[martinw]

I recently bought ten Neodymium disc magnets, 3/4" diameter, 1/8" thick. They are quite impressively strong when in intimate contact. Actually, it is quite a job to prise them apart. Magnet experts will know this, but please bear with me.. they only came in the post today, and I am a magnet virgin.

Anyway, a couple of the little things could quite easily be mutually attracted through a vertical piece of 3/4" MDF without sliding down.

This got me thinking. Yes, magnets do not develop their full force unless really close, but they do develop some that might be useful, at a distance through a non-metallic workpiece like MDF.

The mad idea is to put a grid of far more powerful magnets on the MDF base of a Joe-like machine, and hold the workpiece down with magnets from above. You can move the top magnets to avoid the tool path, at any stage of machining.

A possible refinement might be to add neoprene disks in the bed, above the magnet, to increase lateral friction.

Errh, well I did warn you it might be entirely bonkers.

Please post.

Best wishes,

Martin

[martinw]

http://www.supermagnete.de/eng/magne...oup=blocks_big

Martin

[Geof]

....The mad idea is to put a grid of far more powerful magnets on the MDF base of a Joe-like machine, and hold the workpiece down with magnets from above. You can move the top magnets to avoid the tool path, at any stage of machining.....Martin

I am unwilling to say the idea is totally mad...but here is a reference point...sort of.

We machine 1/4" polcarbonate, about 10" by 12", on a vacuum fixture, this is really machine as in 3/8" cutter engaged full diameter, full 1/4" thickness, 10,000rpm at a feed of (I think) 80 ipm, going around the periphery.

If the vacuum is less than about 20" of mercury, full vacuum is almost 30, we rip the piece off the fixture. Full vacuum is around 14.70psi so 20" is almost 10 pounds for every square inch. If your magnets develop 10 pounds of attraction through whatever is separating them, and you have one every square inch it may work.

ANd after typing this I realised it could be construed as sarcastic but that is not the intent, I am actually trying to give a serious reference point.

EDIT: Correcting typos.

[Zumba]

Not enough force as Geof said.

I've got some lifting magnets that I use for steel plates, tubes, and sheet metal. They're the size of bricks and have load ratings of 400 lbs a piece. Vacuum delivers nearly the same amount of force per unit area and covers every square inch of the workpiece.

Magnets only work with very low cutting forces (surface grinders).

[Verfur]

just thinking what effect would the flux feild have on the stepper motors and or spindle motor. Not to mention the cleaning of a bed of perm. mags would be a pain.

Just thinking out loud.

[ImanCarrot]

I had a look into these neo magnets and on Wiki it says that if you drop one down a copper pipe it will fall dead slow down it.

so I tried it on a 10 ft length of 1/2 inch copper pipe... and bugger me! it does fall down it real slowly! I had to repeat it a few times to beleive it!

What the hell's going on there? the copper is non- magnetic.

I actualy (I know, call me daft) tried it on a plastic pipe to see if the gremlins which normaly inhabit my machine had escaped and were messing about with gravity I was so amazed!

[Geof]

I had a look into these neo magnets and on Wiki it says that if you drop one down a copper pipe it will fall dead slow down it.

so I tried it on a 10 ft length of 1/2 inch copper pipe... and bugger me! it does fall down it real slowly! I had to repeat it a few times to beleive it!

What the hell's going on there? the copper is non- magnetic.

I actualy (I know, call me daft) tried it on a plastic pipe to see if the gremlins which normaly inhabit my machine had escaped and were messing about with gravity I was so amazed!

Eddy current damping.

Used to stabilize the needle movement on delicate instruments; the needle shaft has a thin non-magnetic but conductive disc between the poles of a strong magnet and this disc cannot move fast.

The damping increases with speed and in the needle example drops to zero as the needle settles at its reading so it does not bias the reading like friction damping would.

In your copper pipe example the magnet field around the magnet passes through the copper and because the magnet is falling the field is moving relative to a conductor so it creates currents which circulate in the conductor; eddy currents. The eddy current creates its own magnetic field which interacts with the magnetic field of the falling magnetic in such a way to oppose the motion that is creating the eddy current. The potential and kinetic energy of the falling magnet are actually being turned into heat in the copper pipe.

