A complete set of knowledge about tool points of CNC programming

A must-have manual for CNC!

1. knife point

The tool position point is a reference point on the tool, and the trajectory of the relative movement of the tool position point is the processing route, also called the programming trajectory.

2. Knife setting and setting point

Tool setting means that the operator makes the tool position coincide with the tool setting point through a certain measurement method before starting the NC program. You can use the tool setter to set the tool, its operation is relatively simple, and the measurement data is relatively accurate. After positioning the fixture and installing the parts on the CNC machine tool, you can use the gauge block, feeler gauge, dial indicator, etc., and use the coordinates on the CNC machine tool to set the tool. For the operator, it is very important to determine the tool setting point, which will directly affect the machining accuracy of the part and the accuracy of program control. In the batch production process, it is necessary to consider the repeatability of the tool setting point. It is necessary for the operator to deepen the understanding of the numerical control equipment and master more tool setting skills.

(1) Selection principle of tool setting point

It is easy to align on the machine tool, easy to check during processing, easy to calculate when programming, and the tool setting error is small. The tool setting point can select a point on the part (such as the center of the positioning hole of the part), or a point outside the part (such as a point on the fixture or machine tool), but it must have certain coordinates with the positioning reference of the part relation.

Improve the accuracy and precision of tool setting. Even if the accuracy of the parts is not high or the program requirements are not strict, the machining accuracy of the selected tool setting position should be higher than that of other positions. Select a part with a large contact surface, easy monitoring, and stable machining process as the tool setting point. The tool setting point should be as unified as possible with the design benchmark or process benchmark, so as to avoid the reduction of tool setting accuracy and even machining accuracy due to size conversion, which increases the difficulty of CNC program or part CNC machining.

In order to improve the machining accuracy of the part, the tool setting point should be selected as far as possible on the design basis or process basis of the part. For example, for parts positioned by holes, it is more appropriate to use the center of the hole as the tool setting point. The accuracy of the tool setting point depends not only on the accuracy of the CNC equipment, but also on the requirements of the parts processing. Manually check the tool setting accuracy to improve the quality of the CNC machining parts. Especially in batch production, the repeatability of the tool setting point should be considered, and the accuracy can be checked by the coordinate value of the tool setting point relative to the origin of the machine tool.

(2) Selection method of tool setting point

For CNC lathes or turning and milling machining centers, since the center position (X0, Y0, A0) has been determined by the CNC equipment, the entire machining coordinate system can be determined by determining the axial position. Therefore, only a certain end face in the axial direction (Z0 or relative position) needs to be determined as the tool setting point.

For a three-coordinate CNC milling machine or three-coordinate machining center, it is much more complicated than a CNC lathe or a turning-milling machining center. According to the requirements of the CNC program, not only the origin position of the coordinate system (X0, Y0, Z0) needs to be determined, but also the same as the machining coordinate system. The determination of G54, G55, G56, G57, etc. is related, and sometimes also depends on the operator's habit. The tool setting point can be set on the processed part or on the fixture, but it must have a certain coordinate relationship with the positioning datum of the part. The Z direction can be determined simply by determining an easy-to-detect plane, while the X and Y directions It is determined that the plane and circle related to the positioning datum need to be selected according to the specific part.

For four-axis or five-axis numerical control equipment, the fourth and fifth rotating axes are added, which is similar to the selection of tool setting points for three-coordinate numerical control equipment. Due to the more complex equipment and the intelligent numerical control system, more tool setting methods are provided. , which needs to be determined according to specific CNC equipment and specific processing parts.

The coordinate relationship between the tool setting point and the machine tool coordinate system can be simply set to be related to each other. For example, the coordinates of the tool setting point are (X0, Y0, Z0), and the relationship with the machining coordinate system can be defined as (X0+Xr, Y0+ Yr, Z0+Zr), processing coordinate system G54, G55, G56, G57, etc., just input through the control panel or other methods. This method is very flexible and highly skilled, which brings great convenience to subsequent CNC machining. Once the machine tool collides due to the wrong input of programming parameters, the impact on the accuracy of the machine tool is fatal. Therefore, for high-precision CNC lathes, collision accidents must be eliminated.

