In the application of CNC machine tools for manufacturing products parts of the process, there are many factors affecting the quality of parts, such as CNC machine accuracy, workpiece materials, workpiece heat treatment, processing technology, coolant and tools etc.
Among them, it has been little concern and attention on the accurate setting of the tool parameters, so it will focus on the CNC machine for accurate tool setting method characteristics and development trends.
The basic coordinate relationship in general, there is usually two coordinate system: one is the machine coordinate system, the other is the workpiece coordinate system.
The machine tool coordinate system is the machine tool’s inherent coordinate system, the machine tool coordinate system origin is called the machine tool origin or machine tool zero point.
In order to facilitate calculation and programming, we need to establish a workpiece coordinate system in the machine tool coordinate system.
A point on the workpiece as a coordinate system origin (also known as the program origin) to establish the coordinate system, this coordinate system is the workpiece coordinate system.
In daily work, we try to make the programming datum coincide with the design and assembly datum.
CNC machine tool setting method and in-machine tool setting instrument
Usually, a machine tool’s machine coordinate system is fixed, while the workpiece coordinate system can be established several separately according to the actual needs of the machining process, such as different workpiece coordinate system chosen by G54 and G55 etc.
The purpose of tool setting is that when CNC machining is performed, the path taken by the CNC program is always the path of the tool tip on the spindle.
The trajectory of the cutting tool’s cutter location point needs to be precisely controlled from start to finish in the machine’s coordinate system, which is the only datum for the machine.
The programmer could not have known the exact dimensions of the various tool sizes at the time of programming.
In order to simplify programming, this requires a uniform datum when performing programming and then offsetting the exact length and radius dimensions of the tool relative to that datum to obtain the exact position of the cutting tool tip when machining with the tool.
So, the purpose of tool setting is to determine the tool length and radius values so that the exact position of the tool tip in the workpiece coordinate system can be determined at the time of machining.
1.1 Commonly used methods of off-machine tool setting
A tool pre-setter is a measuring instrument that pre-adjusts and measures the length and diameter of the tool tip.
If the machine is connected to the DNC network, the tool length and diameter data can also be remotely entered into the NC of the machining center as tool parameters.
The advantage of this method is that the tool will be pre-schooled outside the machine, installed on the machine can be used, saving a great deal of auxiliary time.
However, the main disadvantage is that the measurement results are static values, the actual machining process can not be updated in real time to the state of tool wear or breakage, and can not be measured in real time on the thermal deformation caused by the machine tool expansion.
1.1.1 Tool setting by test cutting
Tool setting by test cutting is the manual operation of a machine tool by an operator to make a small amount of cutting of a workpiece before the workpiece is officially machined.
The operator determines the current position of the tool tip by eye and ear, and then proceeds to formal machining.
The advantage of this method is that it is economical and does not require additional investment in tools and equipment.
The main disadvantage is the low efficiency, the operator’s technical level requirements are high, and prone to personal error.
In actual production, there are many derivatives of the test cut method, such as gauge block method, coloring method, etc.
1.1.2 In-machine tool setting
This in-machine tool setting method uses 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
In-machine tool setting gauge is generally composed of sensor, signal interface and tool setting macro program software.
According to the working mode of 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, which can be further subdivided into the following categories according to 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
It is the most common in operation because it does not require a switching component for the counterblade signal and has the best price/performance ratio per unit.
The disadvantage of this tool setting gauge is the dragging of cables, which limits its application and it is suitable for small to medium-sized 3-axis milling machines/machining centers.
Infrared tool setting gauge
The signal transmission range is generally within 6m.
The advantage is the use of encoded HDR (High-Speed Data Transmission) infrared technology which eliminates the inconvenience and potential security threats associated with cable trailing.
It can be removed from the table at any time after tool setting without taking up machining space, and a tool setting gauge can be used on multiple machines to reduce overall costs.
The disadvantage is that it is not cost-effective when used in small machining centers.
Due to its characteristics, this type of tool setting gauge is mostly used in medium-sized machines and large CNC vertical lathes.
Cordless tool setting gauge
The radio signal transmission range is generally above 10 meters.
The advantage is that radio signals are transmitted over a wide range and are less susceptible to environmental influences.
It 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.
This type of tool setting gauge is mostly used for large/heavy machine tools.
The basic principle of 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.
There is no contact force during tool setting, which makes it possible to measure even the smallest cutting tools without worrying about breakage due to contact force.
It 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.
At the same time, 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.
The main disadvantage is the complexity of the structure, which needs an additional high-quality air supply to protect the internal structure.
It also has a high cost, which is 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 update of cutting tool length/diameter:
The dynamic length/diameter measurement of the cutting tool is carried out as it rotates.
The measured parameters include the end runout/radial runout error of the machine spindle, which gives a “dynamic” offset value of the tool during high-speed machining.
At the same time, automatic measurement of cutting tool parameters can be carried out at any time, thus greatly eliminating the “change” of tool parameters due to the 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, when a cutting tool is worn or broken (fractured), it can be difficult for the operator to detect and correct it (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 event 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 treated as 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 of machine tools.
When a machine tool is used for production machining, the thermal deformation of the machine tool is caused by changes in the ambient temperature and the work load, which in turn causes the tool to change at any time.
As a result, the dimensional accuracy of products produced by the same machine in the workshop fluctuates greatly during the morning, noon and night.
With in-machine tool setting gauge, tool parameters can be automatically measured and updated at any time before or during the machining process.
Each measurement is made with the cutting tool set in the current state of thermal deformation of the machine, thus greatly reducing errors introduced by the thermal deformation of the machine.
(4) Measurement and monitoring of tool contours.
In special applications, such as forming cutters, the use of an off-machine tool setting gauge to measure tool contours and determine tool condition can be a time-consuming and complex task.
It also places high demands on the operator’s tool setting skills.
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.