This WAS a 25 tooth #100 pitch Roller-Chain sprocket, but it "wandered" on its shaft, and this was the result!
Step 1: Accepting the Inevitable
A new sprocket is days-away, and the machine it belongs on is vital.
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
Since the edge of the sprocket is trashed beyond repair, I'll need a center-point from which to measure, and scribe. the bore on this sprocket is 2", and luckily, I NEVER THROW ANYTHING AWAY, and I had this 2" Derlin™ spacer that was threaded, but even if I didn't have it, I would have made one out of wood, or possibly cut it out of lexan.
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
As you can see by the spec-sheet, the pitch between rollers is 1.25", and the diameter of the rollers is .750"
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
In this photo, you can see the minimum radius line scribed from the center-point, and the starter-point punched with a center-punch. I then set the dividers to 1.25", and progressively scribe and punch around the radius until I'm back at the start. Now I realise my mistake!
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
Since I'm using a drill-press, I'll try to set it up in the vise to allow for accurate repeatability, but it is just bulky, so it will take time.
In the end, all of the holes were good! (do not spare the cutting-oil!)
Step 6: The Big Holes
This part took the longest. as I only had one 3/4" bit, and I didn't want to foul the cutting-edges, Sooooooooo I was generous with the cutting-oil, and used slow speed, and moderate feed!
Step 7: Figuring-out the Teeth
Now, I decided to use a portable bandsaw rather than a cutoff-wheel mounted on a handheld grinder mostly because I hate cutoff wheels: (I have lots of mercury-amalgam fillings, and the metal dust is like chewing batteries) but equally because the bandsaw is more accurate. so I scribe and punch points between the holes to layout 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
I sawed along the lines: first in one direction until they were all sawed. And then the other.
Step 9: Now to Clean-up the Teeth, and Bevel the Top and Bottom.
Now it's time to get busy grinding all of the rough-spots, and tapering the teeth. They just need a slight taper on the top and bottom to allow the chain to engage them smoothly. the third picture shows this taper.
Step 10: Conclusion
Here's a terrible photo of the finished sprocket right before I reinstalled it.
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!