Assembling the coils

This page describes how to wind the coil and attach the wires to the CoilBobbinWithReboundArrester() to create a coil assembly.

Parts needed:

  • a CoilBobbinWithReboundArrester()
  • around 16 metres of 0.224 mm enamelled copper wire
  • around 2 metres of 22 AWG solid core hookup wire
  • solder
  • a piece of adhesive backed felt 10 mm x 7 mm

Tools needed:

  • A mimphonium coil winder (highly recommended)
  • Soldering iron
  • Wire cutter
  • Wire stripper

The plastic parts and the windings for all the coils are the same for all notes. The only difference is the length of the wire used to connect up to the DM7688 driver board. If you wanted to streamline production at the expense of material cost, you could make all the coils the same and cut them down to length at final assembly time.

Mount the coil onto the coil winder. Gently tighten the central bolt, just tight enough to stop the coil rattling or slipping off the winder.

Turn the coil winder by hand so that the two tags are at the top.

Wrap the wire 3 turns anticlockwise around the left most tag, leaving about 30 mm of wire sticking out. Later on, you will trim this down to 20 mm. As you get more practice, you may be able to go straight for 20 mm at this point. Bend this wire out of the way so that it does not get caught under the winding or in the motor shaft.

Pinch the winding wire between your left thumb and forefinger and start the coil winder from the PC with your right hand. The motor will turn the coil clockwise. If everything goes well, it will seem slow. If you lose concentration and don’t feed the wire properly, things will tangle up very quickly.

As the motor turns, keep the wire taught by pinching the wire, and slowly move back and forth to wind the coil with as evenly and well packed as you can. If you move back and forth too quickly, the gaps between the wires will be too big, and so the magnetic flux density will be lower than it could be. If you move too slowly, the wire will pile up in the same spot, and also cause big gaps.

Keep an occasional eye on the PC screen as it counts turns of the motor. When you get near the end, try and arrange it so that the wire is at the top of the coil when the winding stops. This will avoid a steep diagonal change of direction in the wire at the end which will have a tendency to slip over time and loosen the coil.

Wind the wire around the right tab and snip off the wire with 20 mm of wire hanging off. Remove the coil from the winder and bend the two wires 90 degrees half way along.

Prepare two lengths of solid core 22 AWG wire. These need to be long enough to reach to the DM7688 board that will drive the coils. Depending on your overall construction, the wires for each coil may be considerably different lengths.

Strip 7 mm of insulation off the ends of the 22 AWG wire. Hold the stripped end of the against the middle of the coil wire, and carefully wind the coil wire around the stripped end of the wire. You should now have 10 mm of coil wire leading from the plastic tag and then a few turns of the thinner wire around the thicker wire. Try and space the coil along the full length of the exposed wire rather than bunching it all up at one end. This allows the insulation on the coil wire to burn off without affecting the solder joint.

Once the wires are stripped and wound, heat the joint with a soldering iron, and feed in solder to join the two.

Let the joint cool down for a few seconds, and push the exposed metal into the hole next to the solder tag. The coil wire should be tight against the top of the plastic and there should be no exposed metal.

Glue the wire into place using hold melt glue.

Bend the two wires at 90 degrees to face away from the rebound arrester and twist the two wires around each other. This twisting keeps the two wires together and reduces the amount of electromagnetic interference generated as the coil is energised.

Peel of the backing from the adhesive on the felt and stick it to the end of the rebound arrester. If you can’t find felt with the adhesive already on, plain felt and double sided sticky tape would work just as well. This will eliminate the annoying clicking noises as the hammer hits the rebound arrester.