What Is CNC Machining?

Numerical control machining (numerical control machining) refers to a process method for machining parts on CNC machine tools. The process specifications of CNC machine tools and traditional machine tools are generally the same, but there have also been significant changes. Machining method that uses digital information to control part and tool displacement. It is an effective way to solve the problems of variable parts, small batches, complex shapes, high precision, etc., and to achieve high efficiency and automatic processing.

Numerical control machining (numerical control machining) refers to a process method for machining parts on CNC machine tools. The process specifications of CNC machine tools and traditional machine tools are generally the same, but there have also been significant changes. Machining method that uses digital information to control part and tool displacement. It is an effective way to solve the problems of variable parts, small batches, complex shapes, high precision, etc., and to achieve high efficiency and automatic processing.
Chinese name
CNC machining
Foreign name
numerical control machining
begins at
The needs of the aviation industry
Time
Late 1940s

Basic definition of NC machining

NC machining refers to the processing performed by the control system to issue instructions to make the tool perform various movements in accordance with the technical requirements and processing technology requirements such as the shape and size of the workpiece in numbers and letters. [1] It refers to the process of parts processing on CNC machine tools.
NC machine tool is a kind of machine tool controlled by a computer. The computer used to control the machine tool, whether it is a special computer or a general-purpose computer, is collectively called a numerical control system. The movement and auxiliary movements of the CNC machine tools are controlled by the instructions issued by the CNC system. The instructions of the CNC system are prepared by the programmer according to the material of the workpiece, processing requirements, the characteristics of the machine tool, and the instruction format (NC language or symbol) specified by the system. The numerical control system sends running or termination information to the servo device and other functional components according to program instructions to control various movements of the machine tool. When the part processing program ends, the machine will automatically stop. Any kind of CNC machine tools cannot work without inputting program instructions in their CNC system. The controlled actions of the machine tool roughly include the start and stop of the machine tool; the start and stop of the spindle, the change of the rotation direction and the speed; the direction, speed, and mode of the feed movement; the selection of the tool, the compensation of the length and radius; the replacement of the tool, and cooling Opening and closing of fluid. [2]

Development background of CNC machining

Numerical control technology originated from the needs of the aviation industry. In the late 1940s, an American helicopter company proposed it.
Processing
The initial idea of CNC machine tools, in 1952 the Massachusetts Institute of Technology developed a three-coordinate CNC milling machine. This CNC milling machine has been used for machining aircraft parts in the mid-1950s. In the 1960s, the CNC system and programming work became increasingly mature and perfect. CNC machine tools have been used in various industrial sectors, but the aerospace industry has always been the largest user of CNC machine tools. Some large aviation factories are equipped with hundreds of CNC machine tools, of which cutting machine tools are the main ones. The numerically processed parts include the integral wall panels, beams, skins, bulkheads, propellers of the aircraft and rockets, the mold cavities of the aero engine casings, shafts, disks, blades, and special cavity surfaces of the liquid rocket engine combustion chamber. . In the early stage of the development of CNC machine tools, CNC machines with continuous trajectories were mainly used for continuous trajectory control.
Continuous trajectory control, also called contour control, requires the tool to move according to the specified trajectory relative to the part. Later, we vigorously developed the point control CNC machine tools. Point control refers to the movement of the tool from one point to another, as long as it can finally reach the target accurately regardless of the movement route.

CNC machining operation process

CNC programming

There are two types of NC machining programming methods: manual (manual) programming and automatic programming. Manual programming, the entire content of the program is manually written according to the instruction format specified by the CNC system. Automatic programming is computer programming, which can be divided into automatic programming methods based on language and drawing. However, no matter which automatic programming method is adopted, corresponding supporting hardware and software are required.
It can be seen that the realization of NC machining programming is the key. But programming is not enough. CNC machining also includes a series of preparatory work that must be done before programming and after-processing after programming. Generally speaking, the main contents of CNC machining process are as follows:
Select and determine the parts and content for CNC processing;
Process analysis of CNC machining of part drawings;
(3) Process design of numerical control processing;
Mathematical processing of part drawings;
Writing processing program list;
Make control media according to the program list;
program verification and modification;
First trial processing and on-site problem handling;
Shaping and archiving of CNC machining process files.
In order to improve the degree of production automation, shorten the programming time and reduce the cost of CNC processing, a series of advanced CNC processing technologies have also been developed and used in the aerospace industry. Such as computer numerical control, that is, a small or micro computer is used to replace the controller in the numerical control system, and software stored in the computer is used to perform calculations and control functions. This soft-connected computer numerical control system is gradually replacing the initial numerical control system. Direct numerical control is to use a computer to directly control multiple CNC machine tools, which is very suitable for small batch and short cycle production of aircraft. The ideal control system is an adaptive control system that can continuously change processing parameters. Although the system itself is complicated and expensive, it can improve processing efficiency and quality. In addition to the improvement of the CNC system and machine tools in terms of hardware, another important aspect is the development of software. Computer-aided programming (also known as automatic programming) is when a programmer writes a program in a NC language, inputs it into a computer for translation, and finally the computer automatically outputs a punched tape or magnetic tape. The widely used numerical control language is APT language. It is broadly divided into a main processor and a post processor. The former translates the program written by the programmer to calculate the tool trajectory; the latter compiles the tool trajectory into a part machining program for a CNC machine tool. NC machining is to write programs on the computer before processing the workpiece, and then input these programs to the machine tool controlled by the computer program for instructional processing, or write instructions directly on the machine tool control panel controlled by this computer program. For processing. The process of machining includes: cutting, changing, changing speed, changing direction, stopping, etc., which are all done automatically. NC machining is an advanced method for modern mold manufacturing. Of course, NC machining methods must not only be used for mold parts processing, they are very versatile.

