Holding the press
Tom Shelley reports on a sheet metal forming technique that does away with the need for press tools
By manipulating sheet metal with robotic arms, complex sheet metal forms can be created, with curving folds that avoid the need for press tools.
While applications are mainly seen in low production run automotive body panels at present, the technique could be applied to anything made out of sheet metal. Initial experiments required making lines of weakness along which the folds could be formed, but it has since been found that these are not necessary.
‘Curved folding’ is the brainchild of Gregory Epps, who was showing a striking, folded
model car body at this year’s Royal College of Art summer show. According to Epps, in order to make the model, he took 0.8mm aluminium sheet and then CNC machined a 0.4mm curved groove with a ball-nosed cutter and then made the folds by hand.
“I have been investigating this for 10 years now,” he says. “The method starts with a large piece of stiff felt that I push into shape. I then draw along the curved lines with chalk, and flatten it out and trace the lines on to tracing paper. The lines are then scanned in and drawn as Bezier curves, and used to generate CNC machining instructions.”
The process he uses is not traditional origami, which mostly depends on producing complex constructions using straight folds, but depends on curved folds, which are different.
“Curved folding has certain rules which are dictated by the use of non-stretching sheet material and the homogeneous relationship with other folds,” says Epps. “You will not see a convex surface next to another. Like cupped hands, a convex surface has to sit adjacent to a concave surface.”
In the past, Epps has collaborated with Unto This Last, a small furniture workshop in Brick Lane, London. “I have been working with them for five years, designing and developing in-house products, and more recently spent a year in the manufacturing department.”
Designs undertaken there included a flower lamp assembled from a single piece of polypropylene aeroply, a wine rack and a ‘Ruk Sak’, routed from a single piece of 1.2mm PP.
He has also worked with Peter Testa Architects, based in Los Angeles, fulfilling a commission to design an emergency classroom for on-demand teaching spaces in California. Flat panels are machined from flexible foam and PP panels, which are folded and joined on site. Classrooms can be dismantled and stored when demand is low.
His professed goal now is to take the method and turn it into an industrial process – and discussions with contacts in the small batch size automotive sector would suggest that the most important thing here is to be able to demonstrate repeatability. To do this, Epps has been developing technology that will put the process into the grippers of industrial robots. As an experiment, he glued handles to a piece of aluminium sheet and then found that applying suitable forces to the handles led to the sheet folding along a curve that was not coincident with a pre-machined line of weakness. Making metal sheet deform in a predictable way requires computer modelling. The process soon becomes much too difficult to predict using hand methods and it is for this reason that it has probably not been used commercially before. In a production environment, he envisages robots gripping aluminium sheet using suction cups, or steel sheets using electromagnets.
Epps uses Rhino 3D for his designs, because he finds it very quick to use, and SolidWorks for manufacturing simulations. Once computer modelling is used to predict the behaviour of the deformed sheet so that it can be guaranteed to deform as required, it becomes a rapid manufacturing system, he points out, and one that could be used to produce one-offs, or many-offs, without the need to fabricate press tools.
The SolidWorks deformation modelling is driven by equations, which were obtained by measuring folded pieces of paper to deduce the appropriate ratios. Epps is now working with software academics in Vienna and Canada to perfect this.
Apart from doing away with the need for press tools – which typically represent 70% of car body panel costs – the methodology produces parts that cannot be produced by pressing. At the same time, there are parts that can be produced by pressing that cannot be made by curved folding. The technique cannot be used to make parts with complex curvatures that involve stretching the metal, for example.
“With pressing, you are limited to a minimum draft angle, to be able to get the tool and panel apart,” Epps explains. “But with curved folding, you can make an enclosed volume. One of the interesting things about this method is that it is possible to use it to make parts that crumple in a predictable way, so you could make safer cars.”
Designs made using curved folding do look different. We have all become used to designs with smooth, doubly curved surfaces, but designs of furniture and other artefacts from earlier eras all had corners. What impact do leading edge effects have on aerodynamics? While smooth surfaces tend to be favoured, there are some saloon cars, for example, that are sent to a specialist motorsport engineering company to have corners added to their front ends in order to split air flows.
Epps’ method has already been used to make products that have been sold for money, a matter on which he has this to say: “Originally, I was building a sculpture for a fine art degree and, at the same time, making a mountain bike. Ten years later, I am only just now getting to the reality of this. I would predict another five years will be required to develop the method further, followed by another five years of testing, but sometimes if there is a need, advances can come very quickly.”
Epps has prepared a book about his ideas, ‘Fold Forms’, which will be available this autumn from Tarquin Publications, St Albans, priced £4.95. His core technologies, which have hitherto not been publicly revealed, are protected by patent application.
* Method allows fabrication of complex three-dimensional sheet metal shapes, without the need for press tools
* While it cannot be used to make doubly curved surfaces that require stretching metal, it can make enclosed volumes and fabrications without draft angles
* It has so far been applied commercially to fabrications made from plastic sheet, but computer software is being developed to allow its use in metal fabrication on an industrial basis
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