According to Bill Schwab :
DoN. Nichols wrote:
I don't know about the lathe in question, but I certainly have
power cross feed (and longitudinal feed) in addition to the threading
feed from the leadscrew on my 12x24" belt driven Clausing. The
longitudinal feed reduces wear on the threading portion of the lathe.
Could I trouble you to explain that? I believe you, I just don't
understand how/why.
Every time you cut threads, you are doing it by the halfnuts
clamping around the leadscrew to drive the carriage at a precise speed
related to that of the spindle by a set of gears. This results in wear
in the half nuts and the leadscrew.
A *good* lathe does not use the threads and half nuts for
longitudinal feed when not cutting threads. Instead, there is a keyway
(groove) along the length of the leadscrew, and a collar around it with
a key which rides in that groove. The collar is mounted in bearings
which allow it to rotate with the leadscrew -- but with no wear on the
faces of the acme threads of the leadscrew. The outside of the collar
is a worm gear, mating to another gear in the apron. This gear serves
to turn either the handwheel or the cross-feed crank to move the
carriage and/or the tool relative to the workpiece. It also has the
advantage of being a speed reducing gear, so you get a finer feed than
you do using the leadscrew and half-nuts.
Note that wear on the leadscrew tends to accumulate in the
region where the carriage spends most of its time, but when you are
cutting threads, you may be using the whole length of the bed. Thus the
localized wear causes errors in the feed for threading -- the pitch
changes somewhat as you move from the unworn region into the worn
region.
[ ... ]
I'm not a user of a piston style toolpost -- I've got the
Phase-II wedge style -- but two significant points (other than the extra
rigidity from the way the wedge style locks up compared to the piston)
a
1) The piston style has the lever landing at two fairly large
zones separated from each other by 90 degrees when you switch
between tool holders on the turning dovetail vs the boring/facing
dovetail. This means that your expectations of when it will
lock have to be adjusted between the two tool holder locations.
2) Related, but perhaps more important -- when there is no tool
holder on the toolpost, with a piston style, the lever is free
to swing totally around the toolpost -- which has resulted in
the handle interacting with the jaws on the chuck -- producing
shrapnel as the hard plastic ball grip on the end of the lever
is shattered.
I think you just sold a wedge style post. In fairness though, is there
an argument for simply having a holder in place at all times to avoid
(2)? Or is that a problem too?
Well ... you *have* to remove one tool holder to place another
with a different tool on the toolpost. And it is while turning to put
the first on a shelf or cart and pick up the replacement that you *may*
hit the lever. You have to be only in a fairly narrow range to give
that a chance of the ball handle hitting the chuck jaws -- but you know
Murphy's law. :-)
And even when switching from one holder on the turning dovetail
to a different one on the boring/facing dovetail you have to lift the
one off after loosening before you can drop the other in place with a
piston toolpost because of the 90 degree change in the lock-up position
of the lever from one to the other. You have to lift the tool holer off
to be able to swing the lever enough to allow the replacement tool
holder to go into place.
Now -- you probably could drill and tap the toolpost to install
stops to keep the lever from going too far -- but I have not yet seen
one so adapted.
As for whether it will fit -- the main consideration is the
height of the lathe spindle's center above the top of the compound. The
critical factor is the height of the top of the tool slot above the
bottom of the holder -- on the assumption that the holder will bottom on
the compound unless you slide the toolpost enough to the side so the
whole dovetail hangs over the side -- thus trading off rigidity again.
What makes it critical? Is the idea to be able to "get under" the
center line? Naively, I would think that it would be necessary to reach
at least that high???
You need to be able to get the tool's edge to precisely at or
*slightly* below the axis center for outside turning, or precisely at or
*slightly* above for inside turning. In any case -- you do need to be
able to get the edge down to there. Normally, there is something like a
32nd of an inch between the top of the largest tool shank and the top of
the slot, so making sure that the top edge of the slot can come down to
the center height of the spindle is sufficient to allow adjustment of the
worst-case tool which *should* be in that holder. And -- when you are
given an insert tool which is too big for the slot, you remove material
from the underside of the shank, not the top, to bring the cutting edge
of the insert close to the top of the slot again. (Of course, when
using smaller tools, you have more room to play with -- but you want to
make sure that the biggest tool which will fit is in place you can
adjust the height properly.
Enjoy,
DoN.
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