Machinist

["Lawrence F. London, Jr." <venaurafarm@bellsouth.net>]

Leveling a Lathe - Practical Machinist Thread
http://www.practicalmachinist.com/vb/showthread.php/leveling-lathe-190984.html
The real Leigh

The purpose of leveling a lathe is to return it to the exact position in 
which it was trued at the factory, thus duplicating the location of all 
stresses and pressure points. This can be done most easily by using the 
Earth's center as a reference point.

The best possible configuration is a three-point suspension, with two 
points under the headstock and a single one under the far end. If your 
machines are set up this way you're way ahead of the game.

You'll need a very precise level such as the Starrett 199Z, with 
sensitivity of .0005" per foot (0,04mm/meter). The Starrett catalog 
sheet in PDF format is here 
http://www.starrett.com/download/338_p439_444.pdf

This 15" level is considered the standard of the industry. Comparable 
models from other manufacturers are available in different lengths, with 
15" being most common. Length doesn't matter. The important spec is 
sensitivity. Lesser products (Starrett 98 et al at 0.005"/ft) are not 
sensitive enough for precision work.

Levels are self-calibrating. Check yours for accuracy before you start. 
I've included a test and calibration procedure at the end of this post.

The following general procedure is applicable to most lathes with three 
or four leveling feet. Longer beds with six or more feet present a 
different problem.

Run all of the leveling feet out the same number of turns. This gives 
you working range and roughly squares the machine. You can also preset 
the working height at this time.

Start with two co-planar reference surfaces as close to the spindle nose 
as possible, one behind the other (i.e. across the bed from front to 
back). Be sure that the surfaces you select are finished (either ground 
or scraped) and that they are supposed to be on the same plane. 
(Obviously, all surfaces used must be absolutely clean and free of debris.)

Adjust the rear foot under the headstock to bring that end level. 
Duplicate that adjustment on the rear foot under the tailstock, to keep 
the machine roughly square. Repeat as needed to get the headstock dead 
level.

Put the level (running left-to-right) on a flat bed surface (toward the 
front if possible), and adjust the front foot under the tailstock to 
bring the bed level side to side. If there's only one foot under the 
tailstock, you're done.

If there are two feet... Put the level front-to-rear at the tail end of 
the bed (move the tailstock out of the way), duplicating the position 
used at the head end using the same reference surfaces. Adjust the rear 
foot at that end to set the bed level front to back.

This sequence is repeated (in its entirety) until the results are as 
close as desired. At least two passes would be expected, with no 
adjustments on the final pass which just checks that everything is correct.

I believe the theory behind the procedure is obvious.

The final test of any lathe installation is to turn some stock and check 
that it's straight, i.e. that it does not turn a taper as will happen if 
the bed is twisted.

The classic test is called the "two collar" test, as discussed in "How 
to Run a Lathe" from South Bend Lathe Co. This book has a wealth of 
information. I suggest you buy a reprint from Lindsay Books if you don't 
already have a copy.

There's a huge fallacy with the "two collar" test. Depending on the 
exact nature of the bed twist, you could actually turn a Coke-bottle 
profile, but not realize it because you're only measuring two points on 
the piece.

A better test is to turn a piece of heavy stock perhaps 12" long and 
thick enough that it won't deflect under cutting tool pressure. Mount it 
in a solid chuck and center the axis of rotation at the far end as 
accurately as possible. Don't use a rest or center to support the stock.

Do a couple of skim cuts the full length of the piece, then measure the 
diameter at several points. All should be identical. If not the bed is 
twisted (or possibly quite worn) and corrective action is warranted.

Anyhow, apologies for the long-winded response. The actual effort will 
probably take less time than reading this post.

Not a huge challenge. Good luck with it.

- Leigh

NB. Level checking and adjustment

Any precision level will have an adjustment for setting the vial true. 
This setting can be checked (and adjusted if need be) using a reference 
square and a flat surface.

The reference square is just to provide a positional reference so you 
can turn the level around 180° and put it back in exactly the same 
positon. Any small carpenter square or similar will work.

Tape the square on the surface plate, which should be approximately 
level but precision is not required. Position the level in the inside 
corner of the square. Note the bubble position in the vial, which should 
have several graduation marks.

Rotate the level 180° and put it back in the same position in the 
square. Note this reading and compare it to the previous one.

If they're exactly the same (need not be "dead center"), then the level 
is reading accurately and no further action is required.

If the readings differ, turn the adjusting screw to remove HALF of the 
error. Note the new reading.

Turn the level around and compare it with the previous after-adjustment 
reading. They should be identical. Iterate if needed.


peterve

Leigh
you only took the easy ones with 3 or 4 leveling feet

Actually a lathe with 3 leveling feed needs no leveling because it is 
impossible to bend it square with the aid of gravity
That is the big advantige of 3 leveling feet
But any serieus lathe has at least 6 leveling feet making all much more 
complex You bend the ways lenghtwise like a wave very easy
And a used lathe is even more complex Because of the wear you have in 
the ways it is impossible to have it level everywere So you have to 
compromise
I first get the bubble free floating Then I correct it 1 stripe at a 
time checking at different places If it is a long lathe say 3mtr you 
have to check
the vertical plane also That can be done with a wire and a microscope
I use a spirit 0,02mm/mtr And sorry I never check it I always place it 
orientated the same way Also in exactly the same place and those places 
I stone down before placing a spiritlevel I also clean the level and the 
place I put it every time Yes every time And I do it again if a reading 
is not what I suspected it to be

Peter from holland


The Real Leigh
When dealing with a machine having more than four feet, you must follow 
the manufacturer's instructions for trueing the bed. The taught line and 
microscope that you mentioned are pretty standard for that type of work.

- Leigh


misterT

The way I have always leveled lathes up is to run four of the feet down, 
set the level on the bed running length wise to get it somewhat close 
end to end. Next set it on the crossslide running crossways at the 
headstock end and I set it with the bubble one mark forward of center. 
this will set the front higher and help prevent coolant from running 
under the cross slide into the apron and rusting the gears up. Next run 
the saddle down to the tailstock end and set it the same and keep 
repeating this unitll it reads the same at both ends. Then I take a 
light cut on brass or aluminum to check it, if the end by the chuck is 
larger than drop the front screw at the tailstock end a light bit ( only 
1 degree at a time). If the other end is larger than raise the frone 
tailstock crew a bit untill it is straight. The I run the remaining 
screws down watching the level to see when they start to change it. Lock 
down your jam nuts and you are good to go, you can buy a King way tool 
to do it by the book. But I have set up too many lathes to count and got 
them all less than .0003" in 8" using this method.