Bedford, NH--High-speed machining requires a spindle shaft several times larger than the tool shaft to rigidly hold the tool in its spindle. Unfortunately, bearing power loss and heat generation increase dramatically with increasing spindle bearing diameter. That's in part why, over the past decade, only incremental improvements have been made in rolling element spindles.
Current high-speed spindles designed to mill large monolithic structures from solid metal billets--aluminum aircraft frames, for example--generally operate at speeds less than 30,000 rpm; net cutting power is normally limited to 30 kW or less. Some high-performance units can operate at speeds up to 40,000 rpm, while cutting with 40 kW of power. Twice that performance may be possible, however, by combining the tool shaft with an hydraulic turbine.
MIT spin-off company Aesop Inc. is testing that concept with a patented prototype product called TurboTool(TM). Components include an integrated cutter, axial flow turbine, and speed sensor. Self-compensating hydrostatic bearings formed directly into the tool shaft support the unit within the spindle housing.
During operation, water passes through an inlet diffuser in the spindle housing to stationary vanes, which impart angular momentum to the flow. Impingement on the turbine blades spins the tool. "By rotating the tool shaft itself, cutting speeds of 100,000 rpm are feasible," predicts Principal Engineer Kevin Wasson, adding that "such a tool should deliver 100 kW of cutting power."
Bearings are pressurized with the same water-based coolant used for the cutting process. For instance, high-pressure coolant passes to the radial bearings through a central pressurized annulus. This arrangement, says Wasson, gives the tool extra load capacity, stiffness, and damping capability. It also provides more tool holding rigidity than a clamp-type design.
In addition, high-pressure coolant passes to the thrust bearing located at the rear of the tool. Such a "one-fluid" system not only reduces the cost and complexity of sealing, it lowers power consumption and heat generation. Reason? Viscosity of water-base coolants is about one-tenth that of light weight spindle oil, while heat capacity of water is approximately two times that of oil.
Other potential advantages:
- Higher machining accuracies. Spindles supported with hydrostatic bearings typically exhibit an order of magnitude less error motion than those supported with rolling element bearings.
- Deeper cuts. The squeeze-film damping effect of the hydrostatic bearings and 100,000 rpm cutter tool speed are expected to reduce tool chatter.
- Lower workpiece temperatures. Through-the-tool cooling delivers coolant to the cutting zone at high flow rates.
- Low cost. TurboTool spindle head cost estimates are ten times less than comparable high-speed rolling element spindles.
Additional details...Contact Kevin Wasson, Aesop, Inc., 19 Harvey Rd., #9, Bedford, NH 03110, Tel: 603-644-3664, Fax: 603-644-3668, E-mail: email@example.com.
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