1. Classification of CNC milling machine

Since the industrial revolution, the machine tool industry has undergone earth-shaking changes. Most people know about milling machines, lathes and drill presses, also known as ordinary machine tools. These devices move the tool holder in the correct direction by the skilled worker operating the hand wheel to move the tool holder to the position where the part is machined. Ordinary machine tools require high-quality parts to be processed by operators who have been trained for a long time and have certain operating skills under certain conditions. Relatively speaking, ordinary equipment has low processing efficiency and high cost.

Numerical control equipment has completely or gradually replaced ordinary equipment in many fields. Unlike ordinary machine tools, the process of machining parts on CNC machine tools is carried out completely automatically, and manual processing cannot be performed during the processing. Therefore, all the information of the workpiece to be machined must first be included, including the technological process, the tool movement path and the cutting direction, the displacement amount, the process parameters (spindle speed, feed amount, cutting depth) and auxiliary actions (tool change, gear change, cooling) , clamp, loosen, etc., according to the processing sequence using standard or specified program instructions to write the correct NC machining program, and then input to the control system of the numerical control equipment, and then the control system controls the CNC machine tool to the parts according to the requirements of the NC program. Processing. The so-called numerical control programming generally refers to the whole work process including part pattern analysis, process analysis and design, graphic mathematics processing, writing and inputting the program list, and program verification. CNC programming can be divided into manual programming and automatic programming.

CNC milling machines can perform drilling, boring, tapping, contour milling, face milling, face pocket milling and milling of spatially complex 3D surfaces. The machining center and the flexible machining unit are produced and developed on the basis of the numerical control milling machine, and the main processing method is also the milling processing method.

CNC milling machines can be divided into the following three categories according to the general milling machine classification method:

(1) CNC Vertical Milling Machine

The vertical axis of the CNC vertical milling machine is perpendicular to the horizontal plane. This type of milling machine accounts for most of the CNC milling machines and has the widest application range. At present, the three-axis CNC vertical milling machine accounts for the majority of CNC milling machines, and generally can perform three-axis linkage processing.

(2) Horizontal CNC milling machine

The axis of the spindle of the horizontal CNC milling machine is parallel to the horizontal plane. In order to expand the processing range and expand the function, the horizontal CNC milling machine usually adopts the method of adding a numerically-controlled rotary table or a universal numerically-controlled rotary table to realize the four-axis and five-axis linkage processing. In this way, it is not only possible to process the continuously rotating contour of the side of the workpiece, but also to change the machining position of the part, that is, the so-called station, through a turntable in a single clamping, and to process a plurality of positions or working surfaces.

(3) Vertical and horizontal conversion milling machine

The main shaft of this type of milling machine can be converted, and it can perform vertical machining and horizontal machining on the same CNC milling machine, as well as vertical and horizontal milling machines.

2. The main processing object of CNC milling machine

(1) Plane Parts

The characteristics of flat parts are represented by the fact that the machined surface can be parallel to the horizontal plane as well as perpendicular to the horizontal plane, or it can be a fixed angle with the angle between the horizontal plane; most of the parts processed on the CNC milling machine belong to the flat type parts and flat type parts. It is the simplest type of part in CNC milling processing. Generally, it only needs to use two-axis linkage or three-axis linkage of three-axis CNC milling machine to process. In the machining process, the machined surface is in surface contact with the tool, and the end mill or bull nose can be used for both rough and finish machining.

(2) Surface Parts

The feature of surface parts is that the machining surface is a space surface. In the machining process, the machining surface and the milling cutter are always in point contact. The surface finish is mostly done with a ball-end cutter.

3. The coordinate system of the CNC milling machine

In order to describe the location of points in the plane and space, we first need to define a coordinate system that determines the direction and relative position. The coordinate system of the CNC machine tool adopts the right-handed Cartesian coordinate system. It specifies that the positive directions of the three coordinate axes X, Y, and Z of the rectangular coordinates are determined by the right-hand rule, and the positive direction of the rotation axes A, B, and C around each coordinate axis is determined by the right-handed spiral rule. CNC machining uses a three-dimensional space coordinate system. The three-dimensional coordinate system adds a vertical axis on the basis of the two-dimensional or planar coordinate system. Usually called the Z axis, it is a coordinate axis parallel to the machine tool spindle, as shown in the figure. 1 shows.

