Subject: Tighten the lathe gibs?

From: [email-masked] (Bob Neidorff)
Date: Sun, 15 Sep 1996 03:04:08 GMT

I have a Logan 11" lathe.  My most recent project was to put
a block of steel on the cross slide and a #5 center drill in
the head chuck.  I then fed the steel into the drill.  It did
drill, but the cross slide vibrated quite a bit.  I suspect
that the nature of center drills had something to do with it.
I found that the BACK of the carriage actually was moving up
and down, and was pumping oil in and out of the gap.

I took that as a recommendation to tighten the back gibs.
When I did this a little bit, it stopped wiggling there, but
now the carriage moves much stiffer.  It is so stiff that the
friction clutch for the cross feed has a hard time moving it
even without tool contact!  I can adjust the clutch to work,
but it is quite tight.  Too tight?  How tight is right?  It
has to be tight to make a good cut, right?  I would like to
have the best accuracy that I can for normal spindle turning.
I only tightened those two screws a smidgen, perhaps 1/16 turn.

Anyway, with it tight, the compound still wiggles with the
center drill, but not the carriage back.  I also tightened the
cross slide gibs.  Better, but stiffer, and it still wiggles
with the center drill.  With a 1/4" twist drill or larger,
it cuts fine.

My real question is: did I go overboard with the tightening?
Those levers are all now quite tight, tighter than any machine
I can recall ever using, but I'm not that experienced.  Am I
expecting too much out of a Logan 11" lathe?

Thank you very much for your comments.
--
Bob Neidorff; Unitrode Corporation  |  Internet: [email-masked]
7 Continental Blvd.                 |  Voice   : (US) 603-429-8541
Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-429-8564

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From: Charles Lamont <[email-masked]>
Date: Tue, 17 Sep 1996 22:35:43 +0100

With a reasonably small machine like this, disconnect the feedscrews,
clean and lubricate the slides, and then adjust them so that you can
move them smoothly with two hands.

If you want to use the cross-slide as a boring table you need a method
of locking the slides.  If the lathe is not equiped with clamp screws,
as well as the normal gib adjustment screws, you should fit some
yourself. 
-- 
Charles

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From: [email-masked] (Scott Logan)
Date: 16 Sep 1996 11:53:51 GMT

In article <[email-masked]>, From [email-masked] (Bob 
> I have a Logan 11" lathe.  My most recent project was to put

   <<snip>>

> I only tightened those two screws a smidgen, perhaps 1/16 turn.

Something doesn't sound right here.  I've seen Logan 10" Lathes take a
1" deep cut (turning) in steel, admittedly a very light feed, but this
is how they used to show them off at machine tool shows.

You say "those two screws"; on the cross slide, there should be 4
screws, and on the compound, 3 screws.  Also, do you have cone point
screws? If you are using standard cup point, they will tend to hit the
gib at the top, and cock the gib. Cone points will hit in the middle. 
Let's see if I can diagram this:

Gib //____ 
   //<____  Proper cone point set screw
  //

Gib //____ 
   //>____  Incorrect cup point set screw
  //

Hope this helps, let me know if I can help any more.

--
 ----------------------------------------
 Scott Logan [email-masked]
 09/16/96 06:53
 http://www.loganact.com

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From: Charlie Greenman <[email-masked]>
Date: 18 Sep 1996 12:16:29 GMT

>and down, and was pumping oil in and out of the gap.

I too have an 11" Logan, a model 920. The rear gib screws
lift (distort) that metal plate and that's what provides
the adjustment. As you probably know. To check what's going
on, pull the tailstock all the way back to the rear and
move the saddle as far to the rear as you can. That's good
bed back there. Put a magbase on the rear of the saddle (not 
the cross slide) and put a DTI on the bed and adjust the rear 
gib screws so you can lift the saddle about a thou. Then 
adjust the gib screws so that you just can't lift the saddle. 
Now run the saddle back up toward the headstock (with the DTI
still on the bed) and again try to lift the saddle. The amount
you can lift it is bed/saddle wear. I would not adjust the
gib to be tight when the saddle is up near the headstock if
that binds it as it goes toward the tailstock as that will
rapidly wear your bed.

I use synthetic 10w30 oil on the bed and would not use 
anything heavier than that.

In the FAQ is a description of a technique you can use to
see how much bed wear you have. It's written to show how to 
align the bed but it will also show how much wear you have 
and where it is. I'd try that.

Even if you have little bed wear, you could have a lot of
wear in the base of the saddle. There are 6 surfaces on the
saddle where it meets the bed. If the saddle ways are worn,
then adjusting the rear gib will simply tilt the saddle so
some other part of it is in contact with the bed. 
Unfortunately, tightening a gib is rarely the answer.

