Measuring and testing – Surface and cutting edge testing
Patent
1995-03-07
1997-08-19
Noland, Thomas P.
Measuring and testing
Surface and cutting edge testing
408 16, B23Q 1709, G01N 358
Patent
active
056591314
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cutter fracture detecting system for detecting a fracture of a cutter, and more particularly to a cutter fracture detecting system for detecting a fracture of a cutter which has a plurality of cutting edges.
2. Description of the Related Art
Unmanned operation of a numerically controlled machine tool for a long period of time requires that any fracture of the cutter be detected quickly for a cutter change.
To meet such a requirement, it has been customary to measure the load torque of a spindle, i.e., a drive current of a spindle motor, at all times, generate an alarm when the drive current reaches or exceeds a certain value, stop the machining process, and change the cutter.
However, in the case of a cutter having a plurality of cutting edges such as a milling cutter, the load current of the spindle motor remains substantially unchanged even if one of the cutting edges is broken, and hence it is difficult to detect such a cutter fracture.
SUMMARY OF THE INVENTION
In view of the aforesaid problems, it is an object of the present invention to provide a cutter fracture detecting system which is capable of easily detecting a fracture of a cutter having a plurality of cutting edges even when one of the cutting edges is broken.
To achieve the above object, there is provided in accordance with the present invention a cutter fracture detecting system for detecting a fracture of a cutter which has a plurality of cutting edges, comprising an observer for calculating a disturbance load torque of a spindle motor from a torque command value and a velocity feedback value, a band rejection filter for removing a given frequency component of the disturbance load torque which is determined by the number of cutting edges of the cutter and the rotational speed of the cutter, and a comparator for comparing an output signal from the band rejection filter with a predetermined reference value, and outputting a signal indicative of a fracture of the cutter when the output signal exceeds the predetermined reference value.
The observer calculates a disturbance load torque free of acceleration and deceleration components from a torque command value and a velocity feedback value. The disturbance load torque generally contains a certain frequency component which is determined by the number of cutting edges of the cutter and the rotational speed of the cutter, and such a frequency component can usually be removed by the band rejection filter. In a normal condition, the output signal from the band rejection filter contains a very little frequency component. When even one of the cutting edges of the cutter is broken, its frequency component is varied and falls outside of the rejection band of the band rejection filter. Since the frequency component cannot be removed by the band rejection filter, the band rejection filter produces an output signal of increased level. When the output signal from the band rejection filter is compared with a predetermined reference value, therefore, the fracture of the cutting edge of the cutter can be detected.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a cutter fracture detecting system according to the present invention;
FIG. 2 is a block diagram of a hardware arrangement of a numerical control apparatus for implementing a drilling system according to the present invention;
FIG. 3 is a block diagram of an observer for estimating a disturbant load torque;
FIG. 4 is a diagram showing frequency characteristics of a band rejection filter;
FIG. 5 is a diagram showing, by way of example, a filter input signal when a cutter is in a normal condition;
FIG. 6 is a diagram showing, by way of example, frequency components of a signal inputted to the filter when the cutter is in a normal condition;
FIG. 7 is a diagram showing, by way of example, a filter output signal when the cutter is in a normal condition;
FIG. 8 is a diagram showing, by way of example, a filter input signal when the cutter is in a fr
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Kono Shinichi
Tezuka Junichi
Fanuc Ltd.
Noland Thomas P.
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