooo here it is!