View Full Version : Cutting an internal keyway

10-14-2005, 03:05 PM
I need some opinions, so I'm sure I'm at the right place. http://bbs.homeshopmachinist.net//biggrin.gif

I'm making a new motor pulley for my compressor. The old pulley (and motor shaft) has a 3/16" square keyway. Since I don't have any broaches, or a shaper, I decided to drill out as much as I could in preparation for cutting the rest with a suitable toolbit held in the milling spindle and moved with the quill feed.

I plugged the center hole with a piece of steel, indicated over the parting line between plug and pulley, and drilled a 3/16" hole through. Worked great. I ended up with a beautiful semicircular groove down the side of the pulley hole.

Now for the opinions: Is that good enough? If I shape a key to that curvature on top, and leave it square on the bottom for the keyway in the motor shaft, will it work okay? It seems like it ought to work; I'm just wondering if anybody else has tried this, or if anybody has heard of this being done. It probably won't have quite as much load-bearing capabilitiy as a fully square key...but will it be good enough?

[This message has been edited by SGW (edited 10-14-2005).]

10-14-2005, 03:12 PM
Do it again. This time put it on the shaft in question and drill a hole. Tap in round pin. Done.

10-14-2005, 03:16 PM
how big is the motor? the compressor? the shaft?

you may be able to get away with it. though your pulley hub will probably split when the thing fails.. instead of the key shearing.

these pulleys are usually made of aluminum. if you have most of the meat out, why not just square it up with a file?


10-14-2005, 03:19 PM
This is for a 1 hp motor with 5/8" shaft. The pulley is steel, and pretty wide -- the keyway is about 1 1/2" long. It's for a micro-v drive belt, so it has to be fairly wide anyway, and I added a bit extra.

10-14-2005, 05:07 PM
SGW, I also have a project very simular to yours and I plan on building one of these cut the keyways: http://homepage3.nifty.com/amigos/slotter/slotting_tool-e.htm Besides it would be a good project. I also like Even's idea.

[This message has been edited by lugnut (edited 10-14-2005).]

10-14-2005, 05:10 PM
Once the 3/16" hole is drilled, there is very little material left to remove. Back in the bad old days, a file was employed to accomplish this.

10-14-2005, 10:10 PM
files are for women - cold chisel the bugger out http://bbs.homeshopmachinist.net//biggrin.gif

10-15-2005, 04:04 AM
Do you have a lathe? Why don`t you mount the pulley in the lathe chuck, lock the chuck to prevent rotation, and broach using a boring bar?
You could jerry up something similar for your mill.
Just ensure everything is locked up firm and don`t power up http://bbs.homeshopmachinist.net//biggrin.gif


10-15-2005, 06:51 AM
<font face="Verdana, Arial" size="2">Originally posted by Mcgyver:
files are for women - cold chisel the bugger out http://bbs.homeshopmachinist.net//biggrin.gif</font>

That will work equally well. Caping chisels were used for this purpose.

Sometimes, using the hand tools originally developed for these jobs take less time than setting the job up in a milling machine, and are certainly quicker and cheaper than building or buying yet another tool to sit around wasting space for the occasional use.

10-15-2005, 08:49 AM
JC, i was having some fun with it but I agree. I have little experiece cold chiseling but make good use a decent collection of files.

I feel like yesteryear's craftsmen had a leg up in terms of skill because they were trained to use hand tools so well, so I like to challenge myself on work like that. mind you, before i sound too full of crap i would have done this one with a broach

back to the question, round keys are used I believe. The shear line is pretty much identical, the difference being the angle of force applied to the pulley - the force would be more radial than with square keyway. I wouldn't think it would make much different in a steel pulley

10-15-2005, 09:01 AM
Pulling my new old shaper apart i have not come across one square key yet,where there should be a key they have drilled and tapped the hole and fitted a set screw.

now if it should work on a shaper and last 37 years it might work in your case.

10-15-2005, 01:48 PM
I had pretty much the same idea as Mcgyver; in a steel pulley, with a keyway that long, it wouldn't matter much. Someday I'd like to try it, because drilling down the parting line between plug+pulley sure does make a nice semicicular section keyway, fairly easily.

But conscience (or something) won out, and I planed and filed it out so I could use a square key. Pulley is now on the compressor, it works.

Assuming one has a suitable toolbit though, planing out the corners with the milling machine quill feed is pretty easy.

10-15-2005, 10:18 PM
I also are planning on building this
so I e-mailed Mr Ishimura and he sent me a copy of the attachment with all the dimensions.
If you want the drawing let me know and I can e-mail it to you.


10-16-2005, 07:18 AM
I'm restoring an old belt-drive drill press, and all the "keys" are setscrews, inserted axially 1/2 in the shaft and half in the hub of the gear or pulley. Some of the more experienced guys refer to such an arrangement as a "Dutchman" or "Dutch key". Most cases, there are 2 keys, and they were clearly drilled and tapped after assembly.

10-17-2005, 10:41 AM
cross drill the bloody thing and use a shear-pin http://bbs.homeshopmachinist.net//biggrin.gif