14Phone: 516-328-3970www.technocnc.comTechnical SectionVacuum PumpsVacuum hold-down and which pump to choose can bevery complex subjects. There are several different pumpsavailable for use with a CNC machine and all have varyingspecifications and price ranges. The proper selection of avacuum pump is imperative for optimal machineperformance. To avoid paying too much or not enough,vacuum systems need to be evaluated based on thecustomer’s specific applications rather than cost alone.There are two main items of concern when looking atthe specifications of a pump: vacuum level and flow. Vacuumlevel 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 vacuumhold-down works by atmospheric pressure applying 15 lbs/sq.in. (psi at sea level) in all directions. During a CNCrouting operation, when you use vacuum to remove air fromone side of the material being cut (the underside), then theatmospheric pressure on the other side (topside) increases,in effect, pushing on the material. How much hold-downforce is applied to the material can be calculated by readingthe vacuum level gauge, multiplying that numeral by surfacearea of the material (sq.in.) and multiplying that by 0.5. Forexample, let’s look at a gauge that reads 22"Hg for amaterial workpiece that is 24" square (576 sq.in.).Clamping Force =Gauge Reading ("Hg) x Material Surface Size (sq.in.) x .522 x 576 x .5 = 6336 lbsVacuum flow is the other important specification toconsider when opting for CNC vacuum hold-down. Flowmeasures the volume of air pulled in by the pump. Measuredin cubic feet per minute, vacuum flow is referred to as eitheropen flow(CFM) or specific flow(SCFM). Open flow is themaximum flow without any restriction on the air being drawnby the pump. Whereas, specific flow refers to the level of airbeing drawn by the pump at a specific level of vacuum,usually the optimum level in relation to "Hg. Since SCFM isdirectly related to the vacuum’s level of maximum clampingforce (via the "Hg gauge reading), SCFM or specific flow isthe more relevant measure of vacuum flow.All pump manufacturers have performance curves thatshow the specific vacuum level vs. flow rate. Take forexample, a 10HP pump that is rated for 11"Hg @ 105 cfmand has an open flow rating of 280 cfm. When the open flowreaches 280 cfm, the vacuum level is at 0"Hg, which usingthe clamping force equation above translates into zerohold-down. But, at 105 cfm the pump will not drop below11"Hg until it exceeds that flow rate. At 106 cfm and above,the vacuum level will drop, effectively decreasing yourvacuum hold-down. Diagram 1 illustrates the loss of vacuumas the flow increases.Once the vacuum begins to decrease, the part or partsyou are machining can shift and move due to the loss ofclamping force. This is primarily a concern in nested-basedmanufacturing where flow-through methods are used. Flow-through is the method of using an mdf scavenger board orsacrificial board to protect the CNC machine’s surface. Asacrificial board is made out of porous material so that aircan be vacuumed through the board, helping secure theworkpiece to the table. A sacrificial board (i.e. a sheet ofMDF) is put between the CNC machine’s table surface andunderneath the material being routed. The scavenger boardallows the CNC Router tool bit to penetrate the workpiecewhile the board protects the surface and the vacuum table’sgrid structure.The pump previously mentioned was used to illustratethe value of specific flow rate based on the usage of a 10HPregenerative vacuum blower. This example was the easiestmethod in which to illustrate how vacuum pumps andspecific flow rates operate in the real world. There are manyother pumps to choose from such as rotary vanes, positivedisplacement blowers, and rotary screw pumps. Pricesfrom one system to another can fluctuate a great deal, butthese few are the most commonly used units in the CNCDiagram No. 1
