CNC machining program evaluation and optimization

Abstract: The machining program is in an extremely important position in the operation of CNC machine tools. The article proposes to evaluate the machining program from aspects such as parts processing quality, programming and running costs, production efficiency, etc., and proposes the CAM for the status of CAM post-processing functions. The preparation of the machining program is optimized.

Keywords: CNC machine tool; machining program; evaluation; optimization

1 Introduction

The CNC machine tool processes workpieces according to the part machining program. A good machining program can not only ensure the production of workpieces that meet the requirements, but also give full play to the functions of CNC machine tools, making them safe, reliable, and efficient. The part processing program is an important part of the numerical control system. According to statistics from abroad, 20% to 30% of the reasons for the shutdown of CNC machine tools are due to the inability to compile processing programs. In order to increase the utilization rate of CNC machine tools, programmers should strive to improve the programming capabilities and quickly produce excellent parts processing programs.

2 Evaluation of part processing programs

The processing of a part is by no means the only one. Many programs (programs) are certainly optimal. Can you evaluate from the following aspects:

(1) Ensure that the procedure is correct and the quality of the parts is stable.

(2) The program is easy to debug and modify, and the program has good readability. For example: To change the approach step length of a non-circular curve or the pitch of a curved surface, you only need to modify a certain parameter without having to modify the entire program.

(3) The stability of the program is good. When the tool radius changes or the part installation position changes, there is no need to change the program.

(4) Give full play to system functions to make the program the shortest. For example: the system has a macro check chamber, a command can be programmed into a milling cavity program. If you don't use them, changing to a one-size-fits-all description will inevitably make the program lengthy.

(5) The generality of the program is good. If you have a series of parts, you only need to edit one. The rest can be used as long as you modify the key dimensions.

(6) Low programming cost. To produce a program, the labor costs and machine costs are low. Of course, the cost of labor is related to the programmer's proficiency and it is inconvenient to quantify it. But with a calculator and a computer-aided programming (CAM) system, the cost is comparable.

(7) Low operating costs. Can use three-axis machine tools, as far as possible without four-axis machine tools; can use a four-axis machine with a graduated turntable, try not to use five-axis machine tools. For example, if you are milling a curved groove in a cone, you can consider the turning center. You can also consider a three-axis machine tool plus a rotary axis. The taper surface is angled with a jig so that only a four-axis machine tool is sufficient instead of a five-axis machine tool.

(8) The cost of subsequent processing is low. Such as a mold cavity, the use of a universal ball head tool surface processing, low tool cost, easy programming, but the next process polishing cost is high, and can not guarantee the accuracy. Tooling with a special tooling surface, although the programming and tooling costs are high, the polishing cost is low and the accuracy can easily be guaranteed. To weigh the pros and cons, choose the preferred one.

Specific parts, exactly what kind of processing procedures should be determined according to actual conditions. In the actual programming, we must have a sense of optimization, especially for parts machining programs programmed with CAM. Since CAM's post-processing function is weak, it should be optimized.

3 CAM-optimized parts machining program optimization

Because CAM has a strong graphic math processing function, eliminating tedious mathematical calculations in manual programming, the CAM source program is relatively popular compared to the parts processing program. Due to the differences in CNC systems and machine tools, post-processing of CAMs, despite special post-positioning or universal postpositioning, still has a considerable gap compared to the functions of CNC machine tools. In practical use, if you can not only give full play to the advantages of CAM but also avoid its shortcomings, but also give full play to the functions of the CNC system and the practical experience of the operators, it is necessary to optimize the CAM-programmed parts processing program to make it Produce a high-level part processing program. The optimization of the processing procedure is recommended considering the following aspects.

3.1 Play System Tool Radius Compensation Function

The numerical control system generally has the tool radius compensation function, that is, programming with the part contour, the tool is automatically offset by a radius vector, and the trajectory of the core is automatically calculated by the system. Take the example of Figure 1 as an example.

Figure 1 part processing program from a CAM system. From the CAM-programmed part program (see NC code processing program), the knife-core trajectory of the outer corner is an arc around a sharp corner (substantial B tool compensation), and the numerical control system is generally a straight-line transfer away from the cusp (C knife Complementary), to maintain sharp points, CAM-programmed part program contains the transition section of the outer corner, and the CNC system automatically generates the transition block with the tool radius compensation function, which does not appear in the part program. In this way, the number of blocks is reduced and it is easy to read.

