Thanks for all your feedback on this
I'd planned on using quadrature scales, and also a rotary encoder on the lathe spindle for angular display for live tooling use, and switchable to rpm for general use.
The Arduino solution looks neat, and i already have one of them, but I do like to get hot swarf pinging around, so I'd like the display to be pretty robust. The Shumtech again looks neat, but it's out of stock at the mo, so I'll give them a chase up.
If I went the Mach3 route, is it likely to confuse it if it has no motors connected?
Sorry for what may be dumb questions, but i'm a lot more comfortable with mechanics than electronics...