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Until you have actually handled rare-earth magnets, it is difficult to appreciate the power of their magnetic field. Good ones have a force field that has an attractive power 1000 times the weight of the magnet.

As in most things, there is a quality range in rare-earth magnets. Both the rare earth used and the manufacturing process dictate the strength of the final product. When we were developing the Veritas® Dust Chute (#05J21.10), we had decided to use a magnetic base on it for reasons of convenience and versatility. After a bit of testing, we decided to use four 1/2" diameter magnets, 1/8" thick. We tested several makes, but did not fully appreciate the breadth of the quality range until we came across the nickel-plated neodymium magnets that we finally settled on. One of these little magnets will lift a 10 lb block of steel.

But what is most fascinating about these magnets is the near-endless uses for them. The obvious use is to hold things in position. You can epoxy one of them to your drill press to hold your chuck key. In fact, you do not even have to epoxy it in position. If you put the magnet on flat sheet steel (like the belt cover) it will be more attracted to the sheet steel than to the key. It will still hold the key, but will stay in place when you pull the key free.

The following articles provide additional information on magnets, as well as other uses for them. L.L.



How Strong is a Magnet?

 
What measures a magnet's strength?

There are two measurements that count with magnets. The first is the ability of an alloy to be magnetized, which determines the attractive force. We measure this in Gauss per cubic inch of material at saturation magnetism, a measurement of the strength of the magnetic flux. The second important feature is the permanence of the attractive force, measured in Oersteds. In the world of permanent magnets it is not particularly useful to have a strong magnet that rapidly weakens. The Oersted is a measurement of the amount of coercion required to completely neutralize a magnet. It is usually referred to as the "coercivity" of the magnet. But today neither of these measurements is commonly used; they are multiplied by each other to get a "Maximum energy product" measured in mega (one million) gauss (x) oersteds or MGOe.

MGOe Comparison
Permanent Magnet Material MGOe
Carbon Steel 0.25
Cobalt/Chromium Steel 1
Aluminum Nickel Cobalt (Alnico) 8
Rare-Earth Alloys 50

Note: Ferrites and ceramic magnets fall in the 1 to 6 range.

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What does saturation magnetism mean?

Saturation magnetism is attained when every polarized molecule in the material has the same magnetic orientation – all norths pointing north. This is as good as a magnet gets. Sometimes, however, bad things happen to good magnets, which can cause them to lose their magnetic minds (referred to as Irreversible Loss). Heating magnets beyond their operational temperature, striking them, exposing them to strong magnetic fields, or just old age can all cause Irreversible Loss. Despite the fatal-sounding name for the condition, the losses are recoverable by remagnetization of the magnetic materials. Resistance to demagnetization is called coercivity, for which rare-earth magnets are the champions!

Iron, cobalt and nickel are the only elements that are ferromagnetic at room temperature. Rare earths are alloyed with these materials to increase their coercivity.

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What shape of magnet works best?

Disc magnets provide the highest usable surface area to mass ratio; this shape generally provides the greatest usable magnetic force for the money.

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Materials Used to Make Magnets
 

The rare-earth magnets currently available at the retail level are made of neodymium iron boron (NdFeB). Only three elements are ferromagnetic at room temperature; these are iron, cobalt and nickel. Virtually all other elements increase permanence (coercivity), but any magnet must contain one of the base three to work. The four main magnet types used today are ceramic, alnico, neodymium, and samarium cobalt.

An alnico magnet is made of aluminum, nickel and cobalt. These can be cast (melted, then shaped in a mold) or sintered (fused together by heat and pressure). A magnet that is cast has better magnetic properties than one that is sintered. Although this material can lose its magnetic properties if dropped or struck, the advantage of an alnico magnet is that it can endure temperatures up to 550 °C.

Ceramic magnets are quite hard and brittle. They are made of strontium ferrite and iron oxide, mixed into a ceramic base. For applications under 300 °C, these have lower energy than the other types of magnets, but resist corrosion and demagnetization. Their main advantage is that they are inexpensive.

