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 a
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.

On Sat, 30 Aug 2003 02:10:54 GMT, (Kenneth W. Sterling)
wrote:

Gang,
I have sent some pictures to the dropbox to reference a problem I'm
having where I work.
The links to the pictures in the dropbox a
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.

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

On Sat, 30 Aug 2003 02:10:54 GMT, (Kenneth W. Sterling)
wrote:

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 a
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.

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

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

Visit TonysToolroom for info on Precision Scraping.
***Now with Powerscraping***

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"Kenneth W. Sterling" wrote in message
...
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 a
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.