In the application of CNC machine tools for manufacturing product parts, there are many factors that can affect the quality of the parts, including CNC machine accuracy, workpiece materials, workpiece heat treatment, processing technology, coolant, and tools.
One area that has received little attention is the accurate setting of tool parameters. Therefore, this article will focus on the characteristics and development trends of the CNC machine’s accurate tool-setting method.
In general, there are two coordinate systems involved in the basic coordinate relationship: the machine coordinate system and the workpiece coordinate system.
The machine tool coordinate system is the inherent coordinate system of the machine tool, with the machine tool origin or machine tool zero point serving as the coordinate system’s origin.
To facilitate calculation and programming, a workpiece coordinate system is established within the machine tool coordinate system. A point on the workpiece serves as the coordinate system origin (also known as the program origin) to establish the workpiece coordinate system.
In daily work, the programming datum is typically aligned with the design and assembly datum to ensure accuracy.
CNC machine tool setting method and in-machine tool setting instrument
A machine tool’s machine coordinate system is typically fixed, while the workpiece coordinate system can be established in several ways based on the machining process’s actual needs, such as selecting different workpiece coordinate systems using G54 and G55.
The purpose of tool setting is to ensure that the path followed by the CNC program is the path of the tool tip on the spindle during CNC machining. The cutting tool‘s trajectory’s cutter location point must be precisely controlled from start to finish in the machine’s coordinate system, which serves as the sole datum for the machine.
At the time of programming, the programmer may not know the exact dimensions of the various tool sizes. Therefore, to simplify programming, a uniform datum is needed during programming, and the exact length and radius dimensions of the tool are then offset relative to that datum to obtain the cutting tool tip’s exact position during machining.
Thus, the purpose of tool setting is to determine the tool length and radius values so that the cutting tool tip’s exact position in the workpiece coordinate system can be determined during machining.
1.1 Commonly used methods of off-machine tool setting
A tool pre-setter is a measuring instrument used to pre-adjust and measure the length and diameter of the tool tip.
If the machine is connected to the DNC network, the tool length and diameter data can be remotely entered into the machining center’s NC as tool parameters. The advantage of this method is that the tool can be pre-set outside of the machine, and once installed on the machine, it can be used, saving a significant amount of auxiliary time.
However, the main disadvantage of this method is that the measurement results are static values, and the actual machining process cannot be updated in real-time to reflect the state of tool wear or breakage. Additionally, it cannot measure in real-time the thermal deformation caused by the machine tool expansion.
1.1.1 Tool Setting by Test Cutting
Tool setting by test cutting is a manual operation performed by a machine operator to make a small amount of cutting on a workpiece before it is officially machined.
The operator determines the current position of the tool tip using visual and auditory cues before proceeding to formal machining. The advantage of this method is that it is cost-effective and does not require additional investment in tools and equipment.
However, the main disadvantage is that it has low efficiency, requires a high level of technical skill from the operator, and is prone to personal error. In actual production, there are many derivatives of the test cut method, such as the gauge block method and the coloring method.
1.1.2 In-Machine Tool Setting
This in-machine tool setting method utilizes a measuring device (tool setting gauge) located on the machine table to measure the tools in the magazine according to a predetermined program. The tool length or diameter is then compared with a reference position or standard tool and automatically updated in the corresponding NC cutting tool datasheet.
Cutting tool detection also enables the identification of tool wear, breakage, or correct mounting type.
1.2 In-machine tool setting gauge
1.2.1 Composition of In-Machine Tool Setting Gauge
The in-machine tool setting gauge is generally composed of a sensor, signal interface, and tool setting macro program software.
Based on the working mode of the sensor, the in-machine tool setting gauge can be divided into two categories: contact tool setting gauge and laser tool setting gauge.
The contact tool setting gauge has a repeatable measurement accuracy of 1μm and can be further subdivided into the following categories based on the different signal transmission methods of the tool setting gauge:
- Cable-type tool setting gauge
- Infrared tool setting gauge
- Cordless tool setting gauge
Cable-type tool setting gauge
The reason why it is commonly used is because it does not require a switching component for the counterblade signal, making it a cost-effective option with good performance per unit.
