PDA

View Full Version : Level Accuracy



Paul Alciatore
08-02-2003, 02:42 AM
I need to get a machinist's level. What I want to know is just how accurate does it need to be? And what length is best? I have a South Bend 9 to set up and will likely use it on the mill-drill also. Any advice will be appreciated.

gvasale
08-02-2003, 10:30 AM
The only machine manual have says use a " 'precision ground bulb' level. Carpenter's levels or combination square levels is not sensitive enough." There are 12" european levels that claim sensitivity of .0005" foot available from MSC, which may be quite satisfactory in the $125.00 price range.The only hitch with a long level is what most of us have for parallels for setup may be less than ideal. No connection other than being a paying customer.

[This message has been edited by gvasale (edited 08-02-2003).]

Oso
08-02-2003, 11:25 AM
As SENSITIVE as your work needs to be accurate, and as sensitive as you want the machine accuracy to stay........

If you use an 0.005 per foot level, like a Starret 98 (?) then you may have a couple thous error per foot, in your leveling and maybe in your parts. If you use an 0.0002, your error is less.

The machine will gradually wear to fit the saddle to the shape of the bed, so errors in leveling are reflected in uneven wear, and eventual "rocking horse" saddles.

Takes a long time, depending on use, cleanliness, and lubrication but it happens.

If your machine is used, the previous owner may have already worn it in in a bad way and leveling it may actually make things worse for you. But, it is the only standard for avoiding twist in the bed, so it's best to do it.

0.0002 per foot is best, 0.005 is better than an HD carpenters level............

I don't see where anything over 8" is handy at all, but the bed of my Logan is only around 7" wide. For a bigger machine, it might be handier to be longer in some cases, but then you can always use a beam parallel. It's hard to saw off a bit of a level..........

Cass
08-02-2003, 01:06 PM
I think the emphasis on getting a machine way level is really an effect of trying to get the ways of a machine straight. With a long and not very stiff way such as is the case with conventional lathes it is important to minimize warping of the ways. One way to try to do this is to use a level and check level at increments along the ways. By adjusting the supports until the ways are in the same plane at all test points you have some idea that the ways are straight. I am going over this because my experience has always been with super precision machines that are so stiff and short coupled that we never worried about whether the machine was exactly level but we did worry a lot about slide straightness. All the straightness and squareness measurements were made with laser interferometers. Once we had an application that required sub-arc second resolution on uniformity of angle of an extended surface. The surface was horizontal so "level" applies to the measurement. The required resolution was better than any of the electronic levels we knew of. A creative PhD type thought up an ultra level for the application and we built it and used it effectively. It consisted of a hollow column of steel with a three point base that was ground and lapped flat and nominally square to the axis of the square column. A thin gage foil of steel was suspended from the top of the column with a weight attached to the end to keep the band of foil taunt. A capacitance sensor was positioned to read the location of the foil. The capacitance sensor had a resolution adjustable to as fine as 1/100th of one millionth of an inch. The the "level" thus was a sealed container made of one material and insulated from transient heat. The instrument was positioned and the capacitance gage was zeroed. Measurements at other positions were recorded relative to the first position. Any angular change of the base was indicated as change relative to the foil which remained pointed at earth center. The column had a vacuum port on it to allow extraction of the air but we found that not to be necessary. I have thought since then that a similar "level" could be built which would use a dial indicator. The height of the column was about 12" and the gage strip was about 1/2" wide and about 10" long. Obviously the longer the hanging foil with weight the higher the resolution but also the heavier and more awkward the instrument. The capacitance gage senses on an area so it was not sensivtive to centering which might effect a dial indicator. The capacitance gage is an expensive device costing about $10k at the time (a little cheaper now) so it is too expensive for most people. If you built something like this with a dial indicator or better with an electronic digital indicator I think it would be a useful tool. Laser straightness interferometers would do the lathe line up job more effectively though and autocollimators and alignment telescopes also do a good job.

mklotz
08-02-2003, 01:40 PM
Here's something to think about.

