It is currently 29 Mar 2024, 02:37

Using rare earth magnets

This forum is for any general questions, queries or plain old chinwaggery on Woody stuff in general.

Using rare earth magnets

Postby Eric the Viking » 16 Jul 2021, 07:03

This is barely a woodworking question, but it has relevance to anyone using magnets as box or cupboard latches, etc.

The Problem

Our chest freezer is old but functional. Some years back, one of its hinges disassembled itself, and now doesn't counterbalance the weight of the lid - the other hinge isn't sufficient alone, and the lid falls on your head if you're careless. And that is uncomfortable...

Above the freezer I put up a shelf to take the bigger cooking pots & pans, and that has wooden gallows brackets supporting it. I arranged a loop of cord from one bracket, that could go around the freezer's handle, to hold it up whilst rummaging.

This worked, but it was rather clumsy, especially when trying to unclip it with one hand, whilst trying to hang on to a slippery bag of frozen peas* with the other. So I had a fix, but not a Really Good One.

Then, doing my best Grue impression, I had a lightbulb moment: How about a dangling rare earth magnet instead?

After a bit of fiddling about, adjusting the cord length, etc, I got that to work: two 10mm round rare-earth magnets set into the surface of a wooden 'puck' tht dangles from a shelf bracket.

It's pretty good. The magnets catch the (coil-coated) steel lid nicely, and hold it almost upright whilst you find what you need in the freezer. And you can close it again just by pulling the handle -- no untying necesssary. Supercool!**

20210716_065756.jpg
(108.1 KiB)

20210716_065644.jpg
(123.76 KiB)

20210716_065313.jpg
(116.99 KiB)



The big mistake?

Rule #1 of good DIY solutions is to use what you have and not spend any money (so she tells me!). So I used a couple of 10mm diameter rare earth magnets from stock.

I installed them as I would normally: a blob of 'gloopy' superglue on the flat surface of the magnet, then use the magnet stack (about three or four of them) as a punch, to tap it into place with a soft mallet. Once it was set level with the surface of the wood, slide off the unused magnets. Repeat for the second magnet.

Without thinking, I just used the next magnet in the stack and installed it in the same way. And it does work, with just enough pull to hang onto the freezer lid.

But then (of course!) I realised what might be the Big Mistake: because I simply repeated the steps, both magnets are in the same orientation:

Code: Select all
    ________________  (wood surface)
    |     N   N     |
    |     S   S     |


They are only about 6mm away from each other, and I wonder if I would have got a much stronger pull by flipping one magnet over. It does work, but a stronger pull would have been nicer - more reassuring for the freezer-rummager. And i had to modify the wooden puck a bit to get it to catch on the handle plastic to increase the resistance to sliding off the lid unexpectedly...

The question:

If you use double magnets in cabinet work, do you arrange them as above, or this way?


Code: Select all
    ________________  (wood surface)
    |     S   N     |
    |     N   S     |


Obviously it makes no difference if you have two different bits of iron/steel to attract them to, (or a second set of magnets they catch to), but a single bit, like my sheet steel freezer lid, is affected by both fields at the same time.

It IS still a strong pull. If the team thinks I messed-up, the puck will make a useful chuck-key holder for the pillar drill (and I will make another one, for the freezer lid, but more carefully!).

Answers on a fridge magnet please...

E.

* other vegetables are available.
** see wot I did there?
Eric the Viking
Sapling
 
Posts: 456
Joined: 10 Dec 2020, 21:34
Location: In the downstairs shower, trying to fix the leak.
Name:

Re: Using rare earth magnets

Postby Phil Pascoe » 16 Jul 2021, 07:50

I got one of these, not silly money and a useful bit of cord.
https://www.ebay.co.uk/itm/353256694421
Phil Pascoe
Old Oak
 
Posts: 1094
Joined: 21 Jul 2014, 22:55
Location: United Kingdom Cornwall
Name: phil

Re: Using rare earth magnets

Postby TomTrees » 16 Jul 2021, 08:01

You cannot stack magnets together easily, as they will resist each other.
I'd reckon it would take a lot and be possibly dangerous if it were done somehow.

Recently I made up four magnetic blocks for levelling my bandsaw.
I only had weak pound shop fridge magnets so used three in each, felt about right.
I didn't think about the orientation, and one set of the magnets were odd.

It didn't matter which orientation they were in, the strength was the same.
I had a pair together and would have noticed the pull, as they were side by side also.

I've since drilled a hole in the back and punched it out to allow for stacking.
I might see if another magnet might help sometime, if I need them for something.
They could do with it, as one stacked could be knocked off easily with a bump.
Would be a chance to make two tools for getting a square hole should one only have the cheapies.

A wide flat bit made into a backwards drill, (ala, Frank ford FRETS.COM)
works very tidy if a bit burny, l but never tried anything wider than 20mm.
And the pillar drill if I cant find a flat bit wide enough, which I'm guessing works a lot better
than a snapped bit, as that took some time to nibble away at the bottom of the hole.
https://www.youtube.com/watch?v=_b3CWkHHSYE
Attachments
94134-098a43b4764f77e97b70258c1c3ba778.jpg
(17.32 KiB)
User avatar
TomTrees
New Shoots
 
Posts: 156
Joined: 19 Aug 2014, 23:51
Name:

Re: Using rare earth magnets

Postby Eric the Viking » 16 Jul 2021, 08:40

TomTrees wrote:You cannot stack magnets together easily, as they will resist each other.
I'd reckon it would take a lot and be possibly dangerous if it were done somehow.