(1) Steps to create a machining coordinate system

In order to machine a part on a CNC machine, it is first necessary to determine the position of the workpiece on the machine. Therefore, a coordinate system must be established in relation to the machined part, although the advantage of the CNC machine is that it is allowed on the machine, or on the workpiece, or on the fixture. Any position can be used as a zero point for NC programming to create a coordinate system, but the best solution is to select a position that is both simple and convenient to position. This allows the operator to create a machining coordinate system by pressing several buttons on the control panel. Specific operations can be simply defined as the following steps:

1 Determine the position of the part coordinate system and the orientation of the coordinate axes according to the NC programming coordinate system or the machining coordinate system.

2 Use the positioning surfaces on the parts and fixtures to establish the machining coordinate system.

3 Correct the machining coordinate system and correct the machining coordinate system so that the established machining coordinate system meets the requirements of CNC machining.

(2) To establish the elements of the processing coordinate system

Geometric elements such as points, lines, face corrections, and corrections to the machining coordinate system are very useful. A key factor is the ability to determine the position of features on parts and fixtures and to determine the position of the machining coordinate system. In actual operation, the positioning surfaces and positioning holes on the parts and fixtures are often used as the main means for correcting and correcting the machining coordinate system. This is mainly based on the simple geometrical operation that can link the machine tool coordinate system and the part coordinate system. The following are the three elements for establishing a machining coordinate system.

1 Determine the coordinate plane: Select and align the positioning plane to determine the orientation and position of the work plane.

2 Determine the direction of the coordinate axis: translate or rotate the measured element as the direction vector to determine the coordinate axis direction of the machining coordinate system. The rotation element must be perpendicular to the aligned element. This controls the rotational positioning of the axis with respect to the work plane.

3 Determine the coordinate system origin: as the origin or zero point defining the X, Y, Z axis. 4. Zero point of CNC milling machine

The position of the tool position and the tool position vector of the NC program depend on the position of the machining coordinate system. Therefore, the exact position of the machining coordinate system or the programming coordinate system must be determined before machining the part.

When machining workpieces on ordinary equipment, the operator usually uses the edge or edge of the tool to determine the edge position of the workpiece as the machining zero point, and then uses the scale value of the machine dial or the reading displayed on the digital display to perform simple mathematical operations. The zero point of the working coordinate system is determined, and all the positions use this point as a reference point, which is the origin of the machining coordinate system.

The working principle of the CNC milling machine and the ordinary milling machine is the same: Before machining, the position of the workpiece on the machine tool must be determined, or the value of the dial or the value of the position digital display is used to position the part. Then, the operator establishes the machining coordinate system by pressing the buttons on the control panel of the numerically controlled machine tool, which is also known as the zero point. Only the position of the zero point is determined by the internal operation of the numerical control equipment control system.

Establishing the workpiece coordinate system on the NC milling machine is to determine the exact position of the workpiece in the machining coordinate system. First of all, the concept of two zero points should be understood. They are the origin of the machine coordinate system and the origin of the machining coordinate system.

The CNC milling machine has a reference point, which is usually referred to as the origin of the machine coordinate system or the initial position of the machine tool. It is a fixed reference position set by the machine tool builder on the machine tool and is established by a limit switch or a sensor. The role is to synchronize the machine with the control system and establish the starting point for measuring the machine movement. In practical terms, the machine zero is fixed and is usually at the top right of the machine. When the machine tool is started, the machine tool must execute the canned cycle program that returns to the machine zero point, that is, the initialization program, and then automatically store the offset value between the machine reference point and the machine origin in the machine control unit MCU (Machine Control Unit).

For numerical control programming and numerical control machining, there is also an important starting point is the program origin, which is the geometric reference point defined by the programmer in the numerical control programming, and this point as the origin of the processing coordinate system, which is commonly referred to as the workpiece origin. The workpiece coordinate system is the machining coordinate system that is determined when the part is NC programmed.

5. CNC milling machine offset

(1) The concept of machine offset

The distance between the machine zero and the working zero is called offset, as shown in Figure 2. Each axis has its own associated offset value, which is stored in the offset register of the machine control unit. During machine part NC machining, the machine control unit will always store these offsets and use these offsets to automatically track and move the tool to the correct position. The offset value can also be edited or adjusted in the machine control unit. For example, if 1mm is added to the X-offset value, the entire coordinate system will move 1mm in the positive X direction. This is a common adjustment method for controlling the quality of the workpiece.

