The FxMC was primarily focused on the management of reconfigurable fixtures which can facilitate MC. The types of reconfigurable fixtures that exist were investigated. The fixture design research field
9 was reviewed to ascertain the state-of-the-art and to properly implement the most suitable reconfigurable fixture type in the research undertaken.
A fixture is a device used to physically locate, hold and support a workpiece during a manufacturing process; this may include machining, welding, assembly, inspection and testing. Jigs perform a similar task, while additionally guiding the cutting tool during machining operations [3]. Fixtures are an essential factor in the quality, productivity and cost of a manufacturing process. The design and fabrication of fixtures in a manufacturing system can make up 10 – 20 % of its total cost [4], while a large portion of rejected parts are said to be attributed to poor fixture design [22]. Fixture operations are generally outsourced to an off-site facility to improve the cost-effectiveness of the activity [5];
however, this method is not conducive to a mass customisation production system, due to the concurrent requirements of the fixtures and customised parts. Outsourcing fixture activities would affect the responsiveness of the manufacturing system due to the detachment of the respective activities; this is the issue that the on-demand FxMC aims to address.
Computer-Aided Fixture Design (CAFD) tools are used for the design of fixtures. CAFD is an amalgamation of Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) tools developed for fixture design purposes. Recent approaches to fixture design include Case-Based Reasoning (CBR), Genetic Algorithm (GA) and Neural Networks (NN) [23].
Dedicated fixtures are individually designed for use on a specific workpiece undergoing a particular manufacturing process [3]. Dedicated fixtures are used in mass production, where batch sizes are large enough to warrant the investment of time and capital for a specialised fixture. However, dedicated fixtures are not applicable to the different product types of mass customisation. Reconfigurable fixtures provide a solution to this problem; these include conformable and modular fixtures [24].
Conformable fixtures are designed to hold various types of irregularly shaped parts. These include pin-array fixture technology (Figure 2.1) and phase-change materials (Figure 2.2) [22].
Figure 2.1: Pin-array type fixture [25]
10 Figure 2.2: Phase-change material type fixture [26]
Modular fixtures are the most widely used reconfigurable fixture type [4]. Modular fixtures include grid hole, T-slot and dowel pin. Modular fixtures provide a limited number of combinations in comparison to other reconfigurable fixture types; and compromised stability, accuracy, and efficiency of the fixtures in comparison to dedicated fixtures. However, the performance in those respective categories is superior to the other fixture type (dedicated and conformable, respectively), such that a compromise in flexibility and operability is provided. An IMAO Corporation® grid hole modular fixture is shown in Figure 2.3.
Figure 2.3: IMAO Corporation® grid hole modular fixture [3]
The modular fixture approach has been implemented for both FMS and mass customisation.
Müller et al. [27] developed a system with robot manipulators in place of static modules for the handling of large aircraft parts, where lightweight modules for handling, joining and measuring were used together with an assembly platform; this represents an advancement of the modular fixture idea. A simpler modular fixture design is shown in Figure 2.4. Wallack and Canny [28] developed an adaptive fixture vice, with adaptability via translation along the X-axis. The fixture was capable of conforming to two-dimensional objects.
11 Figure 2.4: Adaptive fixture vice [28]
The fixture design employed in the research undertaken (Section 3.4) was analogous to those displayed in Figure 2.3 and Figure 2.4. A modular approach was chosen due to its cost-effectiveness, applicability to industry and suitability to the cellular manufacturing paradigm (as mentioned in Section 2.5). The modularisation technique was also pertinent to the mass customisation decoupling point (established in Section 2.2).
Bi et al. [12] noted that modular fixtures were identified as useful in mass customisation applications;
however, the scheduling of how to utilise the modular fixture components efficiently in production planning had yet to be investigated. This finding provided evidence of the research gap in the field of study, and the importance of developing the research for the modular fixture type in particular. Further evidence of the gaps in research are elaborated through the scheduling studies reviewed in Section 2.8.2.
The fixture design process was investigated to provide a background for the fixture design implemented in the research (Section 3.4). There are generally four phases in the fixture design process: setup planning, fixture planning, unit design, and verification. These phases are summarised in Figure 2.5.
The main requirements in fixture design are summarised in Table 2.1.
The Physical requirement of fixtures to accommodate the workpiece geometry is in agreement with the necessity of the research to manage reconfigurable fixtures that can adapt to the changeable geometries of customised parts. The contribution of the scheduling method for the fixture manufacturing cell (Chapter 6) is in agreement with the Affordability requirement. The fixture reconfiguration time (or assembly/disassembly time for the modular components of the fixture design described in Section 3.4) is minimised through the techniques used in Stage I (Section 6.7) and Stage II (Section 6.8) of the scheduling method. The idle time for the manufacturing process associated with the part for that fixture is minimised through Stage III (Section 6.9) of the scheduling method.
12 Figure 2.5: Steps to fixture design [29]
Table 2.1: Requirements of fixture design [30]
Requirements Examples
Tolerance Locating tolerances of fixture should be in agreement with design tolerances for parts.
Physical Fixtures should be able to accommodate the workpiece geometry and weight.
Machining areas of the part should be accessible by the associated process.
Affordability
Fixture cost should be kept to a minimum.
Fixture assembly/disassembly times should be kept to a minimum.
Fixture operation time should be kept to a minimum.
Constraint
Fixture should ensure minimum moment and force equilibrium of workpiece.
Fixture stiffness should be sufficient to avoid deformation of either fixture of workpiece.
Usability
Fixture weight kept to a minimum.
Fixture should not cause surface damage of workpiece at interface.
Fixture should provide tool guidance for machining of the workpiece.
Fixture should prevent erroneous workpiece setups.
Fixture should assist in guiding machined chips away from the current process.
Collision prevention
Fixture should avoid tool path/fixture collisions.
The fixture should avoid workpiece/fixture collisions apart from the required interface.
The fixture should avoid fixture-fixture collisions, apart from the required interface.
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