Winding small actuator coils

Martin Newell

August 2004

There has been much written about techniques for winding small actuators, most of which is very helpful. Some of it, though, seems to me to be unnecessarily complicated. So I thought I would add my approach to the problem.

I have wound many 4mm ID 50 ohm coils with 44 awg wire using the following techniques. Items needed are an electric drill with speed control, battery operated elastic-powered plane winder with counter, bobbin as used for tying fly-fishing flies, some spools from fly tying thread, various sizes of brass tubing, soldering equipment, masking tape, cyanoacrylate glue, mold release agent, candle wax, 44 awg wire, 38 awg wire.

Outline of the method: Transfer the 44 awg wire onto a flytying spool, marked in 18ft lengths, using the elastic winder. Using a mandrel made from pieces of brass tubing, mounted in the electric drill, wind the coil using the flytying bobbin, with the previously wound spool. The coil is immersed in a blob of thin CA while winding. Solder lengths of 38 awg wire to the ends of the wound 44 awg, wrap them around the coil together with a stress-relieving tube, and glue with thick CA. Remove the coil from mandrel and you're done.

Using the following mandrel design, with wax and release agent, the coil usually comes off the mandrel quite easily. Make the mandrel from a 1" length of 5/32" (4 mm) brass tubing, split down one side with a hacksaw. Make each end cap from 7/16" lengths of 6/32" and 7/32" tube, one soldered inside the other. File the ends flat and polish with 600 grit emery paper. Mount end caps on the mandrel which is mounted on a 1/8" diameter threaded bolt. Tighten up a wing nut to lock the mandrel to the bolt, and position the end caps apart to leave a 1/8" slot for the coil.

The process of transferring the 44 awg wire from the main spool to the flytying spool is partly because my wire is on a half pound spool which has a lot of inertia, and partly because I want to work with the flytying bobbin for fine control. By cutting 4 slots into the head of the elastic winder, a flytying bobbin can be pushed onto it with a snug fit. Using the counter on the winder, spool off 75 turns of wire and attach it to the bobbin with a small piece of masking tape, but don't cut the wire. You can then wind several more lengths of wire to make several coils, each one demarked with a piece of masking tape. If you break the wire, just discard that section back to the previous tape and start a new section. When you have the number of sections you want, cut the wire and mount the spool in the flytying bobbin. To avoid having to rotate the big spool that holds the 44 awg wire, stand the spool on end and let the wire unwind around the spool. This will put a twist in the wire, but it is so thin relative to its length that this isn't a problem.

To wind a coil, mount the mandrel in the electric drill and rub a thin coating of wax in the slot. Heat it with a small flame while spinning it until the wax melts and flows evenly. Apply some release agent with a small brush, and spin the mandrel until it dries evenly.

Attach the end of the wire to the drill chuck with a piece of masking tape. Slowly wind one layer of wire in the slot. It can be easier to hold the bobbin about a foot from the mandrel. Put a large drop of thin CA onto the single wrap and encourage it to flow around the coil so that it sticks to the sides of the gap by surface tension and forms a waist in the middle. Now wind on the rest of the coil until you meet the next tape marker on the bobbin. Add more thin CA on the way if needed to keep the coil submerged. Blot the excess thin CA with a tissue or cloth. Cut the wire about 2" from the coil.

Attach the 38awg lead wires. Take an 8" length of 38 awg wire. Clean off and tin 1/8" at one end by pushing it into a blob of solder on a fairly hot soldering iron. Put a tiny hook in the end with tweezers. You do not need to clean or tin the coil wire. Hold the coil wire adjacent to the lead wire, overlapping enough to hold them between finger and thumb. Wind the coil wire around the lead wire, starting 1/8" from the end of the lead wire and stopping up against the hook in the end of the lead wire. The hook stops the wraps of coil wire from sliding off the lead wire. Apply soldering paste to the wraps and solder the joint by immersing it in a blob of solder on a fairly hot soldering iron. The insulation on the coil wire will melt and the two wires will be soldered together. Try not to leave any excess solder on the joint. Repeat this process for the other lead wire. It sounds complicated, but it takes less time than reading this paragraph

I have recently learned from the relay manufacturers an easier way to attach the lead wires. Make a small hook on the end of a 2" length of wire. For 6 inch leads, fold the last 6 inches of coil wire back on itself and grip it where the end meets the wire. Insert the hook in the loop so formed and gently pull it tight. Spin the shaft of the hook in your fingers to wrap the folded lead wire onto itself, until the wraps are fairly tight. This strengthened 6 inch end piece will be the lead. Wrap the coil wire to just include the beginning of the doubled part. This is much easier than soldering on lead wires.

Cut a 1/16" length of 1/32" diameter plastic insulation from some thin wire. This will act as a strain relief on both wires. Thread it onto both lead wires. Wrap the pair of coil wire ends onto the coil together until the second solder joint is on the coil. Take care to separate the two solder joints if they happen to lie adjacent on the coil. Slide the strain relief tube up to the coil and glue it to the coil with a small drop of thick CA. The idea is to have both lead wires exit the strain relief tube but not glued to it. You may also want to put a little CA over the exposed solder joints to insulate them.

Next we get the coil off the mandrel. Remove the whole assembly from the drill, unscrew the wing nut and slide the mandrel off the threaded bolt. If you want to be on the safe side, spray a little accelerator on the coil. Gently twist the end caps in opposite directions and one of them will break loose. Remove it. Use the other end cap to push the coil off the mandrel. If necessary, squeeze the mandrel with smooth-jawed pliers to close the length-wise slot in it. This should allow you to push the coil all the way off. Gently separate the remaining end cap from the coil. You may need a razor if it is being stubborn, but usually not if you did a thorough job of preparing the surface. Clean up the coil if necessary and you have a new actuator coil.

Here are some pictures of a coil wound with 52 awg wire (0.0008" diameter) on a 1.5mm mandrel using the techniques above, with a 1mm x 0.5mm magnet.

Here it is next to a Bob Selman actuator for comparison:

And here is a movie of it working.