CNC lathes, also known as Computer Numerical Control lathes, are currently the most widely used and prevalent type of CNC machine tool in China, accounting for approximately 25% of all CNC machine tools. CNC machine tools are mechatronic products integrating mechanical, electrical, hydraulic, pneumatic, microelectronic, and information technologies. They are high-precision, high-efficiency, highly automated, and highly flexible machine tools in mechanical manufacturing. The technological level of CNC machine tools and their percentage in the production and total ownership of metal cutting machine tools are important indicators of a country's economic development and overall industrial manufacturing level. CNC lathes are one of the main types of CNC machine tools, occupying a very important position and receiving widespread attention and rapid development worldwide for decades.
1. Preparation Function (G Function)
The preparation function, also known as the G function (or G code), is a command used to instruct the working mode of the lathe or the control system. The G function consists of the address character G and the two digits following it (00-99). There are 100 functions from G00 to G99, which are used to instruct different actions of the machine tool, such as using G01 to instruct the linear motion of the motion coordinate.
G-codes are divided into single G-codes and modal G-codes. Single G-codes are only valid within the program segment to which they are instructed, while modal G-codes remain valid until other G-codes in the same group appear.
Currently, ISO codes are widely used in CNC lathes both domestically and internationally. However, their standardization is not high, with few codes specifying functions. There are many codes that specify functions but cannot be used for other functions, and many that do not specify functions but may be specified for other functions in the future, or codes that will never specify functions in the future. The functions of CNC system G codes are not consistent, resulting in significant differences in programming between different CNC systems. Therefore, it is necessary to use them in accordance with the specific instructions of the CNC system being used.
2. Tooling Function (T Function)
The tool function, also known as the T function, is used to specify the address word for the tool number and automatic compensation group number used in machining. Its automatic compensation mainly refers to tool position deviation and tool radius compensation. In CNC lathes, the subsequent numbers of the address character T have the following two main stipulations.
① Two-digit convention: For example, the GSK928TA economic CNC system described in this book uses a two-digit convention: the first digit generally represents the tool number, commonly using 9 digits from 0 to 8, where 0 indicates no tool rotation; the last digit represents the tool compensation group number, commonly using 9 digits from 0 to 8, where 0 indicates zero compensation. Example: T23 indicates that tool number 2 is rotated to the cutting position and the third group of tool compensation values is executed.
② Four-digit specification: For CNC lathes or turning centers with a large number of cutting tools, the CNC system generally specifies that the subsequent digits should be four digits. The first two digits are the tool number, and the last two digits are the tool compensation group number or, simultaneously, the tool tip radius compensation group number. Example: T0203 indicates that tool number 2 is moved to the cutting position and the third group of tool compensation values is executed.
3. Spindle Function (S Function)
The spindle speed command function is represented by the address S and the numbers following it. Currently, there are S2 (two-digit) and S4 (four-digit) representations, namely SXX and SXXXX. Generally, economical CNC lathes use one or two-digit codes to control the high and low speeds of a certain spindle gear. For CNC lathes with stepless speed regulation, the given spindle speed (r/min) can be directly represented by the subsequent numbers. In addition, for CNC lathes with constant linear speed cutting function, the S command in its machining program can command a constant speed (rpm) or a constant linear speed (m/min) during turning. That is, during turning, the spindle speed automatically changes with the turning diameter, ultimately maintaining a given constant linear speed.
① Two-digit S: Domestic CNC lathes generally use one or two-digit codes. The GSK928TA CNC system introduced in this article corresponds to the 6-level spindle mechanical shifting (each gear has a high speed and a low speed). S1 is used to specify high speed, S2 is used to specify low speed, and M code is used to specify the spindle rotation direction. M3 is forward and M4 is reverse. Here, high speed and low speed are only relative to a certain mechanical gear of the machine tool.
② Four-digit S: The address S followed by a 4-digit value directly commands the spindle speed (revolutions per minute). For example, S1200 indicates a constant spindle speed of 1200 revolutions per minute. For CNC systems with constant surface speed control, the surface speed after S is constant. As the turning diameter changes, the spindle speed is calculated based on the given surface speed, ensuring a constant relationship between the tool's instantaneous position and the workpiece surface. Use G96 (constant surface speed control command) and G97 (specify spindle speed) in conjunction with the S code to specify the spindle speed. Example: G96S18 indicates a cutting speed of 18 meters per minute; G97S1200 indicates a spindle speed of 1200 revolutions per minute for the G96 cutting speed. For specific spindle function usage, refer to the CNC system's instruction manual.
4. Accessibility Features (M Features)
Auxiliary functions, also known as M functions, are used to instruct the switching action or status of auxiliary devices in CNC machine tools. Auxiliary functions are indicated by address M and its subsequent numbers, which are generally two digits.
Because the address symbols used in CNC machine tools that conform to ISO standards (as shown in the table below) are not as standardized as G-instructions, with fewer specified codes and more unspecified or never-specified codes, M-function codes often vary due to differences in CNC system manufacturers, machine tool structures, and specifications. Therefore, programmers must be familiar with the functional meaning of the M-function instructions of the specific CNC system they are using and should not blindly apply them.
5. Upgrade Function (F Function)
When cutting parts, the speed at which the tool moves is controlled by a specified speed. The speed at which the tool is cut is called the feed. The function that determines the speed is called the feed function, also known as the F function. For CNC lathes, the feed methods can be divided into two types: feed per minute and feed per revolution.
(1) Feed per minute: This refers to the distance the tool travels per minute, measured in mm/min (millimeters per minute). It is independent of the lathe speed and its feed rate does not change with the spindle speed. This differs from the feed rate concept of ordinary lathes. Most economical CNC lathes use this feed method with G94 commands (or without commands). For beginners, the determination of the F function value is often unreasonable, mainly due to a lack of cutting knowledge. The F value can be calculated using the formula F value = lathe speed × selected feed rate. For example, when turning an outer diameter, if the spindle speed is set to 400 rpm and 600 rpm respectively, and the feed rate is selected to be 0.3 mm/rpm, then the F values are F120 and F180 respectively. However, relative to the cutting feed rate, its feed per revolution is consistent. Here, the lathe speed and the selected feed rate are determined based on factors such as material type, diameter, and depth of cut, and are basically consistent with the selection of feed rate for ordinary lathes.
(2) Feed per revolution: This refers to the distance the tool moves in the feed direction per revolution of the lathe spindle, measured in mm/revolution. The feed rate per spindle revolution is directly commanded using the value following F, in conjunction with the G95 command. For example, G95F0.3 indicates that the tool moves 0.3 mm/revolution per spindle revolution, which is exactly the same as the feed rate concept on a conventional lathe. Its speed varies with the spindle speed.
The specified range for the F function value must be set according to the range specified in the machine tool system manual, and must not exceed the specified range.
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