Classification of measuring instruments
A measuring instrument is an instrument with a fixed form that is used to reproduce or provide one or more known values. According to different uses, measuring instruments can be divided into the following categories:
1. Single-value measuring instruments
Measuring instruments that can only reflect a single value. They can be used to calibrate and adjust other measuring instruments or directly compare with the measured value as a standard, such as gauge blocks, angle gauge blocks, etc.
2. Multi-value measuring instruments
Measuring instruments that can reflect a group of similar values. They can also calibrate and adjust other measuring instruments or directly compare with the measured value as a standard, such as line rulers.
3. Special measuring instruments
Measuring instruments specifically used to test a specific parameter. Common ones include: smooth limit gauges for testing smooth cylindrical holes or shafts, thread gauges for judging the qualification of internal or external threads, inspection templates for judging the qualification of surface profiles of complex shapes, functional gauges for testing assembly accuracy by simulating assembly passability, etc.
4. General measuring instruments
In my country, measuring instruments with relatively simple structures are usually called general measuring instruments. Such as vernier calipers, outside micrometers, dial indicators, etc.
02
Technical performance indicators of measuring instruments
1. Nominal value of measuring instruments
The value marked on the measuring instrument to indicate its characteristics or guide its use. Such as the size marked on the gauge block, the size marked on the scale, the angle marked on the angle gauge block, etc.
2. Graduation value
The difference between the values represented by two adjacent scales (minimum unit value) on the scale of the measuring instrument. For example, if the difference between the values represented by two adjacent scales on the differential cylinder of an outside micrometer is 0.01mm, the graduation value of the measuring instrument is 0.01mm. The graduation value is the minimum unit value that can be directly read by a measuring instrument. It reflects the accuracy of the reading and also explains the measurement accuracy of the measuring instrument.
3. Measuring range
The range from the lower limit to the upper limit of the measured value that can be measured by the measuring instrument within the allowable uncertainty. For example, the measuring range of the outside micrometer is 0-25mm, 25-50mm, etc., and the measuring range of the mechanical comparator is 0-180mm.
4. Measuring force
During contact measurement, the contact pressure between the measuring instrument probe and the measured surface. Too much measuring force will cause elastic deformation, and too little measuring force will affect the stability of the contact.
5. Indication error
The difference between the indication of the measuring instrument and the true value of the measured value. The indication error is a comprehensive reflection of the various errors of the measuring instrument itself. Therefore, the indication error is different for different working points within the indication range of the instrument. Generally, a gauge block or other metrological standard with appropriate accuracy can be used to calibrate the indication error of the measuring instrument.
03
Selection of measuring tools
Before each measurement, it is necessary to select measuring tools according to the special characteristics of the measured parts. For example, calipers, height gauges, micrometers, and depth gauges can be used for length, width, height, depth, outer diameter, and step difference; micrometers and calipers can be used for shaft diameters; plug gauges, block gauges, and feeler gauges can be used for holes and slots; square rulers can be used to measure the straightness of parts; R gauges can be used to measure R values; three-dimensional and two-dimensional can be used when measuring small tolerances, high precision requirements, or when calculating form and position tolerances; hardness testers can be used to measure the hardness of steel.
1. Application of calipers
Calipers can measure the inner diameter, outer diameter, length, width, thickness, step difference, height, and depth of objects; calipers are the most commonly used and convenient measuring tools, and are the most frequently used measuring tools at the processing site.
Digital calipers:
Resolution 0.01mm, used for dimensional measurement with small tolerances (high precision).
Picture
Table card:
Resolution 0.02mm, used for conventional size measurement.
Picture
Vernier caliper:
Resolution 0.02mm, used for rough machining measurement.
Picture
Before using the caliper, you need to remove dust and dirt with clean white paper (use the outer measuring surface of the caliper to clamp the white paper and then pull it out naturally, repeat 2-3 times).
