The Coordinate Measuring Machine (CMM) is a precision measuring instrument used in a variety of industries. It is equipped with air brake switches and micro-motion devices on its three axes, enabling precise transmission of each axis. Additionally, the CMM is fitted with a high-performance data acquisition system to ensure accuracy in measurements.
The CMM is commonly used for product design, mold equipment, gear and blade measurements, machinery manufacturing, tooling fixtures, steam mold parts, electronic and electrical equipment, and other precision measurements.
The Three Coordinate Measuring Machine, also known as the Three Coordinate Measuring Instrument, is a device used to measure the geometric shape, length, and circumferential division within a three-dimensional space. It operates by having a detector that can move in three directions along three mutually perpendicular guide rails.
The detector can either make contact with the workpiece or measure it non-contact. The displacement measuring system of the three axes, such as a grating ruler, calculates the points (x, y, z) and various functions of the workpiece through a data processor or computer.
The measuring capabilities of the CMM include dimensional accuracy, positioning accuracy, geometric accuracy, and contour accuracy. This instrument is essential in ensuring the precision and accuracy of measurements in a variety of industries.
Structure type: three axis granite, German movable bridge structure surrounded by four sides;
Transmission mode: DC servo system + preload high-precision air bearing;
Length measuring system: RENISHAW open grating ruler, with a resolution of 0.1 μm;
Probe system: Renishaw controller, Renishaw probe, Renishaw probe;
Machine: high precision (grade 00) granite platform;
Operating environment: temperature (20 ± 2) ℃, humidity 40% – 70%, temperature gradient 1 ℃/m, temperature change 1 ℃/h;
Air pressure: 0.4 MPa – 0.6 MPa;
Air flow: 25 L/min;
Length accuracy mpee: ≤ 2.1 + L/350（ μm)；
Probe ball accuracy MPEP: ≤ 2.1 μm.
The three-axis of the machine uses natural high-precision granite guide rails. This ensures uniform thermodynamic performance across the entire machine and eliminates accuracy errors that may result from differences in thermal expansion coefficients among the materials used for the three axes.
Comparison of granite and aviation aluminum alloy
Aluminum alloy material has a large coefficient of thermal expansion.
Typically, the beam and z-axis made of aviation aluminum alloy material can become damaged and their accuracy can change over time with prolonged use.
The platform of the three-coordinate system is made of granite, and so is the main axis.
The main shaft is constructed of granite, while the beam and z-axis are composed of aluminum alloy and other materials.
The differing thermal expansion coefficients of the three axes can cause distortions in measurement accuracy and stability when the temperature changes.
The three-axis guide rail features a fully granite rectangular structure, equipped with high-precision self-cleaning prestressed air bearings.
This solid foundation ensures the long-term stability of the machine’s accuracy.
The bearings also provide stable and balanced force along the axial direction, contributing to the longevity of the machine’s hardware.
A patented technology of a small-hole air outlet is utilized, with an air consumption of 30L/min.
A condensation area forms in the bearing clearance to counteract heat generated from bearing motion friction and enhance the overall thermal stability of the equipment.
According to physical theory, when gas passes through a circular hole at a certain pressure, friction generates heat.
In high-precision measurements, even small amounts of heat can impact accuracy stability.
However, when the outlet hole’s aperture is smaller than a certain diameter, a condensation effect occurs around the outlet hole, which offsets the weak heat generated by air friction during measurement.
By utilizing this physical principle and the technology of a small-hole gas outlet, the equipment can maintain temperature stability for extended periods, thereby ensuring accuracy stability.
Comparison of CMM bearings of major suppliers
The three axes utilize the original gold-plated grating ruler from Renishaw, with a resolution of 0.1um.
One end of the grating ruler is fixed while the other end is free to expand and contract, reducing its deformation.
The transmission system adopts an internationally advanced design, eliminating any stress deformation of the guide rail to maximize accuracy and stability.
The steel wire reinforced synchronous belt transmission structure effectively reduces vibration during high-speed movement, while providing high strength, speed, and durability.
The software used is PTB’s comprehensive certified industry benchmark, Rational-DMIS, which is powerful and user-friendly, allowing for a focus on product measurement rather than software learning.
2.Basic components of CMM
Utilizes precision inclined beam technology.
Y-direction guide rail:
Employs a unique positioning structure with integral lower dovetail grooves directly processed on the workbench.
