To evaluate the bed or determine how much bed wear has occurred requires a 0-1" micrometer. and some time. First, wipe down both ways from headstock to the right end of the front and rear way. If as usually happens, the right ends of the ways have enough of a varnish build up to see, clean it again until you y have removed all of the varnish buildup. Then determine the unworn dimensions by measuring and recording the thickness and width of the front and rear ways at the extreme right end of the bed where you never work.
Skip say 6" toward the headstock and repeat the measurements. Except on a machine that was routinely used nearly to its limits, the measurements will probably be the same. Continue the 6" skip toward the headstock until you detect measurable wear. Back up 2" and if the wear is measurable, another 2". Start recording the measurements and and continue towards the headstock with skip distances of two inches.
The width error has a 1:1 relationship to the radius error, but the height error does not. In downloads is a paper that I did so long ago that I have forgotten what I called it but that gives the correlation between the height (thickness( error and the radius error for various diameters of work pieces.
