The meat of the thread
A fellow modeler who models in TT scale asked if I had any idea on how to make an antenna for his Erie caboose. The antenna is 36” or 3 feet in diameter. That would be .300” or 3 tenths of an inch. He thought that since I had a CNC mill I could make a mold for plastic injection. I said I’d see what I could come up with.
Here are some pictures of the Erie caboose antenna.
Considering the size that the antenna would be, .300" (7.62mm) in diameter and the size of wired needed, I figured a jig would be the best way. I don't think 3D printing would make the grade on this. We're talking .010" dia. wire being used here. Perhaps a brass part or plastic injection molding might work, but not for perhaps one piece, plus brass casting or injection molding is beyond home brew capabilities. So what to do? Well, a simple round jig will do.
I'm going to employ a method called "Chain Mail" http://www.wikihow.com/Make-Chainmail
I started by making a mandrel approximately .200" (5.08mm) on one end and .300" (7.62mm) on the other. A small hole is drilled through the small dia. part to hold the .010" (.254mm) phosphor bronze wire.
Here's the procedure for making a coil close to .300" dia.
The mandrel is held in the lathe with the wire threaded through the hole in the jig.
The lathe is turned on at a very low RPM, 2 or 3 rpm. The other end of the wire is held with a pair of pliers to give a tight wind on the mandrel. This is the way springs also made.
Once the tension is release the wire un-winds some what. This is why a smaller dia. than .300" is used.
Closer view of the spring/coils.
Next the coiled wire is placed over the .300" dia. part of the jig to facilitate the cutting of a .300" dia. ring of wire.
The end result is a ring close to the .300" dia. of the antenna.
Now that I can make coils of wire to .300" dia. I will need a way to hold it to that dia. and also solder on 6 spokes, plus the pole through the center. I have an idea for a fixture to hold it all together and solder it.
I drew up the antenna in my CAD program ran it through my G-code generator and engraved a piece of aluminium. Not pleased with the outcome. This was the first time I tried using the CNC to mill a circle. I don't like the slightly out of round appearance. Would make for a poor looking antenna.
Also kind of rough using both a "V" groove cutter and then a ball nose cutter.
So what next? Mill the top off to get a nice clean surface and try again.
Time to try out the rotary table. I spend a few hours hand writing code. Dry ran the machine. Looks good. Not going to use a cutter this time. Going to try my carbide scriber I made for engraving walls for a TT scale brass caboose. It gave a perfect circle, but the six legs of the antenna didn't seem to start at the exact center.
Here's how big the jig is. That's a piece of .010" phosphor bronze wire. Once I get the six legs started from center I'll make some small clamps to hold everything together for soldering.
I zeroed in the rotary table perfectly under the spindle center. It now produces a perfect circle.
Next is developing the tiny clamps that will be needed to hold the wires down for soldering.
Had a bit of a set back with the jig. When I went to center punch the jig I inadvertently missed getting it dead center where the spokes meet in the middle. What to do? Re-scribe the jig that's what. In case you'd like to see how I made the holding jig I did a little video. It's speeded up to 4 times of actual speed.
I finally was brave enough to drill a .0225" hole through the 1/4" thick aluminum jig. It was done manually with an Archimedes Drill. What is that? Here's a picture of one.
So here's the hole in the jig, plus a penny for size comparison.
And if you don't believe me here's the drill sticking through the jig. It took about 10 minutes to drill through and I only used one drill.
Next up is making the tiny clamps to hold the wire down for soldering.
I came up with small clamps to hold down the wire. Using .010" (.25mm) piano wire I made three tiny clamps to hold down the circle of phosphor bronze wire.
Next I cut six pieces of wire to a length of .140" (3.556mm) long for the spokes. Flux was smeared on were the wires will be soldered to and tiny slivers of solder were added
Here comes the fun part. Soldering all those parts together. Easy to do. I set the jig on a bigger piece of aluminum and then that was set on a gas stove burner. At the highest setting it only took a few minutes to heat both plates to melt the solder.
A quick run under some cold water and this is the end result. Not 100% but it proves out the jig works.
I can think of one reason not all six spokes soldered, the wire wasn't clean enough. Also when I drilled the .0225" (.5715mm) hole through the center it didn't drill straight though. The slanted hole has nothing to do with the soldering job. Now that I have prove of concept I'm going to make another jig, hopefully with a straight hole through the center.
Well here you go. Third try was the lucky charm.
I cleaned the phosphor bronze wire using a piece of emery paper. Proof positive that clean parts solder much better and neater.
Now on to making the "perfect jig" with a straight hole through the center. WHY? Here's why.
I’ll take a picture when the final “good” antenna is made.
Bernd
