Richard J Kinch wrote:
Jon Elson writes:


If the top of the knee is the big problem, you can make sure the
gibs allow the saddle to move all the way forward and back.
Then, rig a dial indicator to measure rocking of the saddle,
and use the table as a lever to try rocking the saddle. See how
much looser it is in the worn area.


The "shake" test is OK as far as it goes, but the knee-saddle-table system
involves no less than 24 different precision bearing surfaces. A simple
straight edge and feeler gages will map some wear, although jigs and other
standards are needed for complete characterization. For any detailed
measurements you must remove the table and saddle. Mating surfaces with
complementary concave/convex wear maintain rigidity and pass the "shake"
test while degrading positional precision.

While it is possible for the surfaces to wear into mating curves, it
seems that in most cases of one long and one short piece (eg. a long
table on a short saddle) the wear is distributed such that the curves
are very unlikely to actually match. That is certainly the case on
my 1938 bridgeport mill. The underside of the table is concave,
the top of the saddle appears to be a convex curve of shorter radius,
allowing the table to rock slightly at any position.

The Y ways look like they were used as an anvil, apparently by the
daily dropping of wrenches, etc. on them. One of these days I
really have to tear the whole machine down and rescrape it, but I
can't afford to have the machine down for the 6 months it would
take me to do it.

Jon