CNC VACUUM TABLES pg2

Friction between vacuum hold down & CNC part

In every woodworking CNC machining operation, except maybe basic drilling the tool cutter transfers a side load or force to the object being machined. The amount of side load is a function of the geometry, the tooling and the material being cut but that is a tooling conversation that we’ll leave for another day.

In our discussion of vacuum tables, we want to consider instead the friction between the surface of your material and face of your vacuum hold down. CNC implimentation may depend on factoring this into the design of your woodworking vacuum table. You don't want the side load imparted by the tooling to the wood, to unseat your work from the vacuum.

Skiers know about friction...

Any of you out there that are skiers will certainly understand these basic principles. At least those that would like some stopping control ... The magnitude of the friction will be a function of the actual vertical force down on the mating surfaces (how heavy is the skier), the mating material we’re discussing (snow conditions & appropriate wax) as well as the surface area that they share (design of the skis). Heavier weight + snow sticking to ski wax + larger surface area in contact with snow = stopping power!

So holding down a thin, light weight piece (no vertical force) of a slippery wood (low coefficient of friction) like Tigerwood that is only 3” square(limited surface area) will be a pretty good challenge.

The trick is to find ways to increase the friction between the work and the vacuum to make the latter more effective. Sometimes answers are not so easy to find.

In traditional woodworking we often wax the table tops on our planar and router to REDUCE the friction and allow the wood to more easily slide across the work surface. In this instance we are trying for the opposite.

We want to increase the coefficient of friction between the wood and the surface of the vacuum table, … but this works contrary to maximizing the vacuum pressure. The greater the contact between table and work the greater the friction.. this is good. But it is also true, that the greater the surface area of VACUUM the greater the holding power. In most cases you CAN’T HAVE BOTH! Either the vacuum is pulling on the surface of the wood OR it is in contact with the table.

Actually that’s not quite true there is one design of vacuum table where you can have your cake and eat it too but it doesn’t work for all applications!

Bottom line is to consider friction and how, if the thought  is to make a CNC vacuum table, how you might capitalize on this in your design.

Lets look at the basic MDF vacuum table >>