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A brief introduction to the inner and outer diameter micrometer of steel pipes

The outer diameter micrometer and inner diameter micrometer are the most used in the production and inspection process of steel pipes, precision steel pipes and seamless steel pipes. Today we will talk about the inner and outer diameter micrometers.

1. Outside diameter micrometer

Micrometer outside diameter often referred to as micrometer, it is more precise than the vernier caliper length measuring instrument, a common one as shown above, its range is 0-25 mm, indexing value is 0.01 mm. The structure of the outer diameter micrometer is composed of a fixed frame, an anvil, a micrometer screw, a fixed sleeve, a differential tube, a force measuring device, a locking device, and the like. There is a horizontal line on the fixed casing. There is a scale line with a distance of 1 mm on this line and the lower line. The upper scale line is exactly in the middle of the next two adjacent scale lines. The tick mark on the differential cylinder is a horizontal line dividing the circumference into 50 equal parts, which is a rotational movement.

According to the principle of spiral motion, when the differential cylinder (also called the movable scale cylinder) rotates one revolution, the micrometer screw advances or retreats by one pitch – 0.5 mm. In this way, when the differential cylinder rotates one division, it rotates for 1/50th of a turn, and then the screw moves along the axis by 1/50 x 0.5 mm = 0.01 mm. Therefore, the value of 0.01 mm can be accurately read by using a micrometer.

Zero calibration of the outer micrometer
When using a micrometer
First, check whether the zero position is calibrated. Therefore, first loosen the locking device to remove oil, especially the contact surface between the anvil and the micrometer screw should be cleaned.
Check that the end of the differential cartridge coincides with the zero mark on the fixed sleeve.
If not, rotate the knob first until the screw is close to the anvil. Rotate the force measuring device. When the screw just touches the anvil, you will hear rattle. At this moment, stop rotating.
If the two zero lines are still not coincident (the two zero line coincidence marks are: the end of the differential cylinder coincides with the zero line of the fixed scale, and the zero line of the movable scale coincides with the horizontal line of the fixed scale),
The small screws on the fixed sleeve can be loosened. Use a special wrench to adjust the position of the sleeve so that the two zero lines are aligned and tighten the small screws.
The zero adjustment method for micrometers produced by different manufacturers is different. This is just one method of zero adjustment.
When checking whether the zero position of the micrometer is calibrated, the screw and the anvil must be in contact with each other. Occasionally, the backward rotation of the force measuring device does not occur. At this time, you can use your left hand to press firmly against the left side of the anvil on the scale, the right hand against the force measuring device, and then turn the knob counterclockwise with your finger to separate the screw from the anvil.

External micrometer operating method

1) Check the zero point before use:
Slowly turn the trim knob D’ so that the rod (E) and the anvil (A) are in contact until the ratchet sounds. At this time, the zero line on the movable ruler (moving sleeve) should be on the fixed sleeve. The baseline (horizontal line) is aligned, otherwise there is zero error.
2) Hold the scale (C) with your left hand, turn the coarse adjustment knob D with your right hand so that the distance between the measuring rod E and the measuring anvil A is slightly larger than the measured object, put the measured object, and turn the protection knob D’ to clamp the measured object until When the ratchet sounds, turn the fixed knob B to fix the rod and read it.

Outer micrometer reading

When reading, first use the end face of the differential cylinder as the guideline, read the graduation value of the scale line under the fixed casing (only integer in millimeters).
Then use the horizontal line on the fixed casing as the reading guideline to read out the index value on the movable scale. When reading the reading, one tenth of the minimum scale should be read, ie 0.001mm.
If there is no upper scale line between the end face of the differential cylinder and the lower scale mark of the fixed scale, the measurement result is the value of the lower scale line plus the value of the movable scale; if there is an upper scale line between the end face of the differential cylinder and the lower scale line The measurement result should be the value of the lower scale plus 0.5 mm, plus the value of the movable scale.
1, read the fixed scale
2. Read the half scale again. If the half scale line is exposed, note 0.5mm; if the half scale line is not exposed, record it as 0.0mm;
3, read the movable scale again (note the reading). Noted as n × 0.01mm;
4. The final reading result is fixed scale + half scale + movable scale + estimated reading
Since the reading of the spiral micrometer is accurate to millimeters in mm, the spiral micrometer is also called a micrometer.

Determination of zero error of outer micrometer

With the calibrated micrometer, when the micrometer screw is in contact with the anvil, the zero line on the movable scale and the horizontal line on the fixed scale should be aligned. If there is no alignment, a systematic error will occur during the measurement – zero error. If you cannot eliminate zero errors, you should consider their effect on the readings. If the zero line of the movable scale is above the horizontal line and the xth scale line is aligned with the horizontal line, it means that the reading of the measurement is smaller than the actual value by x.100mm. This zero error is called negative zero error; if The zero line of the movable scale is below the horizontal line, and the y-th scale line is aligned with the horizontal line, which means that the reading of the measurement is larger by y.100 mm than the actual value. This zero error is called positive zero error. > For micrometers with zero errors, the measurement result should be equal to the reading minus zero error, which is the object length = fixed scale reading + movable scale reading – zero error.

