Homemade Coil Forms
by David McNamee
Two identical  8.7 x 30 fiberglass forms


    This article covers the construction of fiberglass coilforms. The method shown here is not gospel and may be improved upon and adapted to other sizes, materials, and construction jigs; everyone does things differently depending on the individual situation and what is available. The approach taken is to use the least material possible to accomplish the desired purpose - in this case, minimizing the amount of material intimately associated with a high frequency Tesla secondary winding.

      It may be more economical to make your own coilforms if you find yourself making many coils, can't find exactly what you need, want an odd shape (conical, etc.), or wish to try out different materials without paying custom prices for a one-off item. Initial attempts by first timers might result in a less than perfect appearance, but this is of little concern since most of the coilform is covered with wire anyway;  it is the performance that counts. Other uses also come to mind - telescopes, model rockets, etc.

        These forms are a composite - a catalyzed resin (epoxy or polystyrene) plus a fibrous material (fiberglass, paper, synthetic or natural fabric). The combination of the two results in a material which is strong and lightweight. For this project, I used an epoxy system that had a reasonable working time - 30 minutes, and 9 ounce fiberglass cloth.

        Needed materials: Resin and hardener, mixing cups, stir stick, glass cloth, paper towels, newspapers, squeegee, scissors (or razor blade and steel rule) to cut glass cloth; heavy Mylar (polyester) film, a suitable jig or winding stand, 2 inch clear tape, 80 and 150 mesh sandpaper and block, fine toothed hacksaw, mildly soapy water, washcloth.

     For this project, a section of 8-5/8 inch O.D. fiberglass pipe, 30 inches long, is used as a form. Any material can be used  as long as it is true in end-to-end diameter and not out of round. This particular piece of pipe is a little short to use for this project; it is what I have handy in this diameter, and is the reason for some of the procedures outlined below; a longer pipe could be used. Two 1/2 inch plywood discs the same diameter as the inside of the pipe are placed about one inch in from each end. Each disc has four band saw cuts spiraling in from the edge at a shallow angle; a screw run through each of these cuts causes the wood to expand against the wall of the pipe and hold the disc in place. Each disc also has a 3/4 inch hole in the center, and a 3/8 inch hole to one side. A 3/4 inch steel bar is run through the pipe/disc assembly and placed in a winding stand so that it can be manipulated easily. The 3/4 inch bar is drilled through at several points to accomodate a lock pin made of a short piece of smaller bar bent 90 degrees. One leg of this pin is inserted in the bar; the other end goes in the off-center hole on one of the end discs in the pipe assembly, which is then pushed up against the pin. A 3/4 inch clamp collar on the bar is pushed up against the other end disc and tightened. The pipe is now locked to the bar.

        Two turns of .005 inch Mylar (available from McMaster-Carr) 40 inches wide are wrapped around the pipe without attaching it and the lap seam covered with 2 inch wide clear (Mylar) packaging tape. Mylar is used as a release agent; the resin will not bond to it. The Mylar is now taped to the pipe by carefully applying small strips of packaging tape to the inside edge of the pipe and Mylar. A suitable handle is attached to the bar so that it can be turned as required.

The form ready for glass

        The glass cloth is cut to a length (83 inches for these coilforms) which will allow a little overlap on the last turn, and several inches wider than the desired finished length so that the uneven ends can be cut off later. The number of layers required depends on the thickness of the cloth and the finished wall thickness desired; for this project three layers is adequate. The glass is .009" thick; it bulks somewhat with resin, so the resulting finished wall thickness is about .040".

        Many layers of newspaper are placed on the floor under the winding stand and on the work table; resin has amazing mess-making capability. The laminating is done on a cool day, 70 degrees F or less, to keep the resin working time long. Several ounces of resin is mixed according to the directions on the container. It is best to mix in small batches and to have a helper;  the resin releases heat as it cures; if trapped or not removed, this heat can accelerate the gel time of a large batch, and time is of the essence here. Once started, you cannot stop until done and working while someone else mixes resin makes it easier.

