Re: [CAD_CAM_EDM_DRO] Bridgeport way wear

Polaraligned wrote:

>With all the talk about anti-backlash ball screws, and ground vs.
>rolled leadscrews, Does anyone have problems with way wear when
>CNC'ing? After all, most of these machines are 30+ years old and
>have been used almost daily by some machinist making a living.
>
>

Yup! it is not real serious, but I definitely CAN measure it! I have a
1938-vintage
Bridgeport, serial 1388, and it was before some much-needed updates in
the design.
One of these involves the sinuous oil groove on the top of the saddle.
The original
Bridgeport design fed oil from the right end ONLY. The left end got starved
for oil.

Anyway, there is a measurable concave wear pattern in the middle of the
table,
and the top of the saddle has also worn convex, to match. I have a test
program
I use for checking tram of the head. it mills a circular path of
arbitrary diameter
on a piece of stock, then moves to the exact center of the circle. You
then put a
dial test indicator on an arm on the spindle (indicol) and sweep the
circle. If the
head is out of tram, the left and right, or the front and back will not
be equal.
Well, when it IS in tram, and these points are equal, the front/back
points are
higher on the stock than the left/right. Meaning that the table was LOWER
in the very center than when a few inches away from the center on either
side.
Another test is to put a master precision level on the table and run it back
and forth in the X axis. If the table was straight and flat on the
bottom, it should
move in a perfect straight line. (If you have rubber feet on the
machine, this
test won't work, as the shift in the center of gravity will cause the
whole base
to rock.) Well, mine doesn't travel straight. Also, when the axis
reverses direction,
the table rocks slightly right at the reversal. This indicates the
curvature of the
table and the saddle don't match.

Ahh, it needs scraping, but I can't afford to have the machine out of
service for
the couple months it will take to do it!

Jon