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What is CNC machining?

Numerical control machining refers to a manufacturing method of processing parts on a numerically controlled machine tool. The process regulations of CNC machine tool processing and traditional machine tool processing are generally consistent, but significant changes have also taken place. A machining method that uses digital information to control the displacement of parts and tools. It is an effective way to solve the problems of variable parts, small batches, complex shapes, and high precision, and to achieve high-efficiency and automated processing.
CNC machining

Basic definition of CNC machining

Numerical control processing refers to the command issued by the control system to make the tool perform various movements that meet the requirements. In the form of numbers and letters, the processing required by the technical requirements and processing requirements such as the shape and size of the workpiece is expressed. It generally refers to the process of processing parts on CNC machine tools.

A CNC machine tool is a 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 CNC system. The movement and auxiliary actions of the CNC machine tool are controlled by the instructions issued by the CNC system. The instructions of the numerical control system are compiled by the programmer according to the material of the workpiece, the processing requirements, the characteristics of the machine tool, and the instruction format (numerical control language or symbols) prescribed by the system. The numerical control system sends out operation or termination information to the servo device and other functional components according to the program instructions to control various movements of the machine tool. When the part processing program ends, the machine tool will automatically stop. For any kind of CNC machine tool, if there is no program command input in the CNC system, the CNC machine tool cannot work. The controlled actions of the machine tool roughly include the start and stop of the machine tool; Spindle start and stop, rotation direction and speed change; The direction, speed and mode of the feed movement; Tool selection, length and radius compensation; Tool replacement, coolant opening and closing, etc.


The development history of CNC technology

Numerical control technology originated from the needs of the aviation industry and was proposed by a US helicopter company in the late 1940s.
The initial idea of CNC machine tools, the Massachusetts Institute of Technology developed a three-axis CNC milling machine in 1952. This kind of CNC milling machine has been used for processing aircraft parts in the mid-1950s. In the 1960s, the numerical control 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 milling and boring machines are the main ones. The parts processed by CNC are integral wall panels, beams, bulkheads, and propellers of airplanes and rockets;
The mold cavities of the casing, shaft, disk, and blades of aero engines, and the cavity of the combustion chamber of the liquid rocket engine, etc. The initial stage of the development of CNC machine tools is based on continuous trajectory CNC machine tools, continuous trajectory control.

Continuous trajectory control is also called contour control, which requires the tool to move on a prescribed trajectory relative to the part. Later, we will vigorously develop point-control CNC machine tools. Point control means that the tool moves from one point to another, as long as it can reach the target accurately at the end, regardless of the moving route.

CNC machining workshop

The operation process of CNC machining

NC programming
There are manual (manual) programming and automatic programming methods for NC machining programming. Manual programming, the entire content of the program is manually written in accordance with 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 what kind of automatic programming method is adopted, corresponding hardware and software are required.
It can be seen that the realization of NC machining programming is the key. But programming alone is not enough. CNC machining also includes a series of preparatory work that must be done before programming and the aftermath of programming. Generally speaking, the main contents of CNC machining process are as follows:

⑴ Select and confirm the parts and contents for CNC machining;
⑵ Process analysis of CNC machining of parts drawings;
⑶ Process design of CNC machining;
⑷ Mathematical processing of parts drawings;
⑸ Compile the processing procedure list;
⑹ Make the control medium according to the procedure list;
⑺ Check and modify the program;
⑻ Trial processing of prototype and on-site problem handling;
⑼ The finalization and filing of CNC machining process documents.

In order to improve the degree of production automation, shorten the programming time and reduce the cost of CNC machining, a series of advanced CNC machining technologies have been developed and used in the aerospace industry. Such as computer numerical control, that is, a small or microcomputer is used to replace the controller in the numerical control system, and the software stored in the computer is used to perform calculation and control functions. This kind of soft-connected computer numerical control system is gradually replacing the initial state numerical control system. Direct numerical control uses one computer to directly control multiple numerical control 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 complex and expensive, it can improve processing efficiency and quality. In addition to the improvement of CNC systems and machine tools in terms of hardware, the development of CNC has another important aspect that is the development of software. Computer-aided programming (also called automatic programming) means that after a programmer writes a program in numerical control language, it is input into the computer for translation, and finally the computer automatically outputs it. The most widely used CNC language is the APT language. It is roughly divided into a main processing program and a post-processing program. The former translates the program written by the programmer and calculates the tool path; The latter compiles the tool path into a part processing program of a CNC machine tool.

