In article KfjEb.422665$ao4.1358065@attbi_s51,
Loren Coe wrote:
In article , DoN. Nichols wrote:
In article uG3Eb.72868$8y1.273479@attbi_s52,
Loren Coe wrote:

[ ... ]

this is a basic design feature that marks
the very low end machine, the lack of, i mean. my manual feed requires
turning a handwheel on the end of the lead screw. not very handy.

That also suggests that you don't have half-nuts -- to decouple
the carriage from the leadscrew.

there are half nuts, no thread dial.

If you have half-nuts (in the apron), you really need a
threading dial. And it is possible to *make* one for the lathe. One
quick-and-dirty way is by finding a gear with a tooth pitch which
matches the thread pitch of the leadscrew. You'll have to mount it on a
shaft at a bit of an angle, to make up for the lead of the screw.
Better is to make one which is pseudo-hobbed by a tap which matches the
leadscrew, so you have a better match, and so the teeth are angled
properly to engage the threads on the leadscrew. The number of teeth
on the gear are critical -- and I would have to go to _Machinery's
Handbook_ to refresh my mind on the formulas to determine which tooth
count you want for which leadscrew pitch. (Of course, if you are
cutting imperial threads with a metric leadscrew, or vice versa, the
thread dial is pretty much useless, and thanks to the kind of intervals
used in metric thread sizes (unlike inch sizes, which are mostly powers
of two multiples from a few starting points), you really need a
threading dial with (I believe) four different tooth counts, on a
sliding shaft.

There was an article in _Home Shop Machinist_ (or was it
_Machinist's Workshop_?) on making a threading dial for a lathe which
either never came with one, or which lost it. My Clausing, which was
used throughout its life as a turret lathe, doing threading with
Geometric die heads, did not have a threading dial mounted -- but I
found it (apparently unused) in one of the pedestal drawers. :-)

the lead screw is the y-axis feed
whether power or manual. if you disengage them, you can move the carridge
by hand (literally).

You mean push it along -- no handwheel to move it?

[ ... ]

Well ... actually, this does not have a second (or third) drive
shaft. Some do -- but mine derives the drives from a keyway on the side
of the leadscrew, so it does not use (nor wear) the threads during power
feed operations. A collar with an internal key slides on the leadscrew,
and couples the rotation of the screw into a worm gear, which meshes
with a matching gear in the apron.

Then, there is a lever which can connect this gear either to the
handwheel (for longitudinal feed), or the cross-feed crank (for cross
feed). The actual threads of the leadscrew are only used for threading,
thus reducing wear which can make the system less accurate.

right, i have come to understand that what i have is pretty much a
kludge. the rack gear would be a real bonus (on low-end machines).

Interestingly enough -- the Taig (Peatol in the UK) has a rack
gear and handwheel -- but *not* a leadscrew. You are expected to do
your threading with dies on that machine.

it may not sound like i am happy with this 3n1, but i am, for the
amount of use it gets and my needs. still, it is fun to think about
the better machines and their features.

I understand. I started with a Unimat SL-1000 (the leadscrew
was the *only* way to move the carriage.) I then got an old
Atlas/Craftsman 6x18", which had the rack -- with the handwheel purely
manual -- and all power feed was via the leadscrew -- with a
basket-of-gears threading setup, and a chart on the inside of the gear
cover.

On some systems, the coupling to the power feeds is via a
clutch. On mine, it is a set of gears that engage or disengage with a
lever, which means that you sometimes have to wait for teeth to align
before you can complete the lever motion. The lever rotates a short

i have not tried to use the lead screw clutch when the motor is on,
never had the nerve. if that is okay to do, it might make some things
easier.

Where is this clutch? The ones which I was talking about are in
the apron, used to connect the power derived from the drive screw
rotation to either the cross-feed, or the longitudinal feed handles.
These engage smoothly, and can be set to slip before you are in danger
of damaging your machine.

The ShopTask, however, has a dog clutch (which enages only at
one point in the rotation), to couple the spindle's rotation to the
leadscrew. (It also achieves reverse, by connecting to a
counter-rotating gear with a second dog clutch enaged by the same
lever.)

shaft parallel to the axis of the machine (just in the apron), which can
either move up for longitudinal feed, or (after sliding the lever to
align with a different slot) down for cross-feed.

sounds like a ton of ways for a newbie to break things.

The ones using clutches for the feeds are no problem -- as they
can be set to slip before something is damaged.

On my Clausing, in the normal crossfeed direction, if all else
fails, you will run out of leadscrew before the cross-slide falls off
the carriage. (Fine for facing, but if you're parting, you can run into
the remaining part of the workpiece with the tool block. :-) For
longitudinal feed, you can run into the tailstock, or the headstock (if
you are working with collets, so the chuck is not in the way), and there
is a shear pin in the leadscrew drive from the gearbox to fail before
something serious breaks.

i have left the
drive belt loose, purposely, as a saftly measure until i get more experience.
it has saved me several times already (when running the carridge into to
chuck).

