[TCML] Why is it important to dry the PVC pipe?
quarkster at att.net
Fri Sep 26 18:24:58 MDT 2008
Just a cautionary note on "drying" a PVC (or any plastic) coilform. After any heating, be very careful that your coilform has returned to room temperature before starting the winding process. All plastics have a relatively high coefficient of thermal expansion, and copper magnet wire is annealed and nearly dead soft (no elasticity). If you wind wire on a warm coil form, it will become loose as the form cools and shrinks.
Also, do not try to heat a wound secondary to "dry it" before coating with varnish or epoxy, especially if you have used fine gage (#30 AWG or smaller) wire. The form will expand with the heat, stretch the wire, and the wire will end up sliding loose on the form after it cools and shrinks back to its original diameter.
I learned this the hard way long ago with two beautiful little 3" X 12" matched secondarys I built for a small bipolar twin coil. After laborously winding both acrylic forms with #30 AWG wire, I put them both in a barely warm oven to "dry" them prior to varnishing them. The oven had been pre-warmed, then turned off before the secondarys went in. When I took them out an hour or so later, the windings were grossly loose and both secondarys were ruined.
This thermal expansion problem was reinforced sometime later when I wound a slight larger secondary and experienced a similar problem. Remembering the disastrous results with the 3" secondarys, I dried the bare 3" PVC form in a warm oven, let it cool for a while, and then started winding. The form was just barely warm to the touch. After the coil was completed, I noticed that the windings at the "start" of the coil were slightly loose, and very gradually tightened up towards the finish of the coil. I can only assume that the form continued to cool and shrink as I slowly wound it. By the time I got halfway done, the form had cooled to ambient temperature. The windings towards the "finish" end were still tight, but those at the "start" end could easily slip, even though I had maintained constant tension on the wire during winding by running it through a felt pad squeezed by an alligator clip.
So make sure that your coilform is at room temperature or below before starting to wind it. An interesting experiment might be to wind the secondary while the ID of the form is packed with a bag of ice. After winding is completed and the form warms up, the windings should be under enough tension to absolutely prevent slippage until the coil can be coated.
--- On Fri, 9/26/08, Grant Visser <freeekyg at gmail.com> wrote:
From: Grant Visser <freeekyg at gmail.com>
Subject: Re: [TCML] Why is it important to dry the PVC pipe?
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Date: Friday, September 26, 2008, 1:29 PM
I found it extremely handy to have an elastic band or 3 on the form
right from the start. The first band or two I use to hold the initial
windings and the 3rd I slip/roll along over the freshly wound windings
as I go along. Naturally slip resistant, can't come loose on it's own,
and won't leave any sticky residue even if you only get off the phone
and back to you coiling a week later! *grin*
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