Winding form for lacquer bound coils

[ODM]

Here are some results from experimenting with coil winding forms. I made these winding forms for an induction balance coil with 220 and 110mm diameters. They are made from three circles of plywood, with the middle one finished carefully to have a 90 degree angle to its edge after sanding it, to help with releasing the coil after winding it.

It helps to have sanding paper glued to a block laid on the workbench with a clamp to make sure the sanding is at 90 degrees. There needs to be a slot in the edge for starting and ending a winding.

After making sure the surfaces were smooth and without splinters, I waxed the plywood discs with paraffin (cheap candles) using a hot air gun along the edges which would be facing the windings. The wax surfaces were scraped smooth with an old magnet-stripe card, careful not to scrape the wax off completely and expose plywood, even if some wax soaks into the wood.

The halves were fit together and fastened with small screws and nuts. A strip of baking paper wrapped along the bottom helps with releasing the coil.

Wind coils as usual. I used a manual winder which counts the revolutions, but it's possible to wind using one of the bolts as a handle, using a clamped bolt as an axle. Every layer of windings got a smooth brush of relatively thin shellack and had a minute to dry before the next layer was wound on top of it. This method takes a bit of time, especially with the RX coil, but results in a firm coil which keeps its form with moderate handling.

I let the coils dry for two days before carefully prying the edges open, using a thin carpet stiletto to pop the halves open. I manually rearranged the slightly shifted segments of winding to lay as they were and let the coil dry for some time, brushing on some slightly thicker shellac along the exposed sides of the winding. After it set, I carefully pried the windings off the middle discs of plywood, rearranging shifted windings as before. The baking paper strip was easy to remove, and I applied a coat of thicker shellac to the insides of the coils as well.

Shellac takes a couple days to set perfectly, but dries to the touch in mere minutes since it's ethanol based. It won't attack wax either, and it's easy to adjust the number of windings afterwards as shellac wound coils are easy enough to unwind manually. It scrapes easily off the wax surface just by scraping it off with the same magnet-stripe card.

Shellac is not as stiff as polyester or epoxy would be, but it's firm enough to keep the shape of the coil before potting. Also, it's easier to work with since it dissolves in alcohol even after drying.

Would I use this process for another set of coils, though? It takes a couple days. The results are quite stiff and retain their shape well, but I will probably look into making winding forms from cardboard or fiberglass in the future

Here's a picture of the finished coils, laying atop the side of the larger TX winding form. The other picture shows the three pieces that make up the similar but smaller RX winding form, and the five holes for bolts around the axle hole, for the bolts clamping it together during winding.

[Cyclonite]

Hi ODM,

A very nice and inspiring read indeed, how you make your search coils.

But my first choice would be using Q Dope, and it can be used and applied just like ordinary lacquer, and left to dry by means of evaporation of solvent(s). Composite lacquer needs a hardener, and this process is purely chemically based, and not on the physical process of evaporation like described above. But it can also be a combination of both.

Using Teflon (PTFE) Tape would eliminate nearly all "sticking" problems. This Teflon tape is also used as gasfitting pipe sealant, and is very cheap to buy.

Paraffin Wax or Stearic Acid (cheap candles) may (will) dissolve in the solvent(s) for Q Dope. Alcohol on the other hand should not be problem, but shellac would not be my first choice in maintaining a high Q-Factor. But I have no idea how important maintaining a high Q-Factor would be in a search coil. I wish that I could find a scientific article which addresses this matter. I have seen many photos of very slumpy factory coil designs, which may suggest that maintaining a high Q-Factor is not such a big issue after all. But I am more inclined to think that those who make these search coils are just urinating next to the toilet, thinking that this is just a custom ritual which people have become familiar with. Personally, I think that this Search coil has more hidden powers than that is exploited from it. Just keeping things cheap, and running on old ideas, can hardly be an excuse to justify the excruciatingly high prices that these detectors fetch.

Nevertheless, reading your coil making procedure is a very inspiring read indeed. And hopefully will also motivate further discussion and ideas concerning this matter.

Regards,

Robert

[ODM]

Assuming you mean polystyrene dissolved in acetone or xylene by "Q-dope", it would be useable if the winding forms were made from teflon. I didn't have teflon plates available, but as cutting leftovers it can be fairly cheap at plastics companies.

For IB/VLF detectors, the winding lacquer type should be largely unimportant as long as it is not conductive and permeates the entire winding core for mechanical stability. Even in PI detectors the RLC time constant should be long enough to cover the differences between dielectric absorption in plastic lacquers.

It all depends on what the coils will be used for. If a design requires controlled capacitance and dielectric absorption around a coil, winding lacquer should be considered as well as winding pattern.

Using Q-dope results in low capacitance across the coil (compared to other contemporary lacquers in the early days of radio), allowing for smaller capacitances and higher tuned circuit impedances in radio use. The original use was for adding dimensional stability for basket wound coils and displacing moisture from cardboard forms to make low loss coils.

Personally, I do not believe there is any "unearthed arcane knowledge" about coils or circuits. Plenty of material can be read in electronics and physics publications. A manufacturer would be quick to patent any aspect on coil design if it resulted in better performing coils, since secrets don't usually stay secret too long! The actual trade secrets are details - slight differences in circuit design, which can be read from maintenance manuals and diagrams, or determined by popping open a coil.

