7 replies to this topic

[havlicek]

I got asked about this, and figured it was good general-type info for those who want to cut their own coms.  Most all of it is not specific to the XiPP, so these are my "ten things to watch" when cutting coms.  If you have a bunch of motors that can be user-maintained, a com lathe is a natural.  You don't have to be a machinist to get good results (I sure am not!), and having clean and true coms can make a noticeable improvement in how your motors run.


There's really nothing to it. 

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1) Make sure the cutter is set to the proper height.  Since the arm will be spinning up and then away from you when seated in front of the lathe, the cutter should be *slightly* higher than the center of the com.  For all intents and purposes, the center of the com is the same as the center of the shaft...so you can install your cutter, run it towards the shaft of an arm, and then "eyeball" the cutter tip relative to the shaft.

2) I mark all my coms with a Sharpie.  It does add a bit of lubrication, but what it really does is give you a good indication of what you've cut...and what still needs to be cut.  As you get close, rotate the arm slowly by hand while looking at the com, and you will see some very small areas that still need cutting.

3) When starting your cut, have the cutter away from the com surface and then advance the cutter towards the com *while moving the cutter back and forth slightly.  As it *just* makes contact as indicated by the Sharpie black being removed, move the cutter all the way off the end of the com, and advance the cutter one or two "marks".  Then slowly...but not too slowly, start advancing the cutter across the com until it gets all the way towards the bottom of the com, setting the stop screw so it can't go too far.  Then back the cutter all the way off the top of the com again.

4)  For all subsequent cuts, advance the cutter a couple of marks (2 or 3 at first, later on as you get close to being true...1 or 2).  Don't back off the cutter advance until done, as you'll introduce backlash that will then have to be removed to cut further.  In other words, you will lose your place.

5) The V-blocks have a very small reservoir, with a tiny "channel" cut into them that will allow oil to flow down into the V-groove.  Always keep these oiled.  It will make for better cuts, and prevent the unlikely wear on the arm shaft thar (I guess?) could possibly happen.

6) Since the lathe is powered by a DC motor, how fast the arm spins is dictated by the voltage fed to the lathe motor.  It seems that 5-6V is fine.

7) All copper is not the same, some is harder and some is softer.  If you're suddenly not getting a good smooth finish, you'll have to just try different things to see what combination of arm rotation and cutter feed works best...but make sure the V-blocks are oiled, that can actually make for a slightly rougher cut.  Particularly "soft" coms can make for a sort of "gummy" look to the cut, almost as though the metal is getting dragged out of the way, instead of being cut cleanly.

8) If everything is... and has been working fine, but you suddenly find the quality of the cut has noticeably degraded, it may be time for a new cutter tool.  The diamond tipped tools last for hundreds of coms, but that tip is brittle and fragile and can be damaged by getting careless and casual.

9) Keep a collection of black rubber O-rings (*available in hardware stores for super-cheap) in different sizes.  Different arm O.D.s will require different size O-rings.  The O-ring should be snug and slop-free, but not so tight that they are really difficult to get on.

10) You can do one of two things to adjust the gap between the V-Blocks for different length arms.  You can simply use the allen head V-block locking screws to adjust the blocks towards each other...or use a collection of arm spacers, and even spare oilites (for larger moves) handy.  For very short stack arms, it can become necessary to put a lot of spacers at the rear (tail) of the arm so the O-ring will just be on the stack rear-end, and have the com far enough towatrds the cutter so you don't run out of cutter feed adjustment.

[mreibman]

Brilliant!

 

Love the sharpie idea.

[Geary Carrier]

Thanks John...

[Bill from NH]

I've only had to respace the V-blocks on my Cobra lathe a couple times. I used a 2 mm. drill blank in the V's to keep them aligned while I loosen & retighten the base allen screws.

[Zippity]

I practice all 10 tips  

 

Guess I had a good teacher - may he rest in peace.

[MSwiss]

 

4)  For all subsequent cuts, advance the cutter a couple of marks (2 or 3 at first, later on as you get close to being true...1 or 2).  Don't back off the cutter advance until done, as you'll introduce backlash that will then have to be removed to cut further.  In other words, you will lose your place.

Great thread and advice.

 

#4 is why I use to get cranky, with customers, when they asked me cut a comm on the raceway Hudy.

 

It was painfully slow to use.

 

Among other things, I just wasn't use to a comm with backlash, as at Koford, Stu designed, and made his own, that didn't have backlash.

[NY Nick]

I do only 8 will have to start using a shapie , I like the the drill blank.

Thanks for posting.

[havlicek]

 

#4 is why I use to get cranky, with customers, when they asked me cut a comm on the raceway Hudy.

 

It was painfully slow to use.

 

Among other things, I just wasn't use to a comm with backlash, as at Koford, Stu designed, and made his own, that didn't have backlash.

 

Yeah Mike.  At first, I would forget about the backlash and have to back way off the cutter and then dial-it back in.  Now, and I'm sure it's the same with anyone else, all of the above is so automatic that the backlash doesn't even matter.

***Of course, I left out the "after" part of cutting coms.  Sharpie, copper dust and oil need to be cleaned out of the com segment grooves.  Dragging (*never pushing) the back of a #11 X-Acto blade from the bottom of the com towards/off the top makes short work of that.  The blade body itself is not much thicker than the com slots, but the blade is ground so that the last few millimieters taper to "nothing", MUCH thinner than even the thinnest com slots.