A different way cover

[homebrewed]

Transparent for how long?

Good point. I guess time will tell, if it's invited to stay on the lathe .

Mylar isn't real expensive if it comes to replacement. I'm actually using Duralar, found in art supply stores. A book of 25 11 x 14 sheets costs about $15 through Blick, not including shipping.

 

[homebrewed]

Go old school man, use a micro-switch with a lightweight arm extension mounted on the saddle that becomes activated by the mylar cover being under tension (using the N.C. contacts) to power the motor.

I like it!

 

[NC Rick]

Or, when it goes slack, the marbles are released and...
anything at all to avoid programming or computers!

 

[GunsOfNavarone]

If complicating it, but making it cooler is welcomed, 1 encoder and one motor. The encoder is tied to the motor in a one to one ratio, if the encoder moves 1", the motor moves 1". Probably a bit less complicated than what you have but totally clean/reliable. I think a simple Arduino board aught to do the trick!