I got tired of using my cheap Chinese mill without a DRO.
Linear scales are $400 and up per axis. Complete DRO's run about $1000 and up.
An Omron rotary encoder is $25 with 2000 pulses per rev.
You can't gear it to the ball screw as that has backlash.
So I came up with this scheme.

I am using a $50 Maximite to display the numbers on a PC VGA monitor.
An old  PC keyboard to zero the numbers. I have half a dozen.
A simple BASIC program will get the job done. I will upload the source
code shortly.

First step to a cheap NC mill.

Paul Lamar

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