Lathe bed straightness

So I'm trying to level my new lathe, a 13x40.
I'm using a Starrett machinist level (.005/10").
(I know I'm supposed to be using a master precision level (.0005/10")).
With the level perpendicular to the bed (on both v-ways or on top of
compound slide) there is a 2 division difference when placed at the
headstock vs the tailstock. This lathe is very rigid. I've had
the back tailstock levelling foot off the ground for 2 days
now, and it won't come down at all to remove the 'twist'. The
only conclusion I can come to is that the bed was ground that way.
Is it being off that much going to cause me problems?
With the level parallel to the bed (left to right) it seems the center
of the bed has a small dip. I think this is due to the above situation.
And this one makes no sense to me. With the level on the compound slide
(measuring front to back tilt) moving the cross slide the full range
causes a 3 division change. The only thing I can think of is that
the dovetails are curved front to back, instead of straight, which
would cause an angle change. Is this even an issue?
So for a new expensive Taiwain lathe, would this be considered
normal? If this is the best I can expect, then it will have to do.
I mostly make small parts, so I don't think I'll have any issues
to start. But if I start making bigger parts, will I run into
issues or is this something an experienced machinist has to
deal with normally?
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Don't bother. Do not use a level to set it up. Use the two-collar method described in the south bend book.
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Reply to
jim rozen
Nail the feet to the ground and you will be able to probably pull the lathe to level.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!
Reply to
Bob May
Yes, it will make your life harder, and if you ever need to turn a shaft or drum to constant diameter, you have a real problem. The bed is warped. Is this a hardened bed? Very classic mistake, they mill the bed flat, apply the flame-sprayed hardening, and the bed warps with a dip in the middle. It would take all day to corect that by grinding.
Yes, the cross slide may also be warped by flame spraying. If the motion indicates the bottom of the slide's upper (moving) part is concave, that is likely the cause.
What is really troubling is that the warps are seen with a machinist's level. You will probably see these kinds of errors in most lathes, but you SHOULD need a much better level to pick this up. (I used a Talyvel level that reads tenths of arc seconds when rebuilding my Sheldon lathe, and have the bed error down to about +/- .0003" over the entire bed. That is probably better than most production machines.) But, a dip of more than .001" or so on even a mid-priced machine seems high.
Now, as to how it affects your work. If the bed sag is exactly identical on both front and back ways, a dip of .003" or so is not a very big deal. However, if the error is greater on one way than the other, it tilts the carriage. Put the level reading front/back tilt, and run the carriage left and right, and watch for movement of the bubble. If you can detect more than .001" of tilt in 10" (1/5th of a mark on the vial) it will make a very roughly .001" change in radius, or .002" change in diameter on something you turn. This mode of error is the one that will bite you the most, when you need something to come out consistent diameter.
Reply to
Jon Elson
I agree. Level for this lathe is meaningless. It either cuts straight or it doesn't.
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This is under consideration. I'd have to find some good anchors to put into the concrete floor.
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From reading previous posts I thought this would be an issue. Thanks for verifying it. It also helps to understand why there is a warp. Now I just need to decide what to (not) do with it.
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