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Stack grinding: plusses and minuses


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#1 havlicek

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Posted 07 February 2017 - 12:12 PM

Since I've been asked the question several times as to the "why" about stack grinding, I figured I at least tell my reasons. Stack grinding seems like a no-brainer as one of two obvious ways to achieve a particular airgap and/or "fit" an armature into a particular setup when no stock lams are available for that... the other being honing the magnets in place.
 
There can be at least two problems with stack grinding.  

In extreme cases, the crown of the lams could be weakened so much that a fast arm spinning can cause the tips of the stack to deflect outwards and contact the magnets in a tight setup.

The other one I can think of is that grinding the stack enough so that it all looks almost like one piece of metal can/will result in the lam coating to be removed and cause the lams to short. I think that if you still clearly see the lam ridges on the outside of the stack crowns, this might not be the case, but even if it does happen, it's obviously not a deal-breaker.

On almost all the arms I send out for balancing, I always also ask for at least enough of a "light skin grind" because:

1)  If the lams, shaft and grinding setup are all close to perfect, it could help with balancing, but those are three mighty big "ifs." Then again, if the lams were perfect, there would be no benefit at all as far as balancing is concerned from OD grinding, other than really slightly correcting anything that might have happened when pressing the lams.

2)  Even a little armature spinning really fast causes air turbulence, and is in turn effected by that turbulence. How much of a factor is this? I honestly don't know but I "imagine" it can be.
 
So I figure there are reasons to always grind a stack, even if it's not "necessary" to fit the arm. That's my story and I'm sticking with it, until or unless I get schooled otherwise.


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#2 zipper

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Posted 07 February 2017 - 01:20 PM

I know some "nitpickers" - well, precision machinists - do order their Group 27 arms unbalanced and oversized. Then they grind them to the desired size, cut the comms, and balance them to get better precision than out of the factory.


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#3 Bill from NH

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Posted 07 February 2017 - 01:21 PM

When I've had "skim cuts" done, it was always done so to ensure that the stacks were perfectly round. I've also cut some stacks on my lathe with a diamond bit. I've never had the need to reduce an arm's diameter to fit in a particular setup, but I know others have.

 

As an aside, Camen had a thin-web lam for X12 arms in the early 2000s. These things would fly apart after being drill-balanced, so Camen switched to doing grind-balancing on these particular arms. I have one that looks similar to the old Champion 507 arms.

 

zipper, the late Monty Ohren used to get "raw" arms, like you describe above, from Pro Slot before he began winding his BOW arms.


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#4 gotboostedvr6

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Posted 07 February 2017 - 01:32 PM

John,

You brought up a good point with regards to the process of pressing the laminations together and "things happening."

Possibly the most catastrophic thing that can happen during this process is the arm shaft physically being bent. Stack grinding "fixes" this problem but at what cost? After you dynamically ballance a arm with a press-induced bent shaft wouldn't harmonics induced by RPM cause dramatic inballance?

How do you feel about using pressure to straighten the shaft after pressing?

In your opinion would using a hardened drill blank in place of a regular shaft produce less run-out? How about a high % tungsten carbide shaft?

Dave


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#5 wbugenis

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Posted 07 February 2017 - 01:32 PM

There is quite a history centered around "stack grinding" - from arond 2000-06. Racers are always looking for a part or modification that will be the next "game changer" at the local raceway. Stack grinding easily fits that category.

 

First though we have to discuss another "game changer," the .560" diameter 16D lamination form RJR (Robert Root in 

collaboration with Ron Hershman, who worked closely with RJR at the time). Soon all the racers knew thety had to run a .560" arm in their 16D - go fast or go home. It was a few years before Pro Slot and Monty Ohren came out with their own .560" lamination.

 

I was with Viper 2006-07 - we were eating everyone's lunch with our new .560" lams. (It's really expensive to make a new tool for laminations.)

 

Of course it didn't last. All the manufacturers eventually had their own .560" lamination. BTW, they all used the same stamping house in the midwest.

 

And, all the fast guys, with a few exceptions are, running .560" arms in their 16D motors.