Guide rail mode:
Features self-cleaning preloaded high-precision air bearings composed of four-way static pressure air floating guide rails.
Employs high-performance DC servo motors and flexible synchronous toothed belt drive devices.
Each shaft is limited and electronically controlled, resulting in faster transmission and improved motion performance.
An adjustable pneumatic balancing device enhances the positioning accuracy of the z-axis.
Utilizes an imported dual computer three-coordinate specialized control system.
Adopts Computer Aided 3D error correction technology (CAA) to ensure long-term stability and high accuracy of the system.
Utilizes the powerful 3D-DMIS measurement software package, offering complete measurement functions and online capabilities.
3. Functional principle of CMM
A Coordinate Measuring Machine (CMM) is a type of measuring equipment that consists of a guide mechanism, length measuring element, and digital display device in three mutually perpendicular directions, as well as a worktable that can accommodate workpieces (though they may not necessarily be large).
The measuring head can be moved to the measurement point manually or through motorization, and the coordinate value of the measured point is displayed by the reading device and digital display.
This is the simplest and most basic form of measuring machine.
With this machine, the coordinate value of any point within the measurement volume can be displayed by the reading device and digital display.
The measuring machine’s sampling point sender is the measuring head, which is equipped with grating rulers and reading heads along the X, Y, and Z axes.
The measurement process involves the probe contacting the workpiece and sending a sampling point signal. The control system then collects the coordinate value of the current three-axis machine tool position relative to the machine tool’s origin, and the computer system processes the data.
4. Classification of CMM
According to the structure of CMM, it can be divided into the following categories:
1. Moving bridge type
The most commonly used coordinate measuring machine (CMM) is the mobile bridge type.
The axis, the main shaft that moves in the vertical direction, is guided by a box-shaped frame to move along the horizontal beam.
The horizontal beam, which is perpendicular to the axis, is supported at both ends by two columns, forming a “bridge.” The bridge moves along two guide grooves that are perpendicular to the axis on the horizontal plane.
Due to the support provided by struts at both ends of the beam, the mobile bridge type has a lower minimum deflection and higher accuracy compared to the cantilever type.
2. Bridge bed type
In the bridge bed type, the main shaft moves in the vertical direction and is guided by a box-shaped frame to move along the beam of the vertical axis. The beam moves along two horizontal guide rails in the axial direction, which are located on the upper surface of the pillars that are fixed on the machine body.
This type is similar to the mobile bridge type, as both ends of the beam are supported, resulting in minimal beam deflection and higher accuracy compared to the cantilever type.
Additionally, only the beam moves in the axial direction, which reduces the overall inertia and makes manual operation easier compared to the mobile bridge type.
3. Gantry type
The bridge gantry type, also known as the floor-mounted or door type, is different from the bridge bed type in that it is directly fixed to the floor.
This type of structure offers greater rigidity and stability compared to the bed type bridge and is commonly used in larger three-coordinate measuring instruments.
Each shaft is driven by a motor, providing a large measuring range. The operator can also work within the bridge structure.
4. Fixed bridge type
The fixed bridge type features a main shaft that moves in the vertical direction and is guided by a box-shaped frame to move along the horizontal beam of the vertical axis.
The bridge, or pillar, is fixed on the machine body, while the measuring table moves along the guide rail of the horizontal plane, which is perpendicular to the axis, in the axial direction.
Each shaft is powered by a motor to maintain position accuracy. However, this model is not designed for manual operation.
5. L-shpaed bridge type
The L-shaped bridge is designed to reduce the inertia of the bridge as the shaft moves.
Compared to the mobile bridge type, this design has less inertia in its moving components, making it easier to operate. However, it has a lower level of rigidity.
6. Fixed table cantilever arm type
The movable cantilever type features a main shaft that moves in the vertical direction and is guided by a box-shaped frame to move along the horizontal cantilever beam of the vertical axis in the axial direction. The cantilever beam moves along a guide groove on the horizontal plane, which is perpendicular to the shaft.
This type of structure is open on three sides and is convenient for assembling and disassembling workpieces, as they can extend beyond the table. However, the accuracy is lower due to the cantilever design.
7. Single column movable type
The single pillar mobile type features a main shaft that moves in the vertical direction, and the entire pillar moves along the guide groove of the horizontal plane, which is perpendicular to the shaft, connected to the shaft.
