High-Output, Large-Volume Machining with Complete Flexibility

Aug. 18, 2011
Mikron Multistar NX-24 CNC: a fully programmable rotary machining system
The blank is fed into the first workstation and clamped in the collet. The completely machined workpiece is unloaded from the machine at the last station, so 22 workstations are available.
Using the CNC-programmable Multistar NX-24 rotary machining system is effective when different types of workpieces must be produced in quantities ranging from hundreds to many thousands, with accuracies to the hundredths of millimeters.
The Multistar NX-24 is well suited for producing component parts for automotive systems, medical devices, climate control systems, and timepieces, i.e., where large volumes of different workpieces must be produced to the highest standards of accuracy.

The developers of the new Multistar NX-24 rotary machining system emphasize that users will benefit from its productivity, flexibility and high-precision processing. “With its number of cycling steps, it almost achieves the productivity of fully mechanical machines,” according to Mikron Group. Mikron designs and develops machining systems and cutting tools, as well as automation and testing technologies.

With the Multistar NX-24, all machine axes are programmable, which saves the operator a significant amount of downtime when changing over the from one production series to another.

Mikron SA Agno, the center of the group’s machining business unit, reports that its rotary machine systems are designed to achieve up to 150 cycle steps per minute, meaning they can produce up to 520 parts per minute when equipped with four cycles per machine. It is designed for high-output, large-volume production, but because it is fully mechanically controlled, changeover can take time.

The new CNC-controlled Multistar NX-24 increases flexibility significantly with only a slight reduction in output, so it is effective when different types of workpieces must be machined in quantities from a few hundred up to many thousands, and with tolerances in the range of hundredths of a millimeter. Automotive machining is one likely sector, but others include medical equipment, climate-control systems, and timepieces.

Simultaneous machining at all workstations
All axes are programmable and the control unit's clearly presented HMI simplifies rapid programming and quick product changes. However, the machine still has a range of additional features.

For example, the Multistar NX-24 is equipped with 24 workstations and up to 44 machining units and up to 60 axes. All axes work simultaneously, which is how the machine achieves its extraordinarily high output. Workpieces are machined completely, including processes like deep-hole drilling, workpiece repositioning/reclamping, in-process measuring and compensation, as well as complete deburring. This is how the machine produces, for example, 8.5 for automotive injection nozzles per minute.

Because each axis is CNC-programmable, changeover is done mainly by software programming. Compared with machines that rely on mechanical control, this results in significant timesavings. With different types of workpieces, with maximum diameters of 35 mm and maximum lengths of up to 65 mm, the Multistar NX-24 achieves very attractive per-part production costs.

Mikron is known for high precision machining operations, as for example the special watch screws with M 0.8 threads and corresponding surface finishes that the NX-24 can produce without difficulty.

Machining happens simultaneously on the 24 workstations, each one capable of machining two sides simultaneously — from above and below, from above and from one side or from below and from one side.

Once around the cycle, workpiece finished
On one hand, the machine's compact design means that temperature has less an influence in the machining area. On the other hand, the machine's sturdy construction guards against the effect of vibrations. Turning the rotary table through 15° to the next workstation occurs in 0.6 seconds. Then, indexing pins lock the table with an accuracy of ±2.5 µm for the next machining operation. The table can be equipped with different clamping collets.

The workpiece blank is fed to the first workstation and clamped in the collet; the completely machined workpiece is ejected after the last workstation, meaning that there are 22 workstations available for the machining operation.

The machine can be equipped with different clamping collets, which can be driven from above or below the table. During machining in the individual stations, the collet is locked by the C-axis spindle or with a mechanical index pin.

Collet drives for turning work at up to 7,500 rpm rotate with a precision of ±0.005 millimeters. And all this during millions of indexing cycles. Turning is possible on workpieces with diameters of up to 15 mm. However, collets that are fix during machining can accept workpieces with diameters of up to 35 mm. Collets that are CNC-programmable, which can rotate and interpolate through 360°, can hold workpieces up to 30 mm in diameter. The CNC-programmed axes index through 180° in 0.1 second, for four-axis milling.

Depending on requirements, individual stations can be equipped with different machining units. For example, the CNC-controlled, triple-axis machining unit can produce a number of configured drillings, as well as perform milling and deburring operations. Together with the C-axis, around which the collet chucks are rotated, four-axis simultaneous machining is possible from above as well as from a horizontal direction and also inclined.

Fitting a CNC-controlled twin axis machining unit provides, together with the C-axis, triple axis simultaneous machining. For example, milling operations and drilling, from above or from a horizontal direction. Together with the C-axis, it can also perform outside and internal turning operations.

For small drills and mills, and for deburring the workpiece, high-speed spindles running at up to 32,000 rpm are available.

The machine can be fitted with a CNC deep-hole drilling unit with moving guide bushing, too. The deep-hole drilling unit works from a horizontal direction or inclined from above, either with or without a guide bushing. The high-pressure cooling unit can be programmed so that the coolant is fed with increasing pressure as the depth of the drilling increases. The maximum pressure is 120 bar, which ensures that all chips are flushed out of the hole.

For high-precision turning, including eccentric recesses in bores, Mikron indicates the recess unit is a good solution. But, for many machining operations the single-axis machining unit is sufficient. It offers an excellent price/performance ratio and can be numerical controlled in horizontal, vertical or inclined position.

Complete machining, including quality control
The control unit's interface has self-explanatory graphics that provide a reliable overview of the individual machining stations. At the same time, it simplifies the task of programming individual processes using the graphics. A complete range of machining operations can be programmed and executed with up to four-axis interpolation, including milling, turning, reaming, drilling, deep-hole drilling, tapping, outer-thread turning, profile recessing, precision deburring, and even workpiece washing, and minor assembly tasks.

The possibility of carrying out tasks like 3D deburring directly on the machine means significant cost savings. Rework, as is often necessary when complex workpieces are deburred electro-chemically, always carries a significant cost.

However, complete machining also includes functions like measuring and analyzing the results. By means of continuous measurements, the machine control unit determines in advance when the machining results are approaching tolerance limits (for example, through tool wear or temperature changes.) Based on these results, the control unit detects deviations promptly and automatically for these. Defective components are detected automatically during machining and separated at the unloading station.

The software produces statistical data continuously from the measurements it collects, analyzes it, and informs the machine operator of any deviations from normal operating conditions. This in-process, comprehensive control is important, for example, when producing automotive components for customers with very low tolerance for rejects. For example, the Multistar NX-24 can produce injection nozzles completely on a single machine, rather then on two machines as formerly necessary.

The control unit generates a predictable machine-servicing program, based on machine data that is computed and analyzed continuously. Then, maintenance and servicing can be scheduled for dates when it will be least disruptive to production. Mikron’s remote diagnostics and support reduce the time needed for service.

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