However, one disadvantage of this tool setting gauge is the need for cables, which can limit its application. As a result, it is more suitable for use with small to medium-sized 3-axis milling machines/machining centers.
Infrared tool setting gauge
The signal transmission range of this tool setting gauge is typically limited to 6 meters.
One advantage of this gauge is its use of encoded HDR (High-Speed Data Transmission) infrared technology, which eliminates the inconvenience and potential security threats associated with cable trailing.
Another benefit is that it can be removed from the table at any time after tool setting, without taking up valuable machining space. Furthermore, a single tool setting gauge can be used on multiple machines, reducing overall costs.
However, this gauge is not a cost-effective option for use in small machining centers.
Due to its characteristics, this type of tool setting gauge is primarily used in medium-sized machines and large CNC vertical lathes.
Cordless tool setting gauge
The radio signal transmission range is generally above 10 meters, which offers the advantage of wide-ranging transmission and reduced susceptibility to environmental influences. Additionally, this type of tool setting gauge can be removed from the working table at any time without taking up machining space after tool setting, and can be used on multiple machines to reduce overall costs. It is mostly used for large or heavy machine tools.
The basic principle of a laser tool setting gauge is to use a focused laser beam as the trigger medium. When the laser beam is obscured by the rotating tool, a trigger signal is generated. Unlike contact tool setting systems, laser tool setting systems use non-contact measurement. This allows for measurement of even the smallest cutting tools without worrying about breakage due to contact force. The gauge can measure cutting tool diameters as small as 0.008mm (e.g. drills, taps, or micro-mills) with a self-repeating measurement accuracy of 0.2μm.
Since the tool rotates at high speed during measurement, the measurement state is almost exactly the same as the actual machining state, improving the practical accuracy of tool setting. Due to the laser technology, the tool setting gauge can scan the shape and measure the contour of the cutting tool, as well as monitor the breakage of the individual cutting edges of the multitool.
However, the main disadvantage of the laser tool setting gauge is the complexity of its structure, which requires an additional high-quality air supply to protect the internal structure. It also has a high cost, making it mainly suitable for high-speed machining centers.
1.2.2 Common features and advantages of in-machine tool setting gauge
(1) Automatic measurement and parameter updating of cutting tool length/diameter:
The dynamic length/diameter measurement of the cutting tool is performed as it rotates. The measured parameters include the end runout/radial runout error of the machine spindle, which provides a “dynamic” offset value of the tool during high-speed machining.
Simultaneously, cutting tool parameters can be automatically measured at any time, thereby eliminating the “variation” of tool parameters caused by thermal deformation of the machine. The measurement results are automatically updated in the corresponding cutting tool’s parameter table, completely eliminating the potential risks associated with manual tool setting and parameter input.
(2) Automatic monitoring of cutting tool wear/breakage:
In practice, it can be challenging for operators to detect and correct worn or broken cutting tools (especially with small diameter drills), resulting in further tool loss or even scrap of the workpiece.
Using an in-machine tool setting device, the tool length can be automatically measured once before the tool is returned to the magazine after machining. In the case of normal wear, the wear values are automatically updated to the cutting tool loss parameters.
In the event of excessive wear, the tool can be considered broken (fractured), and the operator can choose between replacing the tool with a new one for the next workpiece or an automatic stop alarm for cutting tool replacement. This improves product quality and reduces cutting tool wear or scrap.
(3) Automatic compensation of thermal deformation in machine tools:
During production machining, changes in ambient temperature and workloads can cause thermal deformation in machine tools, leading to tool variations at any given time. This can result in significant fluctuations in the dimensional accuracy of products produced by the same machine in the workshop throughout the day.
By utilizing an in-machine tool setting gauge, tool parameters can be automatically measured and updated before or during the machining process. Each measurement is taken with the cutting tool set in the current state of thermal deformation of the machine, significantly reducing errors caused by thermal deformation.
(4) Measurement and monitoring of tool contours:
In special applications, such as forming cutters, using an off-machine tool setting gauge to measure tool contours and determine tool condition can be a time-consuming and complex task. It also requires high operator skill in tool setting.
With an in-machine laser tool setting gauge, the laser beam can be used to scan or monitor the tool contour at any time and automatically update the parameters as needed.