If the contour is programmed, the system uses the cutter radius compensation function to control the cutter center. When changing the cutter size, the operator only needs to change the cutter compensation value without changing the program.

CAM To create a cutter program for the contour of the pocket along the part contour, simply set the tool radius to zero. If CAM cannot generate tool compensation G codes, the operator can add tool compensation G codes to the program. The part machining program at this time not only embodies the advantages of CAM's mathematical processing, avoids cumbersome manual calculations, but also embodies the flexibility of the tool radius compensation program.

3.2 Substituting a Circular Interpolation Function Instead of a Straight Line Approximation

In surface machining, CAM is generally approached by a straight line to generate a part program. If it is a symmetrical shape, then generally only the first quadrant of the surface processing program, the rest of the quadrant of the processing system to solve the mirror function. It is the first quadrant that is sometimes very long and exceeds the memory of the system. The author once encountered a program that exceeds the system memory. If the memory is enlarged, it needs to invest 4 to 50,000 yuan, and the utilization rate in the future is very low; if it is processed in sections, the efficiency is low. The curve is in the G18 plane, instead of using an arc to approximate the contour, the number of blocks is drastically reduced, and memory is still left. However, in the program, the tool length compensation and tool radius compensation are required on the Z axis. The operator should check whether the system has this function.

3.3 Using the System's Simplified Programming Function

The system provides a large number of simplified programming functions, such as canned cycles, tool compensation, direct contour programming, scaling and mirroring, coordinate rotation, hole shape description calculation of typical shapes (circumference, matrix), regular shape (circular, rectangular) digging Cavity, irregular shape digging cavity, with island-shaped digging cavity and other functions. If the CAM's post-processing can process the part program according to these functions, the program can be greatly shortened.

For example, the surface processing, only a quadrant of the processing program, the use of the system's mirror function to process the rest of the quadrant, the program is the original 1/4. In sprocket and other repetitive shape contour processing, a shape contour is created by CAM, and the rest is rotated using a rotary function, so that the program can be shortened more considerably. Another example is a rectangular cavity digging procedure. A multi-blade and multi-layer cutting procedure is long. If it can be processed into a dig cavity macro-instruction, only a single procedure can be used to complete the entire digging cavity processing.

3.4 Play System Space Tool Radius Compensation Function

For curved surfaces, CAM generally generates the cutter linear motion machining program. The surface processing generally uses a ball-end tool. If you want to change the tool, you must change the program to bring inconvenience to the machining. If the system has the space 3D tool radius compensation function, CAM can generate the program according to the surface and generate the cutter center vector at the same time. The actual cutter position is calculated by the system according to the cutter center vector. In this way, the tool radius can be adjusted within a certain range to facilitate processing.

3.5 Utilize the user macro program function of the system to shorten the processing procedure of contours, space curves and even curved surfaces of non-circular curves

Taking a non-circular curve profile as an example, CAM generally generates a straight-line approximation program. In the processing, if you want to change the step, you need to reprogram and the program is longer (hundreds of thousands of programs). The correctness of the program can only be determined by graphical display or actual cutting. The change is inconvenient.

If the CAM can generate macros of the system according to the macro format of the system, the processing program is automatically generated by the system. The author encountered an example: the contour is composed of two cycloids and a section of envelope. The first is a program written by CAM. It is also a straight line and a circular arc and is very long. The processing program is about 1000 segments. After switching to macro programming, the macro program is only a few tens of paragraphs, and adjustment and modification are very convenient. After comparison, the operator chose the macro program editing program. Furthermore, when the product has several specifications and an assignment procedure is performed, only a few key dimensions can be entered to change the input value of the dimension. Macro programs are universal and very popular among operators.

3.6 Using Subroutine Functions to Simplify Programming

The CAM programming subroutine program is used to program the main program and subroutine call functions by the numerical control system. Subroutines have a large number of numerical calculation workloads, which are done by CAM. The main program multi-instructions are manually programmed, so that the program is flexible and the programming workload is not large.

4 Conclusion

In summary, the programming functions of the CNC system include basic instructions, such as contour and linear instructions, simplified instructions, fixed cycles, tool compensation functions, scaling and mirror images, coordinate rotation functions, macro instructions, etc. User macro function. CAM generally compiles parts machining programs according to basic instructions, and some use some simplified instructions, such as canned cycles, which fail to make full use of the functions of the CNC system. If the user can optimize the CAM-programmed part machining program and combine the advanced functions of the CNC system with the CAM, an excellent machining program can be created.