I bought a $10 Chicom laser level at HF for a carpentry application. It
projects a laser spot over a considerable distance. I set the level on my
(12" long) surface plate so the laser spot hit a vertical scale fifteen feet
away across my shop. I slipped a 0.004" feeler gage under one end of the
level and noted that the laser spot moved ~1/16" on the scale. Now,

sin x = 0.004/12

But the angle 'x' is tiny so sin x ~= x. So:

x ~= 0.004/12 = 0.000333 radians = 333 microradians (urad)

333 urad is about 68 arc seconds (or, obviously, 0.004" per foot). Detecting
a one arc minute slope for $10 invested in tooling is pretty cool and you
don't need to worry about the vials in the level.

I found that, at 15', reading the beam location to better than 1/16" is
difficult. I have an idea, as yet untried, on how to deal with that. If I
build a simple circuit consisting of little more than a biased
photo-transistor powered by a 9v battery, I can use it as a light amplitude
detector. (The voltage across the biasing resistor will be proportional to
the light intensity falling on the photo-transistor.) If I attach this to a
height gage, I'll be able to measure the height of the maximum signal coming
from the laser light. This will take my bifocals and the laser speckle
problem out of the equation.

I haven't pursued this as a technique for detecting lathe twist but I think it
has potential as an economical intermediate between a coarse, inaccurate level
and a super-expensive machinist level. Perhaps some of you clever guys may
want to explore it.


rbregn
08-02-2003, 01:54 PM
one wat to double check your level is to flip it end for end on your surface and see if your bubble is in the same spot, or it moved. It won't tell you how sensitive it is, just if you should throw it away or not!
rob

Don Warner
08-02-2003, 06:20 PM
This might work for you out there that are electronically endowed. In the May issue of "Nuts & Volts" there is an article on how to build a "tiltometer" by using 2 small cups of mercury 24" apart that can measure the tilt of the earth of 125 mills in 200 MILES. Now that's a level!
Don Warner

Thrud
08-02-2003, 09:30 PM
Oso:
The Starrett #199 is also usable as a straight edge when scrapping. They have a seasoned hand scrapped cast iron base. You my find it too accurate for many applications, but they are ideal for machine set-up. Used them a lot on plastic Injection Molding machines.

Thrud
08-02-2003, 09:39 PM
mklotz:

Even on a high quality laser beam there is more divergance in that distance than you claim to be able to differentiate. Note that a Starrett #199 is 15 arc seconds - accuracy very near to what you suggest the cheesy laser is.

I don't think so.

Cass
08-02-2003, 11:21 PM
An average quality autocollimator has a resolution of 0.1 arc second. That was the standard alignment tool before laser interferometers. The Brunson web site has a tutorial on how to use one.

mklotz
08-03-2003, 01:04 PM
Thrud,

Gosh, I wish you'd told me that what I was doing was impossible before I went
ahead and wasted all that time doing it.

15 arcseconds is "very near" to 68 arcseconds according to you? I don't call
a factor of 4.5 very near even when I'm doing woodworking. The Starrett Master
Precision Level claims 10 arcsec resolution, but it costs $520 - kind of steep
for the hobbyist. Rather than paying that kind of money, I'd be happy to get
within an arcminute cheaply and then take out the last few thousandths of
twist using the standard two-collar test bar.

robert phillips
08-05-2003, 06:31 PM
Hey mk,

double or triple your resolution capability. Throw some surface mirrors in there and artificially lengthen the 'run'. of course you'll have to go back and forth realigning mirrors when the angle is too great. used to do this on a small scale for sensitivity (gain) adjustments on oscillographs. try to collimate the laser spot too with adjustable focus lens. no longer $10.00. but fun to mess with.

nheng
08-05-2003, 06:47 PM
mklotz: You can increase your resolution tenfold or reduce your target distance tenfold. Instead of targeting a flat surface, target a sloping surface. I've done this and shrunk the size of an optical fixture from 72" to 7". Fits a lot nicer on my bench. You'll have an long elliptical spot but who cares (in many cases).

thrud: Missed your reply ... all you gotta do is use the center of mass of the laser spot (after calibrating it and your eyeballs somehow http://bbs.homeshopmachinist.net//smile.gif ).
Den

[This message has been edited by nheng (edited 08-05-2003).]

