Measuring tools in a mechanical factory - mastering them makes you a senior engineer!
Published Time:
2020-02-13
I. Classification of Measuring Instruments A measuring instrument is a device with a fixed form used to reproduce or provide one or more known values. According to their uses, measuring instruments can be classified into the following categories: 1. Single-value measuring instrument A measuring instrument that can only reflect a single value. It can be used to calibrate and adjust other measuring instruments or used as a standard for direct comparison with the measured value, such as gauge blocks and angle gauge blocks. 2. Multi-value measuring instrument A measuring instrument that can reflect a set of values of the same type. It can also be used to calibrate and adjust other measuring instruments or used as a standard for direct comparison with the measured value, such as linear scales. 3. Special-purpose measuring instrument Specifically used to inspect a certain parameter.
I. Classification of Measuring Instruments
A measuring instrument is a device with a fixed form 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 instrument
A measuring instrument that can only reflect a single value. It can be used to calibrate and adjust other measuring instruments or as a standard value to directly compare with the measured value, such as gauge blocks, angle gauge blocks, etc.
2. Multi-value measuring instrument
A measuring instrument that can reflect a set of similar values. It can also be used to calibrate and adjust other measuring instruments or as a standard value to directly compare with the measured value, such as a linear scale.
3. Special measuring instrument
A measuring instrument specifically used to test a specific parameter. Common examples include: Smooth limit gauges for inspecting smooth cylindrical holes or shafts; Thread gauges for determining the qualification of internal or external threads; Inspection templates for determining the qualification of complex surface profiles; Functional gauges for inspecting assembly accuracy by simulating assembly passability, etc.
4. General-purpose measuring instrument
In China, measuring instruments with relatively simple structures are usually called general-purpose measuring instruments. For example, vernier calipers, outside micrometers, dial indicators, etc.
II. Technical Performance Indicators of Measuring Instruments
1. Nominal Value of the Measuring Instrument
The value marked on the measuring instrument to indicate its characteristics or guide its use. For example, the dimensions marked on gauge blocks, the dimensions marked on a linear scale, and the angles marked on angle gauge blocks, etc.
2. Graduation Value
The difference between the values represented by two adjacent graduations (minimum unit value) on the scale of a measuring instrument. For example, if the difference between the values represented by two adjacent graduations on the vernier scale of an outside micrometer is 0.01 mm, then the graduation value of this measuring instrument is 0.01 mm. The graduation value is the minimum unit value that can be directly read from a measuring instrument. It reflects the level of reading accuracy and indicates the level of measurement accuracy of the measuring instrument.
3. Measurement Range
Within the allowable uncertainty, the range from the lower limit to the upper limit of the measured values that the measuring instrument can measure. For example, the measurement range of an outside micrometer is 0-25 mm, 25-50 mm, etc., and the measurement range of a mechanical comparator is 0-180 mm.
4. Measuring Force
The contact pressure between the measuring head of the measuring instrument and the surface of the measured object during contact measurement. Excessive measuring force will cause elastic deformation, while insufficient measuring force will affect the stability of the contact.
5. Indicated Error
The difference between the indicated value of the measuring instrument and the true value of the measured object. The indicated error is a comprehensive reflection of various errors of the measuring instrument itself. Therefore, the indicated errors are different at different operating points within the indicated range of the instrument. Generally, the indicated error of a measuring instrument can be verified using gauge blocks or other metrological standards with appropriate accuracy.
III. Selection of Measuring Tools
Before each measurement, it is necessary to select measuring tools based on the specific characteristics of the part to be measured. For example, length, width, height, depth, outside diameter, step difference, etc., can be measured using calipers, height gauges, micrometers, depth gauges; shaft diameters can be measured using micrometers and calipers; holes and slots can be measured using plug gauges, gauge blocks, and feeler gauges; the squareness of parts can be measured using a square; R values can be measured using R gauges; when measuring parts with small mating tolerances, high accuracy requirements, or when calculating form and position tolerances, three-dimensional or two-dimensional measuring instruments can be used; steel hardness can be measured using a hardness tester.
