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Usable comm diameter


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#1 philk

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Posted 20 October 2019 - 07:02 PM

What would be the smallest diameter that would still be ok to use on a cobalt motor ? In this case a grp27 Koford comm.
Phil Kreuter




#2 philk

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Posted 20 October 2019 - 07:49 PM

Maybe I should say how small diameter before performance suffers.
Phil Kreuter

#3 MSwiss

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Posted 21 October 2019 - 07:27 AM

Performance should stay fine as long as it doesn't fail from being too thin, and peeling.

The way to check that is to face the end so you can see the pattern of the EDM fluting.

We had a micro scope, but an octoscope will work.

Or if you have at optical comparator in your shop, you can magnify the end and measure how much copper is left.

The actual diameter you can go is determined by how precisely the commutator was drilled, on center.

In the case of a 27 arm, and typically running horizontal brushes, of course there is overlap, but probably not a big problem as long as the particular motor doesn't have a lot of COG to overcome at startup.

I do remember having a 27 arm many years ago, that had an extremely concentric comm, and I was running it down into the .170s.

I remember for the last run, switching to an open endbell, with vertical brush hoods.

It was a real, real good runner, with good top end, and tons of grunt, and I'm 95% sure I won that particular race.
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#4 gotboostedvr6

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Posted 21 October 2019 - 08:16 AM

Yup! Ive have plenty of arms that were always good. But the turds always got better as the comm diameter shrunk.
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