Neodymium has one of the highest magnetic properties of any magnetic alloy. Although it is the magnet to use for high-strength applications, one of its drawbacks is that it cannot be used where it will be exposed to temperatures higher than 150 °C, or it will demagnetize.

For high-temperature applications, magnets made of samarium cobalt are used. Even though samarium cobalt is not quite as strong as neodymium, this member of the rare-earth family can withstand temperatures up to 300 °C.

The different types of materials used increase the versatility of magnets. The characteristics of each make it possible to find a magnet suitable for just about any application, from keeping a calendar posted in the shop to ensuring there are no nuts and bolts in industrially processed food.

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Making magnets work harder

Magnets do their best work when focused. The natural field of a magnet is polar radiating loops.
Disc magnets have equal fields (Figure 1). The trick is to get both fields working for you.

A ferromagnetic backing plate placed against one side of the magnet (Figure 2) creates a more efficient path for the flux lines to follow. It also creates a radiating pattern favoring one pole, which effectively points the majority of the magnetic energy in one direction.

When a magnet is placed in a ferromagnetic cup (Figure 3), the cup further magnifies the effect by eliminating the air gap (air is a poor conductor of magnetic fields) and brings both poles of the magnet to grip on the same surface. This is similar in principle to a horseshoe magnet. A rare-earth magnet in a steel cup provides four times the strength of a bare magnet. A cup provides a disc magnet the optimal magnetic flux focus into the smallest gap area.

How much magnetic energy is enough?

When dealing with larger magnets, the magnet's field of influence can exceed the saturation point of thin metal. As an example, a one-inch rare-earth magnet in a ferromagnetic cup requires a force of 28 lb to release it from 1/4" plate steel, but only 14 lb to release it from the steel used in automobile bodies.

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Magnet Cups


 
 
 

The primary reason to use magnet cups is to increase the attractive power of a magnet. The cup will normally increase strength by a factor of 4. Once the cup is screwed in place and the magnet popped in, there is little chance of it ever popping out, whether accidentally or intentionally. However, if you think that you will ever need to remove the magnet and cup, you can file or cut a slot down the side walls as shown.

This gives you the option of inserting a small pointed tool to pry out the magnet. For cups that are counterbored, you would have to bend a small hook on the end of your prying tool.

Although magnet cups are normally installed using wood screws, you have the option of using the equivalent size flat-head machine screw. Where you can drill though your workpiece, this allows you to capture the projecting screw end with a nut. The nut could be counterbored if required, installed with a small socket or nut driver. If this appeals to you, but not the unsightly hole, you can counterbore deep enough to hold both the nut and a matching plug cut with a Snug Plug® cutter. If you ever have to remove the magnet cup, the plug can be drilled out using a drill bit 1/64" to 1/32" smaller than the plug, exposing the nut.

Using machine screws and nuts can also be beneficial when working with softwoods, particularly when they are thin. If the screw doesn’t have sufficient bite, the force that can be exerted on the screw can in some instances rip it out of the wood. Capturing the screw with a nut from the back side eliminates this possibility.

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Magnet Washers

The attractive force of a rare-earth magnet is often limited by the ferromagnetic properties of what it comes in contact with. Although you could use another magnet, the low-cost solution is to use a magnet washer. Unlike plain washers, these are turned flat to ensure intimate contact with the magnet, and are made thick enough to take maximum advantage of the magnetic field that the magnet possesses.

Like the magnet cups, magnet washers can also be installed using machine screws, for improved pull-out strength. (L.S.)

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Magnetic Fields and Pacemakers

There is much concern regarding the effects of magnets on pacemaker devices. Since there is no known insulator for magnetic flux, it is impossible to prevent this force from reaching pacemakers. As a result, current pacemaker design accommodates passive magnetic interference with minimal disruption to the device's function. Pacemakers have a preset default rate based on the patient's requirements at rest. This means that, in the presence of a strong static magnetic field (in excess of 20 gauss), the device will seek the default rate. The presence of a strong magnetic field should not cause the device to turn off or inhibit this default function.