Evan
08-05-2003, 07:45 PM
Cass, It occurs to me that the "Super level" you describe is probably sensitive enough to be affected by the gravitational attraction of nearby equipment. Did you try to make any such determination?

Slightly off topic, I built a Lehman style seisomgraph back in the 80's which is still set up and operational in my basement. A Lehman seismograph is a simple instrument and is basically a tilt meter. Mine is sensitive enough to detect magitude 5 earthquakes or so anywhere in North America and mag 6 just about anywhere in the world. It is so sensitive that it will detect someone merely walking around the outside of the house or a car pulling in the driveway 150 feet away. It easily detects six second microseisms that are thought to be either wave action on the coast 500 kilometers away and/or the effect of atmospheric pressure on the earth's surface. If that sounds unlikely the math shows how it may be so. A change of 25mm on the barometer(about one inch)is about 39 grams per sq cm or about 390 kilos per sq metre or about 3,900 metric tonnes per Hectare (about 2.5 acres) or about 390,000 tonnes per sq kilometre. I can clearly see approaching low pressure cells with the instrument.

Oh yeah, I forgot to mention that I used my SB9 to wind the pickup coil on the siesmograph. 10,000 turns (as per turn counter) of #70 wire as fine as a hair. I managed to do it with out a break on the first try.

[This message has been edited by Evan (edited 08-05-2003).]

mklotz
08-05-2003, 08:18 PM
robert phillips -

Actually I did fold the path back on itself with a surface mirror so the beam
displacement was ~1/8" for a 0.004" shim under the level. That has the added
benefit of putting the scale where the beam position is noted close to the
operator although that's not of great consequence for a one-time operation like
unwinding a lathe.

nheng -

I like the tilted approach. Gotta try that. Despite Thrud's encouragement,
I'm going to build the height gage-mounted beam maximum detector as well.

Cass
08-05-2003, 10:03 PM
Evan,
Since the sensing element and the hanging mass are all inside a steel housing it is pretty well shielded against magnetic fields. The machine it was used on weighed around 9000 lb. and was on active air vibration isolation and the lab was on a very thick concrete basement floor in a temperature controlled environment. It worked very well as far as I know although a technician made the actual measurements and I just looked at the data. I didn't hear any complaints about it being hard to use and the tech was quick to complain on most things. I think a lower resolution instrument of this type might be pretty handy because it would have very high resolution, durable and pretty noise free. A standard LVDT or just a good 50 millionths digital indicator would have enough resolution for most applications.

Evan
08-06-2003, 12:13 AM
Cass,

Gravitational field, not magnetic. You can't shield against a gravitational field.

CCWKen
08-06-2003, 02:03 AM
Geez! Are you guys trying to level a machine or shoot a rocket to Pluto? I put a sheet of glass on my ways, then set a ball bearing ball on the glass. Kept adjusting until the ball didn't roll. I even watched for dust specs and lint!
I think it cost me about $2 for my +/-.0001 level. "Dead On" as far as most are concerned.
Then, of course, there could be gravity sag in the middle of the lathe ways. Or on the ends of the mill table. What would that be? .000001".... Who Cares!

Paul Alciatore
08-06-2003, 02:52 AM
Ken,

I think you just saved me about $50 to $100. But I think I will look for an optically flat piece of glass. Grrrrreat Idea!!!!!

Thanks to all,

Paul A.

Cass
08-06-2003, 09:27 AM
Evan,

I think the gravitational effects on the very small weight and the associated error is negligible. That may have been considered by the guy who had the idea. The device had a comfortable margin of resolution over what the measurement required and on thinking about it I believe any error of the type you suggest could be added out of the system by a reversal technique. Seems the tech would have noticed it if it had been significant. The point of my comments was to suggest that a cheaper and more simple device could be made and still give high resolution. The device could be pushed across a surface and with a swivel incorporated it could check level in any direction with resolution close to an autocollimator. Not very handy for shooting at Pluto.

