Research on on-line measurement technology for CNC machine tools

With the rapid development of the aviation, aerospace and automotive manufacturing industry, high-precision, complex and large parts processing and accuracy evaluation has become a prominent concern in the industry.

Usually, such workpiece products need to be processed, measured and trimmed several times to meet the design requirements.

CNC machine tools as efficient, high-precision manufacturing equipment in the manufacturing enterprises have been widely used, and is moving towards high precision, high efficiency, open, intelligent, complex direction of development.

The goal of the composite is to use a single machine tool as much as possible to complete all or most of the processing tasks, to ensure the accuracy of the workpiece position and improve production efficiency.

Coupled with people’s pursuit of high precision and high efficiency in workpiece machining, on-line measurement technology integrated with CNC machine tools has received widespread attention in actual production.

The traditional offline measurement method, i.e., the inspection method of disassembling and moving workpieces, involves the problem of secondary clamping and positioning, which makes the consistency of machining results and measurement results poor, resulting in longer production cycle and lower productivity.

Disassembly of moving parts for inspection is a major hindrance to the overall efficiency of digital manufacturing.

On-line measurement is a machining and measuring process implemented on the same equipment, the workpiece can be completed after one loading and measuring work, avoiding the secondary clamping and positioning errors.

It can reduce the measurement cost, reduce production time, improve production efficiency and machining accuracy.

The CNC machine tool on-line measurement technology has the characteristics of fast sampling speed and high precision, which realizes the digital data collection and precision evaluation of the workpiece.

Compared with coordinate measuring machining, due to the complex environment and more error influencing factors of CNC machine tools on-line measurement, the coordinate measuring machine is expensive, cost performance and application is far less extensive than CNC machine tools.

Therefore, when the accuracy requirements are not very high, CNC machine tool on-line measurement technology is more advantageous.

CNC machine tool on-line measurement technology is an important part of the processing measurement integration technology, which can expand the function of CNC machine tools and effectively improve the use of existing machine tools to ensure the quality of parts processing.

Therefore, CNC machine tool on-line measurement has been modern manufacturing enterprises attention and application, which also has an important research and application value.

Researchers at home and abroad have carried out a lot of research work on this aspect, and have promoted and applied it in practice.

On-line measurement of machine tool structure 

Modern CNC machine tools in the openness have been greatly improved than previous machine tools, modern CNC system with good scalability and compatibility makes a CNC machine tool contain a certain accuracy of three-dimensional coordinate measurement function possible.

If the machine tool and measurement system are organically integrated, it not only can process parts, but also can realize the workpiece online measurement.

CNC online measurement system consists of two main parts including hardware and software.

Similar to CNC machining systems, their hardware systems mainly include CNC machine tool systems and probe systems.

The software system uses secondary development technology to realize online measurement programming similar to CNC machining programming, and obtains the NC code that drives the CNC machine tool to achieve measurement.

There are 2 main types of CNC machine tool on-line measuring systems:

One invokes basic macros directly, without computer assistance.

The other is based on the machine’s CNC system to provide the CNC instructions, the user needs to develop and compile the application system at any time to generate inspection procedures, which is then transmitted to the CNC system.

In industrially developed countries, probes have basically become indispensable basic spare parts for CNC machine tools like cutting tools, which have been more and more widely used in the field of machinery manufacturing.

Probes used on CNC machine tools are mainly divided into two types:

One is a workpiece measuring probe that is mounted on the machine tool spindle when used with the machined workpiece as the measuring object;

the other is a tool measuring probe that is used with the tool as the measuring object in a fixed position on the machine tool.

Usually, the on-line measurement of the machine tool is a workpiece measuring probe, which can be measured manually or automatically according to the measurement (macro) program.

The CNC on-line measurement system is developed based on the CNC machine tool system and integrated with the measurement system, the measurement process is very similar to the machining process.

Although CNC machine tool on-line measurement has many advantages, the existing online measurement system are mostly dedicated with single measurement function, which can not meet the complexity and diversity of processed parts.

On the basis of the machine tool online measurement system, it is integrated with the CAD system.

After the secondary development of the CAD system, measurement programming and simulation verification are realized, which increases the flexibility and working range of CNC machine tool online measurement and realizes Design-Manufacturing-Inspection (DMI) integration.

The on-line measurement process for machine tools 

1. Principle of operation 

The key component of an inspection system that directly affects accuracy is the probe, a touch probe with the ability to search for advancement is the most commonly used, providing a trigger signal for the CNC to obtain the coordinates of a trigger point.

