1. 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. Different measuring tools according to purpose can be divided into the following categories:
1. Single value measuring tools
It can only reflect a single-value measuring tool. It can be used to calibrate and adjust other measuring instruments or be used as a standard quantity to directly compare with the measured, such as gauge block, angle gauge block, etc.
2. Multi-value measuring tools
It can reflect a group of measuring tools of the same value. It can also calibrate and adjust other measuring instruments or use it as a standard quantity to directly compare with the measured, such as a line ruler.
3. Special measuring tools
A measuring tool designed to test a specific parameter. Common ones are: inspection of smoothness limit gauges for smooth cylindrical holes or shafts, thread gauges for judging the eligibility of internal or external threads, inspection templates for judging the eligibility of complex shapes of surface contours, and the function of inspecting assembly accuracy by simulating assembly passability Rubrics and so on.
4. General measuring tools
In our country, measuring instruments with relatively simple structures are called universal measuring tools. Such as vernier calipers, outside micrometers, dial indicators, etc.
2. Technical performance indicators of measuring instruments
1. The nominal value of the measuring tool
The value marked on the measuring tool to indicate its characteristics or to guide its use. Such as the size marked on the gauge block, the size marked on the scribing ruler, the angle marked on the angle gauge block, etc.
2. Graduation value
On the scale of the measuring instrument, the difference between the value represented by two adjacent engraved lines (the smallest unit value). If the difference between two adjacent engraved lines on the differential cylinder of an outside micrometer is 0.01mm, the graduation value of the measuring instrument is 0.01mm. The division value is the smallest unit value that can be directly read by a measuring instrument. It reflects the accuracy of the reading and also explains the accuracy of the measuring instrument.
3. Measuring range
Within the allowable uncertainty, the range from the lower limit to the upper limit of the measured value that the measuring instrument can measure. 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
In the contact measurement process, the contact pressure between the probe of the measuring instrument and the surface to be measured. Too much measuring force will cause elastic deformation, and too little measuring force will affect the stability of contact.
5. Indication error
The difference between the displayed value of the measuring instrument and the true value of the measured value. The indication error is a comprehensive reflection of various errors of the measuring instrument itself. Therefore, the indication error is different for different operating points within the indication range of the instrument. Generally, a suitable precision gauge block or other measurement standard can be used to verify the indication error of the measuring instrument.
Three, the selection of measuring tools
Before each measurement, you need to select measurement tools according to the special characteristics of the measured parts, such as length, width, height, depth, outer diameter, step difference, etc. You can choose calipers, height gauges, micrometers, and depth gauges; for shaft diameters, you can choose micrometers. Calipers; plug gauges, block gauges, feeler gauges can be used for holes and grooves; right angles can be used to measure the right angle of parts; R gauges can be used to measure R values; measurement fit tolerances are small, high accuracy requirements or shape and position tolerances are required Three-dimensional and two-dimensional are used; hardness tester is used to measure the hardness of steel.
1. The application of calipers
Calipers can measure the inner diameter, outer diameter, length, width, thickness, level difference, height, and depth of objects; calipers are the most commonly used and most convenient measuring tools, and the most frequently used measuring tools in the processing site.
Digital caliper: 0.01mm resolution, used for size measurement with small fit tolerance (high precision).
Table card: resolution 0.02mm, used for conventional size measurement.
Vernier caliper: resolution 0.02mm, used for rough machining measurement.
Before using the caliper, remove dust and dirt with clean white paper (use the outer measuring surface of the caliper to jam the white paper and then pull it out naturally, repeat 2-3 times)
When using a caliper to measure, the measurement surface of the caliper should be as parallel or perpendicular as possible to the measurement surface of the object to be measured;
When using depth measurement, if the measured object has an R angle, avoid the R angle but be close to the R angle, and keep the depth gauge and the measured height as vertical as possible;
When the caliper is measuring a cylinder, it needs to be rotated and the segment measurement is to take the maximum value;
Due to the high frequency of use of calipers, the maintenance work needs to be done as best as possible. After daily use, it needs to be wiped clean and put in the box. A block is required to check the accuracy of the caliper before use.
2. Application of Micrometer
Before using the micrometer, you need to use clean white paper to remove dust and dirt (use a micrometer to measure the contact surface and screw surface to jam the white paper and then pull it out naturally, repeat 2-3 times), then twist the knob to measure the contact When the surface is in fast contact with the screw surface, use fine adjustment. When the two surfaces are in full contact, adjust the zero value, and then the measurement can be performed.
When measuring hardware with a micrometer, adjust the knob. When it is fast to touch the workpiece, use the fine-tuning knob to screw in. When you hear the three sounds of click, click, and click, stop, and read the data from the display or scale.
When measuring plastic products, the measuring contact surface and the screw lightly touch the product.
When measuring the shaft diameter with a micrometer, measure at least two directions and take the maximum value of the micrometer in sections. The two contact surfaces should be kept clean at all times to reduce measurement errors.
3. Application of height gauge
The height ruler is mainly used to measure height, depth, flatness, perpendicularity, concentricity, coaxiality, surface vibration, tooth vibration, depth, height ruler measurement, the first thing to check the probe and each connection part for looseness.
4. Application of feeler gauge
The feeler gauge is suitable for the measurement of flatness, curvature and straightness
Flatness measurement:
Place the part on the platform, and measure the gap between the part and the platform with a feeler gauge (note: the feeler gauge and the platform should be kept tightly pressed without any gap during measurement)
Straightness measurement:
Place the part on the platform and rotate it once, and measure the gap between the part and the platform with a feeler gauge.
