## 1 Introduction
### Related Research  
Ref. [26] dealt with virtual prototyping and optimisation of a heavy machine tool in its pre-production phase. Machine stiffness was chosen as the evaluation criterion with differently modelled machine anchoring methods. In ref. [30], four stages of digital twin in product design are mentioned. These are conceptual design, detailed design, design verification, and redesign. Thus, this paper suggests that there are multiple levels of digital twins in product development. Figure 1: Closed loop engineering system (hardware and software in the loop). There are several papers dealing with the methodology of integration of digital twin methods and CAx computation in the design of complex products. In [31], a method was developed to integrate structure, analytical, and information models to create an interoperability engine. This method was validated on an industrial robot. [PERSON] et al. [32] dealt with optimisation calculations using digital twins in the pre-production phase of product development. Both the physical characteristics of the product and its structural parameters were optimised. [PERSON] [33] provides a comprehensive perspective on digital twins and their role in the fourth industrial revolution, describing a series of applications with detailed case studies of rotating machines. [PERSON] [34] explored the development of the concept of the digital twin, its advancements, and its critical role in the fourth industrial revolution. He introduced the concept of the digital twin from the perspective of academia and industry, discussing its limitations and gaps in research, trends, and technical constraints hindering implementation. Research was performed to answer the question of how the concept of digital twins can support the realisation of an integrated, flexible, and collaborative manufacturing environment, which is one of the aims envisaged by the fourth industrial revolution. [PERSON] [35] optimised the machining conditions of a commercial machine tool with a digital twin. The applied model included physical models of the control system, feed drive systems, and cutting parameters. The digital twin was evaluated based on experiments. A genetic algorithm was used to determine the machining conditions in order to minimise machining time and manufacturing cost. [PERSON] et al. [36] was working on creating a digital twin using MCD and TIA Portal tools. The digital twin was used for virtual debugging. It was shown that the simulation of the virtual model was in agreement with the real product. This approach helped to achieve efficient debugging, monitoring, real-time control, and reduced labor costs. [PERSON] et al. [37] used SIMIT to create a safety instrument system for the W7-X reactor. This system was designed to ensure personnel safety and investment protection. The system was subsequently successfully tested. In ref. [38], SIMIT was used to create a prototype of a processing plant. The result was a model of the processing plant that could be used throughout its life cycle to facilitate changes and optimisation of processes within the plant. Ref. [39] dealt with the creation of basic architecture types that can be used to connect systems such as NX MCD, SIMIT, TIA Portal, and PLCSIM. According to the results, the distributed architecture was the best in terms of project time and PLC code modification. Methodology of Using CAx and Digital Twin Methods in the Development of a Multifunctional Portal Centre  
The aim of this research was to develop a methodology for using CAx and digital twin methods. The proposed methodology should be universally and broadly applicable in machine tool design. The methodology will be verified in the development of a multifunctional portal centre in its pre-production phase (see next section). This is a universal heavy machining centre designed for metal cutting. The application of the methodology is limited only to numerically controlled machines whose control system supports the creation of a digital twin of the machine.