Gear cutting – milling – or planing – Milling – Cutter spindle or spindle support
Reexamination Certificate
1999-11-09
2001-05-08
Howell, Daniel W. (Department: 3722)
Gear cutting, milling, or planing
Milling
Cutter spindle or spindle support
C408S124000
Reexamination Certificate
active
06227777
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to high speed drilling systems for precision drilling of work pieces such as printed circuit boards and the like, and more particularly to systems for drilling very small diameter holes in such work pieces at high speed.
BACKGROUND OF THE INVENTION
Printed circuit boards are typically populated with many surface-mounted circuit devices. Many small holes are formed in the boards to interconnect the layers of the circuit board. Printed circuit boards are also populated with other types of devices which also need holes formed in the boards.
Drilling machines are typically used to drill the holes in the printed circuit boards. One exemplary type of system is described in U.S. Pat. No. 4,761,876, the entire contents of which are incorporated herein by this reference.
There has been a dramatic increase in the hole count on printed circuit boards, which makes the cost of drilling the holes a significant part of the total production cost. In addition, hole sizes are getting smaller. Small drills are more expensive and can not be fed with the same velocity as larger drills. Due to this fact, drilling time and cost are further increased.
To increase the throughput, higher drill bit rotation rates can be employed. However, there is a limit on the spindle rotation rate, which is due to the effect of the large centrifugal forces acting on the spindle rotors at very high rotation rates. Typically, the spindle is fabricated as a solid rod of steel, which will have a growth in the rotor diameter due to centrifugal force at very high rotation rates. Because the rotor typically is supported on air bearings with relatively small gaps between the bearing structure and the rotor, the growth in the rotor diameter will close or significantly narrow these bearing gaps, leading to seizure of the rotor in the bearings.
Drilling spindles typically use a chuck such as a centrifugal chuck to grip the drilling tool while it is being rotated. Centrifugal chucks are advantageous since the tool can be changed without mechanically operating a release mechanism, as there is no gripping centrifugal force when the chuck is not rotated. Small drill applications can have very small drill bit runout tolerances, which can be difficult to achieve with centrifugal chucks in a single grip configuration.
It would therefore represent an advance in the art to provide a spindle capable of extremely high drilling speeds.
It would also be an advantage to provide a centrifugal chuck capable of gripping tools with significantly reduced run-out.
SUMMARY OF THE INVENTION
This invention provides many advantages and features. One aspect of the invention is a spindle capable of operation at 200,000 revolutions per minute (RPM) with a reciprocating shaft design to minimize the moving mass. Another aspect of the invention is a double gripping centrifugal chuck for a drilling spindle to reduce drill bit run-outs.
In accordance with a further aspect of the invention, a combination of an air bearing and a magnetic thrust bearing is employed to reduce the size of the thrust area for better dynamic stability and reduction in stresses of material.
A drilling spindle in accordance with another aspect of the invention includes a ceramic spindle shaft to decrease the moving mass and increase the shaft stiffness for better dynamic stability. The spindle employs a built-in linear motor to provide direct drilling motion, and a permanent magnet DC brushless motor to rotate the spindle shaft.
In one exemplary embodiment, a high speed drilling spindle in accordance with the invention includes a spindle body, a rotatable rotor shaft supported within the spindle body for high speed rotation about a rotor axis. The rotor shaft is fabricated of a ceramic material capable of withstanding centrifugal forces exerted during high rotation rates without significant diametrical growth of the rotor shaft. A rotary air bearing supports the rotor shaft for high speed rotation with low frictional drag within the spindle body. A rotary drive system imparts rotational drive forces to the rotor shaft so as to rotate the shaft on the rotary bearing at high speeds. A linear drive system imparts an axially directed drive force to the rotor shaft to perform drilling movements. A thrust bearing couples the linear drive system to the rotor shaft, and includes an air bearing and a magnetic thrust bearing.
The rotary drive system includes a DC brushless permanent magnet motor, with a permanent magnet mounted within an opening formed in the rotor shaft.
The spindle includes a centrifugal chuck mounted in the rotor shaft, holding a tool having a tool shank in place during high speed rotation to perform tool operations.
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Japanese Patent Abstract, vol. 6, No. 14 (M-108)(892),Jan. 27, 1982 [JP 56-134109],T. Hori.
Electro Scientific Industries Inc.
Howell Daniel W.
Roberts Larry K.
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