The question often arises regarding tolerances on Plain Plug and Ring Gages. The two main issues addressed in our calibration workshops center around:

2. Laboratory Tolerance Requirements: Complaints from people who say that The Calibration Laboratory calibrates and reports their gages using tolerances that are too tight or strict. Some of these companies do not hold their tolerances so tight that they feel they need a tolerance held to millionths of an inch.

Let's address these issues one at a time.

1. Regarding the tolerance requirements: Information for tolerances can be found in ANSI B1.2 or in The Machinery's Handbook. It is listed as American National Standard Tolerances for Plain Cylindrical Gages. The following chart is from the Machinery's Handbook (disclaimer: You should reference The Machinery's Handbook or the ANSI B1.2 standard. We do not recommend using this document as a final decision.)

         Size Range                                          Tolerance Class

Above     to and including         XX                X                Y                Z                ZZ                    

0.020            0.825                     .00002        .00004        .00007        .00010        .00020   

0.825            1.510                     00003        .00006        .00009        .00012        .00024

1.510            2.510                     .00004        .00008        .00012        .00016        .00032

2.510            4.510                     .00005        .00010        .00015        .00020        .00040

4.510            6.510                     000065      .00013        .00019        .00025        .00050

6.510            9.010                     00008        .00016        .00024        .00032        .00064

9.010            12.010                   00010        .00020        .00030        00040        .00080

 

2.    The calibration Laboratory meets or exceeds the requirements of one or more of these standards: ISO 10012, ISO/TS 17025, ANSI/NCSL-Z-540, etc. The Laboratory is required to calibrate your gages to a recognized standard. This requires them to calibrate your plain cylindrical gages to the ANSI B1.2 standard.

 

 

Indicator Calibration

 

Whether you are calibrating a Test Indicator, Dial Indicator or Electronic Indicator, one of your first checks should be a visual check of the contact point to identify any worn or "flat" spots on the contact point. A worn or "flat" contact point will cause the indicator to produce non-linear readings. It is a simple check but one often over-looked.

 

Non-linear readings can also be the result of improper contact point angles. Most test indicators are designed to use parallel to the measured surface. However, if you are using an Intertest Test Indicator, there is a 12° Angle required between the contact point and the work piece. If you are using a Best Test indicator there is a 10° angle required between the contact point and the work pierce. I strongly suggest you read any literature or special instructions that come with your indicator to determine what angle is required. If the literature is not available then check the catalogue or check with the manufacturer for proper specifications.