14Phone: 516-328-3970www.technocnc.comTechnical SectionVacuum PumpsVacuum hold-down and which pump to choose can be very complex subjects. There are several different pumps available for use with a CNC machine and all have varying specifications and price ranges. The proper selection of a vacuum pump is imperative for optimal machine
performance.To avoid paying too much or not enough, vacuum systems need to be evaluated based on the customer’s specifi c applications rather than cost alone. There are two main items of concern when looking at the specifications of a pump: vacuum level and fl ow. Vacuum level is typically measured in terms of inches of mercury (i.e. “Hg). This is the same term used when reading a barometer. So, it’s no surprise that the vacuum utilized in CNC vacuum hold-down works by atmospheric pressure applying 15 lbs/sq.in. (psi at sea level) in all directions. During a CNC routing operation, when you use vacuum to remove air from one side of the material being cut (the underside), then the atmospheric pressure on the other side (topside) increases, in effect, pushing on the material. How much hold-down forceis applied to the material can be calculated by reading the vacuum level gauge, multiplying that numeral by surface area of the material (sq.in.) and multiplying that by 0.5. For example, let’s look at a gauge that reads 22”Hg for a material workpiece that is 24” square (576 sq.in.). Clamping Force = Gauge Reading (“Hg) x Material Surface Size (sq.in.) x .5 22 x 576 x .5 = 6336 lbsVacuum flow is the other important specification to consider when opting for CNC vacuum hold-down. Flow measures the volume of air pulled in by the pump. Measured in cubic feet per minute, vacuum flow is referred to as either open fl ow(CFM) or specific flow(SCFM). Open flow is the maximum flow without any restriction on the air being drawn by the pump. Whereas, specific flow refers to the level of air being drawn by the pump at a specific level of vacuum, usually the optimum level in relation to “Hg. Since SCFM is directly related to the vacuum’s level of maximum clamping force (via the “Hg gauge reading), SCFM or specific flow is the more relevant measure of vacuum flow. All pump manufacturers have performance curves that show the specific vacuum level vs. fl ow rate. Take for example, a 10HP pump that is rated for 11”Hg @ 105 cfm and has an open fl ow rating of 280 cfm. When the open fl ow reaches 280 cfm, the vacuum level is at 0”Hg, which using the clamping force equation above translates into zero hold-down. But, at 105 cfm the pump will not drop below 11”Hg until it exceeds that flow rate. At 106 cfm and above, the vacuum level will drop, effectively decreasing your vacuum hold-down. Diagram 1 illustrates the loss of vacuum as the fl ow increases. Once the vacuum begins to decrease, the part or parts you are machining can shift and move due to the loss of clamping force. This is primarily a concern in nested-based manufacturing where flow-through methods are used. Flow- through is the method of using an mdf scavenger board or sacrifi cial board to protect the CNC machine’s surface. A sacrificial board is made out of porous material so that air can be vacuumed through the board, helping secure the workpiece to the table. A sacrificial board (i.e. a sheet of MDF) is put between the CNC machine’s table surface and underneath the material being routed. The scavenger board allows the CNC Router tool bit to penetrate the workpiece while the board protects the surface and the vacuum table’s grid structure. The pump previously mentioned was used to illustrate the value of specific flow rate based on the usage of a 10HP regenerative vacuum blower. This example was the easiest method in which to illustrate how vacuum pumps and specific flow rates operate in the real world. There are many other pumps to choose from such as rotary vanes, positive displacement blowers, and rotary screw pumps. Prices from one system to another can fl uctuate a great deal, Diagram No. 1