If there is no documentation supporting some design choice, it's valid to ask if it's a design choice or a habit. The same would go for Q-dope as well, since it seems to be only used by amateur radio enthusiasts these days and there is little documentation supporting its use. Professional manufacturers of search coils use "self supporting" wire to wind coils, a winding wire which has a lacquer that sticks together to make a mechanically stable coil.

http://www.cliftonlaboratories.com/c..._compounds.htm

[Cyclonite]

Q Dope is indeed a solution of Polystyrene. Originally a solution in Methyl Ethyl Ketone (MEK), but other solvents like Toluene (Methyl Benzene), Ethyl Benzene, and also Xylene (Ortho (O)-, Meta (M)-, or Para (P)-Di-Methyl Benzene, or mixtures of the same) may be used. I would think twice about using Acetone (Di-Methyl Ketone), for it may (will) dissolve the Copper Lacquer Layer. And please don't make the mistake by thinking that when the Q Dope dries, that it will leave an isolating Polystyrene layer on the Copper wire so that using Acetone doesn't matter. If the lacquer dissolves or swells, then the copper wire may make Galvanic contact BEFORE the Q Dope dries which will make a section of the coil worthless, or even absorb energy from the rest of the coil by acting like a Shunt Inductance Resistor.

I was referring to TEFLON TAPE to prevent the varnished coil from sticking to the mold, and NOT Teflon Plate. Teflon Tape is widely available, and cheap too.

Furthermore, I was not referring to "unearthed arcane knowledge" about coils, but the MECHANICALLY unsound and sloppy construction. Also very bad winding techniques, by overlapping coil input wires with coil output wires in for instance PI detectors. We are talking about hundreds of volts here with a high probability of arcing. Especially coils that have been exposed to many temperature variations, and thus coil expansion and crimp. Also, I am referring to winding techniques that may minimize coil expansion and crimp to high and low temperature variations.

At low frequencies, Q-factors indeed play a minimalistic role, but polystyrene also has a very low thermal expansion coefficient, and would be a good glue, and/or lacquer candidate.

Regards,

Robert

Attachment:

Coefficient of Thermal Expansion of Plastics - Coefficient of Linear Thermal Expansion per ASTM D-696 (Inch per Inch per ºF x 10^-5).pdf (See Polystyrene (HIPS) = 4.5 Inch/Inch/ºF x 10^-5)

Also (Polyester has a extremely low thermal expansion coefficient, and thus is also an excellent choice)

Thermal Expansion Coefficients - Common Plastics.rar

Typical Linear Coefficient of Expansion for Common Plastics.rar

[ODM]

Depending on lacquer type, some transformer wires do withstand acetone. Prolonged soaking in toluene or MEK will harm winding lacquer as well. It's important to look up the type of wire you're using, and if that data is unavailable, some testing is in order

Most laminating epoxies are quite stable with temperature variations to better match the fiberglass strands, making stable potting materials.

[Cyclonite]

Yes indeed, and in the TGSL thread I had already suggested to always first test the Lacquer layer with the Solvent used. The list of lacquer used for the Copper Enamel Layer are quite impressive indeed. With a thermal resistance varying from about 120°C to more than 200°C, and also with many different chemical compositions too, which may be more or less resistant to certain solvents.

I am thinking about using Carbon Fiber with Polyester Resin. Carbon Fiber will probably act like a nice shield too (with embedded bare Copper wires for shielding contact), its also astonishingly strong once Impregnated with resin, and can be used in a very this layer to give Strong and Sturdy Ultra Light Weight Coils.

The GeoTech Health Coil Series, also referred to as the GeoTech Non-Back-Breakers Green-Earth Carbon Series, ... what a back banger will this be in the Search Coil World!

Regards,

Robert

Attachment:

The Carbon Fiber Toilet Coil, ... relieve yourself of that heavy burden!

[ODM]

As long as you can manage a constant electric connection to the carbon fiber cloth and do not make a full enclosed ring, it ought to be fine, but the connections into reinforcing fibers are difficult to guarantee over time.

[Cyclonite]

I have obtained some woven carbon fiber cloth, enough to make a few complete search coils all in one go, without having to past pieces together. By weaving multiple strands of bare copper wire (I may even use fine Ni-chrome wire, or a combination of both) like in a spiderweb in circular patterns, and connecting all strands together by means of intertwining them into one single bundle. Being careful not to make any Thermocouple Junction points by crossing these different wire types together. I want to create minimal thermal expansion differences in contact areas between the Carbon Fiber and the Metallic interface connections.

If you happen to know a local Fishing Rod Manufacturer, and you don't just walk in with a request for carbon fiber cloth, but also try and interest them in what you are using it for, then in most cases you will get this cloth at a deal, or in my case just for free. They also could not resist in giving me a S*H*I*T*L*O*A*D (see toilet seat example above) of Woven Kevlar Cloth (thank you DuPont and Akzo Nobel), which I may use to make my first bullet proof metal detector housing, and telescopic search coil rod with.

Regards,

Robert