Oso
08-06-2003, 10:17 AM
The ball bearing on glass is pretty good, if you know the flatness and roundness. Not very good for comparative measurements, though. But since you are going for level, it may not matter.

I'd be interested in a comparison of that vs a known level.


Cass, I have seen experiments which detected the gravitational attraction of a couple bags of sand. No fancy equipment either, a piece of wood with equal weights on the end, a mirror in the middle, and hung on a length of recording tape.

Bring the sand near, and a light beam reflected in the mirror could be seen to deflect as one of the weights was attracted.

Pretty neat actually.

Evan
08-06-2003, 12:28 PM
Cass,

Just a reminder, a bb and a cannon ball fall at the same speed (in vacuum). The very small weight will be attracted the same as a large heavy weight by a nearby mass such as the headstock or another machine. The reversal technique should work though. One type of gravitometer is sensitive enough to detect the gravity of a passing truck hundreds of feet distant.

Thrud
08-07-2003, 01:03 AM
Oso:
An optical flat would be ideal - get a class 10 ball bearing (carbide, natch). Excellent idea.

MK
Sorry about the screw up on the Starrett #199's accuracy. It is far in excess of most needs. I would not trade mine for anything.

What I was trying to point out is it is difficult to calibrate your device to said accuracies with out the use of an optical Collimator of known precision. Your unit may be "close enough for the girls" but not for any self respecting mad scientist. http://bbs.homeshopmachinist.net//biggrin.gif
(neither a put down, nor discouragement)

Evan
Any two masses in the universe are attracted to one another by gravitation, even though the force may be unmeasurable to us. Ergo, all masses interacts constantly with all the other masses - welcome to chaos in motion. http://bbs.homeshopmachinist.net//biggrin.gif


[This message has been edited by Thrud (edited 08-07-2003).]

Paul Alciatore
08-07-2003, 02:45 AM
<font face="Verdana, Arial" size="2">Originally posted by Oso:
The ball bearing on glass is pretty good, if you know the flatness and roundness. Not very good for comparative measurements, though. But since you are going for level, it may not matter.

I'd be interested in a comparison of that vs a known level.


Cass, I have seen experiments which detected the gravitational attraction of a couple bags of sand. No fancy equipment either, a piece of wood with equal weights on the end, a mirror in the middle, and hung on a length of recording tape.

Bring the sand near, and a light beam reflected in the mirror could be seen to deflect as one of the weights was attracted.

Pretty neat actually. </font>

We did essentially the same experiment in college. Used a ready made beam with two lead weights on the ends suspended by a thread with known modulus of elasticity. It was in a glass enclosure to avoid air currents and had a small mirror on it to bounce a beam of light. Two larger weights were mounted on a concentric pivot and could be swung about 300* from one side of the beam's weights to the other for CW or CCW torque.

The light beam was thrown about 15 or 20 feet across the lab and the displacement was quite small. The accuracy of the measurement was not impressive - 20% or so. But it did demonstrate gravity between the weights.

I would be very distrustful of any such setup that did not shield the arm from air currents as the movement of your sandbags or bodies or even your breath would likely produce a far, FAR greater swing of the beam than the gravity from the sandbags.

As for comparing the glass/ball bearing method to a known level, I have already ordered an optically flat glass plate about 2 3/8" x 7 5/8" and plan to try it in a week or two when my table is complete. Now, if anyone wants to send me a "known good level" I'll be happy to compare them. My feeling is that it should be more sensitive than a 0.005"/ft level. I do plan to experiment with shims to see just how sensitive it really is.

I also ordered a 2" diameter, long focal length, plano concave, negative lens. It has a very shallow concave surface which should be ideal for making a sensitive level. Anyway, I'm going to try. Now, does anyone know what fluids work well in a bubble level?

Paul A.

Thrud
08-07-2003, 03:23 PM
Paul
Ethanol. That is what is used in bubble levels. Dye is sometimes added for visability in cheaper units, but I have no idea what dye is used, but I suspect fabric dye would work.

With your lens a small drop of Mercury might serve well (inverse bubble) - it would be best to seal it with an argon atmosphere to prevent oxidization.

