Advancements in Online Measurement for CNC Machine Tools

With the swift advancement of the aviation, aerospace, and automotive manufacturing sectors, processing and evaluating large, complex, and high-precision parts has become a major concern. Typically, these components must undergo several rounds of processing, measuring, and trimming to meet design specifications.

CNC machines, as efficient and precise manufacturing equipment, are widely used in manufacturing enterprises and are evolving towards higher precision, efficiency, openness, intelligence, and complexity.

The objective of the composite is to utilize a single machine tool as much as possible to accomplish all or most of the processing tasks, ensuring the accuracy of the workpiece position and enhancing production efficiency.

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

The conventional offline measurement method, which involves disassembling and moving workpieces for inspection, has the issue of secondary clamping and positioning, leading to poor consistency between machining and measurement results and resulting in longer production cycles and lower productivity.

Disassembling parts for inspection is a significant barrier to the overall efficiency of digital manufacturing.

On-line measurement is a machining and measuring process implemented on the same equipment, enabling the completion of the workpiece after one loading and measuring operation, avoiding secondary clamping and positioning errors.

It reduces measurement cost, shortens production time, improves production efficiency and machining accuracy.

The CNC machine tool on-line measurement technology boasts fast sampling speed and high precision, allowing for the digital collection of workpiece data and precise evaluation.

Compared to coordinate measuring machines, CNC machine tool on-line measurement has more error influencing factors in a complex environment and is therefore less expensive, less cost-effective and less widely used.

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

CNC machine tool on-line measurement technology is a crucial aspect of processing measurement integration technology, which enhances the function of CNC machine tools and effectively improves the utilization of existing machine tools, ensuring the quality of parts processing.

As a result, CNC machine tool on-line measurement has received attention and application from modern manufacturing enterprises and has significant research and application value.

Researchers both domestically and abroad have conducted extensive research in this field and have successfully applied it in practice.

On-line measurement of machine tool structure 

Modern CNC machine tools have been greatly improved compared to previous ones due to their openness. The good scalability and compatibility of modern CNC systems allow for the possibility of accurate three-dimensional coordinate measurement.

When the machine tool and measurement system are integrated, they can not only process parts but also perform online measurement of workpieces.

The CNC online measurement system consists of two main components: hardware and software.

Similar to CNC machining systems, the hardware components include CNC machine tool systems and probe systems.

The software system utilizes secondary development technology to implement online measurement programming, similar to CNC machining programming. This results in the creation of NC code that drives the CNC machine tool to perform measurements.

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

One way to use basic macros is to do so directly, without relying on computer support.

The other method involves using the machine’s CNC system to provide instructions. The user must develop and compile the application system to generate inspection procedures, which are then transmitted to the CNC system.

In industrialized nations, probes have become an essential part of CNC machine tools, similar to cutting tools. They are increasingly being utilized in the field of machinery manufacturing.

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

There are two types of measuring probes in a machine tool: a workpiece measuring probe and a tool measuring probe. The workpiece measuring probe is mounted on the spindle and used with the machined workpiece as the measuring object. The tool measuring probe, on the other hand, is used with the tool as the measuring object and is fixed in a specific position on the machine tool.

Typically, the online measurement of the machine tool is performed using the workpiece measuring probe, which can be measured either manually or automatically based on the measurement program.

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The CNC online measurement system is integrated with the CNC machine tool system and operates similarly to the machining process. However, the existing online measurement systems mostly offer limited measurement functions and cannot accommodate the complexity and diversity of processed parts.

To address this issue, the machine tool online measurement system is integrated with the CAD system. With the secondary development of the CAD system, measurement programming and simulation verification are made possible, increasing the flexibility and scope of CNC machine tool online measurement and realizing Design-Manufacturing-Inspection (DMI) integration.

The on-line measurement process for machine tools

1. Principle of operation

The accuracy of an inspection system is largely dependent on the probe used. The most commonly used probe for this purpose is a touch probe with the ability to seek advancement. This probe provides a trigger signal to the CNC machine, allowing it to determine the coordinates of the trigger point.