NC machining process analysis

The technical aspects of the numerical control machining of the processed parts are wide-ranging. The following points out the main contents that must be analyzed and reviewed in combination with the possibility and convenience of programming.
1.Dimensions should conform to the characteristics of CNC machining
In NC programming, the size and position of all points, lines, and areas are based on the programming origin. Therefore, it is best to directly give the coordinate size on the part drawing, or try to quote the size with the same reference.
2.The conditions of geometric elements should be complete and accurate
In programming, the programmer must fully grasp the geometric element parameters that constitute the contour of the part and the relationship between the geometric elements. Because all geometric elements of the part outline are defined during automatic programming, the coordinates of each node must be calculated during manual programming. No matter what is ambiguous or uncertain, programming cannot be performed. However, due to inadequate consideration or neglect during the design process, part designers often have incomplete or unclear parameters, such as whether arcs and lines, arcs and arcs are tangent or intersected or separated. Therefore, when reviewing and analyzing the drawings, you must be careful and contact the designer in time when problems are found.
3.Reliable positioning reference
In NC processing, the processing steps are often concentrated, and it is important to position with the same benchmark. Therefore, it is often necessary to set some auxiliary benchmarks or add some process bosses to the blank.
4.Uniform geometry type or size
It is best to use a uniform geometry type or size for the shape and inner cavity of the part. This can reduce the number of tool changes. It is also possible to apply a control program or a special program to shorten the program length. The shape of the part is as symmetrical as possible, which is convenient for programming with the mirror processing function of the CNC machine tool to save programming time.

CNC machining parts clamping

First, the basic principles of positioning and installation
When machining parts on CNC machine tools, the basic principle of positioning and installation is to reasonably choose the positioning datum and clamping scheme. The following points should be noted when selecting:
1. Strive for a unified benchmark for design, process and programming calculations.
2. Minimize the number of times of clamping, and try to process all the surfaces to be processed after positioning and clamping once.
3. Avoid using the manual adjustment processing scheme of the machine, so as to give full play to the effectiveness of CNC machine tools.
Second, the basic principles of selecting fixtures
The characteristics of NC machining put forward two basic requirements for the fixture: one is to ensure that the coordinate direction of the fixture is relatively fixed to the coordinate direction of the machine tool; the other is to coordinate the dimensional relationship between the part and the machine tool coordinate system. In addition, consider the following:
1. When the number of parts processing is not large, the combination fixture, adjustable fixture and other general fixtures should be used as much as possible to shorten production preparation time and save production costs.
2. Only consider the use of special fixtures in batch production, and strive for simple structure.
3. The loading and unloading of parts should be fast, convenient and reliable, so as to shorten the machine's pause time.
4. Each component on the fixture should not prevent the machine tool from processing the various surfaces of the part, that is, the fixture must be open, and its positioning and clamping mechanism components must not affect the cutting during processing (such as collision). [2]

CNC machining processing error

NC machining error number addition is a combination of programming error editing, machine tool error machine, positioning error setting, tool setting error knife and other errors.
That is: number plus = f ( edit + machine + + )
among them:
1. Programming error is composed of approximation error and rounding error. The approximation error is generated during the process of approximating a non-circular curve with a straight line segment or an arc segment, as shown in Figure 1.43. The rounding error is an error caused by rounding the coordinate value to an integer pulse equivalent value during data processing. Pulse equivalent refers to the displacement of the coordinate axis corresponding to each unit pulse. The pulse equivalent value of ordinary precision CNC machine tools is 0.01mm; the pulse equivalent value of more precise CNC machine tools is 0.005mm or 0.001mm.
2. Machine tool error machine is caused by CNC system error, feed system error and other reasons.
3. The positioning error must be generated when the workpiece is positioned on the fixture and the fixture is positioned on the machine tool.
4. Tool setting error The tool is generated when the relative position of the tool and the workpiece is determined.