Numerical control processing is to write a program on the computer before processing the workpiece. Then input these programs to the machine tool controlled by the computer program for command processing. Or directly write instructions for CNC machining on the machine tool control panel controlled by this computer program. The machining process includes: tool feed, tool change, speed change, direction change, stop, etc., all of which are completed automatically. CNC machining is an advanced method of modern mold manufacturing and processing. Of course, CNC machining methods must not only be used for mold parts processing, but also have a wide range of uses.
5-axis CNC machining

Process analysis of the workpiece

The technical issues of CNC machining of the processed parts involve a wide range of aspects. The following is a combination of the possibility and convenience of programming to put forward some main content that must be analyzed and reviewed.
1. The dimensioning should conform to the characteristics of CNC machining
In CNC programming, the size and position of all points, lines, and surfaces are based on the programming origin. Therefore, it is best to give the coordinate size directly on the part drawing, or try to quote the size with the same datum.

2. The conditions of geometric elements should be complete and accurate
In the 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 contour of the part must be defined during automatic programming, the coordinates of each node must be calculated during manual programming. No matter which point is unclear or uncertain, programming cannot be carried out. However, due to inadequate consideration or neglect by part designers in the design process, there are often incomplete or unclear parameters, such as arc and straight line, arc and arc whether they are tangent or intersected or separated. Therefore, when reviewing and analyzing the drawings, you must be careful and contact the designer in time if you find any problems.

3. Reliable positioning reference
In CNC machining, the machining processes are often concentrated, and it is very important to locate them on the same basis. Therefore, it is often necessary to set some auxiliary datums, or add some process bosses on the blank.

4. Uniform geometry type or size
It is best to use a uniform geometric type or size for the shape and cavity of the parts, so that the number of tool changes can be reduced, and the program can be controlled to shorten the length of the program.
The shape of the parts is as symmetrical as possible, which is convenient for programming with the mirror machining function of the CNC machine tool to save programming time.

Clamping of parts

1. The basic principles of positioning and installation
When machining parts on a CNC machine tool, the basic principle of positioning and installation is to choose a reasonable positioning datum and clamping plan. Pay attention to the following points when choosing:
1. Strive to unify the benchmarks of design, process and programming calculations.
2. Minimize the number of clamping times, and process all the surfaces to be processed after positioning and clamping once as much as possible.
3. Avoid using CNC machining schemes that are manually adjusted after shutdown. Should give full play to the effectiveness of CNC machine tools.

Second, the basic principles of choosing fixtures
The characteristics of CNC machining put forward two basic requirements for the fixture:
One is to ensure that the coordinate direction of the fixture is relatively fixed with the coordinate direction of the machine tool;
The second is to coordinate the size relationship between the parts and the machine coordinate system. In addition, the following points should be considered:
1. When the batch of parts is not large, modular fixtures, adjustable fixtures and other general fixtures should be used as much as possible. In order to shorten production preparation time and save production costs.
2. Only consider the use of special fixtures during mass production, and strive to have a simple structure.
3. The loading and unloading of parts should be fast, convenient and reliable to shorten the machine stop time.
4. The parts on the fixture should not hinder the machining of the surfaces of the parts by the machine tool, that is, the fixture should be open. Its positioning and clamping mechanism components can not affect the cutting during processing (such as collision, etc.).

