CP-18 RULES & TAPE MEASURES
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STEEL RULES & TAPE MEASURES

 

A rule is one of the simplest direct reading measuring instruments. They are usually used where accuracy requirements are not greater than 1/64th inch. There are rules with .010 inch graduations, but they are very difficult to use because the divisions are difficult to see. Most people will have enough trouble using the 1/64th scale.

 

 

Other than obvious wear or damage, there cannot be much wrong with a rule. Therefore, one would assume that periodically inspecting for obvious wear and damage would be sufficient for recalibration. At our lab, the old calibration procedure read as such.

 

Our sponsoring activity (a branch of the military) didn’t see it that way. They issued a directive that rules needed to be “calibrated” or inspected for error in the scale. How to accomplish this in a simple manner was a vexing problem. While attending a Mitutoyo gage calibration seminar, the instructor refused to even entertain a question concerning the issue. That was a missed opportunity for Mitutoyo.

 

One obvious way to calibrate a rule would be to use the surface reflection function on a contour projector to verify the accuracy of the scale divisions. This process would be feasible if only a limited number of rules needed to be calibrated. In our case, the amount of rules were too numerous to be handled in this manner.

 

Finally, a very simple solution occurred to us. A one meter long heavy steel rule was purchased from Starrett. The rule was graduated in 1/64th and 1/32nd of an inch on one side. The other side had metric divisions. Starrett was more than pleased to sell us such a rule for approximately $150.00. They provided a serial number and a NIST traceable certificate of accuracy for another $75.00 per calibrated edge. Total cost was $450.00.

 

The certified rule is used as a comparison standard to visually verify rules needing “calibration”. The technician visually inspects for any obvious deviation between the standard and the rule undergoing “calibration”.

 

The same method is used to inspect tape measures. 16 foot and 100 foot certified and serialized tape measures were purchased from Starrett.

 

A couple of years later at another Mitutoyo gage calibration seminar, the same instructor was advising the use of the same method that we came up with independently. The procedure withstood military and ISO-9000 audits.

 

Accuracy for rules was established as “no visible error”. The scale error for tapes is not to exceed 1/32nd of an inch.

 

More detailed information can be found in the following sources:

 

Federal Specification for Rules GGG-R-791

Federal Specification for Tapes GGG-T-106

 

The last section of the procedure covers rules that have had a line specially scribed at a particular inspection point on a part. The scribed line helps the part inspector because the inspector no longer has to “read” the scale. Since there is only one calibration point, a contour projector could have been used as optional calibration equipment.

 

 

CALIBRATION PROCEDURE – HCJ MACHINE COMPANY

 

PROCEDURE NO.

CP-18

 

SUBJECT:                                   STEEL RULES AND TAPE MEASURES                                                                                                                              

 

 

PROCEDURE

 

1.0    REFERENCE DOCUMENTS

 

1.1    FEDERAL SPECIFICATION GGG-R-791H (RULES)

 

1.2    FEDERAL SPECIFICATION GGG-T-106F (TAPES)

 

1.3    CALIBRATION EQUIPMENT ACCURACY LIST.

 

2.0    MATERIALS, EQUIPMENT REQUIRED

 

2.1    CALIBRATION STANDARD (RULE)

 

2.2    CALIBRATION STANDARD (TAPE)

 

2.3    GAGE BLOCKS

 

2.4    MAGNIFYING GLASS (IF APPLICABLE)

 

2.5    TOOLMAKER'S ANGLE PLATE

 

3.0    PROCEDURE

 

NOTE:  WHEN COMPARING A MACHINIST'S STEEL RULE TO THE CALIBRATION

             STANDARD, THERE SHOULD BE NO VISIBLE SCALE ERROR. FOR STEEL TAPES,

             THE SCALE ERROR SHOULD NOT EXCEED 1/32 INCH.

 

3.1    VISUALLY INSPECT SUBJECT GAGE FOR OBVIOUS DAMAGE OR EXCESSIVE WEAR

         THAT MIGHT AFFECT THE FUNCTION OF THE GAGE.

 

3.2    ASCERTAIN THAT THE IDENTIFICATION DATA IS CORRECT.

 

STEEL RULES

 

3.3    PLACE A TOOLMAKER'S ANGLE PLATE ON A SUITABLE SURFACE PLATE.

 

3.4    PLACE THE CALIBRATION STANDARD (RULE) ON THE SURFACE PLATE WITH ONE

         END AGAINST THE ANGLE PLATE.

 

3.5    PLACE THE RULE TO BE CALIBRATED AGAINST THE CALIBRATION STANDARD

         (RULE) WITH THE END OF THE RULE AGAINST THE ANGLE PLATE.  THE

         APPLICABLE GRADUATIONS OF THE STANDARD SHOULD BE ALONG THE SIDE OF

         THE RULE.

