Trial Wooden CNC Table


This first wooden table was based on the flying gantry system with an under-bed leadscrew.
This design is inherently flawed for a number of reasons.
  • Unsupported rails are flexible and permit chatter.
  • The tall gantry ends allow the gantry to rock on the rails.
  • The ends of the gantry prevent wide stock from being milled.
  • The center leadscrew does not support the ends of the gantry, so the gantry racks under low cutting forces.
  • The under table leadscrew is in the way of bed supports, so the machine cannot carry heavy stock.
  • The space under the machine is lost to other uses including fourth axis components and storage.
Flying Gantry Machine
Flying gantry machine.

This machine was made primarily of sheathing plywood.
The rails were black iron gas pipe, and the bearings were 608 skate bearings.
The leadscrews were threaded rods, and the leadnuts were rod couplers.

The machine used 420 oz.in. servos that were driven by Gecko 320s.
 
A leadnut was a rod coupler, which was cut almost all of the way through. Tines were welded onto each side of the cut.
A bolt was threaded into one of the tines; it free-wheeled in the other. This is similar to commercially made bronze split leadnuts.
Tightening the bolt bent the rod coupler, and removed the play between the threads.

This turned out to be a waste of time.
A rod coupler by itself can give backlash in the 0.003 inch range, and any adjusting of the system in the picture just added drag.
Leadnut made of rod coupler
Rod coupler as leadnut.
 
This plastic sign was made with a 0.03 inch bit chucked in a Dremel.
The quality of the cuts was better than expected considering the quality of the table.

However, heavier work was of low quality since everything could flex.

The second image shows two problems.
One is the wavy lines permitted by the machine's flex.
The other is a software problem that sometimes occurred with different DXF files. The extra lines were visible in some programs and not others. The joys of cobbling software.

Small carving in plastic
Sign milled in plastic.

Problems with CNC software
Problems with DXF files.