Traditionally, physical prototypes have been made by hand crafting or by manual mechanical machining as described in subsection 3.2.2 of Chapter three. Thanks to developments in both software and hardware and within manufacturing engineering, it is now possible to produce a physical prototype based directly on a virtual prototype. This brings about one way integration of the two types of prototypes. Two typical technologies in converting virtual prototype to physical prototype are CNC machining and Rapid Prototyping.
According to Gibbs [1984], “numerical control (NC) is the term used to describe the control of machine movements and various other functions by instructions expressed as a series of numbers and initiated via an electronic control system”. Computerised numerical control is the term used when the control system includes a programmable computer. Typically, CNC machining It is a process of removing material from a solid block of metal, plastic or wood to obtain a finished part or physical prototype. In the application of CNC machining, the programmer must deal with every feature in a part, and this can add significant time and cost to the product development process [Wohlers & Grimm 2003].
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In contrast to the material removal process of CNC machining, Rapid Prototyping is a process of material deposition [Grote et al 2001]. Although a relatively recent technology, RP has its roots in topography and photosculpture technologies from the nineteenth century [Prinz 1997]. It is the automatic construction of physical objects directly from CAD data, normally achieved by depositing material in a layer-wise manner. Within the RP process, the part is first created as a 3-D computer model and then sliced into 2-D layers and consecutively fabricated from the first layer to the last, using control schemes to direct the shaping of each layer. Once one layer is created, another layer of material is added, and the entire process is repeated until the completion of the whole part [Otto and Wood 2001, p854]. RP is also referred to as solid freeform fabrication, desktop manufacturing or layer manufacturing technology [Zorriassatine et al 2003]. Currently, there are various commercial RP systems available in the market, such as stereolithography apparatus (SLA), fused deposition modelling (FDM), selective laser sintering (SLS) and 3D printing (3DP) [Ramanath and Chua 2006].
Rapid prototyping allows designers to produce a complex and high quality physical prototype to verify their design [Rouse 1991, Ramanath & Chua 2006]. The relative accuracy of prototypes made by RP (in comparison to hand-made models) can reduce risk in the product development process [Mueller 1999]. Compared to CNC machining, RP can produce physical prototypes with more complicated shapes such as convoluted shapes or parts that are nested within other parts [Efunda 2010]. RP is well known for shortening the product design and development process [Chua 1999] but there are still some pre-processing steps that need to be taken before a model can be built. Data transfer into an RP machine is normally by means of an STL (Stereolithography tessellation language) file. The original CAD model must be converted to the STL format for a specialized computer program
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within the RP machine to analyze and process into the slices used to build the RP model. Both the conversion to STL and the subsequent slicing procedure can lead to some deviation from the original CAD geometry.
CNC machining is a material removal process while RP is a material addition process. In addition, researchers have been investigating another prototyping strategy called “hybrid prototyping”, which use can produce a part through both material removal and addition within the same system. One technology developed based on hybrid prototyping theory is called Shape Deposition Manufacturing (SDM), which is a freeform fabrication process combining material deposition with material removal processes [Amon et al 1998]. Material addition is used to lay down bulk geometry quickly whilst the material removal is used to produce precise geometric features. Therefore, hybrid prototyping systems can provide better accuracy than normal rapid prototyping systems [Zorriassatine et al 2003] and can save prototyping time and cost [Thefreelibary.com 2010].
Compared to conventional hand making and manually controlled machining approaches, CNC machining, rapid prototyping and hybrid prototyping have made significant progress in combining virtual prototyping and physical prototyping technologies. In addition, with the use of computer control, the physical prototype produced with these technologies can faithfully reproduce the VP from which it was built, in terms of appearance, scale and dimension, which allows designers, engineers (and users) to quickly visualize and react to part designs [Stoll 1999]. In contrast, to achieve this level of reproduction by hand crafting would be much more difficult. Faithfulness of reproduction is very significant when testing prototypes with users. For example, if the physical prototype does not look like the virtual prototype, it will be difficult for the designer to present them together to the users, as in their mind, the
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physical and virtual prototype might present different products rather than the same one.
However, product design is an iterative process and the prototyping is no exception. The first physical prototype produced by CNC and RP is usually not the last one. With CNC machining, RP and hybrid prototyping technologies, the processes of virtual prototyping and physical prototyping still occur at different times. A virtual model must be completed before it can be used within the process of CNC or RP to generate a physical part. The physical prototype will then be used for testing with users and any design problems will be identified. Based on these problems, the virtual prototype needs to be modified and be made ready for the next prototyping stage. This iterative cycle can happen several times, with a time delay is incurred during each conversion. In another words, the virtual and physical prototyping processes are not synchronized. There are some problems within this cycle of conversion. Firstly, each time the cycle is repeated, it will add to the product development time, hence increasing costs and potentially delaying the time-to-market. Secondly, because the user cannot see the changes to the prototypes immediately, they will have to come back again and re-evaluate the newly updated prototypes, which will increase the difficulty of user-evaluation experiments. Thirdly, the designers need to modify the virtual prototype according to the changes identified through the evaluation of the physical prototype. Since it is difficult to capture these changes precisely and because the 3D modeling skills of designers are variable, this modification process will not be seamless. If the changes from the physical prototype are not well reflected in the virtual one, the new physical prototype produced from the modified virtual one will still not reflect the user‟s requirements.
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