Can I drill a hole and or thread a magnet?
As a general rule, cast or sintered magnet materials are very hard (RC46). This is 46 on the Rockwell “C” scale, which is harder than commercially available drills and taps, so tools will heat up and become soft and dull, or break, before making a dent in these magnet materials. The plunge EDM process must be used to make holes, and threaded inserts can be epoxied in if threads are needed. NeoForm and other magnets made with an epoxy binder can be drilled, but the process works on the binder, not the magnet material. Threads in these materials can be stripped easily.
Define the EDM process; what are the process limitations?
Two types of EDM (electro-discharge machining) processes are performed; Wire EDM and Sinker EDM.
Sinker EDM is a process typically consisting of a graphite, brass, or copper tube-shaped electrode that is slowly fed into a workpiece that is immersed in oil. A pulsed electrical discharge from the electrode causes sparks to jump to the workpiece, each tearing out a small particle. Oil, also known as cutting fluid, typically flows through the center of the electrode to help flush the particles away from the cutting path. The electrode gradually erodes its way through the workpiece. Surface finishes are typically rougher than in grinding processes, however secondary cuts or slower cutting speeds may improve finish. Tolerances typically range from ±.025mm to ±.127mm depending on the material being cut and the length of the cut (or electrode travel).
Ceramic magnets cannot be machined utilizing EDM because the material does not allow for the “electrical arcing” to take place between the electrode and the work piece.
Wire EDM is a process very similar to Sinker EDM except the electrode carrying the electrical pulse is a very thin (0.010″) zinc-coated brass wire. Whereas the copper electrode gradually disintegrates in Sinker EDM, with Wire EDM the wire electrode simply carries the current to the work piece as it unravels from a spool. With the benefit of the ultra-thin wire, extremely tight tolerances can be held. Surface finishes are similar to Blanchard grinding, however secondary cuts are often made to achieve a smoother finish and even tighter tolerances. Typical tolerances can be held to ±.005mm to ±.127mm.
Just as in Sinker EDM, the work piece must be electrically conductive in order for the EDM process to work. Hard and soft ferrite materials (ceramic magnets) as well as some epoxy-rich bonded magnetic materials cannot be cut utilizing the EDM process. Dexter has both processes in a CNC-capable format for unattended operation.
What are the differences between cast and sintered Alnico parts, and what are the processes?
Cast parts are formed by pouring molten metals, or alloys, into sand or ceramic molds. The molds have sprues and runways to direct the molten metal to one or more female cavities with the desired magnet shape. The cavities are vented for the gasses that form, and there is shrinkage on cooling, so post-casting clean up and / or grinding operations are usually needed.
Sintering is a molecular joining of pure metal powders, or alloys, with heat, but without liquefying them. Wax is blended into the sinter powder before pressing so the pressed parts keep their shape. The binder burns off in the furnace before the sintering process begins.
Automatic presses allow sintered magnets to be formed quickly and continuously, so this is usually a lower cost process, but die pressing places practical limits on size to about 150 grams, and length is limited to roughly 2.5 times the diameter. Sintered parts shrink somewhat when fired, but are very consistent in size so they can often be used as is, or with a minimal amount of post sinter machining. Sintered parts tend to be more homogenous, with much smaller grain sizes and grain boundaries.
Both cast and sintered Alnico magnets must go through a heat treat cycle in a magnetic field to develop their magnetic properties. Magnetic properties of an equivalent size sintered Alnico part were at one time lower than that of a cast part, but the difference has virtually disappeared for most grades and geometries.
What are the expected surface finishes when machining magnetic materials?
Magnets may be shaped using any number of machining processes, such as cutting, grinding, EDM, or even abrasive water-jet. Typically, most magnets are brought to finished size by utilizing a grinding process. Most grinders will produce at least a Ra = .4 µm surface finish on most magnets using a standard grinding wheel.(When grinding Alnico, surface finish would be improved Ra = .2 µm, due to material hardness).
Blanchard grinders will produce at least a Ra = .8 µm surface finish on all magnets using a standard wheel. (Again, Alnico would theoretically be higher [16BL] due to the hardness.) Other machining processes (EDM, water-jet) can be controlled by altering cutting speeds in order to produce similar surface finishes to what is seen in grinding processes. Therefore, the easiest way to remember surface finish grades on magnets is to remember the numbers numbers .4 and .8.
What are the most common cast Alnico quality problems?
Generally, the physical quality of Alnico has improved considerably because this material has been around for a long time and many producers compete for this business. Internal voids and cracks are the most frustrating problems as they may show up only after a lot of work has been done.
With pressed high energy magnets, the orientation is not perfectly parallel or perpendicular to the pressing direction, why?
Orientation is induced by pressing the parts in a magnetic field. The magnetic field is generated by a pair of coils, one on each side of the part. High pressures are used to make the parts so the powders cannot “flow” much in the die cavity, which results in some degree of uneven compression in the parts. Even though the orienting coils are in very good mechanical alignment, a variation in density results in an inhomogenous distribution of energy capacity. Upon magnetizing, this reflects itself as a magnetic moment with the average properties of the part volume, and the effective external magnetic field alignment can differ from that intended.
Only coils with the Helmholtz geometry (mean coil radius = mean coil spacing) generate a very uniform orienting field. This would require coils typically much larger than the space available. The net result is that less than optimal coils must be used and the orienting field curves away from the centerline of the part being pressed. This does not matter much if the whole pressed part is used, or the central section, since the average field alignment is quite good. However, small parts made from the edges or corners of pressed block can exhibit some measurable misalignment of the magnetization axis. It should be noted that we have seldom measured more than a 3° misalignment, and that was usually due to some other factor. An angular offset of the external magnetic moment with respect to part axis is often more the result of part shape than the block orientation.