Advanced Techniques for Rapid Cutting Force Prediction and Control

To tackle the inefficiencies in cutting force optimization using the NC simulation function in VERICUT software, we propose a method for quickly identifying various machining conditions and applying corresponding parameters for NC code optimization. By combining machining experience, cutting parameters, and data from cutting tests, we have established a systematic library of optimization parameters for typical scenarios. This approach allows for the rapid application of simulation optimization processes, enhancing both optimization efficiency and machining accuracy.

 

01 Introduction

Currently, CAM software is commonly used for CNC programming in the machining of aviation parts. This software generates machining programs based on fixed parameters, such as feed rates, retract speeds, and tool lift speeds, combined with defined machining strategies. However, it lacks the capability to adjust cutting parameters instantaneously under varying conditions of cutting volume and cutting force. As a result, NC programs are unable to adaptively adjust feed rates based on the changing cutting allowances and forces throughout each machining path.

In actual machining processes, excessive cutting allowances in localized areas can lead to high spindle loads, which compromise machining accuracy and can potentially cause issues such as tool breakage, chipping, spindle stalling, or inadequate force protection. To ensure smooth machining across all parts, cutting parameters are often set conservatively, which can result in numerous idle toolpaths during the machining of complex cavities. Although the spindle utilization rate remains high, the overall material removal rate of the machine tool tends to be low. This inefficient cutting practice can lead to wasted CNC equipment resources and increased production costs.

To tackle these issues, two methods have emerged in the CNC machining field to enhance the efficiency of CNC programs:

1. Optimizing feed rates based on real-time data collected from parameters such as spindle power, utilizing the OMATIVE adaptive control system installed on the machine tool.

2. Simulation optimization through offline optimization software. For instance, well-known international CNC simulation software like VERICUT and NC SIMUL offer CNC program optimization functions built upon their existing cutting simulation capabilities, making simulation optimization applicable in the cutting machining sector.

Taking VERICUT software as an example, its latest FORCE optimization function utilizes specified material, tool, and machining conditions to maintain the existing tool path and maximum cutting thickness while optimally calculating the feed rate for each program segment. This ensures that the cutting force and air-cut speed in the optimized program remain within specified limits. Upon completion of the optimization, users can view details such as feed rate, cutting force, material removal rate, power, torque, and tool runout to assess the optimization status, ultimately resulting in safer and more efficient CNC programs.

Using the FORCE optimization function requires pre-prepared information on material and tools, as well as an assessment of machining conditions such as the maximum cutting force and cutting thickness that the tool can withstand. Without a comprehensive library of basic optimization parameters, repetitive work may be necessitated, leading to low optimization efficiency and potentially inappropriate selection of optimal parameters. While the FORCE optimization function manages optimization parameters for each tool within a library and provides methods for management and optimization under different roughing and finishing conditions, as well as for varying materials, it often lacks convenience and intuitiveness.

This paper examines a typical flexible production line scenario where the tool type, tool number, and cutting performance remain virtually unchanged. It investigates the FORCE optimization function in VERICUT software, focusing on a method for constructing a tool optimization parameter library and its automatic recognition and intuitive display under different cutting materials and machining conditions. The objective is to improve process optimization efficiency and the accuracy of optimization results by systematically building an optimization parameter library and optimizing its invocation methods.Currently, CAM software is commonly used for CNC programming in aviation parts machining. This software only generates machining programs based on fixed machining parameters such as feed, retract, and tool lift speed, combined with machining strategy definitions. It lacks the ability to adjust cutting parameters instantaneously under constant cutting volume and cutting force conditions. NC programs are unable to adaptively adjust feed rates based on the varying cutting allowances and forces within each machining path. In actual machining, excessive cutting allowances in localized locations often lead to excessive spindle loads, compromising part machining accuracy and potentially causing tool breakage, chipping, spindle stalling, or force protection. To ensure smooth machining of all parts, cutting parameters are sometimes set conservatively. This can result in numerous idle toolpaths during the machining of complex cavities. These idle toolpaths often operate at the machining speed, significantly increasing machining time. While the spindle utilization rate is high, the average material removal rate of the machine tool is low. Inefficient cutting can lead to wasted CNC equipment resources and increased production costs.