John Garner
08-09-2003, 01:37 AM
Paul --

One of the very most precise "level sensors" was the Autonetics 2-axis sensor that was essentially based on a concave lens. The concave surface of the lens had 4 lines at 90 degrees to each other metalized onto the concave surface.

The lens was used to cap a fluid chamber that was almost filled with a conductive fluid; the 4 metalized lines were used as sense electrodes, and the resistance between each of the sense electrodes and a fifth electrode in the base of the chamber was measured and compared differentially across the pairs of sense electrodes separated by 180 degrees.

As I understand it, Autonetics primarily used these sensors in their missile guidance sets but did sell some as stand-alone sensors.

In comparison, the most sensitive circular level vial that I am familiar with is the Fell vial, which is in the 8 to 10 arcsecond-per-graduation range (which corresponds to that of the Starrett 199Z Master Precision Level); the radius of curvature of the concave surface is in the neighborhood of 350 feet on these vials.

As for filling fluid on sensitive level vials, alcohol may not flow freely enough to avoid a "slow vial". Commercial high-precision vials are often filled with ether.

Cass --

I suspect that the pendulum would have taken darn near forever to quit swinging had you evacuated the cylinder on your level sensor; the commercial pendulum-based "electronic level" systems that I'm familiar with need supplemental damping -- sometimes by filling the casing with oil -- to keep from "fluttering".

John

Evan
08-09-2003, 03:51 AM
John,

That Fell Vial would be perfect for leveling my fifth wheel RV. I hate it when my bed isn't level. Yeah, it's Friday night. Where's the beer?

Paul Alciatore
08-09-2003, 02:26 PM
Evan,

Hey, I live in a travel trailer most of the year. House is in Texas: job in Iowa: don't want to sell. I keep my trailer tilted one degree or so to one side deliberately so that the doors stay open and the refrigerator closed. Works for me.

Back to topic. I've been thinking more about the lens-level thing. For a quickie, approximate level, a ball bearing in the upward facing, concave side of the lens would get me in the ball park and then the optical flat would finish the job? The lens I ordered has a radius of about 450mm or about 18 inches so it will not be sensitive enough to reach even .005 per foot. More like .032" per foot. I'm looking for a better lens.

I also need a way to make markings on the lens without adding any significant bumps or grooves that the ball would bump into. Any ideas?

John Garner
08-09-2003, 05:08 PM
Paul --

To veer this train of conversation back toward your original question:

A traditional US/British precision level vial has graduation markings spaced every 0.1 inch -- the rest of the world uses 2 millimeter spacing -- and the human eye can reliably interpolate the position of the bubble to about 1/4 or 1/5 of the graduation spacing.

Taking these facts together, this means that a standard "millwright level" such as the Starrett 98 with 0.005 inch per foot vial graduations spaced at 1/10 inch can be read to about 0.001 inch per foot. A "master precision level" such as the Starrett 199Z with 0.0005 inch per foot vial graduations spaced at 1/10 inch can be read to about 0.0001 inch per foot. Assuming, of course, that environmental issues don't contaminate the measurements . . . no sunbeams, heating and cooling vents, or soft supports allowed.

Then, when it comes to "levelling" a lathe, the primary purpose is NOT to make the bedways horizontal, but is instead to remove twist (often termed "wind" in the literature . . . which I presume is spoken "whined", not "winned").

For this reason, I advocate positioning the level instrument on the lathe carriage so that the vial axis is perpendicular to the bedways.

Move the carriage toward the headstock as far as it will go, then position the level instrument on the carriage so that the vial axis is perpendicular to the bedway. Adjust as necessary to center the bubble in the vial; using shims, a levelling screw, or blobs of oil-mixed modelling clay between the top of the carriage and the bottom of the level instrument's body. (The oil-mixed clay, which is sold in arts-and-crafts stores as "never dry" or "plastic" clay, is super; it is mildly adhesive and will stick to the top of the carriage and the bottom of the level body to help hold the instrument in place on the carriage, and adjusting the position of the bubble in the vial is as simple as pushing the high end of the level body down into the clay a bit.)

