This is a 12" lathe chuck for South Bend lathes.
My question is that the mounting seems to be a simple thread. There is
no "lock" like there is on L type mounts, or on camlock chucks etc.
So, it could happen that the big chuck would unscrew itself during
stopping if RPM was too high, the brake was applied too vigorously, or
something broke inside the gearbox. The chuck weighs about 100 lbs, so
having it bounce around the shop would not necessarily be a great
Am I missing anything?
That's the old style mount. When you had shaft drive with overhead
pulleys there was not any fast breaking or reverse so no problem. But a
quick reversing motor will certainly send it across the room. I've seen
it done with a 6" 3 jaw on a Sheldon Lathe. It's not QUITE as bad as it
looks: the longer you use it on one setup, the tighter is sets itself on
Simply the fact that such mounts are not intended for use with braking
or reversing - at most slow speed light cut reverse if any reverse at
all. I've got a couple - a 2 x 6 on one and a 2-1/2 x 6 on another,
IIRC. Lathes that will swing 20 and 17 inches, respectively, with flat
belt drive and Babbit bearings, about 130 years old.
If you do anything stupid, they will bite you. You either learn to not
do stupid things, or you learn that you need a more modern lathe with a
more idiot-resistant chuck/faceplate mount.
No brakes. And if the gearbox breaks, duck.
Threaded spindles are the traditional design for engine lathes. My 1945 SB
has a threaded spindle. It works fine. Billions of parts have been made
using them, in production and in toolrooms.
Newer locking designs are better. The threaded spindle is an old approach,
as is the rocker-type toolpost, the plain-bearing spindle, plain dead
centers with white lead for lubricant, and so on. They work. They're capable
of really fine machining. Hobbyists will be using them for years to come.
But doing commercial work with one of these lathes today is like delivering
parts in a Model A truck.
Well, I am glad that on my Clausing, I can use brake, reverse etc and
the chuck will never unscrew itself, due to a keyway.
With a 12" chuck, it is plainly a scary thought to use a lathe if it
has a brake, or an ability to power reverse.
Its difficult to tell from this photo. My little Grizzly benchtop lathe has
two clamps that are held on to the chuck with some bolts. The spindle has a
grove into which the clamps engage.
The requisite threaded holes might not show up if the photo was taken at the
You're probably missing the delightful experience of trying to remove a
threaded chuck after it's been on a while! :-) Some have had to take a
parting tool to the back plate to get their chucks off the spindle. I had a
South Bend with a broken back gear tooth, most likely a result of someone
trying to remove the chuck. My 13" South Bend had a D1-4 Spindle that I
much prefer to threaded spindle.
I would prefer anything to threaded spindles... L mounts or D mounts
are both great. I would think that D mounts (camlock) are the best,
due to not being prone to mating surface contaminated with chips.
One of the guys I was in high school metal shop enjoyed spinning up the
chucks on small Southbends and putting the lathe in reverse, causing the
chuck to unwind and drop off and roll across the floor at speed. After a
few occasions he was warned and didn't do it again. Would have seemed a
good idea to not provide reversing on a threaded spindle lathe for novices.
When in Wichita I recall the lecturers mentioning an occasion when a
student had a chuck come loose and roll down between all the subsequent
lathes, causing the students to jump out of the way. All the lathes were
in line and the chuck rolled down the line until it hit the far wall
IIRC. Those lathes were Rockwells, not highly regarded by the
instructors IIRC, but not fitted with threaded spindle either.
I locked out reverse on our 13" Enterprise Lathe for exactly that
reason. It's a cam lock spindle but the forward/off/reverse lever is not
marked for direction nor is it intuitive. I've had novices try to cut in
reverse, even tried for reverse myself in a moment of inattention after
switching from a different lathe.
It will be OK, except if something dumb happens at the higher spindle
speeds. My friend says if it doesn't hit you, it will have a sharp
whistling noise as it flies through the air. That may be an old OSHA
requirement, warning everyone else in the shop to "duck & cover".
The disadvantages have been pretty well covered. One advantage is that it is
a fairly straight forward machining job if you need to make a dummy spindle,
backplate, faceplate, etc..It is therefore also a lower cost system.
Can you put in a spindle lock ?
Something like a C5 threaded tube. Locks by pulling on the inside of
the chuck and on the far end of the spindle a compressing thread, locking it in.
It is probably the most common style of chuck mounting around
for lathes. The various other systems were produced as improvements for
just such reasons.
It would not be a great experience -- but consider that the
machines which use these are typically not supplied with a gear-driven
headstock. Instead, they are belt driven (except in back-gear, where
the speed is too slow to pose a problem, except when doing a deep cut
And the South Bend lathes tended to use flat belts, so the slip
on plug reverse and the like is much greater -- offering a safety
Also -- in the home they are often fitted with single phase
motors, so plug reversing is not an option anyway. :-)
That said -- a friend who used to be a machinist in the Navy
plug reversed a lathe with a *big* chuck and a big workpiece, and it
started to unscrew. He quickly re-plug reversed it which caught up with
the chuck -- but made it *very* difficult to remove that chuck later. :-)
Note that some chucks -- for example some for the Myfords which
are screw thread spindles -- come with provisions to tighten a clamp on
the register of the spindle to allow turning in reverse without the
excitement of a chuck going walkabout.
But you have identified one of the reasons why I converted my
Clausing from 2-1/4x8 threaded spindle to L-00 (by replacing the spindle
with one from eBay.)
Most lathes that will take a 12 inch chuck will not have a very high
spindle speed. Assuming you have more than one lathe, you would use a
much smaller lathe when turning something under 4 inches in diameter
and are using a fairly high spindle speed.
Assuming you are turning a 6 inch diameter part at 180 sfm, that would
be about 120 rpm.