View Full Version : OT Question on magnets

10-31-2009, 09:36 PM
Looking at a site that sells magnets, Some 1/2"OD X 1/8" ID X 1/4" thick are described as Diametrically Magnetized.

What does this mean?

Weston Bye
10-31-2009, 10:28 PM
Most disk and cylindrical magnets are polarized with a north pole on one face and south on the other. However, less commonly, they can be magnetized across the face of the disk so that the north pole is on one point of the "equator" and the south pole is diametrically opposed on the equator.

Magnets can also be pattern magnetized with multiple north-south pairs. I demonstrated this in the Encoders from Refrigerator Magnets article in the Summer 2008 Ditgital Machinist.

10-31-2009, 11:27 PM
Would diametrically magnetized cylindrical magnets make good pick up tools?

10-31-2009, 11:41 PM
A flat magnet will be able to touch both its magnetic poles to any flat area, so on steel the magnetic field will be about as strong as it can be. Check this out by placing various thicknesses of non-magnetic material between the magnet and the steel. You'll find that the 'pull' will seem to develop exponentially stronger the closer the magnet can come to the steel.

The cylindrical magnet can touch either pole onto a magnetizable surface such as a flat on a piece of steel, but won't be able to get more than a line contact with it, so the 'pull' you feel won't be as strong. Diametrically magnetized cylindrical magnets are good for motors, where the steel structure surrounds the magnet and therefore can concentrate the most lines of force through the magnetic path, which is the magnet plus the surrounding steel, usually laminations with gaps for wire to be wound into.

For a pickup tool, maybe it can be good- it depends on what you're picking up, where it has to be able to fit, and how much stuff it has to hold in one go. A round or cylindrical magnet lends itself well to having a sleeve that can slide over it, say a brass tube, that you can rig to carry away the swarf that the magnet picks up without having to fuss so much to remove it from the magnet proper. If the sleeve has a washer on it, swarf can be drawn to the end where the magnet is, then as the magnet is pulled out of the sleeve somewhat, the swarf is caught behind the washer and will pretty much drop off.

The cylindrical magnet is handy to use because it has a hole through it which you can mount it by. I don't think there's an advantage whether it's diametrically magnetized or axially magnetized for this application. Diametrically magnetized, it may be able to lift more from its very end than an axially magnetized one-

11-01-2009, 01:22 AM
What sort of pickup tool? Nuts and bolts or lost anchors?

Weston Bye
11-01-2009, 07:47 AM
The effectiveness of any magnetic field depends on completing the magnetic circuit or circle. In free air, the magnetic field exits the magnet perpendicular to the surface of the magnetic pole, travels along a curved path to re-enter the magnet at the opposite pole, again perpendicular to the surface of the pole.

A diametrically magnetized magnet will have broad, spread out pole regions with short return paths, so the force at the surface of the poles will be relatively weak at any point of line contact and the return path will be relatively intense. For maximum holding ability, the object being held should closely match the shape of the face of the magnet to minimize air gaps.

An axially magnetized magnet, by comparison ,will present a relatively large flat surface of concentrated parallel magnet field with a relatively weak return field.

Ferrous material, iron or steel, is more efficient than air at conducting magnetic flux and can be used to steer magnetic fields around corners. An axially magnetized magnet, centered inside a steel cup with one pole contacting the center of the cup and a sufficient air gap around the cylindrical side of the magnet will provide the best lifting force when contacting a flat steel surface.

Efficiency at lifting objects like a bunch of ball bearings will depend on the relative sizes of the balls and magnets. Ultimately, it all depends on minimizing air gaps and maximizing surface contact area.

11-01-2009, 08:06 AM
What hasn't been mentioned although Weston alluded to it is the use of pole pieces. These are pieces of steel or iron that are attached to the magnet(s) to direct and concentrate the magnetic field. As Weston said, the field must make a complete circuit. Magnetism is just another face of the electromagnetic force and for a current to flows there must be a complete path. Magnetism is always bipolar with a north and south pole and the magnetic flux will always take the path of least resistance to complete the circuit. Air has a high effective resistance with consequently greater losses than a magnetic conductor such as iron. By using pole pieces the distance the field must travel through air is minimized.

Also, by using pole pieces the magnetic field may be compressed and directed in the direction it is needed. A quite recent invention is the Halbach Array which is easily made from a few cubical super magnets and has some very desirable properties. It concentrates nearly all of the field on one side of the array which greatly improves the holding power of the magnet and also makes the magnet one sided which can be an advantage.

It is made by orienting magnets as in the illustration. They must be forced together to make this configuration so a wooden jig in which to restrain them is mandatory. As they are put into position they may be glued together with CA glue and then the entire array bound from end to end with strapping tape. The array may be extended indefinitely and several arrays may be placed in parallel to increase the power while using smaller and cheaper magnets. The array also has the property of producing powerful and stable levitation if it is moving across the surface of a non ferrous conductor such as aluminum or copper.


Weston Bye
11-01-2009, 08:24 AM
The array also has the property of producing powerful and stable levitation if it is moving across the surface of a non ferrous conductor such as aluminum or copper.

Indeed, the magnet induces eddy currents, little rings of electric current inside the metal. These rings of current then establish their own magnetic fields that then interact with the magnet moving past. The eddy currents are short-lived and generate heat within the metal where they exist as the very environment causes them to short circuit.

The magnetic effect can be easily demonstrated by dropping a rare earth magnet down a brass, copper or even aluminum tube. The magnet will demonstrate a considerable delay in falling out of the end of the tube when compared with a similar size and weight piece of steel.

11-01-2009, 10:21 AM
What is special about the Halbach array is that the levitation effect is inherently stable. It is in the testing phase for use on a maglev train. It requires no power and no control systems. In tests it will levitate the train at speeds as low as 5 mph so all the cars need is small low speed wheels since there isn't any way for the levitation to fail at speed.