Gearmotor Antenna Coilform
In this photo, you see the maching underway on the coilform of a new gearmotor antenna being built in my shop. The material for the coilform is fiberglass tubing, and that presented a interesting problem in relation to cutting the 8 tpi required in this fiberglass tube to wind and keep the tinned buss wire.in place on the coilform.
Previous antennas that I have built in my shop have featured Lexan coilforms, and the maching of the Lexan was straight forward. However fiberglass although a good choice for a coilform, being lightweight, having great strength, waterproof, and rf transparent, does not machine very well. When I say that, I mean in regards to cutting the wire-recess to hold the tinned buss wire in place, as cutting the fiberglass with a normal cutting tool, would chip the fiberglass, causing a very poor finish, and I wanted to avoid that. After mulling the problem over for some time, I came up with the solution shown here.
In the photo, you can see the tool that made this possible. This tool is a Foredom die grinder that has a handpiece mounted on the end of a cable that is turnning at 30,000 rpm. The handpiece has a small precision chuck in which I mounted a carbide grinding burr with the required profile to grind the proper contour required for the 14 gage tinned buss wire. Once I had everything set up, I made multiple passes in the lathe, cutting the required 8 turns per inch into the fiberglass coilform the depth that was required. Everything went well, with the end result being a nicely finished coilform.
Oh, one last thing, cutting fiberglass is hazardous to your health, so do not breath in the fiberglass dust that is being expelled from the cutting action of the cutting tool. I had my shop vac running while the maching was taking place, allowing the capture of all fiberglass dust present around the cutting tool area, or in this case, the grinding burr of the die grinder.
Click on the photo for a closer look...