Vacuum workholding is an excellent choice, and often times the only choice, for holding workpieces.  Here are several quick tips to keep in mind when evaluating and/or using a vacuum workholding device.

1. Vacuum Power has a Limit – The most powerful vacuum chucks have a limit of 13-14 pounds of downward holding force per square inch.  This is because the air pressure around us is what actually squeezes the part down onto the chuck.  A vacuum would have no effect in outer space since there is no air pressure.
2. Use Small Cutters when Possible (if you’re not following tip #7) – Small cutters exert less torque, reducing side forces thereby reducing the chance of throwing a part off the chuck.  So, instead of using a 1″ wide cut around a part, use a 1/4″ cut with four passes.
3. Use Sharp Cutters – Sharp tooling reduces side load which also reduces the chances of throwing a part off the chuck.
4. Don’t Use High Helix Endmills – Low helix, or even negative helix cutters will reduce or eliminate any chances of lifting a part off the chuck during aggressive machining.
5. Use Common Sense – Holding a part that is 1 inch square and 4 inches tall is not going to work. Short and wide parts are the best candidates for vacuum workholding.
6. Vacuum Workholding is Often a Secondary Choice – Yes, vises and clamps are the preferred method of holding a workpiece due to higher holding force, however certain part shapes and sizes may restrict their use.  A vacuum chuck may be your only choice, but don’t expect to be able to machine a part as aggressively as when using a vise.
7. Use Workstops Whenever Possible – Though vertical holding force may be high, side forces may easily shift a part sideways off the vacuum surface, especially when machining plastics with a low friction ratings (nylon, teflon, delrin).  Adding pins, side rails or even cutting a shallow pocket for the part to sit in will greatly limit sideways movement.
8. Flexible Workpieces Might be Problematic – Because the rigidity of the workpiece helps maintain a vacuum seal, very thin or very soft materials may be more likely to flex and lift off the vacuum chuck.

Despite the common explanation that vacuum chucks operate by sucking parts down, the truth is quite the opposite. Vacuum chucks rely on the atmospheric air pressure around them to push parts onto their surface.

Atmospheric air pressure exists around 30″ Hg or 15 PSI at sea level (Yes, every square inch of our bodies has 15 lbs of air constantly pressing on us). This is why passengers on planes flying at high altitudes can experience swelling due to lower pressures in the cabin. Cabins are typically pressurized to an equivalent of being at 8,000 ft above sea level. At this altitude there is roughly 11 PSI of air pressure. Translate this into workholding and users at higher altitudes will experience a significant drop in holding force equating to a loss of 0.5 PSI per 1,000 ft above sea level. A high powered vacuum pump operating in outer space would be completely ineffective.

The atmosphere likes to remain at a constant level and it is this principle that causes the wind to blow – air flows from high pressure to low pressure. When objects are tossed around by the wind, they are acting as obstacles to this high-pressure-to-low-pressure flow. When a vacuum chuck is turned on the air pressure decreases below the workpiece which causes higher air pressure above the workpiece to want to fill this low pressure space below it. The workpiece becomes an obstacle which is pressed against the chuck.

The next time someone tells you that a vacuum chuck sucks parts down ask them when was the last time they saw the wind suck over a tree!