What is Slotcar Racing.
By Ray Gardner
Wheelie Car Basics.
by Peter Shreeves
What You Want To Know About Magnets.
By John Sojak, Trik Trax, Inc.
Improve The Handling Of A Slotcar Chassis.
By Ray Gardner
Build and repair a Slotcar Track!
by Ray Gardner with a slight edit by Bob Herrick
Body Painting, Trimming And Mounting Techniques.
By Ray Gardner
An International Affair.
By Dan Green
Last modified: September 29, 2005

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WHAT YOU WANTED TO KNOW ABOUT MAGNETS BUT, DIDN'T QUITE KNOW HOW TO ASK...

BY JOHN SOJAK, TRIK TRAX, INC.

  • A good place to start is at the beginning, right? Okay, so what is a magnet in the first place and where did it come from? Natural magnets are found in an ore that is mined throughout the world called "magnetite," or, more commonly referred to as "lodestone." Lodestone was known as early as 600 BC for the strange, unique property that when it was floated in water on a piece of wood, it always seemed to point in one given direction. Another interesting effect that was discovered about the same time - within minutes, I'd suspect - was when a piece of iron was rubbed across the lodestone, some of the strange, magical properties were transferred into the iron. This was the advent of the first artificial magnet which led to the compass. Around 1600 AD, a crazed scientist by the name of Sir William Gilbert decided to make a study of these strange properties. The rest is history, no pun intended, of course. Today, it's not possible to imagine life without a magnet influencing something we use every day. Think about it...hair dryers, electric motors, automobiles, the kitchen cabinet door, charge cards, stereos, and so on. The list is literally endless. the most important of all of these, however, is the slot car for which life itself exists. There are two basic types of magnets...natural or artificial. Lodestone is a natural magnet and a chunk of steel that has been rubbed across lodestone is artificial. Of these two types, the magnet may be either a temporary or a permanent type. A temporary magnet will lose its magnetic properties rapidly when it is not exposed to a magnetic field. Paper clips are a good example. Once you are through sticking them together with a magnet, they will lose most, if not all magnetic attraction, and thus interest they once had. A permanent magnet, on the other hand, will retain its properties for extremely long periods of time if left alone.
  • The 16D magnets we all know and love are perfect examples of a permanent magnet type. Soft materials such as cold-rolled steel and iron will make temporary magnets, as opposed to hard materials such as a high carbon tool steel that make a permanent magnet. Everyone who has ever tried to demagnetize their favorite screwdriver they use to install the 0-80 endbell screws knows just how permanent a tool steel magnet can be. Another major player in the magnet world is the type of material. The materials used to make today's modern magnets are just about as strange as the effect of magnetism itself. Common materials such as steel have not been used much since about 1920 to make any type of magnet except for children's toys. Today, artificial magnets are composed of a myriad of exotic composite metal alloys such as barium, strontium, and various compounds made of iron oxide, lead/iron oxides, nickel, cobalt, samarium, neodymium, cerium, and so on. All of this confusion can be broken down into three major groups - cast, sintered, and bonded. Each has its own use, characteristics and area of application. I'll discuss each briefly. A "cast" magnet is, literally, cast. That is, the melted metal alloy is poured into a mold. I am sure you have seen many cast magnets. They usually have a rough, grainy finish. Horseshoe and speaker magnets are usually cast and called "alnico magnets" which is a generic name that describes the alloy used to make up the magnet. An alnico is a composite of aluminum-nickel-cobalt. When you look at a cracked edge, it often looks like a bluish/purple/silver mineral crystal. Back in the 60's, alnico magnets were almost exclusively used in slot car motors. They were the most powerful magnet of the day. In about 1968 or so, Raytheon Company introduced a whole new class of cast magnets, the rare earth types...more on this later. The main advantage of a cast magnet is the ease of how it may be shaped to mechanically fit the application. Once the die is made, the magnet is literally poured into any shape it needs to be. The major disadvantage of the cast magnets is that, for the most part, they tend to be heavy for the energy provided. Rare earth types take exception to this rule of thumb. Another serious drawback is the die used to make them, and the smelting to allow the metals together have an extremely expensive up-front tooling cost which is justified only if you are doing a run of billions of the critters.