 
Because magnetic flux is poorly conducted in air, as well as human tissue, a large magnet would have to be very close to the pacemaker to create a problem. A reasonable field strength test can be done with a standard quartz watch, which generally has a tolerance of 10 gauss. To create a field strong enough to stop the quartz movement on a watch, a 1/2" dia. by 1/8" thick rare-earth magnet must be placed directly on the face of the watch. Once the magnet is removed, the watch, like the pacemaker, should return to its normal function without sustaining any permanent damage.

 
The warnings posted near devices such as magnetic imaging equipment are to warn pacemaker and defibrillator patients of the risk of entering an active Electromagnetic Field (EMF), which is a far more serious issue, and very different from the drive force created by a permanent magnet. EMFs, both naturally occurring and man made, have been blamed for everything from health problems to power-grid black-outs, and are an ever-present threat to the function of micro-electronic devices.

EMFs differ from magnetic fields in that they alternate or modulate, and in so doing are able to transmit energy through induction. On a global scale, the earth's EMFs are induced into long runs of electrical utility transmission wire, especially those running north-south. They have been observed at certain times to induce in excess of two million volts across the 1200 km transmission line between the generating station at James Bay and the electrical distribution center north of Montreal. Needless to say, this has created some long, chilly nights for distribution engineers, who wondered why Montreal got dark when the Northern Lights shone brightest.

On the pacemaker level, the short wires contained within the human body are fairly secure from natural forces, but require caution near some of the manmade devices, especially those that operate at high frequencies, such as Magnetic Resonance Imaging (MRI) devices. Over the last decade, appliances such as home computers, televisions and microwaves have become subject to ultra-strict Electromagnetic Interference (EMI) regulations, which have made the world a safer place for cordless phones and Walkmans®, as well as for pacemaker patients. But environments such as radio transmitter rooms, high-frequency welding equipment and especially the powerful magnetrons of MRI equipment remain very dangerous. High-frequency electromagnetic radiating devices create three possible threats:

  1. inducing a destructive voltage level into the device, which could cause permanent circuit failure,
  2. inducing a voltage greater than the operating voltage of the device, which masks the pacemaker's signal, and
  3. inducing a pulse sequence into the heart that is not supplied by the pacemaker, but the outside source.

None of these conditions will be created in the presence of a fixed-pole magnetic field such as that generated by a rare-earth magnet. Our advice to pacemaker and defibrillator patients is to exercise a modest degree of caution when handling large permanent magnets, keeping them from coming into direct contact with the implant area. If any rhythmic change is experienced, move the magnet away from the implant area.

— M.O.
11/98

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The Menace of Magnets

Thousands of you have bought rare-earth magnets from us. Almost invariably, you have found uses for the first few you bought and proceeded to order more. Along the way, you probably found out, as many of us have who have handled them, that you should never carry them in the same pocket as your credit cards or sit them next to your computer, particularly if you happen to be using a computer disk as a coaster. They will wreak havoc with any information stored in magnetic form. Some of us have had the magnetic strip on our credit cards destroyed more than once through inadvertently slipping magnets into pockets. You can substantially reduce their field (and prolong their life) by leaving magnets attached to a piece of iron or steel. Much like the old horseshoe magnets that used to come with a keeper to lay across the tips, rare-earth magnets retain their strength best when in contact with something that is ferromagnetic. However, with rare-earth magnets, it is not nearly as important as it used to be with horseshoe magnets, since the rare-earth magnets have a far higher degree of coercive force — the resistance to loss of strength. Despite all this, rare-earth magnets continue to be well worth having around for their nearly endless uses.

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Magnets, Cans and Toolboxes

When we added 1" diameter magnets (1/8" thick) to our catalog, we did it to expand our range but without putting a great deal of thought into how these larger magnets would be used. We had assumed that they would be put to similar uses as the other magnets and that it would all just be a matter of different scale.

