3.2.1. RMS features
RMS features are highlighted. Table 2 gives the system features of RMS in comparison with a Dedicated Manufacturing Line (flow line) and FMS (Koren Y. , 1999) (Koren Y. , 2006).
Features DMS RMS FMS
System Structure Fixed Adjustable Adjustable
Machine Structure Fixed Adjustable Fixed
System Focus Part Part Family Machine
Scalability No Yes Yes
Flexibility No Customisable General
Simultaneously Operating Tool
Yes Yes No
Productivity High High Low
Lifetime cost Low
(for a single part, when fully utilised)
Medium (For producing medium to high volume new parts)
Reasonable (for producing variety
of parts at low volume) -otherwise high Table 2: Comparison of System Features (Koren Y. , 2006)
- In terms of system and the machine structure, the RMS is adjustable. The modular architecture allows the machines and the modules to (re)arrange itself in any other particular configuration. For DML, the system and machine structure is fixed, whereas the FMS’s system structure is adjustable where the machines can be relocated without changing its module configuration.
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- In terms of System focus, RMS is designed for a particular product part family, well as DML is only designed for one product and FMS for wide variety of products under the machine envelope.
- In terms of Flexibility, RMS has customised flexibility, built around a particular product part family. DML has very low flexibility, as they are designed for only one product for a product life time and FMS have general predefined flexibility required to produce high product variety. - In terms of scalability, RMS can be scaled up/down by adding/removing a particular machine or a module as required. Whereas in FMS, an additional machine unit can be added in the particular configuration. DML cannot be scaled as its technological units are fixed in a standard process.
- In terms of simultaneously operating tools, RMS and DML both allow simultaneously operating tools whereas FMS allows only one tool at a time.
- In terms of productivity, for RMS productivity may be high, depending on the type of the product and demand topology. DML has a very high throughput whereas in FMS, their high flexibility and wide variety of products affects the productivity, considerably making it low. - In terms of lifetime cost, RMS cost is medium depending on the individual costs of the base
machines and reusable modules in the system. The DML’s costs are average depending on the number of workstation and tools on each line and FMS cost is very high as it includes purchasing cost of CNC machines and other automatic handling systems.
Although this table provide an extensive distinguish of the RMS features, some features may be not actually valid. Thus, following table presents the updated information.
Features Dedicated RMS FMS
System Structure Fixed Adjustable Adjustable
Machine Structure Fixed Adjustable Fixed
System Focus Product Product Family Machine
Scalability No Yes Yes
Flexibility No Customisable General
Parallel Tools Yes Possible Possible
Productivity Very High High Low
Cost per part Average Medium Very High
Table 3 Updated System Features
It can be argued that in terms of Parallel Tools, the RMS and FMS may both possibly have simultaneous operating tools. The RMS uses Reconfigurable Machine Tools (RMTs), which adopts the DMT approach and design a machine around a part family or a set of parts (rather than a specific part, so conversion by rapid reconfiguration of the machine is possible) and use CNC technology to drive the machine (Moon, 2006). Thus, RMT has a customised flexibility that makes it less expensive than general purpose CNCs.
RMS embraces the best qualities of DML and RMS systems, i.e. it focusses on product part family with the required throughput and functionality. This focus can allow a designer to plan a system that accommodates different variation of the same part family and can possibly cope with situations where both the productivity and the ability of the system to react to changes are of vital importance. Utilising this approach may possibly give high throughput rate as of DML with required functionality and can prove to be more economical than the general functionality of FMS.
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3.2.2. System Capacity VS Functionality
The system capacity vs functionality of RMS comparatively to DML and FMS is presented. A DML operates at its maximum planned capacity, providing high throughput rate but very low functionality rate (ElMaraghy, 2006). DML “mass produce” the company’s core products or parts at very high volume for an extensive production run. Due to high production rate at very low functionality rate, the throughput of this system is very high, substantially reducing the cost per unit of the product. Whereas, FMS produces variety of products, “mass customising” at a reasonable low capacity. FMS units come with pre-defined flexibility and high functionality, but remains typically underutilised in most scenarios (Mehbrabi, 2002). This reduces their throughput in comparison of DML and makes the system cost high, relatively increasing the cost per part.
Figure 3: System capacity versus Functionality (ElMaraghy, 2006)
RMS are not constrained by the capacity and the functionality. RMS is designed to be scalable and modular. For e.g. in the Figure 3, the RMS can be customised to accommodate different product part families at required functionality and capacity in a production time horizon. Due to this customised nature of the RMS, the residual value of their modular components is expected to increase if they can be reutilised. This “Plug and Produce” practise may potentially reduce the manufacturing costs through diminished expenses for planning as well as can shorten the times for changes, leading in reducing the costs of retrofitting and conversion.
3.2.3. System cost vs capacity
In the manufacturing system cost versus capacity plan, DML cost remains constant at its maximum planned capacity (Koren Y. , 2006). Figure 4 shows the system cost vs capacity. In case of DML, to get additional capacity or to produce an another product, an extra line must be built. This extra added line doubles the capacity and tremendously increases the investment costs. Therefore, in terms of low demands, this extra line built can be idle, hence it is always questionable adding an extra line. FMS is scalable at a constant capacity rate through adding more machines in parallel, which also proportionally increases the cost in parallel for greater capacity. RMS is scalable at a non-constant capacity rate depending on its initial design, the manufacturing equipments and the changing market requirements.
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Figure 4: System Cost versus Capacity (Koren Y. , 2006)
3.2.4. Summary
Dedicated Lines are designed to operate at high production volumes, producing at least 75% of its maximum planned capacity. These systems produce only one product at high throughput rate. FMS can produce wide product variety when there are average demand sizes. If large demand sizes are requested where the products types are limited, then RMS may be a cost-effective solution.