INTRODUCTION
Today, CNC technology largely contributes to improve the
production capacity of industrial companies and is also a
fundamental for them to survive in the ferociously global
competition. It had been reported that more than 70% of
manufacturing businesses in the US and the UK rely on CNC
machines [1]. Those who are skillful in NC machining are needed
in a large scale and given more opportunities in finding good jobs
with higher salaries. For example, more than 300,000 working
opportunities were provided for those skillful in NC machining
all of the China in 2003. As a result, much attention has been paid
on training of NC machining, which involves NC machine
operations and NC programming.
The approaches to NC machining training can be classified
into three categories, that is, physical training mode, simulationbased
mode, and VR-based mode. For the first training mode, a
trainee acquires his/her operation skills by observing, referring
the operation manuals, and then operating on a real machine tool
under the guidance of an experienced operator. The physical
training is an intuitional training mode. A trainee can acquire
good experience and skills both via practical operations and
sensatory feedback. However, problems such as low safety, high
training cost, limited availability of training machines and
personnel, etc. are also brought forth.
To overcome the drawbacks of the physical training mode,
the simulation-based training has been put much effort on in the
past decade. It provides a safe, flexible, and cost-effective training
environment and offers all trainees equal opportunities of
operating various machine tools and learning NC machining
theories on a one-to-one base. Most of all, the machine tool on
which the trainee to be trained in this environment can be
upgraded rapidly as the real one in use. A patent titled
‘‘Machining tool operator training system’’ was issued in USA
on March 7, 2000 [http://www.patentstorm.us/patents/
6033226.html]. The system is an interactive multi-media training
system. It incorporates text, audio, and video graphics animation
and other tools to teach a trainee to use a machine or other tool.
The trainee can input his/her commands to the training system by
various input devices such as a keyboard or a mouse. CANVAS
(Computer-Aided NC-Code Verification and Analysis System), a
dynamic CNC cutting simulation and verification software, was
also launched to simulate the actual process of complete CNC
operations by TurboTek Co., Korea [http://edu.turbotek.co.kr/
products/application/product_02.htm].
Virtual reality (VR), a technology with a sense of high reality
offers potential to develop highly interactive 3D user interfaces for a
variety of engineering applications such as virtual manufacturing,
training [2]. In the VR environment, the trainee can receive sensible
feedbacks in various manners so that it can be evaluated that whether
the desired goal is achieved and whether the planned actions is
actually possible to be performed. The result of a research
demonstrated that VR-based training can obtain a good training
effectiveness [3]. Lin et al. [4] described a VR-based training system
(VRTS) for CNC milling machine operations. With VRTS, the trainee
can acquire a set of skills necessary to perform basic milling
operations. A virtual machining lab was also presented for
manufacturing knowledge learning and skills training based on VR
technology [5]. Although VRTS can present a sense of high reality to
the trainee, it involves a great deal of investment on the hardware such
as VR gloves, stereo glasses, workstation(s) with high computation
and graphic performance. Moreover, it seems impossible for most of
schools to complete their training programs in a limited period on a
one-to-one base with a traditional training approach. Therefore, a