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Old 02-14-2007, 09:51 PM   #1
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Default comm lathe problem

My comm lathe, which has previously worked well, has suddenly developed a problem. I've cut comms on it a dozen times before, without a problem. Now when I cut a comm, the armature starts to jump up and chatter in the V groove at the comm end. I've cut this particular armature previously and didn't have a problem. Any ideas? It has a diamond bit, and it doesn't look like there is any problem there, but perhaps you need a microscope to tell.

Lathe is a Cobro Motor Builders lathe.
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Old 02-14-2007, 10:04 PM   #2
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Make sure that the armature is spinning into the bit not away from it. The other thing to try is cut a different arm. How tight is the o-ring? If it's on the loose side maybe a new one is in order.
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Old 02-14-2007, 10:34 PM   #3
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It is possible that the armature shaft could be bent. Does the comm start to jump even when the bit is away from the comm? If this is the case Either the shaft is bent or it could be that your block is bent or out of alignment.
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Old 02-15-2007, 05:50 AM   #4
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check the blocks for alinment, and do you oil the blocks when you start to rotate the arm before you cut it? that would at least cut down on friction.

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Old 02-15-2007, 07:47 AM   #5
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O ring is on the loose side. Maybe that's the cause. I do oil the blocks. Armature spins fine until the bit touches it, then it starts to "climb" the bit. Hope the diamond bit isn't damaged. I'll have to figure out where I can get another O ring of that size.

Is the O ring supposed to ride the iron laminations on a stock motor (CO27) or the gap between them and ride the windings?
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Old 02-15-2007, 08:14 AM   #6
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if the o-ring is loose, put the o-ring on the stacks of the arm to tighten it up. when you get a new o-ring, run it on the laminations on the windings.

-Zac
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Old 02-15-2007, 08:37 AM   #7
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do not run it on the middle directly on the wires. run it on the iron stack at either side. align the drive pully to line up if needed.
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Old 02-15-2007, 08:41 AM   #8
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just curios; why not run it on the windings? anything wrong with doing that?

-Zac
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Old 02-15-2007, 08:46 AM   #9
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I've been cutting with the o-ring on the windings for years not a problem. Your bit may be chipped. Put a cheap carbide one on and see if it still jumps around while cutting, keep in mind the carbide will need to be run at 1.0 volt higher than with the diamond.

Another thing to check, are you running bearings or v-blocks, if bearings check them out real good you may have a bearing bad.

V-block, make sure you put a dab of oil on them B4 running.
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Old 02-15-2007, 08:51 AM   #10
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Running V blocks. I have checked to see if anything has come loose, causing a change in alignment. Only thing I can think of is that either the bit is chipped, or this armature is different in some way. The comm is .288 diameter. A lot of my old armatures are in the .27x range. Also, I do run the O ring on the windings. Wondering if this one has more compact windings so the O ring isn't as tight? I'll try a different bit, I think I've got one from way back that I used on a full size lathe, I'll see if its the same size.
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Old 02-15-2007, 09:13 AM   #11
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putting the oring on the windings can do weird things. it can create vibrations, it puts pressure on the wires, which can deform them, it has less grip on the arm because its a smaller dia and less surface area the oring can latch on to and slip during cutting. it might also change the rpm from holding onto a sloppy machine wind, but i think thats a minimal effect.

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Originally Posted by Z.Hallett
just curios; why not run it on the windings? anything wrong with doing that?

-Zac
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Old 02-15-2007, 09:29 AM   #12
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Default Lathe Problems

Is the armature spinning toward you or away from you when you are cutting?

How does the cut look on the comm. when it's jumping? It cannot look good

I have to lathes and the arm spins away from me. also make sue bit alignment is correct and that the bit is just above the centerline of the comm.

Does the lathe have a .05 or a 540 can drive motor? I have to use 4 cells on the 540 drive motor with the Cobra, and 3 cells on the .05 19t on the Orion quick set I have.
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Old 02-15-2007, 11:13 AM   #13
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Quote:
Originally Posted by Jaz406
Is the armature spinning toward you or away from you when you are cutting?

How does the cut look on the comm. when it's jumping? It cannot look good

I have to lathes and the arm spins away from me. also make sue bit alignment is correct and that the bit is just above the centerline of the comm.

Does the lathe have a .05 or a 540 can drive motor? I have to use 4 cells on the 540 drive motor with the Cobra, and 3 cells on the .05 19t on the Orion quick set I have.
The armature spins upwards into the bit which has its sharp cutting edge facing downwards. I'm thinking the drag of the cutting tool on the comm should thus hold it down into the V blocks. Have I got my head screwed on right?

I now realize the drive motor is mounted in slots, and I can lower the motor to put more tension on the drive ring. I'll give it another try. The bit, upon carefull examination, looks perfect.
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Old 02-15-2007, 11:20 AM   #14
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Eureka! A perfect cut again. Must have been the drive tension and or position of the drive pulley. Thanks for all the help.
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Old 02-15-2007, 01:31 PM   #15
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Can I ask you guys a question about my lathe. Running a carbide bit but it did it with my old diamond bit. When cutting I see the copper starts to group around the tip and Im worryign this is creating an imperfect cut. Do you think the bit is too high therefore the copper is getting stuck under it.
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