You have to think of this as sort a puzzle. See how much lift
there is in the saddle when it's adjusted for lift as 
described above. Then see how much bed wear you have with
the technique in the FAQ. Then remove the rear leadscrew
bracket, loosen the front gib (two screws) and holding the
bottom of the apron so it doesn't fall, unscrew the two large
allen bolts that hold the apron to the saddle and withdraw
the apron out the rear. Undo the rear gib and the cross-slide
and lift up the saddle and have a look. Wipe down the good
rear bed, put just a bit of spotting blue on all six surfaces
on the saddle, set it straight down gently on the bed, move
it back and forth about an inch and lift it straight up and
have another look. You may see that it is only touching a
little bit and on only two of those surfaces. If so, that's
your problem. Or one of them. What you do then depends on
the condition of the bed, but at the very least buy a copy
of Connelly's "Machine Tool Reconditioning" (see the FAQ).
Correcting this on my lathe took about a year of my time,
but taught me a lot and at the end you have one hell of a
fine lathe. I wouldn't do this with a lathe that is less
quality than a Logan. I really wouldn't. Because it takes
a LONG time. But it sounds to me like you have to take a
very close look at your lathe because it should not be
behaving like this. Get Connelly's book.

Good luck,

Charlie (Oh and you aren't expecting too much out of that
          lathe. It is a lovely lathe.)

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From: [email-masked] (Jonathan M. Elson)
Date: 15 Sep 1996 19:21:17 GMT

Bob Neidorff ([email-masked]) wrote:
: I have a Logan 11" lathe.  My most recent project was to put
: a block of steel on the cross slide and a #5 center drill in
: the head chuck.  I then fed the steel into the drill.  It did
: drill, but the cross slide vibrated quite a bit.  I suspect
: that the nature of center drills had something to do with it.
: I found that the BACK of the carriage actually was moving up
: and down, and was pumping oil in and out of the gap.

: I took that as a recommendation to tighten the back gibs.
: When I did this a little bit, it stopped wiggling there, but
: now the carriage moves much stiffer.  It is so stiff that the
: friction clutch for the cross feed has a hard time moving it
: even without tool contact!  I can adjust the clutch to work,
: but it is quite tight.  Too tight?  How tight is right?  It
: has to be tight to make a good cut, right?  I would like to
: have the best accuracy that I can for normal spindle turning.
: I only tightened those two screws a smidgen, perhaps 1/16 turn.

You adjusted the screws on the outside back of the carriage?
I'm not really familiar with the Logan, but you may have tightened
the WRONG thing.  There is also a guide (or some similar terminology)
that prevents the carriage from lifting UP off the bed, which is
what you suggested when you said it was moving 'up and down'.
Most lathes have a steel plate which rides under a lip on the bed,
and it is often spaced with very thin shims.  What you need to do
is put the rear gib back to where it was before, and investigate
how the vertical gib clearance is set, and perhaps pull one of the
thinnest shims out of the stack.  This will compensate for wear
on the top of the bed and the bottom of the carriage.  The easiest
test is with a dial indicator mounted on the bed (with magnetic
mount), and reading from a flat place on the carriage, ie. the
cross-slide ways.  Just lift up with your hands on the carriage, and
observe the indicator.  A gradual movement of a few thousandths
may indicate bed flexing and lube squeezing out of the gib-bed
gap, but sudden jumping of the indicator shows open clearance
between the vertical gib and bottom of the bed.  Tighten this
clearance in small steps (.0005 to .002" steps, max, unless the wear
is massive all overe the bed), checking frequently across the whole
length of the bed for a tight spot.  When you find the first tight spot,
you can adjust no further (unless you scrape the tight spot down).
You will need to do this for both the front and back rails of the
bed vs. carriage clearance.  The front usually suffers the greatest wear
due to chip pileup and cutting pressure, but in drilling with a
drill in the chuck, the cutting force lifts the rear of the carriage,
so you see it there first.

					Jon

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From: Charlie Greenman <[email-masked]>
Date: 19 Sep 1996 12:02:49 GMT

Scott... I think he's saying his saddle is lifting in
the rear. This isn't a problem (THIS isn't a problem)
with his cross-slide or compound. He's talking about
the two gib screws in the back. The ones that distort
that steel plate gib.

I'd be curious if you think there are problems with
the suggestions I posted on this.

/Charlie

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From: [email-masked] (Alan Frisbie)
Date: 19 Sep 96 09:18:24 PDT

> gib at the top, and cock the gib. Cone points will hit in the middle. 

Thanks, Scott!   This is one of those gems of knowledge that
I would never have thought of.   When I made my T-slotted
cross slide (Metal Lathe Accessories), I never gave this a
second thought.   I am going out to the garage right now to
check it, as well as printing a copy of your note to stick
in my Logan 200 file folder.

--  Alan E. Frisbie               [email-masked]
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
--  Los Angeles, CA 90065         (213) 258-3585 (FAX)