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BIG Daishowa has introduced two air turbine spindles designed to achieve high cutting speeds for efficient metal removal and acceptable tool life when using small diameter cutters.

The RBX5 version operates at 30000-50000 rev/min, while the RBX7 runs at 60000-80000 rev/min.

Available in the UK through NCMT, the air-driven tools come with a variety of BIG-Plus face-and-taper-contact back ends to fit any BT 30/40/50 or HSK-A63/A100 machining-centre spindle.

This is held stationary while compressed air is delivered through a stop block installed on the spindle face to power the turbine.

The air spindle can therefore be exchanged automatically to and from the tool magazine like other toolholders, allowing micro-machining to be mixed with other metal-cutting operations in one uninterrupted cycle.

This contrasts with electrically powered turbine spindles, which require a power-supply lead to be connected manually, preventing automatic tool change.

The air supply of the RBX is turned on and off by M-code, the air pressure determining spindle speed.

For example, 60lb/in2 of air pressure generates 66000rev/min while 70lb/in2 delivers 71000rev/min.

Shank diameters up to 4mm can be held in the air spindle.

Cutting tools down to 1mm in diameter are clamped in a microcollet using a notch-free nut, tightened using a special wrench, to eliminate unbalance at high speeds.

Maximum static TIR (total indicated run-out) is one micron at the collet nose, while dynamic run-out is 4 micron at 80000rev/min.

Moreover, the RBX turbine drive has less than 17 per cent of the thermal growth in the Z-axis compared with a conventional spindle, leading to precise milling, drilling, engraving, slotting, deburring, routing and boring.

The turbine employs ceramic ball bearings and runs quietly at less than 65dB, allowing operators to hear cutting noise from the micro tools and determine if a cutter has broken.

Improvements in productivity and tool life are possible.

For example, the RBX spindle can be used to cut 63 micron walls in aluminium and drill the material without a centre drill.

Even after drilling 3,500 holes in a test cycle, the cutting tool edge showed no wear.

A 50mm carbide drill in the air turbine has been shown to produce 1,200 holes at 50000rev/min, whereas a drill on a machining centre at 12000rev/min produced only 500 holes.

In another comparative machining example, a 10mm ball-nose end mill in an air turbine cut a complex, 2D shape in HRC40 tool steel at 80000rev/min in 120min.

With the tool in the spindle of a 20000rev/min machining centre, the same job took nearly four times longer (450 minutes).

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