Disassembling a mag chuck

One idea, short of a teardown, is to put the whole thing in a 150F oven in a flat pan, and let the grease soften and run out ..

GWE

According to Alan Rothenbush :

To disassemble it, you need some way of replacing the magnetic circuit around the permanent magnet. And I don't know whether this is really possible. Some PM chucks were polarized by an external magnetic pulse applied through the pole pieces after assembly. Disassembling it risks serious weakening of the field strength, leaving you with *two* chucks which are too weak.

I would either try the solvent approach, or perhaps the heat one, as long as you don't go too high in temperature, as that, too, can degauss the permanent magnet.

Of course, if you had a service in town which could re-polarize your chuck, disassembly would be the way to go.

Good Luck, DoN.

In a 50 year old magnetic chuck the magnet will certainly be of the Alnico family. These need to be magnetised in the fully assemmbled state with special rigs. In all normal use magnets do not wear out but they must NEVER be removed from contact with the pole piece assembly.

If it's showing poor holding power it's either not fully mechanically switched 'On'or the working surface is not properly flat and needs regrinding.

In disassembly it's safe to remove any non magnetic item or any iron item that's easily removable by unassisted hand force

Eclipse magnetic chucks use SAE140 gear oil for lubrication.

Jim.

Any chance you could replace the magnet(s) with rare earth magnet(s)? See the Lee Valley site for some shapes and sizes. These are MUCH more resistant to demagnetization and are damn powerfull.

Ted

It's surely occurred to me !

But from other coversations I've had, this may not be so wise a solution.

If I'm really serious, the plan might be to send it away to someone who actually knows what they're doing !

Here's what prompted all of this. It occurred to me that it might be nice to use the little SG to sharpen my woodworking chisels. I've got a fair collection, and all are dull .. at best. I'm not real proud about the dings in some of them, said dings looking distinctly like they were used as cold chisels and not wood chisels.

I could either build a pair of jigs to hold the chisels at the appropriate angles OR I could use my adjustable magnetic block.

It's the same one as is shown at

This unit has ..OK.. holding power when oriented so that the pole pieces match the pole pieces of the mag chuck. However, when oriented at 90 degrees to the chuck (which is the more convenient orientation), it has virtually no holding power at all (not entirely unexpected, I suppose).

I think I'll just build the jigs and be done with it.

Alan

I think those are poled wrong, Ted. The Lee Valley magnets are poled axially while I think the magnet in a mag chuck is poled crosswise.

Yes. A chuck that old will surely have an alnico magnet. If you remove the magnet you will lose some magnetism that can only be recovered by re-poling the whole works after reassembly.

Not easy to answer this one simply. The following may help (or perhaps leave you more comprehensively puzzled!).

Magnetisation of ferrous materials follows a hysteresis curve. This means that as you apply increasing magnetising force (ampere turns in a surrounding coil) flux density will steadily increase until limited by magnetic saturation.

If the magnetising force is then steadily decreased the flux density does not decrease along the same curve as when it was increasing but lags behind it retaining more flux than would be produced by the magnetising force when in the increasing direction.

When the magnetising force has reached zero, because of this lag, some flux density remains. The amount that is left depends on the type of ferrous material - a very small amount for tranformer iron - a large amount for permanent magnet material.

It also depends on the total magnetic circuit reluctance. In practical terms this almost wholly arises from the reluctance of the effective air gap that separates the N and S poles of the sample.

If this reluctance is small, a lot of flux density is retained. This the case of a permanent magnet assembly magnetised after final assembly with the airgap reducing pole pieces in place. If this airgap is now increased, flux density reduces as the magnet moves further down the decreasing limb of the hysteresis curve.

If the original much smaller air gap is restored the flux density returns on a recoil loop which lies between the increasing and decreasing magnetisation curves. The shape of this recoil loop depends on the type of magnet material. With the older tungsten/iron and Alnico steels only partial recovery occurs and there can be a substantial loss in performance. Ferrite and rare earth magnets have a more favourably shqaped recoil loop and can recover almost completely after being exposed to an excessive air gap.

Jim