        One end of the cloth is draped over the form, straightened and aligned. Now you are ready to begin. Work on an easily manageable section, about one quarter of the way around at a time for a form this size.

Ready for resin

Some of the mixed resin is applied by pouring or with a disposable brush along the top of the glass and spread over the cloth with very light pressure on the squeegee, holding or turning the form as needed.

Pouring the resinSmoothing the resin onto the cloth

        In a minute or less, as it soaks in, bubbles may form under the cloth. A disadvantage of the particular cloth used for this project is that it has a tight weave and it is impossible to avoid bubbles; a more open cloth would benefit here.

        The resin is then firmly squeegeed into the cloth, generally toward the uncoated glass side, working out wrinkles and bubbles. Don't skimp on resin, the weave must be well filled. Excess resin is squeegeed to the next section of glass, turning the form as needed. The application and working of resin and glass continues until all the glass is on the form. Watch that the cloth does not run to either side.

Working the resin into the clothSpreading resin onto the next section

        The form is now turned slowly in the direction of glass winding and the squeegee held firmly against it. This removes any remaining bubbles as they are brought around under the glass to the end lap edge and presses all the glass layers down tight. Sometimes a strand of glass will unravel from the edge and should be removed. Stop for a moment, checking and filling dry spots and redistributing excess resin. Any remaining offending bubbles are removed by more rotation/firm squeegee pressure or by pricking with a toothpick or hobby knife; the thousands of really tiny bubbles caused by the resin trapping air as it is worked are of no concern.

Slow rotation/firm pressure to remove bubbles, consolidate layers

When satisfied with the result and the tube needs no more working, the excess resin is smoothed on by slowly moving the squeegee, held at a slight diagonal to the tube axis, with gentle pressure from one end of the form to the other, back and forth while the form is being turned. More resin is added if needed and spread in the same manner to assure a smooth surface.

Distributing excess resin on completed formChecking it out

The form is then turned occasionally until the resin gels, unless you are already at this point! Then it is left to sit overnight and cure. Total time from mixing the first batch until end of final smoothing was about 25 minutes for each form, much less than it took to set everything up to do it. Total mixed resin used for each form was 15 ounces (two batches of 6 ounces resin and 1.5 ounces hardener).

        If resin gets on the hands it can be removed with dish soap applied without water, then rinsed, or one of the pumice-filled hand cleansers.

        The next day the tape is removed from the ends of the pipe, the coilform is slid off the pipe and the Mylar peeled off the inside (to be reused to make another coilform). The coilform is replaced on the pipe and taped in place, and again set up in the winding jig for sanding.  Do not dry sand fiberglass. Fiberglass dust is famous for its itch factor, so clean everything and yourself after the sanding and cutting process. The ridge caused by the final lap of cloth is sanded down and feathered in with 80 grit wet/dry paper and plenty of water (with a few drops of soap) until smooth. An occasional wipe with a damp washcloth clears the residue. The coilform is then turned and wet sanded with the 80 grit until smooth, cleaning as needed. This is followed with a 150 grit sanding.

Wet sanding the lap seamWet sanding the coilform

         After cleaning and drying, the finished length (30 inches) is marked around it and the ends removed with a fine tooth hacksaw held against the coilform as it is turned. A wide strip of thin cardboard or similar material taped around the form helps to guide the blade.  When the blade begins to break through, stop turning the coilform and cut through a small section at a time. The edges are then smoothed with 150 grit sandpaper after being removed from the form.

Trimming endCutting off endEnds removed

        For this project the coilform ends were punched with six evenly spaced 9/32 inch holes to accomodate 1/4 inch nylon bolts. End plates were fabricated from 1/2 inch Sani-lite (HDPE cutting board), edge holes drilled and tapped, and installed. The end plates also have the same hole pattern used in the winding jig discs, so that the coils may be wound on the same jig. Flashover discs can be added but were not installed in these particular coilforms.

Thanks to friend Tim Ferrell for photography assistance, and help with coil winding.

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This part of this site last revised August 15, 2000.