Close

Winding the Heater Coils

A project log for RepKiln

A small and inexpensive kiln for melting metals and firing clay

matt-mosesMatt Moses 10/16/2017 at 05:460 Comments

As described in this earlier project log, the initial design has two heater elements, each 4 ohms, in series, for a total heater resistance of 8 ohms.

Each heater will be made of 5 meters (16ft) of 16AWG Nichrome 80 wire. Each heater will be wound in 250 turns on a 3/16 inch diameter mandrel, with a pitch of 3.4mm (0.134 inches), for a coil length of about 33 inches (84 cm).

The kiln brick assembly contains slots for an additional pair of heater coils if they are needed. These would need to be powered from a separate 110 VAC circuit to avoid blowing the 15 amp circuit breaker.

At first, I was planning to wind the nichrome wire using my little Sherline lathe. I got the idea for this from this video: How to Wind a Coil Spring on the Metal Lathe. This would have involved disassembling the tailstock so the mandrel could pass through it, along with the construction of various jigs and fixtures. It probably would have worked but it would have been tedious.

But then I found these two videos:

Winding Kiln Elements

How to make a Heating Element Coil Winding Machine

The special-purpose coil-winding machines described in these videos are so cool I decided I had to build one myself. I'm very pleased to say that I was able to construct a working coil winder exclusively with stuff I had sitting around. Here's a video of it:


This is the pile of parts I started with:

The square aluminum tube is screwed onto the 2X4. A 3/16 hole through tube and 2X4 guides the 3/16 steel rod mandrel. The small L-bracket is carefully spaced about 1/16 inch away from the surface of the tube. This is for guiding the wire.

The chunk of wood on the left is added to space up the spool. Note the white cable mount (center) used as a wire guide.

Using a file and a hacksaw, a slot is cut lengthwise in the end of the mandrel. This slot holds the wire during winding.

It is hard to see in this image, but on the opposite side of the lengthwise slot (see above) there is a cross-wise slot in the mandrel. This is for wiring on the bushing that clamps the wire in place. Note the semi-circular clearance cut that has been filed in the L-bracket, directly underneath the mandrel.

This shows a closeup of the wire clamped to the mandrel with a small bushing, which has been wired onto the end of the mandrel through the cross-wise slot. Note the spool mounted below. (It turns out the wire was wound too loose on the spool for good feeding, so the spool in its entirety had to be removed and the wire fed loose by hand.)

Drill connected to mandrel and everything set up for winding. Protip: do not knock the drill off the table - it will bend the mandrel. Don't ask me how I know that...

It works! Except for the spool... The spool was removed and the wire loosely fed in by hand by an assistant.

There are about 20 turns per inch, so we need to wind 250/20 = 12.5 inches of tightly wound coil. This will then get stretched out for the final installation in the kiln.

The first coil was a little longer than 12.5, so the second coil turned out a little shorter:

It's easiest to stretch out the coil by clamping one end in a vice and then (carefully) pulling the other end slowly and steadily with pair of pliers.

Discussions