One of the most important aspects of a probe system is its ability to generate a programmed interrupt instruction. When the probe tip comes into contact with the workpiece, the probe system sends an external interrupt request to the CNC machine. This request is initiated by the probe trigger signal.

Once the machine control system receives the interrupt, the positioning system latches the coordinate value of the sphere center of the measuring end to determine the contact point between the measuring end and the workpiece.

Touch probes are known for their high measurement accuracy and are widely used in on-line inspection systems for CNC machine tools. This is due to their simple structure, ease of use, low manufacturing costs, and high trigger accuracy.

The movement of the on-line inspection is controlled by the input of the CNC detection program into the CNC system. However, different CNC systems used by CNC machine tools may have different control methods and programming codes.

2. Probe positioning

For accurate, efficient, and quick completion of every online measurement by the CNC machine, multiple measurement triggers are necessary for a single measurement task.

Three distances must be set depending on the movement of the probe during a single measurement.

(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 being measured.

The probe moves quickly before reaching the pre-contact distance.

(2) Search distance.

This setting determines the maximum distance the probe can travel from the nominal size of the part in the direction of entry into the material being tested.

When the probe is activated during this distance movement, the machine will lock onto the coordinates of the activation point.

During the search distance phase, the probe should be moving at the specified measurement speed.

(3) Fallback distance.

This is the distance that the probe will retract in the opposite direction after making contact with the surface under examination.

Once contact with the surface to be measured has been made, the probe must be withdrawn in the opposite direction to prevent breakage due to excessive movement. The retraction distance must be sufficient to allow the probe to reach the next pre-contact or positioning point safely.

During the retraction phase, the probe will be returned at the retraction speed.

To meet the different requirements of each stage of the probe’s movement, three distances are measured and three speeds are specified: positioning speed, measuring speed, and retraction speed.

The measuring speed should be slow to reduce errors in the measurement and to prevent damage to the stem.

To enhance the efficiency of the measurement process, the positioning speed and retraction speed can be set at a higher value to allow for faster probe movement and reduce measurement time.

To avoid damaging the stylus bar as the probe moves forward after touching the surface, machine tool probing erases the remaining stroke upon receiving a trigger signal.

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When the probe receives a trigger signal during the programmed stroke movement, the current coordinate value is noted and the remaining stroke is skipped to proceed to the next line of code, which is referred to as “remaining stroke deletion.”

Currently, CNC systems typically come with basic measurement instructions, or measurement system development units or personnel can provide some of the packaged measurement instructions for users.

3. Testing path planning

CNC Machine Tool Probing is a system that takes measurements by sampling the surface to be measured. The accuracy of the measurement results is directly impacted by the number and distribution of sampling points, particularly for free-form surfaces. Since it’s unrealistic to sample the entire surface, increasing the number of inspection points is often done to improve the reliability of the results, but this can significantly reduce the efficiency of the measurement.

Therefore, it’s essential to plan an inspection path that is both efficient and accurate. By using machine tool measurement online, the measurement efficiency can be maximized while still meeting the accuracy requirements. The goal is to find 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 results can be achieved using the four-point measurement method. This method is also useful for bore measurement.

Based on the path planning requirements, there are established measurement paths for planar, raised/notched, and angular measurements. In the case of more complex measurements, the programmer may need to develop a CAD system as a secondary tool for interactive measurement path planning and programming within the CAD environment, using basic measurement principles.

Measurement error analysis

In every measurement, there’s always an error present in the measurement value due to various factors. To make the measurement results more closely approximate the real value, it is necessary to account for the measurement error. To achieve this, the measurement process must be carefully analyzed and the sources of error components that affect accuracy must be considered.

In the case of the CNC machine tool online measurement system, which uses the machine as the matrix and integrates the measurement system for generation, errors in the CNC machine tool processing process can also impact the accuracy of the measurement.

The online measurement error of the machine tool is primarily comprised of the probe system error, the positioning error of the machine tool’s moving parts, and the error caused by an unreasonable measuring path.

