View Full Version : Machinist Level

05-24-2008, 01:14 PM
Good Morning Guys,

I have a question about a Machinist Level and I don't know how to ask this question. Bear with me.:o

If I set my Machinist Level horizontally on my milling table, from left to right it reads "level". If I turn the level 180 degrees, the bubble is off. Why?

Years ago, a machinist stated that when I set up my lathe, I should turn the level first one way and then the other. If it doesn't show level both ways then the machine must be leveled until it does. Is this correct?


05-24-2008, 01:19 PM
The level should read the same when rotated 180 degrees even when reading an off-level surface. You level is off. I'd double check for debri or dings first, but that is what an out of cal level does.

05-24-2008, 01:25 PM
Hi Harold,
I'm not sure but if I understand correctly the way you calibrate a precision level is to put it on a level surface, then swap ends as you have done.
If there is an error you adjust it out to zero.
In your case the level may be off and your table is also off.
Adjust your level until both directions read the exact same, then adjust the table to zero.

Forrest Addy
05-24-2008, 02:27 PM
Adjusting precision levels.

There's always some "slope" to a level surface. Even on a slope that has been leveled to single digit arc seconds there will be a residual error. When calibrating a level you turn this slope to advantage.

The "true slope" of a surface is the direction of its maximum departure from true and level. A true level line (for the purposes of calibration of a level) may be taken form a line drawn at 90 degrees from the true slope, that is, "across the slope". Naturally the calibration surface has to be flat and rigid so errors of setting and deflection won't affect the calibration process. A nearly level machine table is ideal. First prepare the machine table by cleaning it and removing all burrs and dings with a slip stone. Do the same for the level's reference surface but be sure you remove no metal. When the surfaces "rings true" as evidenced by gliding the stone around and detecting no "catches" by the sensations in your fingers, you are ready to go.

The max out of level direction may be determined by placing the level and then nudging it around to max the reading, then draw a line 90 degree, etc but I suggest you immediately find the level line by placing the level across the slope and nudging until the level reads the same error regardless of reversal. Draw a line along the edge of the level. This line represents a level line and the level must be set to it for each reading and reversal. "Reversal" means to take a reading, note it then turn the level 180 degrees on its vertical axis and aligning to the level line and taking another reading for comparison.

Different levels are adjusted in different ways. Generally there are a few screws buried under pitch or threaded posts with adjustable nuts used to tilt the vial capsule. When these are adjusted a tiny increment at a time the vial is tweaked into calibration with its reference surface.

Be careful about handling the level. Heat will distort the body enough to seriously affect the readings. There is usually a heavy plastic or wood top or cover on the level that's intended as an insulator. The level should be handled by the insulator. If you have to remove this cover for calibration, handle the level with an oven mitt or a short stack of shop towels to protect the level from the heat of your hands.

When you get repeatable reversal readings after a series of adjustments you are almost home. Let the level sit overnight then return to see if the level has made any changes from settling. If so, your adjustments may have drifted a trifle requiring further tweaking.

Ideally, your reversal error should show a bubble centered within less then the width of a single line. I use one of my head hairs (Some of us still have ours ) tied around the vial as an adjustable curser to detect reversal errors but any fine thread or string will do.

The mark of a pro job in level calibration is to apply a sticker showing calibration date, limits of calibration, the signature of the tech (you) and the due date for re-calibration.

Calibrating master precision levels can a tedious operation. It has to be carried out with scrupulous attention to cleanliness and minimum heat input to the level; don't be dismayed if you bump into frustration. Experienced metrology lab techs have been known to gnash their teeth and stomp off for a cup of coffee when calibrating levels.

05-24-2008, 05:55 PM
Thank You for again sharing your knowledge. A very simple and easy to follow procedure to check and adjust precision levels. Now if only the actual adjustment was as quick and easy:D


bob ward
05-24-2008, 06:14 PM
Hwingo, I've been there recently and chased my tail for a while until I figured out that unlike a standard builder's level, the vial in a machine level is not necessarily parallel to the base of the level.

Catch 22, how do I set the level up properly when I don't have a true surface, and how do I get a true surface when I don't have an accurate level? Thanks to Forrest's post I'll be able to tackle that now.

In the meantime I've been using my unlevel level to get the twist out of the ways on my lathe and it works OK for that, its just a matter of getting the same reading at each end of the lathe.

05-24-2008, 08:20 PM
Leveling a level (???) is relatively easy - as any Land Surveyor will tell you.

First set a cleaned-off level on a cleaned-off flat base - in a vice will do (don't clamp too tightly).

Place the level on and along the length of the flat base.

Adjust (lightly tap) the base until the level bubble is centred in the vial.

Turn the level ed-for-end (reverse it) on the base.

If there is no difference or error the level is OK and no further work is required.

If any error shows on the vial, tap/adjust the base to remove half of the error. Remove the other half of the error by altering the adjustment (screw) for/on the vial.

Reverse the level on the base.

If level is still zero(ed) - job done - nothing more to do.

If there is an error between the vial readings when the level is reversed, continue adjusting the base and the vial as previously. This is a repetitive - and possibly frustrating - process, but it is necessary and is very accurate.

A very light "touch" or "hand" as well as patience is required as this is a very delicate process.

As advised by others, beware of errors introduced by swarf, dirt and temperature differentials. Keep everything clean and handle the metal parts with care (use a cloth or light cloth or leather gloves to minimise temperature errors).

For what its worth, there is no need to actaully (re?)adjust the level if you don't want to. Just use the level on the flat base and set the base so that the level is "centred". End-for-end the level on the flat base. Note the difference in the two vial readings. The "true"/"centred" setting is mid-way between the two vial "readings" - just put a bit of marking tape on the level and mark where the ends of the bubble should be - job done. "Quick and nasty"? Perhaps. Effective and quick? Sure is.

Beware that strong or varying light or temperature can "draw" the "bubble" off-centre/setting - quite significantly at times!! So if you have an error that seems improbable - check light and temperature. I've seen a level vial "drawn" by the sun in varying amounts if the level is left (static) out in the sun all day. Strong lighting in a shop can have a similar effect.
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05-25-2008, 01:17 PM
Hi Guys,

Thanks to all for your replies. We had a storm that knocked out our power and was (until today) unable to respond. Thanks for the various explanations and instructions regarding the level.

Much appreciated.