My clausing 6900 lathe has barely any wear on the ways. And yet, when
I try to turn straight cylinders, I get something like 0.01"
difference along 4-5 inches. How come?

Second is that it chatters when I try to part off. Any suggestions
will be appreciated.

On Tue, 18 Dec 2012 12:27:20 -0600, Ignoramus10903 wrote:

My clausing 6900 lathe has barely any wear on the ways. And yet, when I
try to turn straight cylinders, I get something like 0.01" difference
along 4-5 inches. How come?

Second is that it chatters when I try to part off. Any suggestions will
be appreciated.

What Pete and Gunner said, _except_:

If you're turning between centers, make sure that your tailstock is
centered correctly. If it's not, then you're doing what you _should_ do
to turn a tapered cylinder. 0.01 may be visible just by running the
tailstock up to the headstock and looking at the pointy ends of your
centers.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

"Ignoramus10903" wrote in
message ...
My clausing 6900 lathe has barely any wear on the ways. And yet,
when
I try to turn straight cylinders, I get something like 0.01"
difference along 4-5 inches. How come?

If the work is chucked without other support it will bend from the
cutting force, which after all is enough to rip the metal apart.

If the tailstock supports the work the tailstock may have been offset
to turn a taper and not put back in line with the spindle center axis
accurately.

In either case the lathe bed isn't stiff enough to completely resist
twisting if the floor isn't perfectly planar. The idea is to have the
ways in the same plane from end to end. Otherwise as the carriage
moves it rotates the tool bit in a slow helix instead of a straight
line parallel to the rotational axis.

Although a lathe doesn't have to be level to the Earth to operate
properly, a precision level is common and relatively inexpensive tool
to show if both ends are coplanar, compared to laser optical
equipment.

Second is that it chatters when I try to part off. Any suggestions
will be appreciated.

Mine usually stops chattering when I slow it down.

jsw

On 2012-12-18, Ignoramus10903 wrote:
My clausing 6900 lathe has barely any wear on the ways. And yet, when
I try to turn straight cylinders, I get something like 0.01"
difference along 4-5 inches. How come?

Several possibilities. First, let's assume that you are
holding it only by the chuck (or collet) at the headstock end.

1) Bed is twisted. If you have a Master Precision Level
set it up so first the headstock end is level (measure between
two 1-2-3 blocks resting on the flat ways), then go to the
tailstock and level it. Go back to the headstock end and tweak
it, and again back to the tailstock end. (Don't trust that the
flat tops of the inverted Vs are right -- measure from the flats
that the carriage and the tailstock ride on. (Luckily, with a
Clausing, you have only one V for the carriage, and one for the
tailstock -- on opposite sides of the lathe center.

Don't bother worrying about the level from headstock to
tailstock ends -- unless you are using the chip tray for
recycling coolant, but the front to back should be checked.

And -- if you don't have the Master Precision Level, look up
"Rollie's Dad's Method" on the web and try it.

2) Less likely with the way the Clausing headstock mounts, but it
can be off so the spindle axis is not parallel to the bed.

For the following ones -- the presumption is that the outboard
end is larger than the end near the chuck. With (1) or (2)
above it could be either way.

3) Also (and a bit more likely) the takeup on the bearing ring
at the outboard end of the headstock is a bit loose, so there is
play between the two ends (tapered roller bearings).

4) How are you holding it -- and what is the diameter?

a) If you are holding it in a 3-jaw or 4-jaw chuck, and the jaws
are old and a bit worn, they will grip tightly at the inboard
end, but not so at the outboard end, so the workpiece pushes
back a bit.

b) If the diameter is smaller than 1/4 the length sticking out of
the chuck, then the workpiece bends away.

Now -- if you are holding it between centers and driving it with
a lathe dog:

5) The tailstock offset is set to produce taper (which can be
bigger at either end depending on the direction of offset.

If you look at the tailstock, you will see a hex bolt head on
the side towards you, and another on the side away from you.
These can move the top part of the tailstock relative to the
base plate which slides on the ways. You have to loosen the
clamping to the bed to adjust this.

You want to put a sensitive (tenths) indicator on a short shaft
sticking out of the chuck or collet, and bring the tailstock
close with a dead center (no bearings) in the tailstock ram.
Adjust the offset so you get the same reading on the front and
back of the 60 degree taper as you rotate the chuck by hand.

A quick and dirty way to adjust this is to take that dead
center, and to put some mild steel in the headstock and turn a
60 degree taper on it, and put some thin shim stock, or a
double-edge razor blade between the two. Grip it lightly. If
the centering is right, the blade will be square to the ways.
If too far front or back, you will see the blade tip.

Second is that it chatters when I try to part off. Any suggestions
will be appreciated.

Chatters has several possibilities:

6) Workpiece is too long. Above a 4:1 stickout vs diameter, you
need to support it at the tailstock end with a live center.

7) Bearings loose as in 3 above.

8) Chuck jaws worn, as in (4) above.

9) Gibs loose on cross-slide and/or compound.

Ideally -- the cutting point should be above the cross-slide
dovetail, so it is less likely to tilt it under cutting forces.

10) Something which I have not thought of -- but hopefully someone
else will think to point out.

Good Luck,
DoN.

