How would you machine this?

[pjf134]

I have a aluminum pulley for a car that requires the center hole to be enlarged from 5/8 dia. to 3/4 dia., the trouble is I do not have a 3/4 drill bit and there is not that much area of flat on the inside to grab onto and the part is a little bigger than my lathe chuck. What are my options? Buy a drill bit is one for sure, but how else would you do it? I do have a drill press & lathe and the drill chuck for the lathe takes 1/2" shaft and the drill press takes 3/4" shaft. I do have some bigger drill bits, but not 3/4", and I do have some coming later from a friend, but not now, so I hate to buy one just for this job until I get those from a friend thats cleaning out his shop.
Paul

 

[pjf134]

I forgot to include pic in the post so here it is.
Paul

 

[Hawkeye]

Couple of options:

If you can find a step bit that tops out at 3/4", or one that goes bigger (7/8" is common) but has longer sections to drill thicker material, the seem to center quite well as they enlarge the hole. You may have to drill from one side, then the other to handle the thickness, but it doesn't look too thick.

You could make an aluminum mandrel on your lathe with a removable center piece to fit the existing hole snugly. Use that to hold the pulley while you drill 4 holes to bolt the pulley to the mandrel. Then remove the center guide and bore out the hole (and a bit of the mandrel) to 3/4".

 

[Tony Wells]

I would not recommend using a drill on that. The center hole that accepts the pilot stub on the water pump should fit with only .002 clearance or so. Vibration will kill the pump if it's not running true.
Hawkeye has it right. Make a mandrel with a removable pilot, bolt the pulley to it and after removing the pilot, bore it. Measure the water pump stub for finish size.

 

[Alex]

Just to share a bit of my knowlege, I would use a set of soft jaws & dedicate those for use with extensions, easyest to use if you can get it is aluminium off cuts like 30mm square x 100mm long and just bolt those to the soft jaws.
I can post pic's next week as I have some at work.

Alex.

 

[brucer]

make a fixture with the same hole pattern of the 4 smaller holes, use like 1/4-20 bolts to bolt the pulley to the fixture, that way your holes can be off a little, but try to layout the holes as close as you can, also centerdrill before you drill and tap the 1/4-20 holes.... put the pulley on the fixture backwards, snug pulley on fixture then indicate pulley and then tighten bolts... use largest undersized drill size you have close to 3/4 then bore.. just be easy while machining it..

 

[Zigeuner]

All good advice above. I agree that a drill is not the answer since the larger ones seem to always drill a couple of thousandths oversize. Regardless of the method you wind up using to hold the pulley, I would drill undersize and then bore the last few thousandths to fit.

 

[pjf134]

I did find a old water pump to try it on and went with that measurement and did go a little under for now until I get the right pump to measure, but I did find that the aluminum pulley was a little egg shaped on the center hole and the o.d. The 4 mounting holes seemed to be ok so I used that to machine the center hole. I do think that aluminum is not strong enough for the application though, maybe that is what caused it to be egg shaped.
Paul

 

[pjf134]

I did meet the guy it was for tonight and he thanked me for getting the wobble out. The guy building the motor said it fit nice and tight and he did check for wobble but could not find any, so I guess the way to do it is find another pulley but smaller, then bolt it to the big one but shim the jaws so that the 4 mounting holes run true, a pain but it did work. The 4 mounting holes seemed to be running true, but the center hole was egg shaped. I did have to make a lathe cutter out of 1/4" because the 3/8" ones would not go in the whole way. I did run the lathe by hand when I did the setup so I could see what I was doing, and I did check everything with a dial to make sure it ran true. I was in a hurry to get it done and forgot to take pictures, but maybe the next one. I do need some tooling for my lathe, back plate and some other goodies, just got the lathe in feb. and did a rebuild, so just got it going in may and I have been adding stuff and been getting mat'l. to make some gadgets for it, just takes time and also have 2 CNC machines that I am setting up too.
Paul

 

[pjf134]

gaintwist32,
I just read your post for the first, I missed it before, but I am near Pittsburgh, PA so a little far from you I guess. I have a SB9A lathe that I redid and a Sherline CNC lathe and mill also.
Paul