Calibration procedure for calibration of External micrometer

1. Scope

The calibration procedure is applicable to calibration of external micrometer 0-25 mm, 25-50 mm, 50-75 mm and 75-100 mm.

The Procedure is applicable to micrometers based on screw gauge principle for measurement of external dimensions of least count.

Back to top

2.Purpose

To ensure that the micrometer produces consistently repeated specific measurement results and associated measurement uncertainty.

Back to top

3.Calibration Procedure

3.1 Principle

Accuracy is determined by comparison with calibrated slip gauges and wires. Flatness and parallelism of anvils is determined by calibrated optical flats (flatness l/10).

3.2 Following tests are recommended to be performed

1. Visual inspection

2. Ratchet functioning

3. Zero error

4. Repeatability

5. Flatness and parallelism of anvils

6. Accuracy of scale

After visual inspection the micrometer under calibration is clamped in micrometer clamping stand to prevent temperature changes on account of handling of micrometer during calibration.

The micrometer anvil, slip gauges (mike-check set), standard wires and optical flat are degreased, cleaned with benzene and lint free cloth and left on the surface plate for about two hours so that the temperature of all these is nearly the same.

3.3 Standards required

The measurement standards required for calibration of micrometer are given in table 1.

Table 1

3.4 Support Equipment

The support equipments required for calibration are given in table 2.

Table 2

3.5 Environmental Conditions

The laboratory has a controlled/uncontrolled environment given below:

Temperature (controlled) : 20 ± 2 °C

Relative Humidity (uncontrolled) : 50 ± 10%

Back to top

4. Methodology

4.1 Preliminary Operations

a) Physical condition of micrometer is visually examined for

1. • Ratchet function
   • Zero error
   • (scratch marks, dents etc.)
   • Status of markings of linear and circular scales

Ratchet functioning is checked by moving the ratchet and seeing that it does so smoothly. Zero check is done by moving the movable anvil close to the fixed anvil in contact position and seeing that zero reading is correct or, if not so, zero error is noted (setting of zero is recommended).

b) Acceptability for calibration:

1. • The micrometer having smooth wratchet function lapped & flat anvils, free of scratches, dents and chipped edges can be calibrated by this calibration procedure.

1. • The micrometer with only slightly damaged anvils can also be calibrated.

1. • In such cases (ii) The micrometer can be calibrated by using standard wires instead of slip gauges and optical flat.

c) Criteria for rejection

Any malfunctioning of wratchet and too much visual damage of anvils if any.

4.2 Measurement Procedure After receiving the micrometer, clean it with benzene/methane or any other cleaning reagent which do not leave any residue. Keep the micrometer for temperature stabilization. Standard slip gauges and optical flat which are used for the calibration of micrometer are also cleaned and kept for temperature stabilization.

Following points are considered

1. • Zero setting of the micrometer
   • Error in reading
   • Flatness of anvils
   • Parallelism between anvils
   • Error in scale

(a) Zero error

The zero error of micrometer is checked and if zero error is observed the same may be adjusted by adjusting the barrel to zero of the thimble if possible, zero error should be indicated in the calibration results and compensated for in the final report.

The error in scale is determined by measuring the slips of the following sizes:
2.5, 5.1, 7.7, 10.3, 12.9, 15.0, 20.0, 17.6, 20.2, 22.8 and 25 mm (IS 2967:1983)

(b) Flatness

The flatness error is checked by keeping the optical flat on each anvil under monochromatic light. A number of coloured interference fringes will be seen on their surfaces. The flat is adjusted in such a way so that minimum number of bands are obtained. For the specified tolerance on faltness, maximum four bands should be formed .Flatness is determined as n x l/2, where n is number of bands and l is the wavelength of the light used.

© Parallelism

Parallelism error between two anvils is tested by gripping optical parallels of three different thickness covering different positions of circular scale.
In each case the total number of fringes appearing on both anvils are counted together. Out of three cases, the parallelism assessment is done on the basis of maximum number of fringes.

The flat is placed between the measuring faces and adjusted in a way so that number of interference bands visible on one face are minimum. The number of bands on other face are counted which in general should not exceed 8, for 0-25 mm. The thickness of optical flats recommended are 12.5, 12.625, 12.750 and 12.875 mm.

Parallelism error between the anvils = N x λ/2

Where ‘N’ is the maximum number of fringes appearing on both anvils and λ is the wavelength of light used.

(d) The micrometer is mounted in the micrometer clamping stand.

(e) The temperature before starting the calibration is recorded on data sheet.

(f) The slip gauges are measured one by one in increasing order and then in the corresponding readings of micrometer (estimated to 1/5th division of circular scale are recorded on the data sheet. The same procedure is followed in the decreasing order and readings of micrometer are recorded on the data sheet. This forms one set of measurement. Two such sets of measurements are taken making total of four observations on each slip gauge.

(g) The temperature after completion of measurement is recorded.

4.3 Observations sheet Description of instrument and work assignment

• Name and address of customer
• Letter number and date
• File No. of case allotted
• Purpose (i.e. calibration of micrometer at 20°C)
• Standards and measuring equipment used
• Date on which the calibration was commenced
• Date on which the calibration was completed
• Calibration Report No………
• Date on which calibration report sent to dispatch section
• Date of return of instrument to the concerned section/customer

4.4 Results with uncertainty

EVALUATION of MEASUREMENT UNCERTAINTY 4.4.1 Type A measurement uncertainty (UA)

Where = 12.9001

Type A Measurement Uncertainty UA 4.4.2 Type B Measurement Uncertainty UB

(i)Error due to deviation of mean calibration temperature from 20ºC on account of different coefficients of linear expansions (α) of slip gauges and micrometer for the entire range

(ii) Error due to variation in temperature (± 0.6 ºC) for the entire range

(iii) Error due to standard slip gauges

as reported in the
calibration certificate of slip gauges

(iv) Error due to flatness and parallelism of anvils

(v)Error due to difference of temperature (2 ºC) between slip gauge and micrometer for the entire range,

Total Type B Uncertainty

Combined Uncertainty U

Expanded uncertainty at approximately 95% confidence level (k=2)

Results of calibration:

Maximum error in the entire range of micrometer

Uncertainty of measurement at 95% confidence level

4.5 Traceability

1. • Slip gauges used for calibration of micrometer are traceable to National Standard

1. • Mike check slip gauge set and optical flats are calibrated periodically. (Every two years) Against wavelength of light recommended for the realization of flatness parallelism and length of gauge blocks using interferometric method Or by comparison against interferometric calibrated slip gauges using gauge block comparator.

1. • International traceability is ensured through participating in international intercomparison for measurement of slip gauges by interferometreic method.

4.6 Precautions

1. • The micrometer and slip gauges and other aids should be thoroughly cleaned and wiped off with silk/linen cloth

1. • The system should be thermally soaked for sufficient time to ensure equalization of temperature

1. • Avoid rapid movement of wratchet while measuring the slip gauges

1. • Slip gauges and optical flat should be handled carefully to avoid scratches on these

1. • All the items should be stored properly after calibration.

1. • The items should not be handled with bare hands.

Back to top

5. Staff Authorized to Perform Calibration

As per authorization list

Back to top

6. Location

The measurement set up is located in Room No. ………..

Back to top

7. Calibration Certificate

As per document No………….

Back to top

8. List of Supporting Documents

IS:2967-1983 or ISO…………or BS………….DIN……………JIS…………..