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  • Starrett level calibration?

    I recently purchased a used 12" precision Starrett machinist level from a pawn shop in my area. The level has adjustments how do you know if it is accurate? Is there a way to calibrate it or do I need to send it off to have it calibrated?

    Thanks Mike
    Last edited by gundog; 12-18-2012, 03:48 AM. Reason: spelling

  • #2
    A level is the easiest of tools to test and calibrate. You just take two readings on a reasonably solid and flat surface that you can adjust and that will remain the same between the readings. First place it in one direction and note the position of the bubble. Then turn it end for end and place it in exactly the same position on the flat surface and take a second reading. The bubble will probably be off by different amounts in the two opposite directions readings. You then try to adjust the surface to bring the two readings to as small an amount off center as possible AND adjust the level to make them equally off on both sides. This will both level the surface and calibrate the level at the same time. Repeat these readings and adjustments until the bubble is dead center when read both ways without any further adjustment.

    Of course, with a precision level, getting a solid and flat enough surface can be difficult. For instance, if you are on a workbench on a wood floor, you will easily read changes in the bench due to your body movements, even breathing. So take care in setting up to do this.
    Paul A.
    SE Texas

    Make it fit.
    You can't win and there is a penalty for trying!

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    • #3
      Thanks Paul that makes a lot of sense. I think the flattest thing I have is my mill table or possibly some plate glass I think I have read on here before about how flat plate glass is. My shop floor is concrete.
      Last edited by gundog; 12-18-2012, 04:35 AM.

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      • #4
        For calibrating my levels I have a piece of 1-1/2" square that I set up level in the milling vise and then took a skim cut off one face. It should be long and wide enough to fully support the largest level you're going to be working with. And don't use cold finish material as even a very light cut can relieve the internal stresses and leave you with a curved piece of stock. Once you've cleaned up the bar place the level on it and adjust as Paul suggests. By having it clamped snugly in the vise you have pretty precise control when you're bringing the whole assembly up to true. Once the level is properly adjusted set the bar aside and use it whenever you feel the need to make adjustments. If you got it "flat" the first time there's no need to re-machine each time you use it...
        Keith
        __________________________
        Just one project too many--that's what finally got him...

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        • #5
          A mill table is probably a good choice as the mill will probably have levelers for adjusting it level. But do not assume that a mill table or any other surface is actually flat. Do test it. See if you get consistent readings if the level is moved slightly on it. Also, DO lock down all table movements.

          Also be meticulously clean. An oil film can be several tenths thick and any dust will have a random size.

          Taking multiple readings and checking for consistency is your best approach here.
          Paul A.
          SE Texas

          Make it fit.
          You can't win and there is a penalty for trying!

          Comment


          • #6
            Mike --

            Paul and Keith have given you good, succinct, advise. I'm more long-winded, so here's the "adjustment" part of an essay I wrote a decade or so ago on the old Chaski bulletin board, describing how to replace and adjust the vial of a Starrett 98 millwright level:

            You can adjust the vial assembly relative to the iron body by "reversion", essentially checking the vial against itself. To do this, you need a stable surface plate that is nearly level. How level? Ball-bearing-or-glass-marble-won't-roll-off level is plenty close enough.

            Within reasonable limits, temperature is not important but temperature stability is very important. You can not successfully adjust (or use) a precision level if it is in a beam of sunlight, near a blower vent, or if you've had your fingers on the vial recently. Got that?

            Now, set your level instrument on the surface table and adjust the vial housing until the bubble is centered in the vial. Finger tight on the adjustment screws is good.

            Next, mark the location of the level body on the surface table using a pencil or build a set of fence rails that will provide a nest for the level body. Go get yourself a cup of coffee or a cold soda, giving the level and surface table a half hour or so to come to temperature equilibrium.

            Again examine the position of the bubble in the vial; adjust the vial assembly to nearly center the bubble if necessary. Give the bubble a few moments to settle, and then carefully note the position of the bubble in the vial. Some people note the bubble position in room coordinates ("a quarter division toward the west wall") while others use instrument coordinates ("a quarter division toward the adjusting screw"); pick a method, and call the bubble position in the vial the Initial position.

