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Rotor assembly fundamentals
By K3CZ, Section Mechanical Posted on Sun Jun 26, 2005 at 01:19:33 AM MST
How clean must everything be??

To all you experienced Piggott-type mill builders out there:
I have closely watched the process of mill assembly at a Piggott workshop, and read/reread his blue handbook a hundred times, and still not everything goes according to plan.
Immediate problem: I thot I had the steel disc "thoroly cleaned" with hand wire brush, detergent and paint thinner. However, the completed assembly was unintentially dropped on the concrete floor, and half the magnets/resin block was knocked loose, even tho the magnets had been put in place with Super Glue, per Hugh's latest instructions. (A few light taps with a wooden block and hammer easily dislodged the rest of the magnets/resin, and the resin doesn't seem to stick very well to the magnets, either). I have now tried a power wire brush on the disk, but even that won't get down to bare metal, leaving behind the original gray HRS steel coating. Do I need to use a power grinder to insure resin contact with truly bare metal?
All inputs gratefully received.
Van K3CZ

Rotor assembly fundamentals | 15 comments (15 topical)

Re: Rotor assembly fundamentals (3.00 / 0) (#1)
by windstuffnow on Sat Jun 25, 2005 at 07:56:58 PM MST

For the most part, the rpms these machines run you can just about get away without gluing the magnets in place. The resin holds them in position so they don't slide around while under load. The super glue holds them fine ( just short of droping the disc on concrete ). Under normal circumstances you wouldn't have a problem.

I've laid out discs with no resin or glue and run them in my lathe up to 1000 rpm without any problems. ( I don't recomend doing this by any means because they certainly could come off at any point under extreem load)

I typically use JB weld on mine which holds a little better than super glue but can still be knocked off with a sharp scraper and hammer with a little effort. If your still leary about them you can cut a single layer of fiberglass cloth and lay it over the disc, coat it with resin and it will hold them all in place. cut away the un-needed portion and it makes a nice clean assembly.

Have Fun! Windstuff Ed

Re: Rotor assembly fundamentals (3.00 / 0) (#2)
by Flux on Sun Jun 26, 2005 at 12:59:20 AM MST

I agree with Ed. If you drop it it will fail. Under normal conditions the resin will prevent the magnets moving outwards when the thing spins. There would be some advantage in drilling a few holes through the discs for the resin to key into, but it should not be necessary I have never heard of the cast block ever moving.

I have no faith in superglue, it is just a precaution to prevent the magnets moving during the resin casting process.

I have run test alternators with the magnets stuck on with Loctite Multibond 330 ( not sure if you have it in N America) This seems to be better than epoxy.

Flux

[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#4)
by DanB on Sun Jun 26, 2005 at 07:26:50 AM MST

My thoughts are similar. I've run the rotors up to 800 rpm w/no glue whatsoever and the magnets remained in place, although I expect the thicker magnets we use (and the ones in Hughs design...) may come off sooner.

In my opinion though a wire brush may not be the best route. I like to clean the steel rotors with a belt sander, it gets right down to bare metal and leaves lots of nice scratches in there to help the glue adhere.

Then, before placing the magnets I clean it with laquer thinner and makes sure my hands are clean.

[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#3)
by Norm on Sun Jun 26, 2005 at 06:26:40 AM MST

One more little note here about wirewheel brushes...you can actually be making things
worse...I wirebrushed a mower handle once
then tried to paint it ...and the paint
wouldn't stick...had to use a special solvent
and wet/dry sandpaper to clean the surface.
...I've run across this problem more than
once when painting ...course as far as painting
a house with exterior latex....might be an
entirely different matter...but I still shy
away from wire brushing things that I'm going to
paint.
You can actually solder to glass by loading
a wire brush with lead then wirebrushing the
glass...it's called braiding.( I think...)
( :>) Norm
( :>) Norm

Re: Rotor assembly fundamentals (3.00 / 0) (#5)
by ghurd on Sun Jun 26, 2005 at 08:17:04 AM MST

I found the super glue 'gel' does not hold as good as the regular super glue.
Not sure the gel held much at all, and that was with clean, new machined parts.
G-

Ghurd.info

Re: Rotor assembly fundamentals (3.00 / 0) (#8)
by Norm on Sun Jun 26, 2005 at 01:11:49 PM MST

Ghurd:
I remember you showing me that...and I was
thinking later on if the glue had stuck to the
magnet or the rotor...or no particular preference as far as the glue was concerned
(a clue if it was either of the first two) but forgot to ask you.

