CMMs are widely used in machinery, electronics, instruments, plastics and other industries.CMM is one of the most effective methods to measure and obtain dimensional data, because it can replace a variety of surface measurement tools and expensive combination gauges, and reduce the time required for complex measurement tasks from hours to minutes, which is an effect that other instruments cannot achieve.
Chinese name
CMM
Foreign name
Coordinate Measuring Machine, CMM
Composition
Main engine mechanical system, probe system
Technical difficulties
Get the digital point cloud of the product
Main features
The measurement result has high precision and good repeatability
CMMCoaxiality detection is a problem we often encounter in the measurement work. Coaxiality detection with three coordinates is not only intuitive and convenient, but also has high precision and repeatability of the measurement results of three dimensions, 2.5 dimensions and three coordinates[1]。
The function of CMM is to quickly and accurately evaluate dimensional data and provide operators with useful information about the production process, which is very different from all manual measuring equipment.Place the measured object in the three coordinate measurement space to obtain the coordinate position of each measuring point on the measured object. According to the spatial coordinate value of these points, calculate the geometric size, shape and position of the measured object.
Measuring machine composition
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CMM generally consists of the following parts:
1. Mechanical system of host machine (X, Y, Z axes or others);
2. Probe system;
3. Electrical control hardware system;
4. Data processing software system (measurement software);
usage method
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CMMIt is called CMM for short. Since the first CMM came out in the mid-1960s, with the progress of computer technology and the development of electronic control system and detection technology, it has provided strong technical support for the development of CMM towards high precision and high speed.
CMM can be divided into contact measurement andNon-contact measurementAs well as contact and non-contact measurement, contact measurement is often used to measure machined products andPress formingProducts, metal films, etc.This paper takes contact measuring machine as an example to illustrate several methods of scanning object surface to obtain data points. The data point results can be used for processing data analysis, and can also provide original information for reverse engineering technology.Scanning refers to collecting data points in a specific area on the surface of the object to be measured with the help of the measuring machine application software.This area can be a line, a patch, a section of a part, a curve of a part, or a perimeter at a certain distance from the edge.The scanning type is related to the measurement mode, probe type and whether there is CAD file. The status button (Manual/DCC) determines the available SCAN option on the screen.If DCC method is used for measurement and CAD file is available, scanning methods include OPEN LINE, CLOSED LINE, PATCH, SECTION and PERIMETER scanning.If DCC method is used for measurement but only wireframe CAD files are available, then OPEN LINEAR, CLOSED LINEAR and PATCH scanning methods can be selected.If the manual measurement mode is used, the basic "MANUAL TTP SCAN" mode can only be used.If the probe is rigid in manual measurement mode, the available options are FIXED DELTA, VARIABLE DELTA, TIME DELTA and BODY AXIS SCAN.
The definition of coaxiality tolerance zone of CMM in the national standard refers to the area within the cylindrical surface where the diameter tolerance is t and the 3D image measurement is coaxial with the reference axis of the CMM.It has the following three control elements: ① axis and axis; ②Axis and public axis; ③Center and center.
2.5 dimensionsMeasurement of factors affecting coaxiality
The main factors that affect the coaxiality of 2.5 dimensions include the center position and axis direction of the measured element and the datum element, especially the axis direction.If two cross section circles are measured on the reference cylinder, use their connecting lines as the reference axis.Measure two cross section circles on the measured cylinder, construct a straight line, and then calculate the coaxiality.Assuming that the distance between the two sections on the datum is 10 mm, and the distance between the first section of the datum and the first section of the measured cylinder is 100 mm, if there is a measurement error of 5 μ m between the center of the circle of the second section of the datum and the center of the circle of the first section, then the datum axis has deviated 50 μ m (5 μ mx100 ÷ 10) when extending to the first section of the measured cylinder, at this time,Even if the measured cylinder is completely coaxial with the datum, the result will still have an error of 100 μ m (the coaxiality tolerance value is the diameter, and 50 μ m is the radius).