The measuring table moves in the axial direction along the guide groove of the horizontal plane, which is perpendicular to the axis.
This type of structure boasts good rigidity in both the measuring table and pillar, leading to minimal deformation. Additionally, the linear scale of each axis is positioned close to the measuring axis, ensuring compliance with Abbe’s theorem.
8. Single column xy table type
The single pillar measuring table is of the mobile type, featuring a main shaft that moves in the vertical direction.
The pillar is equipped with a shaft guide groove and is fixed to the measuring instrument body.
During measurement, the measuring table moves along the axis in the direction of the horizontal plane.
9. Moving table horizontal arm type
The horizontal arm measuring table is of the mobile type, with a box frame that supports the horizontal arm to move in the vertical (axis) direction along the vertical pillar. The probe is attached to the cantilever in the horizontal direction.
The pillar moves in the axial direction along the guide groove in the horizontal plane, which is perpendicular to the axis. The measuring table also moves in the axial direction along the guide groove in the horizontal plane, which is perpendicular to the axis and the axis.
This design is an improvement on the horizontal cantilever type, as it eliminates the deflection caused by the extension or retraction of the horizontal arm in the axial direction.
10. Fixed table horizontal arm type
The horizontal arm measuring platform is of the fixed type and has a similar structure to the mobile type.
The measuring table is fixed, and the X, Y, and Z axes move along the guide groove. During measurement, the pillar moves in the guide groove of the shaft, while the sliding table attached to the shaft moves in the vertical axis direction.
11. Horizontal arm moving type
The horizontal arm moving type features a cantilever shaft that moves in the horizontal direction and a box frame that supports the horizontal arm and moves along the column in the axial direction. The column is perpendicular to the axis.
The pillar moves in the axial direction along the guide groove of the horizontal plane, which is perpendicular to the axis, making it unsuitable for high-precision measurements unless the horizontal arm is extended or retracted to compensate for errors caused by weight.
This type of structure is primarily used for vehicle inspection.
12. Ring bridge type
The closed loop bridge type is known for its stability, as the driving mechanism is located in the center of the workbench. This design helps reduce the impact caused by bridge movement, making it the most stable among all three-coordinate measuring instruments.
5. Main advantages
- Surface anodized aviation aluminum alloy;
- High precision self-cleaning air bearing;
- High precision European imported grating ruler;
- Patented technology of precision triangle beam.
6. Application Fields
The closed loop bridge type is widely used in a variety of industries, including automobiles, electronics, machinery, aviation, and military, for the measurement of various objects such as boxes, frames, gears, cams, worm wheels, worms, blades, curves, curved surfaces, hardware, plastics, and more. It is also commonly used in the mold industry.
7. Method of use
Three-coordinate measuring machines (CMMs) are typically classified into three categories: contact measurement, non-contact measurement, and combined contact and non-contact measurement.
Contact measurement is a commonly used method for measuring machined products, pressed products, and metal films. Scanning data points on the surface of the measured object with a CMM is often necessary for analyzing processing data or for reverse engineering purposes.
Using the Foundation-Pro CMM as an example, this article will describe the different common scanning methods and operation steps for CMMs.
The scanning operation of a CMM involves collecting data points in a specific area on the surface of the object being measured using the PC DMIS program. This area can be a line, patch, section of the part, curve of the part, or circumference at a certain distance from the edge.
The type of scan is dependent on the measurement mode, probe type, and whether CAD files are available. The “scan” option on the control screen is determined by the status button (manual/DCC).
If the DCC method is used for measurement and CAD files are available, the available scanning methods are “open line”, “closed line”, “patch”, “section”, and “perimeter” scanning. If only wireframe CAD files are available, the available scanning methods are “open line”, “closed line”, and “patch” scanning.
If manual measurement mode is used, only the basic “manual TTP scan” mode is available. If manual measurement is used with a rigid probe, the available options are fixed delta, variable delta, time delta, and body axis scan.
This article will provide a detailed explanation of the five scanning modes that can be selected when the “utility” menu is entered and the “scan” option is selected under DCC status.
1.Open Linear Scan
Open line scanning is the simplest scanning mode. The probe starts from the starting point, scans along a specified direction with a predetermined step size, and ends at the end point.
Open line scanning can be divided into two cases based on the availability of a CAD model.
