Gear cutting – milling – or planing – Milling – With means to dampen vibration
Reexamination Certificate
1998-03-25
2001-06-05
Briggs, William (Department: 3722)
Gear cutting, milling, or planing
Milling
With means to dampen vibration
C318S575000, C409S131000, C451S011000, C700S177000
Reexamination Certificate
active
06241435
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to vibrational chatter reduction devices usable in machining operations and, in particular, to a universal adaptive machining chatter control fixture.
BACKGROUND OF THE INVENTION
It is generally recognized in the machining industry that problems relating to excessive vibrations, or “chatter”, are among the major factors limiting machining productivity and surface finish quality of a machined workpiece. Chatter is caused by vibrations produced when the cutter of a machining or cutting system, given a particular rotating speed and feed rate, excites a resonance frequency in the system and workpiece. Vibrations are inherent in the machining process and can affect the surface finish of the workpiece. Excessive vibrations limit machining precision, and can be very destructive to a cutting tool and/or a workpiece.
The potential for workpiece chatter depends, at least in part, upon the hardness of the workpiece material. With the increased usage of hard materials such as super alloys, refractory alloys, and many of the titanium alloys, chatter has become a significant problem. The machining of these materials requires much machine operator attention and special care. The machinist must anticipate the possibility for chatter in a set-up phase and attempt to reduce chatter onset as much as possible by applying rigid fixturing. Beyond this procedure, there are a paucity of commercially available means for reducing chatter. Moreover, in practice it often occurs that by the time the machinist hears audible chatter and responds to it, damage may have already occurred in the form of excessive cutter wear (with an associated loss of tolerance of the machined surface due to a worn or broken cutter), poor or unacceptable workpiece surface finish, and increased wear of machine tool bearings and components. Additionally, high-pitched sounds associated with chatter can be annoying and possibly harmful to the operator.
Methods that are presently known in the art for controlling chatter are based upon procedures for controlling the machining process. Automatic chatter control systems which, by example, adjust machining parameters such as spindle (cutter) speed and/or feed rates have been employed to control chatter. Such systems, however, must be designed specifically for, and built into, the particular machine tool of interest.
U.S. Pat. No. 3,671,840, issued to Meyer et al., discloses an adaptive control for a turning machine. Vibrations of a tool or workpiece during a machining operation are measured to produce signals corresponding to actual vibrations in mutually perpendicular directions. The vibration signals are compared with signals corresponding to reference magnitudes in both directions. The results of the comparisons are used to correct feeding of the tool or workpiece in accordance with the comparison results.
U.S. Pat. No. 3,967,515, issued to Nachtigal et al., discloses an apparatus for controlling vibrational chatter in a machine tool utilizing a synthesis circuit that updates workpiece and machine tool positions. A cutting tool mounted on a machine structure is moved into contact with a workpiece which is mounted on the machine structure. A resultant cutting force is reflected back through the tool and machine structure and is then measured by a transducer, which generates a signal indicative thereof. This force signal is applied to the synthesis circuit for continuous calculation of the workpiece displacement in accordance with continuously updated static and dynamic characteristics of the machine structure and the workpiece. Simultaneously, an accelerometer detects the acceleration of one of the cutting tool and workpiece. A signal representative of actual horizontal acceleration of the tool into or away from the workpiece is used to derive a signal representing a corresponding actual horizontal displacement of the tool. An output signal of the synthesis circuit is compared with the horizontal displacement signal. When the two signals are not equal, the difference is used to generate an error signal for controlling a compensatory force actuator that is mounted on the machine structure. The compensatory force actuator comprises a bidirectional, force-delivering assembly having a housing containing a force actuator. The compensatory force actuator mechanically applies compensatory forces to the machine structure to supplement the applied cutting force and thereby adjust the tool displacement to offset and eliminate vibrational chatter.
Another method that is known in the art for controlling chatter is based upon manual control of the machining process. By example, manually operated machines rely upon the machine tool operator to alter the machining parameters once chatter occurs. Cutter speed and feed rates are the two most common parameters that the machinist adjusts when attempting to eliminate chatter. Traditionally, while initially setting up the machine tool assembly, the operator sets the cutter speed, depth and width of cut, and workpiece feed rate for a particular machining pass based upon the operator's experience and finished part requirements. Thereafter, during the machining process, the operator must stop the machine and adjust the feed rate and/or the spindle speed, or remove less material per machining pass in order to reduce chatter. Unfortunately, the need for constant operator vigilance and the time required for the operator to make such parameter adjustments slows the machining process. Moreover, although the chatter is likely to be reduced after the operator has made such parameter adjustments, damage may have already occurred to the workpiece and/or machining tool owing to the chatter that occurred before the operator intervention.
It can be appreciated that chatter reduces the efficiency of the machining process and reduces the quality of workmanship attained. A reduction of chatter in machining processes thus provides for increased machining efficiency and reductions in machining costs.
OBJECTS OF THE INVENTION
It is a first object of this invention to provide a means for performing an automatic, real-time control of machining chatter without the need for operator intervention.
It is a second object of this invention to provide an adaptive machining chatter control fixture that improves machining precision and repeatability and reduces machine tool wear.
It is a third object of this invention to provide an adaptive machining chatter control fixture that enables faster machining to be performed through increased workpiece feed and removal rates.
It is a fourth object of this invention to provide an adaptive machining chatter control fixture that can be mounted or retrofitted to the bed of any machine tool and is independent of the machine tool's existing controller.
It is another object of this invention to provide a means for adaptively controlling machining chatter while accounting for dynamic machining conditions, including changes in the shape and weight of a workpiece being machined.
It is a further object of this invention to provide a means for monitoring chatter-induced translational and rotational motions of a workpiece in a plurality of directions during a machining operation, and for minimizing the chatter by moving the workpiece in directions based upon the monitored motions.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects of the invention are realized by a method for automatically minimizing an undesirable motion of a workpiece during a machining operation, and by a universal adaptive machining chatter control fixture that operates in accordance with the method. The method comprises the steps of mounting the workpiece to a supporting structure or fixture, and then machining the workpiece. During the step of machining, an undesirable motion of the workpiece is sensed in the supporting fixture. Thereafter, a compensa
Haas Edwin G.
Huang Chien Y.
Van Nostrand William C.
Briggs William
Perman & Green LLP
Vought Aircraft Industries, Inc.
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