Manufacturing Processes

Within manufacturing, processes are classified according to six major groups [16]: 1. Primary Shaping Processes: The initial shaping of a product from an amorphous

material. These processes form the general product shape e.g. casting, forging, and rolling.

2. Secondary or Machining Processes: Subsequent processes after primary pro- cessing which improve the basic shape and ensure it meets some of the products specifications e.g. milling, turning, and drilling.

3. Metal Forming Processes: The deformation and displacement of the metal to the required final shape. Achieved using suitable forces, pressures, and/or stresses at temperatures above (hot working) or below (cold working) the metal’s recrystallisation temperature e.g. forging, rolling, extrusion, and drawing. 4. Joining Processes: The joining of parts together to form the product. Processes

can result in a temporary, semi-permanent, or permanent joint, and are widely used within fabrication and assembly e.g. welding, soldering, adhesive bonding, and mechanical fastening.

5. Surface Finishing Processes: Processes which impart the specified surface finish by negligible material removal / addition e.g. polishing, grinding, and painting. 6. Property Altering Processes: Processes which change the material’s micro- structure to alter its mechanical properties e.g. annealing, tempering, and shot peening.

2.2.4

Manufacturing Systems

In general, a manufacturing process can be performed using manual labour, fixed automation, flexible automation, or some combination of these. With reference to Figures 2.4 and 2.5, the most suitable system for a manufacturer is dependent on a number of factors including:

• Availability and cost of labour

• Available capital

• Product variability

• Product life cycle

• Production volume

2.2.4.1 Manual Labour

A manual manufacturing system is performed by human operators with or without the aid of mechanical tools. Manual labour is particularly suited to production lines with high variability due to our natural problem solving ability and high levels of dexterity. This combined with low initialisation costs means that manual labour is typically used to meet low-volume production requirements of SMEs. However manual labour has its drawbacks, such as high operation costs and a human’s inability to work consistently or continuously.

2.2.4.2 Fixed Automation

Fixed automation refers to specialised manufacturing systems that utilise special- purpose equipment to automate a sequence of operations. Since fixed automation is only efficient if the full manufacturing process is automated, individual operations are often performed by custom-engineered equipment to ensure constant and continuous production. Since the performable operations are restricted by the configuration of this equipment, fixed automation is relatively inflexible to changes to the production process. However, this system supports high production rates, which makes it particularly suited for manufacturing products with very high demands and volumes. Since a custom-engineered and dedicated manufacturing system requires significant initial investment, flexible automation only becomes an economically viable option

Figure 2.4: Example of cumulative cost (investment + labour + operating cost) for different types of production, showing the high initial investment required for automation and continual cost of manual labour. The time it takes to recover the cost of the investment is known as the payback period (PB) (generated with reference to [17]).

at these large production volumes thanks to economies of scale. Examples of fixed automation include the machining transfer lines commonly seen within the automotive sector and automatic assembly machines [18].

2.2.4.3 Programmable Automation

Programmable automation refers to productions which utilise equipment that are capable of changing the sequence of operations to accommodate product variations. As its name suggests, the operation sequence is controlled by a program which can be re-coded as necessary. Programmable automation has a relatively low production rate (compared to fixed automation), but is much more susceptible to variations in the production process. While the use of general-purpose equipment requires initial investment, programmable automation is typically used in low- and medium-volume batch production that require program and tool alterations between runs. This changeover means that downtime between batches is a feature of programmable automation. Examples of programmable automation includes computer numerical control (CNC) machines and industrial robots.

Figure 2.5: Example unit assembly cost for different types of production in relation to annual production volume (generated with reference to [19]). The highlighted zone shows the production volumes at which collaborative robots become a financially viable option.

2.2.4.4 Flexible Automation

Flexible automation is an extension of programmable automation. While flexible automation is a relatively new concept and its principles are still evolving, it is currently distinguishable from programmable automation in that it has the capability to switch between part programs and alter its physical setup with no loss in production time. Accordingly, flexible automation facilitates the continuous production of different product ranges or variations.

Similar to the other forms of automation, flexible automation requires high initial investment in order to procure the necessary equipment. However, its elimination of downtime allows flexible automation to achieve better production rates than programmable automation (but still not comparable to fixed automation). This makes flexible automation particularly suited to lower volume production, as highlighted in Figure 2.5. Examples of flexible automation are the flexible manufacturing systems (FMS) for performing machining operations and collaborative industrial robots.

2.2.5

Flexible Manufacturing

Due to global trends such as the increased pace of globalisation, growing competition, reduced product life cycles, and consumer demands for greater product customisation [1], [2], [9], [20] there is an increased interest in smart factories which will adopt more flexible manufacturing processes to improve their customisation capabilities and responsiveness to change.

