Making Coil Forms from PVC Pipe

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PeteS

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I have a HF mini-lathe and hoping to use it for cutting shallow groves (threads, basically) on the outside surface of PVC pipe. In these groves I'll wind 12 to 14 AWG copper wire to create inductors (coils for radio circuits). The groves should be less than or equal to 8 per inch (8 TPI or less). The HF mini-lathes coarsest thread ability is 12 TPI. I was able to set up the A,B,C and D gears to theoretically cut threads under 8 TPI but found out that is was not possible to do so because of the heavy load on the gears/motor due to a bunch of reasons including the friction in the lead screw bearings and the general lack of precision in the whole drive train.

Years ago a friend had a lathe capable of doing this operation and I finally was able to use the coil form that he made for me. The form is made from 3 inch sched 40 ( I think). With the mini-lathe I made plugs for each end so that it would be rotated within a frame. A roller runs alongside and tracks the wire to provide a variable tap point resulting in a variable inductor (in theory from about zero to 30 microhenries) The coil worked out extremely well and I'd like to make my own forms of various pitches, diameters and so forth depending on the application. I would not use 3 inch diameter pipe going forward. Only 2 inch or less.

Any suggestions would be greatly appreciated.

Best Regards

Pete (WB2UAQ)
 
I'll take a guess at it. Although your gearing may set up to cut the 8 TPI you want, the thread dial may not really represent the proper engagement points. That may lead you to see it as inaccuracy in the machine. If that is the case, you may be able to treat it like cutting Metric threads. Never disengage the half-nut. Take several shallow passes. With PVC, you may even be able to turn the chuck by hand easily enough to drive things, if the motor doesn't seem able, but that really shouldn't be the case with plastic.

I don't own one, nor have I ever run one of the little HF lathes, but this is just a suggestion for something to look at. And you might try cutting a 12 TPI as normal just the see if the machine can do that. If so, it should be just a matter of more passes to get the deeper 8 TPI. If it won't do 12, then I would look for something wrong.

73

Have fun with the radios!
 
Maybe you could use a Dremel-type tool post grinder, with a round bit. Slowly cut your groove, at target depth, in a single pass. Minimal load on the change gears and leadscrew that way...








N0UYB
 
CNC is about the only way I can think of, without butchering your lathe. You'd have to have something like a 14X40 to get under 8TPI, and even then you couldn't do oddball stuff like 6.5TPI. CNC will cost nearly as much as a big gear head lathe.

For one off, mark the form every 20deg or so, groove with a hacksaw, then open up with a needle file.

For making lots of these, you need to single purpose your lathe into a coil winder. The company I used to work for modified what looked like SB9's to wind coils and precision resistors. You need full power, very low speed, a modified gear train, and maybe a multi-start lead screw and nut.
 
Thanks for the comments and suggestions!
I'll check back if there are any more ideas,
Merry Christmas and a Happy New Year.
 
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