Finally got the handpiece I've been after all month. I went with the actual Foredom H30 because if you watch the video that I posted when I made the tool holder you can see that Practical Renaissance used a generic version. The generic version had play in it that let it bounce in and out very noticeably and caused a lot of chatter. I was hoping that the real deal would not do that and the Foredom part is ~$60 so it's not cost prohibitive enough to be worth spending a little less on a piece that has already been shown to have a lot of unwanted movement in it.
Here's a look at the 3 jaw chuck that it uses and a side by side with the handpiece that I've been using for hand cutting with the Foredom that uses a collet.
Here are the "1/8 inch" carbide cutters that I picked up for under $15. Looks like a good set, especially at under $15 shipped on eBay. I've had good luck with $10-20 carbide bur sets similar to these in the past and figured they'd fit the bill to cut an 1/8" groove as well as anything.
I set everything up. I should note that this handpiece fit the 1" hole in the tool holder without any shimming (again different from the generic part that PR used in his video). I put a hook in the shelf over my lathe to hold the Foredom's motor.
The chuck needed to be held still so I could make a cut straight across the face of the adapter. I've never done any locking or indexing mod to the mini-lathe, but I have used a simple solution in the past when cutting keyways in new drive gears for the lathe. I took the same approach and put vise grips on the spindle nuts and used a bungee cord to hold it up against part of the lathe. Being up against part of the lathe keeps it from moving one way and the bungee cord provides enough tension to keep it still. Not a rock solid setup, but I didn't plan to put enough load on anything to be at much risk of rotating against the tension from the cord.
I decided to go with this double cut bur for the job. Since the tool holder was drilled while mounted to the slide using a drill bit in the chuck, it is centered (or close enough) without any fiddling.
BTW, the lathe was unplugged and with all switches to off positions to be certain there was no way it could accidentally turn on. Probably a good thing, because I kept reaching for the shutoff switch on the lathe instead of the Foredom controller out of habit.
I didn't want to take a big bite, especially on the first cut. Started out at 0.005" depth and it ran across much easier than I thought it would. Moved up to 0.010" and that went smoothly. Tried 0.015" and that worked fine. Went to 0.020" and it didn't feel as good so I stuck with 0.015" passes. I'd imagine you could take the full depth cut and move the cross slide very slowly, but this was easy enough for me and I liked that it was smooth and cut easily enough that I didn't have to worry much about how quickly I cranked the slide.
Before long I was trying to decide if I wanted to stick with the depth of the groove from my prototype part or make it deeper. The aluminum piece was done at 0.140" and that worked fine, but I figured the head of this steel adapter is around 0.400" and if I cut 0.200" for the groove it should still be strong enough and that would give it more room for any differences from one setup to the next. Here's the aluminum part also marked with a line to show 0.200".
I continued on at no more than 0.015" depth till I reached 0.200".
I checked to see if the 0.125" titanium pin fit into what I had cut and it didn't. Turns out the 1/8" bur was actually 0.116" and cut an 0.119" groove. I forget the size of the original pin, but that may have worked for it. I wanted to stick with the titanium as planned though. Then I started checking other bits in the set that I bought and bits that I already had around. I found a single cut carbide bur that measured 0.120" diameter. I figured if it went just as the previous passes, it should at least up it to 0.123" to get me closer.
I did it in one pass and then measured the groove at 0.123", still more than 0.002" shy of where I needed to be. The groove cut into the test piece was about 0.130 and that didn't feel too sloppy to me so I figured I had a little leeway. In a perfect world perhaps the groove would be a sort of snug fit, but I was trying to work with what I had. Not really sure how much it matters as long as it's not really loose. If it were snug, I guess it could help to alleviate some stress of slack being taken up, but I'd think once the engine started running the pin would probably stay up against the edge of the keyway.
The best thing that I could find was a diamond bur that was around 0.126".
I made some passes with it. Had to do multiple passes because the bit wasn't exactly uniform diameter throughout the length of the cutting area. Ended up around 0.129-0.130" and the titanium pin fit nicely.
In case you're wondering why I cut half a groove instead of doing it all at once, there's an easy answer. Because I'm dumb. lol I would have wanted to do the first couple of cuts on just one side to minimize damage just in case anyway, but beyond that it's because I was using a carriage lock that wouldn't allow full travel of the cross slide. The lathe can be locked to the lead screw to keep it still, but there's play. With a carriage lock the carriage is rigid.
I was going to cut 1/2 and then rotate the piece and try to set it up so it was in line with the previous cut. I figured I could use the motion of the slide in the cut half to help verify that it was straight or had an idea of mounting a degree wheel to the adapter and a pointer on the lathe so I could rotate it 180 degrees exactly. Other thought was to try just locking to the lead screw, but I didn't like that one much even though it may have worked fine.
Then I realized just how dumb I was when I set it up. I forgot that I have spots for the carriage lock on both sides of the carriage. I think I saw a video or pics of the carriage lock mod where someone did it on the side with the adjusters for the slide and I just did it that way initially. Then I went back later after realizing the travel issue and drilled the opposite side. For most use, I actually do like the lock on the back side because it's easier to get to, but it works a whole lot better for this job when I can get all of the travel that I need at once.
I went through the same process to do the second side of the groove, but hit a snag at the end. The diamond bit was worn already.
I got lucky and had another one just like it. These were super cheap bits that I bought many years ago and haven't used a lot, so I wouldn't even know for sure where to get one just like it. I have many similar ones from sets, but I've seen that they range in diameter by quite a bit so who knows what you'll actually get if you tried to order more cheapies.
The new bur allowed me to finish up the groove. I think it turned out pretty well. I was impressed with how well this fairly cheap tool rig worked for this. In the video that the idea was from, the goal seemed to be to spin the lathe and have a spinning tool. I don't quite see the need for that, but I'm sure I'm missing something. To hold something in the chuck and cut a keyway though, I think it's a good setup to have around.
It would be an even simpler process if I just had 1 good cutter that would leave a groove something like 0.125-0.130" instead of making passes with more tools or using a tool to finish that quickly deteriorates. For that reason, I ordered an 1/8" cutter from McMaster-Carr in hope that it will be closer to the size than the cheap bits that I bought.
Ryan's suggestion of drilling through and welding the ends up would have likely worked fine. I think to drill point to point on the hex I'd have a tough time with my drill press getting it started without machining a flat first. Also makes it harder to do a cut to any depth you want. Even with just a 0.140" groove depth, a 0.125" drill will leave material to be dealt with some other way. Then the welding and grinding to box it in. I'm happy with this rig and I think it could help me in other projects so I believe it was worth the effort to make and not expensive. It did take me a few hours to cut the groove, but a lot of that was setup and it was not done as efficiently half and half. The second half didn't take that long since I knew the plan and process.
I'm gonna wait on doing Ryan's Yamaha adapter till I see what shows up from McMaster-Carr. I also left some material on both adapters so I could face them off at the end of the process so I'll still have to do that once the lathe is out of keyway mode.