Lathe turning and parting off questions

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

Probably twisted bed. I get about .002" over that distance, and was figuring on releveling. I got less than .001" after first starting up the lathe in the new shop. Probably settling in. I start with a precision level on both ends. If that doesn't do it, I'll probably look up "Rollie's Dad's Method".
On the other hand, most of the stuff I've been doing lately (sail line reel) is shorter than 1", so the less than .001" taper hasn't been an issue. At least not until I start building some engine kits.
Pete Keillor
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On Tue, 18 Dec 2012 12:27:20 -0600, Ignoramus10903

The bed is twisted.

Something is loose
Gunner
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On Tue, 18 Dec 2012 12:27:20 -0600, Ignoramus10903 wrote:

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.
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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.

Mine usually stops chattering when I slow it down.
jsw
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    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.

    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.
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As many others have said, twisted bed.

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
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On Tuesday, December 18, 2012 1:27:20 PM UTC-5, Ignoramus10903 wrote:

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.

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.
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rangerssuck wrote:

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

============== 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/metallathe/MLA-6.html
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On Friday, December 21, 2012 2:53:55 PM UTC-5, Jon Elson wrote:

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 sligh tly below the center of the workpiece. If the tip is above center, any bend ing 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.
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