Your message was on our answering machine after our power was restored this evening. We had some serious storms in the area and lost power shortly after you called. Anyway, on with it.
The bit I used is a single-flute 1/8". The other parameters are in the header of the gcode file as seen below.
(Generated by SketchUcam V1.3)
(Bit diameter: 1/8")
(Feed rate: 30"/min)
(Plunge Feed rate: 5"/min)
(Material Thickness: 1/2")
(Material length: 3 1/4" X width: 5 3/4")
(Overhead Gantry: true)
(Multipass enabled, Depth = 1/16")
This is from the gcode for cutting one bracket. It's the same in the file for two brackets. At the 30ipm feed rate, I got good chips and no burning.
One thing to check if you haven't already is which way your spindle is rotating. I've mentioned this before, but when wiring according to the published instructions my spindle was spinning the bit clockwise. I didn't catch that at first. Obviously, it makes a big difference between melting the plastic and cutting cleanly.