Tooling up for workholding does not come cheap. The jigs and fixtures required to accurately position any product that a company decides to put into production often require the biggest capital outlay after the machine tool itself.
Figures supplied by trade body the Gauge and Toolmakers Association show that in 1998 UK industry spent more than £300m on jigs and fixtures alone. This amount does not take into account the wider costs, such as those of storage and access. Companies will often employ one person solely to manage these mobile assets, a role that does not directly add value.
For many companies the solution is to request flexible jigs and fixtures from their suppliers. As David Boothroyd, technical manager at jig and fixture specialist Craftsman Tools, says: `We are adding flexibility. Multi-axis machine tools mean we can now do more with one fixture, for example.’
While industry asks suppliers to resolve its jig and fixture dilemmas, academia is investigating longer-term solutions. Professor Nabil Gindy, in charge of the University of Nottingham’s Centre for Responsive Manufacturing, says the aim is to `get away from product-specific fixturing, and move towards reconfigurability’.
Reconfigurability has, until now, meant products involving interlocking blocks that fit together to form different geometries. Developments in this field have yielded a number of modular, adjustable and programmable workholding systems and some companies use a mixture of these. An example is System3R’s modular fixturing system, which has been compared to using Lego.
The main thrust of research into workholding, whether concerned with modular or programmable systems, has been to remove the dependence on bespoke fixturing, which becomes redundant when a particular part is not in production. Three technologies are coming to the fore which could soon make this a reality for some industry sectors: vacuum workholding, low melt alloys and conformable workholding.
Developing existing processes
Although not a new technology – in the past, vacuum chucks have been used to hold irregular shapes – vacuum workholding was limited in that if an operation required a part to be drilled right through, the vacuum holding the part would be destroyed. Rapid prototyping techniques could change that. These can quickly produce inverse geometries of parts to be manufactured, enabling workpieces to be held in place with… vacuum technology. This is because there will always be a close fit between a part and its exact inverse copy, giving the vacuum high integrity.
Costs can vary widely but, according to injection moulding and prototyping specialist JJ Engineering, a fist-sized component using this system could be made for about £150 – remarkably cheap by jig and fixture prices.
Mark Sharman, rapid prototyping manager at JJ, says that there is a need to develop more durable materials to withstand the vibrations generated during machining. The resins typically used for rapid prototyping are too brittle for the drilling, grinding, and milling environments of most machine shops. Laser-sintered steel powder offers offers one solution, but a serious disadvantage is that sintered steel parts must be simple shapes.
Another example of a tried-and-tested materials process technology that is hastening the arrival of non-specific fixturing is that using low melt alloys. Such alloys melt at temperatures of 70-100oC, allowing the alloy to be removed from the component with hot water at the end of the process.
This technique is already used in aerospace manufacture for machining turbine blades, for example. The blade is partially dipped into the low melt alloy, which is then allowed to solidify. The alloy allows the blade to be held firmly without risk of damage as the exposed section is machined.
Nottingham University’s Centre for Responsive Manufacturing, and also the Massachusetts Institute of Technology are separately conducting research into a new application of this basic technique called `free referencing’. In this method the part is encased in low melt alloy which is solidified into a cube.
For machining the part, the cube can be held in place with a simple fixture device, while one side of the cube acts as a datum for the machine to create, in effect, a generic workholding system.
A simplified version of this system, also undergoing trials at Nottingham, requires only that the part be dipped in the alloy to enable a generic work-holding system to be used.
On the down side, critics have pointed out the inherently toxic nature of the alloys used in these processes. Andrew De Vicq, technical director of the Advanced Manufacturing Technology Research Institute, says the alloy process can produce cyanides and other toxic by-products.
The future’s fluid
Sophisticated alloys may seem to offer an advanced workholding solution, even taking into account toxic disadvantages, but conformable workholding could offer even greater flexibility and reduced costs for future products.
This technique uses materials which can automatically modify themselves to the shape of the part, and hold it in the correct orientation while it is machined.
These materials, known as electro and magneto-rheological fluids, become solid when an electrical current or magnetic field is applied to them. This offers the possibility of a completely non-specific workholding technology with a wide range of applications.
The materials are only available from five companies in the world, and are still under development. Research in the UK is centered at the universities of Hull and Sheffield.
In Russia, electro-fluids, which solidify under high voltage and low current, have been used to hold glass lenses in place while delicate grinding operations are conducted.
In their solid state, these materials are extremely strong and allow many machining processes to be undertaken.
Bill Bullough, who is a researcher in Sheffield University’s engineering department, says the solids are so hard they would require a hammer and chisel to remove them.
Polymer-based electro-rheological fluids are best suited for holding non-magnetic workpieces. The workpiece becomes the anode or the cathode in the electrical circuit. When the voltage is switched on, the liquid becomes solid and the piece can then be machined.
Magneto-fluids are used for holding magnetic materials. They require low voltage and high current, combined with a permanent magnet to solidify the liquid, which consists of iron and oil rather than polymers.
Fluids that act as solids may seem exotic but, with modular and programmable fixturing, they offer an exciting avenue of research for any company seeking a universal, low-cost tooling setup to sharpen its competitive edge.
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