Technical SectionPhone: 516-328-3970www.technocnc.com13which can be purchased to aid the vacuum hold-downsystem such as a spindle pressure foot, or roller hold-down.These devices push down on the material while beingrouted thus aiding the vacuum system.Vacuum Table BasicsWhen selecting a vacuum table, the most important thingto remember is how a vacuum table works. The following sixcriteria should be carefully considered when determiningwhether your CNC application could benefit from usingvacuum hold-down.Atmospheric pressure is approximately 15 psipounds/sq.in.). Each square inch of surface areahas a load of approximately 15 pounds on it. (Thispressure is more below sea level and less at thetops of mountains).When we have a box that has no air in it; i.e., almostcomplete vacuum, the top and all other sides of thebox are being loaded with 15 pounds of pressure oneach square inch of surface. If the top of the box is6”x6” then there is 6x6x15=540 pounds of loadevenly distributed on the surface. The box willcollapse 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 apressure of 15 psi (assuming there is a completevacuum). Note that the pressure on the object isbased on the surface area of the hole in contact withthe object being held, not the total surface of theobject being held.It is critical to realize that if the object is beingmachined; i.e., undergoing a side load, the actualforce holding the object in place against the cuttingforce is now the friction between the object and thevacuum table. The magnitude of this relative frictionforce depends on the coefficient of friction betweenthe object and the table, in addition to the actualvertical force on the object. Therefore, if the surfaceof the vacuum table is very slippery; e.g., Tefloncoated, the object will tend to slip no matter howmuch downward force is being applied. If thesurface of the table is nonslippery; e.g., it is rubbercoated, then the object will tend to stay in placeassuming there is enough downward force beingapplied. Note that for small pieces; e.g., smallbrass letters that have a surface area of less than.5 square in., it is unlikely that they can be heldreliably with a vacuum table because the downwardpressure on this object would only be about 7.5pounds. It is important to remember both factorswith respect to securing hold-down:contact surface area between the actualvacuum holes and the object.relative coefficient of friction between theobject and the vacuum table surface.Always consider the air permeability of the objectbeing held down. The permeability is the amount ofair that the object allows to pass through it. Forexample, a sponge is very permeable while a pieceof aluminum is usually not. The permeability of theobject determines whether a vacuum blower or avacuum pump should be selected. A vacuum pumpusually generates a very high vacuum; i.e., thereis virtually no air in the vacuum chamber, butvacuum pumps tend not to draw or pull a greatvolume of air quickly. A vacuum blower tends topull a great volume of air very quickly, but it doesnot necessarily create 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 itschambers?How much leakage will there be in thesystem?How permeable is the material?How quickly will the material be needed toclamp 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)
