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.
For slotting, the clapper must be held rigid. In order to do this I temporarily replaced the slide clamping screw with a short section of all thread - a nut on one end held the slide in place, while a second nut at the other end was screwed out to wedge the tool holder in place.
I had read that immobilizing the clapper is necessary for slot cutting, but as an experiment I tried doing it with the clapper free anyway. I soon found out why immobilizing the clapper is necessary: With the clapper free, the cutter will "skip" over the workpiece for a few passes of the ram, and then suddenly dig in. This is because it is necessary to take very shallow cuts with the slot cutter, and these shallow cuts can sometimes ride over the workpiece without cutting, so that two or three small down-feeds suddenly become one large down-feed.
With the clapper immobilized, the cutter may still "skip"on occasion, but the skips don;t build up to disastrous proportions.
Here is a test cut on a mock pulley (aluminum). Note that the slot is cut upwards, since the lower side is blocked by the vise jaws.
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.