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I know this is a pretty basic question but I am not able to get an answer over the internet. So my question is, how do you read a spirit level. I am talking about the Starrett Machinist Level (98-6 – 6 Inch Precision, 0.42 mm/m / 80-90 seconds Graduation and Sensitivity).

What does 0.42mm/m mean?

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  • Just to note, the level of accuracy this is designed to measure is for metalworking/machining, which deals in tolerances of thousandths or even ten-thousandths of an inch (0.0001"). This is never needed in woodworking on a furniture scale.....
    – Graphus
    Feb 11 at 5:28
  • It just hit me: "Spirit Level Reading" sounds like you've gone to a psychic!
    – FreeMan
    Feb 15 at 17:10

1 Answer 1

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From one of the resources on the internet, Little Machine Shop, this bit of wisdom:

Machinist level

These Starrett Machinist Levels have ground and graduated main vials, and a cross test vial. The 12" level also has a plumb vial. These vials are adjustable to a positive setting and are housed in a satin chrome brass tube with a friction-fit closing cover to prevent breakage.

The base of the precision machinists level features an involute groove running the length of the base, which provides a reliable seat for round work such as pipes or shafting. With the cross test vial, it is possible to simultaneously level in both directions. This prevents inaccuracies in the main vial reading caused by canting the level sidewise on round work.

The main level vials have graduations that are approximately 80-90 seconds or .005" per foot (0.42 mm per meter). There are five, six, or seven lines on each side of the bubble, depending on the base length. Starrett Machinist Levels are the finest levels available, used for precision work that is typically required in industry. They all have these features:

All level bases are made from the finest quality seasoned cast iron and are precision-machined on the reference surface. Nonmachined surfaces have an attractive, black wrinkle finish. An involute longitudinal groove between the bearing flats for accurate seating on round work. This groove has a special involute design, permitting better centering and greater capacity to handle larger rounds. Groove and bearing flats are machined together for maximum accuracy.

If you have the bubble centered on the level, it's level within 5 thousandths of an inch over 12 inches, which is astonishing to my alleged mind. One bubble mark off and your reference becomes 10 thousandths of an inch over the same 12 inches.

You asked about the 0.42 mm per meter, which is applied in the same manner. The distance is larger, one meter compared to one foot, and the corresponding precision figure is appropriately larger. I didn't perform the math, but I expect the two numbers are equivalent.

The 80-90 seconds reference is a measurement of degrees. 360 degrees per circle, 60 minutes per degree, 60 seconds per minute. Another astonishingly tiny reference, but machinists often work in tenths of a thousandths of an inch and require such precision.

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  • "I didn't perform the math, but I expect the two numbers are equivalent." Hehe. Superb Answer, +2 if I could.
    – Graphus
    Feb 11 at 5:29
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    The nice thing about the "missing math" is that you don't even need to do imprecide unit-conversions, just simple divisions! 5/12 = 0.41666 (continued to infinity, rounded to 0.42, thus 0.005 inch / 12 inches corresponds to the 0.42mm / m figure, only by knowing that 1m = 1000mm).
    – D. Kovács
    Feb 11 at 8:19
  • "finest quality seasoned cast iron" I didn't know that cast iron had to be dried before being used in order to prevent warping & checks. I wonder if they're kiln dried?
    – FreeMan
    Feb 11 at 17:36
  • @FreeMan Maybe they mean seasoned like a cast iron frying pan, to get that attractive, black wrinkle finish? Feb 11 at 19:26
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    @FreeMan, the traditional means to 'season' cast iron pieces after they were removed from the sand moulds was to just let them sit for an extended period of time for stresses to naturally dissipate — In some places this amounted to (literally!) just dumping them in the yard and forgetting about them for a couple of years. As you can imagine, as penny-pinching started to factor in more the time castings were left to sit decreased, to the point where eventually it wasn't done at all — and hence all the planes of more recent production that have distorted bases.
    – Graphus
    Feb 14 at 19:24

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