  • Most cast magnetic materials are available in a standard bar or basic shape forms. You, as a magnet user, have to cut the approximate size you need from the bar or shape, then machine the magnet to the exact specifications. This is an expensive, time consuming and very messy process. A "sintered" magnet is molded under high pressure. What happens is the magnetic material - again, there are many - is mixed with a binder and sprinkled into a mold with the help of a shaker, not unlike salting your french fries. This gives an even distribution of material in the cavity. The heat is then turned on and the die halves come together under about 30 tons of pressure. After the binder has set up, the mold is then opened and you have a kind of "hot glue" type of magnet. This process may be done with either a wet or dry binder although dry seems to be preferred because of the speed. The big difference between casting and sintering is the degree of heat involved. Casting is melting and alloying the magnetic materials at several thousand degrees whereas sintering is pressing a powder into shape with the help of a stickit with or without heat. The associated cost of sintering is a fraction of casting. Another big advantage is that the magnet shape and dimensions can be accurately controlled so little, if any, secondary grinding operations are necessary. Lastly, the production rate is very high, lending itself well to automated equipment. The down side to all of this is that 1), not all materials are happy being sintered together; 2), only simple shapes can be made at a "reasonable" cost, and 3) the process must be carefully controlled or flaws will develop in the finished magnet, resulting in distorted fields. A very slight change in the humidity, for example, will cause the magnetic powder to stick together as the die is being filled, resulting in an uneven distribution of powder and binding agent, causing garbage magnets, say perhaps 200,000 pieces worth. 4), Unlike casting, the sintered magnets are usually very brittle. Once again, although not quite as bad as with casting, one needs a half-billion of them to make tooling cost worth looking into. The last type is "bonded." As the name implies, it is bonded together. The process is quite similar to casting without a lot of the folderol. The magnetic powder is mixed with a rubber-like material and poured into a mold or extruded into a sheet, rod, square, etc. These are common refrigerator and business card type magnets...although this in no way implies low quality. Just as with the other two types, the qualify of the magnet depends on the material composition of the magnet. The plastic is just a bonding agent and the process does not use as much heat as the other two. You very well may have a bonded polymer magnet that can lift a '59 Cadillac - fins and all - out of the Grand Canyon. The down side of a bonded polymer is that you usually cannot achieve the same density of magnetic material as with a sintered or cast material. This means, for a given type of magnetic material, a bonded type is not going to be as "strong" as a sintered type. The real applications for bonding used in magnetics falls under the newer rare earth materials where it is not practical to sinter the magnet because of the associated costs or possible changes in the material characteristics because of heat. Another big advantage is being able to cast, by virtue of pouring or injection molding, very intricate magnet shapes held to very close tolerances. A bonded magnet will not shatter or crack but it will chip, tear, or fracture with mechanical abuse.
