mark76 wrote:


I have three thoughts about this, but don't know if any of them would work:

(1) Duct tape is the solution to everything?
Tape up your dirty magnet, and then peel it off... Hopefully the filings stick more to the adhesive in duct tape than they are attracted by the magnet. If it works great. But the worst case scenario is that things get worse when the duct tape leaves adhesive residue on the magnet.... But presumably you can clean that off with Goo Gone or other solvent.

(2) Hot glue?
Same idea as (1). Put on hot glue, let it cool and solidify. Then hopefully it peels off with the metal shavings.

(3) In the future, maybe keep the magnet in a Zip-Lock bag, or wrapped in aluminum foil? Then to clean, just replace the bag and/or foil.

Oh, if that super-magnet needs to leave your house, maybe it can visit my house?

Sincerely,
--Lagrangian

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"What grit sharpens the mind?"
--Zen Sharpening Koan

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P.S. This is both semi-theoretical and semi-practical: many situations involve competing effects of volume to surface area:

(1) The metal particles have an attraction to the magnet which is proportional to their volume.
(Basically true if the particles are close to the magnet, and much smaller than the magnet so that the volume of the particle covers a region of uniform magnetic field.)

(2) The metal particles have an adhesion to the duct-tape/hot-glue which is proportional to their surface area.
(Assuming that if the surface makes good and complete contact with the tape/hot-glue, and assuming that the surface physics of the particles does not change when they very (very, very) tiny, then this is true).)

So the smaller the metal particles, the easier they will be to clean off the magnet surface:
Total Force = c1*V - c2*A

where V is the volume of the particle, and A is the surface area of the particle. The c1 and c2 are constants involving how strong the magnet is, and how sticky the tape/glue is for the metal. I'm using the convention where a positive force sticks the particle to the magnet, and a negative force pulls the particle away from the magnet.

So if the particles are (more or less) spherical we get:
Total Force = k1*(r^3) - k2*(r^2)

where r is the diameter of the spherical particle. The k1 and k2 are constants, and I had them adsorb the constants c1 and c2, as well as constant factors for a sphere's volume and area (ie: sphere volume = (4/3)*pi*(r^3), sphere area = 4*pi*(r^2)).

Finally we notice this: For a tiny number r, its cube (r^3) is smaller than its square (r^2). As r becomes smaller and smaller, the attractive force to the magnet gets small faster than the adhesion to tape/glue. So we reach our final conclusion (given the simplifying assumptions mentioned above):

For removing metal particles from a magnet by tape/glue:
it is easier to remove small particles than big ones.

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In physics, chemistry, and engineering, we very often see effects like this. We call it,"Surface Area to Volume Ratio", "Scaling Laws" (how do things grow/shrink when you change the size r?), and sometimes "Dimensional Analysis" (similar to unit analysis: area versus volume).

The canonical example of surface-area-to-volume ratio are dust explosions (such as in wheat grain silos): Materials burn on their surface. The more surface area a pound of material has, the faster it can burn. The energy released is proportional to the total amount of material (ie: total volume and/or total weight). Particles of powder have surface area proportional to r^2, and volume proportional to r^3. So if you work it out, the surface-area-to-volume ratio of a powder is insanely huge (surface area per pound of material). So if the powder is mixed in air, it can burn insanely quickly. And that means a low-level explosion (not technically a detonation, but a very fast conflagration). After many grain-silos blew up, they started monitoring silo temperature, humidity, and amount of grain-dust.

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Surface-area-to-volume ratio also affects liquid abrasive sprays, such as those used on strops for knife sharpening. In these liquid sprays, you have tiny (sub-micron) particles of abrasive, such as mono-crystaline diamond, poly-crystaline diamond, boron carbide, cubic boron nitride, aluminum oxide, etc. These tiny particles sometimes like to stick together and/or settle out of solution. I won't go into all the details because I'm not a chemist. But you can see how it could matter: particles stick together with their surfaces (surface area), and part of why they settle out of solution is their weight, which is proportional to their volume. If the particles stick together strongly enough, they can, in effect, form a larger grit particle. This is called "particle aggregation". So people like Ken Schwartz optimize their abrasive sprays to minimize particle aggregation by changing particle size, as well as concentration of various additives that affect surface-attraction and viscosity. Many of these effects are not important for coarse grits, but as the grit gets finer and finer (sub-micron), they begin to matter more and more.

I don't know (not a chemist), but believe this is not significant factor in wax-based stropping compounds (because there the particles probably like to stick to wax very much, possibly more than they like to stick to each other, and the particles cannot easily move around in the solid wax). But please don't shoot me if you talk to a real chemist, and he shows me to be wrong.

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The overall point isn't whether the effects surface-area-to-volume ratio are absolutely correct in complete detail for all the situations described. The point is that surface-to-volume ratio is just another tool for how to understand things. Just like energy and force. Depending on the situation, one tool may be better than another.

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I didn't mean to give a mini-tutorial on surface-area-to-volume ratio; but since I did, I think I'll just leave it be. I love being a physics major.

ROFL!!

Hahahah!

