Cutting-Edge Laser Cutting with Integral Mechanical TransmissionAug 26, 2014
Machine builder Proteck selected a Kollmorgen direct-drive system to make its new Sl 2530 seven-axis machine cut flat sheets as efficiently as possible.
Proteck, a Chennai, India-based supplier of machine tools, metal cutting and forming equipment, and other types of capital equipment for manufacturing, as well as its own CAD/CAM software platform, began working with Kollmorgen EU to develop servo motor and drives systems for its own use. When Proteck recently developed its own series of special-purpose laser cutting and CNC machine tools, it selected Kollmorgen as a manufacturing partner. “Our latest generation laser profiling and machine tools have developed from our more than two decades’ experience in supplying and supporting machine tool technologies from our partners, as well as from our increasing in-house design and manufacturing capability,” according to Proteck managing director K. Bala.
Proteck’s initial machine designs were 3-axis laser cutting machines with flying optics, able to process to a wide range of flat sheet material. These small footprint machines included cast-iron beds and single, lower-axis geometry that allowed open three-side access for flat sheets up to 2.5X1.5 meters.
As a development from this machine Proteck introduced a ‘Plug and Play’ model for job shops and fabricators that included all of the features of the previous machines, but used a much lighter and lower-cost welded frame construction.
To make the cutting process as simple to operate as possible, designers looked at the transmission mechanics. Precision-ground ballscrews were an expensive choice for a 2.5-meter travel range machine, and the single-axis/single-screw design with its overhung load requires sturdier, and costly, linear motion guide bearings for adequate support.
Furthermore, to maintain good dynamic performance, the ball screws need to be large in diameter, to allow high rotation speeds —which increases servomotor and servo drive sizes. With the need to produce even longer travels for larger flat sheet work, Proteck and Kollmorgen EU began to address some central questions:
- How to maintain the +/- 10 micron bi-directional repeatability performance and up to 110 m/min max linear speed, required of the machines?
- How to reduce the power requirements and provide increased efficiency?
- How to further drive down powertrain component costs?
The result was the introduction of an innovative rack-and-pinion drive system for the long-travel lower axis of the machine for a new 3-axis bridge laser machining center which includes two synchronized lower X-axes supporting a gantry Y-axis, and a vertical Z-axis.
The initial concept used linear motor technology for the long-travel dual synchronized X-axis, but this was cost-prohibitive for Proteck’s target market. The cost-effective rack-and-pinion design concept was thus developed with the awareness that backlash and repeatability performance would need to be achieved using a novel approach. The special preload mechanism was realized by mounting the pinion and its motor on an eccentric pilot circle and a gear tooth geometry that allowed full-face gear contact and in-situ adjustment to minimize lost motion and hysteresis. This performed more than adequately at the point of the actual rack-and-pinion face — but when combined with a standard rotary servomotor, which required a planetary gearbox to develop the torque required to drive the load — the gearbox backlash, though relatively small, proved prohibitive.
The solution was to design the pinion-to-rack transmission around the Kollmorgen KBM series direct drive motor.
The KBM’s separate rotor and stator packages are supplied together as a unit and mounted directly on the machine by Proteck within its own bearing system and protective cover, with built-in power and encoder connectors. The mounting arrangement directly couples the large-diameter pinion shaft to the rotor as a single assembly, restricting lost motion and potential unwanted harmonics that a flexible coupling may give rise to. The resulting design has been field proven for extremely low backlash and has a dynamic and throughput specification very close to linear servo motor performance. The rack-and-pinion system is also very resistant to debris and does not require the degree of protection that a linear motor system would necessitate for a laser machining arrangement.
The Proteck rack-and-pinion system, combined with the Kollmorgen direct-drive KBM motors has fewer parts, is more energy-efficient, and delivers greater speed and accuracy that reduces the backlash and lost motion to well below the desired specification. As an intrinsic part of the design, Kollmorgen S700 servo drives with their multi-feedback capability are used with both resolver feedback, mounted on the KBM motor for commutation, and magnetic linear scales for the position loop. The feedback protocol is SSI absolute, EnDAT or alternatively BiSS to suit a choice of CNC controllers with set-up and monitoring via EtherCAT which is integrated in all S700 drives. Many optional features of the advanced servo drive may be included in the machine specification.
These include STO (Safe Torque Off) for SIL 2 and 3 safety standards, and Ethernet connectivity for set-up and monitoring. EtherCAT is available too, and will be integrated in future machines for axis synchronization.
With a maximum sheet size of 3.1X1.6 meters, the new generation flat-sheet cutting machines also use Kollmorgen AKM series rotary servomotors with Micron™ series planetary gear heads for the Y-axis, as well as the Z-axis with a brake fitted. For complete compatibility, S700 drives are used throughout the machine. This machine will also benefit in the future from a cost-optimized CNC control solution being developed and supplied by Ackermann.
Developing on from the flat bed, three-axis bridge gantry machines Proteck’s new Sl 2530 laser-cutting machine is a huge, seven-axis ‘walk-in’ machining center with an advanced, flying optics head design that provides laser cutting of vertical and angled sides of formed workpieces. Large, three-dimensional fabricated sheet metal workpieces are fixed to a stationary central table and the multi-rotational laser head is guided to always maintain the optima; perpendicular angle for perfect cutting. KBM direct-drive motors with Proteck’s innovative rack-and-pinion mechanism are used for main horizontal axes of the machine, covering an area of 1.6 X 3.3 X 0.7 meters.
Furthermore the hollow shaft KBM motor is also used as the main drive for the multi-rotational flying optics system – allowing feed-through of the laser delivery and focusing system whilst maintaining the flexibility and precision to provide completely synchronized twin rotary axes featuring nX360 degrees and +/-135 degrees rotational strokes. The machine also uses a complement of AKM servo motors on other axes along with matching S700 drives, allowing Proteck and its customers to reduce spares and inventory costs.