The probe system error is further divided into the static error of the probe, the dynamic error of the probe, and the probe installation error on the machine. The static error includes dead zone and probe positioning errors, which change with changes in probe length, stiffness, and contact pressure. Dead zone error is the amount of bending deformation of the stylus when the probe touches the part, while probe indexing error is relatively small in comparison.

As a result, the static probe error is mainly determined by the dead zone error. The dynamic error of the probe is primarily related to the probe inspection contact speed and the CNC system’s sampling interval.

The probe is mounted on the machine tool’s spindle through the shank, which is matched with the machine tool. However, due to the incomplete alignment of the probe axis and spindle axis, there is a probe installation error, which causes measurement error in multi-directional measurements. This misalignment can be partially compensated for through eccentric calibration of the probe prior to measurement.

Additionally, each working part of the machine tool will produce positioning errors during measurement due to manufacturing errors in CNC machine parts, assembly errors, servo system tracking errors, and gaps, friction, and other factors.

Probe radius error is another major source of error, which can be eliminated through probe radius compensation during data processing. However, in practice, the situation is more complex, with probe radius errors affecting the measurement results, particularly in the case of free-form surfaces.

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To effectively address the many sources of error in the measurement process, an efficient and high-precision error compensation algorithm is crucial. In practical applications, multiple measurements and error compensation can be used to reduce measurement errors and improve accuracy.

Integration of machine tool measuring systems with CAD

CNC machine tool probing, as a typical representative of the M-I mode, has greatly reduced the production cycle. However, it is not linked to the design model of the part, causing inconvenience in the interactive planning of measurement paths. Furthermore, errors can accumulate when reworking based on the measurement results.

After the integration of the D-M-I mode, the error can be compensated, resulting in improved measurement accuracy. In view of the superior accuracy and flexibility of the D-M-I mode compared to the M-I mode, the D-M-I mode CNC machine tool online measurement system has been adopted for machining, measuring, and dressing of structurally complex parts, to enhance measurement and machining accuracy.

The PC is connected to the CNC machine tool, and the integration of the CAD system and CAI software system is mainly carried out on the PC, while the hardware system integration of the NC system and CAI is performed on the CNC machine tool, resulting in the integration of CAD/NC/CAI.

Here is an example of the implementation of a specific measurement system:

Pro/Engineer is selected as the CAD system of the CNC machine tool online measurement system in D-M-I mode, based on the user’s requirements. 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 through simulation.

The relevant functions are implemented via secondary development in Pro/Engineer, where new function menus are added and developed. The machine tool online measuring system is operated by an operator who first loads the target part model into the virtual CNC operating environment.

Next, the virtual operating environment is initialized to establish the relationship between the assembly coordinate system and the actual machine coordinate system, as well as the transformation matrix of each moving part. Finally, the operator selects the measurement surface, plans the measurement path, and simulates the measurement process through the function menu.

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

The system integrates the CAI operating software into the Pro/Engineer environment through secondary development, allowing for seamless connection between the CAD and CAI systems. The Pro/Engineer-based virtual measurement environment simulates the actual process and verifies the reliability of the measurement or machining process.


The Probe System and CNC Machine Tool have been integrated into the Machine Tool Online Measurement System, which can significantly reduce auxiliary time in production, lower the labor intensity for workers, increase production efficiency, and minimize the error caused by scrap rates during offline measurement. This integration fully utilizes the performance of CNC machine tools.

The implementation of machine tool measurement eliminates intermediate links and enhances the accuracy of processing while increasing the processing capacity of CNC machine tools.

Digital data collection of workpieces can be achieved, and the 3D reconstruction of the workpiece surface can be completed with the help of a Computer-Aided Design System at a later stage.

By secondary development of CAD on the D-M-I mode Machine Tool Online Measurement System, the powerful graphic interaction and design capabilities of CAD can be fully utilized. Users can interactively plan measurement paths, making the system easier to use.

This direction of research and application system development has a high practical value and raises the level of CNC machine tool application.

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