Five Axis Machining And Workholding

Five axis machining offers many benefits for injection mold making and is rapidly becoming the norm in CNC milling. One of the problems associated with 5 axis milling is workholding. How do you hold your workpiece to effectively machine 5 sides of the workpiece?

Two of the more common approaches to 5 sided machining are mechanical clamping systems and workholding magnets. It does not make economic sense to invest in a high end 5 axis milling machine and skimp on the workholding. The maximum benefits can realistically only be realized with proper tooling.

 The mechanical systems are more common, and are really like a vise with superior clamping force and a minimum of interference with the cutting area. With five axis machining and a clamping system in place, it is typical to machine the 5 exposed surfaces with a high degree of accuracy.

Small to medium sized workpieces can be held in the clamping system easily. For larger pieces it sometimes necessary to use a riser in order to reach the workpiece.

Because these clamping systems are highly engineered pieces of equipment, the clamping forces are applied where they are most needed. Also, pointed grippers are incorporated in the vise jaws for additional clamping security.

The magnetic solution is also a viable method of workholding. At first glance one would imagine that the machining forces would exceed the magnetic pull on the workpiece, resulting in the part moving, producing scrap.

Yet this is rarely a problem because many 5 axis machines are able to use high speed machining techniques. The high speed method requires higher spindle speeds, lighter chip loads, and faster feeds; all of which result in lower torque and cutter pressure.

One method is to mount the workpiece on a magnetic pedestal, which elevates it and makes it possible to machine all 5 exposed sides. The pedestal is slightly smaller than the workpiece, which exposes all 5 sides for machining. This, of course, reduces error because the workpiece does not require a second set-up to access all 5 sides.

An additional benefit of magnetic workholding is palletization. In a similar manner as EDM pallets, you can move the palletized magnet from the 5 axis machine, to the EDM, gun drill, CMM, or any other machine that is set up for pallets.

The magnets used in this way are electro-magnets, which means that they are activated electrically. Once the magnet is activated, no additional power is required, except to release the magnet. This is advantageous in the event of power outages, as well as when moving the magnetic pallet from machine to machine.

The use of magnetic pallets requires considerable planning and control, but is a huge time saver, when done correctly. Robotic tool changers can also be incorporated in the entire process.

Five axis machining is here to stay and most modern injection mold making shops use it on a daily basis. Finding the right workholding device is critical to success in 5 axis machining.