1. Application of Calipers
Calipers can measure the inner diameter, outer diameter, length, width, thickness, step difference, height, and depth of an object; calipers are the most commonly used and convenient measuring instruments and are the most frequently used measuring instruments in the processing site.
Digital calipers: Resolution 0.01 mm, used for measuring dimensions with small mating tolerances (high precision).
Vernier caliper: Resolution 0.02 mm, used for regular dimensional measurement.
Vernier caliper: Resolution 0.02 mm, used for rough machining measurement.
Before using calipers, remove dust and dirt with clean white paper (clamp the white paper with the outer measuring surface of the caliper and then pull it out naturally, repeat 2-3 times).
When using calipers for measurement, the measuring surface of the calipers should be as parallel or perpendicular as possible to the measuring surface of the object being measured;
When using depth measurement, if the object being measured has a fillet radius, the fillet radius should be avoided but close to it, and the depth gauge and the measured height should be kept as vertical as possible;
When measuring a cylinder with calipers, rotate and measure in sections to take the maximum value;
Because calipers are used frequently, maintenance work needs to be done well. After use every day, wipe them clean and put them in the box. Before use, verify the accuracy of the calipers with gauge blocks.
2. Application of Micrometers
Before using a micrometer, remove dust and dirt with clean white paper (clamp the white paper with the measuring contact surface and the screw surface of the micrometer and then pull it out naturally, repeat 2-3 times), then turn the knob, when the measuring contact surface and the screw surface are about to touch, switch to fine adjustment, when the two surfaces are fully in contact, set to zero, then you can start the measurement.
When measuring metal parts with a micrometer, turn the knob, when it is about to touch the workpiece, switch to the fine adjustment knob to turn it in. After hearing three clicks, stop and read the data from the display or scale.
When measuring plastic products, the measuring contact surface and screw should lightly contact the product.
When measuring the diameter of a shaft with a micrometer, measure at least two directions and take the maximum value in sections. The two contact surfaces of the micrometer should always be kept clean to reduce measurement errors.
3. Application of Height Gauges
Height gauges are primarily used to measure height, depth, planarity, verticality, concentricity, coaxiality, surface vibration, tooth vibration, and depth. When measuring with a height gauge, first check the probe and all connecting parts for looseness.
4. Application of Feeler Gauges
Feeler gauges are suitable for measuring planarity, curvature, and straightness.
Planarity Measurement:
Place the part on the platform and use a feeler gauge to measure the gap between the part and the platform (Note: When measuring, keep the feeler gauge tightly pressed against the platform without any gap).
Straightness Measurement:
Place the part on the platform, rotate it 360 degrees, and use a feeler gauge to measure the gap between the part and the platform.
Curvature Measurement:
Place the part on the platform and select the appropriate feeler gauge to measure the gap between the part and the platform on either side or in the middle.
Verticality Measurement:
Place one side of the part with a right angle on the platform, place a right-angle ruler against it, and use a feeler gauge to measure the largest gap between the part and the right-angle ruler.
5. Application of Plug Gauges (Pins):
Suitable for measuring the inner diameter of holes, slot width, and gaps.
When the hole diameter of the part is large and there is no suitable pin gauge, two plug gauges can be overlapped, and the measurement can be performed in 360 degrees. Fixing the plug gauge on a magnetic V-block can prevent loosening and facilitate measurement.
Hole Diameter Measurement
Inner Hole Measurement: When measuring the hole diameter, through-hole is considered qualified, as shown in the figure below.
Note: When measuring with plug gauges, insert them vertically; do not insert them at an angle.
6. Precision Measuring Instrument: Two-Dimensional Measurement
Two-dimensional measurement is a high-performance, high-precision, non-contact measurement instrument. The sensing element of the measuring instrument does not directly contact the surface of the measured part, so there is no measurement force from mechanical action; the two-dimensional measurement projects the captured image to the computer's data acquisition card via a data cable, and then the software displays the image on the computer monitor; it can measure various geometric elements (points, lines, circles, arcs, ellipses, rectangles), distances, angles, intersections, and form tolerances (roundness, straightness, parallelism, perpendicularity, inclination, position, concentricity, symmetry) on the part, and can also perform 2D outline drawing and output using CAD. It can not only observe the workpiece contour, but also measure the surface shape of opaque workpieces.
