I have a large DC motor that need the copper commutator turning and would like some advice from the machinists, as it is an expensive motor to bugger up.
I have various insert type tooling so if any one can give me the approximate feed and speeds etc.
The comm. is 2.5" in dia and there is a .015 thou brush groove I need to skim down to. I don't know wether this qualifies as an interupted cut? the bars are .18" wide with a .015" gap between them.
TIA
Al.

When machining copper it is sometimes best to use a tool with several degrees of positive rake, up to 10 degrees possibly. Hand ground high speed steel tools may be best because insert carbides just do not have a nice sharp edge. Speed can be low around 100 sfpm, depth of cut just maybe 0.005 and feed even less than that. On a job like this you are not in a hurry. Before taking a final cut check the gaps between the segments; if you have not machined down into insulation you might find the copper burrs over badly on the trailing side of the cut and can even bridge the gap. After machining you will probably have to undercut all the gaps to take the insulation down below the copper surface.

Thank for the advice, Yes I have undercut before, so I know how that goes, it's just the machining I was not sure of.
Thanks again.
Al.

Does anyone actually use a undercutting machine? I have allways poor boyed it with a modified hack saw blade.

Darek

I guess motor rewinders may now use the power tool ones, I was just looking for a off the shellf hand tool to replace the one like yours (ground hacksaw blade ).
A place called Vensel in Ill has one, I will have to call them on Mon.
Al.

I hate to get OT, but how do you all growl your armatures?

Darek

I have a buddy who takes them in to the shop he works at, for the odd one I need tested.
Al.

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.