Winding nice tight coils on a lathe

Tricks of the trade

Winding nice tight coils on a lathe

Postby Doug Coulter » Fri Dec 13, 2013 4:51 pm

Well, it's not an official coil winder - for one thing, it lacks a turns counter. Which turns out not to be so bad in real life, at least for the stuff I do.
Here, I'm winding an inductor, or perhaps a transformer primary, for the HV line in the fusor. Since I don't yet have a clue what the desired frequency to make this "tube" oscillate at is - for most Q of course - I figured I'd start on the low end with around 4.5 millihenries.

These are the jigs I use:
JigA.JPG
Coil winding jig for lathe

From a few angles. All it really is, is a way to hole a spool of wire oriented the right way (it's a very loose fit on the "axle"), and a stick of wood with a small eye (to make the wire come to the final spot going straight) and a slot in a piece of soft wood - the final wire guide. Here's some more views:
JigB.JPG

Jig3.JPG

In this last one, you can see the bead of "hot glue" I stuck along the length of the form to hold all the wire in place. This isn't strictly needed, but I do it anyway.
I always do use the hot glue at each end, as having it all unwind if you "oops" is kind of a pain, and a start-all-over-again type of proposition ($).

I wind at 1 tps (60 rpm), my lathe's slowest speed. This particular coil was using the threading feature of the lathe at 80 TPI, and using #34 wire, so there's (in theory) about 4 mils in between each turn (the diameter of the wire is a bit over 7 mils). This is a fairly good number for standing off arcs, since the theory is that the volts will divide equally among the around 620 turns of wire shown here. Or at 50v at one end, arc at the other, about 83v/turn. Heck, the insulation alone should take care of that and not even need airspace. But real life says - have the air space just in case.

And, here's the result, all ready to stick in there and try stuff. I've not yet measured the millihenries, but it should be on the close order of 4.5 (neglecting end effects of my HV system adapters and so on), from the # of turns calculated by the effectice series resistance...and those nice wire tables.
Coil.JPG
I may shellac this, but then again, I may hacve to rewind it for lower inductance, stick ferrites inside for more - dont' know, that's why we call what I do research. If I knew what I was doing, it'd be tech.


You'll note that you can see a bit of "outie" sticking out of one end of the coil, and while you can't see it - there's a matching "innie" at the other end. These are about 1.12" diameter for the outies, and about 1.15" diameter for the innies, and I use a few set screws to hold things together in use. This way, it's all "one stick" that I then insulate and shield - it's more or less a big piece of hardline coax in use, which saves my shop a ton of EMI in arcs, and makes things safer in general.

ProTip:
You decide how tight you want it. For this one, since I was using pretty thin wire, I provided winding tension with a fingertip on the source roll of wire, and a pair of fingers guiding the wire between the coil and the jig - to wipe off any dirt and to feel any nicks or whatever. If you're doing this with say, #10 wire, you'll have to be pretty strong to provide enough tension....practice a little before going for the real deal.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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Re: Winding nice tight coils on a lathe

Postby Jerry » Tue Dec 17, 2013 2:50 am

I tried rewinding a motor once and did this on my lathe, used a retroreflective sensor and a counter to count the wraps on the form.

Too bad it didn't work in the end, turns out the laminations were shorted.

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Re: Winding nice tight coils on a lathe

Postby Doug Coulter » Sat Dec 21, 2013 11:05 am

A lot of times (including my little example) you spend more time making the coil form to spec than winding it, eh? Too bad you had bad laminations after doing that work! I think I'm going to adopt your idea for a turns counter for those things that matter (most of them). I'd really like to be able to turn pies with "universal" winding (for low parasitic C), but making that tool (a specially geared cam driven off the main drive to weave back and forth the width of the pi with an odd gear ratio) seems a bit past my speed (and time to do it). Maybe I can pick up a universal winder surplus.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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