What we had not anticipated was the effectiveness of these magnets in a whole range of new tasks. For example, these can be used to hold a tin box with wrenches in it to the side of a table saw or a tractor fender. The tremendous grip afforded by these magnets will stop metal containers from sliding down a flat surface unless they are very heavily laden.

Square metal boxes are ideal. Frequently, such boxes arrive with candy or special nuts inside. If they have a top, so much the better; you can make waterproof toolboxes out of these. All you have to do is put one or two of the 1" diameter magnets inside the box with heavy washers or a strip of steel at least 1/16" thick as a backer. The magnets will hold the box in position, yet make it easy to transfer from one machine to another. This is particularly handy when repairing machines since the nuts and bolts removed can be kept in such a portable container.

Coffee cans with a slightly flattened side also work well in this use. The side needs to be flattened to eliminate the gap between magnet and can.

If you have any farmer friends, you can immediately endear yourself to them by giving them at least one tin or box for each tractor that they own. Something the size of a coffee tin is ideal because it will hold vise grips, pliers, punches and a hammer. The tin only needs to be flattened for a 1-1/2" wide strip from top to bottom. This lets you put the magnets and the backing material inside the can, and then stick the can to any flat ferromagnetic surface.

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Quick-Attach Features

We have four neodymium magnets in the base of our Veritas® Dust Chute (#05J21.10). We have found that if we want to attach the dust chute to something that is not iron or steel, all we have to do is put a couple of flat-head screws in place (with the same spacing as the magnets) and the Dust Chute cheerfully clings to them. You can use the same principle to hang almost anything, anywhere. The principle is particularly useful if you have a wooden work table where you would like to use the Dust Chute, or anything else with magnets. You can glue washers in place or insert screws to make the wood "attractive".

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Knife Holder

I have always found it jarring to put a sharp knife on an ordinary magnetic knife holder. The blade is held against two steel strips. It is not really a question if this will cause dulling, just when it will happen.

Regardless of how careful you are, sooner or later the sharp edge will hit a bar. On the other hand, wooden knife blocks clutter up a counter, even though they are kinder to knives.

There is a third choice. You can make a wooden knife holder using rare-earth magnets. The magnets are strong enough that you can put a layer of wood between them and the knifes. The knives are held securely in place, but never get damaged because they only ever come in contact with wood, not steel nor the magnet. The simplest way to do this is to drill 1/2” holes to within 1/16” or so of the face of your wooden bar. Whether you use a brad-point drill or a forstner bit, the center brad will probably put a hole in the face of the bar. You can put filler in this if you wish, but the whole job looks more professional if you drill it out to 1/4” or 3/8” and put a wooden plug in it. The job looks neater and the plugs mark exactly where the magnets are in the wooden bar. This makes a far better knife rack than you can buy in any store.

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Curiosities

Among the endless things you can do is make a plumb bob that will not hang plumb. (Put a magnet in the plumb bob with the North pole facing down and one in the base with the North pole facing up.)

A standard plumb bob and one of the rigged ones are shown. Great for teaching principles of magnetism.

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Hiding the Spare Key

If you have ever used a magnetic key container to hide a key on your vehicle, you will know that some of them have very weak magnets. When you go to recover your key, you may find that it has dropped off.

Among the hundreds of uses for powerful rare-earth magnets is to hide the spare key. By wrapping a key and one of these magnets in silicone tape (#23K30.01), you will have a waterproof film over both the magnet and the key, and you can hide it anywhere on the vehicle. Now your only problem will be to remember where the spare key is hidden.

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Getting your magnetic money's worth

A magnet placed in a cup can do four times the work of a bare magnet. Magnet prices are proportional to the magnetic alloy quantity.

Rare-earth alloys are brittle and chip easily; cups provide protection.

Rare-earth magnets are more efficient and survive best when set to work in ferromagnetic cups. The cost is also less per unit of attractive force.

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