FIVE-AXIS MACHINING

In three-axis machining, the fixture is designed to hold the part in one position. A five-axis machine has more flexibility in its movement to reach all surfaces, but it can only do so if the fixturing or workholding allows access.

With five-axis machining attention must be paid to clearance and eliminating interference between the workpiece fixturing and the machine table, spindle housing and cutting tools. This is especially evident when machining a small part on a large table. Clearance and interference problems occur because small parts tend to be concentrated toward the center of the machining table. The edge of the table can become an impediment as the table pivots.

With five-axis machining attention must be paid to clearance and eliminating interference

Elevating the Workpiece

One way to mitigate interference issues is to use longer tools, but that limits how aggressively you can machine. It is also common practice to elevate the workpiece by using vices with long jaws which are available from a number of manufacturers. (Pictured, Schunk's Kontec vise is designed for more secure 5-axis machining). These devices elevate the workpiece while clamping on a narrow band at the bottom of the workpiece to provide maximum access to the 4 sides of the part as well as the top. These vices provide a flexible solution for many parts but will have limitations in heavy machining applications where a high degree of stability is required. Some vises just lift the part while others are built for more secure machining.

More recently workholding systems designed for five-axis machining are becoming available that elevate the part and offer secure workholding. A good example is Big Kaiser’s Unilock five-axis workholding system. The system has a clamping knob (retention knob) and the clamping chuck. The knob is attached to the underside of the fixture and the chuck is installed on a base on the machine’s table. The workholding system that attaches onto that knob is designed to be robust but also small, so it fits into the footprint of the workpiece, providing access to all edges of the workpiece. It also includes a modular stacking system that gives users the ability to change the height of the workpiece in relation to the table.

Dovetail and Custom Fixtures

Dovetail fixtures are another workholding solution for five-axis machining, best applied to relatively small parts. The dovetail has enough holding power that it can hold parts well within their footprint, giving complete access around all five sides of the workpiece. The challenge with this type of fixturing is that it doesn’t work as well on curved surfaces, and it can’t be used on parts with varying widths. This can lead to overhang that is detrimental to the overall stability of the part.

Custom fixtures do have their place, particularly for repeat production work or families of parts. For example, the fixture shown here features four posts with their tops angled at 45 degrees. This provides sufficient spacing and clearance around parts when they are installed in the dovetail clamps that top the posts. This fixture was built by Powill Manufacturing and Engineering (Phoenix, AZ).

The dovetail has enough holding power that it can hold parts well within their footprint, giving complete access around all five sides of the workpiece.