Main features of CNC machining

At the beginning of numerical control machine tools, aircraft parts with complex profiles were selected as processing objects, and the key to solving common machining methods was difficult. The biggest feature of NC machining is to use punching tape (or magnetic tape) to control the machine for automatic processing. Due to the different characteristics of aircraft, rocket and engine parts: aircraft and rocket parts and components are large in size and complex in profile; engine zero and components are small in size and high in accuracy. Therefore, the CNC machine tools used in aircraft, rocket manufacturing and engine manufacturing departments are different. In the manufacture of aircraft and rockets, large-scale CNC milling machines with continuous control are mainly used, while in the manufacture of engines, both CNC machines with continuous control and point-controlled CNC machines (such as CNC drilling machines, CNC boring machines, machining centers, etc.) are used. .

Centralized CNC machining process

NC machine tools generally have a tool holder and a magazine that can automatically change tools. The tool change process is automatically performed by program control. Therefore, the process is relatively concentrated. Concentrated process brings huge economic benefits:
Reduce machine tool footprint and save workshop.
Reduce or no intermediate links (such as intermediate testing of semi-finished products, temporary storage and handling, etc.), saving time and labor.

CNC Machining Automation

When CNC machine tools are used for processing, there is no need to manually control the tools, and the degree of automation is high. The benefits are obvious.
Reduced requirements for operating workers:
A senior worker of an ordinary machine tool can not be trained in a short time, but a CNC worker who does not need programming has a very short training time (for example, a CNC lathe can take a week and write a simple machining program). In addition, the parts processed by CNC workers on CNC machine tools are more accurate and time-saving than the parts processed by ordinary workers on traditional machine tools. Reduce the labor intensity of the workers: CNC workers are mostly excluded from the machining process during the machining process, which is very labor-saving.
(3) Stable product quality: The automation of CNC machine tools eliminates human errors such as fatigue, carelessness, and estimation of workers on ordinary machine tools, and improves product consistency.
High processing efficiency: The automatic tool change of CNC machine tools makes the processing process compact and improves labor productivity.

High flexibility in CNC machining

Although the traditional general-purpose machine tool has good flexibility, it has low efficiency; while the traditional special machine has high efficiency, but has poor adaptability to parts, high rigidity and poor flexibility, and it is difficult to adapt to the fierce competition brought by the market economy. Frequent product modifications. As long as the program is changed, new parts can be processed on the CNC machine tool, and it can be automated, and has good flexibility and high efficiency, so the CNC machine tool can adapt to market competition well.

Strong CNC machining ability

The machine tool can accurately process various contours, and some contours cannot be processed on ordinary machine tools. CNC machine tools are particularly suitable for the following occasions:
1. No scrapped parts are allowed.
2. New product development.
3. Processing of urgently needed parts.

CNC programming

NC machining program structure

A program block is a continuous block that can be processed as a unit. It is actually a program in a CNC machining program. The body of a part machining program consists of several program segments. Most blocks are used to instruct the machine to complete or perform an action. The block is composed of size, non-size and block end instructions. When writing and printing, each program block generally occupies one line, and the same is true when the program is displayed on the screen.

CNC machining program format

A conventional machining program consists of a start character (single-line paragraph), a program name (single-line paragraph), a program body, and an end-of-program instruction (normally a single-line paragraph). There is also a program terminator at the end of the program. The program start character is the same character as the program end character:% in the ISO code and ER in the EIA code. The program end command can be M02 (program ends) or M30 (end of paper tape). CNC machine tools generally use stored programs to run. At this time, M02 and M30 have the same thing in common: after completing all other instructions in the block, they are used to stop the spindle, coolant and feed, and reset the control system. M02 and M30 are completely equivalent when used on some machine tools (systems), while they are used on other machine tools (systems) with the following differences: When using M02 to end the program, the cursor stops at the end of the program after the automatic operation ends; When M3O is used to end the program running, the cursor and the screen display can automatically return to the beginning of the program after the automatic running is completed. Once the start button is pressed, the program can be run again. Although M02 and M30 allow one block to be used with other program words, it is better to list them separately or share one block with the sequence number.
The program name is placed before the program body and after the program start character, and it is usually on its own line. There are two forms of the program name: one starts with a prescribed English word (multiple O), followed by several digits. The maximum allowable number of digits is specified by the manual, and two-digit and four-digit are common. This form of program name can also be called a program number. Another form is that the program name consists of English words, numbers or a mixture of English and numbers, and a "-" sign can be added in the middle. This form makes the user's naming program more flexible. For example, the third process of machining the flange of part number 215 on the LC30 CNC lathe can be named LC30-FIANGE-215-3, which will be used and stored. And retrieval. The form of the program name is determined by the CNC.
%
O1001
N0 G92 X0 Y0 Z0
N5 G91 G00 X50 Y35 S500 MO3
N10 G43 Z-25 T01.01
N15 G01 G007 Z-12
N20 G00 Z12
N25 X40
N30 G01 Z-17
N35 G00 G44 Z42 M05
N40 G90 X0 Y0
N45 M30
%

