CHAPTER 20 ■ COUPLER
This explains how the wall size and two of the four tube sizes were determined. The other two tubes
have larger diameters. Those tubes slide over the smaller tubes for added strength and to provide enough
thickness to add a screw hole.
Measuring and Cutting the Tubing
The tubing comes in lengths longer than needed for the coupler. The desired length of the individual tubes
needs to be determined and then cut accordingly.
Determining and Marking Tube Lengths
The motor shaft sticks out 15 mm. The motor-shaft half of the coupler should be a bit shorter, say 13 mm, to
avoid rubbing against the gearbox cover as the shaft rotates.
Measure two 14 mm lengths on the^3 / 16 -inch diameter tubing. Mark the two locations to cut with a
fine-point felt-tip permanent marker (see Figure 20-7). Why 14 mm? Didn’t we just decide we wanted a
13 mm length? Yes, but about a millimeter is going to be lost during cutting and sanding, so a 14 mm mark
results in a 13 mm cut and finished tube.
Measure two 23 mm lengths on the three larger-diameter tubes. These tubes will span the entire length
of the coupler, about 22 mm, after cutting and finishing. Cut the tubes longer if desired, but a wider robot
will result.
Cutting Tubing
There are a number of tools that can cut tubes. A fine-toothed hacksaw works, although the thin walls of the
tubes usually bend or deform. You can insert a suitably sized wooden dowel within the tubing during cutting
to reduce warping, but there are tools that are superior to a hacksaw for cutting small tubes.
Tube Cutter Tool
A tube cutter is a tool designed specifically for cutting tubes (see Figure 20-8). A tube is placed between
a rotating blade and two rotating rollers. Twisting a knob pushes the two rollers against the tube, forcing
the tube into the blade. By rolling the tube and twisting the knob over and over, the blade eventually cuts
through the entire circumference of the tube.
Figure 20-7. 3 / 16 -inch diameter tubing held beside the motor shaft to mark the desired length