One of the most critical features of a probe system is the ability to generate a programmed interrupt instruction, so that when the probe tip is in contact with the workpiece under test, the probe system sends an external interrupt request to the CNC machine tool (the interrupt request is provided by the probe trigger signal).

Once the interrupt is received by the machine control system, the coordinate value of the sphere center of the measuring end is latched by the positioning system to determine the coordinate value of the contact point of the measuring end and the workpiece.

Touch probes have higher measurement accuracy than other probes, touch probes are widely used in the CNC machine tool on-line inspection system with the advantages of simple structure, ease of use, low manufacturing costs and high trigger accuracy.

On-line inspection movement is realized by the control of the CNC detection program input to the CNC system.

As the CNC machine tool uses different CNC system, its control method and programming code etc. are different.

2. Probe positioning 

In order for the CNC machine to complete every on-line measurement accurately, efficiently and quickly, multiple measurement triggers are required during a single measurement task.

Depending on the movement of the probe during a single measurement, three distances need to be set.

(1) Pre-contact distance.

This distance refers to the distance from the center of the probe to the nominal size of the contact point on the surface of the part to be measured.

The probe moves rapidly before it enters the pre-contact distance.

(2) Search distance.

This distance sets the maximum distance of the probe starting from the nominal size of the part along the direction of entering the material of the tested part.

If the probe is triggered during this distance movement, the machine will lock on to the coordinates of the trigger point.

During the search distance phase, the probe should move at the given measurement speed.

(3) Fallback distance.

This distance is the distance that the probe will retreat in the opposite direction after contact with the surface under test.

Once the probe has made contact with the surface being measured, the probe needs to be withdrawn in the opposite direction to avoid breaking from excessive movement, and the retraction distance must be large enough to allow the probe to reach the next pre-contact or positioning point safely.

During the retraction distance phase, the probe is returned at the retracting speed.

In order to meet the different requirements of each stage of the probe’s movement, three distances are measured and three speeds are included, namely positioning speed, measuring speed and retracting speed.

The measuring speed should be small to reduce errors in the measured value and to avoid breaking the stem.

In order to improve the efficiency of the measurement process, the positioning speed and retraction speed can be set at a higher value to ensure that the probe moves at a faster speed to reduce the measurement time.

To avoid the probe breaking the stylus bar as it moves forward after touching the surface being measured, machine tool probing erases the remaining travel after receiving a trigger signal.

Remaining stroke deletion, i.e., when the probe receives a trigger signal during the movement of the programmed stroke, the current coordinate value is noted and then skipped to continue the next line of code.

At present, CNC systems are generally provided with basic measurement instructions, or measurement system development unit or personnel will also provide part of the measurement instructions that have been packaged for the users.

3. Testing path planning 

CNC machine tool probing is a system in which measurements are taken by sampling.

The number and distribution of sampling points directly affect the measurement results, especially for free-form surfaces.

It is unrealistic to sample the entire surface to be measured, so in order to improve the reliability of the measurement results, the number of inspection points is usually increased, but the high accuracy will also greatly reduce the measurement efficiency.

Therefore, it is crucial to plan an efficient and accurate inspection path.

When planning the inspection path, the machine tool measurement on-line can increase the measurement efficiency as much as possible on the basis of meeting the requirements of measurement accuracy, i.e. the shortest measurement path with the fewest number of measurement points.

In cylindrical surface measurement, for example, the probe is positioned on the centerline of the surface, and high precision measurement results can be obtained using the four-point measurement method.

This measurement method is also applicable to bore measurement.

Guided by the path planning requirements, there are defined measurement paths for planar, raised/notched and angular measurements.

In the case of complex measurements, the programmer has to develop the CAD system as a secondary tool for interactive measurement path planning and programming in the CAD environment based on basic measurement principles.

Measurement error analysis 

In any measurement, there is always an error in the resulting measurement value due to various factors.

In order to make the measurement results more accurately approximate the real value, it is necessary to compensate for the measurement results, so the measurement process should be carefully analyzed and consider the sources of error components that affect the accuracy of the measurement.

As the CNC machine tool on-line measurement system used the machine as the matrix and integrated measurement system to generate, the errors in the CNC machine tool processing process will also affect the measurement accuracy during the measurement process.

The on-line measurement error of the machine tool mainly includes the system error of the probe, the positioning error of the moving parts of the machine tool, and the error caused by the unreasonable measuring path.

