Magnets

I don't care for using magnets in molds. For the most part its unnecessary.
Once you get a feel for the mold it gets to be pretty easy to slide a hook
or a keeper into its slot. Even if you have to wear glasses like I have had
to for the last decade.
Sometimes though the only way to get a piece of hardware to stay in place is
with a magnet. This leads me to why I felt the need to post this. I
learned recently that rare earth magnets have a failing. Above a certain
temperature they loose their magnetism and that temperature is much lower
than you might think. A little higher and they lose their magnetism
permanently. These temps are below the melting temperature of lead. Now a
mold rarely gets as hot as lead. Even if you set it on top of the lead pot
to preheat, but it does get pretty hot. Hot enough that you have to wear
gloves to handle it safely. Maybe closer than you might like to
degmagnetizing temperature of the magnet.
I didn't research other magnets as I like the strong holding force of even
tiny rare earth (neodymium) magnets for these problematic applications. I
discovered that for a little more money there were higher heat rated rare
earth magnets. I check a couple sources. MSC didn't have any in a size I
wanted. McMaster had one size that was useful. K&J had them much cheaper
than McMaster, but they were further away and only shipped UPS. I ordered
from both McMaster and K&J Magnetics.
I actually had two molds that I couldn't come up with any other solution
for. The first was done with the McMaster magnets and like other magnets I
have ordered they were a little under their specified dimensions. They also
did not seem to have a hard coating like most rare earth mags I have bought
in the past. I just glued them in place with JB weld. The K&J magnets
came in a few days later, and I miced them. I took measurements on 2 or 3
out of each pack. They were all within 0.0005" of target diameter, and
about the same for target length. They also had the nice shiny nickel
coating I am used to seeing. I machined tapered holes (.001 over at the
top to .001 under at the bottom) to press the magnets into and they fit
perfectly. I expect if you rapped the mold sharply on the bench you could
knock them free, but neodymium magnets are pretty fragile. I didn't want to
break them when pressing them in.
Another thing learned. I hope it helps somebody else.
Reply to
Bob La Londe
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Neodymium rare-earth magnets, the common type, can't tolerate much heat. Normally, their max working temperature is in the range of 80 deg C - 120 deg C, with the latter being types designed specifically to take the heat (I think they have cobalt in them, or maybe something else).
In military applications and all sorts of higher-temp applications, you'll find that the samarium-cobalt rare-earth magnets are more commonly used. They can handle up to 350 deg C.
And good ol' ferrite magnets are usually good for 250 deg C. They, too, are used instead of Nd magnets where higher temperature tolerance is called for. Their strength is a lot lower but they're also a lot cheaper.
Reply to
Ed Huntress
If you were desperate enough you could run a piece of steel out the side and slip a solenoid coil over it. -jsw
Reply to
Jim Wilkins
If you were desperate enough you could run a piece of steel out the side and slip a solenoid coil over it. -jsw
*** My first reaction was literally to laugh out loud, but of course you are right. I had this visualization of some poor tackle maker wondering why his jig mold looked like a miniature Russian EMP generator and why in the heck he would have to plug it in to the wall. LOL.
Reply to
Bob La Londe
That definitely hit me in the same way, Bob. Reading the pic you painted right afterward sent me directly to the tissues.
Happy Halloween, you culturally appropriating racists.
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Reply to
Larry Jaques
Stop disrespecting undead-Americans and traditional Celtic religion (witches) too.
Reply to
Jim Wilkins
I once designed and built a custom four place magnetic lab stirrer with electrically heated aluminum jackets for a sample workup. I needed the samples at 200 C with uniform temperature.
Due to the temperature, I went with samarium cobalt magnets for the drivers. I recall them being relatively inexpensive. They were fragile. I mounted them almost entirely enclosed within aluminum mounts.
Worked great through the thin ss baseplate and ss sample vessels.
Pete Keillor
Reply to
Pete Keillor
Speaking of the undead, I rented The Mummy and was aghast to find that it was somewhat of a zombie movie, with not a single bare anything on the gorgeous mummy queen goddess. Output from Hollywood has gotten more and more dull as they go PC.
Reply to
Larry Jaques
Ya gotta check those ratings first. It was only PG-13...
For a low budget hoot check out this trailer for Machete:
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There was a sequel too, haven't gotten to it yet ;-)
Reply to
Leon Fisk
Egyptian queens don't age well:
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Reply to
Jim Wilkins
I saw the original, and it was a hoot. Bloody to the extreme, but some lovely ladies wore nothing. The sequel trailer looked awful so I passed on it.
Reply to
Larry Jaques
Darned if you're not right.
Reply to
Larry Jaques

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