Build - Key Slot Cutter

[Dec. 2019]

As part of a small bit of repair work, I need to cut a key slot in a pulley so I made a slot cutting tool for my Atlas shaper. Although I have a tool holder for round bars for my shaper, I do not currently have any round cutter holders to go with it. In addition, I needed to cut a fairly large slot in a small opening, so a typical cutter holder was not going to work.

My solution was to take a piece of 1/2" OD drill rod (O-1) and machine and grind a cutter on one end that would be small enough to fit into the axle hole in the pulley I needed to slot.

I mounted the drill rod in a square collet holder, and using my mill I machined the basic rectangular shape for the cutter. You can't see it in the picture, but the collet holder is sitting on a 2-degree angle plate; this will provide relief for the cutting tool in use.

Here is the plan for the tool (click to enlarge). Note that the side relief is created later on the grinder - the mill work just provides the basic rectangular profile.

Here is the tool as it comes from the mill.

Prior to heat treating, some side relief is added using the bench grinder. Grinding to final shape will be done after heat treat. Note that I also ground a fillet into the point where the cutter end meets the main section of drill rod.

Another view prior to heat treat.

The cutter end of the rod was heated with a MAPP torch and then quenched in oil. After quenching the tool was tempered in a 450 degree F oven for 1 hour.

Cutter after hardening.

Cleaned up with some scotch brite. Following cleaning the final cutter shape was ground on the bench grinder. I ended up adding more front relief in the final grinding.

Here is the cutter mounted in the shaper, along with a test cut in aluminum.

Another view of the test cut - note the cutter orientation.

One last view of the test cut.

Here is a photo of the repair I needed to make the cutter for: A steel toothed belt pulley. More details on this oil pump shaft and pulley repair here.

Afterword

The cutter in this build was made to cut a 1/4" slot. In use, I found that it worked easily in aluminum, and with some difficulty in steel. In both cases it is necessary to take very shallow cuts. In cutting steel, I found myself feed 0.001" to 0.002" on each ram stroke. I judge the tool to be workable but not ideal for steel, and quite workable for aluminum (and probably also brass, although I did not test brass).

In hindsight, it would probably have been better to make the tool in a width of 0.1" instead of 0.25" - I could then use it to cut narrower slots, as well as wider slots in increments. A narrower tool would also have the advantage of having lower cutting pressure, which should make cutting easier and would also reduce the stresses on smaller work-pieces.

While I still need to make or acquire a more conventional boring bar type cutter holder, this tool will be a useful addition to my shaper, and I suspect I will be making future variations on it as the need arises.

Additional Note

[Dec. 20, 2019]

After I wrote the above, I ran across this illustration in the Atlas &B shaper manual. Possibly I saw this earlier and forgot I had seen it, or maybe I just re-invented a somewhat obvious tool design.