I never thought I'd learn so much about magnetism on a forum about knife sharpening!

The magnet has left the house already. It was too dangerous for the electronics and other stuff. When I made the photograph of the magnet, I was about 50 cm removed from it, and still it drew sparks from the flash light! I didn't dare to get it in a radius of 1 meter from a computer, a phone or even my stereo.

It was quite cheap, however. I bought it here. That shop is in Germany, but I imagine it is sold for similar prices in the US.

So I cannot try any new tricks to remove metal filings. What I did try was the duct tape (3M cellotape), which had too little power . The magnet came in a small plastic bag and if I removed the bag, the metal filings simply stayed where they were relative to the magnet. That is, they ended up on top of the magnet .

What probably would have worked was to pack the magnet in a larger plastic bag, use it and then turn the bag inside-out before removing it from the magnet.

I now simply do what I always did to prevent metal filings from getting into the pivot of a knife and to get the WEPS clean: a wet cloth and, if necessary, some tape over the pivot.

But please get a magnet and experiment more! (The shop has a 160 kilogram magnet on sale tight now ) And please do found all your findings theoretically. Love it!

Wow, actually, the fact that it drew sparks from your LED flashlight at 50cm...! That sounds scary. I own a lot of electronics, so I think my reaction would be the same as yours: Sell it away. ^_^;

Sincerely,
--Lagrangian

www.supermagnete.de/eng/project348

Removing iron filings from magnets from the German site.

I use some small but powerful magnets on the bottom of the rods on MY WEPS. They do attract very fine filings. I have to think that it is better on the magnet than in the rod pivots .
I can ususlly just rub the fine powder off with my fingers, or sometines a ScotchBrite pad and a brisk fast rubbing motion.

Phil

Hi PhilipPasteur: Thanks for that link
I didn't know that some of the rare earth magnets are sensitive to heat (it can destroy some of their magnetic alignment). That link you posted suggests warm wax kneaded in your hand first, because hot wax on the magnet directly would harm it! Pretty interesting

When experimenting with these strong magnets, guys, please be careful, don't hold a really strong magnet & try to remove shavings out of the pivot area. Imagine a small 20lb magnet pulling your finger into a razor sharp blade because you held it too close. Once you get the magnet close enough to steel it will yank it out of your grip before you know it. Now a 38kg magnet is 83lb's, even more dangerous. I also got a magnet stuck on a brand new blade & it was impossible to pull the magnet off & I had to slide it off the edge of the blade which made a very noticeable long scratch in my brand new just sharpened knife.
The best way to keep shavings out of your pivot while sharpening is to tape up the pivot area.

I was going to order some bigger magnets from that German site but surprisingly they do not ship to USA. They have almost every other country on their list except America.

Here is a place that ship to the US
www.kjmagnetics.com/products.asp?cat=10

They have about anything you would want in magnets. I have purchased from them several times.

Any magnet will lose its magnetic properties if you get it too hot. Heat scrambles the carefully aligned poles of the molecules that give it the force. Of course the Common N type of Neodymium magnets can get pretty warm without losing too much of their magnetism.

From the site above:
"If a magnet heated above its maximum operating temperature (176°F (80°C) for standard N grades) the magnet will permanently lose a fraction of its magnetic strength. If they are heated above their Curie temperature (590°F (310°C) for standard N grades), they will lose all of their magnetic properties."

If you go to the SH grades their max operating temp is 302ºF (150ºC). I have soldered wires to these to make balance charging harnesses for Lithium Ion batteries. I don't think warm to even hot (liquid) wax would bother them in small amounts and when removed quickly.

www.kjmagnetics.com/products.asp?cat=167

Phil

BTW these are what I am using for the WEPS:

www.kjmagnetics.com/proddetail.asp?prod=D34-N52

They are small, but at N52 pretty strong for their size. They will not fly to your knife blade from 6 inches away, and you could pull them directly off of the blade without sliding them, if they ever got attached. They just sit on my paddle rods right above the pivot and attract lots of very fine metal shavings.

Hi PhilipPasteur: Thanks for the info and those links!
I used to look at magnets from www.unitednuclear.com, but clearly the selection from www.kjmagnetics.com is much better.

But even if I do play with magnets, I still don't have a nice way to address the non-magnetic abrasive dust which comes off of the sharpening stone.

Sincerely,
--Lagrangian

I take the arms of periodically and soak them in hot water with dish washing liquid in it. I then blow the joints out with hot water from my sink sprayer. I have a tooth brush that I dedicated to joint cleaning, but after the soak and rinse I don't see any residue, so I have not had to brush out the joints with the brush. I then blow the water out of the joints with compressed air and let them sit on paper towels. I let them sit overnight if I am done sharpening for the day. I then spray them with WD-40, wipe them down with paper towels to remove the excess and lube the joints with "superlube". I wipe off any excess, and I am ready to use the arms.

This will get them pretty darn clean. How often you do this is directly proportional to the level of OCD that you are afflicted with

Depending on how much I sharpen, I go through this process once every week or so. If I was sharpening for 6 to 8 hours per day, I might do it every day.

Phil

That was AWESOME, thank you!