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.
How Strong is a Magnet?
measures a magnet's strength?
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.
|Permanent Magnet Material
| Cobalt/Chromium Steel
| Aluminum Nickel Cobalt
Note: Ferrites and
ceramic magnets fall in the 1 to 6 range.
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
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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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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
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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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
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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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
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 doesnt 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
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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
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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
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:
- inducing a destructive voltage
level into the device, which could cause permanent circuit failure,
- inducing a voltage greater
than the operating voltage of the device, which masks the pacemaker's
- inducing a pulse sequence
into the heart that is not supplied by the pacemaker, but the outside
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.
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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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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
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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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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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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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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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