(1) Without CAD Model:
If the measured workpiece does not have a CAD model, first input the nominal values of the boundary points. Open the “boundary point” option in the dialog box, click “1” to input the data of the starting point. Then, double-click “d” to input the new X, Y, and Z coordinate values of the direction point (the coordinate point indicating the scanning direction). Finally, double-click “2” to input the data of the end point.
Next, input the step size. Enter a new step length value in the “Max Inc” column in the “Direction 1 Tech” column in the scan dialog box. Finally, verify that the set direction vector is correct, which defines the normal vector of the surface of the first measurement point after the start of scanning, the cross-section, and the surface normal vector of the last point before the end of scanning. Click “create” after all the data has been entered.
(2) With CAD Model:
If the measured workpiece has a CAD model, click the corresponding surface of the CAD model with the left mouse button at the start of scanning, and the PC DMIS program will generate a point on the CAD model and mark it as “1”, the starting point. Then, click the next point to define the scanning direction. Finally, click the end point (or boundary point) and mark it as “2”. Connect the line between “1” and “2”.
For each selected point, the PC DMIS program will enter the corresponding coordinate value and vector in the dialog box. After determining the step size and other options (such as safety plane, single point, etc.), click “measure” and then “create”.
2. Closed Linear Scan
Closed line scanning mode allows for the scanning of the inner or outer surface of a workpiece. It only requires two values, the “starting point” and the “direction point” (PC DMIS takes the starting point as the end point).
(1) Data Input Operation:
Double-click the boundary point “1” to enter its position in the edit dialog box. Double-click the direction point “d” to enter its coordinate value. Select the scan type (“linear” or “variable”), enter the step size, and define the touch type (“vector”, “surface”, or “edge”).
Double-click the “initial vector” and enter the vector at point “1”. Verify the section vector. After entering other options, click “create”.
You can also touch the first measuring point on the surface of the workpiece using the coordinate measuring machine’s control panel, then touch the direction point. The PC DMIS program will automatically input the measured value into the dialog box and calculate the initial vector.
After selecting the scanning control mode, measuring point type, and other options, click “create”.
(2) Closed Line Scanning with CAD Model:
If the measured workpiece has a CAD model, confirm “closed line scanning” before measurement. First, click the starting point of the surface to generate symbol “1” on the CAD model (when clicking, the surface and boundary points are highlighted to help select the correct surface). Then, click the scanning direction point.
PC DMIS will provide the corresponding coordinates and vectors of the selected points in the dialog box. After selecting the scanning control method, step size, and other options, click “create”.
3. Patch scan
The Patch Scanning Mode allows you to scan an area, rather than just a single scanning line.
This scanning method requires at least four boundary point information, including the starting point, direction point, scanning length, and scanning width.
The PC DMIS can calculate the triangular patch based on boundary points 1, 2, and 3, which are defined by the basic or default information. The scanning direction is determined by the coordinate value of point D.
If you add a fourth or fifth boundary point, the patch can be either square or pentagonal in shape.
When using the Patch Scanning Method, make sure to select “Closed Line Scanning” in the checkbox to scan closed elements such as cylinders, cones, grooves, etc. Then, enter the start point, end point, and direction point.
The end point position represents the upward or downward distance moved during the scanning of the measured element.
The section plane vector can be defined by the starting point, direction point, and starting vector (which is typically parallel to the measured element).
Three methods for defining patch scanning are introduced, using the creation of quadrilateral patches as an example:
(1) Coordinate Value Entry:
- Double-click boundary point “1” and enter the starting point’s X, Y, and Z coordinate values.
- Double-click boundary direction point “d” and enter the scanning direction point’s coordinate value.
- Double-click boundary point “2” and enter the scan width to define the first direction.
- Double-click boundary point “3” and enter the scan width to define the second direction.
- Click “3”, then press the “Add” button to display the fourth boundary point in the dialog box.
- Double-click boundary point “4” and enter the end point’s coordinate value.
- After defining the step size and maximum step size required for scanning, click “Create”.
(2) Touch Testing Mode:
- Select the “Patch Scanning” mode and touch the first point at the desired starting point using the coordinate measuring machine’s probe.
- The coordinate value of this point will be displayed in the “#1” item of the “Boundary Point” dialog box.
- Touch the second point, which represents the end point of the first scanning direction, and its coordinate value will be displayed in the “d” item of the dialog box.
- Touch the third point, which represents the width of the scanned patch, and its coordinate value will be displayed in the “#3” item of the dialog box.