(2) Machine coordinate system setting and offset

Most CNC machines can now create multiple workpiece offsets to define multiple workpiece coordinate systems. In fact, even if you need to create multiple workpiece coordinate systems to process the same workpiece, you do not need to completely re-create the working coordinate system. This task can complete the offset of the workpiece by setting the G code or G command. The most commonly used coordinate system setting instruction is G54, and other coordinate system setting instructions are G55 to G59. The X axis offset value and Y axis offset value shown in Figure 3 are the working coordinate system set by the G54 command.

Other working coordinate system settings, such as the working coordinate system defined by the G55 command, can be stored in the same coordinate work plane, such as G54. As shown in Figure 4, the G55 command is also known as the creation of another workpiece coordinate system G55. .

The creation of another workpiece coordinate system command code can be a common G54-G59 or other G code, depending entirely on the G code definition format set by the machine manufacturer for the offset value. This code must be used in your part program to define offsets or coordinate systems. Calling the G code that defines the offset of the workpiece is usually in an absolutely safe position as in the beginning of the program:

O1111; N5 G54 G90 G40 G70; or after tool change: N20 M06 T09; N25 G54 G00 X50.0 Y20.0 Z100.0;

Setting the work offset must be done by the tool tip or tool edge contacting the part or other methods. The defined workpiece axis offset values ​​X, Y, Z are stored in the machine control unit's register. If you look at the offset record on the control panel of the CNC machine, it is shown in the following table.

DesignationX-offsetY-offsetZ-offset

G54-30.221-65.8640

G55-7.987-33.366-9.873

G56-15.765-7.832-35

G57-50.352-0.788-8.963

(3) The role of workpiece offset

In the numerical control programming process, in order to avoid the size calculation, the work coordinate system needs to be properly translated and rotated several times. General CNC machine tools can preset six (G54～G59) workpiece coordinate systems. The coordinate offset values ​​of the coordinate origins of these coordinate systems with respect to the machine origin are stored in the machine control unit and remain after the machine returns to zero or initialization. Exists, once the G command is specified in the program, the CNC system specifies and recalls the coordinate system as the current workpiece coordinate system. The origin of the workpiece coordinate system is the current program origin, and the workpiece movement coordinate values ​​in the subsequent program are relative to each other. Coordinate system origin coordinate value.

(4) Z coordinate offset and tool length offset

Z-axis coordinate offset value The Z-axis offset value setting is complicated by the tool mounted on the spindle because the offset value is the Z-axis offset value between the machine tool origin and the workpiece coordinate system origin. It is not the Z-axis offset value from the milling cutter edge to the workpiece coordinate system. Relatively speaking, the X and Y axis offsets are relatively simple to measure and set, because the operator can try to align the center line of the machine tool spindle with the edge of the workpiece. There are some differences in the setting of the Z coordinate offset value because the tool is mounted on the spindle of the CNC milling machine and the influence of the tool on the Z axis offset value has to be considered. Tool length offset is an effective way to solve this problem. When the control system executes the instructions in the program so that the Z-axis reaches the specified horizontal position, the control system must do the work to add the value of the coordinate point in the program and the tool length offset value to the Z-axis coordinate offset value. . E.g:

The NC program executed by the control system is: G01 Z-100.0; the Z coordinate offset value is: -12.5; the tool length offset value is: 35.8; then the control system executes the program segment quickly and performs the following mathematical operations: Machine position Z = -100.0+(-12.5)+35.8=-76.7

The spindle then moves the tool to this position.

The machine tool control system only executes the machine tool absolute position of the motion control point, that is, the absolute coordinate value of the machine tool coordinate system, and all the other numerical values ​​just make the numerical control programming and the tool setting become simple.

When running the NC program, the CNC will complete the tool length offset work by automatically moving the tool away from the workpiece by a suitable distance according to the tool length offset value.

In the machining process, in order to control the depth of cut or perform trial cut processing, a tool length offset value is often used to control the depth of cut of the tool without revising the NC program.

6. Tool parameter preset

Tool presets are often used to set the newly installed tool parameters including length offsets, diameters, etc. The setting methods include test cutting method, internal tool setting method in the machine tool, external tool setting method in the machine tool, and the most common method of measuring tool setting tool is described below.

(1) Tool setting method

The tool setting tool measuring method is an external tool setting method of a machine tool. Its purpose is to set the tool parameters in advance outside the numerical control machine tool without stopping the numerical control machine tool and manually setting the tool to touch the workpiece to set the tool parameters.