  • Of the three types - there actually are several more - there is another, further classification of either metallic and ceramic. A metallic magnet is obviously made from a metal or alloy of metals. A ceramic magnet - some refer to this type as a ferrite magnet, (either term is correct) - are usually made of metal oxides or families of oxides. They are called "ceramic" because their physical properties resemble those of porcelain - hard, brittle and tend to fracture under an impact. The metallic types will chip or dent randomly. Ceramics tend to fracture in a long, regular split along a weak or stress line in the material. You may have a metallic sintered magnet...the metal allow was powdered then sintered into the final magnet...but, it would be unusual, although not unheard of, to have a cast ceramic. When someone says they have a polymer magnet, they don't have something made of weird exotic material from Mars. The word "polymer" is a chemist's way of saying nothing more than the word "plastic" and you now know that it is a bonded type of magnet, right? You may have a metallic plastic magnet. Sounds like a contradiction in terms, but it really isn't. It's just an allow of magnetic metals that have been powdered and suspended in a plastic binding agent. Most, if not all, 1/24th slot car magnets are of the sintered ferrite, or ceramic - whichever you prefer - types. Some newer 1/64th HO magnets are of the "polymer" (bonded plastic) types. Way back in the 60's, cast alnico magnets were mostly used until Champion, along with Mura and others, pioneered the "dot" series of magnets that were of sintered ferrite material. Parma, about 1974, if memory serves me, introduced a samarium cobalt that replaced the stock 16D magnets. Yes, Virginia - a cobalt 16D from Parma! The cost of a sintered, or cast cobalt the size of a 16D magnet would have made the National Debt look like a seven year old's allowance by comparison. These Parma critters were a bonded metallic plastic magnet with rare earth compounds being the active material. What I would give to have a case or two of them now...!
  • The rare earth (cobalt, neodymium, etc.) types deserve a discussion apart from everything else. First off, the "rare earth" is a very poorly chosen name. Most of the materials that fall into this category are neither "rare" or of a material type implied by the term "earth." Somebody, somewhere, decided to call the elements on the periodic table between 58 and 71 "rare earth." I have no idea why. Cobalt is specifically not a rare earth, even under this definition, but confusion persists. It doesn't matter if you call an orange an apple as long as you are consistent about it! Anyhoo...a rare earth type of material is very unusual in its properties. Some elements, when combined in certain ways, exhibit a synergy of one, or more of their physical properties. That is, the effect taken as a whole is greater than the effect of each individual property added together separately. Rare earth magnets exploit the magnetic synergetic properties of the materials used. Most of the rare earth types are a hybrid of casting and sintering. The magnetic materials are allowed in a melting pot under an inert gas atmosphere such as helium or argon, then cast into an ingot. The ingot is then milled or crushed into very small particles, typically a few microns in size. This powder is the magnetically aligned (zapped) during the sintering pressing, again under an inert atmosphere and heat treated. This yields a small but very powerful magnet with closely controlled mechanical dimensions, ala strap can types. Because of the many special steps involved in manufacturing, they tend to be somewhat expensive per piece. As with everything, you pay for performance. In case you are interested, the actual "rare earth" elements, according to my periodic table, are in the Lanthanides group and specifically are Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Galolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, and Lutetium. Some texts refer to Lanthanum, Scandium, and Yttrium atomic numbers 57, 21, and 39 respectively, as being grouped in the general "rare earth" class used with magnetics...although this is incorrect. Of these materials, Holmium, a bright silver, very soft metal, is the most potentially powerful, but this is theory. There is no known way to magnetically charge (zap) it. Neodymium is next to impossible to charge without very specialized equipment, and, in theory, Holmium is 14 times more difficult, provided you can find enough of the stuff. It indeed is a very "rare earth" substance. Simple, huh? Next we look at some of the terminology involved with magnet and magnet manufacture. You may have heard of "orientation" at one time or another. This simply means that the magnet prefers to be magnetized in one direction. For example, in a slot motor, if the "north" pole is on the outside of the curved surface, the "south" pole will then be inside of the curve. The poles are through the thickness of the magnet, right? By the same token, the poles could very well be created through the length of the magnet rather than the thickness. The motor wouldn't be at all happy but this could be done simply by putting the magnet in the charging fixture end to end rather than through the thickness. An unoriented magnet would be happy as a lark in a meadow either way. If the magnet was made of an oriented type material with the orientation being through the thickness, you could create the north and south poles through the length, but they would not want to stay there. The poles would shift at the slightest provocation (shock, heat, etc.) to the direction of the orientation...in this care the thickness. You cannot create an orientation. It's done at the magnet mill during manufacture. The only difference between an oriented magnet and an unoriented magnet of the same material is whether they exposed it to a magnetic field - essentially "zapped" it - when it was in the mold, cooling...being heat treated after being pressed...or drying, if it was of a polymer type.
