Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
2003-03-31
2004-10-26
Picard, Leo (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C703S002000, C700S280000
Reexamination Certificate
active
06810302
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a method for the systematic prediction of stable high speed cutting parameters for machining titanium.
(2) Description of Related Art
Titanium alloys are used extensively in manufacturing helicopter components because of their excellent combination of high specific strength, which is maintained at elevated temperature, high resistance to corrosion, fracture resistance characteristics and extensive ductility, especially at high strain rates. Despite the excellent properties of titanium alloys, their machinability is generally considered poor due to the following inherent properties. First, the high strength maintained at elevated temperatures with a low modulus of elasticity impairs the machinability of titanium. Second, large amounts of heat are generated at the tool/workpiece interface adversely affecting the tool life because titanium alloys have thermal conductivity 13 times less than aluminum.
Third, machining of titanium produces typically shear-banded (segmented) chips due to poor thermal properties. These chips cause a sudden force fluctuation from a peak value to a minimum value. The rapid force fluctuation causes a hammering on the tool face at the tool tip in the vicinity of cutting. This phenomenon accelerates the tool chipping process as the cutting speed increases and reduces tool life to a fraction of a second.
Fourth, the segmented chips roll onto the tool face and have a short time of non-sliding contact. During machining the low thermal conductivity and high strength of titanium, alloys create high temperatures leading to high rates of tool wear.
Lastly, titanium is very chemically reactive, and has the tendency to weld to the cutting tool during machining, which leads to chipping and premature failure.
An existing method for controlling the tool chip interfacial temperature consists of a high-pressure coolant jet applied at the tool-chip interface. The high-pressure coolant is delivered through internal coolant passages and an array of discrete nozzles that eject the coolant onto the cutting edge at a predetermined mass flow rate and impingement pressure.
Yet another method for controlling the tool chip interfacial temperature consists of a high-pressure coolant jet applied at the tool-chip interface. A thermal-mechanical High Speed Machining (HSM) model is used to predict the interfacial temperature as a function of cutting speed, coolant flow rate, and coolant application angle. Based on the predicted temperature, the optimal integral nozzle configuration is designed. The nozzle shape is optimized through a definite element model for predicting interfacial temperature isotherms with the objective of minimizing their values.
Model predictions and experimental results show that the shaped nozzle creates a correspondingly shaped jet, which is more effective at removing heat from the tool-chip interface, thereby reducing the tool chip interface temperature. Although the high-pressure coolant applications evacuate the chips very efficiently and reduce the tool temperature, which allows the increase in the cutting speed and consequently the feed rate 10 times, tool life is very low. The main mechanism of tool failure is chipping. The fluctuation of the cutting forces due to chip segmentations is one of the main reasons for this chipping. High rigidity of machine tool, use of high feed, and low rake angle can mitigate tool chipping. The segment spacing of the chip is equal to the feed (or uncut chip thickness) and is governed by the rake angle. Increasing the segment spacing reduces the frequency of impact and increases the area of contact so that the forces will be less concentrated on the tool tip.
This solution cannot be generalized for any tool/workpiece/fixture system. In many applications the feed is constrained due to workpiece flexibility, which can cause chatter. Also, in an operation such as face milling, the flexibility of the workpiece fixture does not allow for high feed as the cutting speed increases.
High speed milling of titanium is limited because of the dynamic behavior of the tool/workpiece/fixture system and the loads on the tools. Vibration can occur if the tooth passing frequency (No. of flutes or inserts *spindle speed−rpm/60) matches the frequency of anyone component of the tool/workpiece/fixture system. This type of vibration is usually referred to as forced vibration.
With reference to
FIG. 1
, there is illustrated a mass experiencing a single degree of freedom under forced vibration excitation. The amplitude of motion depends upon both on the amplitude of the force and on the frequency of the force. A low frequency of excitation force causes a displacement determined by the familiar “static” stiffness (F=kx). As the excitation frequency increases, so does the amplitude of the displacement, up to the “resonance”. At resonance, the frequency of the excitation force matches the natural frequency. At resonance, the amplitude of the displacement is much larger than at low frequency. For excitation frequencies higher than the natural frequency, the amplitude of the displacement decreases. The forced vibration, as seen in
FIG. 2
, is termed as a Frequency Response Function (FRF), where &ohgr; is the frequency of the exciting force and &ohgr;
n
is the natural frequency of the system. As illustrated, the figure on the right is a plot of the displacement occurring at the natural frequency while the plot on the left illustrates the different levels of displacement given a level of excitation wherein the maximum displacement occurs at the natural frequency. The natural frequency is represented as:
&ohgr;
n
{square root over (k/m)}
,
Where k is the stiffness and m is the mass of the system.
In general,
FIG. 2
shows that the Frequency Response Function (FRF) describes how a tool/workpiece/fixture system will vibrate in response to different frequencies of excitation. The FRF is a measurable function, and it can be used to compare and predict the performance of cutters and machine tools. There is a very high correlation between the FRF and the amount of speed and power that can be used in a milling operation.
Whereas “single degree of freedom” systems have 1 natural frequency, “multiple degree of freedom” systems have 1 natural frequency for each degree of freedom. Each natural frequency has a corresponding characteristic deformation pattern (mode shape). Vibration in “multiple degree of freedom” systems may be thought of as a sum of vibrations in the individual modes.
With reference to
FIG. 3
, there is illustrated the wavy surface
31
produced on a workpiece
33
when a milling cutter or tool
35
makes a pass resulting from the tooth passing frequency. When a subsequent pass is made, the cutter
35
removes material from an existing wavy surface and at the same time leaves behind a new wavy surface. The regeneration of waviness causes a steady input of energy from the milling spindle drive into vibration at the cutting edge. The chip that is created by this cut carries both the waviness from the previous pass and that translated over by the current pass.
If the new cut leads to a chip with constant thickness (i.e. the waviness of the chip is in phase), it creates a stable cut as illustrated in FIG.
4
. If the waviness generates variable chip thickness (i.e. the waves are out of phase as illustrated in FIG.
5
), this translates as variable forces on the cutting edge and eventually as vibration. This leads to the most undesirable vibrations in milling, specifically, self-excited chatter vibrations.
Chatter, the self-excited vibration between the workpiece
33
and the cutting tool, is another common problem during high speed machining and titanium. It significantly limits the machining productivity, adversely affects the surface quality, accelerates the premature failure of cutting tools, and damages the machine tool components. In general, it is observed that chatter cannot occur at the tooth passing frequency or any of its harmonics because
Darcy, Jr. Paul
El-Wardany Tahany
Fitzpatrick Peter
Harris William C.
Torok Michael S.
Bachman & LaPointe P.C.
Garland Steven R.
Picard Leo
Sikorsky Aircraft Corporation
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