 

Now to the stack grinding part;

 

Lou Pirro of Grand Prix Model Raceway in Schenectady, NY, acquired a large batch of C-can magnets that were mistakenly ground 0.020" too thin.

 

What to do? Lou called Ron at RJR  and asks him to make up some Group 12, Contender, and Super Wasp arms on the .560" blank but ground to 0.540". No problem for RJR as they had a big Crystal Lake OD grinder.

 

 

(It became quite a struggle for Monty using a tool post grinder on his Grisley lathe until he got a surface grinder and spin fixture.)

 

Arms were fast - but maybe news does not get out of Schenectadty so quick.

 

Soon I am working with Viper and Lou asks for some .540" arms. Viper resists but he is eventually persuaded.

 

We have a Northeast ISRA race on the Viper King track in New Jersey and Jay Kisling is running one of the new .540" Viper Group 12 arms in his car. He is fast, seriously fast, and kicks everyone's butts.

 

Game change!

 

Soon everyone was running the .540" arms in ISRA races and they are all ground down from the .560" lamination.

 

That's my story and I'm sticking to it!

 

Bill


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#6 MSwiss

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Posted 07 February 2017 - 02:07 PM

All the manufacturers eventually had their own .560" lamination.

 

BTW, they all used the same stamping house in the midwest.

 

Bill,

 

All manufacturers had .560" dia. arms? I don't think that was the case with Koford.

And my way less than perfect hearing will attest that not all lamination are (or at least, were) stamped at Larry's Lamination City.

Just as I recall you saying recently in another thread, as far as stack grinding, the rounder the stack, the easier the arm is to balance.

Years ago, I remember a local racer coming in with two or three arms,his buddy, who owned a machine shop, "precision" OD ground for him.

They were so far out of whack, they couldn't be balanced. You could put the arm in the balancer three times, and without ever drilling it, you would get three different poles to drill.


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#7 gotboostedvr6

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Posted 07 February 2017 - 02:23 PM

Drill blank shafts are notorious for being out of round. This causes balancing machines to misinterpret data.

Not saying this was the issue, just food for thought.
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#8 wbugenis

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Posted 07 February 2017 - 02:27 PM

Mike,

 

Koford was never a player in the 16D game as were RJR, Pro Slot, and BOW, hence no .560" laminations.

 

Koford stamped their own laminations. Lou mentioned this to me when he visited Stu. I should have  been more specific.

 

And yes, you need to start with a reasonably straight shaft (0.0003" TIR or less), don't bend the shaft when you press the stack, heat the blank for powder coating, and epoxy cure... and grind with as little TIR as possible. Too much TIR (Total Indicated Runout) will make balance difficult or impossible. Precision is possible, but not without care and the appropriate measuring tools.

 

I am an advocate of guys getting an indicator and V-blocks (low $$ on eBay) but that is another can of worms.


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#9 MSwiss

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Posted 07 February 2017 - 02:29 PM

The stacks were at least .002" or .003" out of round.

Mike Swiss
 
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#10 wbugenis

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Posted 07 February 2017 - 02:32 PM

Bent shaft or hole not centered in lam?

 

Yes, the balancing machine is not happy with that much run-out.


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#11 MSwiss

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Posted 07 February 2017 - 02:40 PM

I too, should have been more specific.

They were existing used arms that ran fine, but got reground because his machinist friend convinced him he did super-precision work.

I'm not sure if he used a 3-jaw chuck, or just went too fast, without both ends of the arm supported.


Mike Swiss
 
IRRA® Components Committee Chairman
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#12 wbugenis

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Posted 07 February 2017 - 02:45 PM

Yeah, those 3-jaw chucks; I cringe everytime I see one.

 

I fear that some will decide to cut their commutators on a mini lathe with a 3-jaw chuck.

 

Way too much run-out even with the average collet. 


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#13 olescratch

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Posted 07 February 2017 - 03:43 PM

OK! Better find another way around this process.
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#14 havlicek

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Posted 07 February 2017 - 04:35 PM

There's a lot of information going on above, for those interested... pay attention!