- Click “3” and select “Add” to add the fourth point to the list.
- Touch the end point and the dialog box will close.
- Finally, define the scanning line spacing and step length in both directions.
- After selecting the scanning touch type and required options, click “Create”.
(3) CAD Surface Model Mode:
- This scanning method is only applicable to workpieces with a CAD surface model.
- Select the “Patch Scanning” mode and left-click the CAD working surface.
- Highlight “1” in the “Boundary Point” dialog box and left-click the scanning start point on the surface.
- Highlight “d” and click the surface to define the direction point.
- Click the surface to define the scanning width (#2).
- Click the surface to define the width of the scan (#3).
- Click “3”, select “Add”, add the additional point “4”, highlight “4”, click to define the scanning end point, and close the dialog box.
- After defining the steps in both directions and selecting the required options, click “Create”.
4. Section Scan
The Section Scanning mode is only applicable to workpieces with a CAD surface model.
It enables scanning of a specific section of the workpiece.
The scanned section can be along the X, Y, or Z-axis direction or at a specific angle with the coordinate axis.
Multiple section scans can be performed by defining the step size.
You can set the boundary point of the section scan in the dialog box.
By pressing the “Cut CAD” conversion button, you can locate any holes in the CAD surface model and define its boundary line in the same way as Open Line Scanning.
The PC DMIS program will automatically adjust the scanning path to avoid any holes in the CAD surface model.
To cut the CAD surface model by user-defined surface, follow these steps:
- Enter the “Boundary Point” option.
- Enter the “CAD Element Selection” box.
- Select the surface.
- Select the “Cut CAD” option without clearing the “CAD Element Selection” box.
At this point, the PC DMIS program will cut the selected surface to find any holes.
If there are no holes defined in the CAD surface model, it’s not necessary to select the “Cut CAD” option. In this case, PC DMIS will scan according to the defined start and end boundary points.
For complex CAD graphics with multiple surfaces, different surfaces can be sectioned in groups. The number of groups is restricted to the local CAD surface models.
5. Perimeter scan
The Perimeter Scan method is only applicable to workpieces with a CAD surface model.
This scanning mode uses the CAD mathematical model to calculate the scanning path, which is offset from the boundary or outer contour by a user-selected distance.
To create a boundary scan, follow these steps:
- First, select the “Boundary Scan” option.
- If it’s an internal boundary scan, select “Internal Boundary Scan” in the dialog box.
- When selecting a working surface, start the “Select” checkbox, highlight each selected surface, and exit the checkbox after selecting all desired surfaces.
- Click on the surface to determine the scanning starting point.
- Click on the same surface to determine the scanning direction point.
- Click on the surface to determine the scanning end point. If no end point is given, the start point will be used as the end point.
- Enter corresponding values in the “Scan Structure” edit box (including “Value Added”, “CAD Tolerance”, etc.).
- Select the “Calculate Boundary” option to calculate the scan boundary.
- After confirming that the deviation value is correct, press the “Generate Measuring Point” button. The PC DMIS program will automatically calculate the theoretical value of the scanning.
- Click “Create”.
6. Application points
(1) To improve the accuracy of data acquisition and efficiency of measurement, it is important to choose the appropriate scanning measurement mode based on the specific characteristics and modeling requirements of the workpiece being measured.
(2) The clamping position of the workpiece should be planned carefully to facilitate the measurement process and the movement of the probe. To ensure modeling accuracy, try to arrange the probe so that it completes scanning measurements of all objects at once when clamping the workpiece.
(3) The selection of scanning measurement points should include key points of the geometric information of the workpiece’s contour and measurement points should be added appropriately at parts with significant curvature changes.
8. Data management
1. Data conversion
Tasks and Requirements for Data Conversion:
(1) Convert the measurement data format into the IGES format recognized by CAD software and save it with a product name or a user-specified name after combining.
(2) Data with different products, different attributes, and different positioning that is prone to confusion should be stored in separate files and should be organized and separated in IGES files.
The data conversion is performed by the Coordinate Measurement Data Processing System.
For the operation method, refer to the software user manual.
2. Relocation and integration
During the process of product surveying and mapping, it’s often not possible to measure the product’s geometric data in the same coordinate system for various reasons.
The first reason is that the product size exceeds the measuring machine’s stroke.
The second reason is that the measuring probe cannot reach the opposite side of the product.