As shown in Figure 2.6, manufacturing flexibility can be considered a part of agile manufacturing, which refers to the organisation of company’s processes and structures to enable a swift response to changing consumer needs and markets. Within this hierarchy, flexibility incorporates the production of pieces and components within workstations, cells, and factory segments. In this sense, flexibility within manufactur- ing has been defined as the, “operative ability of a manufacturing or assembly system to switch with minimal effort and delay within a pre-defined family of work pieces or sub-assemblies by re-programming, re-routing, or re-scheduling the same system” [21]. Similarly in other work, manufacturing flexibility has been described as the ability to adaptively respond to changing circumstances or environmental uncertainty with minimal impact on time, effort, or performance [22]–[25].

As summarised by Jonsson in [25], manufacturing flexibility can be further broken

Figure 2.6: The hierarchy of production levels showing the relationship between changeover ability, reconfigurability, flexibility, transformability, and agility (adapted from [26]).

down by phases (e.g. value chain), hierarchy (e.g. machine level), or objects (e.g. products). Regardless of the type, flexibility has three elements [24], [25], [27]:

• Range: The range of products that can be produced by the manufacturing system

• Response: The ease (time, cost, etc.) at which the manufacturing system can respond to each product within this range

• Uniformity: The ability of the system to handle changes without impacting performance (e.g. quality and profitability)

Within this thesis, flexible manufacturing is the term used for a manufacturing envi- ronment that requires all or some of the three elements identified above. Accordingly, to perform within this environment a manufacturing system must be capable of responding to changes at the macro level (e.g. in product volume and mix) and micro level (e.g. disturbances in product geometry or material properties) [25]. With reference to Figure 2.6, flexible manufacturing requires flexibility, reconfigurability and changeover ability to enable the production of sub-products at station, cell, or segment level.

2.2.6

Manufacturing Assembly

Assembly is a particularly prevalent and important part of flexible manufacturing as it accounts for up to 30% of a product’s manufacturing costs and 50% of its manufacturing time [28]. Of the different manufacturing systems presented in Section 2.2.4, manual labour is best suited for production lines with high variability and consequently is the most widely used assembly approach within this area. This is particularly true within SMEs, as small-scale, low-volume production currently makes manual assembly the most economically viable choice.

Within manual assembly, there are eight layouts commonly used which help to categorise the assembly process (see Figure 2.7). These layouts are dependent on the size of the parts being assembled and the tools required, but in general their floor space is minimised in order to optimise the assembly by reducing transport costs.

Figure 2.7: Assembly layouts defined by Boothroyd [29] (small to large scale): (a) Bench layout, (b) multi-station layout, (c) modular assembly layout, (d) custom assembly layout, (e) flexible assembly layout and (f) large multi-station layout. Other assembly layouts not shown are the very small assembly layout (e.g. clean room assembly) and assembly on site.

Accordingly, small-scale assemblies typically utilise the bench and multi-station layouts that do not require bending, turning, or walking during the process, while large-scale assemblies utilise the flexible and large multi-station layouts that allow parts to be stored, oriented, and fabricated with greater ease. Large-scale assemblies tend to require additional tools for transport and alignment, which further increases their required footprint. An assembly line is most commonly used within manual assembly. In this setup, the product moves along an automated line while operators perform their assembly tasks at designated workstations (i.e. multi-station layout). To optimise a manual assembly’s layout, the complete process can be decomposed into Primary Assembly Processes (PAP) and Secondary Assembly Processes (SAP) [30]. The former refers to all operations which directly contribute to the formation of the product i.e. part mating, energy costs and items of information, while the

Figure 2.8: Bench assembly layout showing an operator’s PAP andSAP areas based on the assembly position (AP). For assembly of relatively small parts, the PAP area has a radius of 35 cm (taken from [30]).

later refers to those operations which are required during an assembly but do not directly contribute to the final product’s realisation e.g. material handling, transfers, re-grasping, etc. From [30], a distance threshold exists which identifies movements as either PAP or SAP. Figure 2.8 shows this distance threshold within a typical bench assembly layout. This transition point occurs at a distance of 35 cm for small-scale assemblies and 45 cm for large-scale assemblies. PAP and SAP are typically measured using time, and can be used to define the efficiency of an assembly process:

Ea=

P AP

P AP +SAP x100 (%) (2.1)

In addition to optimising an assembly’s layout, the product being assembled and the operations being performed can be optimised to minimise assembly time and maximise profits. One valid approach to achieving this is to utilise established Design for Assembly (DFA) methods.