 

-john


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John Havlicek

#15 Cheater

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Posted 07 February 2017 - 04:38 PM

Yepper. I was banging on that Bugenis character earlier today about posting more here. He just likes to lurk, you know... LOL!


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#16 gotboostedvr6

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Posted 07 February 2017 - 06:18 PM

lurker.jpg
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-David Parrotta

#17 havlicek

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Posted 07 February 2017 - 06:55 PM

Yepper. I was banging on that Bugenis character earlier today about posting more here. He just likes to lurk, you know... LOL!

 

Yeah... he is a lurker alright, Greg!  I sure hope he's not compiling some sort of dossier on me, because I don't have enough money to skip town!   :D

 

-john


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#18 MSwiss

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Posted 07 February 2017 - 07:04 PM

Bill,

 

Whatever happened to Viper Jerry?

 

Any contact at all?

 

I wonder if he's changed his name to Lamborghini Jerry by now. :laugh2:


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Mike Swiss
 
IRRA® Components Committee Chairman
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17B West Ogden AveWestmont, IL 60559, ( 708) 203-8003
mikeswiss86@hotmail.com (also my PayPal address) 
Note: Send all USPS packages and mail to: 5858 Chase Ave., Downers Grove, IL 60516
Make checks out to Chicagoland Woodworking, Inc.


#19 wbugenis

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Posted 07 February 2017 - 07:12 PM

Mike,

 

No Idea. Last I saw him he was into kayaking - sold all the slot car stuff to me.

 

Maybe Kayak Jerry. That was years ago now.

 

Bill


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#20 Cheater

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Posted 07 February 2017 - 08:04 PM

Funny. In my younger years I was a semi-serious kayaker myself. Been down most of the navigable water you see in Deliverance.
 
Still have a handmade wooden kayak paddle that is a fabulous work of art. I ought to sell it, but it is such a great object. It's almost 55 years old...

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Never forget that first place goes to the racer with the MOST laps, not the racer with the FASTEST lap


#21 MSwiss

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Posted 07 February 2017 - 08:14 PM

Been down most of the navigable water you see in Deliverance.


That explains quite a bit...
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Mike Swiss
 
IRRA® Components Committee Chairman
Five-time USRA National Champion (two G7, one G27, two G7 Senior)
Two-time G7 World Champion (1988, 1990)
Eight-time G7 King track single lap world record holder (pointless era - LOL) 
17B West Ogden AveWestmont, IL 60559, ( 708) 203-8003
mikeswiss86@hotmail.com (also my PayPal address) 
Note: Send all USPS packages and mail to: 5858 Chase Ave., Downers Grove, IL 60516
Make checks out to Chicagoland Woodworking, Inc.


#22 Rob Voska

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Posted 08 February 2017 - 09:04 AM

Still have a handmade wooden kayak paddle.
 
Probably pretty heavy now that it's had plenty of time to petrify...  :victory:
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#23 wbugenis

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Posted 08 February 2017 - 10:11 AM

My son and I made a couple of these a few years back:
 
william's kayak.jpg
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William Bugenis

#24 Dan Miller

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Posted 08 February 2017 - 01:47 PM

Another important reason to grind stacks is to make the surface as smooth and uniform as possible. The lines of magnetic force that come off an armature radiate straight out from the lamination edges. If the laminations are left unground you have lines of force emanating in many random directions. Grind the lamination edges smooth and this radiation will be at 90 degrees to the stack surface and you will have a more efficient armature.
 
Drill blank shafts ground properly are round. Poorly ground and they will triangulate and this is hard to measure without using an indicator and V-blocks. Handheld measuring with micrometers will not do. They should be within .0005" TIR over 2.00" straight out of the centerless grinder. That is a reasonable starting point. When you process an armature there are many things that stress and bend the shafts. You should straighten them three or four times in process.
 
John mentions shorting out the laminations by grinding the edges smooth and sort of mashing the lamination material together. That is real and has a negative effect but we must tolerate that fault.
 
Grinding has other benefits already mentioned above.
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