Sprocket Overlay

[DaveInMi]

I just ordered a “weld on” sprocket with 36 teeth. I want to make it an overlay sprocket fitting a 27 tooth sprocket. That will require boring the 36 tooth to equal the 27 tooth OD. Then I need to make 4 to 6 half holes to line up with half holes where the chain rollers fit in the 27 tooth sprocket. Bolts go through these holes to hold the two sprockets together. My thinking is that roller chain does not require as close tolerances as gears. I have considered making a hub for the 36 tooth sprocket so that I could bolt through where the chain rollers go but I hope there is an easier way. I would like some suggestions for the following: How do I hold the new sprocket so the bore will be concentric with the pitch circle? How to locate the half holes. I have a 13” lathe and mill but no rotary table.

 

[Wreck™Wreck]

What is an "Overlay Sprocket" and also what is a a "Half Hole", please explain.

 

[willthedancer]

Get a piece of 3/8 birch plywood. Using a common center, layout two pitch circles and put the appropriate size holes in a few places around the circles. Put short pins to locate the smaller, and some longer pins to locate the larger sprocket. That should get you pretty close. You'll need to put pins on all 4 quadrants on both sprockets to get it close to concentric.

 

[DaveInMi]

An overlay sprocket fits over a sprocket so that bolts fit through where the chain rollers would go on the smaller sprocket to hold the two together. If you think about the hole that the bolt goes through, half of it is in the larger sprocket and half of it is in the smaller sprocket where the chain roller would have fit. Thanks for the plywood idea. Thanks for the plywood idea. I knew there were more simple ways to approach this.

 

[Tozguy]

Dave,

Interesting job.
I am curious about how you hold the big sprocket and what tool you use to bore it out to fit over the small sprocket.
May I ask how you are going to cut the half holes in the outer sprocket once their location is established?
Thanks, Mike

 

[rgray]

Only place I've ever seen this was on a Honda 90. Big sprocket that went over existing rear sprocket for low gearing. Worked well and I think it was held with only 4 bolts.

 

[Randall Marx]

The first thing that comes to mind for me is to locate the large sprocket by its bore, then locate and make the holes, then bore the sprocket through the holes so it fits over the existing sprocket. This could all be done on the mill, at least in my mind.

 

[Tozguy]

Disc brakes on many motorcycles are mounted in a similar fashion. This type of mounting system allows the brake disc to expand and contract from heat without warping the mounting flange.

I have lowered the drive ratio on a motorcycle as rgray mentioned above and it works great. Dave is right in that concentricity is not as critical for chain and sprockets as for gears within reason.

 

[DaveInMi]

What I'm thinking now: Check to see if the bore of the larger sprocket is concentric with pitch circle. If it is, indicate on the bore and use four jaw chuck on the outside of the sprocket (36 teeth). Bore the larger sprocket to match the smaller sprocket outside diameter. The roller diameter of #50 roller chain is .400". I'll use .375 bolts. Clamp smaller sprocket to mill table. Put a .375 pin in mill collet so that it touches the bottom center of gullet for roller. Place the larger sprocket over the smaller and clamp it to the mill table. Remove the smaller sprocket and use a .375 end mill to cut the matching half circle in the large sprocket. Replace the smaller sprocket on the mill table and clamp it Rotate outside (large sprocket) about 5 teeth and reclamp. Remove the smaller sprocket and machine the next half hole. Continue until there are pockets for 5 bolts. What do you think?

 

[Tozguy]

Sounds good but I wonder how hard your sprocket is.
Because; last week I held a 16 tooth 525 engine sprocket in a 4 jaw as you mentioned above. There were 2 teeth touching each jaw. My objective was to cut the splines out of the sprocket.
The sprocket is VERY hard and tough! My carbide boring tool cut it but it put tremendous strain on the tool holding system. There was not enough rigidity in the tool holding and the carbide chipped.
I ended up grinding out the sprocket splines.

The idea of milling a half hole in the sprocket will depend on how hard the sprocket is, what tool you use and how ridgid your mill is.

Spreading 5 bolts over a 27 tooth sprocket will work but might look funny. Depending on how much meat is left on the 36T sprocket it might be a good idea to avoid having a bolt hole opposite a gully on the outer sprocket if possible.

Is there any merit to cutting the tips of the teeth on the inner sprocket a bit before boring out the outer sprocket to fit?