View Full Version : Why does a lathe need to be level?

02-20-2008, 08:02 AM

I have been reading and enjoying this board for more than a year now and about 9 months ago I bought an Sieg X3 milling machine and a C6 lathe. I haven't used either of them very much because I was too busy with other things but now I am starting to use both of them a bit. I am completely new to machining, in fact my X3 the first milling machine that I have actually seen in real life. Right now I am building a bench for both the milling machine and the lathe. I have read posts on here and at CNCzone.com about people using very accurate levels and such when setting the machines up but I never understood exactly why being level was so important.

Can anyone tell me why it is important for the machine to be level?


tony ennis
02-20-2008, 08:17 AM
I think that's just the easiest way to make sure the bed isn't twisted.

02-20-2008, 08:51 AM
The level is used to ensure that all parts of the lathe are in the correct plane relationship to all other parts. It doesn't matter a bit if the lathe isn't oriented "level" to the force of gravity. You could mount it on a wall if it suited you and in fact a lot of machining center lathes are in effect so mounted.

02-20-2008, 08:55 AM
Lathe beds are design to be level in order to turn the work true to the bed. A twisted bed will yield a tapered turn. Might not be noticeable on your smaller HSM machines but on larger machines it can kill the project completely. Think of what will happen to the carriage as it rides over a twisted bed. That's why we use master precision levels to get that bed as close to perfect as possible. ;)

02-20-2008, 08:55 AM
Leveling is just a convenient means to an end -- ensuring that the lathe bed isn't twisted. And it's not the final authority, either; the ultimate test is turning a sample piece of work and seeing if it's parallel.

Another way of setting up a lathe, which doesn't use a level at all, is "Roland's Father's Technique," introduced to the world by Roland Gaucher of the New England Model Engineering Society. You can read about it here: www.neme-s.org/Rollie's_Dad's_Method.pdf

02-20-2008, 09:01 AM
Can anyone tell me why it is important for the machine to be level?

its not, however like Tony said, its a very convenient reference to make sure there is no twist in the bed - it possibly can twist under its own weight or even as the casting ages and releases stresses and for sure when its bolted down - this changes the position of the cutting tool to the axis over the length of the bed and hence the ability to turn a cylinder. keep in mind leveling is done with say a Starrett master precision level where accuracy is measured in tenths per foot, not a regular carpenters level - so a little bit matters.

is this a small bench top lathe? if so a lot of this doesn't confront you imo. If it does confront you, you have to design your bench as part of the solution - there's no point in leveling a lathe on a wood bench for example. Lathe should be bolted to the bench, and bench to the floor.... one of the sets of feet needs to be adjustable.

For a bench top lathe I would do this; get one of the cheapo composite granite surface places. drill, or have drilled four holes (figure out how or take it to monument guy), epoxy in some threaded pieces and bolt the lathe to the surface plate. The ways should be parallel to the bottom of the feet so you lathe is now without twist. the extra granite will also add vibration absorbing mass. if you are able to get access to a precision level, check for twist (does the level read the same at headstock as at tailstock) and shim under the feet IF necessary, but i doubt you will have to.

02-20-2008, 09:07 AM
Lathe beds are design to be level in order to turn the work true to the bed.

Don't confuse "level" and "in the same plane". Level means perpendicular (more properly "normal") to the local force of gravity. That isn't the only thing a level is used for. The level provides a consistent reference to a known vector that is used as a reference in aligning the parts of the lathe. A small lathe doesn't need to be actually level.

Note: For truly large equipment it is different as the larger the machine the more it can be distorted by it's own mass. They are designed to be installed level in the true sense.

02-20-2008, 10:19 AM
As Tony and Evan stated, the task of "leveling a lathe" is only to remove any twist from the bed. It is not necessary for the whole lathe to be exactly level. If that were so it would be imposible for lathes to be used on ships at sea. The only reason you try to keep the bubble centered and therefore "level" is to make it easy to get the bed twist out.

02-20-2008, 11:34 AM
Some lathes do need to be level, this for the oil to flow properly. Other than that "level" is just a convenient way to make sure that the whole length of the bed is in the same plane. Once level (and periodically checked to keep it so) it's also convenient to use in setting up some types of work.

02-20-2008, 06:02 PM
OK - I agree that getting the lathe level (ie in the horizontal plane) makes it much easier to see it if the lathe bed is as it should be in the vertical plane - ie the level actually measures/shows any discrepancy in or deviation from the horizontal plane.

