2/3 Axis Grating CNC Milling Digital: http://bit.ly/2WJ61Ht (code: BG134012, 12%off)
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My mill has had several types of DRO's fitted over the years.
When funds were tight I added X, then Y and finally Z of the digital scales, like the ones used on digital calipers. The Z one was still in use.
A number of years ago I was give a Anilam 2 axis system removed from a Bridgeport mill. The scale were much too log but I was able to dismantle them and shorten them, including carefully cutting the glass scale. Eventually the display stopped working so I replaced it with the one shown on the video.
I spotted this 3 axis system at very low price and decided to fit it to the mill and put the 2 axis one on my lathe. Look out for a video on this installation.