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I'm trying to build a sort of three-axis stepper motor that consists of a sphere with a permanent bar magnet embedded in it surrounded by a cube (slightly smaller than the sphere, so that the sides of the sphere protrude) of ferromagnetic material. On each pair of opposite sides to the cube is a coil of wire, surrounding the hole in the cube through which the sphere protrudes.

By forcing current through a combination of the wire loops, the sphere can be magnetically induced to align itself along any axis. I've tried to build a prototype without using the ferromagnetic cube, but the field from loops of wire alone is too small to easily and safely cause the sphere to rotate.

My first few attempts have been to make a mold of the proper shape and fill it with iron filings, then pour some liquid that can dry or otherwise cure to a fixative (preferably non-conductive so eddy currents aren't a problem) to hold the filings in place- it is then easy to laminate the surfaces of the material so that corrosion of the iron, if not induced by the fixative itself, is not an issue. However, the only material I have on hand to make the mold out of is PLA plastic, which has a low tolerance for heat. I've tried a few different varieties of wax, but all that I've been able to lay hands on melt at too high a temperature for the PLA to stand or else are too cohesive and don't seep in between the iron filings, and I've had similar problems with various glues.

Can anyone suggest either a possible fixative or an alternative moldable ferromagnetic material to use?

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  • \$\begingroup\$ Have you tried silicone? Or is it not rigid enough? \$\endgroup\$ Dec 17, 2015 at 1:58
  • \$\begingroup\$ ...you know, somehow I had not tried it. I have no idea how it managed to slip my mind. I've set some up right now, and while I need to wait for it to cure before I'm sure it seems to be working well. It's worthless as a filling, since it doesn't flow at all, but mixing it with the filings directly makes nice clay-ish material that seems to mold pretty well. Thank you for the suggestion. I would still like to know if anyone can come up with a more fluid solution, since that would definitely be better if only from the insulation perspective, if that makes sense. \$\endgroup\$
    – P...
    Dec 17, 2015 at 2:13
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    \$\begingroup\$ It may or may not meet your needs, but steel-filled epoxy is available in putty form at your local hardware store (typically in the plumbing aisle or else adhesives), and liquid form at specialty suppliers or online. \$\endgroup\$ Dec 17, 2015 at 3:04
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    \$\begingroup\$ I'd try levitating the sphere on an air cushion. \$\endgroup\$ Dec 17, 2015 at 4:24
  • \$\begingroup\$ If you can live with the static field, fridge magnet material can be obtained quite easily from a variety of sources. (Been there etc..) \$\endgroup\$
    – Andy aka
    Dec 17, 2015 at 9:01

4 Answers 4

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It sounds as if you are needing moldable flux concentrator for hobby purposes. Professionals in induction heating have several moldable flux concentrators available. They are quite high in price, but performance is very good. Search Flux Concentrators such as Fluxtrol or Ferrotron.

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  • \$\begingroup\$ I appreciate the suggestion, but I'm sorry to say that at this point such solutions are far beyond my means. I have no doubt that this or something similar would be the ideal way to go. \$\endgroup\$
    – P...
    Dec 18, 2015 at 1:08
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You can get two pot epoxy embedding compounts which may be flowable enough. | Increasing temperature will make epoxy more flowable and also accelerate setting times. By reducing setting compound amounts in 2 pot mixes you may be able to combine flowability and acceptable setting times.

Polyurethane plastics (one version is seen as clear varnish spray) are atmospheric water setting (relatives of cyanoacrylate glues) and can be very 'runny'.

Cyanoacrylates may do what you want (at a cost) and can be of very low viscosity.

Not flowable but if you want to hasten setting of silicone rubber then mixing it with cornflour greatly increases setting rate - this is because the CF provides intimate access to the small amount of water needed for setting distributed throughout the mix rather than it having to permeate through the SiR at mm's per day rates.

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  • \$\begingroup\$ I've actually tried Cyanoacrylates, with no success (too little and the result crumbles easily, too much and it is impossible to remove from the mold), and what I've read suggests that most plastic (urethane or polyurethane) fixatives are likely to suffer this same problem. I'm going to see if I can get a hold of some epoxy to rate its performance- it seems like it could go either way. \$\endgroup\$
    – P...
    Dec 18, 2015 at 1:03
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Marine epoxy ... good stuff. (Doesn't have to be the very expensive West Systems either). It's a 2-part compound, mixed as per instructions, usually 2 parts resin to 1 part hardener. (Usual warnings : can cause allergy, use rubber gloves. Mix too much, say >150ml, too warm, or too much hardener, and you'll see an exothermic reaction and almost instant setting!)

Unlike hardware store epoxy, it's a viscous liquid (like varnish) rather than a paste in its just-mixed state, for 20 minutes or more at lower temperatures. It's common to use chopped glass, fumed silica powder, microspheres etc as filler material, to thicken the consistency from a liquid anywhere up to a stiff paste.

But for a ferromagnetic material you could use any of: iron filings, iron powder, powdered ferrite, or 1mm ball bearings (not stainless steel, which is usually non-magnetic). Pour into a mould to set overnight. After about a week you can machine, grind, polish to final size.

You didn't say if you needed to charge it as a permanent magnet : that would be more difficult, though you could embed permanent magnets in the component.

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  • \$\begingroup\$ Thank you for the suggestion- while I don't think it's likely to help me at the moment (I lack the tools to properly finish it, and I don't expect it would release easily from the mould I have on hand) it is definitely something to look into for the future. \$\endgroup\$
    – P...
    Dec 18, 2015 at 1:00
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Two part urethane molding compounds have quite low viscosity and cure quickly. They are thermoset polymers so I would expect reasonable temperature performance. There are suppliers such as Smooth-on in the US who have a large array of products for casting.

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  • \$\begingroup\$ The problem with urethane for my purpose, as with many other rubber and plastic fixatives, is that I do not have nor do I have the means to produce at the moment a solid, non-porous mould to cast from, which makes removing it a problem. Thank you for the suggestion, though. \$\endgroup\$
    – P...
    Dec 18, 2015 at 0:55

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