Standard spindle speeds on UK machining centres go up to about 12,000 rpm, according to John Dickens, area sales manager for PCM Tooling (UK), which distributes the OMG range of mechanical spindle speeders.

While that speed may be sufficient when using a 15 mm diameter end-mill, it may not cut it, literally, when spinning a 1 mm diameter tool, since that tool's speed at the cutting edge will be 15 times less.

"Small-diameter end-mills on standard machine tools can't run at the right cutting data," says Mike Jones, Iscar Tools (UK) technical manager. When using a spindle speeder, he says: "They will get better tool life, better surface quality and reduce cycle times and improve productivity."

Jones adds that speeders are relevant for any operations that use small diameter tools, including work on large components that might need, for example, a 0.2 mm break edge chamfer.

All things being equal, faster spindle speeds yield better surface finish, smoothing edges and reducing burrs, since the faster tool has the opportunity of more passes to remove imperfections. This may be particularly important when machining non-metals, such as composites or wood, whose tricky grain structures can be pulled apart by low cutting speeds.

So when using small tools (up to about 6 mm, but possibly less), spindle speeders sold in the aftermarket can help improve performance for much less money than a machining centre upgrade.

In terms of design, there are essentially three types of spindle speeder: the ones that rely on a mechanical gearbox mounted on the spindle, forming a continuous mechanical linkage between motor and spindle; those that rely on an electric motor mounted on a stationary spindle, and those that rely on a fluid-powered turbine mounted on a stationary spindle.

The first type, exemplified by OMG's range, converts every spindle rotation into a greater number of turns on the tool. The second type of model, for example Nakanishi units, use an externally-powered electric motor. The third type of speeder spins a turbine connected to the tool by drawing on the force of fluid flow, either from an external supply of compressed air (Air Turbine Tools – ATT) or the machine's high-pressure coolant (Iscar).

IN MORE DETAIL
The MO range of spindle speeders from Italian manufacturer OMG (01424 753 174) contains a system of planetary gears with ratios ranging from 1:8 (its fastest model, the MO10.HS, goes up to 35,000 rpm, with a maximum capacity 10 mm diameter tool) down to 1:4 (its highest torque model, the MO34, has a maximum tool diameter of 34 mm). All of the speeders come in size 40 or 50 tapers, and have the option of through-tool coolant or coolant supplied from the torque arm mounted on the side. The company also offers a range of specials.

Japanese tool manufacturer Nakanishi (TS Technology, 01923 221155), offers much higher speeds with its electric motor-driven speeder range: up to 80,000 rpm for the six HES810 models (available in 30, 40 and 50 tapers of different kinds), with a tool capacity of up to 4 mm diameter. Also offered is a 50,000 rpm version (the HES510 series).

These speeders incorporate a 350 W electric motor in the spindle attachment, operated by a controller device that displays speed and other parameters. A motor cable connects motor and controller, with a quick-release connection in case the spindle unit were to accidentally turn. Since the motors generate heat during operation, they are also fitted with a cooling air inlet. For that same reason, NSK specifies reduced torque levels for continuous duty over the speeder range.

The performance of Nakanishi speeders is plotted on a chart of power and torque versus speed. Although each model's performance is unique, they all show similar patterns. As the unit starts from zero, it consumes power and the spindle speed increases. Most often, but not always, increasing the speed reduces the available torque.

Air turbine spindles from ATT (001 561 994 0500; no current UK dealer) replaces the electric motor with a pneumatic turbine, and the electrical cable with an air hose. Just like a pneumatic wrench doing up the wheel nuts of a car at the garage, vanes in the spindle turbine spin under air pressure and turn the tool. Exhaust air vents out of the unit. The system requires pneumatic pressure of 6.2 bar and an upstream air filter/extractor is required to protect the device from impurities in the air supply. Speeds range from 40,000 rpm on the 6 mm diameter capacity 650 series, up to 65,000 rpm on the 3 mm diameter capacity 602 series.

Since there is no control system for these units, each model starts up and runs to a fixed speed. The company states that an O-ring governor mechanism keeps the motor running within 20% of rated speed to 80% of rated power, which runs from
1.04 kW on the largest 650 X (the X indicates a double air turbine) down to 0.11 kW for the 602 running at 40,000 rpm.

According to ATT, the through-tool coolant path might be able to carry compressed air through the machine; failing that, the air hose attaches directly to the spindle. Also available is an automatic toolchanging mounting assembly, a block and collar arrangement that fits to the machine above the spindle without interfering with other tools. The block accepts the air hose, but an internal valve switches off the flow of air when another tool is being used.

Iscar's new SpinJet relies on the force of high pressure coolant to turn the tool, in this case a 0.6 mm diameter tool

The newest entrant in the spindle speeder market is Iscar's (0121 422 8585) Spinjet HSM Jet Spindle, launched in 2015. Like the Air Turbine Spindle, it relies on flow, but rather than that of compressed air, this device draws on machine coolant. Through-tool coolant of between 20-40 bar and 12 l/min flow rate generates spindle speeds up to 40,000 rpm (maximum capacity of 1.5 mm diameter tooling), and supports tooling capacities of up to 3.5 mm diameter in steel and 6 mm in aluminium (20,000 rpm). Like the ATT device, an upstream filter is incorporated to protect the turbine mechanism.

Iscar speeders need no external cables or pipes running to the spindle, which means Spinjet heads can be stored in the tool magazine, changed using the automatic toolchanger and so used like standard tools.

Three models run at 20,000, 30,000 and 40,000 rpm; the 20,000 rpm model has the greatest torque. Reduced speeds are possible by lowering coolant pressure, with a sensor monitoring spindle speed and duration transmitting that information wirelessly to a receiver.

Jones at Iscar is hopeful that the new speeder will find a place in the UK market. "There have been a lot of enquiries and quite a few demonstrations to Formula 1, electronics manufacturers and companies doing small diameter work. Market interest is promising," he concludes.

SPECIAL-PURPOSE SPINDLES

Air- and motor-driven spindles are also available from Nakanishi (01923 221155), for driven tooling on CNC lathes.

Its air-operated spindles rotate at up to 160,000 rpm for CNC lathes, with tool capacity of 40 mm (Xpeed series), and up to 150,000 rpm for CNC mills, but only for small diameter end-mills and drills (HTC series). They have the disadvantage of only one operating speed, although 1:4 and 1:16 gearboxes are available.

Nakanishi also makes brushless DC motor-driven spindles offering variable speeds. They include the iSpeed 3, promising up to 80,000 rpm for CNC lathes, and the E 4000 series that offers up to 1 Nm torque and speeds of up to 40,000 rpm.

First published in Tooling & Workholding supplement, May 2015