Google 'Magnetic Retarder', Magnetic Braking', 'Eddy Current Braking' and see what you find. Really, really large versions of the needle damping disc are used as transmission brakes on trucks (lorries) and buses in Europe.

Also Google 'Faraday Generator'

[ImanCarrot]

That is truely amazing Geof! thanks man, I love learning new things.. will be glued to Google all afternoon now hehe

[Geof]

That is truely amazing Geof! thanks man, I love learning new things.. will be glued to Google all afternoon now hehe

Not amazing...just applied Physics.

I forgot to mention: Old fashioned speedometers and direct drive tachometers work on a similar principal. In this case it is a spinning magnet very close to an aluminum disc attached to the needle with a return spring. The faster the magnet spins the stronger the eddy current torque on the disc and the further it moves against the spring.

AC Induction motors work on the same principal: In this case the magnetic field, created by the electric current through the coils on the poles, passes from pole to pole inside the motor; you can think of it as the magnetic field rotating. The rotor is made out of both conductive bars and soft iron laminations and the eddy currents induced in the conducting part in turn creat a strong magnetic field because the laminations form a core. The rotating magnet field from the poles not only creates the magnetic field around the rotor it drags it along with it. There has to be a slight amount of slip because if the rotor moved as fast as the field rotation there would be no relative movement between the rotating field and the conductors in the rotor hence no induced currents. The rotating magnetic field of the rotor also interacts with the field coils and creates a voltage opposing the applied voltage so when the rotor is spinning freely with minimum slip the opposing voltage is maximum so the motor draws less current. As the rotor slows down under load this voltage declines, the current increases and the magnetic drag gets stronger so the motor can drive the load. Incidentally the laminations in the soft iron core for both the field coils and in the rotor are to prevent an undesireable effect of eddy currents and that is heating. Eddy currents are induced in the iron because it is a conductor but because it is a poor conductor it gets hot; the laminations, which are separated by an insulating film, limit how large the eddy currents can travel in the iron and reduce heating losses.

All in accordance with Newton's laws, and Maxwell's, and Henry's, and Ohm's and other probably that I have forgotten.

And this consitutes a hijack I guess.

[martinw]

Dear Geof,

Thanks for the copper pipe explanation. I have some 22mm OD copper pipe that just lets my 20mm OD magnet inside. It really does fall slowly. I then tried electrically earthing (grounding) the pipe and it still fell slowly. I thought it might "destroy" the eddy currents but it seems not.

ImanCarrot... here is magnetic levitator using rotating copper cylinders....
http://www.youtube.com/watch?v=glCNP6qH_Dc.

I can quite see that the mad magnet idea is never going to compete with a vacuum as regards strength, but , for thin workpieces and light passes, I think it might have some applications. Those magnets are fiercely strong (eg 2" by 2" by 1" has a strength of 220 lb).

Thanks, everybody for your comments.

Best wishes,

Martin

[Geof]

...I then tried electrically earthing (grounding) the pipe and it still fell slowly. I thought it might "destroy" the eddy currents but it seems not...

The currents are not going anywhere, they are true eddies just going around in circles. The plane of the circle depends on the orientation of the magnet, conductor and the direction of motion. If the preferred plane intersects only a small distance of copper then the size of the circle is limited and the effect is not as strong.

If you could constrain your magnet so that it is across the tube while it slides down and then so that it is parallel you may find it moves faster in one of these orientations. The magnetic field certainly intersects with the tube differently.

[martinw]

Dear Geof,
Through the swirling mists of time and cerebral fog come the three finger motor and generator rules. Was it Thompson?

Best wishes,

Martin

[martinw]

Errh, no it wasn't. It was Fleming. I just checked. Sorry.

Best wishes,

Martin

[Geof]

Errh, no it wasn't. It was Fleming. I just checked. Sorry.

Best wishes,

Martin

I have to admit I did also.

And I just realised you have designed, well maybe not you because it was the root vegetable guy that dropped the first magnet down a pipe, however you can take some credit because you started the whole thing...and now I am lost; oh yes, the two of you have helped design my self damping gate closing mechanism. I have a steel weight on a cable running over a pulley that closes a big sliding gate; but it speeds up and closes with a crash. If I stick a bunch of these magnets in the weight and turn it into a big magnet then I can let it fall inside an aluminum tube and it should be damped.

