On Tue, 21 Jun 2011 04:29:36 -0500, Karl Townsend
wrote:

On Tue, 21 Jun 2011 06:02:24 GMT, "Harold & Susan Vordos"
wrote:


"Andrew VK3BFA" wrote in message
...
OK, whats the preffered way of doing the horizontal and vertical
alignment of the headstock and tailstock. I have a calibrated bar and
finger indicator. No machinists level, so will take it as a given the
bed is flat and true.

Is it between centres each end. (my first choice.)

If you did it using the 3-jaw chuck and the dead centre at the
tailstock method, as I have seen in places, would this not just
introduce more error into the process as the 3-jaw certainly aint
true ...and it would give a different offset every time you chucked
something up anyway.

Replies appreciated,

Andrew VK3BFA.

To not level the lathe isn't a wise decision. Only when the bed is level
(in a common plane, even if it's not really level) can you do any meaningful
adjustments. If you rely on the chuck and tailstock, you may get the
reading to come true by tweaking the bed, or it may indicate it's true when
it's not, but that doesn't translate to a properly setup lathe. Beg,
borrow, or steal a decent level to get started. You can have minor twist
in the bed and not be any the wiser. If you happen to have, all your
setup time is wasted. Remember, the real purpose of a level isn't to
*truly* level the lathe, although getting it level is easier than trying to
remember how much error you have end to end, so you can duplicate the error.
The real purpose is to insure that the bed isn't twisted.

Harold

Based on my experience with one lathe, I'm no longer a great fan of
leveling. I bought a very expensive level for our Mazak M4 lathe,
16,000 lb. 22" X 72", and leveled it perfectly.

Then chucked a 30" long 6" diameter rod. The idea here was the biggest
thing that could be run without a tailstock. A light cut showed a
taper. Started raising one of the legs on the tailstock end and got it
to cut true. Anyway, level shows twist, machine runs right.

I own the level and would probably get the next lathe close. But the
proof is in how it cuts. IMHO, skipping the level step on small lathes
might not hurt that much.

Karl


Much of this has been said before but it's perhaps worth
summarising.

With normal ( anything short of Gigantic! ) lathes errors arising
from the direction of gravity are negligible so absolute level is
unimportant. Bed level to within a few degrees is more than
good enough. However, dependent on how it is mounted, the long
narrow length of the bed is relatively easily twisted.

A precision level is commonly used to check bed twist by noting
the change in bubble position as it is placed at different
positons along the bed . This has to be done pretty carefully to
be sure that dirt is not affecting the measurement and that the
chosen surfaces are accurately representative of the surfaces
that actually guide the carriage. It's also necessary for the
lathe to be first levelled to within the very narrow range of
available bubble movement.

An alternative method is to first place the level on the
carriage and adjust with shims to approximately centre the
bubble. The carriage can now be traversed over its full range.
The bubble movement now directly indicates the twist (and wear
errors) in the guidance surfaces.

While it's interesting to know how the bed is behaving, the
usual end aim is to set up the lathe so that the tool as
supported by the carriage moves accurately parallel to the axis
of rotation of the headstock spindle i.e. it cuts a true constant
diameter workpieces. This can be done elegantly and accurately
By " Rollie Dads Method". This does not require a precision level
or even an accurately straight test bar.

Whichever method is used, care is needed in the method of
correcting the twist. While shims or screw adjustment between
the lathe and a very solid stiff surface would seem desirable
it needs extraordinary fine adjustment to reach the desirable
tenths accuracy . In addition, undesirable stresses result
unless the mounting surface has both similar thermal expansion
coeficient and thermal time constant.

Many manufacturers use the simpler solution of mounting the lathe
on a comparatively flimsy sheet metal cabinet. In this case the
torsional stiffness of the cabinet is much less tha the lathe
bed and this means the effect of screw or shim adjustment is
divided between the cabinet and the bed and it becomes much
less critical.

The twist adjustment should be between the bed feet and the
cabinet. The location of the cabinet feet is relatively
unimportant and may even be rubber mounted.

Jim