Aluminum Line Gets Radical

Customizations to a standard CNC controller have helped special-purpose machinery builder MKM International (Bad Oeynhausen, Germany) to engineer a radical integrated single-stage processing line for aluminum profiles.

The automated end-to-end machining solution replaces an old multi-stage, semi-manual production process. Dual five-axis robot heads machine aluminum profiles up to 9 m long on all six sides and in a single pass. Combined with bar code identification of materials, the new line dramatically increases production speed for a leading window and door manufacturer, while also eliminating sources of human error that were possible on the previous process.

A single Axium CNC platform from NUM Corporation (Napierfield, IL) 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 customize the CNC controller to optimize 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 much 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 very tight movements, while the stiffness of the system ensures that precision and accuracy is very high.

Compactness was also behind a second major modification: the integration of a specialized third-party spindle motor with NUM's digital drive and designed-for-purpose high-speed bus. A single CNC kernel controls the dual 5-axis robot heads and machine.

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, they 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 aluminum 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 optimize machining performance.

"Their support 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. "Many CNC makers charge large sums for engineering modifications, or can be unwilling to make them at all for low volume applications. Their willingness to give us direct access to the CNC development team helps us to create special-purpose solutions more quickly and economically."

"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.

The servo drives and motors complement the CNC kernel with motion control capability that is highly optimized for machine tool applications. Accuracy and fidelity of motion are central to the control loop, and are achieved by a number of unique 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 – an almost unprecedented speed in this application sector. This is combined with motors with optimized 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. This control over the complete motion axis chain secures an unmatched performance with regards to speed and accuracy where it matters most: at the direct mechanical interface to the machine.

MKM International GmbH, Königstraße 128, D 32547 Bad Oeynhausen, Germany, +49 (0)57 31 – 17 51 51, www.mkm-international.de.

NUM Corporation, 603 East Diehl Road, Suite 115, Naperville, IL 60563, 630-505-7722, www.num.com.