KURZUS: Finommechanika és optika a járműtechnikában

MODUL: I. modul

Lesson 1: Definitions related to measurement

| Purpose of learning this lesson: Definitions related to measurement are taught within the scope of the subject with the aim of understanding and mastering the basics of measurement.
To be mastered by necessity: A highlight of the lesson is the understanding of the different measurement methods and errors that occur during measurement and how to handle/correct them.
Degree of freedom of bodies
Visualisation of 3 main axes and 3 main planes using CAD software
Figure 1
Figure 1 illustrates the 3 main axes of the coordinate system and the 3 main planes defined by them. In general, in the field of machining, the upward pointing of the "z" axis is the positive direction of the vertical coordinate (as a counter-example, in the case of BIESSE woodworking machining centers, the positive direction is downward for the "z" axis). Although it is not standardized, it is advisable to define the positive direction of the "z" axis as upwards, following the example of the world's leading machine manufacturers, and also making the work of the technicians and engineers working on it easier.
The degree of freedom of bodies is 6 based on displacement along 3 principal axes (x, y, z) and rotation along 3 axes (x, y, z).
Degree of static determination: 3
Metrological division of measuring instruments:
Basic measuring instruments: used to set or check other instruments. Standards: they represent a fixed value of one or more quantities. International basic units: record the specified value of certain physical quantities. National basic units: a base unit that is the same as an international unit.
Metrological subdivision of measuring instruments:
Measuring instruments for use: used in production, development, research. Control instruments: measuring instruments used for production control. Verification measuring instruments: measuring instruments used for periodic checking of the accuracy of utility and verification measuring instruments.
Basic measurement concepts:
due to dimensional changes during thermal expansion, measurement should be made at 20 °C, the measurement must be carried out several times on the same component in the same position until at least two identical measurements are obtained.
Measurement errors
Absolute errors: the total difference between the exact and measured value can be positive or negative H abs = h exact − h meas Relative error: gives an idea of the accuracy of the measurement, the absolute error is compared to the correct value H rel = H abs − h exact Regular errors:
| 1. | their magnitude and sign can be determined,
| 2. | the measurement result can be corrected for this error,
| 3. | sources:
instrument errors (scale inaccuracy, heading error, zero point error, etc.), wear and tear of the measuring instrument, temperature variation. Accidental errors:
| 1. | occurring confusing errors,
| 2. | by taking a sufficient number of measurements and averaging the results appropriately, the most probable value of the measurement can be determined.
Gross errors:
| 1. | results from wrong or incorrectly chosen measurement,
| 2. | can be detected by taking several measurements.
Read out errors:
Reading error and incorrect measuring device use
Figure 2
Read out errors are quite common in the field of engineering measurement design. One of the sources is the incorrect positioning and reading of the measuring instrument, in the case of unexperienced measurers, but most often reading errors are introduced into our measurement for convenience, e.g. when reading the dial indicator, we do not read perpendicular to the scale, but below the scale, which introduces a measurement error into our measurement.
There are several problems with the measurement illustrated in Figure 2:
1. | the scale of the measuring device does not start from the end of the measuring device, but about 3 mm inwards, so knowing this value we have to correct our measurement (read 47 mm + 3 mm=50 mm).
2. | The scale on the measuring device is on the wrong side.
3. | In this case the reading will be incorrect even under an angle (see highlighted view A).
The correct reading, measurement method and use of the measuring device are illustrated in Figure 3:
Figure 3
"Disturbing" scale
Figure 4
In the Figure 4 drawing, which illustrates the manual body measurement of the Car-O-Liner company, students often encounter reading errors that can be attributed to serious inattention. The INCORRECT responses to the measurement were as follows:
3.2 cm, 32 cm, 2.8.
The correct answer is 28 mm. It is important to check the method of measurement, in this case the starting point of the scale and the location of the reading. In the engineering world, the pronounced number - e.g. 42 - is 42 mm. The most correct way is to use the unit of measurement, because when working in, for example, woodworking, the pronounced dimension is understood in cm - e.g. 3.2 means 3.2 cm, i.e. 32 mm.
Incorrect measure
Incorrect positioning of a measuring device.