NC machining segment format

The rules for the arrangement of words, characters, and data in a block are called the block format. In the history of CNC, fixed sequence format and delimiter (HT or TAB) block format have been used. When these two program segment formats have passed, the word address variable segment format is widely used at home and abroad, also known as the word address format. In this format, the program word length is not fixed, and the number of program words is also variable. Most CNC systems allow the sequence of program words to be arranged arbitrarily, so it belongs to the variable block format. However, in most cases, for the convenience of writing, inputting, checking, and proofreading, the program words are used to be arranged in a certain order in the program block.
In the programming manual of CNC machine tools, the detailed format is used to classify and specify the details of programming: the characters used in programming, the order of the program words in the block, and the word length. E.g:
/ NO3 G02 X + 053 Y + 053 I0 J + 053 F031 S04 T04 M03 LF
The detailed format classification description of the above example is as follows: N03 is the block number; G02 indicates that the processed track is a clockwise arc; X + 053, Y + 053 indicates the end coordinates of the processed arc; I0, J + 053 indicates the processed circle The center coordinates of the arc; F031 is the processing feed rate; S04 is the spindle speed; T04 is the tool number of the tool used; M03 is the auxiliary function command; the LF block end command; / is a jump selection command. The function of skip selection instruction is: under the premise of unchanged program, the operator can choose to execute or not execute the program segment with skip selection instruction in the program. The selection method is usually achieved by moving the jump selection switch on the operation panel to ON or OFF, so as to realize the execution or execution of the block with "/".

CNC machining execution program

This is sometimes encountered when programming a machining program: a group of blocks appears multiple times in a program, or it is used in several programs. We can extract this group of blocks and store them separately after naming. This group of blocks is a subroutine. A subroutine is a piece of processing program that can be called by appropriate machine control instructions. It generally has independent meaning in processing. The processing program where the instruction that calls the first-level subroutine is called the main program. The instruction of the subroutine is also a program segment. It generally consists of the subroutine call instruction, subroutine name, and number of calls. The specific rules and formats vary with the system. For example, it is also "call subroutine 55 once". M98 P55. ", While the American AB company system uses" P55x ".
Subroutines can be nested, that is, layer by layer. The relationship between the previous layer and the next layer is the same as the relationship between the main program and the first subroutine. The maximum number of layers that can be set is determined by the specific CNC system. The form and composition of the subroutine are roughly the same as the main program: the first line is the subroutine number (name), and the last line is the "end of subroutine" instruction, with the subroutine body between them. However, the main program end instruction is used to end the main program and reset the CNC system. Its instructions have been standardized, and each system uses M02 or M30. The subprogram end instruction is used to end the subprogram, return to the main program, or the previous subprogram , Its instructions are not unified in various systems, such as FANUC system using M99, Siemens system using M17, the United States A-B company's system using M02 and so on.
User macros (programs) can be used in CNC machining programs. The so-called macro program is a subprogram that contains variables. The instructions that call the macro program in the program are called user macro instructions. The function that the system can use the user macro program is called user macro function. You only need to write a user macro command during execution to execute its user macro function.
The biggest characteristics of user macros are:
Variables can be used in user macros;
You can use calculus, turn statements and various functions
Variables can be assigned using user macro commands.
CNC machine tools use group technology to process parts, which can increase batch size, reduce programming volume, and improve economic efficiency. In group processing, parts are classified, and processing programs are compiled for this type of parts, without the need to write a program for each part. When processing the same type of parts with only different sizes, the main convenience of using user macros is that you can use variables instead of specific values. In actual processing, you only need to assign the actual size values of the parts to variables with user macro commands.

CNC machining processing principles

The processing of the previous process must not affect the positioning and clamping of the next process [2] .
Inside, then outside, that is, the internal cavity (inner hole) is processed first, and then the external shape is processed.
(3) It is best to carry out the same machining process with the same installation or using the same tool to reduce the errors caused by repositioning or tool change.
In the same installation, the process with less influence on the rigidity of the workpiece should be performed first.

