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Machinist level specs seconds vs tenths/10"

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AGCB97

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#1
I'm looking at level specs of various levels.
Starrett gives 0.42 mm/m or 80 to 90 seconds
How does this compare to .0005/10"
My math seems to not work

Thanks
Aaron
 

RJSakowski

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#3
10 seconds of arc equals approximately .0005"/10" or .0006"/12". A second of arc is 1/3600 degrees. The sine of 1 second of arc is 4.848 x 10^-6 (.000004848) That times 10" is .00004848". For very small angles, the sine, tangent, and arc in radians (2π radians equals 360º) are close to equal so 10 seconds of arc = .00048"/10".
 

mikey

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#4
Per Starrett, Main level vials have graduations that are approximately 80-90 seconds or .005" per foot (0.42mm per meter)
 

AGCB97

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#5
Per Starrett, Main level vials have graduations that are approximately 80-90 seconds or .005" per foot (0.42mm per meter)
Should that be .0005" per foot?
 
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