Outer diameter micrometer is often abbreviated as micrometer. It is a more precise length measuring instrument than vernier calipers. A common one is shown in Figure 2.4-1. Its range is 0-25 mm, and its graduation value is 0.01 mm. The structure of the outside micrometer consists of a fixed ruler frame, measuring anvil, micrometer screw, fixed sleeve, differential cylinder, force measuring device, locking device, etc. There is a horizontal line on the fixed sleeve. The line and the bottom have a row of scale lines with a spacing of 1 mm. The upper scale line is exactly in the middle of the two adjacent scale lines below. The scale line on the microtube is a horizontal line that divides the circumference into 50 equal parts, and it rotates.
According to the principle of spiral motion, when the micrometer cylinder (also known as the movable scale cylinder) rotates once, the micrometer screw advances or retreats by a pitch of 0.5 mm. In this way, when the micrometer cylinder is rotated by one division, it has rotated 1/50 of a turn. At this time, the screw moves along the axis by 1/50×0.5 mm = 0.01 mm. Therefore, the value of 0.01 mm can be accurately read by using a micrometer.
Zero calibration of outside micrometer
When using the micrometer, first check whether its zero position is calibrated, so first loosen the locking device to remove oil stains, especially the contact surface between the measuring anvil and the micrometer screw should be cleaned. Check whether the end surface of the microtube coincides with the zero scale line on the fixed sleeve. If it does not coincide, first turn the knob until the screw is close to the anvil, and rotate the force measuring device. When the screw just touches the anvil, a click will be heard. Click sound, then stop rotating. If the two zero lines still do not overlap (the sign that the two zero lines overlap is: the end of the microtube coincides with the zero line of the fixed scale, and the zero line of the movable scale coincides with the horizontal horizontal line of the fixed scale), the fixed casing The small screw on the upper part is loose. Use a special wrench to adjust the position of the casing so that the two zero lines are aligned, and then tighten the small screw. The zero adjustment methods of micrometers produced by different manufacturers are different. Here is only one of the zero adjustment methods.
When checking whether the zero position of the micrometer is calibrated, the screw must be in contact with the measuring anvil. Occasionally, the backward rotation of the force measuring device may not separate. At this time, the left hand palm can be used to force the left side of the measuring anvil on the ruler frame, and the right hand palm against the force measuring device, and then use your fingers to rotate the knob counterclockwise to separate the screw from the measuring anvil.
The reading of the outside micrometer
When reading, first take the end surface of the microtube as the guideline, read the graduation value of the scale line under the fixed casing (only integers in millimeters are read out), and then take the horizontal horizontal line on the fixed casing as the reading standard Line, read the division value on the movable scale, the reading should be estimated to one-tenth of the smallest scale, that is, 0.001 mm. If there is no upper scale line between the end face of the microtube and the lower scale line of the fixed scale, the measurement result is the value of the lower scale line plus the value of the movable scale; for example, there is an upper scale line between the end face of the micrometer tube and the lower scale line , The measurement result should be the value of the lower scale line plus 0.5 mm, plus the value of the movable scale, as shown in Figure 2.4-2, the reading is 8.384 mm, and the reading in Figure 2.4-3 is 7.923 mm.
The movable scale of some micrometers is divided into 100 equal divisions, and the pitch is 1 mm. The fixed scale does not require a half-millimeter scale, and each division of the movable scale still represents 0.01 mm. Some micrometers have a movable scale of 50 equal divisions, but there is no half-millimeter scale on the fixed scale, so it can only be estimated with the eye. For the micrometer with zero error eliminated, when the front face of the micrometer tube is exactly in the middle of the two lines of the scale line under the fixed scale, if the reading of the movable scale is between 40-50, the front edge does not exceed 0.5 mm, and the scale is fixed It is not necessary to add 0.5 mm to the reading; if the reading on the movable scale is between 0-10, the front end has exceeded half of the two adjacent scale lines of the lower scale, and 0.5 mm should be added to the fixed scale.
Judgement of zero error of outside micrometer
After the calibrated micrometer is in contact with the measuring anvil, the zero line on the movable scale should be aligned with the horizontal line on the fixed scale, as shown in Figure 2.4.4. If it is not aligned, a systematic error will occur during the measurement-zero error. If zero errors cannot be eliminated, their influence on the readings should be considered. If the zero line of the movable scale is above the horizontal line, and the x-th scale line is aligned with the horizontal line, it means that the reading during measurement is x.100 mm smaller than the true value. This zero error is called negative zero error, such as As shown in Figure 2.4-5B, its zero error is -0.03mm; 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, it means that the reading during measurement is better than The true value is y.100mm larger. This kind of zero error is called positive zero error, as shown in Figure 2.4-5 C, its zero error is +0.05 mm.