 

3.6    COMPARE THE GRADUATIONS ON THE RULE TO THE GRADUATIONS ON THE

         CALIBRATION STANDARD.  USE A MAGNIFYING GLASS IF NECESSARY.

 

HOOK RULES

 

3.7    CHECK THE "HOOK" TO INSURE THAT IT IS SECURE.

 

3.8    HOOK THE END OF THE HOOK RULE OVER THE END OF THE CALIBRATION

         STANDARD (RULE).

 

3.9    COMPARE THE GRADUATIONS OF THE HOOK RULE TO THE GRADUATIONS OF THE

         STANDARD.

 

STEEL TAPES METHOD NO. 1

 

3.10  HOOK THE END OF THE TAPE OVER THE END OF THE CALIBRATION STANDARD

        (RULE).

 

3.11  COMPARE THE GRADUATIONS OF THE TAPE TO THE GRADUATIONS OF THE

         STANDARD.

 

NOTE: THE ABOVE TWO STEPS CHECK THE ACCURACY OF THE TAPE WHEN USING THE

            HOOK. IT IS ONLY NECESSARY TO INSPECT THE FIRST PORTION OF THE TAPE

            TO VERIFY THE ACCURACY OF THE HOOK. THE REST OF THE TAPE IS

            INSPECTED AS FOLLOWS.

 

3.12  PLACE A SUITABLE ANGLE PLATE ON THE FLOOR.

 

3.13  PLACE THE END OF THE CALIBRATION STANDARD (TAPE) AGAINST THE ANGLE

        PLATE.

 

3.14  PLACE THE END OF THE TAPE TO BE CALIBRATED AGAINST THE ANGLE PLATE

         WITH ITS GRADUATIONS ALONG SIDE OF THE CALIBRATION STANDARD (TAPE).

 

3.15  COMPARE THE GRADUATIONS OF THE TAPE TO THE GRADUATIONS OF THE

         STANDARD.

 

TAPES METHOD NO. 2 (OPTIONAL METHOD FOR STEPS 3.12-3.15)

 

3.16  PLACE A TOOLMAKER'S ANGLE PLATEON A SUITABLE SURFACE PLATE.

 

3.17  PLACE THE CALIBRATION STANDARD (RULE) ON THE SURFACE PLATE WITH ONE

         END AGAINST THE ANGLE PLATE.

 

3.18  PLACE THE TAPE TO BE CALIBRATED AGAINST THE CALIBRATION STANDARD

        (RULE) WITH THE END OF THE TAPE AGAINST THE ANGLE PLATE.  THE APPLICABLE

        GRADUATIONS OF THE STANDARD SHOULD BE ALONG THE SIDE OF THE TAPE.

 

3.19  COMPARE THE GRADUATIONS ON THE TAPE TO THE GRADUATIONS ON THE

         CALIBRATION STANDARD.

 

3.20  ONCE THE FIRST THREE (3) FEET OR ONE (1) METER OF A TAPE HAS BEEN

         CALIBRATED, CALIBRATE THE REMAINING TAPE AS FOLLOWS:

 

3.21  PLACE THE CALIBRATION STANDARD (RULE) ON THE SURFACE PLATE.

 

3.22  PLACE THE TAPE TO BE CALIBRATED AGAINST THE STANDARD.  1 FOOT OR

        200mm OF THE PREVIOUSLY CALIBRATED PORTION OF THE TAPE SHOULD BE

        INCLUDED AS A REFERENCE POINT.

 

3.23  COMPARE THE GRADUATIONS OF THE TAPE TO THE GRADUATIONS OF THE

         STANDARD.

 

3.24  REPEAT STEPS 3.22 THROUGH 3.23 UNTIL THE TAPE IS COMPLETELY

         CALIBRATED.

 

STEEL RULES MARKED WITH SCRIBED LINES

 

3.25  THE FOLLOWING STEPS ARE FOR RULES THAT ARE SPECIALLY MARKED WITH

         SCRIBED LINES INDICATING A PARTICULAR INSPECTION POINT .

 

3.26  BUILD UP A GAGE BLOCK STACK TO THE SPECIFIED DIMENSION.

 

3.27  PLACE A SUITABLE ANGLE PLATE ON A SUITABLE SURFACE PLATE.

 

3.28  PLACE THE RULE VERTICALLY AGAINST THE ANGLE PLATE.

 

3.29  PLACE THE GAGE BLOCK STACK AGAINST THE RULE AND COMPARE TO THE

        SCRIBED LINE.

 

3.30  ACCEPT/REJECT ACCORDINGLY.

 

3.31  UPDATE GAGE RECORDS.

 

 

APPROVAL

REV.

X

 

 

 

 

CP-18

INITIALS

DATE:

X-XX-XX

 

XXX

 

 

SHEET  X OF  X