How to bore out step pulley

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martik777

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I need to bore this out from 22mm (.865) to .875". Not much to hold on to on the small end with a 4 jaw so I machined the outer Vee wider to .150" which is still not ideal. I was considering extending it 1/2" or so by tapping and attaching a round plate to the small end with a 3/8" screw. The pulley is cast iron 4" long. What do ya think?

pulley.jpg
 
Does it have a through hole or a blind hole?

If it's a through hole I would just set it on something flat with a hole in the center and then use step clamps around the rim to hold it down, small end facing up.

If it's a blind hole, then I probably would make a fixture. Have a hole through the fixture that's slightly larger than the second step, and then counterbore so that the first step with lay flat inside of the counterbore. Then, hold it down with step clamps or whatever, with the large end facing up.

Either way, once you secure the pulley to your mill table, just indicate the bore and drill/ream away.

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It's blind and it is not a flat surface so I'm not sure how to get it perpendicular with the bore. Also clamping down on the tapered V grooves may be unstable. That's why I wanted to bore it on the lathe.

pulley2.jpg
 
How deep does the blind hole have to be? Could you bore the small end deep enough to mount it on a mandrel?
 
It looks to me that you could machine 3 (or 4) V-ways in 6061 and use these between the chuck jaws and the V-groove to hold the pulley.
Done in the 4-jaw, you can dial in on the original bore, or dial in on the pulley.
 
or you can use dowel pins that fit about 1/2 way into the grooves. Be aware of precession (? wobble), so think about putting a 22mm shaft in your drill chuck and using that to support the pulley as you set it up in your 4 jaw. Then indicate the pulley bore both radially and longitudinally (axially?). I would also suggest reaming it as that's more likely to follow the existing bore than cut an offset bore like a boring bar would.

It's not trivial to do this, I screwed one up once. Not by alot but enough to be visible. If you want to uber careful about it after cutting the bore you'd want to mount the pulley to an arbor made between centers and skim cut the faces of the Vs :)
 
What size lathe do you have? Or do you have a way to clamp it to your mill, and use a boring bar?
 
This is a challenging exercise in work holding. The reamer followed up by truing the vee grooves while mounted on a 7/8" mandrel sounds like the best idea.
If I had to bore the hole, I would probably cast a sacrificial material around the vee grooves. Then I would make a 22 mm shaft and mount the pulley and turn the sacrificial material true to the shaft.. then flip the part, remove the shaft and bore to 7/8". Then mount the pulley on a 7/8" shaft and cut away the sacrificial material.
Another way would be to use a faceplate and clamping set. I would turn, face, and bore a block out of hardwood so the pulley would rest on the face of the largest or second largest step and the smallest face would be floating. Then, I would use a 22 mm shaft to dial in the pulley, remove the shaft, and bore the hole. Then I would mount some stock in my three jaw and turn to 7/8" diameter and mount the pulley on this mandrel and true the vee grooves.
A lot of work! The pulley would have to be very important.
 
Thanks for the suggestions. I'll mull them over for awhile. The blind hole is 2.5" deep. The pulley is part of a 3 pulley setup on my mill. I am using a single pulley on the motor for now with a VFD but would like to use the original pulley for more lower rpm ranges.

Even if I mess it up I can always rebore larger and make a sleeve. I may in fact do this so I can re use this pulley on the original 22mm shaft motor

What kind of reamer would I need? Hand, chucking etc?
 
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