magnetic chuck problem

Gang, I have sent some pictures to the dropbox to reference a problem I'm having where I work. We use a grinder to "flatten" the cutting edges
of 4-winged knives used in a meat grinder (to make hamburger). The machine being used has a 7" circular magnetic chuck, 90Vdc with concentric rings. The knife is placed in the center of the round chuck with the cutting edges up, and the stone (8" x 3") is spinning and lowered down to flatten the cutting edges (some of which are stellite) and then the knife is taken to another grinder to "relieve" the flattened portion to product a cutting edge which will press flat against a circular disk with holes in it for the hamburger to press through. The problem we are having is that there is not enough metal on the backside of the knife for the mag chuck to get a good grip onto - and the knife will sometimes slide away from the center of the mag chuck. This causes problems with the grind - and also sometimes the knife will just fling off the chuck and hit the side wall of the grinder. As the center of the mag chuck is "probably" the weakest point of the mag chuck (which is fed the 90vdc through slip rings under the coolant box) I was wondering if a round magnetic chuck with parallel laminations would be stronger magnetically than what we currently are using. I have cleaned the slip rings, checked the voltages and the brushes and everything is about as good as I can get it. Many times we will grind the back side of the knife to get a better magnetic grip on the knife. A gig such as a plate with a square post sticking up would probably also work, but there are about 6 different sizes and we would have to make/have made 6 different "post plates" - we are trying to avoid having to do that.... Any suggestions? The links to the pictures in the dropbox are: http://www.metalworking.com/dropbox/magchuck.txt
http://www.metalworking.com/dropbox/magchuck1.jpg
http://www.metalworking.com/dropbox/magchuck2.jpg
http://www.metalworking.com/dropbox/magchuck3.jpg
Thanks for any suggestions. Ken.
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On Sat, 30 Aug 2003 02:10:54 GMT, snipped-for-privacy@usaor.net (Kenneth W. Sterling) wrote:

Couple of things come to mind, Ken. The first is that it appears the squarish base of the cutter doesn't quite reach to the concentric pole immediately outside the center pole, except at the very tips. You've just thrown away a good deal of your holding power with an air gap. A conventional rectangular magnetic chuck with closely spaced alternating parallel poles would most likely straddle more of the base. If you are stuck with the circular chuck, then quarter circle pole pieces made out of soft iron to extend the annular ring magnetic flux up into the body of the cutter and thence to the central pole would be helpful - would also provide an anti- torque arm to resist the grinding wheel torque. The only touchy thing would be in making the base of each quarter section the same thickness as the annular ring it abuts or you'll lose some of the flux to leakage to the other two poles on each side.
Best wishes, Mike
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On Sat, 30 Aug 2003 02:10:54 GMT, snipped-for-privacy@usaor.net (Kenneth W. Sterling) wrote:

You are right - a round magnetic chuck with parallel pole pieces would produce more holding power on your workpiece setup but the difference may not be large enough to solve your problem. The main immobilising force on the magnetic chuck is the mechanical friction between the workpiece surface and the chuck surface. The magnetic field force is almost all downward with little sideways component so it doesn't matter a lot whether you try to break away in the direction along a pole piece or at rightangles to it.
However steels have a tendency to remember and resist any change in the flux pattern through them (hysteresis). Because of this effect, the breakaway force required to move a steel workpiece can be slightly higher at rightangles to a pole piece than when trying to move it along a pole piece. It's easily checked by trying to move a small sample of the workpiece material firstly along and then at right angles to the pole piece. The sample always bridges the pole piece gap of course. We're not talking about actual movement but the point where stick/slip friction lets go.
We don't know whether the chuck is succeeding in magnetically saturating the workpiece. Up to the point where saturation is reached the down force is proportional to field strength squared. A 10% increase in chuck voltage could possibly produce a 20% increase in down force. It's easily checked by feeding the chuck from a variac or auto transformer. It would also produce a 20% increase in the chuck winding temperature rise so long term increase could be limited by overheating. However if it does produce a useful increase it shows that the chuck is not really doing it's job. A new chuck using modern magnet steels might produce the extra flux you need.
Another attack is to modify the grinding process to reduce the breakaway forces - for example move the chuck sideways and continuously slowly rotate it so that the four arms are ground sequentially instead of simultaneously.
You mentioned that a square post jig could solve the problem but was not really practicable because of the varied sizes of workpiece. A fairly simple kludge would be to produce four slugs of mild steel which are dropped on the chuck face anywhere between the four wings of the workpiece not necessarily touching them. As soon as the grinding forces start to move the workpiece it will come hard up against one or more of the slugs and remain safely anchored there for completion of the grinding process.
Jim
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The problem probably isn't with your chuck. You are trying to grip too small of a suface area. I would think you are better off making a mechanical fixture to hold the cutter, something with a square post and a bolt/washer.
--
Tony

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