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Maximum flexibility for metal cutting

Article-Maximum flexibility for metal cutting

Maximum flexibility for metal cutting

Stand-alone or dedicated? That's a question the world's major automakers want answered. Once, the global market for automotive powertrain manufacturing systems was the exclusive domain of dedicated metal cutting transfer lines. Now, intense cost pressures and unpredictable markets are placing more focus on the production flexibility of stand-alone systems.

"Instead of investing 30 or 40 million dollars in a dedicated transfer system based on traditional demand predictions," explains Philip Szuba, director of research and new product development at Lamb Technicon, "customers would rather invest in flexible systems. They would like the option of redeploying their asset base or capital to make some other product in a quick retool fashion."

By placing three linear axes under the machine spindle, engineers can configure Jaguar as a transfer machine module in a high-volume production system or as a stand-alone HMC.

Unlike dedicated transfer lines that would require extensive retooling, stand-alone machining centers readily accommodate different components. Instead of replacing fixtures and heads, the operator simply changes the NC program and loads new tools in the toolchanger. So why the indecision between stand-alone or dedicated systems? One reason, says Szuba, is the high price flexibility exacts.

"Traditionally, the cost of flexibility has been paid for in terms of reduced system availability, difficult system diagnostics, and increased maintenance requirements." This, he points out, is the result of attempts to adapt stand-alone machine designs to the very different demands of the serial production environment.

The optimum solution, Szuba volunteers, would be a family of cost-effective, single-spindle, horizontal machining centers (HMCs) configurable as both transfer machine modules and stand-alone machines. Such machines, in fact, are now available. Developed by Lamb Technicon for their customers in the automotive, diesel manufacturing, pump, and compressor industries, "Jaguar" HMCs are capable of high-force, high-speed machining, and they are optimized for a range of materials from aluminum to cast iron and even compacted graphite iron (CGI).

High speed. The Lamb Jaguar is available as a ram-style, three- or four-axis (with rotary B-axis table) HMC, or as a three-axis transfer machine module. Two models are offered, one with 650 x 650 x 500-mm X, Y, and Z axis travels, and the other with 1,030 x 850 x 560-mm travels. Both models offer full 1-G acceleration/deceleration and 45 m/min rapid traverse speed on all three axes. Pallet size for the smaller machine is 500 x 500 mm; for the larger model, the pallet measures 850 x 630 mm.

Locating three linear axes of motion under the spindle permits incorporation in a transfer line, where spindle movement in the Z direction accommodates fixtured parts. A high-voltage servo drive, supplied by Rexroth Indramat, coupled with a high-torque, low-inertia, rare-earth magnet dc servomotor, also from Indramat, drives the X-axis ball screw (from Rexroth Star) for high dynamic performance. Two Rexroth-supplied servo-driven ball screws drive the Y axis, eliminating the need for a pneumatic counterbalance cylinder.

"Recent advances in servo and ball screw technology," Szuba explains, "help us achieve low-mass drive systems for high acceleration." Such advancements, he says, include 700V drives for more power, low-friction recirculating ball screws, and contoured trackways vs box-way axis guides.

Jaguar's Indramat control system (others can be supplied per customer demand) comprises open-architecture, PC-based design using the Microsoft Windows NT V4.0 operating system. A SERCOS interface permits communication with virtually all of today's intelligent drives, providing the flexibility to handle the full range of customer drive preferences. Additional control system features are transformerless high-voltage bus drives; Interbus, Profibus, 24V dc I/O; and IEC-rated, UL-, CSA-, and CE-approved components.

Szuba notes that the combined strength of the Rexroth group of companies (see box) helped Lamb meet its cost/performance targets for the new Jaguar machines. "Because Rexroth is our prime supplier for all motion control components, we were able to negotiate prices that were better than any one of the divisions could have given us individually." Another important consideration, he states, was Rexroth's worldwide presence, and the company's ability to support all of Lamb's international customers.

High stiffness. Given the increasing trend toward replacement of cast-irondrivetrain components by aluminum within the automotive powertrain industry, Jaguar HMCs offer interchangeable spindle options: A baseline spindle designed for machining aluminum and lighter cuts in cast iron and steel; a 24,000 rpm spindle for high-speed machining of aluminum; and a high-torque spindle capable of high-force machining of tougher materials like cast iron and steel.

The high cutting forces required to machine tough materials like cast iron require, in turn, high structural rigidity. Attaining the necessary stiffness for five- to six-micron accuracy without sacrificing the low-mass, high acceleration needed for fast machining operations, says Szuba, was one of the hardest challenges in designing the Jaguar.