To address this issue, two methods have emerged in the CNC machining field to improve the efficiency of CNC programs: 1. Optimizing feed rates based on real-time data collected from parameters such as spindle power, using the OMATIVE adaptive control system installed on the machine tool. 2. Simulation optimization using offline optimization software. For example, mainstream international CNC simulation software such as VERICUT and NC SIMUL have introduced CNC program optimization functions based on their existing cutting simulation capabilities, making simulation optimization capabilities applicable in the cutting machining field. Taking VERICUT software as an example, the latest FORCE optimization function, based on the specified material, tool, and machining conditions, maintains the existing tool path and maximum cutting thickness to optimally calculate the feed rate for each program segment. This ensures that the cutting force and air-cut speed in the optimized program are within specified limits. After optimization is complete, the user can view feed rate, cutting force, material removal rate, power, torque, and tool runout to assess the current optimization status, ultimately resulting in a safer and more efficient CNC program. Using the FORCE optimization function requires pre-prepared material and tool information, as well as an assessment of machining conditions such as the maximum cutting force and cutting thickness that the tool can withstand. Without a comprehensive basic optimization parameter library, repetitive work is often required, optimization efficiency is low, and optimal parameter selection is often inappropriate. While the FORCE optimization function manages the optimization parameters for each tool into a library and provides management and optimization methods for different roughing and finishing conditions, as well as for different materials, it lacks convenience and intuitiveness.

This paper examines a typical flexible production line scenario where tool type, tool number, and cutting performance remain essentially unchanged. It investigates the FORCE optimization function in VERICUT software, based on a tool optimization parameter library construction method, and its automatic recognition and intuitive display under different cutting materials and machining conditions. The goal is to improve process optimization efficiency and the accuracy of optimization results by systematically constructing an optimization parameter library and optimizing its invocation methods.

 

02 VERICUT Tool Library Optimization Parameters and Management Methods

The FORCE function in VERICUT software offers an extensive library of fundamental optimization parameters tailored for various materials. This library enables the accumulation of machining experience based on the cutting parameters for different materials and the tool specifications used with specific CNC equipment. By conducting analyses and screenings within the FORCE function, along with targeted cutting tests, preliminary recommendations for optimization parameters can be created for each tool. In practical applications, these parameters can be iteratively optimized.

VERICUT software links these optimization parameters to tools within the tool library, facilitating both management and easy retrieval. Changes in machine tools, tool specifications, cutting materials, and machining conditions will lead to different optimization parameters.

For example, Figure 1 illustrates the optimization parameter management in VERICUT software. For tool T10, optimization parameters for roughing and finishing operations on 7075 and 2024 aluminum alloys can be managed within the library. Other tools can be similarly managed within the same tool library. When the optimization function is enabled, the optimization process can be controlled by defining specific VERICUT command calls in the CNC program. For instance, you can define the comment statement: (VERICUT-OPTIPATH DESC= T10 End Mill D 10R1_AF1250-rough) to select and activate the first set of optimization parameters from the four available sets.

In certain situations, such as when a process engineer wants to disable a specific finishing optimization program, they can use the “on” or “off” statement in VERICUT-OPTIPATH to control the optimization process.

However, this management approach has its drawbacks. When a part has multiple machining programs and complex processes, the roughing and finishing operations are displayed with the same tool number in the results. This overlap makes it challenging for process engineers to clearly differentiate between them. As a result, it becomes difficult to accurately identify the status of each program and assess the current optimization results, particularly when one program utilizes the same tool for both roughing and finishing. Figure 2 illustrates the management of optimization parameters in VERICUT.

03 Improved VERICUT Tool Optimization Parameters

To improve the recall of optimization parameters by process engineers, we can create a tool optimization parameter library that organizes these parameters independently for different machine tools and materials.

To help identify the conditions for optimization, we categorize tool magazine positions for conventional CNC machine tools, which typically require only a few dozen tool positions. The tool positions can be divided into three categories during optimization simulation: positions 1-100 for roughing, positions 101-200 for semi-finishing, and positions 201-300 for finishing. This categorization allows for different machining conditions associated with the same tool specification to be mapped to distinct tool numbers.

To implement this management system, it is essential to integrate command customization and development capabilities within VERICUT software, as well as to add necessary annotation-based auxiliary commands to the CNC program.

 

3.1 Optimization parameter change of FANUC CNC system

(1) In the tool library, each tool is now defined by three tool numbers instead of just one. These tool numbers are T1 for the semi-finishing tool, T101 for the finishing tool, and T201 for the roughing tool. The management of optimization parameters under different working conditions for these tool numbers is illustrated in Figure 3. The optimization parameters specific to roughing, semi-finishing, and finishing are configured accordingly for each respective tool number.

