Magnetic Chucks Revisited

Impressive results.

I have a few of these kicking around - might give it a try.

Regards, Tony

Cool! My mill's table is so small (1 T slot), that your vise would be great.

In the previous thread

How do your eccentrics engage the backing plate?

You sawed & filed the magnet's original mounting plates to be thinner. Is that necessary? IOW, what is the purpose of that?

The chuck in the video had pieces to prevent x,y motion - is yours strong enough to not need them?

Could you send this to the DropBox, so it will be available for "all time"? Stuff loaded to places like Photobucket tends to get deleted after a while.

Thanks! Bob

You would break them apart if you wanted to build a more conventional design with a sliding magnet assembly, as opposed to this design where the magnet assembly is simply moved away from the top plate. The magnets are effectively horseshoe magnets with very short arms, which is just right for this approach.

I believe that the eccentrics merely hold the magnet plate away from the top plate - the magetic attraction is sufficient that the magnet plate doesn't fall to the bottom. The brass pads at either end serve as bearing surfaces and also help to keep the magnet plate centered.

Smaller, not thinner, if I read his post correctly. The point is that you want to be able to mount the magnets as close to each other as possible, but you still need enough of the original backing plate to allow you to attach them.

Regards, Tony

The magnets do not need to be broken into two pieces because,this design with its mild steel magnet mounting plate and top surface pole pieces, properly utilises both the NS and the SN magnetised areas . Mild steel can be regarded as an extremely low resistance magnetic conductor so this configuration turns the flat HDD magnet into a short powerful horshoe magnet.

The eccentrics are not used as in the steam engine valve gear sense but as cylindrical cams. The magnet assembly is always strongly attracted to the top pole pieces and considerable force is needed to pull it away. This is provided by the cylindrical cams bearing on L shaped brass extensions at each end of the magnet assembly.

As rescued from a HDD the magnets are bonded to odd shaped backing pieces which locate them with the required gap. The full thickness of the backing piece behind the magnet is still required but the outer parts should be cut away to permit the modified magnets to be mounted as close together as possible on the mild steel mounting plate. The standard backing piece is not thick enough to carry the maximum flux of this design and it needs the extra thickness of the 1/8" mild steel mounting plate. This is easily demonstrated. As removed from a HDD the backing is inert - it doesn't behave as a magnet. If the magnetic circuit is now closed by a lump of mild steel in contact with the magnet face the backing piece saturates and N and S poles appear on the back surface.

The hold down force in this design is nearly strong as in professional chucks so stops are not normally needed for light grinding (0.0005" downfeed). Very thin workpieces and even light milling require stops of some sort. Surface grinder magnetic chucks usually have an end plate that can be lowered or raised to act as a stop. It's usually quicker to backup any problem workpiece by a much larger thinner piece(s) of mild steel spanning several pole pieces.

Posted in the DropBox under the heading Magnetic Chuck

Jim

OK

Alright - I was thinking that the assembly had to be pushed both ways. I can see the brass bearing plates in the 2nd pic, MagnetAssysideview

OK, I don't know why I was thinking thickness removal (that wouldn't have been an easy job).

Ah - *surface grinding*. The video showed the chuck being used on a mill & that's what I thought yours was for. And what I was planning on. Would it be useful on a mill, with x/y stops? Or, would it be the case that, even with the stops, you would be limited to light milling?

Thanks

Bob

x/y stops would be needed for milling. In this case increase the thickness of the mild steel pole pieces to 1/4"and provide as many tapped holes as is convenient to secure x/y stops.

Jim