Sometimes a project calls for the manual turning of a shaft by some means other than simply attaching a handle or a knob. For example, you have some shaft geared down that you want to rotate by known discrete amount or steps, maybe this device is out of convenient reach.
You could fit a stepper motor and some electronics to send a known number of step to the motor or perhaps such electronics is not your forte or the environment is not conducive to reliable electronics.
I had a requirement some time ago to adjust the components of an aerial tuning unit for a HF transmitter that was mounted at the aerial mast some distance from the operating position. Stepper motors were obvious candidates but the high intensity RF fields would play havoc with electronic circuits, so I made a manual stepper control.
Here is the mockup I used to test the concept (sorry about the dust)!
The manual control is a multi position switch which is wired to energise the coils of the stepper motor in sequence, the stepper motor is from an 8" floppy drive but one from a printer would be just as suitable. The terminal strip has nothing on it except diodes to protect the switch contacts and a couple of sockets for plugging in the 12V supply.
Connect the power to this board and turn the knob the motor turns too in the direction and at a ratio according to the design.
Supposing you had a 12 position switch (not hard to find at the usual electrical and radio suppliers) and you had a 7.5 degree stepper motor you would be able to configure these so that one turn of the switch makes 12 steps of the motor, or 90 degrees. So you have a nice 4:1 reduction, with the same type of switch and using half stepping you would have an 8:1 reduction and so on according to your choice of motor and switch.
Of course the switch does not have to be turned manually, consider for example that you mounted a switch on your mill spindle and the stepper on a feed screw you would then have power feed in a set ratio to spindle speed and you could change the ratio by switching out some of the switch contacts. Staying with your 12 position switch and the 7.5 degree motor you could reduce the feed ratio down to 48:1 or even 96:1. Of course the feed would stop or start with the spindle.
Almost all multi position switches have a stopper to prevent 360 degree rotation but except for the modern all sealed plastic ones you can usually easily remove the stopper, check before buying.
