Ignoramus20788 wrote:
Tried to estimate the bed wear today.

I did the following: placed a digital indicator (0.001mm resolution)
with the magnetic base on the carriage. Its point was resting on the
non-worn part of the bed where the tailstock rides.

I moved it all the way away from the headstock and reset the dial to
zero.

I was hoping that as I moved the carriage along, the indicator would
show a positive number, as the bed would go lower due to its wear
closer to the tailstock.

It indeed happened. The number on the indicator started rising from
zero and rose to 0.05 millimeter (or 0.002 inch).

I repeated the procedure several times, every time the reading would
return to zero as I returned the carriage back away from the
tailstock, and the max reading was consistently 0.05 or 0.06mm, or
0.002 inch.

So, my obvious question is how to read this number, do you think that
the wear is bad or not bad?

That is quite good for a well-used lathe. I had about .013" on
my Sheldon 15" lathe, and decided that was much worse than I
could live with. Making estimates of the overhang and lever
arm, I calculated that it would put a .016" taper or barrel
shape in a 1" diameter part. That was totally unacceptable to me.

What you have to do is repeat the test with the indicator at the
REAR way, riding on the reference surface of the tailstock way
(assuming a classic American-pattern lathe). If the wear shows
a roughly similar change, you are in great shape, as the wear is
roughly symmetrical front and back. Also, gravity and the
distortion caused by flame hardening the bed may have put a sag
in the bed, and your measurement technique may be picking that
up, too. (My Sheldon's 6-foot long bed had a .003" sag in the
middle, probably caused by the stresses induced from the flame
hardening.) The inverted-vee changes the force vectors causing
more wear on the front way than the back, and the dribbling of
chips increases wear on the front way, too. But, the critical
thing is the relationship between the front and rear way. If
the wear is relatively similar, the taper on parts will not be
large. Assuming the front way has all the wear and the back is
unworn, then you might get as much as .005" of taper on the
diameter of a part. Measure the distance between the two ways,
and measure the distance between a line front-to-back between
the ways and the spindle centerline. On most lathes, the height
of the spindle above the ways is very roughly similar to the
spacing between the ways.
You can work out the trig for your particular machine.

If the wear is similar front and back, then the tool will just
move up and down, not in and out, and the taper will be vastly
smaller.

Jon