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What's the best way to secure the ends of single layer windings on a plastic form?

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Herr Zapp

Sat Oct 03 2009, 05:32AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Anders -

I sent my epoxy-coating procedure to you as a PM; possibly too long to post in the Forum.

Regards,
Herr Zapp

Bamacoiler

Mon Oct 05 2009, 12:49PM

Registered Member #1628 Joined: Wed Aug 06 2008, 08:48PM
Location: Huntsville, AL USA
Posts: 95

Herr Zapp,
Would it be posssible for you to copy and paste that epoxy coating procedure in a pm to me please?

jpsmith123

Mon Oct 05 2009, 07:01PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well I'm beginning to think that polyethylene (PE) might not have been the best choice for a coil former. From what I'm reading, basically nothing sticks to it very well...not epoxy...not insulating varnish...nothing.

I was going to use Krylon "Sprayon" insulating varnish, which would probably hold the coil together, but probably would not bind the coil to the coil form very strongly or reliably, if at all.

Has anyone ever tried any kind of large diameter heat-shrink tubing?

Steve Conner

Mon Oct 05 2009, 07:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Herr Zapp, please feel free to post your method. It can be as long as you like, as long as it's not more than 400 pixels wide.

I've made a few coils with PE formers, and you're right, nothing sticks to it! I used electrical varnish similar to Glyptal. It doesn't really stick to the former, but it binds the turns together. Even so, I found that some turns can wrinkle up if the temperature changes greatly (PE has a very high coefficient of thermal expansion)

In the end, I covered the whole coil in a tight wrapping of black electrical tape to hide the mess, and it's worked fine, making sparks more than 3 times its length.

doctor electrons

Mon Oct 05 2009, 09:52PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Herr Zapp wrote ...

electrons -

You wrote: "30% is usually enough for thread engagement".

30% of what?

jp -

It would be extremely difficult to drill and tap good quality blind holes in a hollow cylinder with only a .25" wall thickness, even if you have a setup with a precision depth stop (milling machine, etc) and a special plug (bottoming) tap. Your UHMW material has very low tensile strength, so you will need all the threads you can get, and you will need to be carefull with screw torque to avoid thread stripout or screw breakage.

If you DO decide to use a bolt-on copper plate, I'd suggest just drilling completely through the coilform wall and tapping the hole with #6-32 or #8-32 threads, which should give you at least six full threads of engagement. Secure the plate using nylon screws, and apply a sealant (RTV silicone, etc) to the threads when you install the screws. Done carefully, this should provide good dielectric isolation between the plate and the inside of the coilform.

The secondary in the photo is coated with a single application of Envirotex Lite epoxy (eti-usa.com) , applied while the rotating secondary was still mounted on the winding jig. I can provide a detailed procedure for applying the two-part epoxy if you are interested. Done correctly, it yields a near perfect mirror finish, like the secondary is encased in a glass sleeve.

Regards,
Herr Zapp

30% thread engagement, 30% of the thread to thread contact surface. Hole depth doesn't matter. Its printed on every tap and drill chart.
I was simply stating that as long as 30% of the threads are engaged, it shouldn't matter it you have 100 or 10. 30% will ensure proper hold when the fastener is tight. If you are worried about it breaking use a fastener of the same, or as close as you can get to the tapped material.

doctor electrons

Tue Oct 06 2009, 12:17AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Instead of just saying how i make my coils. Here are some shots of a coil i just wound for a 4mhz project. I used the tack paper like i said in my first post. You will notice that there are some small wrinkles in the tack paper. Once the wire is wrapped tightly around the form, the wrinkles are pressed out and the winding looks great. This coil does not have end terminations yet because i need to pull off some turns to tune it. There is no polycrylic on it yet either. Note how the ends stay neatly in place which leaves me room to make modifications or add terminations without the hassle of the coil unravelling. The form is simply 2 pvc repair couplings held together by a small piece of pvc pipe. After i finish tuning it will be when i apply the coating. (i also had the couplings in my refer prior to winding and put it back there 3 times before completing it. The 3 photos are of the progress over about 2 hours. You can tell which one is where it was complete enough to move on.

Herr Zapp

Tue Oct 06 2009, 01:50AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

electrons -

(Drifting slightly off topic, but ....)

Double-check your tap drill chart; unless its a typo, no tap drill chart would provide tap drill diameters that yould yield 30% thread engagement. The resulting threads would be so weak as to be nearly useless. The "standard" thread engagement value is 75%.

More detailed tap drill charts will give different ranges of engagements, based on material characteristics. Typical percentages are 75% for plastic, brass, and aluminum, and 50% - 81% for ferrous alloys. The ferrous alloys can get away with less engagement because their higher tensile strength can withstand higher shear loading.

The number of threads engaged also IS important, especially when the fastener and the tapped hole are different materials. The tensile load of a fully torqued fastener is spread (although not uniformly) across all the threads in engagement. If you don't have enough threads engaged, the threads in the weaker material will fail (example: very high strength, high-tensile alloy steel headbolts engaging tapped holes in a relatively weak aluminum engine block).

In jpsmith's original post, he was dealing with a UHMW polyethylene coilform, which has very low tensile strength and is not a good material to try and cut threads in, especially if the material is relatively thin, or if the fastener diameter is small, or if a fine thread pitch is used.

Regards,
Herr Zapp

doctor electrons

Tue Oct 06 2009, 02:17AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Checked it! Its still there! Its not diameter Herr Zapp. It is 30% of depth. I will find a link to clarify this. The effective engagement notes are at the bottom of the chart. Also, i suggested that he use a material as close to the tapped hole as possible. IE a nylon bolt threaded into a 1/8" thick piece of material such as pvc, uhmw, oak, acrylic, steel, aluminum, or even glass would hold just fine at 30%. Fine thread or not, 30% is 30%. If you do the math you will understand what thread will hold in what thickness of tapped material. Obviously 30% engagement is impossible with a 1/4-20 bolt in a 1/16" plate. As i do not want to insult anyone, like i don't want to be insulted, rest assured i am not attacking your intelligence. I happen to be very impressed by that coil picture you posted and i commend you for your fantastic work. However, 15 years of machining experience tells me that i happen to be correct.

jpsmith123

Tue Oct 06 2009, 02:25AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I wonder how well varnish and epoxy will work with CPVC pipe?

I was at first considering using CPVC pipe...the main reason I went with PE instead was because of its better electrical characteristics; primarily the much lower dissipation factor.

I suppose I should do some more detailed calculations regarding power loss and temperature rise for the two materials.

Herr Zapp

Tue Oct 06 2009, 03:00AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

jp -

Based on CPVC's chemical and physical properties it should be very similar to PVC, as far as the ability to create adhesive bonds. However, the electrical dissipation factor is 3-4 times HIGHER than PVC, so it's not clear that CPVC would provide any benefit over dirt-cheap thin-wall PVC pipe.

When using adhesive bonding, it's always a good idea to use fine (~280-320 grit) wet-or-dry silicon carbide abrasive paper to remove the surface gloss from plastic materials prior to applying the adhesive. This not only helps remove any invisible surface contamination, but gives a microscopically rough surface for the adhesive to "grip".

If you try a CPVC coilform, you might want to test the ability of your epoxy or varnish to bond to it, using a small cut-off piece as a test coupon. Let us know what you find.

Regards,
Herr Zapp

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