The system error of the probe is divided into the static error of the probe, the dynamic error of the probe and the error of the probe on the machine Installation error, etc.

The probe static error includes dead zone error and probe positioning error, which changes with the probe length, stiffness and contact pressure change.

Dead zone error is the amount of bending deformation of the stylus when the probe touches the part.

Probe indexing error is relatively small compared to the dead zone error, so the static probe error is mainly determined by the dead zone error.

Probe dynamic error is mainly related to the contact speed of the probe inspection and the sampling interval of the CNC system.

The probe is installed on the spindle of machine tool through the shank which is matched with the machine tool.

Due to the incomplete alignment of the probe axis and spindle axis, there is probe installation error, which causes the measuring error in multi-directional measurement.

The misalignment of the probe and the spindle can be partially compensated by the eccentric calibration of the probe before measurement.

Due to the manufacture of CNC machine parts, assembly errors, servo system tracking errors, as well as gaps, friction and other factors, each working part of the machine tool will produce positioning errors when measuring movement.

In addition to this, the probe radius error is a major source of error and can be eliminated through probe radius compensation during data processing.

The situation is more complex in practice, the probe radius errors are introduced into the measurement results, and it is more obvious in the case of free-form surfaces.

An efficient and high-precision error compensation algorithm is a key issue to address the many sources of error in the measurement process.

In practical applications, multiple measurement and error compensation can be used to reduce measurement errors and improve measurement accuracy.

Integration of machine tool measuring systems with CAD

CNC machine tool probing, as a typical representative of M-I mode, has greatly shortened the production cycle.

In practice, however, it is not connected to the design model of the part, which leads to inconvenience in the interactive planning of measurement paths.

In addition, errors accumulate when reworking based on measurement results.

After the integration of the D-M-I mode is realized, the error can be compensated to further improve the measurement accuracy.

In view of the higher accuracy and flexibility of the D-M-I mode compared to the M-I mode CNC machine tool online measurement system, for the machining, measuring and dressing of structurally complex parts, we have adopted the D-M-I mode CNC machine tool on-line measurement and machining to improve the measurement and machining accuracy.

The PC is connected with the CNC machine tool, the integration of the CAD system and the CAI software system is mainly completed on the PC, and the hardware system integration of the NC system and CAI is completed on the CNC machine tool, thereby achieving the integration of CAD/NC/CAI.

The following is an example of the steps to implement a specific measurement system.

According to the user’s requirements, Pro/Engineer is selected as the CAD system of CNC machine tool on-line measurement system in D-M-I mode.

The actual machining measurement environment is simulated by building CNC machine and workpiece models in a Pro/Engineer environment, where measurement and machining trajectories are planned and verified by simulation.

The relevant functions are implemented through secondary development in Pro/Engineer, where new function menus are added and developed.

The machine tool on-line measuring system is operated by an operator who first loads the target part model into the virtual CNC operating environment.

Subsequent initialization of the virtual operating environment, the purpose of which is to establish the relationship between the assembly coordinate system and the actual machine coordinate system in the virtual operating environment, and the transformation matrix of each moving part.

Finally, the operator selects the measurement surface, measurement path planning, and measurement process simulation according to the function menu.

When the part to be machined (measured) needs to be modified, it can be re-operated simply by activating the part in the virtual environment and then reactivating the entire virtual environment after the modification is complete.

The system integrates the CAI operating software into the Pro/Engineer environment through a secondary development technique, allowing a seamless connection between the CAD and CAI systems.

The Pro/Engineer-based virtual measurement environment is built to simulate the actual process and to verify the reliability of the measurement or machining process.

Conclusion

Probe system and CNC machine tool integrated into the machine tool online measurement system, which can significantly reduce the production of auxiliary time, reduce the labor intensity of workers, improve production efficiency and reduce the offline measurement error caused by the scrap rate, giving full play to the performance of CNC machine tools.

The application of machine tool measurement can reduce the intermediate links, ensure processing accuracy and improve the processing capacity of CNC machine tools.

The digital data collection of the workpiece can be realized, the 3D reconstruction of the workpiece surface can be realized with the help of computer-aided design system at a later stage.

Based on the D-M-I mode machine tool online measurement system, through the secondary development of CAD, it can make full use of CAD’s powerful graphic interaction capabilities and design functions, and the users can carry out interactive measurement path planning and facilitate the application of the system.

This direction of research and application system development has a greater application value, and enhance the level of application of CNC machine tools.

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