Keyway Cutter design and project


@John_Bump posted a video of a keyway cutter mounted to a lathe toolpost that got me thinking. And modeling. I really should have one in my shop, and I enjoy making tools anyway.

I had just accidentally purchased (don’t ask…) an extra #3 MT quick change tool holder for the toolpost on my lathe, and thought that maybe I could use it as the basis of my tool. Here’s an animation of what I came up with:


The basic idea is a shaft with interchangeable bits at the end. The bits can have different sizes and shapes for different keyways and materials; very little rake for brass, somewhat aggressive rake for aluminum, moderate rake for steel. A simple linkage to push the shaft through a sleeve in the toolholder, and the cutting edge of the bit lined up with the center of the shaft. After learning a lot of FreeCAD, particularly how to use Assembly4 for animation so that I could test that the range of motion would work, I made some drawings and got to work.


I made my first part, the sleeve, out of 1117 cold-rolled steel. (1117 is a sulfur-bearing steel that machines fairly well; I’d rather use sulfur-bearing than lead-bearing steel like the ubiquitous 12L14 because lead.)

This took me two attempts.

The first try was frustrating. I started by cutting a very good #3 morse taper on my first ever attempt copying the tailstock taper, and was really proud of myself. Then I tried using a 7/64 aircraft drill as a pilot drill, and managed to bury the tip and break off the first inch or so of it right in the middle of the part. I spent way too much time trying to rescue that part before deciding that if I’d cut a good MT#3 taper once, I could do it again, and that I should start over again with another chunk of stock.

For my second attempt, I used a 3/16" x 6" aircraft drill and tried carriage drilling. I drilled 1/10" per pass between cleaning out the flutes. I used power feed for the carriage at the slowest feed (1117 doesn’t work-harden as far as I know).

Sadly, the bit wandered, and the exit hole was slightly off center after traveling nearly 5" through the stock. I didn’t measure, but maybe 1mm?

I decided that was likely to work well enough, and invested the time to cut another #3 MT taper. Again it was a close fit, so I guess I figured this out. In retrospect, it was a mistake to cut it early as it limited where I could hold the part. I should have skimmed the surface to make a good concentric surface, and cut the taper after I had a straight hole. Live and learn!

To fix the wandering hole, I used a two-flute ⅜" end mill in each end to make a centered hole about 1" deep. Then I used a ⅜" drill to follow the hole from one direction, and I could see a much more accurate transition where the drill met the milled hole from the other direction. Maybe 0.1-0.2mm? Then I drilled 7/16", reamed 7/16, and finally had a straight enough hole. I drilled with a 39/64" (1/64" undersize for reaming ⅝") Silver & Deming bit, which is stiff and was also long enough to go straight through. I drilled 1cm between cleaning chips from the bit to avoid them packing.

Then I pulled out my H7 reamer, forgot to put a chamfer on the hole, and started to ream. After about 1cm, I pulled the reamer and saw and felt bad chatter marks. Yes, they really are as bad as they look:

I turned the work around, pulled out a chamfer bit, and tried to chamfer with the bit. That chattered too, so I switched up and used a good carbide boring bar to cut a 45° chamfer. I had to work out all the chatter with the boring bar, so I ended up with a comically oversized chamfer that you can see in the picture. Reaming from the chamfered end was smooth, and while I can still see and feel the chatter marks in the other end, the ⅝ drill rod is a good fit.

I had intended to put an oil groove inside that end, but discovered that I didn’t have a tool capable of putting a groove inside a ⅝" hole. The fit for the drill rod is loose enough that the oil grove probably isn’t necessary. I can spend a few hours to grind a new tool if I need to, but I won’t do that unless it’s a problem.

I put the sleeve in its tool block and put it in the mill to drill out the oil hole. In retrospect I should have done that before final reaming, but I did manage to clean up the hole anyway.

Sadly, the bearings on my lathe are going, so I’ll probably have to put completing this project on hold until I change the bearings. That’s likely to be its own substantial undertaking; no clear idea how long it will take.

The PDFs of my drawings, such as they are, and the realthunder FreeCAD files, are all available at:


I finally found a thread on hobby-machinist that explained why it wandered.

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