Z-axis balance adjustment
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The Z-axis balance of the measuring machine is divided into heavy hammer and pneumatic balance, which is mainly used to balance the weight of the Z-axis and make the driving of the Z-axis stable.If the pneumatic balance switch is actuated by mistake, the Z-axis will lose balance.Handling method:
1) Turn the angle of the measuring seat to 90, 0 to avoid touching the measuring head during operation.
2) Press the "emergency stop" switch.
3) One person holds the Z axis with both hands and pushes it up and down to feel the balance effect.
4) One person adjusts the air pressure balance valve, and the adjustment amount is smaller each time. Two people can coordinate to adjust the Z-axis balance to the same feeling of upward and downward.
The end of travel switch is used for the end of travel protection and HOME of the machine.The end of travel switch generally uses contact switch or photoelectric switch.The switch type is most likely to change the position when the shaft is pushed by hand, resulting in poor contact.The switch position can be properly adjusted to ensure good contact.The photoelectric switch shall be checked to ensure that the position of the insert is normal, and the dust shall be removed frequently to ensure its normal operation.
matters needing attention
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Proper useCMMIt plays a key role in its service life and accuracy, and the following issues should be noted:
1. Before lifting the workpiece, the probe shall be returned to the coordinate origin to reserve a large space for the lifting position;The lifting of workpieces shall be stable and shall not impact any component of the CMM.
2. Properly install the parts and ensure that they meet the isothermal requirements of the parts and the measuring machine before installation.
3. Establish a correct coordinate system to ensure that the established coordinate system meets the requirements of the drawing, so as to ensure the accuracy of the measured data.
4. When the program runs automatically, it is necessary to prevent the probe from interfering with the workpiece, so it is necessary to increase the inflection point.
5. For some large and heavy molds and inspection tools, the workbench should be lifted down in time after the measurement to avoid the workbench being in the bearing state for a long time.
Factors affecting products
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There are many kinds of mechanical parts of CMM. What we need daily maintenance is the parts of the transmission system and air circuit system. The maintenance frequency should be determined according to the environment of the CMM.Generally, the measuring machine in a fine measuring room with a good environment is recommended to be regularly maintained every three months. If there is a lot of dust in the operating environment and the temperature and humidity of the measuring room cannot fully meet the requirements of the operating environment of the measuring machine, it is recommended to carry out regular maintenance once a month. For the regular maintenance of the measuring machine, the factors affecting the measuring machine should be understood:
1. Impact of compressed air on measuring machine
1) To select a suitable air compressor, it is better to have an air receiver, so that the air compressor has a long working life and stable pressure.
2) The starting pressure of the air compressor must be greater than the working pressure.
3) When starting up, first turn on the air compressor, and then turn on the power.
2. Influence of oil and water on measuring machine
Since compressed air plays a very important role in the normal operation of the measuring machine, it is very important to repair and maintain the air circuit.There are the following main items:
Check the pipeline and filter before using the measuring machine every day, and drain the water and oil in the filter and the air compressor or air tank.
Wipe the oil stain and dust on the guide rail every day to keep the air flotation guide rail in normal working condition.
3. The influence of air conditioner wind direction on the temperature of the measuring machine
Variable frequency air conditioners shall be selected as far as possible for the measuring machine room.The frequency conversion air conditioner has good energy saving performance, and the most important thing is its strong temperature control ability.Under normal capacity, the temperature can be controlled within ± 1 ℃.
Since the temperature of the air from the air conditioner is not 20 ℃, the air must not be directly blown to the measuring machine.Sometimes, in order to prevent the wind from blowing on the measuring machine, the wind direction is turned to the wall or one side. As a result, one side of the machine room is hot while the other side is cool, and the temperature difference is very large.
The installation of air conditioners should be planned, and the wind should be blown to the main position in the room. The wind direction should form a large cycle upward (the measuring machine cannot be blown), so that the indoor temperature should be balanced as far as possible.
If possible, air ducts shall be installed to send air to the top of the room through double-layer orifice plates, and the air return port shall be at the bottom of the room.This makes the airflow flow irregularly, which can make the temperature control of the machine room more reasonable.
