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Why are LM, SI, and TR tolerances no longer available?

Actually, the old tolerances for Grades LM, SI, and TR are still around—just changed slightly and renamed 
for the reasons below.

ISO has in recent years, recommended that any uncertainty of measurement be subtracted from stated 
tolerances.  This is to insure that delivered product is within specifications.
The old tolerance grades LM, SI, and TR were introduced in the 1950’s.  At that time manufacturing 
tolerances were commonly stated in catalogs and brochures.

Unfortunately, the uncertainty of measurement for these gages was larger than anticipated and was 
greater than the stated tolerances.   It was impossible to arrive at a zone of assured conformance.

When an angle gage was returned for recalibration, this problem was handled by creating “Used Block” 
tolerances.  These “Used” tolerances were the “manufacturing” tolerances plus the uncertainty of 
measurement.
Grade
"Manufacturing" 
 or "New Block"
Tolerances
Uncertainty of
Measurement
"Used Block"
Tolerances
LM
±0.2
0.6
±0.8
SI
±0.5
1.0
±1.5
TR
±1.0
3.5
±4.5

The “Used Block” tolerances for the old LM, SI, and TR grades are comparable (with allowances for 
rounding) to the new Reference, Calibration, and Working grade tolerances.
Old Grades
New Grades
Grade
"Used Block"
Tolerances
LM
±0.8
SI
±1.5
TR
±4.5
Grade
"Used Block"
Tolerances
Reference
±1.0
Calibration
±2.0
Working
±5.0
As we recently added the calibration of angle gage blocks to our accredited calibration services, we felt it 
was time to change the tolerances of our angle gages to bring them into line with current practices.  We 
believe these new tolerances will provide a more realistic reading of these gages when they are being used.

When purchasing new angle gage blocks to the Reference, Calibration, or Working grades, the new blocks 
will have readings that are less than the stated tolerances in our catalog to allow for our uncertainty of 
measurement.   But for these grades, we will no longer have “New Block” and  “Used Block” tolerances.

When older gage blocks made to the LM, SI, or TR grades are returned to us for recalibration, we will
continue to use the “Used Block” tolerances shown in in the tables above.

				
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©2008, Webber Gage Division, The L. S. Starrett Co, Cleveland, Ohio , 44145
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