The third reason is that data is missing after the workpiece is removed and needs to be re-measured.
In such cases, it’s necessary to measure each part of the product in different positioning states (i.e., different coordinate systems), which is known as Product Relocation Measurement.
In modeling, data from different coordinate systems in different positioning states must be transformed into the same coordinate system, which is called the Integration of Relocation Data.
For complex or large models, multiple positioning measurements are often required in the measurement process.
The final measurement data must be relocated and integrated multiple times according to a specific conversion path, to convert data measured in each positioning into measurement data under a common positioning benchmark.
Relocation integration principle
There is a discrepancy between the measurement data after the workpiece is moved (relocated) and the measurement data before the movement.
To integrate the relocated measurement data into the data before the movement, a shape that can be measured both before and after relocation on the workpiece (referred to as the Relocation Benchmark) must be established. As long as the measurement results of the shape after relocation match the measurement results before relocation through a series of transformations, the relocated measurement data can be integrated into the data before the movement.
The Relocation Benchmark serves as a link in the integration of relocated data.
PID control stands for Proportional, Integral, and Differential control.
The system’s response process to the position error can be determined by analyzing the relationship between the system’s stability, rigidity, and positioning error.
A lower value indicates a more stable system with reduced oscillation, but with weaker rigidity and larger positioning error.
On the other hand, a higher value results in better rigidity and smaller positioning error, but the system may experience oscillation.
The control of the static positioning error caused by friction and load is determined by the relationship between the control value and the arrival time at the theoretical position.
A lower value results in a longer arrival time.
A higher value increases the likelihood of oscillation at the theoretical position.
The parameter provides stability and damping to the system by avoiding excessive error change.
A lower value results in a faster response of the system to the position error.
A higher value results in a slower response of the system.
9. Daily maintenance
To prevent “holiday syndrome” it is necessary to change the management mode of the CMM.
The CMM is composed of complex components, including mechanical parts, electrical control parts, and computer systems.
It is important to properly maintain the CMM while using it to measure workpieces, in order to extend its lifespan.
The following explains the basic maintenance of the CMM from three perspectives.
There are various types of mechanical components in a Coordinate Measuring Machine (CMM). To ensure proper functioning, it is important to perform daily maintenance on both the transmission system and air circuit system components.
The frequency of maintenance should be determined based on the operating environment of the CMM. In fine measuring rooms with optimal conditions, regular maintenance is recommended every three months. However, in environments with high levels of dust or where temperature and humidity do not meet the requirements for proper operation, maintenance should be performed on a monthly basis.
For the regular maintenance of measuring machines, the factors affecting the measuring machines should be understood:
Influence of compressed air on measuring machine
To choose an appropriate air compressor, it is advisable to incorporate an additional air tank. This will enhance the longevity and stability of the air compressor’s pressure.
It is important that the initial pressure of the air compressor is higher than the required operating pressure.
When starting the equipment, it is recommended to first activate the air compressor, and then turn on the power supply.
Influence of oil and water on measuring machine
Compressed air is crucial for the smooth functioning of a measuring machine, so it is essential to properly maintain the air circuit.
The following tasks should be performed regularly:
- Before using the measuring machine each day, inspect the pipes and filters and drain any water and oil from the filter and air compressor/air tank.
- Clean the filter elements of the attached filter and pre-filter at least every three months.
- Shorten the interval if the air quality is poor. Over time, oil pollution can clog the filter elements, reducing the actual operating air pressure of the measuring machine and compromising its normal operation.
- Regular cleaning of the filter elements is necessary.
- Daily wiping of the guide rail to remove oil stains and dust will help keep the air floating guide rail in good working condition.
To protect the guide rail of measuring machine, good working habits shall be formed
To ensure the safety of the guide rail, place cloth or rubber padding underneath.
After work or completion of parts, be sure to clean the guide rail.
While using the measuring machine, strive to maintain a consistent ambient temperature in the measuring room as during calibration.
Keep in mind that electrical equipment, computers, and personnel all generate heat. During installation, arrange electrical equipment, computers, etc. to be at a sufficient distance from the measuring machine.
Strictly manage the measuring room and minimize the presence of extra personnel.
The management of the operating environment for high precision measuring machines should be especially stringent.
Influence of air conditioning wind direction on temperature of measuring machine
For the air conditioning in the measuring room, it is best to choose a variable frequency air conditioner.
