Subject: Precision(?) levels

From: [email-masked] (Barry Wolinetz)
Date: 23 Jan 95 07:42:17 -0500

Hi all;

Thanks to those who responded to my question about my vibrating lathe.

In the responses were several mentions of precision levels for setup.  
I was pondering how to precisely level my lathe without a "precision" level
and came up with the following idea.

Make a rigid isoceles triangle with a "flat" piece of angle iron for the base.
Precisely locate the center of the base using basic geometry (construct
the perpendicular bisector to a line) and hang a plumb bob from the apex.
The taller the triangle, the more sensitive the reading (so it seems to me).

Does this sound reasonable?  Has any one else tried a similar idea?
I know that levels of this sort were used to build many ancient structures
(pyramids, cathedral notre dame,...etc) but  is it accurate enough for
machine tools?

Thanks

Barry
--

Barry Wolinetz                       My opinions, Only!  I would not
The Northern Trust Co                claim to speak for my employer in 
Chicago, Il                          most matters discussed here!

Give some kid(s) a break, Be a Foster Parent!

---

From: [email-masked] (Ron Ginger)
Date: Mon, 23 Jan 1995 20:00:27

In article <[email-masked]> [email-masked] (Barry Wolinetz) writes:
>Subject: Precision(?) levels
>From: [email-masked] (Barry Wolinetz)
>Date: 23 Jan 95 07:42:17 -0500

>and came up with the following idea.

I think the idea of level is misssing the mark. the lathe could be setup on a 
steep downhill incline and still turn true if the bed is aligned. Level is 
just one way to detect alignment, and Ive never been convinced  it was 
particulary accurate.

Put a test rod in the lathe, turn two colars on it and mike them. adjust the 
bed till it turns true.

There wer lathes on shipboard machine shops that turned true, and they sure 
werent level !

Ron Ginger
my signature is http://www.ultranet.com/~ginger

---

From: [email-masked]
Date: 24 Jan 1995 02:56:04 GMT

>Hi all;
[...]

>The taller the triangle, the more sensitive the reading (so it seems to me).

I imagine you want to level your lathe to 0.002 or 0.003 inch per foot, so
if you want to see the plumb bob move (relative to a reference mark) by a
nice visible 0.02 or 0.03 inches, you have to make the triangle 10 feet
tall. You'll want to go have a cup of coffee while you wait for it to
stop swinging. I suppose this would work adequately with a 6-foot-tall
triangle if your eyes are good. 

Be a lot simpler to borrow a machinist's level from a friend; heck,
maybe he'll let you borrow that shiny new Talyvel...

                      ...when I sit down to write
Don Holly             my intentions are always blameless however deplorable
                      the ultimate effect of the act may turn out to be.
                                           --Joseph Conrad

---

From: [email-masked] (David Adams)
Date: 24 Jan 95 10:49:58 CST

> (pyramids, cathedral notre dame,...etc) but  is it accurate enough for

In theory, the horizontal angular precision will be the same as the
vertical precision you can obtain when reading the closeness to the
center line.  In other words if you can be certain that you are with
in 1/10th of a degree from the center line then (assuming your
triangle etc., are build precisely) you can be certain that you are
level within 1/10th of a degree. etc.  This will tell you how big
you need to build your triangle.  Suppose you want 1/10th of a degree
to make a 1 milimeter swing, then:

sin(pi*1/900) = 1mm/X 

Where X is the height of the triangle in millimeters, so that each
side will be 2*sqrt(3)/3 times X.

sin(pi/900) = about .00349...  so that X must be

286.47mm  or 28.6 centimeters tall.  Not too un reasonable.

Each side would be about 330.8 mm long.

For 1/100th of a degree of precision we have:

sin(pi/9000) = 1/X

or X = ~ 2864.8  with each side = ~ 3307.9 mm or 3.3 meters long.

So having a 1/10 of a degree sweep out 1 mm seems quite reasonable,
but 1/100th of a degree sweeping out 1mm is not very reasonable.

If 1/10th of a degree sweeps out 1 mm you can probably read one more 
digit or close to it.  So I say it is a reasonable project if you 
are not asking for 1/1000th of a degree of precision.
(In theory.)

How about making the level be a perfect tetrahedron 28.6 cm tall?

(How could you construct such a thing with the accuracy desired?
Also would not temperature changes affect the shape enough to
alter the reading??  Could it be made of some temperature
invariant material??  Is this an appropriate use of INVAR??)

-- 
	-David  [email-masked]

---

From: [email-masked] (Russ Kepler)
Date: Tue, 24 Jan 1995 14:36:14 GMT

In article <[email-masked]>,
>(pyramids, cathedral notre dame,...etc) but  is it accurate enough for

This wouldn't have one tenth the accuracy that a cheap 4" machinist's
level will have unless you build it up with a level of precision
better than your lathe bed.  I'd suggest either springing $60 for a 6"
machinist's level or finding a machinist who has or can borrow a good
level.

-- 
Russ Kepler posting from home  [email-masked]

                      Please don't feed the Engineers

---

From: [email-masked]
Date: Thu, 26 Jan 95 23:02:54 -0500

A good setup level indicates .0005"/foot and they are expensive.
 
You can setup your lathe this way. This works I've done it. You have to put
some means of adjusting the feet of the lathe bed that you can adjust. The
last chapter of Tubal Cain's Workholding in the Lathe has a beautiful
description of this. Get the lathe fairly level and somewhat tightened down.
Get a good round rod that is very round and the same diameter end to end.
A nontrashed car strut will work. Put the rod in the 3-jaw chuck. Get something
like the Starrett last word and put it on it's magbase and the magbase on
the compound. Rotate the rod in the 3-jaw and adjust the compound using
the cross-slide such that the swing on the dial exactly straddles the 0.
Say there is 6thou total runout at the chuck. You'd want 3thou on each
side of the 0.
 
Run the carriage down toward the tailstock and again rotate the bar. The
actual runout you get is not important. What is important is that the
reading on both sides of the 0 is the same. If there's 20 thou total
runout, you'd want 10 thou on each side of the 0. If it is say -8 and
plus 12 that means that the right front leg (tailstock end, nearest you)
is high. Lower it a bit. Go back to the headstock and again set the dtil
so that it straddles the 0, move to the tailstock end of the bar and check
it again. When you get it such that the magnitude straddles the 0 at the
chuck end of the rod (the carriage as close to the headstock as you can
get it) and it straddles the 0 at the tailstock end, you've aligned the
lathebed. Now, here's the pisser. You can see, quite literally, how much
(if any) wear there is in the bed by checking the rod along the way. You
will probably find that there is some wear up near the headstock. And you
can see how much wear there is by seeing the difference in the straddle
on the dti. If you have a .0001 dti, all the better.
 
What is important on the rod is that it is round and smooth and the same
diameter. It does not matter if it is straight as you know damned well that
it's going to be swinging around the headstock axis no matter how bent! The
only problem with a really bent rod is the runout might go beyond the
range of your dti. You might want to read this a couple of times and think
about it before doing it. :-)   This is not originial with me (I'm not
smart enough. It is, I believe, original with Arthur Gaucher, a retired
tool and die maker who has forgotten more about this stuff than I shall
ever know.
 
Good luck.