Curvature measurement:
Place the part on the platform and select the corresponding feeler gauge to measure the gap between the two sides or the middle of the part and the platform.
Verticality measurement:
Place one side of the straight angle to be measured on the platform, and place the square square against it on the other side, and measure the largest gap between the part and the square with a feeler gauge.
5. Application of plug gauge (stick needle):
It is suitable for measuring the inner diameter, groove width and clearance of holes.
The part has a large aperture. When there is no suitable needle gauge, the two plug gauges can be overlapped, and the plug gauge can be fixed on the magnetic V-shaped block according to the 360-degree measurement to prevent loosening and easy to measure.
Aperture measurement
Inner hole measurement: when the hole diameter is measured, the penetration is qualified, as shown in the figure below.
Note: When measuring the plug gauge, it must be inserted vertically, not diagonally.
6. Precision measuring instrument: the second element
The second element is a high-performance, high-precision non-contact measuring instrument. The sensing element of the measuring instrument is not in direct contact with the surface of the measured part, so there is no mechanical measuring force; the second element transmits the captured image through the data line to the data acquisition card of the computer through the projection method, and then Imaging on the computer monitor by the software; various geometric elements (points, lines, circles, arcs, ellipses, rectangles), distances, angles, intersections, geometric tolerances (roundness, straightness, parallelism, perpendicularity) on the parts can be performed Degree, inclination, position, concentricity, symmetry) can be measured, and the contour can also be drawn with CAD output in 2D. Not only can the contour of the workpiece be observed, but also the surface shape of the opaque workpiece can be measured.
Conventional geometric element measurement: The inner circle in the part shown in the figure below is a sharp corner and can only be measured by projection.
Observation of electrode processing surface: The lens of the two-dimensional element has the function of magnification for roughness inspection after electrode processing (image magnified 100 times).
Small size deep groove measurement
Gate inspection: During mold processing, there are often some gates hidden in the groove, and various testing instruments cannot measure. At this time, rubber cement can be used to paste on the glue mouth, and the shape of the glue mouth will be printed on the glue. On the top, and then use the two-dimensional measurement of the size of the mud print to get the gate size.
Note: Because there is no mechanical force during the two-dimensional measurement, try to use the two-dimensional measurement for thinner and softer products.
7. Precision measuring instruments: three-dimensional
The characteristics of the three-dimensional element are high precision (up to μm level); universal (can replace a variety of length measuring instruments); can be used to measure geometric elements (in addition to the elements that can be measured by the two-dimensional element, it can also measure cylinders and cones) , Form and position tolerance (in addition to the form and position tolerance that can be measured by the two-dimensional measurement, it also includes cylindricity, flatness, line profile, surface profile, and coaxiality), complex surface, as long as the three-dimensional probe Wherever it can be touched, its geometric size, mutual position and surface profile can be measured; and data processing can be completed with the help of a computer; with its high precision, flexibility and excellent digital capabilities, it has become an important part of modern mold manufacturing and quality assurance Means, effective tools.
Some molds are being modified without 3D graphics files. The coordinate values of various elements and the contours of irregular curved surfaces can be measured, and then exported by drawing software and made into 3D graphics based on the measured elements, which can be processed and modified quickly and without errors. (After setting the coordinates, you can take any point to measure the coordinate value).
3D digital model import comparison measurement: in order to confirm the consistency of the processed parts with the design or find the abnormal fit in the process of assembling the fit model, when some curved surfaces are neither arcs nor parabolic, but some irregular curved surfaces , When geometric element measurement is not possible, you can import the 3D model and compare the measurement of the part to understand the machining error; because the measured value is a point-to-point deviation value, it can facilitate rapid and effective correction and improvement (the data shown in the figure below are actual measured values Deviation from theoretical value).
8. Application of Hardness Tester
Commonly used hardness testers include Rockwell hardness tester (desktop) and Leeb hardness tester (portable). The commonly used hardness units are Rockwell HRC, Brinell HB, and Vickers HV.
Rockwell hardness tester HR (desktop hardness tester)
The Rockwell hardness test method is to use a diamond cone with an apex angle of 120 degrees or a steel ball with a diameter of 1.59/3.18mm, which is pressed into the surface of the material to be tested under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to different material hardness, it can be divided into three different scales to express HRA, HRB, and HRC.
HRA adopts 60Kg load and the hardness required by the diamond cone indenter, and is used for extremely hard materials. 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. It is used for materials with lower hardness. For example: annealed steel, cast iron, etc., alloy copper.
HRC is the hardness obtained with a load of 150Kg and a diamond cone indenter, and is used for materials with 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 microscope analysis. The diamond square cone indenter with a load of less than 120kg and an apex angle of 136° is pressed into the surface of the material, and the diagonal length of the indentation is measured. It is suitable for the hardness determination of larger workpieces and deeper surface layers.
Leeb hardness HL (portable hardness tester)
Leeb hardness is a dynamic hardness test method. When the impact body of the hardness sensor impacts the workpiece, the ratio of the rebound velocity to the impact velocity at 1mm from the surface of the workpiece is multiplied by 1000, which is defined as the Leeb hardness value.
Advantages: The Leeb hardness tester manufactured by Leeb hardness theory has changed the traditional hardness test method. Since the hardness sensor is as small as a pen, it can directly perform hardness detection on the workpiece in various directions at the production site by holding the sensor. Therefore, it is difficult for other desktop hardness testers to perform.
