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CALIBRATION / RECALIBRATION & REPAIR SERVICES
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All our reference standards are calibrated directly by NIST.  Statistical methods are used to control and monitor all our
reference and transfer standards.  Our detailed documented histories allow us to give the lowest uncertainties possible.

Reported measurement uncertainties are based on a 95% confidence level (two standard deviations).  Factors included
in the stated uncertainty are derived from a detailed error analysis, and are based upon experimentation whenever
possible or industry consensus from estimates derived from NIST publications.  Experimental checks of the stated
uncertainties are made using laboratory comparisons involving both internal repeatability studies and external comparisons
with other calibration laboratories.
LINEAR GAGE BLOCKS
Commercial Calibration procedures are performed by direct comparison to master gage blocks of similar size.
Information about the calibration is recorded by our computer systems that includes the materials of the gage blocks,
pressure and tip radius of the comparator, the temperature of the gage blocks, and the environmental conditions during
the calibration.  Individual applied correction factors are recorded as well as the results of any calculations.  The
uncertainty is computed for each measurement based upon the current conditions during the calibration.  The validity of
the calibration is tested by comparing the standard deviation of three measurements against an expected standard
deviation derived from historical data.  Compare our commercial calibration pricing here.

Master Calibration procedures are also performed by direct comparison to master gage blocks, but a second master,
sometimes refereed to as a control block is also used in the process.  The purpose of the second master is to generate
known difference readings from the first master which are used to control the measuring process using SPC techniques.
The validity of the calibration is tested by comparing the standard deviation of three measurements against an expected
standard deviation derived from historical data as well as comparing the expected average and standard deviation of the
measurements of the control blocks.

Laboratory Calibration procedures are performed to achieve the lowest possible uncertainty of measurement, but
only those blocks with the best geometry (parallelism and flatness) and of Webber croblox® material may be tested.
Laboratory Calibrations are restricted to Webber rectangular style croblox® gage blocks of Webber grades LM or AA,
GGG Grades 0.5 and 1, and B89 Grades 00 and K.  When a test block is compared to one of our master blocks of the
same croblox® material, the correction factors for temperature and deformation are significantly smaller.   This greatly 
reduces the uncertainty of measurement.

Laboratory Calibration procedures are performed as three separate sets of Master Calibrations using different operators,
master gage blocks sets, and control blocks.  Each set of measurements must pass all the statistical tests required of a
Master Calibration.   The three sets of readings are averaged together to give the reported result. 

The table below gives a comparison of our uncertainties for the calibration procedures described
above.
.                    Uncertainties (in microinches, k=2)                           Uncertainties (in microinches, k=2)
            For gage blocks 4-inches and below in length          For gage blocks 5-inches and above in length
Type of
Calibration
Typical for
1-inch block
Minimum
or Best
  General Uncertainty
  Formula
Commercial
3.0 µin.
2.4 µin.
U = 2.0 + 1.0L
Master
2.0 µin.
1.8 µin.
U = 1.2 + 0.7L
Laboratory
1.5 µin.
1.4 µin.
U = 0.7 + 0.7L
Type of
Calibration
Typical for
10-inch block
  General Uncertainty
  Formula
Commercial
10.0 µin.
U = 5.0 + 0.35L
Master
7.0 µin.
U = 3.5 + 0.25L
Laboratory
n/a
n/a
.                    
                    Uncertainties (in micrometers, k=2)                           Uncertainties (in micrometers, k=2)
            For gage blocks 100mm and below in length          For gage blocks 125mm and above in length
Type of
Calibration
Typical for
25mm block
Minimum
or Best
  General Uncertainty
  Formula
Commercial
0.08 µm
0.06 µm
U = .05 + .001L
Master
0.05 µm
0.045 µm
U = .03 + .0007L
Laboratory
0.04 µm
0.035 µm
U = .018 + .0007L
Type of
Calibration
Typical for
250mm block
  General Uncertainty
  Formula
Commercial
0.25 µm
U = .13 + .00035L
Master
0.18 µm
U = .09 + .00025L
Laboratory
n/a
n/a
The actual uncertainty of measurement may vary due to the condition of the gage block being measured. 
Explanation of the Linear Gage Block Calibration Certificate
ANGLE GAGE BLOCKS and TRUE SQUARES
Calibrations are performed by direct comparison to master angle gage blocks of similar size using a pair of autocollimators 
and a fine rotary table.  First, the autocollimators are set using the master angle block. A "null" autocollimator reads the base 
of the angle block and the "read" autocollimator reads the hypotenuse or the angle of the angle block.  The angle block to be
calibrated is substituted for the master angle on the rotary table.  The rotary table is adjusted until the "null" autocollimator 
again reads zero.  The difference of the "read" autocollimator plus the calibrated deviation of the master angle is the error of
the test angle block. 
Grade of Angle Block
Dimensions of Angle Block
Best Uncertainty
Reference or Lab Master
1" x 2"
0.6 seconds
Calibration or Inspection
1" x 2"
1.0 seconds
Working or Tool Room
5/8" x 4"
3.5 seconds
The actual uncertainty of measurement may vary due to the condition of the angle gage block being measured. 
The flatness of the base and hypotenuse has a large effect on the uncertainty of measurement.
OPTICAL POLYGONS
Calibrations are performed by direct comparison to a master indexing table using an autocollimator. 
Best Uncertainty
1.0 Second