Once the bubble is roughly centered in the vial with the carriage at the headstock end of the bedways, carefully note the actual position of the bubble relative to the vial graduations. Then troll the carriage down to the tailstock end of the bedways and adjust the tailstock-end jackscrews/wedges/shims to move the bubble to the same position in the vial.

Having done that, troll the carriage back up to the headstock end and check the bubble position. You'll probably find that the bubble has shifted a bit in the vial, so you'll need to reiterate the whole process a couple of times until there isn't any significant change in bubble position between the headstock and tailstock end of the bedways.

If the lathe is long enough to have "center" mounting feet, you'll want to repeat the adjustment process after rotating the level instrument on the carriage so that the vial axis is parallel to the bedways so that you can detect (and eliminate) "humping".

As for the level instrument itself, I think that the French-made EDAT levels distributed by SPI are great values. I have one with a 20 centimeter (8 inch) iron body with a 10 arcsecond-per-2-millimeter-division vial that I bought new for about $125, and it works like a charm for me.

For what it's worth, the EDAT and similar levels with "10 arcsecond per 2 millimeter graduation" vials are significantly less sensitive than the Starrett 199Z and similar levels with "0.0005 inch per foot per 1/10 inch graduation" vials.

John

Oso
08-10-2003, 12:34 AM
For what it's worth, I recommend NOT using the carriage as the level base.

It has no definite relationship to the bedway, that is an adjustment (gibs) in the first place, and does not account for wear, in the second.

Best is to find a surface or pair of them that is(are) likely unworn (see Connoley book) and use that (or them). The tops of V-ways are often unworn, and the level can be put across them.

There isn't a guarantee that the carriage is flat on the ways. If the ways have "wind", it probably isn't.

It may be contacting solidly on two corners only, and rocking.

Sprocket
08-10-2003, 11:42 PM
CCWKen - Any thought about an "optimal" size for the bearing? or does it really matter. I would think that up to a point, larger would be better,(we are still talking bearings here) but no real idea. What size do you use? anybody else have thoughts on this? seems like a very useful tool. Cheap and effective.

[This message has been edited by Sprocket (edited 08-10-2003).]

darryl
08-11-2003, 12:44 AM
I can't help but wonder is some random radioactivity in the glass, or in the metal lathe bed would affect the rolling of the ball, once the leveling is close. Or a random falling of dust onto the ball, or a slight wind from your body parts moving around, etc. What about the slight shifitng of the earth on which the floor, and ultimately the lathe bed itself, is resting- also, what of your body weight coming and going, giving a slight but measurable deflection to the floor, then the stand, then the bed- seems to me that accuracy within 10 billionth of an inch would be unkeepable, even it it's attainable at one setting sitting. How about the effect of the slight changes of the earth's magnetism on the magnetorestrictive components within the cast parts of the lathe- sheesh! Then there's temperature changes due to the sun rising in the east, and setting in the west. And isn't there a preferred orientation which works best, like headstock to the north, or something? What happens to all that accuracy once the switch is flipped, and the motor jerks to life, sending spasms of multi-micron waves rippling down the lathe bed? Gosh. I think I'll go down to the shop now and see if I can get those rivet holes drilled within a tad of center.

Oso
08-11-2003, 12:55 AM
Darryl.....got the point......

Presumably though, if the poster wants a good level, he wants to measure accurately.

See Forrest's posts about body or sunshine heating of parts. You jesting post is actually truer than you think..

Evan
08-11-2003, 02:23 AM
Cripes,

The point here is not to "LEVEL" the lathe but to make it straight from headstock to tailstock. You get it almost right with the level and then turn a bar. That will tell you if the head and tail are aligned. Then put shims under the feet. Crap, I can tell if the tailstock is aligned with the headstock just by looking, put a head centre and a tail centre and run them up to each other. If you can turn a bar the full length of your lathe and have it at the same dimensions at both ends then it must be OK.

BTW, a lathe should be set up to turn slightly concave on a face, perhaps a .0001 or .0003 or so from edge to centre.

