Rapid
Traverse Technology
and Trends Spotted By The Editors of Modern Machine Shop
Boosting Machine Spindle
Speed For Micromachining Applications
|
| Air-powered
spindles allow conventional machine tools to achieve spindle
speeds as fast as 80,000 rpm for micro- machining applications.
The ribs on this aluminum part measure 0.0025 inch thick and
0.157 inch tall. They were machined at 70,000 rpm and 59 ipm
using a 0.02-inch-dia-meter end mill. |
Small-diameter end
mills and drills used for micromachining require very high rpm to
achieve sufficient cutting speed and material removal rates. There
are different ways that shops can add high-speed capacity to allow
them to machine micromolds, electronic devices, medical parts and
other such tiny components. One option is to purchase a dedicated,
high speed machining center. A more economical alternative is to use
an auxiliary, high speed spindle that installs and operates in a conventional
machine’s locked spindles. Versions
of these auxiliary spindles use compressed air as the power source.
However, some of these air spindle designs do not lend themselves
to automated machining operations. That’s because air lines
must be attached manually each time the spindle is loaded into the
machine. The RBX air spindle from Big Kaiser Precision Tooling offers
adjustable speeds ranging from 30,000 rpm to 80,000 rpm for tools
having diameters smaller than 0.06 inch. It also provides an untethered
compressed air interface to allow automatic changeovers using the
machine’s ATC. When not being used, the air spindle is stored
in a tool carousel pocket as would any toolholder.
 |
| This
air spindle can be changed out using the machine’s ATC
and stored in a carousel pocket as would any other toolholder.
The compressed air that powers the spindle is delivered through
a stop block installed on the machine’s spindle face. |
The
RBX is compatible with VMCs that have CAT 40, BT 30 or HSK-A63 spindle
tapers. Compressed air is piped through the machine and delivered
to a stop block that installs on the machine’s spindle face.
When the RBX is loaded into the machine, a position pin slides into
the stop block to provide a sealed path for compressed air to reach
the air spindle. The air supply is controlled (turned on or off) by
M-code, and spindle speed is determined by the air pressure. For example,
60 psi of air pressure delivers 66,000 rpm, while 70 psi will generate
approximately 71,000 rpm.
Accurate
dynamic runout at high rpm is just as important for micromachining
as fast spindle speed. The RBX uses a microcollet system that provides
a maximum runout of 1 micron at the nut. The collet nut design has
no flats or slots, so vibration at high rpm is reduced (a custom ratcheting
wrench that grips the entire periphery of the nut is used to change
tools and collets). While the RBX accommodates tool diameters smaller
than 0.06 inch, it can clamp shank diameters as big as 0.159 inch.
Its microcollets are sized in 0.004-inch increments.
In
addition, excessive spindle Z-axis thermal displacement can reduce
micromachining accuracy. The RBX’s turbine drive has 1/6 the
thermal displacement of a conventional spindle during use, the company
says.
Micromachining
also presents difficulties recognizing when a cutting tool has broken.
Because the RBX air spindle generates less than 65 dB, the noise generated
by the cutting tool can be heard giving operators a better chance
of determining if a tool has broken.