CNC machining processing route

CNC lathe feed processing route refers to the path that the turning tool moves from the tool setting point (or the fixed origin of the machine tool) until it returns to that point and ends the machining program, including the path of cutting and cutting, cutting, etc. Cut the empty travel path.
The feed route for finishing is basically performed along the contour of its parts. Therefore, the focus of determining the feed route is to determine the feed route for rough machining and empty stroke.
In CNC lathe processing, the determination of processing routes generally follows the following principles.
The precision and surface roughness of the workpiece to be processed should be guaranteed.
Make the processing route the shortest, reduce the idle travel time, and improve the processing efficiency.
Simplify the workload of numerical calculation and simplify the processing procedures.
For some reusable programs, subroutines should be used.
Making the machining program have the shortest feed route can not only save the execution time of the entire machining process, but also reduce some unnecessary tool consumption and wear of sliding parts of the machine's feed mechanism. The types and implementation methods of the shortest feed path are as follows.
Shortest cutting feed path. The shortest cutting feed path can effectively improve production efficiency and reduce tool wear. When arranging the shortest cutting feed route, it is also necessary to ensure the rigidity and processing technology of the workpiece.
The shortest empty itinerary.
Use the starting point. An example of a general case of roughing in a rectangular cycle. The setting of the cutting point A is to consider the need to change the tool conveniently during the machining process such as finishing, so it is set at a position far from the blank, and at the same time, the starting point and the cutting point are coincided.
Set the tool change point. In order to consider the convenience and safety of the tool change, sometimes the tool change point is also set far away from the blank.
Position, then when the second tool is changed, the empty stroke route when finishing turning must be longer; if the tool change point of the second tool is also set to the midpoint position, the empty time can be shortened. Travel distance.
Arrange the "Return to Zero" route reasonably. When manually compiling a machining program for complex contours, in order to simplify the calculation process and facilitate checking, the programmer sometimes sometimes returns the end point of each tool after machining, and executes the "return to zero" operation instruction to return it to the tool setting point Position before performing subsequent procedures. This will increase the distance of the feed path and reduce production efficiency. Therefore, when reasonably arranging the "return to zero" route, the distance between the end point of the previous knife and the start point of the next knife should be as short as possible. Or zero to meet the shortest feed path requirements. In addition, when selecting the return to the tool set point instruction, as long as no interference occurs, the x and z axis bidirectional simultaneous "return to zero" instructions are used as much as possible. The "return to zero" route of this function is the shortest.
(3) The step cutting feed path of the large allowance blank. Lists the cutting progress of two types of too much stock
Give the route. It is the wrong step cutting path. It is cut in the order of 1 bucket 5. The remaining margin is equal for each cutting. It is the correct step cutting feed path. Because under the same amount of knife.
Continuous cutting feed path for part contour finishing. The finishing of the part contour can be arranged with one or more finishing operations. The finished contour should be continuously processed by the last tool. At this time, the advance and retract positions of the tool should be selected appropriately. Try not to arrange people and cut out or change the tool and stop in the continuous contour to avoid sudden changes in cutting force And destroy the equilibrium state of the process system. As a result, scratches, sudden changes in shape, or stuck knife marks may occur on the part contour.
Special feed route. In NC turning, generally. The longitudinal feed of the tool is fed along the negative direction of the coordinate, but sometimes it is not reasonable to arrange the feed route in its normal negative direction. It may even damage the workpiece.

Advantages and disadvantages of CNC machining

NC machining has the following advantages: The number of tooling is greatly reduced, and complex tooling is not required to process complex shapes. If you want to change the shape and size of the part, you only need to modify the part processing program, which is suitable for new product development and modification. Stable processing quality, high processing accuracy and high repeat accuracy, adapt to the processing requirements of the aircraft. The production efficiency is higher in the case of multi-variety and small batch production, which can reduce the time for production preparation, machine tool adjustment and process inspection, and reduce the cutting time due to the use of the optimal cutting amount. It can process complex profiles that are difficult to process by conventional methods, and even machining parts that cannot be observed. The disadvantage of NC machining is that the cost of machine tools and equipment is high, requiring a high level of maintenance personnel.