The overall stiffness goal was set with the ram fully extended, for a value considerably in excess of that normally found in ram-style HMCs. Mass and stiffness budgets were then assigned to discrete component parts and subassemblies. By using a combination of computer technologies including FEA, FMEA, and 3D CAD, Lamb was able to refine the various Jaguar elements to meet design targets.

Back-to-back coupled bearings on the axis guides, for example, reduce the bending moment of conventional guides. Three guides on the Y axis result in an exceptionally stiff, yet lightweight column, while four guides on the Z axis ensure almost zero deflection.

Stiffness without mass, concludes Szuba, raises some interesting possibilities. Diesel engine manufacturers, he points out, would like to machine parts from compacted graphite iron because the material's high strength compared to cast iron allows thinner walls and subsequent weight savings. Forces required to cut the tougher CGI, however, are very high, demanding the most rigid of stand-alone HMCs.

"Diesel engine manufacturers want to experiment with CGI, but don't want to retool a transfer line for the new material," Szuba says. "Jaguar's flexibility with regard to spindle type and parts configuration could prove a big advantage in getting CGI engines to market first."

Machine tool makers migrate to integrated solution suppliers

Hannover, Germany-Single sourcing is a new business initiative influencing manufacturing and business practices across automotive and machine tool sectors. Reducing the number of suppliers is a key part of the drive to cut development costs and time-to-market, while lowering inventory and keeping a tighter control of the supply chain. A parallel development giving some suppliers the edge is the ability to combine various components into an integrated solution before they are sold to the customer.

One such OEM supplier is the rebranded Rexroth group of companies (formerly Mannesmann Engineering), comprising the divisions Rexroth Hydraulics, Brueninghaus Hydromatik, Lohmann + Stolterfoht, Rexroth Indramat, Rexroth Star, Rexroth Mecman, and Rexroth Foundry. At this year's Hannover Fair, Dietmar Straub, the company's former CEO of Automation, now CEO of Mannesmann Dematic, announced Rexroth's new approaches of "drive and control" and "total connectivity". Rexroth had been practicing this approach since early 1999 with the then newly established Rexroth Automation Systems (RAS) giving customers integrated solutions.

RAS aims to achieve the integration and automation of standard Rexroth components as well as offering the flexibility of integrating third-party components. A typical integrated system from RAS is a toolchanger. This system makes it easier to develop and operate the new generation of HSC automatic universal machining centers. It increases the availability of tools-in-stock to the work process. The toolchanger's other main function is to remove the used tools from the machine's motor spindle as quickly as it puts them in.

Waldemar Fuchs, divisional manager at RAS, says, "We are a separate division for automatic systems, assembly, and pick-and-place systems. We try to build systems using Rexroth components but, although RAS is a profit center, we also use other company's components if an appropriate Rexroth component is not available. Since RAS' establishment in early 1999, we have been offering solutions for tool-making machines. In particular, we have been constructing tool-changing magazines for the Deckel Maho Gildemeister (DMG) GmbH Group.

For decades, DMG has been one of Indramat's main customers, and more recently also for Rexroth Star and Rexroth Mecman. At the beginning of 1999 we received orders to co-develop a new tool changers for them."

In the joint development with DMG for a CNC control system, RAS developed a fully automatic handling system which features a combination of motor-driven axes from Rexroth Star, an Indramat motor with a Profibus fieldbus interface, and vertical cylinders from Rexroth Mecman. The toolchanger is fitted onto the automatic universal machining centers beside the working area. It can hold 120 tools at a time, each measuring as long as 300 mm with a maximum weight of 15 kg.

Typical installation time of a tool is about 6 seconds. Because of the tool changer's modular construction it is possible to adjust the movements across the x, y, and z axes to adapt the changer to different configurations of machine. The Rexroth Automation Systems is able to modify this subsystem on every tool machine without any extra costs.

A major argument in favor of the integrated approach is that several of RAS's customers do not have the time or resources to source all accessories or to make every part of their value-added offering. Fuchs adds, "Establishing RAS has also enabled us to look for new customers. Customers now come and see that there's a new way for us to work together. New business since the development of RAS has included the likes of Gildemeister and EMAG so it has been important in the industrial sectors related to automated handling solutions, feeders, and pick-and-place systems." EMAG designs and manufactures inverted-spindle vertical lathes and associated cutting technologies.

Rexroth AG
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