Precision CNC machining
Machining error

CNC machining error △ number:
It is composed of programming error △ editing, machine tool error △ machine, positioning error △ fixed, tool setting error △ and other errors.
Namely: △number plus=f (△editing+△machine+△fixed+△tool)
in:
1. Programming error △ is composed of approximation error δ and rounding error. The approximation error δ is produced in the process of approximating a non-circular curve with a straight line segment or a circular arc segment, as shown in Figure 1.43. The rounding error is the error produced by rounding the coordinate value to an integer pulse equivalent value during data processing. Pulse equivalent refers to the displacement of each unit pulse corresponding to the coordinate axis. Normal precision level CNC machine tools, the general pulse equivalent value is 0.01mm;
The pulse equivalent value of more precise CNC machine tools is 0.005mm or 0.001mm, etc.
2. Machine tool error △Machine is caused by CNC system error, feed system error and other reasons.
3. The positioning error △ is always caused when the workpiece is positioned on the fixture and the fixture is positioned on the machine tool.
4. Tool setting error △ tool is generated when determining the relative position of the tool and the workpiece.

NC programming

Program structure
A program segment is a continuous group of words that can be processed as a unit, and it is actually a section of a program in a CNC machining program. The main body of the part processing program is composed of several program segments. Most program segments are used to instruct the machine tool to complete or execute a certain action. The program segment is: It is composed of size words, non-size words and block end instructions. When writing and printing, each block generally occupies one line, and the same is true when the program is displayed on the screen.

Program format
Conventional CNC machining program is composed of start symbol (single row), program name (single row), program body and program end instruction (generally single row). There is a program end character at the end of the program. The program start character and the program end character are the same character:% in ISO code, ER in EIA code. The program end instruction can be M02 (program end) or M30 (paper tape end). CNC machine tools generally use stored programs to run. At this time, the common points of M02 and M30 are: After finishing all other commands in the block, it is 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 the following differences are used on other machine tools (systems): When the program is ended with M02, the cursor will stop at the end of the program after the automatic operation ends; When using M3O to end the program operation, the cursor and screen display can automatically return to the beginning of the program after the automatic operation ends, and the program can be run again by pressing the start button. Although M02 and M30 are allowed to share a block with other program words, it is better to list them in a single block, or share a block with the sequence number only.

The program name is located before the main body of the program and after the start of the program, and it usually occupies a line on its own. The program name has two forms: One is composed of prescribed English characters (usually O), followed by several digits. The maximum allowable number of digits is stipulated by the manual, and the two common ones are two digits and four digits. This form of program name can also be called a program number. Another form is that the program name is composed of English characters, numbers or a mixture of English and numbers, and a "-" sign can be added in the middle. This form makes the user naming procedure more flexible. For example, the program for the third process of machining the flange with the part drawing number 215 on the LC30 CNC lathe can be named LC30-FIANGE-215-3. This brings great convenience to use, storage and retrieval. The form of the program name is determined by the CNC system.
%
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
%

CNC machining technology
Segment format
The rules for the arrangement of words, characters and data in a program segment are called block format. The fixed sequence format and separator (HT or TAB) block format have been used in the history of CNC. When these two program segment formats have passed, the word address variable program 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, program words are used to arrange in a certain order in the program segment. The programming manual of the CNC machine tool uses a detailed format to classify and stipulate the details of the programming: The characters used in the programming, the sequence and word length of the program words in the program segment, etc. E.g:

/ NO3 G02 X+053 Y+053 I0 J+053 F031 S04 T04 M03 LF

The detailed format classification of the above example is as follows: N03 is the sequence number of the program segment; G02 means that the processing track is a clockwise arc; X+053, Y+053 indicate the coordinates of the end point of the arc to be processed; I0, J+053 represent the center coordinates of the arc to be processed; F031 is the processing feed rate; S04 is the spindle speed; T04 is the tool number of the tool used; M03 is an auxiliary function command; LF block end instruction; / is a skip selection instruction. The function of the skip selection instruction is: On the premise that the program remains unchanged, the operator can choose whether to execute or not execute the blocks in the program with skip selection instructions. The method of selection is usually by turning the skip selection switch on the machine operation panel to ON or OFF to realize the non-execution or the execution of the block with "/".

execute program
This situation is sometimes encountered when compiling a CNC machining program: A group of program segments appear multiple times in a program, or it is necessary to use it in several programs. We can extract this group of program segments, name them and store them separately, this group of program segments are subroutines. A subroutine is a section of processing program that can be called by appropriate machine control instructions, and it generally has an independent meaning in processing. The processing program where the instruction to call the subroutine of the first level is located is called the main program. The instruction to adjust the subroutine is also a program segment, which is generally composed of the subroutine call instruction, the subroutine name, and the number of calls. The specific rules and formats vary from system to system. For example, it is also “call subroutine No. 55 once”, FANUC system uses “M98 P55.”, and American A-B company system uses “P55x”.

