Machining a large timing pulley - work holding question

[Reddinr]

So, I've got one of these and I need to turn off the extended hub, bore out the middle to fit a 1.75" shaft and add tapped radial holes for set-screws (not in that order). It seems simple enough. I plan to hold the hub on an indexer to do the set-screw holes, then hold in a 4-Jaw chuck for the rest. I want to avoid gripping the flanges because they are thin. What are some ideas for holding the piece for removing the extended hub and boring, while avoiding marring the teeth?

The tooth notches are about 1/8" wide and the flange is ~0.25" higher than the notch valley or 0.15" higher than the top of the tooth. Just stick some small blocks in there straddling two teeth? The part was not cheap which is why I'm second-guessing myself.

 

[Latinrascalrg1]

So, I've got one of these and I need to turn off the extended hub, bore out the middle to fit a 1.75" shaft and add tapped radial holes for set-screws (not in that order). It seems simple enough. I plan to hold the hub on an indexer to do the set-screw holes, then hold in a 4-Jaw chuck for the rest. I want to avoid gripping the flanges because they are thin. What are some ideas for holding the piece for removing the extended hub and boring, while avoiding marring the teeth?

The tooth notches are about 1/8" wide and the flange is ~0.25" higher than the notch valley or 0.15" higher than the top of the tooth. Just stick some small blocks in there straddling two teeth? The part was not cheap which is why I'm second-guessing myself.

View attachment 307139

If you have an old belt Could you use a piece big enough to wrap the pulley so the jaws dont come into contact with the pulley and also give them a flatish surface to close on?

 

[JimDawson]

I would hold that by the hub to bore to 1.75, then mill off the hub later. That way you won't mar the teeth. You could hold it in a couple of wood v-blocks to mill off the hub. Don't try to grab it by the flanges, I made that mistake once. (it is possible to put them back on ) When setting up, indicate on the teeth.

Where possible I normally buy taper lock type pulleys, that way you can get a bushing that fits the needed bore. https://www.mcmaster.com/timing-belt-pulleys

 

[Reddinr]

Are the spacer blocks I mentioned a bad idea? With 40 teeth, the thing is symmetric for a 4 jaw I think. I have to mill off the hub and hold it somehow when doing that. Had not thought about V-blocks. I don't have an old belt to use. Might that be too spongy anyway?

 

[RJSakowski]

I did almost exactly these operations on a smaller scale a week ago. I mounted the pulley by the hub n my four jaw and machined the bore. Rather than set screws, I broached a keyway, using single point tool in the mill. Finally, I mounted the pulley in the lathe and face the pulley to remove the hub. There is no requirement for concentricity in the hub removal, only that the finished face be perpendicular to the pulley axis.

 

[main_cogg]

I would suggest a shop made expanding mandrel for holding on to it for turning off the hub. Here’s a couple I’ve used in the past. The splits were done with a bandsaw, nothing fancy.

 

[RobertB]

I would grip it by the hub, bore the hole, then hold it internally to face off the hub

 

[Reddinr]

Great ideas! Unfortunately, my jaws are too big for an inside grip by about 1/4".

Based on the ideas above, here's my plan:
1. Using a mandrel (that I already have) for the original ID, drill and tap for set-screws using indexer.
2. Grip the hub and bore the ID to 1.7xx"
3. Grip the OD of the pulley using four spacer blocks and face-off the hub. The back of the chuck can support the opposite face so I think all will hold fast.

Thanks for the great ideas! BTW, just bought a rotary table/indexer from Griz. and am hoping it gets here in a few days so I can do this! That has been on my wish list for a long time now. It seems to get fairly high marks. I have my fingers crossed.

 

[ChrisAttebery]

You're drilling/tapping 4 holes for set screws. I'd turn a mandrel to fit in the 1.75" bore and use the set screws to grip it. Then grab the mandel in your 4 jaw and turn off the remaining hub. Just take light cuts and I think it would be fine.

 

[benmychree]

I would chuck the hub, bore to size, and press in a mandrel and face off the hub, right up to the mandrel without cutting into it, then use a scraper to remove the burr and slightly chamfer the bore.