CNC machining tool selection

CNC machining selection tool

1.Principle of selecting CNC tools
Tool life is closely related to the amount of cutting. When setting the cutting amount, a reasonable tool life should be selected first, and a reasonable tool life should be determined according to the optimization goal. Generally divided into the highest productivity tool life and the lowest cost tool life, the former is determined based on the goal of the smallest single-piece man-hour, the latter is determined based on the goal of the lowest process cost.
When selecting tool life, consider the following points according to the complexity of the tool, manufacturing and sharpening costs. Complex and highly accurate tool life should be selected to be higher than single-edged tools. For indexable cutting tools, due to the short tool change time, in order to give full play to its cutting performance and improve production efficiency, the tool life can be selected lower, generally 15-30min. For multi-tool machines, combined machine tools and automatic processing tools with more complicated tool loading, changing and adjusting, the tool life should be selected higher, and the tool reliability should be guaranteed. When the productivity of a certain process in the workshop restricts the increase of the productivity of the entire workshop, the tool life of the process should be selected to be lower. When the whole plant expenses shared by a certain process unit time are large, the tool life should also be selected Lower. When finishing large pieces, in order to ensure that the tool is completed at least once, and to avoid changing tools during cutting, the tool life should be determined according to the accuracy of the part and the surface roughness. Compared with ordinary machining methods, CNC machining places higher requirements on cutting tools. It not only requires good quality and high precision, but also requires stable dimensions and high durability. It also requires easy installation and adjustment. Meet the requirements of high efficiency of CNC machine tools. The tools used on CNC machine tools often use cutting tool materials suitable for high-speed cutting (such as high-speed steel, ultra-fine-grained carbide) and indexable inserts.
2.Select CNC turning tools
CNC turning tools are generally divided into forming turning tools, pointed turning tools, arc turning tools and three types. The forming turning tool is also called a model turning tool, and the contour shape of the machined part is completely determined by the shape and size of the turning tool blade. In CNC turning processing, common forming turning tools include small radius arc turning tools, non-rectangular slotting tools and thread cutting tools. In numerical control machining, it should be used as little as possible or not. A sharp turning tool is a turning tool characterized by a straight cutting edge. The tip of this type of turning tool is composed of linear main and auxiliary cutting edges, such as 900 internal and external circular turning tools, left and right end turning tools, grooving (cutting) turning tools, and various external and internal turning edges with small chamfers Hole turning tool. The selection method of the geometric parameters (mainly the geometric angle) of the pointed turning tool is basically the same as that of ordinary turning, but it should be considered in combination with the characteristics of CNC machining (such as machining route, machining interference, etc.), and the tool tip itself should be considered. strength.
The second is the arc-shaped turning tool. The arc-shaped turning tool is a turning tool characterized by a circular-shaped cutting edge with little roundness or line contour error. Each point of the arc edge of the turning tool is the tip of an arc-shaped turning tool. Therefore, the cutting point is not on the arc, but on the center of the arc. The arc-shaped turning tool can be used for turning the inner and outer surfaces, and is especially suitable for turning all kinds of smooth connecting (concave) forming surfaces. When selecting the turning radius of the turning tool, the radius of the arc of the cutting point of the two-point turning tool should be less than or equal to the minimum radius of curvature on the concave contour of the part, so as not to cause shallow machining. The radius should not be too small, otherwise it will not only be difficult to manufacture, It can also damage the turning tool due to the weak strength of the tool tip or poor heat dissipation of the tool body.
3.Choose a CNC milling tool
In NC machining, the inside and outside contours of milling plane parts and milling planes are usually flat-bottomed end mills. The empirical data of the relevant parameters of the tool are as follows: First, the radius RD of the milling cutter should be less than the minimum radius of curvature Rmin of the inner contour surface of the part, generally RD = (0.8 to 0.9) Rmin. The second is the machining height of the part H <(1 / 4-1 / 6) RD to ensure that the knife has sufficient rigidity. Third, when the bottom of the inner groove is milled with a flat-bottomed end mill, since the bottom of the groove needs to be overlapped for two passes, the radius of the tool's bottom edge, Re = Rr, that is, the diameter is d = 2Re = 2 (Rr). Take the tool radius as Re = 0.95 (Rr). For the processing of three-dimensional profiles and variable beveled contour shapes, spherical milling cutters, ring milling cutters, drum milling cutters, cone milling cutters and disc milling cutters are commonly used.
Most of CNC machine tools use serialized and standardized tools. The tool holders and cutter heads of indexable machine tools such as external turning tools and end turning tools have national standards and serialized models. For machining centers and automatic tool changers Machine tools and tool holders have been serialized and standardized. For example, the standard code for tapered shank tool systems is TSG-JT, and the standard code for straight shank tool systems is DSG-JZ. In addition, for selected tools, Before use, strict measurement of the tool size is required to obtain accurate data, and the operator inputs these data into the data system, completes the machining process through program call, and processes qualified workpieces.

CNC machining point

Where exactly does the tool move to the specified position? So at the beginning of the program execution, the position where the tool starts to move in the workpiece coordinate system must be determined. This position is the starting point of the tool relative to the workpiece when the program is executed. So it is called the program starting point or starting point. This starting point is generally determined by tool setting, so this point is also called tool setting point. When programming a program, the position of the tool setting point must be selected correctly. The principle of tool setting point is to facilitate numerical processing and simplify programming. Easy to find and correct during processing; small processing errors caused. The tool setting point can be set on the processing part, it can also be set on the fixture or the machine tool. In order to improve the machining accuracy of the part, the tool setting point should be set on the design basis or process base of the part as much as possible. In actual operation of the machine tool, the tool position of the tool can be placed on the tool setting point by manual tool setting operation, that is, the coincidence of the "tool point" and the "tool setting point". The so-called "tool position point" refers to the positioning reference point of the tool, and the tool position point of a turning tool is the tip or the center of the arc of the tip. The flat bottom end mill is the intersection of the tool axis and the bottom surface of the tool; the ball end mill is the ball center of the ball head, the drill bit is the drill point and so on. With manual tool setting operation, the tool setting accuracy is low and the efficiency is low. Some factories use optical tool setting mirrors, tool setting instruments, automatic tool setting devices, etc. to reduce tool setting time and improve tool setting accuracy. When a tool change is required during processing, the tool change point should be specified. The so-called "tool change point" refers to the position of the tool holder when the tool is rotated. The tool change point should be set outside the workpiece or the fixture, so as not to touch the workpiece and other components during the tool change.