Subroutines can be nested, that is, layer by layer. The relationship between the upper level and the next level is the same as the relationship between the main program and the first level subroutine. The maximum number of layers that can be set depends on 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), the last line is the "subroutine end" instruction, and between them is the subroutine body. However, the function of the main program end instruction is to end the main program and reset the CNC system. Its instructions have been standardized, and all systems use M02 or M30; And the function of the subroutine end instruction is to end the subroutine, return to the main program or the upper level subroutine. The commands are not unified in various systems, such as M99 for FANUC system, M17 for Siemens system, M02 for system of American A-B company, etc.

User macros (programs) can be used in CNC machining programs. The so-called macro program is a subroutine containing variables. The instruction that calls the macro program in the program is called the user macro instruction, and the function that the system can use the user macro program is called the user macro function. When executing, you only need to write the user macro command to execute its user macro function.
The biggest features of user macros are:
● Variables can be used in user macros;
● You can use calculation formulas, turning statements and various functions
● You can use user macro commands to assign values to variables.
The CNC machine tool adopts group technology to process parts, which can expand the batch size, reduce the amount of programming, and improve economic efficiency. In group processing, the parts are classified, and the processing program is compiled for this type of parts, instead of editing a program for each part. When processing the same type of parts with different sizes, the main convenience of using user macros is that variables can be used instead of specific values. When it comes to actual processing, just assign the actual size value of this part to the variable with the user macro command.

CNC machining principles
⑴ The processing of the previous process cannot affect the positioning and clamping of the next process.
⑵ Inside first, then outside, that is, the inner cavity (inner hole) is processed first, and then the outer shape is processed.
⑶ The process of machining with the same installation or using the same tool should be carried out continuously to reduce errors caused by repositioning or tool change.
⑷ In the same installation, the process that has little effect on the rigidity of the workpiece should be carried out first.

Processing route
The feed processing route of the CNC lathe 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 this point and ends the processing program. Including cutting paths and non-cutting empty travel paths such as cutting and cutting out by the tool.
The feed path of finishing is basically carried out along the part contour sequence. Therefore, the focus of determining the feed route is to determine the feed route for rough machining and idle stroke.
In CNC lathe processing, the determination of the processing route generally follows the following principles.
① It should be able to ensure the accuracy and surface roughness of the workpiece to be processed.
② Make the processing route the shortest, reduce the idle travel time, and improve the processing efficiency.
③ Try to simplify the workload of numerical calculation and simplify the processing procedure.
④ For some reusable programs, subroutines should be used.
Make the processing program have the shortest feed route, not only can save the execution time of the entire processing process. It can also reduce unnecessary tool consumption and wear of the sliding parts of the machine tool's feed mechanism. The types and implementation methods of the shortest feed route are as follows:
⑴ The shortest cutting feed path. The shortest cutting feed path can effectively improve production efficiency and reduce tool loss. When arranging the shortest cutting feed route, it is also necessary to ensure the rigidity of the workpiece and the requirements of processing technology.
⑵ The shortest empty trip route.

① Use the starting point of the machining skillfully.
An example of a general situation of rough turning using rectangular loop mode. The setting of tool point A is to take into account the need for convenient tool change during finishing machining and other machining processes, so it is set at a position far away from the blank. At the same time, coincide the starting point with the tool setting point

② Ingeniously set the CNC tool change point.
In order to consider the convenience and safety of tool change, sometimes the tool change point is also set at a position far away from the blank. Then, when the second knife is changed, the idle travel route during the fine turning is bound to be longer; If the tool change point of the second tool is also set at the non-point position in the middle, the idle stroke distance can be shortened.