Maintenance
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The composition of CMM is relatively complex, mainly including mechanical parts, electrical control parts and computer system.At ordinary times, when we use the CMM to measure the workpiece, we should also pay attention to the maintenance of the machine to extend the service life of the machine.The following focuses on the compressed air maintenance and guide rail maintenance[2]。
Compressed air maintenance
Because the compressed airCMMThe normal operation of the air supply system plays a very important role, so it is very important to repair and maintain the air supply.There are the following main items:
1. Daily useThree coordinatesMeasuring instrumentCheck the pipeline and filter before discharging the water and oil in the filter and the air compressor or air storage tank.
2. Generally, the random filter andPre filterFilter element of.The cycle of poor air quality should be shortened.Because the filter element of the filter is also blocked by oil pollution while filtering oil and water, a little longer time will reduce the actual working pressure of the measuring machine, affecting the normal operation of the CMM.Be sure to regularlyClean the filterfilter element.
Guide rail maintenance
CMMThe guide rail is the benchmark of the measuring machine, and the normal operation of the measuring machine can only be ensured by maintaining the air floating block and guide rail.In addition to frequently wiping with alcohol and absorbent cotton, care should be taken not to place parts and tools directly on the guide rail for the maintenance of CMM guide rail.Especially the granite guide rail, because of its brittle texture, any small bump will cause damage. If not found in time, debris will damage the flotation block and guide rail.To develop good working habits, use cloth or rubber pad to ensure the safety of the guide rail.
Wipe the guide rail after the work or the upper part is finished.Wipe the oil stain and dust on the guide rail every day to keep the air flotation guide rail in normal working condition.
application area
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It is mainly used for measuring boxes, racks, gears, cams, worm gears, worms, blades, curves, curved surfaces, etc. in machinery, automobile, aviation, military industry, furniture, tool prototypes, machines and other small and medium-sized accessories, molds and other industries, as well as in electronics, hardware, plastics and other industries. It can precisely detect the size, shape and geometric tolerance of workpieces,So as to complete the tasks of part inspection, contour measurement, process control, etc.
Mold industry
CMM is widely used in the mold industry. It is a modern intelligent tool for design, development, detection, statistical analysis, and is also an effective tool for unmatched quality and technical assurance of mold products.Three coordinate measuring machines mainly used today include bridge measuring machines, gantry measuring machinesHorizontal arm measuring machineAnd portable measuring machines.The measurement methods can be roughly divided into contact and non-contact.
If there is any deviation in the matching between the core and cavity of the mold and the guide sleeve of the guide pillar, the deviation value can be found through the CMM for correction.After the core and cavity contour of the mold is processed and formed, many inserts and local surfaces need to be processed on the electric pulse through the electrode, so the quality of electrode processing and non-standard surface quality become the key to mold quality.Therefore, it is necessary to measure the shape of electrode with CMM.CMM can use the input of 3D digital model to measure and compare the positioning, size, relevant geometric tolerance, curve and surface of the finished mold with that of the digital model, and output a graphical report to visually and clearly reflect the mold quality, so as to form a complete inspection report of the finished mold.In the case that some molds have been used for a period of time and wear needs to be corrected, but there is no original design data (i.e. digital analog), the point cloud can be collected by the section method, output in the specified format, and shape after the probe radius is compensated, so as to achieve the original repair effect.
When the contour of some surfaces is neither arc nor parabola, but some irregular surfaces, the surface can be made manually with putty or plaster as the base embryo.Then, use the CMM to measure the section lines, characteristic lines and parting lines on each section, output in the specified format, model after the probe radius compensation, and smooth the curve in the modeling process, so as to design and manufacture a new mold.
CMM, with its high precision, high flexibility and excellent digital capability, has become an important means of modern manufacturing industry, especially mold industry design, development, processing and manufacturing and quality assurance.