I really believe that somewhere there is a Metal
Prep just like car body professionals use before
they put Bondo or primer on a car....
....besides I thought some of you guys used JB Weld ??
I think if you could just smear a bead of
JB Weld around the edges of the of the magnet
that will keep it from sliding around mainly
...plus the JB Weld is using it's sheer strength
more than cohesion...they taught us that in
Welding school to make use of sheer strength.
A poor weld, if designed well, will be stronger
...IE..hold together better than a good weld
..poorly designed.
( :>) Norm
[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#9)
by ghurd on Mon Jun 27, 2005 at 09:17:50 AM MST

It did not seem to care.
Some stayed on the magnets,
some on the rotor.

JB Weld crawls all over the magnets!
Made a heck of a mess.

G-
Ghurd.info
[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#11)
by windstuffnow on Mon Jun 27, 2005 at 10:14:32 AM MST

Ghurd, If the JB is ooozing and crawling you used way to much. A very small amount is all that is needed. Just enough to discolor the disc.

The magnet is going to pull itself to the disc squeezing out alot of material. This creates a micron layer of JB under the magnet and the ooze is simply wasted. You only need enough to create this micron layer and fill any scratches or deformaties on the steel.

Also, by using a very small amount you don't have to hold it in place while its setting but you still have ample amount of time to locate the magnet before it sets. Super glue is a one shot deal and is better used with a magnet jig.
Have Fun! Windstuff Ed
[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#12)
by ghurd on Mon Jun 27, 2005 at 10:38:31 AM MST

Ed,

I used way too much JB Weld.

The super glue 'gel' seemed to take a long time to set.
Made it slippery too.
Was hard to get the magnets to sit with the right skew,
they seemed to align themselves to the rotor very slowly.
It was a little tiny conversion. Even 0.020" wrong was too far off.

I'm just using epoxy now.
G-
Ghurd.info
[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#13)
by ghurd on Tue Jun 28, 2005 at 10:37:15 AM MST

And last night, a rotor slammed into a stator.
(the magnets were bare when I got them, so I painted them with regular epoxy)
The epoxy came off 1 magnets face in one large chip.
G-
Ghurd.info
[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#6)
by K3CZ on Sun Jun 26, 2005 at 09:16:18 AM MST

Again, thanks to all those who proffered advice from their experiences. It is little details like this that can often make or break a first-time project. I do seem to remember from Hugh's workshop that whoever prepared the disks did use a power sander to reach bare metal, and when we had trouble with magnets shifting during resin cure (do NOT stand the assembly on edge while curing!), considerable effort was required to grind the drifted magnets out of the embedment, and the resin showed no signs of breaking loose. (The rotor was successfully repaired, and the project moved forward to completion.) Anyhow, this time I will reach bare metal before I locate the magnets and pour resin. (The super glue may not be necessary, but the gel does provide a bit of a lubrication layer for shifting the magnet into final position). Sidebar: I am going to find some bronze or other non-magnetic tools to work on these things; I am still nursing several bloody pinched finger skin events!
VAN K3CZ

Re: Rotor assembly fundamentals (3.00 / 0) (#7)
by Ungrounded Lightning Rod on Sun Jun 26, 2005 at 10:19:32 AM MST

Stainless steel is non-magnetic, too.

Even if you can't find stainless tools you can improvise pry bars and the like with stainless rods.

You can use stainless screws, washers, and wingnuts for holding things down near the magnets (like clamps and wooden alignment jigs) without having the magnets grab them out of your hand).

[ Parent ]

Re: Rotor assembly fundamentals (3.00 / 0) (#10)
by DanG on Mon Jun 27, 2005 at 09:21:00 AM MST

I saw an antique brass adjustable wrench, like the ones they gave w/ Model A's but made entirely out of brass, sell for $95 USD at auction in rural Missouri. Good luck finding them...

Re: Glue (3.00 / 0) (#14)
by normthehandyman on Wed Jun 29, 2005 at 03:44:38 AM MST

superglue has the advantage the bond is prety much insantaneous especially if you use kicker. However it will not withstand any shock loading, like droping anything glued with it. If you have clean steel, fibreglass resin will stick to it as good as anything available on the market, but is not necessarily the best bond to a plated Neodmium magnet.
The best bonding glue I have ever come across for dissimiler metals is called "Toughened acrylic adhesive" Penloc would be an available brand and Bostick in the UK though there are others. It is a 2 pack adhesive and generally mixes 1:1 ratio. At 20 degC it takes about 5 mins to cure. It is expensive. But once you've used it all other glues are 2nd best with the exception of temp resistance.

At the end of the day any glue is a compromise.

Re: Rotor assembly fundamentals (3.00 / 0) (#15)
by Devo on Wed Jun 29, 2005 at 08:25:15 AM MST

Permatex cold weld seems to work fine as well.

As Ed has stated with JB weld , very little is needed. I put it on the mag then put the mag on the rotor.

Devo

Rotor assembly fundamentals | 15 comments (15 topical)

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