View Full Version : Magnetics and hardened machine surfaces

11-08-2004, 03:27 PM
Every time I stick a magnetic base or a powerful "mighty-mag" on a machine surface, I worry about magnetizing a "region" of the hardened surface and attracting steel swarf. I haven't seen it happen yet.

Has anyone seen long term problems with this on larger surfaces? It's a PITA on tools and I run a demagnetizer on them.

11-08-2004, 03:28 PM
Just the other day I was worrying the same thing when I was using my mag base on my south bend. So far I havent noticed any swarf sticking there.

11-08-2004, 03:47 PM
I may be mistaken, but I was under the impression that you couldn't impart magnetic properties to just a "part" of a composite ferrous model - but, the magnetic moment along a line of flux can vary accoring to the composition of the material.

OK...Now we need a professor to sort this out.

11-08-2004, 06:48 PM
I had a similar impression but recalled that magnetic recording, although it certainly uses optimized materials, is doing just that.

Just finished a few experiments while waiting for all the replies to come in. My two magnetic field testers consist of a small compass and a 10 penny nail hanging from 6" of string.

On the rear way of the lathe (hardened ways), I traveled with the compass and noticed some strong points of varying polarity. I demagnetized this same length and verified that the field was gone. Then, I put a magnetic base on that way and turned it on and off a few times. Sure enough, that region took on a north and south pole visible as you pass the compass along it. The two strongest points were enough to hold the 10 penny nail as the string was moved away about 10 degrees from vertical.

The significance of this is that fine steel swarf should certainly be held even better than a nail of that size. It will hang under the ways or cling to surfaces where you don't normally brush or wipe. Stuff that is visible cleans right off but it's the hidden stuff that concerns me.

Incidentally, the cross slide exhibited the same (and even stronger) results when the magnetic base was clamped to it with one edge of the base aligned with the edge of the cross slide. Makes sense since you are closing the magnetic field near that edge.

Not gonna lose any sleep over the phenomena but needless to say, I ran the demagnetizer up and down that area as well as the cross slide as soon as the experiment was over http://bbs.homeshopmachinist.net//wink.gif


[This message has been edited by nheng (edited 11-08-2004).]

11-08-2004, 07:36 PM
Now that you mention it, I've noticed some extra "fuzz". My tool demagnetizer is too large and heavy to try and use. Do you think one of those old TV degauzing coils would work? I've got one of those somewhere.

11-08-2004, 09:27 PM
A sharp whack with a hammer (I guess rubber mallet for the ways) can demagnetize sometimes. Apparently the whack can be enough to change the orientation of the atoms.

I guess I should be careful about orienting a lathe north-south, huh? Might wind up magnetizing it that way :-D

11-08-2004, 10:15 PM
If that were so, then Evan might need to worry about that Aurora http://bbs.homeshopmachinist.net//biggrin.gif

I picked up a VHS tape eraser on closeout at Radio Shack for $19.95. It has a pretty strong field but a whacky 10 second on, 3 minutes off (or something like that) to allow it to cool. It does work well on tooling, qctp holders, etc. but if the thermal switch cuts off while you're using it, all your tools face north http://bbs.homeshopmachinist.net//biggrin.gif

11-08-2004, 10:37 PM

11-08-2004, 10:37 PM
Darn! Another double post. Sorry.

[This message has been edited by Arcane (edited 11-08-2004).]

11-08-2004, 10:47 PM
I saw somewhere,sometime that an old electric hair trimmer can be used as a de- magnetizer. The way I remember it the blades are removed and the moving magnet is exposed . Does that sound right?? My workshop de-mag is also too large and heavy for mobile use. I`ll see if I can dig out the web site .

11-08-2004, 10:55 PM
Ken, It may work well for small stuff. I made one out of an $8 aquarium air pump. It had a permanent magnet on a rubber bellows, swinging over an open magnetic core. Yanked the core out and insulated the wiring. It runs cool but the field is strong enough to demag small tools (screwdrivers, scribers, etc).

Paul Alciatore
11-08-2004, 11:02 PM
Yes, it is definitely possible to magnetize a small part of a larger object. Remember the old wire recorders that used a steel wire instead of tape. Regions of almost any size that we work with can be magnetized or not in a larger object. Some industrial magnets are deliberately made with many poles.

Yes, a sharp blow can demagnetize some metals or even magnetize them if they are placed in a strong magnetic field while being struck. Some of the first magnets were made this way (with natural lodestones providing the field) But this is not 100% on the first blow. Depending on the alloy, it may take several or even many blows to do even a half way job of demagnetizing. I would never suggest doing that to a precision machine. Not a good idea.

The videotape erasers are probably the best way to demagnetize. Professional models that are made for larger sizes to tape would be best. One thing to remember when using them is that they must be switched on at a distance of a few feet away and brought to the object and slowly moved over the entire area to be demagnetized. Then they MUST be withdrawn slowly to a distance of a few feet BEFORE turning them off. Failure to do it thus may lead to the object being magnetized instead of demagnetized.

Now I gotta go check my lathe and mill/drill.

Paul A.

11-09-2004, 01:41 AM
Magnetizable materials have a property of resisting the influence of a magnetic field to some extent. It takes a certain minimum strength to magnetize them, and to demagnetize. If the magnetic field is applied without rapping on the material, then it won't magnetize as much, but if you rap the magnet or part of the magnetic structure on it, more of the field will remain when you take the magnet away. The better of the transformer core materials will magnetize very easily, can be magnetized to a high degree, but won't retain the magnetism once the current is removed. Lathe beds, etc, are made with material that basically has the opposite of these properties, that is, they won't magnetize that easily, but will retain some magnetism for a time. It will fade since the material isn't hard enough to resist the magnetic domains randomizing again.
I imagine if one were to bang a magnetic base onto the bed, that it would leave a temporary field behind. Even then, I doubt it would remain for long.