(3) The main reasons for the collision:

a. Incorrect input of the diameter and length of the tool;

b. The size of the workpiece and other related geometric dimensions are incorrectly input and the initial position of the workpiece is incorrectly positioned;

c. The workpiece coordinate system of the machine tool is set incorrectly, or the zero point of the machine tool is reset during the machining process, resulting in changes. Most of the machine tool collisions occur during the rapid movement of the machine tool, and the collisions that occur at this time are also the most harmful and should be absolutely avoided.

Therefore, the operator should pay special attention to the initial stage of the machine tool executing the program and when the machine tool is replacing the tool. At this time, once the program is edited incorrectly and the diameter and length of the tool are entered incorrectly, collisions are likely to occur.

At the end of the program, the tool retraction sequence of the CNC axis is wrong, then a collision may also occur. In order to avoid the above-mentioned collision, the operator should give full play to the functions of the five senses when operating the machine tool, and observe whether the machine tool has abnormal movement, whether there is sparks, whether there is noise and abnormal noise, whether there is vibration, and whether there is a burning smell. If an abnormal situation is found, the program should be stopped immediately, and the machine tool can continue to work after the machine tool problem is solved.

3. Zero drift phenomenon

The phenomenon of zero-point drift is caused by the influence factors of the surrounding environment of the CNC equipment. Under the same cutting conditions, for the same equipment, using the same fixture, CNC program, and tool to process the same parts, an inconsistent processing size occurs. or reduced accuracy.

The phenomenon of zero point drift is mainly manifested in a kind of precision reduction phenomenon in the CNC machining process or can be understood as the inconsistency of the precision during CNC machining. The phenomenon of zero-point drift is inevitable in the process of CNC machining. It is common for CNC equipment. Generally, it is greatly affected by the surrounding environmental factors of the CNC equipment. In severe cases, it will affect the normal operation of the CNC equipment. There are many reasons that affect the zero drift, mainly including temperature, coolant, tool wear, large changes in spindle speed and feed rate, etc.

4. Tool compensation

After a certain period of CNC machining, tool wear is inevitable, which is mainly manifested in the change of tool length and tool radius. Therefore, tool wear compensation also mainly refers to tool length compensation and tool radius compensation.

5. Tool radius compensation

In part contour machining, since the tool always has a certain radius, such as the radius of the milling cutter, the motion trajectory of the tool center is not equal to the actual trajectory of the part to be machined, but needs to be offset by a tool radius value. This offset is customary It becomes tool radius compensation. Therefore, the radius value of the tool must be considered when performing CNC machining of the part contour. It should be pointed out that the UG/CAM CNC program is programmed with an ideal machining state and an accurate tool radius, and the tool movement trajectory is the tool center movement trajectory, without considering the state of the CNC equipment and the degree of tool wear on the CNC machining of parts. influences. Therefore, no matter for contour programming or tool center programming, the realization of UG/CAM NC program must consider the influence of tool radius wear, and use tool radius compensation reasonably.

6. Tool length compensation

On CNC milling and boring machines, when the tool is worn or replaced, when the position of the tool tip is not at the programmed position of the original machining, the change in size must be compensated by extending or shortening an offset value in the length direction of the tool to avoid Ensure that the processing depth or the position of the processing surface still meet the original design requirements.

7. Machine tool coordinate system

The nomenclature of the coordinate axes of the CNC machine tool is that the linear motion of the machine tool adopts the Cartesian coordinate system, and its coordinates are named X, Y, and Z, which are commonly referred to as the basic coordinate system. The movement of rotating around the X, Y, Z coordinate axes or the coordinate axis parallel to the X, Y, Z coordinate axes is called the A axis, the B axis, and the C axis, respectively. Press the right hand in the positive direction of A, B, and C. The spiral law is established.

Z axis: The main axis that transmits the cutting force is usually specified as the Z coordinate axis. For machine tools with tool rotation, such as boring machines, milling machines, drilling machines, etc., the axis of tool rotation is called the Z axis.

