Machine design update

Machine design update

With IMTS ’96 approaching rapidly, builders are rethinking the components they put in their machine tools. Their goal is to be able to show machines that redefine what users mean by short cycle times. Cincinnati Milacron (Cincinnati), for example, has added Monorail guideways from Schneeberger AG, (Switzerland) to its large-capacity Magnum 800H and 1000H horizontal CNC machining centers. The guideways and their integral linear encoders let the HMCs move at 709 ipm (18 m/min) traverse and feedrates and machine complex geometries to micron accuracies, while providing strength (6148-lb Z-axis thrust) and rigidity comparable to box ways.

Each machine’s all-gear 50taper spindle offers high cutting torques of 0.16 hp/rpm-830 lb-ft maximum-for heavy roughing and 6500-rpm speeds for fine finishing, with 10,000 or 15,000 rpm optional. Depending on the model, the HMCs can handle parts up to 61″ (1549mm) diam and 6600 lb (3000 kg) on pallets up to 1000 x 1250 mm, and shuttle parts on and off the carrousel-type automatic workchanger at 1000 ipm (25.4 m/min).

Harry Meyer, Magnum project engineer, reports that the Monorail guideway differs from conventional linear units in these ways:

Roller bearings provide greater rigidity and damping, twice the loadbearing capacity of comparable ball bearings, and 10 x the theoretical operating life. The design eliminates most adjustments and preventive maintenance typical with box ways and roller elements.

An optional integrated magnetic encoder scale enhances accuracy and repeatability from +/- 0.0002″ and +/- 0.00008″ (from 0.005 and 0.002 mm) to 0.000016″ and +/- 0.00006″ (0.00041 and 0.0015 mm). The Monorail measurement system also resists 20 g vibration out to 2 kHz. This allows heavy interrupted cuts without risk of losing the signal.

George Jaffe, Schneeberger’s vice president, operations and engineering, adds that no alignment is necessary during installation because engineers align the Monorail’s magnetic grating with the bearing carriage during manufacturing. The system also is easy to clean, in part because the sensor reads magnetically, not optically, and can be wiped without causing damage. Operators can remove the scanning unit from the bearing carriage for cleaning and reinstall it in minutes without need for a calibration specialist. Circle 208.

To reduce cycle times in transfer lines and other applications, Rexroth Corp. has combined a BTL-3 serial-output magnetostrictive transducer from Balluff (Florence, KY) with its Indramat four-axis CLM numerical control, proportional valve, and PVNC valve interface card to produce a CNC hydraulic axis. Gunther Nunweiler, manager, Rexroth Industrial Hydraulics Machine Tool Group, reports that, although the hydraulic axis’s stated accuracy is +/- 0.001″ (0.025 mm), early drive applications have achieved accuracy near the system’s +/- 0.0005″ (0.013mm) resolution.

The CNC hydraulic drive is a turnkey, plug-in package functionally similar to an electric servodrive. The drive has a 300:1 standard speed range, 100 m/sec acceleration time base, and 2000-ipm (50.8 m/min) velocity. It works with ordinary hydraulic power units and has a 48″ (1219 mm) standard cylinder length, with standard bore sizes of 2, 2.5, and 3.25″ (51, 64, and 83 mm), and pressures to 3000 psi (20.7 MPa).

The CLM control offers 3000 lines of program and accepts both incremental and digital absolute encoder feedbacks, so users can multitask other drive types with the hydraulic cylinder to combine the best attributes of each. With 64 inputs and outputs available, it can perform PLC tasks, or communicate within a hierarchy of PLCs, CNCs, and factory computer systems.

The BTL-3 linear transducer is reportedly the first magnetostrictive transducer for which mapping and microprocessor compensation will have linearized output. It is highly resistant to shock (50 g one-time hit) and vibration (6 g at 10-150 Hz) that develop in reciprocating hydraulic mechanisms. Sealed to IP 67 (NEMA 6) requirements, it is immune to chips, dirt, oil, and wear. Signal format is asynchronous serial interface, a format supported by most major controllers.

The magnetostrictive transducer uses pulse timing to measure the position of a magnetic marker that slides along, but isn’t in contact with its shaft. Normally, the traveling magnet mounts to the hydraulic cylinder’s piston with the shaft extending down inside the rod. The transducer consists of a small-diameter ferromagnetic tube, called a waveguide, with a conductor wire inside. The waveguide is sealed inside a stainless steel tube, damped at one end, and coupled to the transducer’s electronics at the head end. Sending current through the conductor momentarily magnetizes it, inducing an answering pulse in the waveguide at the location of the magnetic marker. By measuring the time between the start of the current and the returning answer pulse, the system then can establish marker position.

A common hydraulic axis application is in automotive transfer lines. For example, when peck-feeding drilling heads, the axis increases the machine’s rapid rate and keeps the tool in the work longer. In a broaching application cutting a blind spline, the cylinder drives a tool with 10-ton force at constant velocity to a programmed position. Other applications include transfers and positioners, programmable grippers and fixtures, and metal feeding and forming systems. Circle 209.

Copyright Society of Manufacturing Engineers Jul 1996

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