# When balancing in-line arm goes wrong.



## Jisp (Oct 19, 2008)

So, with the weather warming up on my side of the pond and forever inspired by Mr Hall’s builds I have a custom build project in mind for Spring. I had a Mabuchi motor set aside and decided to balance the arm, polish the comm and apply Grungerockjeepe’s endbell venting before going any further. Gotta start with a decent donk right? In theory yes but let me loose on one of these little critters and before long it all gets out of shape. Polishing and endbell venting went well, balancing did not.

A while back I had an arm from an AFX G+ that was horribly out of balance and so did some major drilling, my first attempt at this method on an in-line. I used the parallel razor blade method for balancing. Was quite surprised at how much material I had to remove to get it balanced but so be it. The Mabuchi arm was only very slightly unbalanced and decided to use the same method I had used for the G+.........

I take the arm and very lightly clamp it in my bench drill vice with the offending pole facing up. I have aluminium inserts that sit over the steel jaw faces for delicate work like this and support the arm from underneath. Using a pin vice with a sharp drill bit I make a starter mark and then use a cordless drill to remove the material. While drilling the Mabuchi arm I could feel that something was wrong because the drill tip began wobbling around and wouldn’t stay centred.

The laminations of the pole had slipped sideways against one another. The photo shows the G+ arm on the left successfully balanced while the Mabuchi arm shows the step in the laminations. The arm now reads 10.1, 10.2 & 20.4 ohms.... no good.



I have some questions if anyone can help.

1.	Above all, I don’t understand why the laminations pushed in the opposite direction of the drill bit rotation. I would expect the opposite to occur particularly if the drill bit had grabbed, which it didn’t. Any thoughts?
2.	How do you guys balance an in-line arm? I considered using a rotary tool to cut a slot along the length of the pole but decided on drilling instead.

Thanks for any help.

Cheers,
Michael.


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## LDThomas (Nov 30, 1999)

I use a triangluar file and cut a groove crosswise. (Perpendicular to the shaft.) Creates less windage...


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## slotking (May 27, 2008)

with the drill bit, you still have to push kind of hard, the lams are pressed on so that is why they moved.

Like larry, I cut sideways, but use a cutting wheel on a dremel
less pressure is needed.

I think also allows for easier fine tuning


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## SDMedanic (Apr 21, 2011)

A Dremel with a cutoff bit would be much easier to use for this application than a drill.


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## slotking (May 27, 2008)

I forgot
some time I cut long ways as well

with the dremel & cutting wheel, it does not matter
both ways are easy


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## slotking (May 27, 2008)

oh
yeah
forgot to mention

it so easy, I hold the arms with my fingers when I balance them


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## Jisp (Oct 19, 2008)

Thanks guys. You've confirmed what I suspected but wasn't sure. Prior to doing it I couldn't recall seeing photos of it being done with a cut off wheel but have seen many photos of it done with a drill. I figured it was "just the way it's done" and there was a good reason for it (unknown to me).

Live and learn hu. It's a pity, I hate trashing perfectly good parts in the learning curve. I should have posted a question to begin with.......

Thanks again,
Michael. :thumbsup:


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## slotking (May 27, 2008)

good luck:thumbsup:

just an fyi 
I have several kinds of cut off wheels
what I like best are the small thin ones.

they break easy if you bend them, but never had 1 break while using it.


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## pshoe64 (Jun 10, 2008)

I use a Dremel drill press with the 7103 diamond engraver/shaper bit. Very high RPM and light pressure, removing a small amount at a time, checking the balance in between grinding sessions. The arm is held in place by a piece of hardwood I shaped to hold the arm still on its side and a rubberized clamp.

I still trying to develop a better mousetrap to hold down the pancake arms to balance those. I use the grinding wheel method for those.

-Paul


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## Bill Hall (Jan 6, 2007)

I'm a sicko...I generally just plod along carefully with a hand file and try to keep things smooth with regard to windage in the airgap. You can kinda tell how much material has to be removed by how fast it falls through your poising tool.

I polish the stacks by spinning them against some 320 or 600 and recheck balance.


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## Jisp (Oct 19, 2008)

I appreciate your thoughts guys, thanks.

Mike, the only time I've ever had a thin cut off wheel break in use was if I pushed it way beyond it's limit. The limit is soon learned. If you've never tried them, I reckon the Dremel brand wheels #420 are the best I have used. They are pricey but are harder wearing and seem less prone to breaking.

Paul I actually used a diamond ball point to try and clean the mess up after the drill bit. That was before I retested each pole and found one was up the creek. How have you found the drill press? I've eye balled them in the tool shop but never out of the box. I love my bench drill press but it's pretty awkward for tiny work.

Bill, there were two files (& a Dremel) within easy reach when I performed my butcher work. It may be that it comes down to the type of arm. A G+ drilled perfectly for me, not so the TCR.

The poor little guy..... he never had a chance once I got hold of him. He's still not talking to me. :freak:

Cheers,
Michael. :thumbsup:


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