With a micrometer, you have to take account the enamel coating.
The measurement was with the enamel coating on. At .0149'' it was closer to the expected coated range of 27 Ga (.0149" to .0155'') than to 28 Ga (.0132'' to .0139'') I tried paint stripper on it but the coating didn't come off. I was hesitant to scrape it off because there is so little difference in diameter between the two gages. For other reference, the existing coil measures about 1-3/16" Dia. and 3/8" thick.
I wound the coil this afternoon. I used a long #8 screw as the arbor. I had to "shim" the central square box that the arbor ran through with some small pieces of sheet metal to keep the spool square to the screw. I used a large fender type washer on the opposite end to contain the free end of the coil. It was not easy to keep the wire evenly spaced as I did the winding. I did the best I could. Note that it taped the loose end to the bottom of the spool and wound new wire over the tape buffer rather than risking scraping any coating off that lead. I carefully wound the first layer by hand and kept it reasonably even. After that, I unspooled about 10" of wire from the stock coil and held light tension on it, guiding the wire from side to side as it accumulated on the new coil. I ran the drill at a creeping speed till my hand was close to the spool and arbor, and then repeated the process. When the coil was about the same diameter as the original, I cautiously loosened the nut and started to pull the fender washer away, but the wire looked like it might start to spring off the loose end of the coil, so I tightened it back up. I painted some clear lacquer on the coil and will try removing the washer and coil from the arbor when this is completely dry. I may put a coat of lacquer on the loose end of the coil once the washer is removed. Then it's on to reassembly and testing assuming the coil stays together.
Yes, but did it work?
What gauge did you end up with, how many feet, how many turns, etc.?
I used 27 Ga wire. I didn't count turns and just wound it until it was about the same diameter as the old one. I bought 400 ft of wire that should have been enough for more than one coil and it was. I'm getting about 6 ohms resistance from the coil and that fits with the 5-7 ohms for the factory coils earlier in this discussion. I won't know if it will work until I get it reassembled. Any tricks for getting the coating off the magnet wire? Scraping with a knife didn't work very well. Pulling it through a folded piece of emery paper worked, but it seemed slow to get it to bare wire and cleaning of the varnish was inconsistent.
I "burn" off the coating by feeding it into a blob of molten solder and lots of flux. Takes, say, 5-10 seconds and you end up with a nicely tinned 1/2" end. I hope that makes sense. I'd think there are youtube video of this...but if not I can make one if that would help. I've done this many times and has always worked for me.
I discovered today that I had put too much height on the coil. I need to remove the wire and rewrap the coil. The plastic core must touch the underside of the track.
Well, I completed the rewind and installation and it kind of works. I used jumpers from my tubular 3 rail track to power the Fastrack section and test it with the voltage reading about 17 V on the meter. Some couplers are attracted to the magnet and it doesn't have enough oomph to grab some other couplers. I don't have an action car with a center magnetic stem to test that action. I'll have to wait until it's installed on my grandson's set again with his power supply to see if it works like it used to.
I used 27 Ga wire rather than 28 Ga like the older tubular track couplers are reported to be wound with. Would magnetic attraction have been stronger with more turns of the smaller diameter wire? I was not able to keep the turns of wire even on the coil when wrapping by hand under power even when it was turning at a creeping speed. Thus there are not as many turns of wire as there could have been. I might have been able to wrap it more evenly if I had wound it completely by hand, but it would have taken hours to wrap the coil. The top of the magnet core is not completely even with the two outer rails. It's not lower by much - an estimated 1/64". Is that a significant amount with respect to the magnetic action on couplers / action cars? I was able to gain a little additional height by pushing the two central metal core spacers up a little above the two on the outside.
I don't know that I'd want to consider this done and a complete success. It's more of a proof of concept that it can be done and needs refinement. I think this repair would now take about an hour to an hour and a half. Probably not economically feasible for those of you doing commercial repairs, but for the hobbyist, it's doable. When I first asked the question I hoped that somebody else had already done this and had worked out the best way. A different spool and core rather than reusing the one from the Fastrack might be a way to allow more rapid and even winding. Cannibalizing the magnet from an old piece of RCS track would be another way. Photos of the bottom with the rewound magnet installed and the top as it exists now are below.
Yes, more turns of smaller wire yields a stronger magnetic field. I know that in the Kadee world, using DC instead of AC with their e-m uncouplers, also produces a stronger field at the same voltage. Just be sure you use a good quality momentary contact switch, that reliably turns off when released, or you'll still have meltdowns. And only hold the button for a second or so to prevent heat buildup.
Bill in FtL
Well, more turns of smaller wire "might" yield a stronger magnetic field, it all comes down to the number of ampere turns on the coil. For the same applied voltage, that statement is not necessarily true as the smaller wire and more turns will have more resistance and also more inductance. This will result in less current flow at a given voltage.
Also, the fact that the coil is lower now due to melting has significantly reduced the magnetic force, the force drops as to the square of the distance, so a small gap can have a large effect. That coil used to be slightly above the track plane, now it's visibly below the track plane. That has had a major effect on the magnetic force to the couplers.