"Dave Hinz" wrote in message
...

I am so glad to see someone write this. I've been struggling to get
better than that on some finish work. Am I just expecting too much?
Everything seems adjusted right, but I'm getting movement
from....somewhere. Nature of the beast? Flex? Cosmic rays? What
makes that be the inherent limitation of a tool type?

Are you having trouble hitting a thou on one-off work or just production
work? With care you should be able to hit a thou on one-off work, depending
on the work you're trying to do.

The major weakness of the Bridgeport and knockoffs is the two-way knuckle
joint on which the head sits. These joints are the bane of the machinist who
just wants to mill square faces and drill perpendicular holes. During heavy
cutting operations including facemilling, flycutting, endmilling, and
boring, the head is likely to go out of square. It is absolutely necessary
to square the head before doing any fine work, and I do it before starting
*each* day additionally.

After the knuckle joints, there are the brakes and gibs on the axes. While
it is good practice to always use the brake, one should understand that
neglecting the gibs will result in the axes changing location/orientation
when the brake is used. When the gibs are properly set, the brake should
have only minimal effect on the position and squareness of an axis. This can
be easily checked using a dial test indicator.

I'd say paying attention to the knee break is of significant importance.
Always square the machine, and run it, with the knee locked (the lever sits
below the knee crank, on the knee). Due to the weight of the knee and table,
having the gib incorrectly adjusted can result in major squareness and
vibration issues.

Obviously the x and y axes gibs must be properly adjusted, and their
respective brakes must always be used when the axis is not being actuated
during a cut. When the x and y gibs are adjusted incorrectly, the table can
be rocked by hand without too much trouble. This will obviously cause
accuracy, surface finish and cutter life issues.

Lastly, don't cheat with the quill - always cut with the quill retracted up
in the head (you need the exercise anyway).

HTH.

Regards,

Robin