X-axis: The X-axis is usually parallel to the workpiece clamping surface and perpendicular to the Z-axis. For machine tools with rotating tools, such as horizontal milling machines and horizontal boring machines, when viewed from the tool spindle to the workpiece, the right-hand direction is the positive direction of the X-axis. When the Z-axis is vertical, for a single-column machine tool such as a vertical milling machine, the Looking from the tool spindle to the column direction, the right-hand direction is the positive direction of the X-axis.

Y-axis: The Y-axis is perpendicular to the X-axis and Z-axis, and its direction can be determined according to the determined X-axis and Z-axis, according to the right-hand Cartesian coordinate system.

The definition of the rotation axis is also in accordance with the right-hand rule, the rotation around the X axis is the A axis, the rotation around the Y axis is the B axis, and the rotation around the Z axis is the C axis. The coordinate axis of the CNC machine tool is shown in the following figure: Picture

The machine origin is the coordinate origin of the machine coordinate system. There are some fixed datum lines on the machine tool, such as the center line of the spindle; When the coordinate axes of the machine tool manually return to their respective origins, the position of the machine origin can be determined by the distance between the reference line and the reference plane on each coordinate axis component, which is described in the instruction manual of the CNC machine tool.

8. Part machining coordinate system and coordinate origin

The workpiece coordinate system, also known as the programming coordinate system, is a coordinate system established by the programmer with a fixed point on the workpiece as the origin when compiling the part processing program. The origin of the part coordinate system is called the part zero point (part origin or program zero point), and the tool path coordinates during programming are determined according to the coordinates of the part outline in the part coordinate system.

The origin of the machining coordinate system is called the adjustment point in the machine tool coordinate system. During processing, the part is installed on the machine tool with the fixture, and the clamping position of the part is fixed relative to the machine tool, so the position of the part coordinate system in the machine tool coordinate system is also determined. The distance between the measured part origin and the machine origin is called the part zero offset, and the offset needs to be stored in the CNC system in advance.

During machining, the part origin offset can be automatically added to the part coordinate system, so that the CNC system can determine the absolute coordinate value during machining according to the machine tool coordinate system. Therefore, the programmer can ignore the actual installation position and installation accuracy of the part on the machine tool, but use the offset function of the CNC system to compensate the position error of the part on the machine tool through the offset value of the part origin. This function is very convenient to use. The position of the part coordinate system takes the machine tool coordinate system as the reference point. Multiple part coordinate systems can be set on a CNC machine tool, and they are stored in G54/G59, etc. The part zero point is generally set at the design datum and process datum of the part. , for easy size calculation.

General CNC equipment can preset multiple working coordinate systems (G54~G59), these coordinate systems are stored in the machine tool memory, and the working coordinate systems are all based on the machine origin as the reference point, which are represented by their respective offsets from the machine origin. , it needs to be input into the CNC system of the machine tool in advance, or the coordinate system set before processing.

The machining coordinate system (MCS) is the positioning reference for all the tool path output points of part machining. The machining coordinate system is represented by OM-XM-YM-ZM. With the machining coordinate system, when programming, there is no need to consider the installation position of the workpiece on the machine tool, as long as the programming is performed according to the characteristics and dimensions of the workpiece.

The origin of the machining coordinate system is the workpiece machining zero point. The position of the workpiece machining zero point is arbitrary, and is selected by the programmer according to the characteristics of the part when compiling the CNC machining program. The workpiece zero point can be set on the workpiece to be processed, on the fixture or on the machine tool. In order to improve the machining accuracy of the parts, the workpiece zero point should be selected as far as possible on the machining surface with higher precision; in order to facilitate data processing and simplify programming, the workpiece zero point should be set on the design basis or process basis of the part as much as possible. For symmetrical parts, it is best to The zero point of the workpiece is set on the center of symmetry, which is easy to find and easy to check.

9. Clamping origin

The clamping origin is commonly found in CNC machine tools and machining centers with a rotary (or swinging) table, such as a rotary center. The offset from the reference point of the machine tool can be measured and stored in the origin offset register of the CNC system for the CNC system. Origin offset calculation.

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