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darryl
02-18-2013, 12:48 AM
I'm combining a couple of topics in this new thread to save the hassle of looking through the old ones.

Anyway, I got my ring bored out ok, then switched to hss and faced it to length. After going through the threading hassle in the 8 inch disc with multiple re-positioning of the cutter, etc, wrecking some crappy inserts doing it, that part is done. Now I have the ring fitted to the lathe spindle and the disc mounted up to it, epoxied together using pc-7, and four bolts installed with the heads just under flush with the disc surface. Bolts are also epoxied in place. This is going to be a custom faceplate, just to give a little clearer picture of the thing. I now have it faced to my satisfaction.

The step I'm working on now is turning the edge. I got my favorite boring bar set up in the most rigid mounting possible, and the opposite side is prepared as a cutting edge now as well as the usual side. I can't cut with the usual edge, as that would mean turning in reverse, and the disc would just unscrew. This is a carbide bar ground as a boring tool. So in I go with the carbide- prior to this step I took the disc to the belt sander and took off a fair amount of material to both make it a little rounder and to get rid of as much of the hard skin as practical. I'm able to get this cutter to bite, and everytime a hard spot comes around I'm getting some pretty good fireballs coming off. Kind of cool actually! The rear of the carriage is lifting off the rear way because of the forces being generated, so I find I'm having to keep a ton of hand pressure down on it to keep the cutter in 'metal removal' mode- otherwise it just pushes away. With the cut being so far away from the spindle axis, the pressure is ahead of the front way- thus the carriage is rocking.

I just wanted to post a couple thoughts on this whole thing. First, some time ago I was having problems with the carriage rocking. There is a tab at the back of the carriage to prevent it from rising off the rear way, but it's a very fine adjustment- in order to have the carriage move freely along the bed, there has to be a bit of play left. I have modified the area to give some spring pressure downwards on the rear way, and that's been a really good mod- it lets the carriage move along easily but maintains the contact. One source of play eliminated- up to the point where cutting pressures overcome the spring.

Second, by mounting the toolpost onto a solid riser on the carriage, I've taken some play and some spring out of the equation. This makes a big difference when the cut is going to take some real power and you don't want the cutter flexing all over the place. The result is going to be closer to ideal. Now when I'm cutting on the edge of this disc and it's an interrupted cut, the cutter stays more or less where it should be instead of bouncing in and out and then just rubbing across the tough spots.

Third, because I have a DC treadmill motor on this lathe, and it's driven by a variable voltage supply (not a speed control) I can get a range of speeds while leaving the belt in one position. This means that when I want a slow speed I can just click the switch down a few notches and the motor runs slower. But I've reached a limit here- with the force required to push this hard edge past the cutter, I'm running out of torque because the motor isn't getting enough voltage to keep the torque up. As a result I've got the speed range up a bit, and the SFM is probably too high for the job. Being an interrupted cut, the rpm picks up quickly between hard spots. It's not the ideal way for things to be. I already have the belts arranged for slowest rpm, and I have another arrangement for very slow rpm, but nothing in between- nothing in the range I need for this operation anyway.

I'm definitely exploring the limits of this lathe and the drive system. I'm getting the job done, and the little fireballs look cool :) I hope I don't get any dropping on my skin :(

Jaakko Fagerlund
02-18-2013, 05:08 AM
I just wanted to post a couple thoughts on this whole thing. First, some time ago I was having problems with the carriage rocking. There is a tab at the back of the carriage to prevent it from rising off the rear way, but it's a very fine adjustment- in order to have the carriage move freely along the bed, there has to be a bit of play left. I have modified the area to give some spring pressure downwards on the rear way, and that's been a really good mod- it lets the carriage move along easily but maintains the contact. One source of play eliminated- up to the point where cutting pressures overcome the spring.
Use two sets of bolts: one set sets the space between the carriage and the gib strip and the others tighten it. Other option is locking nuts on the bolts.

darryl
02-18-2013, 09:17 AM
That would let me set the play fairly precisely. I'll have to look and see if I can still do that.

I did get the new faceplate finished up tonight. I ended up using the tool post grinder to put the final touch on the edge. Once I started that process, I realized that the carriage travel would come to a limit before the grinding wheel got all the way across the edge. There was no way to get more motion from the carriage, and also no way to move the wheel out any further. It worked out anyway since I could use the ground portion as a guide while finishing the material removal on the belt sander. I faced the back of the disc as well as the front, to keep it in perfect balance. This disc will end up with a lot of tapped holes in it, probably placed according to jobs which come up requiring strategically placed mounting points. I've run into that several times, where a regular faceplate just isn't the thing for the job.