CNC machining cutting consumption

In NC programming, the programmer must determine the cutting amount for each process and write it into the program in the form of instructions. Cutting amount includes spindle speed, back-eating amount and feed speed. For different processing methods, different cutting rates need to be selected. The selection principle of cutting amount is: to ensure the machining accuracy and surface roughness of parts, to give full play to the cutting performance of the tool, to ensure reasonable tool durability, and to give full play to the performance of the machine tool to maximize productivity and reduce costs.
1.Determine the spindle speed
The spindle speed should be selected according to the allowable cutting speed and workpiece (or tool) diameter. The calculation formula is: n = 1000 v / 7 1D where: v? Cutting speed, the unit is m / m movement, determined by the durability of the tool; n one spindle speed, the unit is r / min, D is the workpiece diameter Or tool diameter in mm. For the calculated spindle speed n, select the speed that the machine tool has or is close to.
2.Determine the feed speed
Feed rate is an important parameter in the cutting amount of CNC machine tools. It is mainly selected according to the machining accuracy and surface roughness requirements of parts and the material properties of tools and workpieces. The maximum feed speed is limited by the stiffness of the machine and the performance of the feed system. The principle of determining the feed rate: When the quality requirements of the workpiece can be guaranteed, in order to improve production efficiency, a higher feed rate can be selected. It is generally selected in the range of 100-200mm / min; when cutting, machining deep holes or machining with high-speed steel tools, a lower feed speed should be selected, generally in the range of 20-50mm / min; when machining accuracy, the surface When the roughness requirements are high, the feed speed should be selected smaller, generally in the range of 20-50mm / min; when the tool is in empty stroke, especially when the distance is "zeroed back", you can set the CNC system settings Maximum feed rate.
3. Determine the amount of back knife
The amount of back-eating knife is determined according to the rigidity of the machine tool, the workpiece and the tool. Under the condition that the rigidity allows, the amount of back-eating knife should be equal to the machining allowance of the workpiece as much as possible. In order to ensure the quality of the machining surface, a small amount of finishing allowance can be left, usually 0.2 to 0.5 mm. In short, the specific value of the cutting amount should be determined by analogy according to the machine tool performance, relevant manuals and actual experience.
At the same time, the spindle speed, cutting depth and feed speed can be adapted to each other to form the optimal cutting amount.
The cutting amount is not only an important parameter that must be determined before the machine tool is adjusted, but whether its value is reasonable or not has a very important impact on the processing quality, processing efficiency, and production cost. The so-called "reasonable" cutting amount refers to the cutting amount that makes full use of the cutting performance of the tool and the dynamic performance of the machine tool (power, torque) to achieve high productivity and low processing costs while ensuring quality.

CNC machining operation safety

CNC machining civilized production

NC machine tool is a kind of advanced processing equipment with a high degree of automation and a complicated structure. In order to give full play to the superiority of the machine tool and improve production efficiency, it is important to manage, use, and repair the NC machine tool. The quality of technicians and civilized production are particularly important . In addition to being familiar with the performance of CNC machine tools and being proficient in operation, operators must also develop good working habits and rigorous work style of civilized production, and have good professional quality, responsibility and cooperation spirit. The following should be done during operation:
Strictly abide by the safe operation regulations of CNC machine tools. Do not operate the machine without professional training.
Strictly abide by the system of commuting and shifting.
(3) Make good use and management of machine tools, and have a strong sense of work responsibility.
Keep the environment around the CNC machine clean and tidy.
Operators should wear work clothes and work shoes. Do not wear or wear dangerous clothing.