Reasonably arrange the "return to zero" CNC route. In order to simplify the calculation process and facilitate the verification when manually compiling the processing program of the complex contour, the programmer sometimes executes the "return to zero" operation instruction for the end point of the tool after each cut is processed. Make it all return to the position of the tool setting point, and then execute the subsequent program. This will increase the distance of the feed path and reduce production efficiency. Therefore, when rationally arranging the "return to zero" route, the distance between the end point of the previous cut and the start point of the next cut should be as short as possible. Or zero to meet the shortest feed path requirement. In addition, when selecting the command to return to the tool setting point, under the premise of no interference, use the x and z axis bidirectional “return to zero” command at the same time as much as possible. The function "return to zero" route is the shortest.

⑶ Step CNC cutting feed path with large margin. Two cutting feed routes for blanks with too much margin are listed. It is a wrong step cutting route: cut in the order of 1 bucket and 5, and the margin left for each cutting is equal, which is the correct step cutting feed route. Because under the same cutting thickness.

⑷ Continuous cutting feed route for finishing part contour. The finishing of the contour of the part can be arranged with one or several finishing processes. The finished contour should be processed continuously by the last cut. At this time, the forward and backward positions of the tool should be selected appropriately. Try not to arrange cutting and cutting or tool change and pause in a continuous contour, so as not to damage the balance of the process system due to sudden changes in cutting force. Cause scratches, shape mutations or retained tool marks on the contour of the part.

⑸ Special feed route. In CNC turning processing, under normal circumstances. The longitudinal feed of the tool is fed along the negative direction of the coordinate, but sometimes it is unreasonable to arrange the feed route according to its normal negative direction. It may even damage the workpiece.

Pros and cons

CNC machining has the following advantages:
① The number of tooling is greatly reduced, and complex tooling is not required for processing parts with 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.
② The processing quality is stable, the processing accuracy is high, and the repeat accuracy is high, which is suitable for the processing requirements of aircraft parts.
③ In the case of multi-variety and small batch production, the production efficiency is higher, which can reduce the time of production preparation, machine tool adjustment and process inspection, and the cutting time is reduced due to the use of the best cutting amount.
④ It can process complex profiles that are difficult to process by conventional methods, and even process some unobservable processing parts. The disadvantage of CNC machining is that the cost of machine tools is expensive and requires a high level of maintenance personnel.
Choosing the right CNC machining tool

CNC tool selection

1. The principle of choosing CNC tools
Tool life is closely related to cutting volume. When formulating cutting parameters, the reasonable tool life should be selected first, and the 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 according to the goal of the least single-piece man-hours, and the latter is determined according to the goal of the lowest process cost.

When choosing tool life, you can consider the following points according to the complexity of the tool, manufacturing and grinding costs. The tool life of complex and high-precision tools should be selected to be higher than that of single-edged tools. For machine clamp indexable 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 to be lower, generally 15-30min. For multi-tools, modular machine tools and automated machining tools with complex tool installation, tool change, and tool adjustment, the tool life should be higher, and the reliability of the tool should be ensured in particular. When the productivity of a certain process in the workshop limits the improvement of the productivity of the entire workshop, the tool life of the process should be selected lower. When the cost of the whole plant shared by a certain process unit time is large, the tool life should also be selected lower. When finishing large parts, in order to ensure that at least one pass is completed, and to avoid changing the tool in the middle of cutting, the tool life should be determined according to the accuracy of the part and the surface roughness. Compared with ordinary machine tool processing methods, CNC machining puts forward higher requirements on cutting tools. It not only needs to be rigid, high precision, but also requires stable dimensions, high durability, and easy installation and adjustment at the same time for breaking and discharging performance, so as to meet the high efficiency requirements of CNC machine tools. The selected tools on CNC machine tools often use tool materials suitable for high-speed cutting (such as high-speed steel, ultra-fine-grained cemented carbide) and use indexable inserts.

