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Winding coils on lathes (was coil coating)
Hi List
I just thought I'd share my experience of winding my 4" coil on a perspex
former using a lathe.
Reinhard wrote......
>
>
> Total agreement. I would NEVER wind a coil on my lathe. Esp. not
> if I would be coating it with epoxy, etc (running the lathe to prevent
> sags and runs). A lathe is an (expensive) piece of high tech
> equipment and should not be abused for such purposes. It only
> took me about 2 1/2 hrs to build a electrical winding rig out of
> scrap parts, I had laying around. It was a most beneficial aid in
> secondary construction. Another problem with winding a coil on
> a lathe: This is EXTREMELY difficult to do, if you wind the coil
> alone. You will never be able to stop the lathe in time (mine
> goes down to 60 rpm), if the wire tangles, etc. If the wire gets
> caught or tangles somehow, it WILL snap before you can stop
> the lathe. My lathe spins down slowly. It doesnīt have a quick
> stop. (Iīm not sure if any lathe has a quick stop, because you
> have high inertial energy "stored in the lathe" esp at low (high
> gear ratios) turning speed).
>
The technique I used to wind the coil was to put the lathe in "back gear".
This makes the chuck rotate *very* slowly. Certainly slowly enough to wind
a coil. The real trick to this however, is to disconnect the chuck from the
drive. This is done, on my lathe (a myford super 7) by opening up the drive
belt cover and just turning the semicircular location key on the drive
flange so that the chuck is independent of the drive. I say independent,
although the residual friction between the chuck and drive means that this
acts as a low friction clutch. So.................. the chuck\secondary
former will turn at the back gear speed but can be stopped quickly in the
event of a winding problem simply by catching hold of it with your hand
(the drive keeps turning). I must disagree with Reinhard when he says this
is very difficult. I found it to be simplicity itself - it took me about 10
minutes. I also applied a spray varnish coating to the wound coil while it
was turning on the lathe after masking the exposed bed etc.
hope this helps
Steve Crawshaw
>
> > An outer coating is not necessary if the winding is fair (smooth,
> >tight, no overlaps). It will make the coil more rugged. I generally
> >use one, but it does detract slightly from performance. Use a
> >clear oil-based, varnish, shellac, or epoxy coating. You can sand
> >between layers, if you want to achieve a thick, high-gloss, coating.
>
> And here I TOTALLY disagree. Here is how I prepīd my secondary.
> The bare form was sanded inside and out. Then, I washed it down
> with water and soap with a clear rinse to finish it off. I force dried
> the former, wiped it down with acetone and coated it on the inside
> with PU varnish. This dry, acetone, PU process was repeated for
> the outside of the former. Then I wound the wire on the former. I
> used a thin epoxy resin (24 hr hardening time) to coat the wire/
> former. After it was hard (I actually waited 48hrs), this was
> sanded smooth and I applied a second coat of resin. This, too,
> was sanded smooth. Then I applied two more coats (with sanding
> inbetween) of PU varnish. Due to the thin nature of the resin and
> PU varnish and the constant turning, I have almost no air bubbles
> (you will need a magnifiying lens to see them) and the coating
> is very smooth and glass hard. The reason I used epoxy AND PU
> varnish is easy to explain: The epoxy is thick (and will really embed
> the wire) and very hard, whilst the PU (on the outside) is slightly
> resilent and wonīt crack if you pick up (or maybe even bump
> something into) the coil. Now to the performance aspect (which
> is the place where I really disagree with you): My coil is powered
> by (a) 7.5kV neon(s). At the time I am running 1275VA and getting
> continious 57" sparks with an occasional 63 incher. Input Joules
> are 3.77J (~16nF cap for 15kV coilers). I use a flat primary with
> a 11.8" inner diameter. This means I have around 2" clearance
> between primary and secondary. The primary is RAISED ~4.5"
> ABOVE the lowest secondary turn. While I do still have to
> measure the true coupling factor, I am pretty certain it is on the
> high side. During one run, the primary tilted and I got hefty
> flashover between primary and secondary. While this only lasted
> a short time (I shut down immediately), I examined the coil and
> found not the slightest mark. If the coil had not been coated or
> just a single coat (to keep the wire in place) of PU varnish, I
> am most certain, it would have been trashed during this run.
> I have had this happen with a non coated coilform (2.5"), run
> on much lower power (200VA) and the wire (cotton covered
> magnet wire) was charred, more or less, instantly. The
> measured (!) Q of my coil is ~200 (196 to be exact).
>
> My bottom line on coil coating: I donīt think a coating will ruin
> / hurt the coilīs performance at all. As a matter of fact, I
> would even consider it a must, esp if one runs at high kVA
> and or coupling levels. It WILL definately save the coil wire
> and former from injury during a mishap (which will eventually
> happen).
>
> Coiler greets from Germany,
> Reinhard
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