Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

I tried to add up all the toroid inductors and transformers I've wound over the years and came up with four hundred. Probably 90% of these are wound on 0.5 inch diameter cores, either powdered iron or ferrite.

Almost all of these have been wound by holding the core between the fingers of one hand and threading the wire through with my other hand. After winding a half dozen cores, my fingers are usually stiff and it's time for a break.

I've tried clamping the core in a small soft-jaw vise, and found that worked after a fashion, but the vise gets in the way of the windings.

My latest project, a band-reject filter, Inductor Choice for Band Reject Filter, has seen me winding more toroid inductors. This time, however, I decided to build a clamp to hold the core securely whilst providing clearance for winding. The photo below shows the clamp I made.

The clamp is made from two pieces of 0.5 inch x 0.5 inch Delrin (acetal resin). The clamp's nose is 0.2 inches across and is curved to match the core's inner diameter.

The two sections are held together with 6-32 cap screws.

In order to provide a downward clamping force on the core, I added three stainless steel washers between the upper and lower clamp bars in the rear cap screw.

The recess in which the core rests is cut with a 0.5 inch diameter so that the core makes contact with the clamp over its full length. The retaining pocket is about 0.040" smaller than the core height so that the clamp can generate an adequate retention force.

The clamp lip is milled away in both the vertical and horizontal dimensions so that wire can be wound right up to the clamp. I designed the clamp to occupy about 12.5% of the core circumference with the thought that value is typical of the open space between winding start and stop.

In addition to the clamp, I made a small wire needle from a piece of scrap printed circuit board. The needle is sized to fit through a 0.5 core's inside diameter even with the core fully wound. The needle allows me to grip the wire without straining my fingers and lets me apply more uniform tension when winding the core, particularly with small diameter wire.

The photo below shows how the clamp and needle are used. The clamp is held in a Panavise, but I'm considering making a dedicated arrangement to hold it in place.

I've added a small diameter hole crosswise through the clamp's upper jaw to hold the wire in place. The end of the starting turn can be seen sticking out to the left of the needle's center. Having the winding start firmly anchored makes a  big difference in comfort as it's no longer necessary to tightly grip the core and wire for the first few turns.

The tools I used in making the clamp are a bandsaw to cut the stock and a milling machine to cut the jaws and to form the nose relief  plus a 6-32 tap. I'll probably make another clamp for T68/FT68 size cores. The next one I make will probably have a longer nose. I might also consider milling an angle on the top bar to remove the need for washers. Otherwise, the original design works pretty well.