[artdutra04]

Originally Posted by JABianchi

I love the idea of making our own sprockets with a CNC mill, but have a couple questions for the more experienced:

(1) What's the best way to keep the sprockets from moving around on the last pass? (Especially if you are doing a tool change for finishing...) Are you using a vaccum table or another method?

(2) What's the easiest way to get the hex-shaped bore for the sprocket? Is the fillet from the CNC mill negligible or do you need to file or broach it after milling?

Thanks for any advice!

I haven't milled any timing belt sprockets personally on a CNC mill, but I have machined a lot of 25 pitch roller chain sprockets out of 7075 aluminum when I was in college and the process is similar. I would make 25p sprockets in three operations:

1) Drill the center hole (usually 0.375in or 0.5in and mill out everything except for the teeth profile itself. On small sprockets where there were no additional holes beyond center hole, I would mill a very slight double-D pattern into the hub to align to the fixture plate in step 2. Here's a photo taken before I milled the double-D:
http://team228.org/media/pictures/view/6819

2) Take the plate out of the vise and put the pre-made fixture plate into the mill. The fixture plate would have tapped holes for either 0.375in or 0.5in bolts, and would have a double-D pocket in it to keep the sprocket from rotating. I would then bolt down the piece from step 1 and mill out the outer profile of the sprocket. Here's a photo:
http://team228.org/media/pictures/view/6818

3) As the final step, I would remove the sprocket from the fixture plate, flip it over, and use three equally-spaced #6/32 screws to bolt the sprocket to the fixture plate so I could face-mill off the double-D pattern and bring the overall width of the sprocket down to 0.405in to match COTS sprockets

After step 3, I would then hex broach the center holes.

Generally since I was making these sprockets at quantities of at least 20 at a time, I would do step one for every sprocket, change to the fixture plate, then do all step 2, then do all step 3. Sometimes if the machine shop was empty I would use three CNC mills, one for each step to speed up the process.

Edit: I almost forgot but I have "made" belt sprockets on a CNC mill and lathe before. To save time, instead of milling the profile myself, we bought timing belt pulley stock from SDP-SI, cut it down into wafers, then did the final machining on the CNC mill and lathe before hex broaching. Generally I never bothered adding flanges if at least one of the two sprockets had them. Here's two photos:

http://team228.org/media/pictures/view/6888
http://team228.org/media/pictures/view/6885