The tool setting tool has a pre-assembled taper hole matched with the tool holder. When the tool is set, the tool is firstly installed on the tool handle, and then the tool holder with the tool is inserted into the matching taper hole of the tool setting tool. The tool is then measured using a contactless optical system, which is usually a profile projector that allows the operator to measure tool parameters with a large magnification. The optical magnifier on the tool setting tool can focus the blade edge of the tool, record the tool length offset value after the focus, the tool nose radius and the diameter value, etc., as shown in Figure 5; finally, all the tool parameter values ​​are manually adjusted. Input or through the system to the machine control system offset value register, so that accurate NC machining can be started.

(2) The composition of the external tool setting tool

● Shank positioning mechanism

The tool holder positioning mechanism corresponds to the standard holder, which is the reference for measurement. Therefore, it must have high precision and be the same as the positioning reference of the machining center to ensure the consistency of measurement and use. The positioning mechanism includes a spindle with a high rotational accuracy, a transmission mechanism for rotating the spindle, and a pretensioning mechanism 3 for tensioning between the spindle and the tool.

● Probe and measuring mechanism

There are two kinds of touch probes, contactless and non-contact. The contact probes directly touch the main measuring points (highest point and maximum outer circle) of the tool; the non-contact type mainly uses optical methods to project the tool onto the screen for measurement. . The measuring mechanism provides the Z-axis and X-axis dimension values ​​at the cutting point of the cutting edge, that is, the axial dimension and radial dimension of the tool, the measured readings are mechanical (such as the vernier ruler), there are also digital and optical .

● Measurement data processing device

The function of the measurement data processing device is to print out the measured value of the tool automatically, or to be networked with a superior management computer, perform flexible processing, and achieve automatic correction and compensation.

Compared with other methods such as manually touching the tool or testing the edge of the workpiece, the method of measuring tool parameters using the tool setting tool is more accurate and faster. In general, manually moving the spindle to make the tool contact or try to cut the edge of the workpiece is limited by many conditions, such as the accuracy of the actual tool, the precision of the workpiece test cutting edge, and the operator's measurement technique. On the other hand, using a tool setting tool to measure the tool diameter offset is more accurate than simply entering a tool diameter value. The operator and programmer must take into account spindle rotation accuracy because the tool rotates with the spindle during actual machining.

The system equipped with a high-precision external tool setting tool has more functions and is more convenient to use. For example, the tool parameters are automatically transmitted to the machine tool control system through the data exchange system and directly become the tool parameters required by the operator. In this way, the operator does not need to manually input the tool parameters into the machine control unit, thus avoiding human error.

The external tool setting tool is mainly used to measure the length, diameter, tool shape and angle of the tool. The main parameters of the tool stored in the machining center's tool magazine must have accurate values. These parameter values ​​must be taken into account when programming the machining program. When a new tool needs to be replaced due to tool damage during use, the main parameter values ​​of the new tool can be measured with the external tool setting tool so that the deviation from the original tool can be grasped, and then the normal processing can be ensured by modifying the tool compensation value. In addition, with the external tool setting tool can also measure the tool cutting edge angle and shape parameters, which will help improve the processing quality.

(3) Precautions for measuring tool parameters

● Problems that should be noticed when using the tool setting tool

Use a standard mandrel for calibration before use. Each tool setting tool is equipped with a standard tool setting spindle, which must be properly protected against rust or deformation by external forces. Axial and radial dimensions must be calibrated before each use.

● Correction of measured values ​​of static tool parameters

There is a difference between the statically measured tool size and the actual machined size. There are many factors that affect this difference. The main ones are:

• Precision and rigidity of tools and machine tools.

• The material and condition of the workpiece being machined.

Coolant and cooling system status.

· Use the skill and skill level of the knife tester.

Due to the above reasons, the statically-measured tool size should be larger than the actual size after machining. Therefore, a correction value should be considered when the tool is set. This should be pre-selected by the operator's experience, generally 0.01 to 0.05 mm larger.

● Correction of the origin of the workpiece coordinate system

After the part alignment or fixture positioning fixture, the coordinate value of the workpiece's programming origin in the machine coordinate system must be correctly measured and then input to the offset register. When trial cutting is performed, due to factors such as on-site environment, such as repeated positioning accuracy of the machine tool. Even if the same program is used to process, the actual machining size may also have a large offset due to the change of machining conditions. At this time, the actual measurement results can be corrected until the technical requirements of the part are satisfied, ultimately resulting in the tool movement path and numerical control programming path. To completely overlap, if necessary, in order to improve the machining accuracy of the workpiece, you can also use the above method for tool compensation correction.

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