CCWKen
08-11-2003, 04:19 AM
Evan:
You've got a "level" point there. After I leveled mine, I still had to adjust the tailstock over by .002" and tighten it. This may have been caused by rough handling or no handling at the factory. At 40", .002 is not much. At point to point, this is conciderable and may not be noticed by bringing dead centers together. I always put my test bar in and check it frequently.
As Daryl notes, ya never know how barametricly modified magnetic flux of the moon will affect the job.

Sprocket:
I used a 1/2" Stainless Steel ball because I had a few around. (Pretty accurate stand-offs too) These are grade 24, which means they are spherical to .000024". I don't have anything to measure that close, so I'll take their word. (The size is acurate to +/- .0001.) In any case, even a fingerprint will distort the "roundness" of the balls so handle with care. Make sure everything is CLEAN!

Forgot to mention: You can get very "flat" glass at the hardware store. Distortions can usually be seen, if you hold it at an angle. Pick through the scraps (cutoffs) until you find a CLEAR piece. 6x6 is big enough. I got mine for free. Optical glass is ground, float glass is FLAT to about .00015 in the middle of the batch!



[This message has been edited by CCWKen (edited 08-11-2003).]

nheng
08-11-2003, 09:29 AM
Evan: I read about the slight concavity designed into lathes in an old (1937) book by Smith from MIT. Is this still done on manual machines today ? Is it built into the cross slide ?
Den

Bill Cook
08-11-2003, 11:19 AM
Precision pendular horizon generator

A lot of talk about precision levels on the machinist boards, and an interest in levers and balances got me thinking. Wouldn't it be fun to make one?
I didn't expect it to have any practical use other than satisfy a yen - and it doesn't.
In the interest of keeping it fun, I decided that it couldn't waste much of my life:
1. No machining.
2. Very little measuring
3. No sketching - just do it!

The levers are 2' long, made mostly of 3/16 CRS. The upper lever pivots on sheet metal "knife edges" on the right. Friction reduction was accomplished with a few strokes of a mill bastard and a few drops of oil.

The pendulum - a 1" drive 2 3/4" socket. The other sockets were moved in and out to balance the levers and adjust the sensitivity.

The span of the beam is 2'. It's difficult to see, but there is a .005" feeler guage under the right hand end of the beam in the picture - .0025" in 12".

The inset on the left is with the feeler. The one on the right is without - right hand end of beam lowered .005". The pointer moved about .1 inch consistently.

http://home.att.net/~skytop19/wsb/media/157166/site1012.JPG

bc

Evan
08-11-2003, 12:24 PM
As far as I know all manual lathes should be set to turn a slightly concave face. The reason this is done is because as one turns toward the center of the work when facing the loads increase thereby springing the tool and holder away from the work. This is also done because in almost every case a slightly concave face is far more preferable than a convex one when it comes to fitting parts.

If the lathe does not have carriage adjustments then it can be set by positioning the right side feet ever so slightly to the rear of the headstock end, less than .001, in effect racking the bed ever so slightly. In this case a very slight twist of the bed (again less than .001) will compensate and allow the tailstock to track true along the length of the bed. These are really tiny adjustments beyond the capability of most levels to measure.

See here for an explanation of setting up the carriage, section 2.1:
http://easyweb.easynet.co.uk/~chrish/tadjust.htm#adjustments



[This message has been edited by Evan (edited 08-11-2003).]

Paul Alciatore
08-11-2003, 02:48 PM
John,

Sounds like exactly the kind of info I was looking for. Thanks. Never having used a "precision" or "machinists" level, I did not have a good feel for these details although I suspected that one could achieve better than rated accuracy just as you can read between the lines on most measuring instruments.

I fully realised that in "leveling" a lathe, you really want flatness, and not necessairly perfect levelness. I made that point on another BB recently where someone was trying to say that a level that had no adjustments and therefore could not be zeroed was useless and should be thrown away. First point is that all high quality levels are adjustable. Second point is that even if it is off by a small amount, it can still be used to bring the lathe to a flat state if not truely level. The sensitivity is more at issue than perfect level-ness.

