As routers become more accurate and capable they are used increasingly to machine parts that, in the past, typically were machined on CNC milling machines. Routers with large machine tables and coupled with highspeed spindles are ideal for machining thin and small parts from sheet or plate material.
The challenge has been the workholding. A vacuum table is a good way to hold a sheet in place. However, because of the insufficient surface area of the part to render the vacuumholding power effective, it does not allow for small parts to be cut in one setup.
Keeping the parts attached to the sheet with tabs is also common. This requires the extra steps of removing the tabs to separate the parts from the sheet plus handwork to de-burr the parts. Tab removal often leaves unacceptable marks on the part contour. Also, tabs require more space between the parts so more material is required. For highvolume production, such as aerospace components, this is a major cost factor.
Sometimes, marginally sized parts can be held in place by vacuum and routed in one step without using tabs if the feedrate is reduced sufficiently. Unfortunately this also reduces the production rate.
Double-sided tape is also often used to hold small parts securely in place. The disadvantages are the difficulty of removing the often very fragile parts from the tape without distorting them, and the residue from the tape’s adhesive requires a cleaning step. Adhesive spray is another option, but this is messy and residue on the parts also requires a cleaning step.
“A product is now available in the U.S. that overcomes many of the difficulties of holding small, thin and delicate parts when machining,” Gerd Bode president of Stackmaster Engineering Corp. said. Vilmill, available in North America exclusively from Stackmaser, is used as a substrate between the sheet or plate material to be machined and a suitable vacuum table or vacuum chuck where it is held securely in place by the vacuum. “It acts as a holding fixture and can double as a conveyor belt for automatic loading and un-loading. With vilmill, the feedrate of appropriately powered machines can be significantly increased over that possible with other holding techniques,” Bode explained.
Vilmill is only 0.010-in. thick and has an adhesive coating on one side. The adhesive is activated by the heat generated in the machining process. Parts are bonded to the vilmill approximately 0.025 in. around the periphery and are held firmly in place during machining. They can be peeled off easily when machining is finished. The adhesive leaves no detectable residue.
“Vilmill is becoming increasingly popular not only in the aerospace and electronics industries and for routing control panels, but also for machining name tags and with users of small routers and engraving machines, as well as with jewelry makers,” Bode explained.
For machining metal, a lean mist is used to keep the chips from building up on the router bit. For plastic, only a soft air blast is used to blow away the chips. Since the heat that is generated in the milling process is needed to activate the vilmill adhesive, no coolant can be used. This limits the use of vilmill to machining non-ferrous materials, such as aluminum, brass and plastics.
Vilmill is available in 100-meter rolls in widths of 1.22 m (48 in.), 1.5 m (59 in.) and 1.8 m (71 in.). The 1.8-m wide product is available by the yard. If purchased by the roll, vilmill costs 50 cents per square foot.