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Last update 12/08/2010

Winding the Work Coil

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since 08/17/09
Winding 3/8" copper tubing into a tight spiral coil is no easy task. It requires the combination of a trick and a jig.  On this page I present the process.

The trick is to fill the tubing with dry sand.  After you've filled the tube with sand, vibrate it with something like the back end of an engraving pen.  The sand must be packed as tightly as possible.  Flatten and roll the ends over to seal in the sand.  The sand makes the tubing act like a solid rod of copper.  It will not collapse or distort, no matter how tight the bend radius.

The jig and its use is presented here.


The jig consists of only 2 pieces.  An axle, called the mandrel, that sets the coil's inside diameter and the combination bearing and work rest/guide.  For the coil at hand, I used a 1" machine shaft and a 1" rigid coupling, both obtained from McMaster-Carr.  I don't happen to have a vice at the moment (can you imagine a nerd without a vice?) so I welded the coupling to a piece of plate and clamped it to the bed of my medium duty box van using C-clamps.

A small length of bolt comprises the work rest. Another piece is welded to the axle to form the retainer.  The part of the work rest that contacts the tubing is contoured to fit the diameter of the tubing using a Dremel tool.


This shows another view of the mandrel and work rest

Yet another view.


Here the tubing is positioned to start the bend.  It greatly helps to put a little moly-loaded grease on the tubing and the rest/guide.  Be sure to leave plenty of excess tubing at the start so that you will have the leverage necessary to bend the end to shape to fit the capacitor

The first turn is made.  This is the hardest.  The mandrel is being turned with a pipe wrench.  A handle was not welded on because the axle has to slide out of the bearing to remove it from the finished coil.  Someone with a mill-drill or milling machine could come up with a much better crank but for me the pipe wrench worked fine.


Several turns have been wound here.  You'll notice a couple of things. One, I cut the head off the bolt that formed the rest/guide.  Too much friction. The result of too much friction is the denting of the tubing under the mandrel piece.  A well lubricated bearing and well lubed tubing require little effort to bend.  An almost mirror finish on the part of the rest/guide that contacts the tubing helps a lot.



Winding is complete.


The capacitor is held up to the coil to assist with bending the coil leads.  Beneath the coil is visible an early version of a Litz coil that I wound from enamel coated motor winding wire.  It proved the efficacy of Litz wire and demonstrated that I ought to get some real manufactured wire. 

The major advantages of the copper tubing coil is that it can be made with hardware store materials and it can be water or air cooled.  These benefits outweigh the added efficiency that the Litz wire provides.