Note:
1. When using the caliper to measure, the measuring surface of the caliper should be parallel or perpendicular to the measuring surface of the object to be measured as much as possible;
2. When using depth measurement, if the object to be measured has an R angle, it is necessary to avoid the R angle but close to the R angle, and the depth gauge and the measured height should be kept as vertical as possible;
3. When measuring a cylinder with a caliper, it is necessary to rotate and measure in sections to take the maximum value;
Because the caliper is used frequently, the maintenance work needs to be done to the best. After daily use, it needs to be wiped clean and put in the box. Before use, the accuracy of the caliper needs to be checked with a gauge block.
2. Application of micrometer
Picture
Before using the micrometer, you need to remove dust and dirt with clean white paper (use the micrometer to measure the contact surface and the screw surface, hold the white paper and then pull it out naturally, repeat 2-3 times), then twist the knob, and when the contact surface and the screw surface are about to touch, use fine adjustment instead. When the two surfaces are completely in contact, adjust to zero and you can measure.
When measuring hardware with a micrometer, adjust the knob, and when it is about to touch the workpiece, use the fine adjustment knob to screw it in. When you hear three sounds of click, click, click, stop, and read the data from the display or scale.
When measuring plastic products, just measure the contact surface and the screw until it touches the product lightly.
When measuring the diameter of a shaft with a micrometer, measure at least two directions and measure the maximum value in segments. For the micrometer in measurement, the two contact surfaces should be kept clean at all times to reduce measurement errors.
3. Application of height gauge
Height gauge is mainly used to measure height, depth, flatness, verticality, concentricity, coaxiality, surface vibration, tooth vibration, depth, height gauge measurement, first check whether the probe and the connection parts are loose.
Picture
4. Application of feeler gauge
Flatness measurement:
Place the part on the platform, and use the feeler gauge to measure the gap between the part and the platform (Note: the feeler gauge and the platform should be kept in a tight state without gap during measurement)
Picture
Straightness measurement:
Place the part on the platform and rotate it for one circle, and use the feeler gauge to measure the gap between the part and the platform.
Picture
Bending measurement:
Place the part on the platform, select the corresponding feeler gauge to measure the gap between the two sides or the middle of the part and the platform.
Picture
Vertical measurement:
Place one side of the right angle of the zero to be measured on the platform, and let the square ruler close to it on the other side, and use the feeler gauge to measure the maximum gap between the part and the square ruler.
Picture
5. Application of plug gauge (needle gauge):
Applicable to measuring the inner diameter, slot width and gap of the hole.
Picture
When the hole diameter of the part is large and there is no suitable needle gauge, two plug gauges can be overlapped and fixed on the magnetic V-shaped block for measurement in 360-degree direction to prevent looseness and facilitate measurement.
Picture
Inner hole measurement: When measuring the hole diameter, penetration is qualified, as shown in the figure below.
Picture
Note: When measuring the plug gauge, it needs to be inserted vertically, not obliquely.
Picture
6. Precision measuring instrument: Secondary element
Secondary element is a high-performance, high-precision non-contact measuring instrument. The sensing element of the measuring instrument does not directly contact the surface of the measured part, so there is no mechanical measuring force; the two-dimensional element transmits the captured image to the computer's data acquisition card through the data cable by projection, and then the software forms an image on the computer monitor; it can measure various geometric elements (points, lines, circles, arcs, ellipses, rectangles), distances, angles, intersections, form and position tolerances (roundness, straightness, parallelism, verticality, inclination, position, concentricity, symmetry) on the parts, and can also be used for CAD output of 2D contour drawing. Not only can the contour of the workpiece be observed, but also the surface shape of opaque workpieces can be measured.
Picture
Conventional geometric element measurement:
The inner circle in the part in the figure below is a sharp angle and can only be measured by projection.
Picture
Electrode processing surface observation:
The lens of the two-dimensional element has a magnification function. The roughness inspection after electrode processing (magnification 100 times image)
Picture
Small size deep groove measurement:
Picture
Gate detection:
During mold processing, there are often some gates hidden in the groove, and various detection instruments cannot measure them. At this time, you can use rubber clay to stick on the rubber mouth, and the shape of the rubber mouth will be printed on the rubber clay. Then use the second element to measure the size of the rubber clay print to get the gate size.