 

(2) Program Identification

In general, the NC program does not clearly identify roughing and finishing working conditions. To correctly specify the optimization parameters for different working conditions, it’s essential to define the necessary optimization instructions for the NC program before running simulation optimizations. To ensure that the subsequent operations of the NC program on the machine tool are not affected, these optimization instructions should be included as comment lines. The simpler the definitions, the easier they will be for process programmers to use.

To achieve this, three comment commands are utilized: (ROUGHING), (SEMI-FINISHING), and (FINISHING) to indicate the respective working conditions. These commands should be placed on a separate line following the tool call instruction. Additionally, keywords such as ROUGH, rough, roughing, and ROUGHGING can be defined as needed to help identify the optimization intentions of the process programmer.

 

(3) The CNC manufacturing process of converting a single tool into different tool numbers based on the identification of working conditions is outlined here. In the tool call instruction of the NC program, only the initial tool number is specified. It is essential to translate the current tool number into the appropriate tool number for the defined roughing and finishing conditions. Additionally, instructions related to roughing and other processes need to be transformed into subroutine calls, where the tool number conversion will be implemented.

For example, when a command like (FINISHING) appears in the program, the system will automatically change it to a subroutine call instruction, such as M98P9103, which activates the call to subroutine O9103. The method for replacing different working conditions with subroutine calls is illustrated in Figure 4.

 

In the subroutine O9103, initialize the global variable TOOLNO_BASE to 200. You can define this global variable in the system environment in a manner similar to how CGTech_Var NCK INT TOOLNO_BASE is initialized to 0. After initializing the variable, perform the necessary parameter assignments and value transfers to correctly call the new tool number. The following code example illustrates this process.

O9103
TOOLNO_BASE = 200
#4120=#4120+TOOLNO_BASE
#4320=#4120
M06T#4120
H#4120
D#4120
M99

The simulation optimization interface now features a tool change list that displays tool numbers corresponding to various working conditions. By switching between different tool numbers, process engineers can easily compare optimization results, enabling more focused analysis and decision-making. Figure 5 illustrates the optimization results and tool usage associated with this new processing method.

 

3.2 Optimizing Parameters in the Heidenhain CNC System

In the Heidenhain system, tool definitions and program identifiers in the tool library are managed similarly to how they are in the FANUC CNC system. This is done using the Heidenhain CALL PGM method. An example of a Heidenhain subroutine call is illustrated in Figure 6.

 

When assigning the same tool to different tool numbers based on the working condition identifier, the simulation environment of the Heidenhain CNC system cannot be implemented in the same manner as that of the FANUC CNC system.

In the Heidenhain system, when the TOOL CALL command is executed, the currently referenced tool number is assigned to the custom variable CGT_SELECTED_TOOL within the VERICUT software. Since this variable is defined as a string type, it cannot be directly used for numerical calculations. To obtain the current tool number, you can change the type of CGT_SELECTED_TOOL from CGTech_Var NCK STRING to CGTech_Var NCK INT. However, because the TOOL CALL command cannot pass the tool number via a variable, a logical judgment method must be used in a subroutine to implement the selection of different tool numbers. The O9103 subroutine is used to call the finishing tool, as demonstrated below.

BEGIN PGM O9103 MM
Q1 = 200
FN 1: Q2 = Q1+CGT_SELECTED_TOOL
FN 9: IF +Q2 EQU 201 GOTO LBL 1
FN 9: IF +Q2 EQU 202 GOTO LBL 2
…… FN 9: IF +Q2 EQU 215 GOTO LBL 15
……
LBL 1
TOOL CALL 201 Z
GOTO LBL 100
LBL 0
LBL 2
TOOL CALL 202 Z
GOTO LBL 100
LBL 0
……
LBL 15
TOOL CALL 215 Z
GOTO LBL 100
LBL 0
……
LBL 100
END PGM O9103 MM

 

04 Conclusion

Cutting force simulation optimization is an advanced technology that enhances custom aluminum CNC machining efficiency, process safety, and stability. Currently, manufacturers in the machining industry are closely monitoring its development, and some have already initiated preliminary applications.

This paper discusses the practical application of cutting force simulation optimization using the FORCE function in VERICUT software for CNC machining. It explores the creation, quick access, and easy analysis of a cutting force simulation optimization parameter library that is integrated with a tool library. This method allows for systematic management of tool optimization parameters and provides an intuitive display of simulation optimization results under various machining conditions.

Overall, this approach significantly improves the productivity of process engineers, facilitates the adoption of cutting force simulation optimization technology in CNC machining, and holds considerable potential for application in flexible machining production lines. Its prospects for widespread application and promotion are promising.

 

 

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