Precision coil winding

For making small electromagnetic actuators, winding coils can be a significant time sink. Industrially, this process is performed at high rates of production at tiny scales, just look at the selection of minute wire-wound inductors on Digikey. Achieving good results with small diameter wire is thus possible, but not readily achieved with a general purpose winding machine.

This project is an attempt at a customizable micro-traverse for very precisely laying coils onto a magnetic core. In the application I'm working on currently, I need a pair of coils wound onto a common core with opposite handedness. I could wind them one at a time, but the results have been cleaner and more balanced if they are wound simultaneously. Thus, I decided to make a dual wire device. I started by mimicking coil winding traverses I had seen for larger coils, which use pulleys to handle the wire.

These pulleys ended up being too bulky to get really close to my cores and didn't guide the wire precisly enough. I searched around for micro-coil winding machines, and saw that guide tubes are used to get very close to the winding surface. I decided to use this approach, but I wanted interchangeable tips for different wire diameters, adjusting the wear of the wire, etc. There are endless varieties of inexpensive dispensing tips available (e.g., from McMaster-Carr) with varying lengths, diameters, and materials but sharing a common connection type: the Luer-Lok. This connection uses a single pitch of threads and a mating taper. I re-printed my traverse with a connection for these tips, using a pair of dispensing tips matched to some 32 gauge wire.

For the motion system, I'm practicing my flexural and exact constraint design. Each tip rides on a tall hinge flexure, oriented so 3D printer filament traverses the flexure. By adjusting the lengths between the lead screw nut and the flexure and the flexure and the dispensing tip, we can very easily change the resolution and travel range of the device.

Each hinge is actuated by a NEMA11 stepper motor sitting at the back of the device. The stepper shaft is coupled to a 100 TPI microadjusting screw and the mating bushing is pressed into the 3D print. These microadjusting screws are very inexpensive for the precision they provide. I buy them from Kozak Micro. This screw comes with a hardened ball at its end to provide a repeatable contacting surface. I use this ball against the planar end of the stepper shaft. Any misalignment of the bushing, which translates into a gyration of the ball, doesn't crease any axial displacement, as the ball simply travels over the planar surface. The screw is coupled to the shaft using some thick heat shrink, which allows this radial movement, while providing a surprisingly stiff torsional coupling. The entire mechanism is preloaded (e.g., the ball against the shaft end) by a tension spring.