Micrometer precautions

1. The micrometer is a precision measuring instrument. Use caution when it is used. Be gentle. Do not let it hit and collide. The thread inside the micrometer is very precise. When using it, pay attention to: 1 The knob and the force measuring device can’t use too much force when they are turning; 2 When turning the knob to make the micrometer screw close to the object to be tested, it must be changed to the force measuring device and can’t rotate. The knob makes the screw press on the object to be measured; 3 When the micro screw and the anvil have jammed the object to be tested or tighten the locking device, the knob must not be forcibly turned.
2. Some micrometers are equipped with a thermal insulation device on the scale to prevent the manual temperature from causing the frame to expand. During the experiment, you should hold the heat insulation device and touch the metal part of the foot frame as little as possible.
3. When using a micrometer to measure the same length, it is usually repeated several times and at different positions, and the average value is taken as the measurement result.
4. When reading, pay attention to whether the mark on the fixed scale representing half a millimeter has been exposed.
5. When reading a reading, the one thousandth digit has an estimated reading number and cannot be easily thrown away. Even if the zero point of the fixed scale is exactly aligned with one of the scale marks of the movable scale, the “000” should be read as “0”.
6. When the small anvil and the micrometer screw are brought together, the zero point of the movable scale and the zero point of the fixed scale do not coincide and a zero error will occur. The zero error should be corrected. That is, the zero error value is removed from the reading of the last measured length.

Micrometer use maintenance

1. Check if the zero line is accurate;
2. The workpiece must be wiped clean when measuring;
3. When the workpiece is large, it should be measured on a V-shaped iron or flat plate;
4. Clean the measuring rod and anvil before measuring;
5. Use a ratchet device when tightening the sleeve;
6. Do not loosen the back cover to avoid causing the zero line to change;
7. Do not add ordinary motor oil between the fixed sleeve and the movable sleeve;
8. After use, wipe oil, put it in a special case, and put it in a dry place.

Inner diameter micrometer
Inner diameter micrometer
As shown, it is generally similar to the outer diameter micrometer. So don’t make too much introduction.
Measurement methods
Inner diameter micrometer for precision measurement of inner dimensions (single unit and post)
Correct measurement method
1) When the inner diameter micrometer is used for measurement and use, it must be connected with the micrometer head with the largest size of the post, and then be connected to the measuring contact in order to reduce the bending of the axis after connection.
2) The amount of change in the micro-head fixing and releasing should be observed during the measurement.
3) In daily production, when the hole is measured with an inner diameter ruler, the measurement surface of the measurement contact is supported on the surface to be measured, and the differential tube is adjusted so that the measuring surface on the side of the differential tube swings within the radial section of the hole. The minimum size. Then tighten the fixing screws and take out the readings. There are also direct readings without tightening the screws. This poses a problem of attitude measurement. Attitude measurement: The consistency between measurement and use. For example, when measuring 75-600/0.01mm inner diameter ruler, when connecting the long bar to the micrometer, the dimension is larger than 125mm. The difference between the tightening and the tightening of the fastening screws is 0.008 mm, which is not only the attitude measurement error.
4) The bearing position should be correct when measuring with an internal micrometer. The long dimension of the internal diameter after the length of gravity deformation, involving the straightness, parallelism, verticality and other geometric errors. The size of its stiffness can be specifically reflected in “natural deflection.” Theoretical and experimental results show that the stiffness determined by the cross-sectional shape of the workpiece has a great influence on the gravity deformation after the support. For example, if the length L of the inner diameter ruler with different cross-sectional shapes is the same, the error of the measured value of the inner diameter ruler can meet the requirement when it is supported at (2/9)L. However, the bearing points are slightly different and the change in straightness is larger. Therefore, the change in the straightness when the support position is moved to the maximum support distance in the national standard is called “natural deflection”. In order to ensure rigidity, in China’s national standards it is specified that the bearing point of the inner diameter ruler should be at (2/9)L and 200 mm from the end face, that is, the minimum amount of change in the measurement. And check the inner diameter ruler every 90°
Once measured, the indication error should not exceed the requirements.

Error Analysis

Error Analysis

The direct measurement error of the internal diameter ruler includes the force deformation error, temperature error, and indication error of general measurement, reading aiming error, contact error, and zero error of the length measuring machine. Influencing the measurement error of inner diameter ruler, the main factors are the force deformation error and temperature error.
Source: China Seamless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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