April 6, 1998 Design News
DESIGN APPLICATIONS
From the regional editors
Turbine to double spindle speed
Integrated into the tool
shaft, new design avoids drag losses of conventional
machine tool spindles.
David J. Bak, International Editor
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 TurboToolTM.
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.
Additional details?Contact Kevin Wasson,
Aesop, Inc., 19 Harvey Rd., #9, Bedford, NH 03110, Tel:
603-644-3664, Fax: 603-644-3668.
Other Applications
Cutting of titanium aircraft parts
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