Magnetic Matters

Magnetic workholding has made moldmakers more efficient on the shop floor, but choosing the most appropriate technology and achieving optimal results requires a deeper understanding of the mechanics, benefits and limitations of the different available solutions. Three of these types of magnetic workholding devices are permanent magnetic, electromagnetic and electro-permanent magnetic chucks.

• Permanent magnetic chucks. This type of workholding does not require electricity for activation or de-activation, but is instead mechanically activated with a simple lever. The magnets inside the chuck are always active, but when the lever is in the “off” position, the magnetic field is contained inside the chuck. When the lever is rotated to the “on” position, a mechanical grid slides and allows the magnetic field to extend outside the chuck and into the steel part that is being machined. This mechanical control means that a power failure will have no effect on the chuck’s hold on a part. Also, the permanent magnetic chuck does not emit any heat, so the part it is holding will not experience any heat distortion.  There are a few drawbacks to this type of workholding, however. Wear and tear on the internal moving parts and magnets will shorten the life of the permanent chuck, and its maximum size is typically restricted to 1000mm by 300mm, primarily because of the great mechanical force that would be required to activate it if it were any larger.

• Electromagnetic chucks. Unlike in a permanent magnetic chuck, there are no actual magnets inside an electromagnetic chuck. Instead, a magnetic field is created when an electrical current is applied to a coil wrapped around a mild piece of steel inside the chuck. When there is no electrical current running through the coil, there is no magnetic field. 

  A benefit of this type of workholding is that there are no moving parts inside, so there is less internal wear and tear. In addition, the magnetic field height and the holding force of the chuck can be fine-tuned by applying more or less power. A disadvantage of an electro-magnetic chuck, however, is 
  that constant power is needed in order for it to hold the part, and this constant power can overheat the chuck and cause heat distortion of the parts. Also, a power failure will cause such a chuck to release a part, potentially causing an accident. 

• Electro-permanent chucks. This is a mix of permanent magnetic and electromagnetic technologies. The inside of an electro-permanent chuck contains actual magnets composed primarily of alloys of aluminum, nickel and cobalt (alnico), as well as a coil that is wrapped around those magnets. A one- to two-second programmed electrical impulse sent into the coil will magnetize the alnico magnets, enabling the chuck to hold the part. With a reverse impulse, the alnico magnets become completely inactive, allowing full demagnetization of the parts being machined. 

  Some electro-permanent magnetic chucks also will contain additional neodymium magnets to increase holding force. These neodymium magnets cannot be fully demagnetized, however, leaving residual magnetism in the part. These chucks are more specifically called compensated electro-permanent systems.

  The primary benefit of using electro-permanent chucks in moldmaking is that they provide full access to the five sides of a mold. Because the part is magnetically clamped to the chuck, there is no need for vises that might get in the way of the tool path and require multiple workholding setups. These chucks allow for face milling, edge milling and even through-drilling in one setup. 

  Additionally, electro-permanent chucks do not require a constant power source to maintain activation, so the absence of power cords allows for full range of motion during machining with a moving or swivel table. Power is only needed for the two-second impulse that either magnetizes or demagnetizes the chuck. The chuck holds the part even during a power failure, and it remains cold, so there is no heat distortion of the parts. And, like the electromagnetic chuck, there are no moving parts inside, eliminating internal wear and tear.

  Naturally, with every pro comes a con. Electro-permanent chucks are more expensive than electromagnetic and permanent magnetic chucks, and their magnetic fields and power can be difficult to fine-tune. This is because the strength of electro-permanent chucks increases in irregular steps, unlike electromagnetic chucks, in which the increase in strength relates directly to how much power is running through the coils. 

Of course, magnetic workholding solutions aren’t limited to the three described here. The world of magnetics is always evolving, and great advances already have been made in the manufacture of lifting magnets, workholding magnets, filtration systems and magnetic quick-mold-change technology.