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Servo actuators control rapid tooling process

Edited by the Engineeringtalk editorial team Oct 24, 2005

Unimatic Engineers has built a bespoke milling machine for an innovative company developing a fundamentally new way of creating moulds for plastic and metal forming.

Unimatic Engineers has built a bespoke milling machine for an innovative company developing a fundamentally new way of creating moulds for plastic and metal forming.

The concept pushes the boundaries of rapid prototyping into a new area becoming known as "rapid tooling".

Where previously manufacturers in sectors as diverse as aerospace, automotive, packaging, mass transit and construction, have had to invest months and vast quantities of money into developing new moulds or tool sets, Surface Generation's new rapid tooling capability will reduce this outlay by 60-90%.

"The setting-up overhead traditionally associated with processes such as compression mounding, metal pressing, pattern making, vacuum forming, spark erosion and casting stands to be slashed to a fraction of current levels", says Ben Halford, MD of Surface Generation.

"The method is even expected to prove economically viable for modelling, pattern making and other pre-production and rapid prototyping processes".

Instead of laboriously machining moulds from solid blocks, the new procedure 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 have written software that takes a standard CAD file and reinterprets 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", explains Halford.

"In fact there is a bit more to it than that: 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 the space to rotate".

"With each pin set at the desired height the close-packed matrix is then reformed".

The machine then uses the CAD file to mill across the top 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 standard Isel servo actuators to provide the x-, y- and z-axes for the milling tool.

The tool, of course, also needed a rotary axis for cutting operations.

"We then had to design another rotary axis for turning the pins to their set height", explains Terry Gibbins, the Unimatic engineer who partnered with Halford, "and this needs x-, y- and z-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 required for separating the pin matrix for rotational setting of the individual pin heights.

"Initially we struggled with how to do this but in the end developed a commendably simple mechanism based on through shaft pneumatic actuators".

"We are now in the process of developing a new larger machine for Surface Generation and interestingly 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 electrically driven alternatives too".

The new larger machine has become necessary because of interest shown by the UK aerospace sector and because several companies in America have seen the concept and want to adapt it for their own specific uses.

New development funding has been sourced, as has the backing of both PERA (Production Engineering Research Association) and Cambridge University.