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press fit for a magnet...
2

press fit for a magnet...

press fit for a magnet...

(OP)
Howdy folks
I have a simple (but not so) problem. We are in production with a solution that is not ideal. We need to attach a small magnet (diam.100"+/- .002, cylindrical) to the end of a stainless rod (.050"+/-.000005, seriously).

the solution that the previous engineer went with was to machine a brass socket, where the rod presses in one end, and the magnet is glued into the other end. problem being the magnets are highly polished and dont adhere well, and that gluing in production is not ideal for several other reasons.

we want to press fit the magnets also, using plastic for the socket. the difficulty is that with the magnets tolerance, I am having trouble finding a plastic that will not yield at the worst case tolerances. we are not doing volumes that would justify molding these parts. of course, creep is also a concern.

so the question is: is there a machinable plastic I should look at that could handle ~.005" diametrical press fit at .100" while maintaining a hold on the magnet over time? UHMW looks good on paper, but I cant find any info on creep resistance, and knowing how "unstable" it is, I do not have warm fuzzies spec'ing it.

thanks!

RE: press fit for a magnet...

Just bouncing an ideal...
have you concidered an extended lenght, undersize brass socket and then deforming to expand?

RE: press fit for a magnet...

Before recommending a plastic we need to know a lot more about the environment. Temperature range, atmosphere, fluids, etc.

Some nylons when conditioned to 50% relative humidity have quite large elongations but they are going to relax if the temp is too high. A small amount of glass fill (~13%) will reduce the elongation but help with the high temp.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: press fit for a magnet...

Why a press fit?

Why not a snap fit?

Maybe a 2-piece plastic socket that is welded together to contain the magnet?

Maybe mold the magnet right into the socket?

RE: press fit for a magnet...

(OP)
no extreme conditions- it's bench-top equipment for labs. No autoclave or refrigerated applications. that said, it will need to survive in the tropics without AC (India, China it's common for labs and schools to not be equipped with AC). So warm humid air is likely.

I (think I) need a low modulus material to accommodate the wide range of press fit interference possibilities. plus low creep. the glass fill will accomplish the latter, but will it accommodate the former?

RE: press fit for a magnet...

(OP)
Tick, that woudl be great, but the magnet needs to protrude from the socket so that it had direct contact with a ferrous plate. we are trying to avoid molding as our volumes are not quite high enough. plus, it's a neodinium magnet- so max temp is around 175F before field strength takes a hit.

Kenat, good point, I am looking for it's COF now... may just test it insitu empirically. I just dont have time to let it sit for years to make sure creep performance is ok...

I just had a thought of using a soft "film" instead of the glue (teflon tape, parafilm etc) thus retaining the brass piece.

might be more labor than manufacturing wants to deal with tho...

will test after lunch!


RE: press fit for a magnet...

Why not counterbore the rod then heat up that end of the rod? With the rod end still hot, insert the magnet then let it cool. Will it crush the magnet when it cools I don't know.

Tunalover

RE: press fit for a magnet...

Plastics have large coefficients of expansion compared to metals, so the press fit will be low at high temps and high at low temps. You are looking for glass-filled acetals to get the range and strength.

Look at:
https://www.kjmagnetics.com/blog.asp?p=sticky-busi...
http://www.cyberbond1.com/adhesive-solutions/magne...

You need to make sure the thickness of the adhesive film is large enough to take the shear load from differential expansion.

The most likely adhesive problem is lack of cleaning, but there are some retaining compounds designed to tolerate oil contamination. You might contact MasterBond and 3M and Henckel-Locktite.

Too bad manufacturing is stick in the mud over modern technology.



RE: press fit for a magnet...

Another idea is if the stainless steel rod is magnetic, perhaps a cup of the same stainless steel type can be made in which the magnet can be housed and held in place just by magnetism.

RE: press fit for a magnet...

I think I misunderstood the "socket" when I mentioned swagen the brass (I was thinking magnet with hole.

Just throwing out a thought. I had a mechanic show be the "trick" to using super glue. I can't remember the locktite designation, but the trick is to use 70% rubbing alcohol to clean, then once the two pieces are together (with glue), use a misting sprayer and lightly wet around the joint. the wicking of the alcohol carries the water the super glue needs to set.
I used this to attached ground key stock to polished harden hydraulic stems (outside the working stroke of course) for taking terst measurements.

RE: press fit for a magnet...

I second the above: LocTite has an activator (or probably multiple) that helps curing time and adherence.
Maybe talk to a loctite/henkel sales rep or the technical service department?

RE: press fit for a magnet...

Could you go with a thin plastic coating on the magnet which should be easier to adhere to?

RE: press fit for a magnet...

can you tap a hole in the magnet (and thread the SS rod) ?

another day in paradise, or is paradise one day closer ?

RE: press fit for a magnet...

Is it an option to just roughen the surface(s) of the magnet that is being bonded to the steel?

Han primo incensus

RE: press fit for a magnet...

how about freeze fit (interference) ?

another day in paradise, or is paradise one day closer ?

RE: press fit for a magnet...

How do they make those screwdrivers with the telescoping magnet to pick up dropped screws? Those work pretty well.

RE: press fit for a magnet...

Using an interference fit between a metal magnet and a plastic housing will never work over the long term. Even the best plastics, like Torlon or Vespel, will creep/relax over time under sustained loading. And any press fit will be lost, causing the magnet to work free.

If it is not practical to produce a suitable interference fit between the magnet and a non-ferrous housing, the next best option is to use some type of anaerobic adhesive to secure the magnet in a non-ferrous metal housing.

RE: press fit for a magnet...

Why not have the magnet vendor grind the OD to a more realistic tolerance (or find another vendor)?

Bruce
http://accuratus.com

RE: press fit for a magnet...

Slip fit magnet into hole with a drop of JB weld, worked for me.

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