I now work in a shop that repairs large industrial motors. Have been there about five months.
Our average comm is 12-inch diameter. Present resurface method is using carbide bit ground down to about 45 degrees horizontally with about 30 degree relief underneath, and as sharp a tip as we can grind. We run zero rake X and Z axis, turn slowly using small feed, and take light cuts. Don't know the exact X Y and Z numbers, because we have no set numbers. We do each comm by look and feel. It leaves a surface that can be polished down with 120 cloth followed by 400 cloth followed by a rock to almost a mirror finish. If we do it right, we get little copper hanging over the edge of each segment.
All our equipment is old. We use a toolpost-mounted undercutter whenever possible, and can undercut a 12-inch comm in about 90 minutes. We have to manually line up each groove. Some comms don't run exactly parallel to the shaft axis, so we have to compensate by shimming one end of the undercutter slide bars. It is an entirely old-fashioned way of doing things, but we make it work. We allow 12 hours to undercut, resurface, and restore the dust groove in a comm with average wear.
(Sidebar: Last week I undercut a 5-inch comm by hand and it took 11 hours just to undercut. That's why we use a motorized undercutter whenever possible.)
Being an insert addict from my previous shop, where we did a lot of high-speed production jobs making parts from blueprints, I would like to figure out a way to use carbide inserts for turning a comm.
Responses in another thread recommend 20 - 30 degree negative X rake, with similar slow-speed-light-feed so I might try that with a TNMG4X1 mounted just below horizontal on the next comm. Will report results.  |