--
Remove oil spill source from e-mail
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

In article ,
Ignoramus10903 wrote:

My clausing 6900 lathe has barely any wear on the ways. And yet, when
I try to turn straight cylinders, I get something like 0.01"
difference along 4-5 inches. How come?

As many others have said, twisted bed.


Second is that it chatters when I try to part off. Any suggestions
will be appreciated.

I had a long saga about this a few years ago. The answer was that
everything that could be loose was loose. When I tightened everything
up, parting off became easy. Turned around, ability to part off is a
good test of a lathe.

RCM thread titles "Trepanning and Parting Off" and "Clausing 5914
chatter -- solved at last" in 2008 give the saga.

Joe Gwinn

On Tuesday, December 18, 2012 1:27:20 PM UTC-5, Ignoramus10903 wrote:
My clausing 6900 lathe has barely any wear on the ways. And yet, when

I try to turn straight cylinders, I get something like 0.01"

difference along 4-5 inches. How come?


nothing to add here, except that a precision level is your friend. Remember, the object is not about getting the lathe *level*, it's about getting the ways coplanar.

Also, make sure that there are no chips or dirt caught under the headstock.


Second is that it chatters when I try to part off. Any suggestions

will be appreciated.

I don't think I saw anybody suggest this: Lock the carriage down before you part off. Also, you only want the minimum length of parting tool unsupported - just enough to reach to the center of the part.

rangerssuck wrote:


I don't think I saw anybody suggest this: Lock the carriage down before
you part off. Also, you only want the minimum length of parting tool
unsupported - just enough to reach to the center of the part.
Important, but I'd like to expand on that. There are many ways to
mount the cutoff tool holder to the compound, and what you want is to
have the compound rest and swivel as close to under the tip of
the cutter as you can. It is not just the overhang of the tool
itself, it is the whole stack that matters. When the tool tip
is far in front of the swivel, it allows many lathes to suffer
a forward rocking of the compound assembly, that causes the tool
to bite into the work. This is bad for any turning operation, but
much worse on cutoff. If the compound swivel is as far forward
as possible, then this forward rocking should be reduced, at least
by shortening the lever arm. The best case is when the tool tip
is maybe less than 1 inch in front of the swivel center and also
centered left-right over the swivel.

Jon

On Fri, 21 Dec 2012 13:53:55 -0600, Jon Elson
wrote:

rangerssuck wrote:


I don't think I saw anybody suggest this: Lock the carriage down before
you part off. Also, you only want the minimum length of parting tool
unsupported - just enough to reach to the center of the part.
Important, but I'd like to expand on that. There are many ways to
mount the cutoff tool holder to the compound, and what you want is to
have the compound rest and swivel as close to under the tip of
the cutter as you can. It is not just the overhang of the tool
itself, it is the whole stack that matters. When the tool tip
is far in front of the swivel, it allows many lathes to suffer
a forward rocking of the compound assembly, that causes the tool
to bite into the work. This is bad for any turning operation, but
much worse on cutoff. If the compound swivel is as far forward
as possible, then this forward rocking should be reduced, at least
by shortening the lever arm. The best case is when the tool tip
is maybe less than 1 inch in front of the swivel center and also
centered left-right over the swivel.

Jon
===============

For the less massively constructed lathes, common in home
shops, a rear mounted upside down tool holder for the cutoff
blade that mounts directly to the cross slide rather than
the compound [top slide] can be helpful.

For a picture of one we designed and built as a class
project click on
http://mcduffee-associates.us/machining/rearcoth.htm.
As the Brits might say -- it worked a treat.

Casting kits are available
http://www.statecollegecentral.com/m...the/MLA-6.html

On Friday, December 21, 2012 2:53:55 PM UTC-5, Jon Elson wrote:
rangerssuck wrote:





I don't think I saw anybody suggest this: Lock the carriage down before

you part off. Also, you only want the minimum length of parting tool

unsupported - just enough to reach to the center of the part.

Important, but I'd like to expand on that. There are many ways to

mount the cutoff tool holder to the compound, and what you want is to

have the compound rest and swivel as close to under the tip of

the cutter as you can. It is not just the overhang of the tool

itself, it is the whole stack that matters. When the tool tip

is far in front of the swivel, it allows many lathes to suffer

a forward rocking of the compound assembly, that causes the tool

to bite into the work. This is bad for any turning operation, but

much worse on cutoff. If the compound swivel is as far forward

as possible, then this forward rocking should be reduced, at least

by shortening the lever arm. The best case is when the tool tip

is maybe less than 1 inch in front of the swivel center and also

centered left-right over the swivel.



Jon

And even further to that, you want to make ABSOLUTELY sure that the tip of the cutoff tool (or any other cutting tool, for that matter) is at or slightly below the center of the workpiece. If the tip is above center, any bending forces (of the entire assembly from the tool tip to the lathe bed) will tend to pull the tool deeper into the work, where it will stick, bend more, finally cut and then spring back - the very definition of chatter. If the tool is at or below center, the bending forces will move the tool tip away from the surface - no harm done.
Wasn't there a thread here a few years ago about parting off and the sound of sizzling bacon? I believe that had a very good description of the setup and procedure for a successful operation.