CNC machining operation regulations

In order to use CNC machine tools correctly and reasonably, reduce the incidence of its failures, and operation methods. Operate the machine only with the consent of the machine management.
Precautions before starting
1) The operator must be familiar with the performance and operation method of the CNC machine. Operate the machine only with the consent of the machine management.
2) Before the machine tool is powered on, check whether the voltage, air pressure and oil pressure meet the working requirements.
3) Check whether the movable part of the machine tool is in a normal working state.
4) Check whether the worktable is offside or over-limit.
5) Check whether the electrical components are firm and whether the wiring is disconnected.
6) Check whether the ground wire of the machine tool is reliably connected to the workshop ground wire (initial startup is particularly important).
7) Turn on the main power switch after you have completed the preparations before starting up.
Precautions during startup
1) Strictly follow the startup sequence in the machine manual.
2) Under normal circumstances, the machine must first return to the reference point of the machine during the startup process, and establish the machine as the standard system.
3) After the machine is turned on, let the machine run idle for more than 15 minutes to make the machine reach a balanced state.
4) After shutting down, you must wait for more than 5 minutes before you can turn it on again. You must not randomly start or shut down frequently without special circumstances.
Precautions during debugging
1) Edit, modify and debug the program. If it is the first trial cutting, dry run must be performed to ensure the procedure is correct.
2) Install and debug fixtures according to process requirements, and remove iron filings and debris from each positioning surface.
3) Fix the workpiece according to the positioning requirements to ensure correct and reliable positioning. Do not loose the workpiece during processing.
4) Install the tool to be used. If it is a machining center, the position number of the tool in the tool magazine must be exactly the same as that in the program.
5) The tool is calibrated according to the programmed origin on the workpiece to establish the workpiece coordinate system. If multiple tools are used, the remaining tools are compensated for length or tip position.

3D 3D simulation CNC machining

3D technology can make the operation of CNC machine tools more accurate, avoid the damage of the instrument, and ensure the accuracy and efficiency of product processing. Through a series of complicated algorithms, the working trajectory of the model is calculated, and the numerical control machining such as metal processing and metal cutting is realized.

Higher Vocational Teaching Materials for CNC Machining

Basic Information
[2] Title: Comprehensive training on assembly CNC machining
Level: Higher Vocational College
Package: Electronic courseware
Author: Cao illustrious
Publisher: Machinery Industry Press
Publication time: 2013-08
ISBN: 978-7-111-42694-3
Folio: 16
Price: $ 32.0
brief introduction
This book uses environmental protection agency-"wind drive" as the main teaching project. Each project in the textbook has several specific tasks. Students are required to complete the modeling of the mechanism, animation design, numerical control process analysis, and tool path setting. , CNC machining, etc., the process of using the teaching materials is the process of completing tasks. Through this mode, the teaching effect of project leadership, task driving, and training guaranteeing skills is achieved. There are six items in the book, which are the design of the driven mechanism of the "wind drive", the numerical control of the base parts, the numerical control of the camshaft parts, the numerical control of the shaft sleeve parts, the numerical control of the impeller shaft parts, and the multi-axis numerical control of the "Hercules Cup". It is divided into 26 work tasks, of which projects 1 to 5 are set up around "wind drives", which require the use of UG, CAXA manufacturing engineers, and VERICUT to complete the part modeling, tool path setting, and NC machining. Project 6 is an expansion project that requires the completion of the tool path setting and NC machining of "Hercules Cup" parts. The project involves multi-axis machining technology, including the use of POWERMILL to complete the multi-axis machining tool path setting of parts and the operation of DMG multi-axis CNC machine tools. Etc., with certain advanced nature.
This book is a practical numerical control technology book. It is not only suitable for teaching books on numerical control and electromechanical majors in vocational colleges, but also can be used as a reference for engineering and technical personnel engaged in numerical control processing in mechanical processing enterprises and engineering research institutes.
table of Contents
Foreword
Project 1 Design of driven mechanism of wind drive
Task One: Understanding Wind Drives
Task 2: Design of driven components of wind drive
Task 3 Assembly and Animation Design of Wind Drive
Design of task 4 wind drive working animation
CNC machining of project two base
Task-Base Modeling
Analysis of process tooling of task 2 base
Task three base tool path settings
Processing of four bases
CNC machining of project three camshafts
Task-Camshaft Modeling
Analysis of Camshaft Tooling for Task Two
Programming of three camshaft turning
Task four cam profile toolpath settings
Task five machining of camshafts
CNC machining of the project
Task-Sleeve Modeling
Analysis of Two-Sleeve Process Tooling for Task
Programming of three-axis sleeve CNC turning program
Task 4-axis sleeve milling toolpath settings
Processing of five-axis sleeves
CNC machining of project five-blade impeller shaft
Modeling an Impeller Shaft
Analysis of Process Tooling for Impeller Shaft in Task Two
Programming of NC Turning for Mission Three Impeller Shafts
Task four blade and positioning slot tool path settings
Task five machining of impeller shaft
Project Six Expansion Training-"Hercules Cup" Multi-axis CNC Machining
Task 1: Analysis of Process Tooling for "Hercules Cup"
Design of the processing strategy for the second "Hercules Cup"
NC machining of task three "Hercules Cup"
Appendix: Teaching related forms
references

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