Magnetic Indicator Back

One way to evade the issue, and increase holding force, is to make a cup of mild steel into which the rare earth magnet is installed with a resilient rubbery adhesive (so the magnet won't fracture over time). This will focus the magnetic fields onto the open side of the cup.

Joe Gwinn

That is not a bad idea. I'd need to look into how much the "iron" reduces the effective magnetic field.

Typically when I use magnets for holding I have a light press fit and high temp epoxy with a very slight air gap between the magnet and the surface it is holding or holding to. Magnetic paper holders, hardware holding, box closures etc. It does reduce the hold very slightly over a surface to surface contact, but only a few thousandths is needed to prevent impacts.

I'm not crazy about the rubber cup idea unless it is a very high shore number to reduce likelihood of movement. Probably not a huge issue, but...

That can be done here as well.

It's not a rubber cup, it's a steel cup. It is not a press fit. There should be a reasonably large clearance between magnet OD and cup ID, the clearance being filled with slightly soft epoxy, needed to allow for both the fragility of rare-earth magnets and the difference in linear coefficients of thermal expansion.

.

.

Joe Gwinn

I was pointing out that I know how to set a magnet in aluminum and I do it as part of fairly common processes in my shop. Some of those items experience quite a lot more thermal range than the average shop environment casting parts from media that ranges from 350F to 850F. The magnet itself typically does not experience those temps directly (It would lose its magnetism), but in use experiences a thermal range as the aluminum dissipates heat. I really don't think the heat range of from maybe 30F (worst possible case if I left the doors open at night here in the desert) to 130-140F (when the doors are closed and its a hot summer day) will cause an issue in a short period of years. I do believe that impacts will cause the magnets to break and crumble because I have seen it happen. I can simpley set two on my desk and let them pull together to see that.

FYI: I buy the higher thermal rating magnets from K&J Magnetics. They can still shatter if impacted, but they keep their magnetism longer in thermal cycling environments.

I am quite aware that you were referring to two different things. I even spoke about the two different things separately. Iron in regard to bending the magnetic lines of force, and weakening the effective magnetic field, and rubber due to its propensity to flex and even sometimes flow under various forces. I was not so confused to believe that the rubber and the steel (mostly iron) were the same part.

I even acknowledge your overall idea, and commented more that I needed to investigate or was uncertain for those reasons about the exact application in practice.

I go one step further and note that its possible that even if the rubber does flex friction of the back body may prevent it from being an issue. Still I would go with a very high shore hardness if I implemented a rubber thermal growth compensation mechanism. Your ideas do merit consideration. Just pointing out the things that appear obvious to me.

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

One that's obvious to me is that the magnet has to be retained in the cup firmly enough that it doesn't pull out and stay attached to the work surface. I think that means the steel cup bottom should be fairly thick and quite flat and smooth inside and the potting compound should bond well to the magnet.

I don't have a good suggestion for the compound because so many things I've tried have failed. Actually my best results so far are from Gorilla-taping a button magnet to the end of a same-diameter rod. It's not perfect but it fails without breakage and is easily repaired.

There are epoxy types that are intended for attaching neo magnets to steel or aluminum cups. I'd roughen the cup surfaces that will be glue bonded. Everything must be *clean*, such that water does not bead up. Hot TSP (the real kind, not the no-phosphorus kind) will work.

Joe Gwinn

I've got a buddy who makes (and sells) a specialty grinding accessory that is held in place with magnets. Like me he has found that the right press fit in aluminum works really well. Generally neither of us use an adhesive. If I were to use an adhesive I would likely abrade the coating on the magnet and the inside of the hole, and use something like Devcon 2 ton epoxy. Alternatively if I was really concerned about differing thermal expansion I might switch to something like Flex Coat epoxy typically used in fishing rod building.

One thing to note is I've found polyester resin seems to stick to aluminum better than epoxy. This is purely anecdotal. I have secured parts to be machined to a sacrificial aluminum back plate with epoxy before. I also make some aluminum molds for casting polyester resin into a shape. I can break the bond with epoxy, but a fully cured polyester bond is difficult to remove even with mild cleaners like acetone, alcohol, or mineral spirits. On the aluminum polyester molds I say in the description that the user MUST USE A MOLD RELEASE, and "If you glue your mold shut I won't replace it."

I would note that the previous paragraph is based on anecdotal observations. I have not deliberately tried to glue aluminum together or to something else with a polyester resin. I seem to recall original Gorilla glue is a polyester glue, but I could be mistaken. Of course they now make a range of adhesives including their own claim to the best cyanocrylate(s). LOL.

Yes I know I mentioned bedding in epoxy in a previous post, but that's only when there is a problem fit.

Aluminum is pretty soft. There is no reason one cannot use a brass or aluminum spacer between magnet and steel cup, with everything held together with glue.

But beware too-thin glue layers between dissimilar materials - thermal cycling may tear the glue bond. The thicker the bond gap, the less the mechanical stress on the glue layer.

Epoxy is more sensitive to surface cleanliness than polyester, if I recall. But polyester ought to be workable as well.

Gorilla Glue is a polyurethane, if I recall. It does stay flexible, but tends to foam badly with moisture. I'd read the data sheet.

Joe Gwinn

Yes, I believe that's correct. The foaming is triggered by moisture, and is intended to improve its gap-filling behaviour.

Another PU glue that is/was available in the USA as an inexpensive construction adhesive (gun tube) that doesn't foam as much is "PL Premium".

Clifford Heath