Sunday, December 25, 2011

Torsion Box for CNC Router Table

I have been off on other workshop projects for several months, but I am now ready to get back to the CNC Router.

I have been investigating a torsion box assembly bench for general use in my shop. A torsion box provides a stable flat surface on which to assemble projects and furniture. Here are some links:,or.r_gc.r_pw.,cf.osb

This last is a google image search of torsion boxes and includes some designed for CNC routers.

While I was investigating, I looked closely at the CNC router table surface that had already been built and found that it was no longer flat. The table surface is an integral part of our CNC router design providing flatness, rigidity and mass. For these reasons the table surface will be replaced by a 32" x 24" x 2.5" torsion box.

What is a torsion box? If you build a box with 6" sides, like a sand box and glue plywood skins, top and bottom, you have a torsion box. Placing the box between two saw horses and a load at the center, the top skin will try to compress and bottom stretch. As long as the spacing between sheets is held fixed the box will resist bending and its strength goes as the cube of the thickness. So we need to build a box with a grid of spacers inside, a honey comb to maintain constant thickness.

Building a torsion box is a chicken and egg problem. For the torsion box to be flat it must be built on a flat surface. So to build a flat surface you must first make a flat surface. Look at the first two links above and you can see how they solved that problem. Things are simpler for us since the surface is small; therefore, instead of jointing and planing 2x4's, I obtained three inexpensive 4ft levels from Harbor Freight for $7 each. These levels will take the place of the 2x4's and eliminate the need for a jointer and planer which I don't have.

Our torsion box will be built from MDF with the sides and core using 1/2" material and the skins 1/4" sheet.