Hmm. The Bendix Ferrand made a DRO. I used on in the late '60's. It had a nixie tube readout and a box of printed circut boards big as an apartment freezer. It had dial switches to enter offset, axis reversal, acis zero. The Bendix Ferrand was accurate as hell and almost trouble free. Good system.
Back when, I ran a mill with an optical DRO that had been around forever. The scale was mirrored and coarsely graduated. You turned a dial and the little cursor lined up with the grad to give you thousandths. PITA but you could position within 0.001". SIP had an optical scale sytem built in since the late '30's as I recall.
Back in the day, many larger machnes had precision made troughs for micrometer length standards. The lenght standards were selected from a large tray and clipped to the troughs. The micrometer one was set for the decimal inches. The machine had little dial indicators on the troughs that registered against the length standards. You conld make reliable settings to +/- 0.0003" if you were careful.
The all-electronic DRO's began in the 1970's and over the years evolved to their present form. As the tecnology advanced new features were added til now we have grid reorientation, bolt circle, expert system resources, etc.
I ran a 10 year old mill n 2000 that had all electronic handwheels. What ever you dialed you got and there was a big display on a screen to tell you what it was. You could run it full CNC, teach and play, or straight manual. I loved it but it was in high demand for CNC batch parts manufacture.
Intereting topic. Maybe there's a book to be written "The Evolution of the DRO and its Effect on World Manufacturing."
