European machine tool manufacturers are only a few years away from a leap forward in productivity in the high-precision, mass-customisation of complex components. The PopJim project has developed and tested novel ‘joint interface modules’ (JIMs) – modules that are integrated into turning and milling machines to reduce the occurrence of ‘chatter’ (vibration-induced defects on the surface of metal parts that are a major cause of production losses).

Chatter arises during the metal-removal process because the overall rigidity of the machining system is not optimised. When implemented in industrial environments, PoPJim’s results will offer a step up in productivity and competitiveness for EU machine-tool makers and a step forward towards the goal of ‘zero defects’.

Fixing a disconnect

PoPJim addresses a ‘disconnect’ between machine-tool manufacturers and their customers – the metal component manufacturers. “In 30 years of research we have not seen a single commercial application from machine-tool manufacturers for monitoring and controlling vibration in an industrial environment,” says project leader Professor Mihai Nicolescu of the Royal Institute of Technology in Sweden.

“Solving the issue of vibration and chatter has been left to the users, who retrofit sensors to detect problems and then adjust the cutting process – often making it less efficient and productive,” Nicolescu explains.

Users are forced to come up with multiple process trade-offs to meet the growing demands for more accuracy, precision and complexity, while product variants are increasing. “Current solutions are sub-optimal,” he says, “But PoPJim’s ‘plug and produce’ solutions for tuning-out vibrations allow fewer process parameter changes and raise performance and productivity.

Stiffer solutions

While an aircraft wing and other mechanical structures are designed with strength in mind and accordingly can safely move up and down by half a metre, a machine tool is designed not to deform more than a few micrometres. “Machine tools are built for rigidity and are therefore over dimensioned in terms of strength, however they have several fixed and movable joints, where connected mechanical elements can deform,” says Nicolescu.

Some 90% of the movement in machine tools arises in these joints, he adds: “They allow vibrations to propagate that can result in chatter and other problems, but they can also provide nodes for damping any vibrations – so they are an ideal place for ‘smart design’ solutions.”

The partners developed and tested JIMs that are fixed between the different parts of machine tools. The modules allow the operator to tune the vibration-damping capacity and rigidity of the whole machine, thus reducing vibrations and chatter. Several types were developed and tested.

‘Passive’ JIMs rely on inserting a material – such as a metal-polymer composite – into the joint, which improves damping capacity. ‘Active’ JIM prototypes included such things as tuneable piezoelectric, pneumatic and hydraulic elements, which allowed the rigidity and damping properties of the joint to be varied. Piezoelectricity is the electric charge that accumulates in certain solid materials when mechanical stress is applied.

In addition, the partners developed a highly-robust ‘multi-hop’ wireless network system for remote management of multiple JIMs in a harsh and noisy factory-floor environment.

Competitive edges

Testing JIMs on an industrial machine revealed excellent results. For fast material removal a JIM-equipped machine produced two to three times the cutting rate, thus doubling or tripling productivity. Significantly, the same machine then produced the final high-quality, chatter-free finish demanded by the end product. Normally this is achieved with two separate machines – again an indication of the potential savings from PoPJim.

“There is huge global competition among machine-tool manufacturers,” explains Nicolescu, “our PoPJim modules are perhaps two to five years away from a commercial prototype, but the project has shown the benefits of industrial-academic cooperation – showing European manufacturers how new, innovative concepts can help them gain a competitive edge.”

Plug & Produce Milling toolholder