Commutator tolerences

rec.crafts.metalworking Commutator tolerences

What are typical tolerences for commutators of ac/dc motors used in circular saws? The motor of one needs turning of it's commutator.

Hul

Reply to
Hul Tytus
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Manufacturing tolerances are not an issue when turning a damaged commutator. Turn it so it "cleans up" and make sure there are no bridges (shorts) between any of the bars. It is a good idea to scrape between the bars with a dull tool to assure they aren't bridged. Turn only enough to restore smoothness. Practically speaking there is no diameter that is too small as long as the brushes make proper contact. New brushes are probably in order. You will notice that the arc surface of the new brushes isn't quite the same as the outside of the commutator. The brushes will soon "run in" to make proper contact.

Bob Swinney

Reply to
Robert Swinney

The typical limit for consumer grade saws is .0004 TIR, and .0002 between adjacent bars. For industrial saws it was .0002 TIR and

0002 bar to bar. Typical production was far below these limits.

John Normile

Reply to
John Normile

Reply to
Machineman

Thanks for the info. These tolerences are tighter than expected and, with them in mind, some attempts to clean the commutator (file, sandpaper...) will come before turning.

Hul

John Normile wrote: : The typical limit for consumer grade saws is .0004 TIR, and .0002 : between adjacent bars. For industrial saws it was .0002 TIR and : 0002 bar to bar. Typical production was far below these limits. : : John Normile : : : On 19 Aug 2004 06:05:22 -0400, snipped-for-privacy@shell1.iglou.com (Hul Tytus) : wrote: :>rec.crafts.metalworking :>Commutator tolerences :>

:>What are typical tolerences for commutators of ac/dc motors used in :>circular saws? The motor of one needs turning of it's commutator. :>

:>Hul :>

:>-- :>- for email, put the word "keep" in subject line - :

Reply to
Hul Tytus

Many thanks all - often pays to ask.

Hul

Reply to
Hul Tytus

You are less likely, in my opinion, to end up out of round turning them than you are filing them on the lathe. Cheers Trevor Jones

Reply to
Trevor Jones

My experience is with larger industrial DC motors (commutators to 18" in diameter or so), not power tool universal motors, but I am sure there are a lot of similarities. The TIR and bar to bar limits referenced by Mr. Normile are a bit tighter than I am used to, but not by a lot. I have heard of using diamond tools for turning commutators, but most comutator turning I have seen was done with carbide inserts. The copper itself is relatively soft and could probably actually be done better with HSS tooling, but the mica between the copper segments is hard. Undercutting the mica below the commutator OD is usually done with a small rotary cutter not unlike a small slitting saw mounted on a carrage on the lathe. Undercutting can be done by hand with special tools or even with an old hacksaw blade, but this would be an unrealistic amount of work on a big commutator. Some small motor commutators use "flush mica" meaning the mica is not undercut and the brush must be abrasive enough to keep the mica worn flush. I have no personal experience with "flush mica."

My first question might be why your circular saw commutator needs turning? Grooving or a "threaded appearance" are not ideal, but the really do not hurt very much as long as the commutator is round and has acceptable TIR. If this is the only problem, you might be better off leaving the commutator alone.

Most power tool armatures I have seen have molded commutators. This type of commutator is not repairable -- meaning if a copper segment is loose or if there is a short, the commutator is scrap. Many of these small molded commutators also appear to have relatively thin copper segments in the radial direction and there may not be much allowance for resurfacing the commutator.

When everything is working properly, there should be a uniform gray, brown, or black film in the commutator surface where the brushes ride. Ideally, the brush should ride on this film, not on bare copper. The film provides a low friction surface and minimizes copper and brush wear.

My thoughts for what they are worth, Mill

Reply to
MP Toolman

Reply to
Robert Swinney

Wow! That might work for a science fair project, but I find it difficult to believe it would work for serious use. A wood shell will not stay round with heat and humidity changes. "Glue" in pure tensile at commutator operating speeds and temperatures would be suspect. What about bar to bar insulation?

Mill

Reply to
MP Toolman

Sorry! I should have included the phrases "experimental" and "laboratory project" - maybe even "proof of concept". Lots of things we use today were conveived as wood models.

Reply to
Robert Swinney

Circular saw armatures have a no load speed in excess of 20,000 RPM. Enough said!

John Normile

Reply to
John Normile

The saw required a manual spin to start 1 time out 10. The color of the commutator was blackened, which looked slightly insulating. Thanks for your info about the color.

Also, after about 1 second of releasing the trigger, a strong braking action would take place. This appeared to be a speed governed switch shorting a resistor. Saw no sign of that on disassembly. Recently, the braking action delay grew longer and often the braking didn't occur at all.

Any suggestions are most welcome.

Hul

MP Toolman wrote: : My experience is with larger industrial DC motors (commutators to 18" in : diameter or so), not power tool universal motors, but I am sure there are a lot : of similarities. The TIR and bar to bar limits referenced by Mr. Normile are a : bit tighter than I am used to, but not by a lot. I have heard of using diamond : tools for turning commutators, but most comutator turning I have seen was done : with carbide inserts. The copper itself is relatively soft and could probably : actually be done better with HSS tooling, but the mica between the copper : segments is hard. Undercutting the mica below the commutator OD is usually : done with a small rotary cutter not unlike a small slitting saw mounted on a : carrage on the lathe. Undercutting can be done by hand with special tools or : even with an old hacksaw blade, but this would be an unrealistic amount of work : on a big commutator. Some small motor commutators use "flush mica" meaning the : mica is not undercut and the brush must be abrasive enough to keep the mica : worn flush. I have no personal experience with "flush mica." : : My first question might be why your circular saw commutator needs turning? : Grooving or a "threaded appearance" are not ideal, but the really do not hurt : very much as long as the commutator is round and has acceptable TIR. If this : is the only problem, you might be better off leaving the commutator alone. : : Most power tool armatures I have seen have molded commutators. This type of : commutator is not repairable -- meaning if a copper segment is loose or if : there is a short, the commutator is scrap. Many of these small molded : commutators also appear to have relatively thin copper segments in the radial : direction and there may not be much allowance for resurfacing the commutator. : : When everything is working properly, there should be a uniform gray, brown, or : black film in the commutator surface where the brushes ride. Ideally, the : brush should ride on this film, not on bare copper. The film provides a low : friction surface and minimizes copper and brush wear. : : My thoughts for what they are worth, : Mill : :>

:> One other thing is surface finish. Most commercial refinishing of :>cummutators is done with diamond tools and will give you an almost :>mirror finish. If you can't use a diamond tool then very sharp positive :>rake carbide or cermet inserts. Because copper is so gummy you have to :>watch for tears in the material. These will seriously reduce the life :>of the brushes. :>

:>John Normile wrote: :>> The typical limit for consumer grade saws is .0004 TIR, and .0002 :>> between adjacent bars. For industrial saws it was .0002 TIR and :>> 0002 bar to bar. Typical production was far below these limits. :>> :>> John Normile :>> :>> :>> On 19 Aug 2004 06:05:22 -0400, snipped-for-privacy@shell1.iglou.com (Hul Tytus) :>> wrote: :>> :>>>rec.crafts.metalworking :>>>Commutator tolerences :>>>

:>>>What are typical tolerences for commutators of ac/dc motors used in :>>>circular saws? The motor of one needs turning of it's commutator. :>>>

:>>>Hul : :

Reply to
Hul Tytus

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