These small tuning capacitors are used in transistor radios and crystal set radios. They have two sets of vanes inside the plastic box connected to the three wires on the outside (the center connection is the common). In a typical radio this device is usually is attached to a thin plastic disc that goes to the tuning mechanism cord and dial. The capacitor does not have a standard 6mm shaft so you can't simply fit a control knob on the device. Here I have designed a 3D printed shaft with a simple control knob on the top. The knob fits over the fitting on the capacitor and is secured using a 20mm 2.5M bolt that passes through from the top (via a counter sunk hole).