Figure 5
During the measurement, it is extremely important to ensure the correct position of the measuring instrument on the piece to be measured. A reading on a gauge that is at an angle (Figure 5) will be incorrect. One of the most common measurement errors of this type occurs when measuring depth with a caliper. To ensure that the gauge is perpendicular to the bottom plane, this must be accomplished in one position on two main planes. As illustrated in Figure 5, in the front view (upper part of the figure), the gauge makes a wrong angle with the piece to be measured. In the top view of Figure 5 (lower part of the figure), the gauge also makes a wrong angle with the piece to be measured, so that the measurement result will be incorrect, i.e. more than the actual size is measured.
Incorrect position of measuring device on the piece to be measured
Figure 6
Figure 6 illustrates a gauge positioning where the position is now only one main plane out of alignment with the one illustrated in Figure 5.
Correct measuring
Correct use of a measuring device
Figure 7
The measurement illustrated in Figure 7 will show a true value due to the correct position of the measuring device. In this case, the measuring instrument is perpendicular to the two main planes.
Caliper
Their grouping:
Operating principle:
| 1. | mechanically operated, with vernier scale,
| 2. | master vernier caliper,
| 3. | digital caliper with capacitive transducer.
Measuring limit: 100, 125, 200, 300, 500, 750, 1000 mm. According to reading accuracy: 1/10, 1/20, 1/50 mm reading accuracy. By design:
| 1. | pocket calipers,
| 2. | twin-bladed fine-adjustment vernier calliper,
| 3. | height gauge.
Caliper parts
Caliper
Figure 8
1. fixed lower jaw for outside dimension, 2. moving lower jaw for outside dimension, 3. fixed upper jaw for inside dimension, 4. moving upper jaw for inside dimension, 5. depth rod, 6. vernier scale, 7. main scale, 8. thumb screw, 9. lock screw.
Pre-measurement (caliper)
1. | Check the measuring range of the caliper:
| 1. | 0-125,
| 2. | 0-150,
| 3. | 0-200,
| 4. | 0-300,
| 5. | etc.
2. | Testing the accuracy of the caliper (0.05mm is typical).
3. | Checking the operating fineness of the caliper (not fixed).
4. | Calibrate the caliper as required with an etalon.
Using a caliper
Measuring external dimensions: Use the caliper with your right hand. With our thumb finger, we move the moving mechanism while the piece to be measured is positioned between the measuring spikes. With our thumb, we gently touch the moving caliper to measure the external dimension. The correct position of the pusher on the piece must be completed in two planes.
Internal size measurement: In this case, the piece to be measured is measured with the measuring spout used for the inside dimension. With the inch gauge, the movable gauge is pushed against the piece to be measured, taking care to keep the gauge on the axis of symmetry and perpendicular to the hole, for example.
Depth measurement: When measuring depth, the depth gauge part of the caliper is lowered to the position to be measured. In this case, it is also important that the inclination of the caliper is completed simultaneously on the two main planes of the hole.
Reading the caliper
Caliper reading
Figure 9
When reading the main scale, we look at the position of the 0 value of the nonius scale. If the value 0 of the noun scale coincides with the value of the master scale, then it is an integer value. If the value of the nonius scale 0 does not coincide with the value of the master scale, then we look for a value on the nonius scale that does not coincide with a value on the master scale. In this case, the whole millimetre is read before the value 0 of the nonius scale and the decimal value is read from the nonius scale. As shown in Figure 9, 5mm is read on the master scale, while 0.05mm of the scale is read 0.05mm of the scale, so the dimension is 5.05mm.
Master vernier caliper
Master vernier caliper
Figure 10
When measuring internal dimensions with a master vernier caliper, it is important to add twice the value shown on jaws. The reading on the main scale refers to the measurement of the external dimension. When measuring the inside dimension, if the reading on the caliper is 62 mm as shown in the picture, this means 72 mm (Master scale: 62 mm+ fixed measuring jaw: 5 mm + moving measuring jaw: 5 mm = 72 mm).
Depth gauge
Depth gauge
Figure 11
The depth gauge's jaw is longer than of the caliper. Its thickness is 2-3 times than on the caliper, ensuring a stable fit on the surface, which increases positioning accuracy by an order of magnitude.
Self-checking questions:
1. What measurements can I take with a caliper?
2. Is it possible to measure with a caliper with high accuracy (0.01 mm)?
3. How many degrees of freedom does a body have?
4. What is the concept of a standard?
5. Do I have to take care about the correct position of the measuring device on the piece to be measured?
6. What unit of measurement is used in the "engineering" world?
7. What is the degree of static determination?