Magnetic Chucks Revisited

A little while back there was interest in the miniature magnetic chuck shown on youtube

http://www.youtube.com/user/tryally#p/u/30/avUkGTNqUtk
.
This chuck used three sets of triple stacked computer HDD magnets. Unfortunately the magnetic design is not optimum so it can only achieve a fraction of the hold down power ofprofessional magnetic chucks. However the idea seemed worth following up. The results are shown in
http://i801.photobucket.com/albums/yy291/pentagrid/MagbeticChuck.jpg
http://i801.photobucket.com/albums/yy291/pentagrid/MagnetAssysideview.jpg
http://i801.photobucket.com/albums/yy291/pentagrid/Spacerlayout.jpg
This again uses HDD magnets, only two pairs this time but now arranged to achieve a properly closed magnetic circuit.This results in a big increase in hold down force.
With a 22mm diameter 1/2 " thick mild steel test piece the results were;-
youtube triple magnet design with 2mm thick top plate 4Kg
New design 17Kg
Eclipse 18"x 6" Chuck 20Kg
The JPGs are largely self explanatory but a few points may help.
The unwanted parts of the original HDD magnet mounting are sawn and filed away leaving just enough meat to bolt through to the 1/8" mild steel mounting plate.. This plate is part of the magnetic circuit and must be at least 1/8" thick.. Brass end pieces bear on the eccentrics and also locate the magnet assembly centrally within the chuck body
The chuck top layout depends on the actual magnet size. A 5/32" wide brass spacer mounts acros the center line of the chuck. The centre line between the two sets of magnets lies directly under this spacer.Two more 5/32"spacers are added, each directly above the centre line of the arc of the magnet pair that it covers. 1/8" thick mild steel pole pieces are then machined to the correct widths to fit in the empty spaces.
To deal with the larger "magnet switch off" forces the youtube cranks are replaced by a pair of 7/16" dia 3/16 offset eccentrics running on 1/4" bearings, To keep the overal height low these eccentrics are located beyond each end of the magnet assy The eccentrics move 180deg for full "on" to full "off.". magnetic forces retain them in either the "off" or "on"positions. "full off" is a misnomer. The chuck is still partly on but with reduced downforce for workpiece removal.
Jim
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On Mon, 14 Dec 2009 16:50:40 +0000, snipped-for-privacy@yahoo.com wrote:

Impressive results.
I have a few of these kicking around - might give it a try.
Regards, Tony
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Cool! My mill's table is so small (1 T slot), that your vise would be great.
In the previous thread (http://tinyurl.com/ydxaobn ), you said that hdd magnets were really pairs with opposing poles (NS-SN) & to break them apart. But you haven't done that?
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
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On Mon, 14 Dec 2009 22:34:39 -0500, Bob Engelhardt

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
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On Mon, 14 Dec 2009 22:34:39 -0500, Bob Engelhardt

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
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snipped-for-privacy@yahoo.com wrote:

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
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On Tue, 15 Dec 2009 12:52:18 -0500, Bob Engelhardt

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
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