Variable frequency air conditioning has excellent energy-saving capabilities and, most importantly, strong temperature control abilities. At normal capacity, it can regulate the temperature to within ±1 ℃.
Note that the air from the air conditioner may not be at 20 ℃, and should not be directed directly at the measuring machine. To prevent this, the wind direction may be redirected towards a wall or side, leading to a large temperature difference in the room.
The air conditioner should be installed in a planned manner, blowing air to the main area of the room. The wind direction should be upwards to create a large cycle (not towards the measuring machine) and balance the indoor temperature as much as possible.
If feasible, an air duct can be installed to deliver air to the top of the room through double-layer orifice plates, with the return air outlet at the lower part of the room. This will create irregular airflow and make the temperature control in the measuring room more efficient.
Influence of switching time of air conditioner on temperature of machine room
You are required to activate the air conditioning system at your workplace every morning and turn it off at the end of the day.
Once the temperature in the standby room has stabilized for approximately four hours, the measuring machine’s accuracy will also become stable.
However, this operating procedure significantly hinders the efficiency of the measuring machine, making it challenging to guarantee accuracy during both winter and summer.
This will also have a substantial effect on the measuring machine’s normal stability.
Influence of machine room structure on machine room temperature
To maintain a constant temperature in the measuring machine room, thermal insulation measures should be implemented.
In case there are windows, double-paned windows should be installed and direct sunlight should be avoided.
Using a transition room will help reduce temperature loss.
The air conditioning system in the machine room should be of comparable capacity to that of the surrounding room.
If the machine room is too large or too small, it will present difficulties in temperature control.
In areas with high humidity in the South or during the summer or rainy season in the North, sudden shutdown of the cooling air conditioner can cause rapid condensation of water vapor in the air on the low temperature parts and guide rails of the measuring machine, leading to serious corrosion of the air floats and some parts of the machine, affecting its service life.
Excessive humidity can also cause corrosion or short circuits in the circuit boards of computers and control systems.
Low humidity can severely impact the water absorption of granite and cause deformation.
Dust and static electricity can harm the control system.
Therefore, the humidity in the machine room must be controlled within 60% ± 5%.
Poor sealing and high air humidity in the measuring machine room are the primary causes of high humidity.
In areas with high humidity, the machine room should have better sealing and dehumidifiers should be added if necessary.
To address this, the management mode should be changed from “cleaning before holidays” to “cleaning during work,” and the air conditioner and dehumidifier should be turned on to remove moisture.
Regular cleaning of dust in the computer and control system will reduce or prevent potential problems.
Using standard parts for machine inspection is effective but relatively cumbersome and can only be performed periodically.
A more convenient method is to use a representative part, compile an automatic measurement program, and carry out multiple measurements after verifying the machine’s accuracy.
The results can be calculated according to statistical laws, and a reasonable value and tolerance range can be recorded.
The operator can frequently check this part to determine the machine’s accuracy.
Adjustment of Z-axis balance
The Z-axis balance of the measuring machine is divided into weight and pneumatic balance, which helps balance the weight of the Z-axis and ensures its stable operation.
If the air pressure balance switch is accidentally triggered, the Z-axis will become unbalanced.
To resolve this issue, follow these steps:
- Rotate the measuring base to 90 degrees to prevent the measuring head from making contact during operation.
- Activate the “emergency stop” switch.
- One person should physically hold the Z-axis and move it up and down to assess its balance.
- Another person should adjust the air pressure balance valve, making small adjustments at a time.
Two people can work together to adjust the Z-axis balance until it feels balanced when moving up and down.
The end-of-travel switch serves to protect the machine and establish its home position.
Typically, the end-of-travel switch is either a contact switch or a photoelectric switch.
The contact switch is prone to changing position when manually pushing the shaft, leading to poor contact.
To ensure good contact, the switch position can be properly adjusted.
When using a photoelectric switch, it is important to check that the insert’s position is normal and to regularly remove any dust to maintain its proper operation.
10. Use and safety precautions of CMM
Only individuals who have received training and obtained operational certification are allowed to operate the CMM.
Before starting the machine each day, the control cabinet can only be opened once the air supply pressure meets the requirement: the air supply pressure must be ≥ 0.65 MPa, and the machine air pressure must be ≥ 0.4 MPa.
If the height of the oil-water mixture in the triplet water storage cup exceeds 5mm, the water must be manually drained.
