Holding Small, Thin Parts for Effective Machining on a CNC Router

A non-woven porous material with a special coating enables vacuum chucks and tables to hold sheets of metal and other materials more effectively on CNC routing machines. The material, called Vilmill, 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.

Article From: 4/30/2010 Modern Machine Shop, ,

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Workholding on a CNC router can be a challenge because vacuum plates or tables have limitations. A material with unusual properties helps overcome these limitations to make CNC routing more productive.

 

In the past, CNC routers have primarily been used to cut contours and 3D shapes in wood, plastic and foam for signs, plaques and more. Over the years, however, routers have become more accurate and capable and are used more and more to machine parts that were once machined on CNC milling machines. The larger machine table sizes of today’s routers, coupled with the high speed spindles, makes them an ideal choice for machining thin and small parts from sheet or plate material.
 
With these machines, however, the challenge has always been the workholding. A vacuum table is one good way to hold a sheet in place for machining on a router, but this method faces some limitations and disadvantages. An unusual solution has been developed to overcome the shortcomings of vacuum workholding. The solution uses a thin, porous material placed between the vacuum plate and the workpiece stock. The material allows vacuum pressure to hold the stock in place while a special coating keeps parts lightly bonded so that they remain in place once cut free from the plate. According to developers, this material makes routers more widely applicable for productive machining of thin and small parts.
 
One of the problems with vacuum workholding is that it often does not allow small parts to be cut in one setup, because the surface area of the part may not provide sufficient holding power. Keeping the parts attached to the sheet by way of tabs is a commonly used method to overcome the limitations of vacuum workholding for these applications. This method requires the extra steps of breaking the tabs to separate the parts from the sheet and deburring the parts where the tab was attached. This step often leaves unacceptable marks on the part contour. Additionally, the use of tabs may require more space between the parts and so more material is required. For high-volume production, such as in the aerospace industry, this presents a major cost factor.
 
Marginally sized parts can sometimes be held in place by vacuum and routed in one step without the use of tabs if the feed rate is reduced sufficiently. Unfortunately this reduces the production rate.
 
Double-sided tape is also frequently used as a means to hold small parts securely in place. However, it is often difficult to remove the fragile parts from the tape without distorting them, and reside from the tape’s adhesive often requires a cleaning step. Adhesive spray is another option, but it is messy and residue on the parts requires a cleaning step, as well.
 
The new workholding material, called Vilmill, is an alternative that combines vacuum and adhesive workholding properties. It 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.
 
This non-woven material is 0.010-inch thick and very porous. When placed between a suitable vacuum plate and the workpiece, it increases the workholding power significantly in two ways, developers say. It provides an even vacuum over the entire surface of the vacuum fixture, not just over the small holes in it. The real benefit, however, comes from the adhesive coating on one side. The material is placed on the vacuum plate with the adhesive side facing up. The plate stock to be machined is then placed on top of it. Unlike the adhesive on double-sided tape, the coating is not sticky until it is activated in the milling process by the momentary heat generated by the cutter. The machined parts are thus lightly bonded along the periphery approximately 0.020 to 0.030 inch wide on both sides of the cut. According to developers, finished parts can easily be peeled off with no visible adhesive residue left to be cleaned.
 
Initially the vacuum is the sole method of securing the plate or sheet in place as there is sufficient surface area. All pre-machining, such as 3D and pocket milling, can be done on all parts as they are nested on the sheet. The milling of the part contours can be done in one step on thinner material. Thicker plates may require more than one cut. When the part contours are being cut, the router bit or end mill will cut about 0.003 to 0.005 inch into the Vilmill in order to keep it as one sheet, because it is now the holding fixture, which itself is being held securely in place by the vacuum.
 
Developers report that the product 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. Several router manufacturers are now offering routers with vacuum tables for use with this workholding material.
 
Because the heat that is generated in the milling process is needed to activate the adhesive coating, no fluid coolant can be used. This limits its use to machining non-ferrous materials such as aluminum, brass, copper and plastics, including glass reinforced printed circuit boards. For aluminum, a lean lubricant mist from a focused nozzle, positioned close to the cutter, is recommended to keep chips from building up on the cutter. For plastic only a soft air blow is used to keep the chips airborne for easier and more effective chip collection.
 
Some benefits of using this approach to workholding on a router are:
 
  • Increases production by allowing feed rates of appropriately powered machines to be dramatically increased, especially on small, thin and otherwise hard to hold parts
  • Parts are held securely in place for machining
  • Vilmill provides even vacuum over the entire surface and thereby increases the holding power of a vacuum chuck or router vacuum table
  • No tabs for part-holding required
  • Savings on material. With no tabs, the parts can be nested closer together.
  • No clamping or re-clamping
  • Better edge finish (part contours are machined in a single setup)
  • Parts can be removed easily without distorting even fragile ones
  •  No parts clean-up needed (there is no detectable residue after machining)
  • Environmentally friendly (no wood dust from MDF board and no spray adhesive)
  •  Vilmill can double as a conveyor belt for automatic loading/un-loading
  • Low cost (approximately 50 cents per square foot if purchased by the roll)

 

This product is available in rolls of different widths: 1.22, 1.5 and 1.8 meters (48, 59 and 71 inches). There are 100 meters (109 yards) to a roll. As an option, it can be used much like a conveyor belt to automate production by pulling new sheets onto the machine table and finished parts off. Stackmaster Engineering, the exclusive distributor of Vilmill in North America, recognizes that not every job or shop requires a full roll. They will sell the 1.8-meter (71-inch) wide Vilmill by the yard.

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