2. Choose tools for CNC turning
The commonly used CNC turning tools are generally divided into three types: forming turning tools, pointed turning tools, arc turning tools and three types. Forming turning tools are also called prototype turning tools, and the contour shape of the machined parts is completely determined by the shape and size of the cutting edge of the turning tool. In CNC turning processing, common forming turning tools include small radius arc turning tools, non-rectangular turning tools and threading tools. In CNC machining, the forming turning tool should be used as little as possible or not. The pointed 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 secondary cutting edges: Such as 900 internal and external turning tools, left and right end face turning tools, grooving (cutting) turning tools, and various external and internal turning tools with very small tip chamfers.
The selection method of the geometric parameters of the pointed turning tool (mainly the geometric angle) is basically the same as that of ordinary turning: However, the characteristics of CNC machining (such as machining route, machining interference, etc.) should be fully considered, and the strength of the tool tip itself should be taken into account.

two, Arc-shaped turning tool. The arc-shaped turning tool is a turning tool characterized by an arc-shaped cutting edge with a small roundness or line profile error. Each point of the arc edge of the turning tool is the tip of the arc-shaped turning tool. Accordingly, the tool position point is not on the arc, but on the center of the arc. The arc-shaped turning tool can be used for turning inner and outer surfaces, and is especially suitable for turning various smooth connection (concave) forming surfaces. When selecting the arc radius of the turning tool, it should be considered that the arc radius of the cutting edge 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. In order to avoid machining interference, the radius should not be too small, otherwise it will not only be difficult to manufacture, but also the turning tool will be damaged due to weak tool tip strength or poor heat dissipation capacity of the tool body.

3. Choose tools for CNC milling
In CNC machining, flat-bottomed end mills are commonly used for milling the inner and outer contours of plane parts and milling planes. The empirical data of the relevant parameters of the tool are as follows:
1. The radius RD of the milling cutter should be smaller than the minimum radius of curvature Rmin of the inner contour surface of the part, generally RD=(0.8-0.9) Rmin.
2. The machining height of the part H< (1/4-1/6) RD to ensure that the knife has sufficient rigidity.
3. When milling the bottom of the inner groove with a flat-bottomed end mill. Since the two passes at the bottom of the groove need to be overlapped, and the effective radius of the bottom edge of the tool is Re=R-r, that is, the diameter is d=2Re=2 (R-r), and the tool radius is Re=0.95 (Rr) during programming. For the processing of some three-dimensional profiles and contours with variable bevel angles, spherical milling cutters, ring milling cutters, drum milling cutters, tapered milling cutters and disc milling cutters are commonly used.

Most of the CNC machine tools use serialized and standardized tools. There are national standards and serialized models for the tool holders and heads of indexable machine-clamped external turning tools and face turning tools. For machining centers and machine tools with automatic tool changers, tool holders have been serialized and standardized: For example, the standard code of the tapered shank tool system is TSG-JT, and the standard code of the straight shank tool system is DSG-JZ. In addition, for the selected tool, strict measurement of the tool size is required to obtain accurate data before use. The operator inputs these data into the data system, and completes the processing process by calling the program, thereby processing qualified workpieces.

CNC tool point

From where does the tool start to move to the specified position?
Therefore, 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 compiling the program, the position of the tool setting point must be selected correctly. The principle of tool setting point is: Facilitate numerical processing and simplify programming. Easy to align and easy to check during processing; The processing error caused is small. The tool setting point can be set on the machined part, it can also be set on the fixture or on the machine tool. In order to improve the machining accuracy of the parts, the tool setting point should be set as far as possible on the design datum or process datum of the part.

In actual operation of the machine tool, the tool position point of the tool can be placed on the tool setting point by manual tool setting operation: That is, the coincidence of "tool position point" and "tool setting point". The so-called "tool location point" refers to the positioning datum point of the tool, and the tool location point of the turning tool is the tool tip or the center of the tool tip arc. The flat-bottomed end mill is the intersection of the tool axis and the bottom of the tool; the ball-end mill is the center of the ball, and the drill is the point. Using 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 the tool needs to be changed during processing, the tool change point should be specified. The so-called "tool change point" refers to the position of the tool post when it rotates to change the tool. The tool change point should be located outside the workpiece or fixture, and the workpiece and other parts should not be touched when changing the tool.
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