How would you do this?

[mickeyf]

Short question:
How would you cut an axial slot all the way through the wall of a very thin walled cylinder?

Longer version:
I’m replacing a single phase motor on a drill press with a 3 phase motor and VFD. The shaft on the original motor and pulley bore are 5/8”, the new motor shaft is 12mm, slightly smaller. Simple enough, make a sleeve. So I did, allowing about ½ a thou larger in the bore of the sleeve for a snug push fit, and ½ a thou less on the outside diameter for a similar fit in the DP pulley.

But I didn’t want to lose the keyway, and the wall thickness of the sleeve is less than 2 mm. I didn’t see likely success by cutting the keyway either before boring the inside or turning down the outside of the sleeve – even with a very light cut I was afraid that interrupted cut would grab.

What I finally did was to make a sacrificial plug the same diameter as the shaft the sleeve would slide onto, and a hair shorter so that the sleeve itself rather than the plug would be gripped by the vice when cutting the keyway. I was also nervous about the rotational force of an end mill causing issues. A woodruff cutter would have been better than an end mill (or doing it on the shaper) since all the cutting force would have been axial to the sleeve, but that would have required an awkward setup on a vertical mill.

I did it, it worked, but I’m wondering what other approaches might have been taken?

 

[YotaBota]

Making a plug was my first thought. I think the only other way would be to roll a sleeve with the correct thickness material, maybe shim stock.

 

[Dan Dubeau]

I probably would have done it your way. I like the sacrificial shaft method. Another similar method would be to cut a slot in your sleeve, but leave the ends intact to constrain the key to the shaft like a Pratt and Whitney key. Or maybe leave one end (the motor side) fully intact. If the ends are machined square, you could clamp it inline with the vise screw to slot it, without the need for the sacrificial shaft. Go easy on the feed, but should be fine. It's basically just clearance for the key, not a precise alignment feature. You could scribe it, and cut it with a hacksaw too....

An even different way would be to strip the motor down, weld build up the shaft, and remachine it to 5/8".....slightly more involved, and way overkill here.

Also another way would be to bore out the pulley a bit bigger and fit a plug bored to 12mm with a fresh keyway machined.

A few different ways to skin this cat, but I think you picked the best/most appropriate one for the job here.

 

[whydontu]

I’ve done this many, many times, matching metric valve shafts to imperial actuators.

Cut the tubing longer than it needs to be allowing 2x the key width. Put the tube on the milling table, held down at the ends with the tube sitting lengthwise in one of the table slots. If you’re concerned about the tube flattening push a piece of round bar through it and clamp the round bar.

Use a two-flute end mill about 3/4 the size of the slot - for a 1/4” keyway I use a 3/16” mill. Start the slot centre on the tube, cut slowly and end the cut before you get to the end of the tube. Move out 1/32” out to get to 7/32” width, across to the other side to get your finished 1/4” keyway.

Cut the tube to length. You can leave one end as-is, it keeps the key from sliding out.

Keep in mind the sleeve doesn’t transmit any load, it’s just there to keep the shaft and pulley concentric. The key supports all the torque, so even if the slot in the tube is sloppy it willl still work.

 

[mickeyf]

If you’re concerned about the tube flattening push a piece of round bar through it and clamp the round bar.

Pretty much what I did with my sacrificial plug. But using the slots in the table would have likely been better than putting it in the vise aligned with the vise screw, which is what I did.

 

[Susquatch]

You did what I did. The only difference is that I added some loktight gap filler to make sure the sleeve and pulley didn't move. Looking at your motor and pulley, might even be the same application.

I enjoyed reading what others suggested too. So, thanks for posting.

 

[Dan Dubeau]

Squatch brings up a great point I missed last night. Securing the sleeve. Drilling and tapping another set screw 90* to the existing one would be a good addition also.