Hello,

I am in the process of designing a CNC router with a 4’x4’ cutting area. It will be built mostly with fiberglass cloth, epoxy, and foam.

The design question I have is in regards to the gantry. The geometry that I have chosen for the gantry is a hollow semi-cylinder (which will be fabricated with biaxial fiberglass at +\- 45 degree angles over a foam core).

This is my reasoning:

1. The weakest link in a router is often the gantry.

2. The weakest link in the gantry is often torsional strain.

3. A circle has a much better polar moment of inertia than a rectangle. However, using a whole cylinder makes it difficult to mount the linear rails to, and offsets the spindle from the gantry’s axis of rotation, giving the spindle extra leverage.

4. A square tube has a worse polar moment of inertia given the same perimeter as a semi-circular tube and stresses the skin unequally, putting a higher proportion of stress on the corners of the tube.

5. A semi cylinder gives me a flat surface on the front to mount the rails/ball screws to, and gives me better resistance to torque than using a rectangle tube.


For now I am only soliciting advice for this one problem, I.e., the merits of a semi-cylinder versus a rectangular tube. Discussing other aspects of the build would require many cans of worms to be opened. I will post a full build thread once I am ready to commence the build in order to get feedback on the overall design.

Am I missing something?


Thanks!