surface finish and magnetic attraction

apparently surface finish has a noticeable impact on how well a magnet sticks. is there any data available on this? (should i polish the chuck on the grinder?)

dian said:

apparently surface finish has a noticeable impact on how well a magnet sticks. is there any data available on this? (should i polish the chuck on the grinder?)

Click to expand...

It's not surface finish so much as there being any air gap at all, no matter how thin between the magnetic pole and the magnetic item.

The only info I ever saw was on an electro magnet we use on our crane, it gives holding weight on cast, sawcut, milled, ground, ect.. Polishing your surface grinding chuck,the holding power difference would be nil, plus the odds of you making it "un-flat" are greater than the holding power you would gain.

Agree a mirror finish would not do a whole lot for holding and would be more difficult to grind..Still some top grinder hands try for best finish..

This should only be tried buy a very seasoned grinder hand who knows he/she can do that with not burning or even heating the grind.

I have an old orange (clay)wheel that would give a mirror finish. likely need wet and and a few hours to grind my chuck with that wheel.

Except for show-off it would do little good.

The reason that smooth mating surfaces are held more tightly by magnetic force than rough/non mating ones is twofold:-
1) The force depends on the rate of change of magnetic field strength with separation distance.
2) Steels typically have about 1000 times more field through them than air for a given level of excitation.

This leads to well mating surfaces having a higher field going through/between them than ones with a lower percentage of contact area. it also leads to them having a much sharper drop in magnetic field between them on separation. This results in the very strong force holding parts together that don't seem to be all that magnetic in terms of holding washers/iron filings etc.

One place this is used commercially is the thermocouple that sticks into the flame of a furnace's pilot light. When you hold the button down for a time after lighting the flame, you are forcing the ground mating faces of a solenoid together. Once the thermocouple is warmed up, it generates enough voltage and current to keep the solenoid closed, It can do this with only about 15mV and a couple of mA. Not a lot compared with a 110V solenoid, but it couldn't possibly move the solenoid armature, only hold it after it's been held closed.

We see this in transformer cores. A tape of magnetic material, usually silicon steel but there are a lot of others, is wound on a form and given an overnight anneal and the form is pushed out.

Its magnetic qualities can be tested by wrapping wire through it and excited with AC or a pulse with a ballistic galvanometer. Toroid coils are made that way, but most cores are cut in half with an abrasive wheel, the pre-made windings slipped on the legs, and the core pieces strapped together. This is a relatively inexpensive way to make a transformer, but they never come up to the theoretical performance. I have assembled transformers usiing spiral hose clamps and watched the permeability increase as I screwed it tighter and tighter, but it never gets up to the max possible. Lapping the cut faces together helps. Some high end manufacturers make a rectangular tape wound core starting with a narrow strip which gradually gets wider, then narrower at the end, making a more or less round cross section. They make a spool of two halves that snap together with gear teeth on an end. After winding and annealing, they install the spool and wind the wire with it spinning on the core. They get good performance, but at a greatly increased cost. People also make distributed gap cores by wrapping short lengths of strip with the ends overlapping so the transfer of flux from strip to strip of a perhaps .012" thick strip is spread over several inches, keeping the flux at any one spot low. Of course, flexing the strip inevitably causes some plastic deformation, deteriorating magnetic qualities and the lapped joints are not quite the same as a solid connection. As in every endeavor, TANSTAAFL rules.

Re polishing your chuck, it will help, but if it is holding a ground piece, it is resting on peaks and valleys. If you polish the part flat and smooth, there will be some improvement, but it will be controlled by a lot of other factors such as the saturation flux of the part. If the chuck already puts as much flux through the part as it can carry, there will be no improvement.

Bill

PS Mark, thanks for the explanation of furnace thermocouples. I always wondered about that.

OT:
I once had some stellite thin parts that would not grind, stellite is a non mag aloy.
Double back would have worked but I didn't have any,
So I put a little milk under the parts blocked them in and they ground Ok.

Reason I tried the milk was becaue I was thinking Elmers glue but didn't have any.

here is some data i found on permanent magnets: 20µ air gap = 90% of max. holding force, 100µ gap = 60%.

Yes improving surface finish will decrease the air gap - better surface finish means lower peaks and valleys. But don't lose sight of the macro to improve the micro - in other words, be careful not to cause local high and low spots when polishing or you'll find yourself with a net loss of holding power rather than a gain.

It would be unwise to use a buffer for instance. Lapping would be the way to go if you need to do this. I would absolutely keep in mind the comment above from Bill about the workpiece too though. Unless both surfaces are brought to the finer surface finish you probably won't see much gain.

Another thing to consider is the coefficient of friction. Get these surfaces too smooth and your workpiece may start slipping and sliding around much easier. I like a fairly coarse chuck grind myself. Holds better in my experience. For grinding that resistance to slipping is more important to me than absolute max holding power.

Also keep in mind the specs you listed. 20 microns is almost a thousandth of an inch. Most ground parts are already WAY flatter than that. So your gain might be from 95% holding power to 98% holding power by doing this... Worth it? Not for me.

How flat is the magnet though? Doesn't help much to make your steel super-flat and polished if your magnet is not.

Regards.

Mike

The science of friction and things rubbing against each other is called "tribology". It is very complex, spilling over into many other fields. The short description is that electrons from one object link to another piece and require energy to pull them apart. I have a book by a couple of Russians who studied it in relation to machining. They insulated the bit from the workpiece and found that they could read an electric current between them generated by the friction of cutting.

Wiki has a good section on the subject-

Tribology - Wikipedia

Well worth reading.

Bill

Hold some 123 blocks together and pull them apart. That force is not magnetism, it is atomic forces, like gravity because the atoms are almost touching if both faces are perfectly flat and polished. I have no idea how flat they have to be for the forces to be noticable.
Bill D

Bill D said:

Hold some 123 blocks together and pull them apart. That force is not magnetism, it is atomic forces, like gravity because the atoms are almost touching if both faces are perfectly flat and polished. I have no idea how flat they have to be for the forces to be noticable.
Bill D

Click to expand...

Do it in a vacuum see if it behaves the same. :-)
...lewie...

dian said:

apparently surface finish has a noticeable impact on how well a magnet sticks. is there any data available on this? (should i polish the chuck on the grinder?)

Click to expand...

Dropbox - Eclipse Permanent Magnet Chuck Users Handbook.pdf - Simplify your life

Section 1

Lewie said:

Do it in a vacuum see if it behaves the same. :-)
...lewie...

Click to expand...

Will Gauge Blocks Stick In Vacuum? - YouTube