Conventional Geometric Element Measurement: The inner circle in the part shown in the figure below is a sharp corner and can only be measured using projection.
Electrode Machining Surface Observation: The lens of the two-dimensional measurement has a magnification function for roughness inspection after electrode machining (100x magnification image).
Small Size Deep Groove Measurement
Gate Detection: In mold processing, there are often gates hidden in grooves, and various detection instruments cannot perform measurements. In this case, rubber clay can be applied to the gate, and the shape of the gate will be imprinted on the clay. Then, the two-dimensional measurement can be used to measure the size of the clay imprint to obtain the gate dimensions.
Note: Because there is no mechanical force during two-dimensional measurement, thinner and softer products should be measured using two-dimensional measurement as much as possible.
7. Precision Measuring Instrument: Three-Dimensional Measurement
Three-dimensional measurement is characterized by high precision (achieving μm level); versatility (can replace various length measuring instruments); it can be used to measure geometric elements (in addition to the elements that can be measured by two-dimensional measurement, it can also measure cylinders and cones), form tolerances (in addition to the form tolerances that can be measured by two-dimensional measurement, it also includes cylindricity, planarity, line profile, surface profile, coaxiality), and complex surfaces. As long as the probe of the three-dimensional measurement can reach, its geometric dimensions and relative positions, and surface profile can be measured; and data processing is completed with the aid of a computer; 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.
For some molds that are being modified and do not have 3D drawings, the coordinates of each element, the contour of irregular surfaces, can be measured, and then exported using drawing software and a 3D graphic can be created based on the measured elements, allowing for fast and accurate processing and modification (after coordinate setting, coordinates of any point can be measured).
3D Model Import and Comparison Measurement: After the part is processed, to confirm consistency with the design or to find abnormal mating during assembly fit mold, when some surface contours are neither circular arcs nor parabolas, but irregular surfaces, and geometric element measurement cannot be performed, a 3D model can be imported and compared with the part to understand the processing error; since the measured value is the point-to-point deviation, it can facilitate fast and effective correction and improvement (the data shown in the figure below is the deviation between the measured value and the theoretical value).
8. Application of Hardness Testers
Commonly used hardness testers include Rockwell hardness testers (desktop) and Leeb hardness testers (portable). Commonly used hardness units are Rockwell HRC, Brinell HB, and Vickers HV.
Rockwell Hardness Tester HR (Desktop Hardness Tester)
The Rockwell hardness test method uses a diamond cone with a 120-degree apex angle or a 1.59/3.18 mm diameter steel ball to press into the surface of the material being tested under a certain load, and the material hardness is determined from the depth of the indentation. Depending on the material hardness, three different scales can be used to represent HRA, HRB, and HRC.
HRA is the hardness obtained using a 60 kg load and a diamond cone indenter, used for materials with extremely high hardness, such as cemented carbide.
HRB is the hardness obtained using a 100 kg load and a 1.58 mm diameter hardened steel ball, used for materials with lower hardness, such as annealed steel, cast iron, and copper alloys.
HRC is the hardness obtained using a 150kg load and a diamond cone indenter, used for materials with very high hardness. For example: quenched steel, tempered steel, through-hardened steel, and some stainless steels.
Vickers hardness HV (mainly for surface hardness measurement)
Suitable for microscopic analysis. A diamond square pyramid indenter with a load of up to 120kg and an apex angle of 136° is pressed into the material surface, and the diagonal length of the indentation is measured. It is suitable for larger workpieces and deeper surface layer hardness determination.
Leeb hardness HL (portable hardness tester)
Leeb hardness is a dynamic hardness testing method. The ratio of the rebound velocity to the impact velocity of the impact body of the hardness sensor at 1 mm from the surface of the workpiece, multiplied by 1000, is defined as the Leeb hardness value.
Advantages: Leeb hardness testers, based on Leeb hardness theory, have changed traditional hardness testing methods. Because 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 on the production site, which is difficult for other desktop hardness testers to achieve.
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