MKM International has used customised CNC controllers for an automated end-to-end machining solution at a window and door manufacuturer's, replacing an old multi-stage, semi-manual production process. Dual five-axis robot heads machine aluminium profiles of up to 9 m long, on all six sides and in a single pass. Combined with bar code identification of materials, the new line increases production speed while also eliminating sources of human error that were possible on the previous process.
A single Axium CNC platform, from NUM, controls the entire machining centre, including the 10 motion axes on the robot heads plus various sensors and actuators.
The Axium platform was chosen for its processing power, but also because NUM was willing to work with MKM on the control system and customise the CNC controller to optimise performance.
One modification is the development of an interface to a harmonic drive motor, which combines a hollow shaft servomotor with backlash-free gearing. The harmonic drive is exploited on MKM's 5-axis robot heads, and provides the machine tool with a higher than usual range of performance, fidelity of motion, and compactness compared with alternative approaches such as belts or planetary gearboxes. The resulting compactness of the robot head allows it to make tight movements, while the stiffness of the system ensures precision and accuracy.
Compactness was also behind a second major modification: the integration of a specialised third-party spindle motor with NUM's digital drive and designed-for-purpose high-speed bus.
When MKM first started discussions with the customer to specify the new type of machine, the original concept was for a single robot machining head and a 'flip over' work holder, allowing the head to machine all the faces of the profiles in two steps. However, as MKM gained knowledge of the process, it saw that productivity and precision could be increased by providing two robot heads, so that all six faces could be machined in one pass. This would eliminate a complex materials handling element.
NUM is MKM's CNC technology partner and the two companies have cooperated together on numerous special-purpose machine projects. After discussing the new aluminium processing concept, the parties worked out a way of controlling both heads using a single Axium CNC controller for cost effectiveness, as well as agreeing to modify the motor interfaces to optimise machining performance.
"Support from NUM helps us compete with the largest machine tool builders, even though our products are often made in single or small volumes," says Michael Koehler, manager of MKM International.
"We focus on small to medium-sized OEMs, and a number of machine tool segments including special-purpose automation," adds Alexander Helmes of NUM. "Our success is built on our products, but also on a partnership ethos. We are happy to work alongside the machine tool company and take complete responsibility for the control system, as in this instance where we integrated third-party products to create the best solution."
Various NUM products are used on the machine including an Axium CNC kernel, HP servo drives and motors, and I/O modules. The spindle motors and harmonic drives are sourced from third parties. NUM is also providing the user interface panel. MKM programmed this to provide a simple application-specific control and monitoring interface for users.
NUM's servo drives and motors complement the CNC kernel with motion control capability that is optimised for machine tool applications. Accuracy and fidelity of motion are central to the control loop, and are achieved by a number of measures including a high-speed proprietary bus between the CNC kernel and drives, and fine interpolation of the movement profile by closing the positioning loop, at drive level, at 5 kHz. This is combined with motors with optimised torque ripple and high inertia characteristics to suit the requirements of the mechanics in this segment.
In general, automation is a question of how fast you can get from A to B. But, for CNC operation, path accuracy is even more important. NUM's control over the complete motion axis chain secures performance, with regards to speed and accuracy, at the direct mechanical interface to the machine.