If the machine air supply pressure is normal, but the pressure at the triplet cannot be adjusted to the normal value, the filter element needs to be replaced.
The operating environment for the CMM should have a temperature of 20 ± 2°C and a relative humidity of 40-75%.
The regulated power supply should have an output voltage of 220 ± 10V.
No objects should be placed on the machine’s guide rail.
Do not touch the guide rail’s working surface with your hands.
Before starting the machine each day, clean the three-axis guide rail surface with a high woven pure cotton cloth dipped in anhydrous alcohol, and the machine can only be operated once the guide rail surface is dry.
Alcohol should not be used to clean the painted surface or the grating ruler.
The startup sequence is as follows:
- Turn on the power box
- Turn on the total air source
- Turn on the cold dryer
- Turn on the air valve
- Turn on the control cabinet’s power supply
- Turn on the pedestal controller
- Turn on the motor when the operation box light is on (the emergency stop key must be released)
- After the system’s self-inspection is complete, start the measurement software, zero the three axes (go home), and enter the normal working state after automatic completion.
The machine must be returned to the zero point after each startup.
Before returning to the zero point, move the probe to a safe position to ensure that there are no obstacles during the probe’s reset and upward movement in the Z-axis.
When replacing the probe, use the special tools provided with the machine and calibrate the new probe:
- Start the software before opening the measurement software (recommended method)
- If the measurement software is turned on, press the emergency stop key switch on the operation box and turn it on after replacement.
If a probe error message dialog box appears, close it (or enter 18.104.22.168 in the web address input column to view the error history and message). This message will be automatically cleared during the next startup.
During manual operation, press the slow key when approaching the sampling point.
When rotating the probe, calibrating the probe, automatically changing the probe, or performing any other operations, ensure that there are no obstacles in the probe’s path of movement.
When the program is not in use or not programmed, set the speed on the operation box to 0.
During the first operation of the program, reduce the speed to 10-30% and monitor whether the operation track meets the requirements.
When handling and placing workpieces, first move the probe to a safe position, and ensure that the workpieces do not collide with the work table, especially the machine’s guide rail surface.
The shutdown sequence is as follows:
- Move the Z-axis to the left, front, and top of the machine and rotate the probe angle to A0B0.
- Clean the work surface.
- Turn off the pedestal controller, the control cabinet’s power supply, the air valve, the dryer, the main air source, and the power box in that order.
Steel standard balls that are not used for an extended period of time should be sealed with oil to prevent rust.
When securing a workpiece with an inlay on the granite work surface, the torque should not exceed 20 Nm.
If any abnormal conditions are detected (excluding error messages related to probe replacement), record the error information displayed by the software, contact the technical service department of Hexcon via fax or telephone, and do not perform inspection or maintenance without guidance and permission.
Do not install any software that is not related to the three coordinates on the computer to ensure the system’s reliable operation.
The air conditioner should be left running 24 hours a day, and its maintenance should be performed in the autumn to ensure the normal operation of the three coordinates.
FAQs about CMM machine
What is CMM?
The three-coordinate measuring machine is commonly referred to as a measuring system that determines the three-dimensional coordinates of the surface points of a workpiece through the relative movement of the probe system and the workpiece.
It is also known as a coordinate measuring machine (CMM) or three-coordinate measuring instrument.
Does the temperature have a great influence on the measurement results of the CMM?
The CMM is a complex measurement system that combines light, machinery, electricity, computer, and control technology, which means there are many factors that can affect the measurement results’ uncertainty.
However, for medium and small coordinate systems, the main factor affecting the measurement results’ uncertainty is deviation from the standard measurement temperature (20°C) of the ambient temperature. To achieve accurate coordinate measurement results, the ambient temperature must be strictly controlled within the range specified by the coordinate machine’s instructions.
What items of CMM need to be calibrated and how long is the re calibration interval?
The current calibration standard for three coordinates is JJF1064-2000 calibration specification for coordinate measuring machines, which specifies that the calibration items are length measurement indication error and detection error.
It is recommended to calibrate once a year.
When does the CMM need to calibrate 21 errors?
The 21-point error is the foundation of the CMM’s accuracy, and its calibration is complex.
Even though it is not specified in the standard, calibration of the 21-point error is necessary under the following circumstances: when accepting a new machine, when the length measurement error calibration result falls outside of the tolerance range, after the coordinate machine has been relocated, and after the coordinate machine has been repaired.