I hope I can remember this when I get time to build it.

[martinw]

Dear Geof,

As regards "fun with magnets", I think that they started a "curious path" in an a lot of people hereabouts a long time ago.

Best wishes,
Martin

[Greolt]

The mad idea is to put a grid of far more powerful magnets on the MDF base of a Joe-like machine, and hold the workpiece down with magnets from above. You can move the top magnets to avoid the tool path, at any stage of machining.

I just read through this post and yes for something like 12mm ply it most likely would be useless.

BUT what about something like PCB or engraving plastic which maybe only 1.5mm thick. I think it might work quite well there.

I use a vacuum hold down device a lot and it works great for heavy stiff materials but not so well for thin flexible materials.

They hold well enough. They are not going to move. However because the gasket material must be compressed a bit,

thinner materials tend to sit up around the edges. Where they extend past the gasket.

No good for PCBs as they must sit very flat. Same for engraving plastics to get constant depth lettering etc.

Now I know there are other styles of vacuum hold down but that is not the point here.

How about a surface, say MDF, with magnets glued into holes to leave them flush and matching magnets placed on top of material.

Or even a steel surface with magnets on top of material anywhere needed. They could be simply moved out of the cut path mid job.

What do you think? I think I might buy some magnets.

Greg

[lgalla]

Martin,Hellow,
Your link for the super magnets calls them"The skin pincher"I sure know that line.Put one in your palm,another on top of your hand and presto magnetism through your hand untill one slips at 100mph and you get pinched.I put them away after that.Can these also erase the wife's credit card?Be a pity!
Larry

[martinw]

Dear Greg and Larry,

I didn't explain it, but the idea was to let the magnets into the MDF base of a router with a thin (say 1/16") piece of "grippy" rubber stuck to the top of the magnet. The top surface of the grippy rubber would be just proud of the base when uncompressed, and flush when compressed. The idea of the grippy rubber is to increase lateral friction between the base and the workpiece. Another set of magnets would sit on top of the workpiece.

As regards strength, one of the graphs in the website above suggests that you might get about 35% of full strength with an air gap of 1/4". I have no idea how that thickness of MDF workpiece compares to an air gap, but at a mad guess, (and I have considerable expertise in that particular area..) I think they might have similar properties. Even at 35% of full strength, that is still a hefty hold-down force, with some of the larger magnets.

If you want to experiment, there are plenty of people on e-bay shifting them. One word of warning.. they are extremely brittle. Never let two magnets fly at each other unrestrained. They will almost certainly shatter into very sharp shards.

Larry, thanks for the comments about your wife's credit card. I had a good chuckle..

Best wishes,

Martin

[fatal-exception]

I have somewhere in the neighborhood of 100 rare earth magnets in my shop, the biggest measuring 2x2x1/2". As mentioned above, they don't have enough latteral force to hold the work piece, it's easy to slide even the biggest ones off of each other.

Mostly, they hold pieces of paper to metal cabinets, and in some cases tools to the machines they belong to. I wish they could hold 3/16" perspex, and they can if the piece is constrained around the outside, but not for profiling the outside, unconstrained.

BTW, I think the MythBusters showed the magnet in copper pipe on one of their wacky experiments. (gotta love the show though....)

Paul

Edit: you can also buy all shapes an sizes from http://www.wondermagnet.com/ - excellent service and shipping.

[martinw]

I have somewhere in the neighborhood of 100 rare earth magnets in my shop, the biggest measuring 2x2x1/2". As mentioned above, they don't have enough latteral force to hold the work piece, it's easy to slide even the biggest ones off of each other.



Paul

Edit: you can also buy all shapes an sizes from http://www.wondermagnet.com/ - excellent service and shipping.

Dear Paul,

Try this.

Get some double stick tape, the carpet stuff. Stick it to the magnet, and then stick some 1/16" closed-cell neoprene foam on top. It only needs to be the size of the magnet. Now try moving the magnet laterally with another magnet on top of a "workpiece" . You may me quite surprised as regards the increase in lateral holding capacity.

I'm absolutely no expert, but I tried it yesterday.

Best wishes,

Martin