Virtual Machining

By Niraj M Wanikar, Chief Editor, CNCTimes.com, 2013-09-14 06:37:27

By K.P. Karunakaran, R. Shringi and Amit Kumar Singh

Computer Graphics Laboratory, Department of Mechanical Engineering
Indian Institute of technology Bombay, Powai, Mumbai 400076, INDIA

Verification of NC programs through trial machining is risky, slow and costly. Virtual machining using software tools can overcome this difficulty. Optimization of cutting parameters achieved through this cuts down machining time and improves quality and tool life. But existing virtual machining methods are all inexact.

1. Introduction

The sources of errors in NC programs are many and varied. They range from relatively trivial syntax errors to subtle interactions with work-holding fixtures. Debugging NC programs on the shop floor is expensive, time consuming and sometimes dangerous. Hence, NC users and developers have sought computational aids for program development and verification from the earliest days of NC.

CAD/CAM/CAE packages available today offer total solutions to the users by incorporating modules for modeling, visualization, analysis and manufacturing. Almost all CAM modules can generate efficient NC cutter paths with minimal user interactions. Some of them are even able to generate the cutter paths automatically for simple geometries of rotary or prismatic nature. However, the correctness of the NC programs generated by these packages cannot be guaranteed until a component is produced on a CNC machine using these programs and inspected. An unproved NC program may have movements that may cause collisions with the fixture elements, dig into the component, create undercuts, gouge the component or leave excess material. An erroneous NC program may also produce out of tolerance dimensions, poor surface finish or have wasteful movements. The task of ascertaining the correctness of the NC programs of a component is called NC Verification or Tape Proving.
The physical tape proving methods have the following drawbacks:

:: They are slow and tedious.
:: The machine tool is utilized less efficiently during tape proving.
:: The programmer and operator are underutilized and fatigued.
:: These methods may be dangerous for the operator and can cause damage to the machine and cutting tools.

Efforts have been going on for a long time on two fronts to eliminate or minimize the time and money spent in tape proving. Researchers in the area of Computer Automated Part Programming (CAPP) have been trying to develop intelligent cutter path generation systems that can produce error-free NC programs thus alleviating the need for verification altogether. Such a system would require vast knowledgebase (KB) and database (DB) related to machining and fantastic computing power to scan through these in order to obtain a 'zero-defect' NC program of a practical component. Furthermore, these knowledge and data bases themselves need to be updated to keep pace with technological growth such as the development of new cutter materials, high speed machining etc.. It appears too optimistic to expect such software to be available in the near future. This prompted people to work in the development of software that would simulate the machining process virtually on a computer screen. These Virtual Machining (VM) systems emulate the CNC machines. While the graphic simulation available in most CAM packages show only motions, the VM systems can depict the material removal as well. In a VM system, one starts with the virtual model of the work piece and as machining progresses, the geometry of the work piece gets updated by subtracting the volume swept by the cutter during each motion. It is possible to model even the machine tool and fixture elements so that even collisions can be detected.

Since the user gets a realistic visual feel of the machining process in a VM system, it helps in predicting errors on the computer screen itself. This system can automatically do the verification both to ascertain the safety of machining (detection of collision etc.) and geometric conformance to design. It is also possible to optimize the technological parameters (spindle speed and feed rate) from the geometric characteristics of the material removal process.

2. Classification of NC Verification Systems

NC verification is the process of ascertaining the correctness of cutter movements before performing the actual machining. Table 1 presents the various methods used for NC verification. Any of these methods can be either an online or an offline method depending on whether the activity is carried out on the actual CNC machine or on an offline computer or a desktop CNC machine. Methods such as Z-neglect machining, scaled-down machining, dry-run machining and alternate-material machining come under this category. On the other hand, scaled-down machining, dry-run machining and alternate-material machining can be carried out offline on a separate cheaper machine. Virtual machining is obviously preferable to physical machining.

i. Kinematic NC Simulation (KinSim)
ii. Volumetric NC Simulation (VolSim)

KinSim is very popular among CAM packages because of its low computational complexity. As the cutter moves, it is drawn at the end of each position. The cutter may be depicted simply by its tip or axis or it may be drawn fully in 2D or 3D. Some of these kinematic displays are shown in Figure 1. KinSim can depict only motions and not material removal. Therefore, physical NC verification may be necessary before releasing the NC program for regular production.

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