A few weeks ago Stefan Gotteswinter posted a video on thread milling.
I have had an interest in this for a long time but never got round to really pursuing it. I have the high speed spindle from my old CNC router and thought this would be a good use for it. The first thing I needed to do was mount it in a tool holder, This involved making a mounting bar from mild steel, to pick up the existing fasteners on the spindle. I wanted as much metal in the tool holder as I could get, that meant the spindle centre was really low and I ended up having to add some shim on top of the tool post to get it aligned.
You can buy commercial thread milling cutters, normally they are used in CNC mills. They all seem to be carbide and expensive. As the spindle has an ER11 collet I am limited to a maximum tool shank of 7mm diameter. I wanted to see if I could make my own cutter from 6mm silver steel, so turned a blank on the lathe.
We could now spend a few hours talking about helix angles and tip geometry till the cows come home. At this point I had no idea if this would work, in my mind I saw all sorts of problems, such as the gully of the thread being trench cut like a wood saw in timber. No back rake on the cutter leading to rubbing or pounding of the work, and chip evacuation. Hmmm starting to see why I didn't have a go earlier. Even watching Stefans video left me with doubts about thread quality.
Ok, what else you going to do other than twiddle thumbs, so I set up a 5C collet block on the mill and proceeded to mill the cutter. Other than the size and being unable to see what I was machining it all went well, thank you DRO.
I heat treated this cutter and hardened it in oil, I find it less likely to crack or warp, more of that later. Some quick measurements and it was still all true, on for grinding. I used my T&C grinder, sadly realising I couldn't grind -60 degree, so had to order and use a diamond disk to grind the back of the cutter, the outer face was ground with a diamond cup wheel.
The result was pretty good, though this cutter was a bit stubby having just chucked some material and gone for it. I made a second cutter and this time water hardened it, it warped, slightly. Fortunately, when it got ground the warp made no difference, but I will stick to oil for future cutters. The cutters I have made are really for nonferrous materials and ideal for plastics like Delrin.
The first test cut in brass was examined under a microscope and to say the least I wasn't that impressed with what I saw. Bear in mind it was a 0.7mm pitch thread on 16mm stock. I wasn't going to waste further brass on making a matching part, now this is where my assumptions got me. I next tried making a part in Delrin, this time at 1mm pitch so I could see them more easily. It didn't look bad, and once I made the cap it actually fitted quite well. The brass thread I made looked far worse for being under the light I used to illuminate it. The worry over thread profile wasn't without merit, the thread isn't bang on 60 degrees as no helix compensation, but it does work and is very close. I do see some very fine chatter, but under a microscope. The threads are cut in two passes, you can cut in one pass, but I can't get the big lathe to run slow enough, so two passes upto 1mm pitch, the maximum pitch for this cutter. The smallest pitch I can cut is 0.45mm, but things get iffy on the lathe at that scale, you need to compensate for backlash and make sure the gear box is loaded up before cutting the thread. I can thread away from a shoulder or cut a groove with the thread mill and thread from that. Run the lathe in reverse and thread away from the part to avoid crashing.