Not possible to leave it loose on my Clausing. There are three
belts in parallel between the free-rotating pulley on the spindle shaft
(which is locked to the bull gear with a pin, or coupled via the back
gears) down to the countershaft in the pedestal. From that there is a
single belt, going on one of five different grooves from the end of the
countershaft to the motor spindle. There is a lever which lifts the
motor to slack this for shifting belts, but that is way too lose to run.
When you move the lever to the side, to engage the belts, the weight of
the motor takes over, and won't stop moving the motor until the belt is
tightened by the motor's weight. (There is a set of springs to catch
the motor if the belt breaks. :-)

...
Those which I have seen with *three* shafts actually use that
third shaft (key coupled to a lever on the carriage) to turn the spindle
motor forward, stop, or reverse, operating switches inside the pedestal,
so there are no wires run out to the carriage. This is more important
with a longer bed, which does not allow you to reach both the carriage
controls and the switch on the headstock easily -- at least when working
near the tail end of the lathe. Very useful when something happens
other than what has planned.

that makes a lot of sense, and gives me some idea of the variety of
features extent. there is a definite safety issue with longer beds.
just opening the half-nuts may not always be desirable?

If the workpiece is starting to work loose in the chuck, or one
between centers is starting to bow, you want to stop that spindle motor
*right* *now* -- not just the feed. On one lathe equipped with the
carriage mounted lever, there was another panic stop means which was
even quicker. There was a stomp bar across the distance between the
pedestals, and pressure on that anywhere along its length, would turn
power off the motor, and apply a spindle break. (Needless to say, this
was *not* a threaded spindle, in which the chuck would be likely to
unscrew under such a panic stop. :-) I had to use that only once, but I
was really glad to have it. The cut was spewing chips too hot to let me
get to the headstock -- or even to the lever on the carriage. :-)

Note that if a large workpiece was starting to work loose in the
chuck, I would not want to pass the chuck to get to the controls on the
headstock anyway. :-)

.... interesting design clinic deleted ........

I've only done threading once on a machine without half-nuts -- a
ShopTask belonging to a local friend, to test it out, as he was having
problems getting good threads from it. Part of the problem is that the
spindle speeds don't go low enough to make reaction times reasonable

yes, no back gears. same here, but i do have room to do something, some-
day.

Note that you are limited in the torque you can get with just
pulleys and belts doing the stepdown. Gears have that nice no-slip
advantage.

my hope is that the dc motor may mitigate the need for them. another
someday project (the motor i ordered is an "open-frame").

Hmm ... you'll have to fix *that* feature (with a housing of
your own, and a fan blowing filtered air through it), or your motor is
likely to stop suddenly, spitting sparks, as some chips get into the
brushes. (Unless you plan to only turn plastics and other
non-conductive materials. :-)

(at least without the optional low-speed pulley set, which he had not
installed). The other is that the LED countdown lights are harder to
get used to for me than the threading dial, and instead of half-nuts, it
has a dog clutch to engage the threading feeds.

this sounds like a newer machine? leds?

It was an add-on to an older ShopTask (which is probably not too
different from what you have). There is a lot of metal housing the
belts and gears, and on the shaft driving the leadscrew, there is a set
of reflective markers picked up by the "threading dial" -- an add-on
feature which comes standard on the later ShopTasks). It sort of serves
the purpose of the threading dial -- especially with the dog clutch
driving the leadscrew -- to make sure that you engage the dog clutch in
the right position. (Apparently, it is *not* a single position engage,
which does not make much difference when using it for power feeds, but
it makes a big difference when using it for threading. :-)

it seems like half-nuts are
a very basic feature, w/or w/out thread dial. i have no thread-dial,
so have to keep them engaged while reversing out of the cut. for very
occaisional use, okay. for anything more it's a non-starter. --Loren

As long as you have an imperial thread leadscrew, and are
cutting imperial threads, or metric for both, you can use it, with the
help of a threading dial. You *can* make a threading dial for the
machine. I would check the mailing lists and web-based fora for the
model of lathe you have -- I'll bet that someone has made a threading
dial for them. (The ShopTask LED "threading dial" was designed and
first built by one of the users, to get around just this problem. But
the slowest speed is still a potential problem in reflex time. For
threading (except very fine pitch threading), I tend to use the back
gears. The latest thread that I cut (a 1"x12 inside thread) was cut
using the back gears but the same pulley setting which gives me 650 RPM
without the back gears -- which works out to be 100 RPM in the back
gears. With a coarser thread pitch, I would go down to the lowest belt
speed in combination with the back gears, which gives me 35 RPM. I'll
also use that if I have a very narrow landing zone when threading to a
shoulder, or especially for inside threading to a shoulder.

For a *really* fine thread (e.g. 40 TPI or finer), the leadscrew
is turning slowly enough, even at 650 RPM, for my reaction times to work
out well.

Enjoy,
DoN.
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