  • Magnetic saturation is just as it sounds. What happens when a towel is saturated with water? It can absorb no more, right? There is no point in trying to use it to pick up some more water because it will not. So it is with magnetic saturation. When you saturate a magnet, it contains all of the magnetic field it can hold...as strong as it can and ever will be. The point of saturation varies, just as the strength of the magnet does, from magnetic material to magnetic material, but will not vary for a given type of material. In lieu of going into quantum physics about spin theory, streamlines and equi-potential contours, and myriad other such nasty topics, a much simpler way to explain what happens when a material becomes magnetically saturated is to imagine that material as being made up of millions and millions of microscopic bar magnets, each with a north pole on one end and a south pole on the other. When these bar magnets are in a random order, there is no discernable magnetic poles on the material because all of the individual poles within the material tend to cancel one another out. When you expose the material to an external magnetic field, some...not all...of these bar magnets begin to align to the field. Opposites attract, like poles repel. What actually "flips" is the way in which an electron orbits in an atom. A funny thing happens when you remove the field and the material is of the permanent magnetic type I spoke of earlier. The domains (little bar magnets) that flipped, stay flipped. Now, there may be 1% of all the magnetic domains with their north poles pointing in a given direction. Guess what? The material will now begin to show a north pole and a south pole. Hit it again with a little stronger external field. Now, and other 3% flipped, so 4% of the total number of domains are now aligned in one direction. The material now shows stronger poles on its ends. As Dr. Frankenstein said, "MORE POWER!" and eventually 100% of the magnetic domains will align themselves with the external field. The material is then magnetically saturated. You can keep pouring on the power, but once all the domains flip, there will be no more strength to be had from your new magnet. I had a bumper sticker on my old Volkswagen Bug that sums up magnetic saturation theory well..."That's all there is, and there ain't no more!" As a rule of thumb, the more powerful a magnet is, the more energy it takes to saturate it. This may seem pretty elementary, but when you consider some of the energy levels of modern neodymiums, and one puts together the mathematics as to how much power is required to saturate one of those little critters, one begins to envision a power company substation in one's back yard...seriously! To date there are only two companies that I know who have equipment that will insure complete saturation of a neodymium-iron-boron magnet of any useful size to the slot car world. You don't want to know what such a size magnet would cost, let alone the equipment to charge it. Saturation is ideal, but zapping your magnet at any energy level is better than not zapping it at all. It can't hurt, provided attention is paid to the poles, and could help.
  • At one time or another you may have heard someone say, "Yeah...I had my magnets gaussed at 6000" or similar uninformed dribble. Do you recall when you were in school and you sprinkled iron filings on a piece of paper over a magnet? The filings arranged themselves in many arcs from one pole to another. What you were seeing was a representation of the magnetic field (flux) of the magnet. The actual magnetic effect does not really consist of lines of force, but it is a good representation. The more lines, the more powerful the magnet. If we could count these lines we would have a number that would be proportional to the strength of the magnet. A gaussmeter does this for you. One gauss is equal to one line of force per square centimeter. Remember how those iron filings seemed to get fewer the further away from the magnet they went? The number of lines of force per given area actually decreases as one gets further from the magnet. Also, there are no flux lines directly over the poles, hence no filings. That piece of paper only represents one horizontal slice of the actual flux surrounding any magnet. Picture what you see on the paper, but in 3D. Not unlike a ripe apple, huh? Don't laugh. It actually is very close to the flux field that surrounds any magnet. The top and bottom of the apple are the poles. With this in mind, how can 6000 gauss have any meaning at all if it is not specified exactly where that reading was taken? The point I am trying to make is that a gaussmeter is a very valuable tool used to investigate magnetic fields, but it will tell you absolutely nothing useful unless you pay attention to what you are doing. You must take any measurements at very definite, repeatable points around any magnet to identify the field, paying attention to the probe's angle, relative to the field. You then plot the result in either 3D or with several horizontal slices on the same graph paper to get any idea of what you actually have. I have used a 3x3x3 block of acrylic plastic, milled to accept a magnet, and drilled on the face of a 1/2 inch grid. Have your probe marked off in 1/4 inch divisions. Take readings at each hole in 1/4 inch depths and write it down. Yes...it takes a lot of time. Who said it wouldn't? I have seen one guy sort motors by just sticking the probe in a can and saying, "This motor will be faster than that motor." Gag me with a gaussmeter! I just watched and chuckled to myself. Flux density measurements are very critical if anything is to be gained by them.