				
OPTICAL CUBES
Calibrations are performed by using the closure method.   Using autocollimators, each 90° angle of the cube is compared to
the other three 90° angles on the axis of rotation which by definition must equal exactly 360 degrees.
Best Uncertainty
0.6 Second

				
OPTICAL FLATS
Calibrations are performed by direct comparison to a master optical flat.   The flat to be calibrated is placed on a master flat 
and viewed with a monochromatic light source.  The interference pattern that is generated is interpreted by the operator to 
fractions of a bandwidth and converted to microinches or micrometers.
Flatness:  Best Uncertainty
General Uncertainty Formula
3.0 µin. or 0.08 µm
U(µin.) = 3.0 + 0.15 of Reading
U(µm) = .075 + 0.15 of Reading 
REFERENCE BARS AND WEBBER DIGI-CHEK II® HEIGHT GAGES
Calibrations are performed in the vertical position by direct comparison to a master gage stack using a precision indicator.
The stack to be calibrated is placed along side the master gage stack on a transfer table in a controlled calibration chamber.
The transfer table allows the stacks to be moved to a fixed indicator rather than moving the indicator to the stacks on a
surface plate.   

The transfer table is two parallel plates separated by closely matched ball bearings.  This allows for nearly parallel movement
of the top plate.  The surfaces of the transfer tables are calibrated using optical flats and held to less than a fringe band.  This
can be compared to the tolerance of the best surface plates which typically have flatness tolerances of 100 microinches 
(2.5 µm).   By using a transfer table, the error of movement of the stacks is reduced to only a few microinches or a fraction of
a micrometer. 

NOTE:  WHEN PREPARING AN UNCERTAINTY BUDGET WHEN USING THESE GAGES, PLEASE CONSIDER THE
UNCERTAINTY FROM:
       1)  OUR CALIBRATION OF THE GAGE, 
       2)  TEMPERATURE  (COEFFICIENTS OF EXPANSION OF MATERIALS AND TEMPERATURE GRADIENTS)
       3)  THE SURFACE PLATE BEING USED IN YOUR MEASUREMENTS.   
Typical Uncertainty
at 25 inches (625mm)
General Uncertainty Formula
75 µin.
1.88 µm
U(µin.) = 10 + 2.5L
U(µm) = .25 + .0025L 
MODEL 258 DIGI-CHEK Micrometer HEIGHT GAGES
Calibrations are performed in the vertical position by direct comparison to a master gage stack using a precision indicator.
The stack to be calibrated is placed along side the master gage stack on a surface plate in a controlled calibration chamber.
Best Uncertainty
100 µin. or  2.5 µm
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©2017, Webber Gage Division, The L. S. Starrett Co, Westlake, Ohio, 44145
INDEX
calibration
All new sets shipped from our factory are calibrated and provided with a Certificate of Calibration traceable to NIST. Webber Gage can recalibrate your linear gage blocks, angle gage blocks, optical polygons, optical cubes, optical flats, reference bars and height (step) gages. Our recalibration services provide you with low measurement uncertainty at a low cost! We not only recalibrate Webber Gage gage blocks, but also blocks manufactured by Mitutoyo, Brown & Sharpe, Fowler, Pratt & Whitney, Johannson, Ellstrom and many others. Additionally, we can recalibrate accessory pieces such as half round jaws, straight jaws and base blocks as well as special size gage blocks. Depending on usage, regular recalibration of gage blocks should be considered to maintain gage accuracy. To receive a preliminary quotation for recalibration service, e-mail us at sales@starrett-webber.com and remember to include the number of blocks, style (rectangular, square, etc.), material, if known, desired calibration grade and if there are any accessories requiring recalibration.

Repair services are available for: 258 DIGI-CHEK and DIGI-CHEK II height gages, Reference Bars, long blocks and angle gage blocks.