Customized Spindle Motor and CNC for Cutting Stone

April 28, 2011
Large dimension, five-axis machining center for fine-detail production and reproduction
OMAG’s Blade5 machining center for stone is based on an application-specific 5-axis CNC system from NUM, and driven by an powerful, liquid-cooled spindle motor also developed by NUM.
The new five-axis machine center involves a large, overhead gantry-style structure that supports multi-axis machine head. The gantry’s horizontal X- and Y-axes are based on linear guides that employ ball screw drives and guide blocks fitted with recirculating ball bearings. These combine high load-bearing capabilities with smooth, backlash-free positioning. The vertical Z axis, which carries the machine head, uses the same technology and can raise the head 3 meters above the base of the system, for cutting and shaping huge stone workpieces.

A customized spindle motor and application-specific CNC system made it possible for OMAG to design and build a novel five-axis machining center that combines the functions of a former with those of a lathe — all for cutting and processing designs in stone.

OMAG is a 30-year-old Italian company that specializes in machinery for processing marble, granite, prophyritic and sedimentary stone, and glass. The new OMAG’s Blade5 machine establishes a new level of flexibility for processing stonework by supporting capabilities like sawing, milling, routing, turning, sculpting and polishing. One critical attribute is its high-performance spindle motor with a very large peak torque capability of 400 Nm.

The powerful spindle motor accelerates all aspects of stone processing. The machine — with a large working area of 5×5 meters – also can execute a variety of shapes and profiles direct from CAD/CAM files, making it equally suited to creating one-off pieces or volume production. It features a laser scanning option, too, allowing digitization and subsequent replication of finely featured work such as stone artwork.

The spindle motor, and the CNC system for the machine's five positioning axes – which are fully interpolated – were supplied by NUM, which has had a technology partnership with OMAG for more than 20 years.

NUM develops a series of high-end CNC applications, including hardware and software, together with drives and motors, and other core components that impact overall system performance. NUM also integrate third-party products in application-specific solutions.

“A willingness to partner with a constructor of special machines and share the risk of creating specific application and CNC solutions is at the heart of NUM's business philosophy,” explained Marco Battistotti of NUM Italy. “In OMAG’s case, there is a strong cooperation in the technological development for new machines."

OMAG's Blade5 machine is based on a large, overhead gantry-style structure, supporting an innovative multi-axis machine head. The gantry’s main horizontal X- and Y-axes are based on linear guides that employ ball screw drives and guide blocks fitted with recirculating ball bearings. These combine high load-bearing capabilities with smooth, backlash-free positioning. In the largest configuration of Blade5, both axes have a stroke length of up to 5 meters, giving machine operators a very large 25 square meters of working area.

The vertical Z axis, which carries the machine head, employs the same type of technology and can raise the head 3 meters above the base of the system, so huge stone workpieces can be accommodated. The stone being processed may be held in a fixed position on a large 3.5×2 meter worktop, or rotated by a lathe attachment for turning purposes.

The machine head spindle will accommodate a range of cutting and forming tools, including a diamond cutting blade up to 1.6 meters in diameter. The head contains two positioning axes, which enable the spindle to be inclined by up to 140 degrees to the vertical and rotated laterally by as much as 380°, so the process has exceptional flexibility.

The gantry’s X, Y, and Z axes, as well as the dynamic inclination and lateral rotation of the machine head, are controlled by NUMDrive C servo drives and NUM BPX brushless motors; all five axes are capable of fully interpolated motion, under overall control of a NUM Axium Power CNC system.

This all-digital control approach helps to ensure highly accurate and repeatable processing of the workpiece; the cutting coordinates are downloaded as ISO files from the machine’s host PC to the Axium Power CNC system. An optional laser scanning system makes it possible to retrieve three-dimensional measurements from a finished piece of work with “extreme” accuracy – a statue, for example – and convert that data into an appropriate ISO file automatically, allowing precise replication of art work.

Also, to achieve the highest possible standard of finishing, OMAG developed a special macro for the program to evaluate dynamic motion control commands from the Axium Power CNC system in real time, to correct for tool wear and ensure optimal contact with the workpiece at all times.

The machine head spindle is driven by an extremely powerful liquid-cooled synchronous motor, specifically developed for this application by NUM. It supplies the motor’s rotor and stator as separate components, and OMAG then integrates them in the machine head, including bonding the rotor directly to the spindle to maximize rigidity. The motor, which is driven by a NUMDrive C unit and an MDLL power supply, has a continuous power rating of 25 kW at 2,000 rpm, a maximum speed of 8,000 rpm, and a peak torque capability of 400 Nm.

OMAG’s PC-based CAD/CAM software includes a library of machine tools that can be freely requested by the operator; the physical tools are maintained in a rack to the side of the work area. Typically, these will include milling and turning tools, as well as one or more cutting blades.

Tool changeover is handled completely automatically; when requested by the operator, the Blade5 machine automatically parks one tool and then places another on the spindle – which uses the industry-standard ISO 50 taper. The machine’s intuitive human-machine interface (HMI) is comprised of a large touch-sensitive monitor, keyboard and dual joysticks for manually controlling movement when required.

“OMAG’s Blade5 exemplifies the benefits of close collaboration between machine designers and numerical control specialists,” according Marco Battistotti. “The various configurations of Blade5 are proving extremely popular in the market.”