  • Magnetic flux density (field strength) is affected by many external forces. Heat is not as serious as one might expect. Typically, a ferrite magnet will lose 3-5% of its strength by being elevated from room temperature to 100 decrees Celsius...the boiling point of water. When it cools, the strength returns to very near normal, so this is a reversible effect. There is a point where a magnet will lose all of its strength, never to return until rezapped. This is called the "Curie temperature." For a ferrite magnet, this is somewhere about 450 degrees C, about 850 degrees Fahrenheit...well below "red hot," but somewhere just above "blue hot." A cobalt curies at about 800 degrees C. Have you ever wondered by neodymium magnets are not used in slot cars? The curie temperature is about 300 degrees C, much to low to be useful for much of anything.
  • Another issue with magnets and heat is soldering. Most irons used for slot car building sit somewhere near 1000 degrees F which is well above the curie temperature for a sintered ferrite or cobalt magnet. Don't bake the motor with the iron or you will kill the magnets. The silent killer of magnets is called "contact demagnetization." Every time you allow a magnet to stick to something, the magnet loses energy! The effect is not as serious on the poles as it is on the sides or ends. Have you ever wondered why the little metal bars you throw away are placed across the open end of a horseshoe magnet? The reason they were placed there was to close the magnetic flux path so it would not lose its flux density by being handled. So it is with all magnets. If you have an exceptionally good set of magnets you would like to preserve, keep them together to close the flux path and do not allow them to stick to anything. A plastic box with foam will do nicely. (Isn't that how most cobalt and expensive magnets are packaged when they come from the manufacturer and distributor?) The worst possible way to package magnets is in a poly bag, stapled to a card where they are allowed to contact everything and stick together. Even sliding the magnets in the can will lose some flux density. By removing the armature, the flux path will be partially opened and the magnets will be weakened to a degree. Packaging, handling, and resulting contact, demagnetization are all necessary evils. Just keep in mind that when building a motor from scratch, a magnet recharge (zap) is certainly called for. Impact has an effect on he flux strength, but it really isn't significant...typically less than .001% per wall shot. What may have a more profound effect is the magnet sliding in the can because of the impact. It is best to Loctite or Superglue your magnets in place in the motor can.
  • So what does all of this mean to the average slot car owner? Well, on the face of it, very little, unless you are willing to take on some homework. I hope I have cleared up some confusion about magnets and magnetic properties. The study of the magnetic effect itself lends well to the days of alchemy and witch hunts. Most slot car owners believe that a magnet is a magnet, and if I dig one out of the bottom of the box and shove it in the can, the motor will run. In part, this is true...the motor will run. How well remains to be seen. As with any high performance sport, attention to details is what wins races and makes fast cars. Take some time and investigate "What if???" Keep a notebook because, at least for me, pale ink seems to work a lot better than a good memory. Armed with a basic understanding of magnets and magnetic properties, and with a little research on your own, you can transform the most overlooked and least understood component of your slot car to the "edge" over the competition.
  • Magnets may be the inside secret behind a motor that runs like a banshee! Think about it...and then tinker... John Sojak