Thanks, but I wasn't trying to stack them in the wood - the safest place to keep unused ones is in a stack on the side of my tool drawers, so they won't magnetize anything else unintentionally. I like strong magnets where I can see them!

In use it's easier to glue the end magnet in place (flush), and only then separate it from the others in the stack.

But I think I should have taken the second magnet from the opposite end of the stack, so its polarity was opposite to the first one.

I probably wasn't clear enough with my question - would I have got a stronger grip with one north and one south pole outwards, or does it not really matter?

I feel an experiment coming on...
Eric the Viking
Sapling
 
Posts: 456
Joined: 10 Dec 2020, 21:34
Location: In the downstairs shower, trying to fix the leak.
Name:

Re: Using rare earth magnets

Postby novocaine » 16 Jul 2021, 08:57

Now your asking. From a very fragile memory of magnetic fields in engineering a lot of years ago.
No you wont get a stronger pull by having one flipped. The magnets are acting independent of each other in this configuration with some interaction of the field around the middle of the 2 (negligable difference).

Stacking them will give approximately double to pull fpr eacj magnet until you reach the diameter of the magnet.
Carbon fibre is just corduroy for cars.
novocaine
Old Oak
 
Posts: 2560
Joined: 26 Nov 2020, 10:37
Name: Dave

Re: Using rare earth magnets

Postby 9fingers » 16 Jul 2021, 09:54

I suspect that you would get a greater pull from putting a soft iron pole piece behind the magnets and having opposing poles outwards - effectively making a horse shoe type magnet and then the freezer lid will complete the magnetic circuit.

Less easy to make though!

Bob
Information on induction motors here
https://drive.google.com/file/d/1dBTVXx ... sp=sharing
Email:motors@minchin.org.uk
User avatar
9fingers
Petrified Pine
 
Posts: 10038
Joined: 21 Jul 2014, 20:22
Location: Romsey Hampshire between Southampton and the New Forest
Name: Bob

Re: Using rare earth magnets

Postby novocaine » 16 Jul 2021, 10:00

9fingers wrote:I suspect that you would get a greater pull from putting a soft iron pole piece behind the magnets and having opposing poles outwards - effectively making a horse shoe type magnet and then the freezer lid will complete the magnetic circuit.

Less easy to make though!

Bob

Yes, i think that would work. Good thinking Bob.
Carbon fibre is just corduroy for cars.
novocaine
Old Oak
 
Posts: 2560
Joined: 26 Nov 2020, 10:37
Name: Dave

Re: Using rare earth magnets

Postby Eric the Viking » 16 Jul 2021, 10:59

It may be a practicality:

I have a lot of electrical transformers, so can probably demolish to use the core laminations. The thought did occur to me whilst making the thing. Easy enough to split the "puck" in half down the sides and make a shallow recess to take one or more laminations - once stuck together it wouldn't show. Since the force is the inverse of the square of the distance apart (magnet to target), I'd still want the magnets flush with the outer surface, but that's easy enough to do

Also, obviously, in that circumstance, flipping one magnet (to achieve the second of my diagrams) would be necessary. It ought to be slightly better than 2X as strong, too, as that would make a complete magnetic "circuit".
Eric the Viking
Sapling
 
Posts: 456
Joined: 10 Dec 2020, 21:34
Location: In the downstairs shower, trying to fix the leak.
Name:

Re: Using rare earth magnets

Postby Cabinetman » 16 Jul 2021, 18:25

No you’re fine doing it the way you did it, the only time you ever have a problem is when you put magnets on the door and the frame – blindingly obvious that they have to be the right way round, and I fit them in the holes the same way you do in a pole, except I use a hot glue gun, after fitting the first magnet I always mark the second one so I know which way round to put it in the hole.
The safety pack that comes with these magnets scares the hell out of me, when it’s talking about not getting your self (hand) between two very strong magnets as the closer they get together The harder they pull, it doesn’t bear thinking about but then I’m never likely to have such powerful magnets at my disposal ha ha.
Cabinetman
Old Oak
 
Posts: 3196
Joined: 11 Oct 2020, 07:32
Location: Lincolnshire Wolds + Pennsylvania
Name: Ian

Re: Using rare earth magnets

Postby Eric the Viking » 16 Jul 2021, 22:53

I used to work in a factory that assembled 5.25" hard disks. The head actuating magnets for those are amazing - I still have a few - handle with care, but VERY useful. Best kept in plastic bags too, as you can never get iron filings off them once they have been attracted.

A friend worked in that section, and had a couple of them, separated by just a business card, on his desk. He turned up at a meeting one day with a sizeable blood blister. Apparently he'd been on a long phone call and without really thinking had picked them up to play with.

The other really strong magnets I've found are the ones from magneto-optical disk drives. Those are not CD/DVDs (no permanent magnets there), but the industrial SCSI read/write units with the disks in cartridges. They were used in big data libraries as they were faster than tape drives. The actual magnets are tiny but have a hugely strong field, by far the strongest permanent magnets for their size I have ever seen.

The other day I (stupidly) tried to un-jam a stuck Silva compass with one of the hard disk magnets. It worked, but I unintentionally reversed the polarity of the compass needle!

E.
Eric the Viking
Sapling
 
Posts: 456
Joined: 10 Dec 2020, 21:34
Location: In the downstairs shower, trying to fix the leak.
Name:


Return to General Woodworking

Who is online

Users browsing this forum: No registered users and 12 guests