First, the measuring machine can provide quality assurance for the mold industry, and is the best choice for mold manufacturing enterprises to measure and test.The flexibility of the measuring machine in dealing with different tasks and its own high accuracy make it an arbiter.While providing dimensional data for process control, the measuring machine can provide additional performance such as incoming product inspection, machine tool calibration, customer quality certification, gauge inspection, processing test and optimization of machine tool settings.The highly flexible CMM can be configured in the workshop environment, and directly participate in all stages of mold processing, assembly, mold testing, and mold repair, provide necessary detection feedback, reduce the number of rework and shorten the mold development cycle, thus ultimately reducing the cost of mold manufacturing and bringing production into control.
Second, the measuring machine has strong reverse engineering ability and is an ideal digital tool.Through the combination of different types of probes and measuring machines with different structures, the 3D data and geometric features of the workpiece surface can be obtained quickly and accurately, which is particularly useful for mold design, sample replication, and repair of damaged molds.In addition, the measuring machine can also be equipped with contact and non-contact scanning probes, and use the powerful scanning function provided by PC-DMIS measurement software to copy the CAD model of complex workpiece with free-form surface shape features.Without any conversion, it can be directly recognized and programmed by various CAD software, thus greatly improving the efficiency of mold design.
Specifically, when using measuring machines to complete design and testing tasks in mold manufacturing enterprises, we should pay close attention to many factors, such as the selection of measuring datum, the calibration and selection of probe, the planning of measuring points and positions, the establishment of coordinate system, the influence of environment, the influence of local geometric features, CNC control parameters, etc.Each of these factors is enough to affect the accuracy and efficiency of the measurement results.
Automobile industry
CMM is a measuring system that detects the three-dimensional coordinates of the workpiece surface points through the relative movement of the probe system and the workpiece.By placing the measured object in the measuring space of the CMM, the coordinate position of each measuring point on the measured object is obtained using the contact or non-contact detection system. According to the spatial coordinate value of these points, the software performs mathematical operations to calculate the geometric size, shape and position to be measured.Therefore, CMM has the characteristics of high precision, high efficiency and versatility, which is to complete various auto partsGeometric measurementAnd quality control.
Auto parts have the characteristics of high quality, large batch and different shapes.According to different parts measurement types, they are mainly divided into three categories: box, complex shape and curve and surface. The configuration of each type of relative measurement system is different, and it is necessary to make mutual matching and selection from the host, detection system and software of the measurement system.
Engine manufacturing
The engine is composed of many parts of various shapes. The manufacturing quality of these parts is directly related to the performance and life of the engine.Therefore, it is necessary to carry out very precise detection in the production of these parts to ensure the accuracy and tolerance of the products.In the modern manufacturing industry, high-precision comprehensive measuring machines are more and more used in the production process, so that the goal and key of product quality gradually change from final inspection to the control of the manufacturing process. Through information feedback, the parameters of processing equipment are adjusted in time, so as to ensure product quality, stabilize the production process, and improve production efficiency.
In the selection of traditional measurement methods, people mainly rely on two measurement methods to measure box type workpieces and workpieces with complex geometry, that is, to detect box type workpieces through CMM;Through special measuring equipment, such as specialGear detectorSpecial cam detection equipment is used to measure workpieces with complex geometry.Therefore, for enterprises engaged in the production of workpieces with complex geometric shapes, quality control enterprises that complete the above products need not only to configure general measuring equipment, such as coordinate measuring machines, general standard measuring tools and gauges, but also need to configure special testing equipment, such as gear special testing equipment of various sizes and types, cam testing equipment, etc.This often leads to the need for measurement departments of enterprises to allocate various types of measurement equipment and professional testing personnel engaged in measurement operations. The utilization rate of measurement equipment is low, and enterprises bear high costs for training measurement personnel and the use and maintenance of measurement equipment;Enterprises cannot realize flexible and universal measurement and detection.Therefore, reducing the measurement cost of enterprises, the training cost of measuring personnel, the use and maintenance cost of measuring equipment, so as to improve the efficiency of measurement and detection, and enable enterprises to have real-time quality control capability in the production process, which will be related to the adaptability of enterprises in market activities, and will play an important role in helping enterprises establish and maintain good market reputation.
