Subject: [LATHE] Powerfeed lead screw disconnect

From: [email-masked] (Dion Hollenbeck)
Date: Wed, 9 Feb 1994 15:44:43 GMT

I have only used one lathe and last night was watching a Rudy Kouhoupt
video showing a Bridgeport lathe and it brought up a question.  I am
used to having a lever which is used to engaging/disengaging the
carriage to the lead screw under powerfeed conditions.  It appeared
that the Bridgeport had a knob which had to be turned several turns to
engage the lead screw.  It brings up several questions since I have
lost access to the lathe I was able to use and will eventually be
buying my own.

1) What other kinds of lead screw engagement systems exist?
2) On the Bridgeport, how do you disengage quickly?  Or if you can't
	disengage quickly, how do you accommodate situations like
	taking a turning cut down into a shoulder and stopping exactly
	at the shoulder?
3) Other than condition, what types of functionality should one look
   for or avoid when buying a lathe (new or used).
4) Are there any books or videos available on lathe (or machine tool)
   purchase. 

-- 
Dion Hollenbeck (619)455-5590x2814     Email: [email-masked]
Staff Software Engineer                       [email-masked]
Megatek Corporation, San Diego, California    ucsd!megatek!hollen

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From: [email-masked] (Ed Richley)
Date: 9 Feb 1994 18:09:58 GMT

On lathes which derive power feed from a slotted lead screw
(South Bend, in particular) there is a clutch mechanism
consisting of a set of "washers" which, when squeezed together,
engage the feed mechanism. This is important, because this clutch
can be engaged to various degrees. This is the reason for the
little knob that ol' Rudy was turning. If your carriage happens to
hit a hard stop, it's better to have the clutch slip than to trash
gear teeth. In later years it was discovered that this litle knob
could be replaced by a lever to perform the same squeezing action. 
This was an option on SBL equipment, and maybe became standard later on.
Note that this is in contrast to Hardinge which has a separate motor for
the power feeds, or any lathe that uses the lead screw thread for feed 
(a real bad idea...).

Ed Richley
[email-masked]

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From: [email-masked] (f.w.kerfoot..iii)
Date: Wed, 9 Feb 1994 18:32:03 GMT

>From article <[email-masked]>, by [email-masked] (Dion Hollenbeck):
> 

Fundamentally, what is needed is a way to engage and disengage a friction
clutch between two slowly rotating members. The knob is a relatively
easy way. The back member of the clutch has a threaded shaft which
protrudes through a pack of friction clutch elements. A knob, running
on this threaded shaft, squeezes the clutch elements. The knob and the
threaded rod turn slowly toggether as the feed progresses, but this
does not interfere wih tightening or loosening. 

Lever clutches do the same process of squeezing a clutch pack, but they
do it with a cam, and thrust bearings to allow the lever to sit still
even though the elements being pushed against one another are rotating
slowly.

I can imagine lots of other ways to engage a friction clutch, but these
are the only two I have actually seen on a lathe.

> 	taking a turning cut down into a shoulder and stopping exactly

I don't know about the Bridgeport lathe (actually, I didn't realize that
Bridgeport ever made lathes), but my 10" South Bend has a "knob clutch".
Mine is a 1957, and in 1958 South Bend converted that lathe to a lever
actuated clutch design. When I started using the lathe, and I discovered
that there was a means to update my lathe to the lever clutch, I spent
some time considering spending the $200 or so on the upgrade package,
or perhaps fabricating something myself. After using the lathe more,
I have come to the conclusion that the knob clutch is not a significant
disadvantage. At least for my lathe, it takes 1/8 - 1/4 of a turn of
the knob, not several turns, to engage or disengage. You can keep
turning the knob for several turns (in the "disengaging" direction)
but nothing more happens after 1/4 turn. You can turn the knob 1/4 turn
in about the same time as moving a lever, it is just a somewhat less
intuitive motion. The accompanying slow rotation of the knob as you
approach the point where you want to disengage also is slightly
disconcerting until you get used to it. Overall, however, I don't think
the difference between a knob clutch and a lever clutch is a very big
deal. I certainly wouldn't reject the purchase of an otherwise suitable
lathe because it had a knob clutch. This of course assumes that other
lathes with knob clutches act about like mine, but I think that is a
reasonable assumption based on the way they work.

>    for or avoid when buying a lathe (new or used).
You want a lead screw to allow thread cutting. There are occasional
examples of lathes without them.

Power longitudinal and cross feeds are very useful. I know some 9"
South Bend's came without this (the so-called C series).

A "quick change" lead screw gearbox, as opposed to manual change gears,
is a significant advantage (in terms of time- you can do the same
work, although more slowly, with manual change gears).

Look for decent sized graduated dials on the cross feed and compound.
Some early lathes had very small dials, which made accurate readings
more difficult.

You want a "back gear" capability on the headstock, to allow very slow
turning. I don't off hand know of a general purpose metal lathe which
didn't come with this. In addition, you want some way to select a range of
spindle speeds, in addition to the back gear, either a set of cone pulleys
for a belt, or a "transmission" in the case of a geared-head lathe.
Again that is pretty common, but who knows what you might come across
at an auction or wherever.

If you are buying used, make sure everything is there (tailstock,
compound, motor, etc). New or used, consider the accessory package
(chucks-3 jaw and 4 jaw, faceplate, lathe dogs, centers, tool holder
and tools, tailstock chucks, collets are a nice extra).

A taper attachment is a nice extra. You will not use it often, but
occasionally it is hard to do something any other way.

A tailstock with a graduated dial on the handle is a nice extra for
drilling to relatively precise depths.

Frank Kerfoot
[email-masked]