Technical SectionPhone: 516-328-3970www.technocnc.com13which can be purchased to aid the vacuum hold-down
systemsuch as a spindle pressure foot, or roller hold-down. These devices push down on the material while being routed thus aiding the vacuum system.Vacuum Table BasicsWhen selecting a vacuum table, the most important thing to remember is how a vacuum table works. The following six criteria should be carefully considered when determin-ing whether your CNC application could benefi t from using vacuum hold-down.Atmospheric pressure is approximately 15 psi pounds/sq.in.). Each square inch of surface area has a load of approximately 15 pounds on it. (This pressure is more below sea level and less at the tops of mountains).When we have a box that has no air in it; i.e., almost complete vacuum, the top and all other sides of the box are being loaded with 15 pounds of pressure on each square inch of surface. If the top of the box is 6”x6” then there is 6x6 x 15 = 540 pounds of
load evenlydistributed on the surface. The box will collapse or bend in if it is not strong enough.A vacuum table; i.e., a box with holes in it, has the “holes”pulling down on the object above it with a pressure of 15 psi (assuming there is a complete vacuum). Note that the pressure on the object is based on the surface area of the hole in contact with the object being held, not the total surface of the object being held.It is critical to realize that if the object is being machined; i.e., undergoing a side load, the actual force holding the object in place against the cutting force is now the friction between the object and the vacuum table. The magnitude of this relative friction force depends on the coefficient of friction between the object and the table, in addition to the actual vertical force on the object. Therefore, if the surface of the vacuum table is very slippery; e.g., Tefl on coated, the object will tend to slip no matter how much downward force is being applied. If the surface of the table is nonslippery; e.g., it is rubber coated, then the object will tend to stay in place assuming there is enough downward force being applied. Note that for small pieces; e.g., small brass letters that have a surface area of less than .5 squarein., it is unlikely that they can be held reliably with a vacuum table because the downward pressure on this object would only be about 7.5 pounds. It is important to remember both factors with respect to securing hold-down:contact surface area between the actualvacuum holes and the object. relative coefficient of friction between the object and the vacuum table surface. Always consider the air permeability of the object being held down. The permeability is the amount of air that the object allows to pass through it. For ex-ample, a sponge is very permeable while a piece of aluminum is usually not. The permeability of the object determines whether a vacuum blower or a vacuum pump should be selected. A vacuum pump usually generates a very high vacuum; i.e., there is virtually no air in the vacuum chamber, but vacuum pumps tend not to draw or pull a great volume of air quickly. A vacuum blower tends to pull a great volume of air very quickly, but it does not necessarilycreate a very “complete” vacuum. How big a vacuum pump or blower should I select? Consider the following questions when deciding:How large a part is being held down?How large is the vacuum table and its chambers?How much leakage will there be in the system?How permeable is the material?How quickly will the material be needed to clamp and unclamp?1) 2) 3) Clamping hole in vacuumtable. The area of this holedetermines the clampingload on the object, not thesurface area of the object.Vacuum TableObject being held4) 6) 5) a) b) c) d) e) a) b)
