Pushing the boundaries into rapid tooling

A new method of generating mould tools very quickly direct from CAD data is currently being developed. London-based company Unimatic Engineers has built a custom milling machine for an innovative company that has developed a fundamentally new method of creating moulds for plastic and metal forming. The concept pushes the boundaries of rapid prototyping into a new area that is becoming known as 'rapid tooling'. Unimatic says that where previously manufacturers have had to invest months and vast sums of money into developing new moulds or tool sets, Surface Generation's new rapid tooling capability will reduce this by some 60 to 90%. Managing director at Surface Generation, Ben Halford commented: "The setting up overhead traditionally associated with processes such as compression moulding, metal pressing, vacuum forming, spark erosion and casting stands to be slashed to a fraction of current levels. The method is even expected to prove economically viable for modelling, pattern making and other pre-production rapid prototyping processes." Rather than spending time machining moulds from solid blocks, the new process is based on a matrix of closely packed square section pins, each of which can be raised or lowered on a screw thread so that collectively they form a first approximation of the required mould surface. The tops of the pins are then milled to remove the steps between pins and create the smooth continuous surface of the mould. "Working with Unimatic, we've written software that takes a standard CAD file and re-interprets the desired mould surface as a set of pin positions or heights. This controls our machine to spin each pin on its thread to the correct height," explained Halford. "Because the pins are square and have to abut their four neighbours to create a contiguous surface, we have to temporarily separate each pin from those adjoining it so that it has space to rotate. With each pin set at the desired height, the close-packed matrix is then reformed," he added. The machine then uses the CAD file to mill across the top of the surface of the pins to create the finished surface of the mould. Unimatic created the machine based on components supplied by machine tool maker ISEL. A special base bed had to be designed and to this were added ISEL servo actuators to provide the x-, y- and z-axes for the milling tool. The tool also needed a rotary axis for cutting operations. Terry Gibbins, the engineer at Unimatic who worked with Halford commented: "We then had to design another rotary axis for turning the pins to their set height and this needed x-, y- and z-axis capabilities of its own. All of these axes were driven by relatively lightweight stepper motors because the duty did not justify the use of full servos." More axes were needed for separating the pin matrix for rotational setting of the individual pin heights. "Initially, we struggled with how to do this," explained Gibbins, "but in the end we developed a commendably simple mechanism based on through shaft pneumatic actuators. "We're now in the process of developing a new larger machine for Surface Generation but this time we can base the design on a new large format ISEL machine that has just been launched. I think we'll be keeping the pneumatic mechanism for separating the pin matrix, although we are toying with the idea of using electrically-driven alternatives too." Unimatic says that a requirement for the new larger machine came about because of interest expressed from the UK aerospace industry. Also, several US companies have seen the concept and would like to adapt it for their own specific use. New development funding has been sourced and the project has the backing of PERA (Production Engineering Research Association) and Cambridge University.