East Texas Woodturners' Association

Vacuum vs Pressure Graph

Updated Nov10, 2003

Vacuum chucks for lathe turning are growing in popularity among woodturners.  Understanding the relationship of vacuum and exerted pressure is important to woodturners to avoid crushing delicate objects while holding for turning.  

The force exerted on the object being turned is both a function of the vacuum applied and the area of contact with the object.  Contact area refers to the square inches in the plane of contact between the object being turned and the vacuum chuck.  For example, a 5" diameter bowl bottom making contact with a 4" diameter seal on the vacuum chuck would have a contact area of 

4 x 3.14 = 12.56 sq in.  

Vacuum is nothing more than negative pressure, i.e., instead of positive pressure pushing upon an object, it is a negative pressure pulling upon an object.  With positive pressure, the more area the pressure is applied to, the more force that is exerted on the object away from the pressure source.  With negative pressure, the more area the vacuum is applied to, the more force that is exerted on the object toward the vacuum source.  As the vacuum pressure increases, the force pulling upon the object increases proportional to the area of contact.  

Vacuum is measured in inches of mercury with a manometer.  The graph below shows the relationship of vacuum in inches of mercury verses the area of contact and is expressed in pounds of pressure.  Bowl diameter refers to the diameter of the bowl-vacuum chuck contact area.

Example: If a 5" diameter bowl is making sealing contact with 4"diameter of the vacuum chuck, then using the lower curve, 10" of vacuum exerts 50 pounds of negative pressure.  Again using the lower curve, if the vacuum is raised to 20", the force exerted increases to about 120 pounds of negative pressure.  If an 9" diameter bowl is making sealing contact with 8" diameter of the vacuum chuck, then 10" of vacuum exerts about 225 pounds of negative pressure (3rd curve from bottom).  Raising the vacuum to 20" increases the pressure to nearly 500 pounds of negative pressure. Thus, doubling the contact diameter of the bowl-vacuum chuck increased the negative pressure exerted on the bowl from 120 pounds to nearly 500 pounds, an increase of over 400%  If the object being turned is thin-walled, but has a large contact area, a large vacuum can cause the object to implode.  

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