Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1996-07-26
1998-06-30
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318432, 318434, 31856811, B25J 918
Patent
active
057739498
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a cutter fracture detecting system for detecting a fracture i.e., any debilitating damage or breakage, of a cutter used in a numerically controlled machine tool in which the cutter and a workpiece are moved relatively to each other.
BACKGROUND ART
Some conventional numerically controlled machine tools have a sensor mounted on a table or a cutter holder for detecting a fracture of a cutter which may be brought about by an abnormal load imposed on the cutter when the cutter cuts a workpiece. Each time a workpiece is to be machined by a cutter, the cutter is actuated to move to the position of the sensor, and if the sensor fails to detect the cutter, then the cutter is judged as being broken. The use of the sensor is, however, disadvantageous in that it is difficult to install the sensor in position due to space limitations around the table, and the need for a signal processing controller associated with the sensor in addition to a numerical control apparatus results in a system which is structurally complex and highly costly.
Another process of detecting a cutter fracture is based on the assumption that a cutter fracture is caused by an excessive load applied to a location where a component operates, e.g., a spindle rotates or a cutter moves along an axis. A fracture of the cutter is detected based on monitored loads on the spindle and a Z-axis. Specifically, a disturbant load torque is estimated from the torque of a motor which is required to rotate the spindle, and another disturbant load torque is estimated from the torque of a motor which is required to feed the cutter. A fracture of the cutter is detected from the estimated disturbant load torques. For details, see Japanese patent application No. 5-138293.
According to the above process of detecting a cutter fracture by monitoring loads on the spindle and the Z-axis, when the estimated disturbant load torques exceed a predetermined reference torque, the cutter is judged as being worn, and when the estimated disturbant load torques fall short of another predetermined reference torque or exceeds still another predetermined reference torque, the cutter is judged as being fractured. In most cases, a cutter fracture can be detected by detecting an excessive load on the operating spindle or axis.
However, an analysis of actual cutter fractures has indicated that the cutters are broken under various different conditions, and some of the cutters are fractured under loads on the operating spindle or axis which are sufficiently lower than a maximum load that is applied in the normal cutting process, and hence they are fractured even though enough wear has not been developed thereon. Consequently, it has been found out that cutter fractures may not necessarily accurately be detected simply by monitoring loads on the operating spindle or axis.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of the above problems. It is an object of the present invention to provide a cutter fracture detecting system capable of detecting a fracture of a cutter with high accuracy.
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 in a numerically controlled machine tool in which a workpiece and the cutter move relatively to each other, comprising still axis disturbant load torque estimating means for estimating a disturbant load torque acting on one or a plurality of axes with respect to which the workpiece and the cutter do not move relatively to each other, comparing means for comparing the disturbant load torque estimated by the still axis disturbant load torque estimating means with a predetermined reference torque, and machine shutdown command means for outputting a command signal to stop the numerically controlled machine tool depending on a result of decision by the comparing means.
With the above arrangement, a disturbant load torque on one or the plurality of axes with respect to which the
REFERENCES:
patent: 5568028 (1996-10-01), Uchiyama et al.
patent: 5570256 (1996-10-01), Schoen et al.
patent: 5587915 (1996-12-01), Nagatomi et al.
Nagatomi Takashi
Sakaguchi Miwako
Fanuc Ltd.
Lockett Kim
Shoop Jr. William M.
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