What was once considered a black magic has become a more calculable science. Still, many variables affect the clamping force of a magnet. Some of these variables include: material composition, surface finish, contact area, vibration and cutting forces.

An important fact to understand about magnetic clamping is that all magnets have 100 percent clamping force directly away from the face of the magnet, but only 20 percent clamping force against slide forces. This is the coefficient of friction. Often people will improperly judge a magnet’s ability to hold for machining by striking a magnetically clamped part on the side with a mallet.

Unfortunately, this does not accurately replicate machining forces. Cutting forces are usually multi-directional – including down into the table. Serious manufacturers of milling magnets offer cutting formulas to aid in the determination of viable cutting parameters.

In some cases, cuts on a magnet will exceed conventional clamping due to reduced vibration or increased holding force. In other cases, cuts may have to be reduced for magnetic clamping. In these cases though, the additional cut time may be offset by either a reduced number of setups or increased feedrates. The addition of physical stops can increase stock removal by up to two times over not using stops.

Mild steel is the most receptive material to magnetic clamping. Tool steels are 15 to 30 percent less permeable and cast iron is 30 to 50 percent less permeable than mild steel. Therefore, moldmakers need to be aware that cutting calculations need to be adjusted when other material, such as tool steel, are considered. Magnetism does not like air gap, whether caused by surface finish or vibration.

Air gap is any space between a workpiece and the surface of a magnet caused by uneven workpieces, rough finish on a workpiece, nonmagnetic spacers like shims, or vibration. Therefore the better the surface finish and proper pole placement – to avoid vibration – the greater success one will have using a magnet.
Rough finish material can be milled on a magnet if the proper cutting calculations are used. Some magnets have greater magneto motive force (MMF) – a magnet’s ability to bridge air gap – and are more suited to regular usage on rough surfaces.

Otherwise, a simple down rating on the cutting forces will suffice. Keep in mind, however, that a permanent electro or a permanent milling magnet will not suddenly lose its clamping force. Therefore, if a magnet is overpowered by a cut, the part simply slides across the face of the magnet. This prevents serious damage to the part, cutter and machine as well as injury to anyone near. If a cutter runs through a conventional clamp or if the clamp or vice becomes loose, the part will fly.

Vibration on a magnet can be caused by numerous conditions. One cause is too much overhang or too much space between poles.

As a rule of thumb for standard cutting calculations, overhang should not exceed the thickness of the part, and the distance between the poles should not exceed 2.5 times the part thickness. Another cause for vibration is an air gap caused by a non-flat workpiece or a rough surface finish. Some magnets offer self-leveling devices to address a non-flat workpiece. Rough surface finish is best handled by adjustments in the cutting parameters or through the uses of a magnet designed for rough finishes.

A critical point to understand regarding magnetic clamping is the area in contact with the magnetic poles. It is not the number of total poles in contact with the workpiece that determines the holding power, but rather the number of equal north and south poles (polarity) that are in contact that determine the clamping force. If the area of a part is in contact with more of one polarity than the other, the extra magnetic contact actually interferes with the ability to machine the part. Magnets offering magnetically balanced north and south poles combined make for easier setups.

Magnetic clamping offers as many benefits for the small shop as it does for the production machining shop. Magnets can offer full access to all five faces of a part in one setup. For example, a magnet laid flat on a pallet of a horizontal mill will allow the machine to mill around all four faces and drill into all four faces in one setup – an ideal squaring up process.
In addition, the use of magnets also makes programming easier. No longer is there a need to coordinate clamp location with the program.

Magnets reduce vibration, therefore tooling is freer to run faster and longer. Through the use of top tooling or modular poles, a block of steel can be faced, edged and drilled through in one setup. With greater access to a part, a machine can run longer unsupervised (lights out) – resulting in higher productivity.

Milling on a magnet benefits both VMC and HMC operators through setup reduction and improved safety as well as reducing the risk of losing your qualified locator by reducing the number of setups. Magnetic setup includes loading parts, indicating parts and energizing the magnet. Energizing a magnet requires less than one second of electricity. In comparison, many injuries during setups are a result of reaching while tightening a clamp, or of trying to support work on a horizontal machine while tightening a clamp.

Other advantages of milling on a magnet include consistent clamping force. Regardless of the strength of an operator, the clamping force of a magnet is a constant.

Most milling magnets offer pole extenders to raise a part off the surface of the magnet. This allows not only for milling, but also for through drilling. There also are new individual magnetic setup blocks that can be located anywhere under a part and used as a milling magnet or a magnetic work support with conventional fixturing.