            Now pick up the whole level and turn it 180 degrees. Set it down on the surface table and either nest it against the fences or adjust it to be within the pencil outline. Give the bubble a few moments to settle in the tube. Now examine the bubble to see where it is in the tube, which we’ll call the Rotated position.

            If the bubble is in the same place relative to the room ("a quarter division toward the west wall" OR in the "opposite" place relative to the instrument body ("a quarter division away from the adjusting screw"), the vial-to-base adjustment is correct. In this fortunate circumstance, all you need to do is snug the vial adjustment screws without moving the bubble.

            If, more probably, the bubble is not in the right place, you'll need to adjust the vial housing. Use the adjustment screws to move the bubble all the way to its "should be" position, which is midway between the Initial and Rotated positions, finger snug the adjustment screws, and go through the whole test-and-adjust cycle again. Keep at it, and you'll get it right.

            For what it's worth, more sensitive level instruments usually have a side-to-side vial adjustment in addition to the up-and-down adjustment. This adjustment allows you to rotate the vial so that its lengthwise axis is parallel to the long edge of the instrument's body. Why? Well, if the vial is not parallel to the long edge of the body the bubble will change its lengthwise position in the vial with small fore-and-aft tilts of the instrument.

            John

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            • #7
              Originally posted by John Garner View Post
              For what it's worth, more sensitive level instruments usually have a side-to-side vial adjustment in addition to the up-and-down adjustment. This adjustment allows you to rotate the vial so that its lengthwise axis is parallel to the long edge of the instrument's body. Why? Well, if the vial is not parallel to the long edge of the body the bubble will change its lengthwise position in the vial with small fore-and-aft tilts of the instrument.

              John
              I would like to add that many precision levels have a side to side *vial* to ensure the surface is level in both directions.

              When calibrating however, you should rotate the level 90 degrees when leveling the surface and use the master vial to ensure the surface is 'level' in both directions before adjusting the calibration of the level itself.

              If its not level in both directions, rotating it 179 degrees instead of 180 degrees (or any other small random amount of error) will cause a small error in the reading due to the slant on the other axis.

              And yes, Do wipe everything down and consider that just the heat of you touching the level can distort it enough to throw readings off.

              I have heard of people shining a halogen lamp on a precision level and watching the bubble move.
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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              • #8
                The surface you are setting the level up on does not have to be level. What you are doing is getting the level to read the same from both directions. Once it read s the same both ways it is properly "leveled". You are calibrating the level, not the piece it is setting on.

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                • #9
                  Originally posted by kf2qd View Post
                  The surface you are setting the level up on does not have to be level. What you are doing is getting the level to read the same from both directions. Once it read s the same both ways it is properly "leveled". You are calibrating the level, not the piece it is setting on.
                  That is certainly true but the test surface has to be level enough for the bubble to be readable. For a high precision level, that is pretty close to level. This also assumes that the vial is uniform on both ends, which is true for a quality level.

                  The sensitivity of the level can be checked by placing narrow shims under one end at the time.
                  Don Young

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                  • #10
                    Originally posted by John Garner View Post
                    Mike --


                    Now, set your level instrument on the surface table and adjust the vial housing until the bubble is centered in the vial. Finger tight on the adjustment screws is good.

                    Next, mark the location of the level body on the surface table using a pencil or build a set of fence rails that will provide a nest for the level body. Go get yourself a cup of coffee or a cold soda, giving the level and surface table a half hour or so to come to temperature equilibrium.


                    John
                    I'm going to disagree with John here. I used to travel with precision levels and the first thing you do is check the level for calibration. Go through the setup John describes for locating the level on a surface. Then rest the level and allow it to come to temp then rotate the level 180 and see if you get the same reading. If you do not get the same reading then go through the procedure. Do not start out by loosing the adjustments, that is step two (or 4).

                    As for needing a level surface, it is not required. It only has to be level enough to float the bubble. If you set the level on a surface and it reads two marks to the right in either rotation then your calibrated.

                    lg
                    no neat sig line

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                    • #11
                      I'll nod in complete agreement with larry_g's point that adjustment features should not normally be loosened before checking vial adjustment. The excerpted portion of my narrative was specifically addressing initial adjustment of a vial-and-carrier assembly after installation of a new vial, and therefore does not exactly correspond with the situation Mike (the OP) asked about.

                      John

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