Picture
Note: Because there is no mechanical force when measuring with the second element, try to use the second element to measure thinner and softer products.
7. Precision measuring instruments: three-dimensional measuring instruments
The characteristics of three-dimensional measuring instruments are high precision (can reach μm level); versatility (can replace a variety of length measuring instruments); can be used to measure geometric elements (in addition to measuring elements that can be measured by two-dimensional measuring instruments, it can also measure cylinders and cones), form and position tolerances (in addition to measuring form and position tolerances that can be measured by two-dimensional measuring instruments, it also includes cylindricity, flatness, line profile, surface profile, coaxiality), and complex surfaces. As long as the probe of the three-dimensional measuring instrument can reach, its geometric dimensions and relative positions, surface profiles can be measured; and data processing can be completed with the help of computers; with its high precision, high flexibility and excellent digital capabilities, it has become an important means and effective tool for modern mold processing and quality assurance.
Picture
Some molds are being modified without 3D drawings. The coordinate values of each element and the contour of irregular surfaces can be measured, and then exported with drawing software and made into 3D graphics based on the measured elements, which can be processed and modified quickly and accurately (after the coordinates are set, the coordinate values of any point can be measured).
Image
3D digital model import and comparison measurement: For finished parts, in order to confirm the consistency with the design or find abnormal fit during the assembly of the fit mold, when some curved surface contours are neither arcs nor parabolas, but some irregular curved surfaces, it is impossible to measure the geometric elements. The 3D model can be imported and compared with the parts to understand the processing error; because the measured value is the point-to-point deviation value, it is convenient to make quick and effective corrections and improvements (the data shown in the figure below is the deviation between the measured value and the theoretical value).
Image
8. Application of hardness tester
Commonly used hardness testers include Rockwell hardness tester (benchtop) and Leeb hardness tester (portable). The commonly used hardness units are Rockwell HRC, Brinell HB, and Vickers HV.
Image
Rockwell hardness tester HR (benchtop hardness tester):
The Rockwell hardness test method is to use a diamond cone with a vertex angle of 120 degrees or a steel ball with a diameter of 1.59/3.18mm to press into the surface of the material under test at a certain load, and the material hardness is calculated from the indentation depth. According to the hardness of the material, it can be divided into three different scales to indicate HRA, HRB, and HRC.
HRA is the hardness obtained by using a 60Kg load and a diamond cone indenter, and is used for materials with extremely high hardness. For example: cemented carbide.
HRB is the hardness obtained by using a 100Kg load and a hardened steel ball with a diameter of 1.58mm, and is used for materials with lower hardness. For example: annealed steel, cast iron, etc., alloy copper.
HRC is the hardness obtained by using a 150Kg load and a diamond cone indenter, and is used for materials with very high hardness. For example: hardened steel, tempered steel, quenched and tempered steel, and some stainless steel.
Vickers hardness HV (mainly for surface hardness measurement):
Suitable for microscopic analysis. Press into the material surface with a load of less than 120kg and a diamond square cone indenter with a vertex angle of 136°, and measure the diagonal length of the indentation. It is suitable for hardness determination of larger workpieces and deeper surface layers.
Leeb hardness HL (portable hardness tester):
Leeb hardness is a dynamic hardness test method.
The ratio of the rebound velocity of the impact body of the hardness sensor when it impacts the workpiece to the impact velocity when it is 1mm away from the workpiece surface multiplied by 1000 is defined as the Leeb hardness value.
Advantages: The Leeb hardness tester manufactured according to the Leeb hardness theory has changed the traditional hardness testing method. Since the hardness sensor is as small as a pen, the sensor can be held in the hand to directly test the hardness of the workpiece in various directions at the production site, which is difficult for other desktop hardness testers to do.