I love your clay idea. As for using the carriage, at first thought I would disagree with Oso's comments. It seems to me that the whole purpose of leveling the lathe is to have the CARRIAGE travel in a straight, untwisted path. So, what better place to measure from? My lathe is perhaps 45 or 50 years old. Who knows how many dings or burrs are present on the top of the ways? The whole idea of the V ways is that the headstock, tailstock and carriage all ride on the broad faces of the Vees, not on the top edges. The more you think about, the more clever and durable it seems. Anyway, the carriage carries the cutting tool and that's what needs to move on a straight line so that seems to be the best place to measure from.

As for other influences, you may be absolutely correct. I may need a cover over the ball bearing if I use that idea. And, yes, fingerprints can mike in at several tenths. But as for the gravational attraction of my body, I really do need to lose some weight but... No seriously, I really doubt that it will matter. Heat, sunlight, any movement of the table, any movement of heavy nearby objects, etc. can definitely matter.

PS: Grizzly seems to have a special on their levels. They claim 0.0005" per 10" for the 8" model and they are adjustable (I downloaded the manual). Don't know the spacing of the graduations. I may just break down and get one.


<font face="Verdana, Arial" size="2">Originally posted by John Garner:
Paul --

To veer this train of conversation back toward your original question:

A traditional US/British precision level vial has graduation markings spaced every 0.1 inch -- the rest of the world uses 2 millimeter spacing -- and the human eye can reliably interpolate the position of the bubble to about 1/4 or 1/5 of the graduation spacing.

Taking these facts together, this means that a standard "millwright level" such as the Starrett 98 with 0.005 inch per foot vial graduations spaced at 1/10 inch can be read to about 0.001 inch per foot. A "master precision level" such as the Starrett 199Z with 0.0005 inch per foot vial graduations spaced at 1/10 inch can be read to about 0.0001 inch per foot. Assuming, of course, that environmental issues don't contaminate the measurements . . . no sunbeams, heating and cooling vents, or soft supports allowed.

Then, when it comes to "levelling" a lathe, the primary purpose is NOT to make the bedways horizontal, but is instead to remove twist (often termed "wind" in the literature . . . which I presume is spoken "whined", not "winned").

For this reason, I advocate positioning the level instrument on the lathe carriage so that the vial axis is perpendicular to the bedways.

Move the carriage toward the headstock as far as it will go, then position the level instrument on the carriage so that the vial axis is perpendicular to the bedway. Adjust as necessary to center the bubble in the vial; using shims, a levelling screw, or blobs of oil-mixed modelling clay between the top of the carriage and the bottom of the level instrument's body. (The oil-mixed clay, which is sold in arts-and-crafts stores as "never dry" or "plastic" clay, is super; it is mildly adhesive and will stick to the top of the carriage and the bottom of the level body to help hold the instrument in place on the carriage, and adjusting the position of the bubble in the vial is as simple as pushing the high end of the level body down into the clay a bit.)

Once the bubble is roughly centered in the vial with the carriage at the headstock end of the bedways, carefully note the actual position of the bubble relative to the vial graduations. Then troll the carriage down to the tailstock end of the bedways and adjust the tailstock-end jackscrews/wedges/shims to move the bubble to the same position in the vial.

Having done that, troll the carriage back up to the headstock end and check the bubble position. You'll probably find that the bubble has shifted a bit in the vial, so you'll need to reiterate the whole process a couple of times until there isn't any significant change in bubble position between the headstock and tailstock end of the bedways.

If the lathe is long enough to have "center" mounting feet, you'll want to repeat the adjustment process after rotating the level instrument on the carriage so that the vial axis is parallel to the bedways so that you can detect (and eliminate) "humping".

As for the level instrument itself, I think that the French-made EDAT levels distributed by SPI are great values. I have one with a 20 centimeter (8 inch) iron body with a 10 arcsecond-per-2-millimeter-division vial that I bought new for about $125, and it works like a charm for me.

For what it's worth, the EDAT and similar levels with "10 arcsecond per 2 millimeter graduation" vials are significantly less sensitive than the Starrett 199Z and similar levels with "0.0005 inch per foot per 1/10 inch graduation" vials.

John </font>