We are trying to create custom length timing pulleys. We ordered stock from Stock Drive Products (25 tooth GT2.) We need to cut it to size, face it, and center drill a .500 hole so that we can broach it later. When we put the stock into the lathe only two of the three jaws make contact with the outside of the pulley, the third is in between two teeth. Does anyone have suggestions as to how to cut and face the pulley stock?
Thanks.
Use a collet.
I got a great lesson back in my early career and that is the rule of three. For a gear/pulley to align in a 3-jaw check it requires number of teeth to be divisible by three. this allows an equal number of teeth to be resting on each jaw of the chuck. Otherwise you will run into the problem you are seeing and will have to use a 4-jaw chuck and manually dial it in.
I’d go with this as well, or a four-jaw chuck.
You can also make a psuedo-collet of sorts to us as an adapter with a 3-jaw.
Get a chunk of aluminum and bore it out to where the Id is just barely larger than the pulley, and size the OD such that the walls are pretty thin.
Clamp this collet in the 3-jaw with the pulleystock inside, and it should work.
We do this on the fly probably 5 times per season for various parts.
If you’re a lazy machinist who doesn’t want to find a collet or make one, can you just wrap it in a thin layer of tape or a piece of belt?
Honestly with I thought to wrap it in scrap timing belt, because that’s a really good way to grip it, but securing it side to side on the pulley stock might be hard
Judging from the recently posted engineering flowchart, I believe this is a situation for duct tape?
I would warn against just tape. If you are going to make a psuedo-collet I would go with Adams approach. We have done things like this before and it works well, though we have used PVC or something we can compress against
My reasoning is that you need a surface that you can bear against such that the part does not slip inside the chuck. Tape IMHO may not provide enough rigidity.
We’ve previously done this, but with a thicker wall. We then bandsaw a slit into the side of it so it can fully collapse.
You’re introducing some runout when you do this, but we generally only did this when we wanted to face hubs off of sprockets or gears, where runout didn’t matter.
Neither would be acceptable from a safety or accuracy standpoint. They simply cannot grip the workpiece well enough.
Another idea for a collet of sorts, assuming a 4-jaw is not available, is a band of metal about 3/32" thick, bent into most of a circle the size of the workpiece. The ends are left unconnected and separated by a distance about 1/6 of the circumference. You grip at all three jaw locations onto solid metal, which can contract to grip the workpiece. The metal is thick enough to not distort from the clamping forces, and since it rides on the teeth it will maintain concentricity.
For facing it’s not important, but for boring the center hole it is.
Ah, yes: Don’t “drill” the center hole, bore it with a boring tool, since a drill will wander and make an off-center hole, almost guaranteed.
You could use a mill and make soft jaws for a vice. Band saw to length and face on the mill using the soft jaws to hold the pulley. Or you could send out the job and have it cold sawed.
You can machine an arbor to press into the bore. Use larger diameter stock so you will form a shoulder. Keep its length close to bore length and machine along with pulley width reduction.:] :]
PS You can wrap the pulley with aluminum sheet to form a shell so the 3 jaw chuck will seat against the same thickness material when bridging the odd tooth location. Have also made aluminum slip rings with a thin slot cut that the chuck jaws can compress onto the pulley teeth equally. Make the bore of this adapter slightly smaller than the pulley ID for a tight fit. Do not use tailstock/drill to open up bore. Get a boring tool for proper concentricity.
I’d consider making a “key” that somewhat fit the gap between the teeth. It could be aluminum or soft copper wire. Hammer and file to fit somewhat, the height being a bit hit-or-miss. Don’s idea is better if you have the material. Be safe!