Fixing rare earth magnets in place

[Eric The Viking]

The application isn't the gentle jewellery box lid catch at all, it's holding up a pivoting trap door in a tower scaffold plywood deck. These will have to pull really hard.

I've got some suitable magnets from Axy - 20mm diameter. A pair of these, with a steel keeper behind, and sticking to a clean steel plate will do it nicely - hold the trap open at 90 degrees for rigging the deck, but allowing it to release reasonably easily when it needs to close.

Because there needs to be a very strong pull (quite a lot of leverage is possible against the magnetic field), I need to set the magnets flush, probably into a block of hardwood. I can't have anything between the surface of the magnet and the steel plate they'll engage with. I guess if I did put them in blind holes, I could stack the magnets to increase the field strength, but I'd rather not if its avoidable.

Has anyone done this just by glueing them in with epoxy? Axminster sell some with countersunk holes through the middle to take screws or bolts: this would work but they are really expensive considering.

Any thoughts, anyone?

E.

 

[Wuffles]

I wouldn't hesitate to use epoxy for that.

What shape are the magnets you're using?

 

[MattRoberts]

Yep, epoxy is what you need. I've done it myself just fine.

I've also bought some of the countersunk ones from Amazon or eBay - can't remember the cost, but they were under a tenner for 10

 

[oakmitre]

I've glued with JB weld with good results.

Epoxy is probably the best solution because rare earth magnets tend to be quite brittle. Fully seating and supporting them with epoxy may stop them from flexing and shattering.

 

[Eric The Viking]

Thanks chaps - they're 20mm dia. x 3mm disc magnets, and pretty strong if I can complete the magnetic "circuit". The drilled Axminster equivalents would be more than two quid each, which I think is a bit excessive.

I'll use epoxy, and probably let them into a beech block, glued/screwed onto the underside of the decking, which I can sand down to get the exact fit. I'll try to arrange a bit of float, like cupboard catches, so the magnets grip really well.

The trap has to stand open (vertical) to install the decking into the assembled tower framework, I have tried to fit it with the trap closed, and it's nigh-on impossible for one small person to manage.

The whole catch arrangement has to go into a parallel gap of 10mm, between the trap and the platform's heavy-duty rectangular aluminium "chassis", when the trap is opened to 90 degrees - it's stood off by the hinge thickness mainly.

It's a real PITA, but if I can get it to work well it will be the most elegant solution.

Thanks again.

E.

 

[woodpig]

I've glued a 20mm x 2mm magnet on a work light with epoxy and it's in place.

 

[No skills]

Pound shop epoxy - unless its going to get very hot, then buy something more expensive - and curse when that's no better and you should of brought west systems :lol:

 

[Lons]

Sorry for hijacking the thread but just in case of interest to anyone.

A great source of very strong rare earth magnets are old computer hard drives also available from pvr recorders. Pretty simple to extract and lever the magnets from the holders they're stuck to. Better than throwing them away.

 

[custard]

If you go for the ones with a hole in then be cautious screwing them in, they're extremely brittle and split very easily under even the slightest pressure.

 

[Eric The Viking]

Thanks Custard - I was going to Araldite into 20-ish mm holes, as it should be the easiest.

 

[Homerjh]

suppose with the hole it allows the epoxy to make a better hold once cured.

would roughing the surface of the magnet help the adhesion?

 

[Myfordman]

My experience of using magnets in this way is that you must ensure that the two parts come together as two surfaces with no misalignment which would cause a line contact instead as the attraction is very much reduced. The problem is exacerbated with multiple location for the magnets
I invariably arrange for the magnet or the keep to float in some way.

hth

 

[n0legs]

For what it's worth I used brass screws to fit mine (obviously they're the ones with the counter sunk hole) into my tool cabinet. I didn't want to waste the "field" by magnetizing the fixing screw.
Good point made by Custard, well worth remembering =D>

 

[Phil Pascoe]

I wonder if it's presumed they will be left loose/floating?