A straight edge will show if the item being assessed by use of the straight edge is straight or not.

I see a lot of discussion and concern about using the level to get the bed "flat" in the vertical plane and the disasters it can cause if it is not flat - ie tapers and who knows what.

Have you ever thought to put a straight edge on/along the front face/edge of the lathe bed to see if it is straight or bent in the horizontal plane - ie whether it forms "bends" and/or "ess-es" like a snake.

I suggest you'd better as there is no way that using a level on "top" faces will find it unless you turn the lathe over on its back (or front).

That is what an Engineer's straight edge is for.

It is often - but not always - in the form of an arch with a flat - usually scraped or ground - base and is used to scrape flat long lengths - like lathe and machine beds. But it does have other uses!! - see above for checking in the horizontal plane for errors in "vertical" (and in any) plane.

A lathe could be "dead flat" (or straight-untwisted) in the vertical plane using an Engineers Level but if the lathe bed was distorted horizontally, the lathe tool may well repeat the distortion on the job in a lathe that was "Passed" as "OK" with an Engineer's Level.

So, lets stop repeating - rote or "parrot" fashion, or as a Monastic Chant - all the "old ways" - as if they are Psalms, Gospel or "Holy Writ" - and think of what we need to achieve and if what we are doing achieves or meets that requirement.

J Tiers
02-20-2008, 09:05 PM
I claim that RDM, as mentioned above, won't do the trick alone......

You have to START level, or close, in order to avoid unknowingly turning the machine into a pretzel chasing the errors.

The problem is that there is no outside frame of reference, so no one part of the machine can be verified as free of error and then used to aline the rest.

Other than that, it is functionally equal to the "two collars" test. Nothing magic about either one, neither depends on any precision test pieces being in existence. RDM may be a bit more sensitive to "droop" than the two collars test, but......

Obviously, the proper use of a level can assure the bed is not twisted.

Starting from that, RDM, or two collars test, etc, can be used to assure that the spindle axis is in fact parallel to carriage motion.

Without that start point, you have no idea what actual error (of the several possible) you need to correct.

No RDM or two collars etc test can really distinguish between a spindle pointed wrong, and a bed that is warped. It can find SOME possible errors, but cannot find ALL possible errors in any convenient manner.

Surveying could, but that introduces an external reference equivalent to a level.

So the bottom line is that with a level, you can easily test for bed twist, AND for a bed that is "hogged", "sagging", or bent in the lengthwise direction. It naturally must be a very sensitive level.

As oldtiffie suggests, when the bed is "level", your job is NOT done. Two collars, or RDM, etc is still necessary to check OTHER points.

However, once the bed is known correct, the orientation is of no consequence, level or not, unless the mounting is such as to distort the bed, or allow a very different sag to occur that distorts the bed. For instance, upside down would be likely to change the sag and throw the bed "out" compared to a prior measurement which was done right side up.

And moving the machine would probably cause a different twist due to the new "set" of the feet on the floor. So once leveled, the machine is best not moved. If leveled while attached to a large block of concrete, and all moved at once, your machine should be OK. But practical it isn't.

02-20-2008, 09:46 PM
Thanks JT.

Agreed - entirely - again.

Good post.

02-20-2008, 10:11 PM
First thing to do is turn a test bar and see if it needs anything.If it ain't broke don't fix it applies here.If the bed is short and the machine light it may not need anything other than being approximately level with a carpenter's level.

02-20-2008, 10:24 PM
First thing to do is turn a test bar and see if it needs anything.If it ain't broke don't fix it applies here.If the bed is short and the machine light it may not need anything other than being approximately level with a carpenter's level.

Quite right weird.

A good 24" or 36" machined Carpenter's box level is accurate to within 1/32" (or in metric terms, within 1.0mm in 1,000) which is 0.0573 arc degree = 3.4377 arc minutes. That is as good as many digital or vernier protractors and less than half the 0.10 arc degree (6.0 arc minute) that most digital levels are capable of. And they are quite cheap and very good value for money.

The level needs to be checked for adjustment of course.

A good "Chippie" (Carpenter) can easily get to within half of that with a good box level.

I have both a Machine Frame Level and a machine level both accurate to 0.02 mm/meter which are both a PITA to use and quite unnecessary.