TwinDrive rack-and-pinion system selected for gantry mills

Nicolas Correa's milling and boring machines offer a combination of high performance, versatility and reliability at a competitive price. Nicolas Correa considers itself a world leader, with more than 600 of its bridge-type milling machines operating worldwide.

The gantry architecture is said to represent the best balance between the dynamic behaviour of the machine and the level of precision it can achieve. In addition to the well-known robustness of its machine, Nicolas Correa has exceptional experience and technical know-how in five-axis milling. Now the company has updated its range of bridge-type milling machines with a new high-performance family called Versa.

These models are equipped with a full range of interchangeable heads, among which is the mechanical universal head with a resolution of 0.02deg and a rated speed of 6000 rpm (UAD). This patented head is claimed to offer better repeatability and locking force than conventional systems.

Worthy of note is the Correa | Versa travelling table model, which has a work envelope of up to 14,500 x 5,000 x 1,500mm. The Correa | Versa M travelling bridge version and the Correa | Versa MW variant with adjustable crossbeam offer even greater versatility for the machining of large structural elements and tools while retaining the rigidity and precision specific to the gantry architecture.

This new family of machines incorporates the most recent advances on the market in terms of the guide systems and axis drives, as well as the structural elements that offer greater dynamics, precision and damping in machining work. Using the most appropriate drive system is therefore a keypoint and Nicolas Correa has found in Redex Andantex a specialist supplier of rack-and-pinion drive systems for machine tools. The X and Y axes are therefore fitted as standard with this manufacturer's drive systems.

On the X axis, these drives are capable of moving a mass of 50T with an acceleration of 1.8m/s2, although they are capable of moving a mass of 70T depending of the customer's needs. Speeds in the X and Y axes are 30m/min.

Eliminating backlash

To eliminate backlash in the two axes, Nicolas Correa has opted for the TwinDrive system. It consists of a master-slave electronic configuration (electrical preload), with two motors. The rigidity of the reducers, together with the 'true integrated pinion concept' (which is the subject of a Redex Andantex patent) provides the Versa machine with KV values as follows:

• X-axis: KV=1 with 50,000kg of moving mass
• Y-axis: KV=2 with 9200kg of moving mass

As a consequence, the machine is able to reach the highest dynamic levels and optimal control response, resulting in greater positioning precision. A closed-loop system is implemented using a Heidenhain linear encoder achieving the following repeatability and precision:

• X-axis: repeatability = 5 microns; precision = 7 microns
• Y-axis: repeatability= 4 microns; precision = 6 microns

The Versa X axis moves with an acceleration of 1.8m/s2, resulting in a 50T travelling table with a repeatability of 5 microns.

Racks are manufactured by Swiss company Gdel, a Redex Andantex business partner in a number of countries. High-precision racks are an excellent technological partner for the servo-reducers, providing what is claimed to be the lowest pitch error on the market in tempered and rectified commercial racks.

Javier Hernando, I+D Director of Nicolas Correa, comments: "With this new range of milling machines, we have incorporated Redex Andantex rack-and-pinion drives in the Y axis for the first time and proven that interpolation with the X axis has improved compared with the previous ball screw solution."

Nicolas Correa also fits Redex Andantex rack-and-pinion drive systems in its Anayak high-speed floor-type HVM-HS moving column milling machines. This range is designed for machining medium and large-sized parts.

Hernando adds: "Working with Redex Andantex, we have found the perfect partner that meets our expectations, not only in terms of product quality and reliability, but also in terms of their before- and after-sales service, which provides us with significant added value."

New TwinDrive KRPX

Amongst other machine-tool OEMs, the answer to Nicolas Correa's needs in terms of racks-and-pinions drive systems led Redex Andantex to increase its TwinDrive range of products with the new KRPX. This new version will be officially introduced at EMO 2009.

A TwinDRIVE KRPX consists of two planetary reducers, designed to be mounted on both sides of a standard machine frame. Fixation is made by oversized screws to ensure a rigid connection with the machine frame. Used as an extension to the main frame of the machine, the KRPX housing is made of high-quality spheroidal graphite cast iron. Its structure has been optimised with FEM analysis methods to avoid deflection during acceleration. This smart system has been designed to eliminate any risk of mounting system deflection.

The new KRPX version was original created to simplify the servo-reducers' assembly design for fixed-bed and travelling column machines, although, as in the X axes of the moving table Correa | Versa model, other configurations can take advantage of this new concept. When backlash is eliminated on the relevant axis by means of a master-slave system with electrical preload, the placement of a reducer backed up against each side of the column often involves eliminates intermediate assembly elements that can decrease the global stiffness system and complicate both the design and the assembly of the servo-reducers.

Anayak has incorporated this approach into the HVM travelling column milling machines, leading to a saving of 2T in weight in the basic design of the column.

Correa has incorporated the technology in the X axis of the new Versa range and, because there is no need to design and manufacture a mounting system, the implementation is much simpler and more cost-effective (cost savings are up to 15 per cent).

Like all rack-and-pinions servo-reducers from the Redex Andantex range, the KRPX version includes the right-angle variant and all possibilities for motor flanges as standard options.