Fixing a Permanent Magnetic Chuck for the Surface Grinder

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I have been looking around for a cheap magnetic chuck, (or power supply for the one I’ve already got) so that I could put my surface grinder to use. I ran across this one at the CAMS Yard Sale really cheap, and bought it to play around with. Its a permanent magnet type, and I was told it was a little weak. There is an Allen bolt on the side which operates a cam that releases the magnet. The action of the cam was stiff, and the mag didn’t seem to release. I figured it couldn’t be all that hard to fix.

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This is a shot of the front of the chuck, with the mag release on the right.


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Heres a shot of the bottom. There are 10 allen head bolts holding the cast iron bottom to the segmented top. Time to get a look at the inside.


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Usually, once I get something apart I can figure out how it operates. This time is different. The cast iron base is insulated by the segmented (magnetic) top by a non-magnetic strip (stainless steel by the looks of it). The allen bolt in the side has a cam on the end that rides in a trunion that is bolted to the top magnet. This is the confusing part; the top is securely bolted to the bottom, so the cam SHOULDN’T be able to rotate at all. The solid part that the trunion bolts to appears to be part of the top magnet, but something tells me that this must be a moveable part (I just can’t get it to move with the chuck apart). I pulled the brass strips off the solid section, and it has copper strips that correspond to the strips on the top. I’m thinking this “solid section” is held tight when  the magnet is energized, and releases when the segments are slid out of position when the cam rotates the trunion to one side or another.


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The cam shaft and the trunion. With the base bolted to the top, the only part that I can see moving is the part that the trunion is bolted to.


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This shot shows the “stop plate” that keeps the allen bolt from rotating 360 degrees.  This would limit the cam rotation, so that the bolt would rotate to “ON” and “OFF” only. Its worn out, so there is no way to know where the cam is positioned. I’ll have to make a new one.


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Heres a close up of the stop plate, with the wear.


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There was a thin piece of shim stock under the stop plate. Looks like I’m not the first one to have a go at this chuck.


My first plan of action is to get this thing cleaned up and see if I can find any info on how these chucks work. Unfortunately, there are no manufacturer’s markings on it, so I don’t know how much I’ll find. I think I’m kinda on my own with this one. Any help will be appreciated. If anybody knows anything about these chucks, please let me know.


Tomorrow, I clean……………


I finally got some time to work on this chuck again, and pretty much finished it up. After I cleaned and painted the parts, it pretty much sat for a while.


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I found some nice hard stainless about the right thickness, and hacked out a stop plate for the shift-shaft for the magnet, and whipped one out on the mill.


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Here it is installed on the chuck body (yes, thats a crack right on the shift shaft bore). If I was real picky, I would have made a thicker one, but just shimmed this one with a piece of a razor blade (it was pretty close, anyway).


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This is the trunnion block (attached to the segmented top) that the shift shaft rides in. When the eccentric on the shift shaft is rotated, it moves the plate, and breaks the magnetic field (and releases the magnet).


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Time to bolt all the parts back together. Once I had it all tightened down, I scraped the paint drips off the bottom so it would fit flat on the grinder table. I might just give the bottom a light grind if its not flat.


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It tested out pretty good. I couldn’t get it to release before, and now it does. The action seems a bit smoother (not dramatically), but it DOES release. The round block is just a drop off the last project, and is squared on the bottom end, so it is pretty flat and grabs the chuck tight. Once released, it comes off with only a little effort. I don’t think these chucks ever REALLY release, but the residual magnetism is very slight.


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I decided to treat the top with a little Evapo-Rust to get some of the rust off, before grinding. I would guess the rust would clog a fine stone pretty quickly. I’m going to let the thing soak overnight, and get a look at it. Theres still some more work to do, but I got another hitch in my gitty-up as a result of a visit I had from another CAMS member the other day.


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Tim Boan stopped by the other day to look at a band saw I had for sale. We got talking about surface grinders, and as he was walking out, he spotted my “old” chuck, (thats it in the pic above) and asked if it was for sale. I told him the story about how I got the grinder with everything BUT the power supply for the chuck. Now, anyone that knows me knows I have NO electrical/electronic skills AT ALL. Any success I have had are usually preceded by sparks and magic smoke. Thats how I test stuff. Not really to code. I told him about the trials and tribulations of trying to get the chuck operational.

This is when Tim says: “Hey, I used to be an Electrical Engineer, want me to look at it?”……………………………Uhhhhhh, Sure!  Well it took Tim about five minutes to get the thing working, and make a few suggestions as to how to get it working better. I have to dig up a few parts yet, and come up with some kind of enclosure for the controls, but I’m pretty excited to have it working (REALLY working). Tim explained a few things about how they operate, and made things a whole lot clearer to me. The lack of a working chuck was the only thing that was holding me up from using the grinder. I’ll be making some time to get that squared away ASAP. I’m now in the envyable position of having two magnetic chucks for my grinder. I suppose that the permanent chuck with be used for bench work, and the electric will live on the K O LEE.  Thanks very much, Tim. Thats one less hurdle for me to clear.


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Well, it cleaned up pretty good. Once I get a few other projects out of the way, I’ll drop it on the grinder and see how it goes. More later…………

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