Method and Apparatus for Controlling Drilling Operation

The present invention relates to a drill control method and apparatus, and, more particularly, to a method and apparatus for controlling drilling operations when holes are drilled through a composite workpiece which is made up of a plurality of materials of different machining properties.

Ordinarily, holes are drilled by rotating and feeding a drill at a constant speed. However, in the case where holes are drilled through a composite workpiece made of a plurality of laminated materials of different machining properties, a drilling condition suitable for one of the materials may not be suitable for another material, and boundary-layer peeling or chipping phenomena tend to occur thereby resulting in deterioration of the quality of the drilled holes. In addition, the usable life of the drill tends to be short, and selection of the drilling conditions is extremely difficult.

In order to eliminate the difficulties described above, there has been developed a drilling control method wherein drilling conditions, such as the feed speed and the rotational speed are set beforehand, which are suitable for each material; each material is drilled constantly under the set conditions; the terminating point of drilling of the material is detected from the time required or from an amount of the speed of the drill; the drilling condition is then changed to another one suitable for the material of the next layer and the drilling operation is carried out continuously by changing the drilling conditions one after the other as described above.

For controlling the rotational speed of the drill, there has been proposed a method for controlling the spindle speed for the drill (for instance, refer to Japanese Patent Laid-Open (KOKAI) No. 274843/1986). In this method, a variable speed motor is used for driving the spindle, and this motor is controlled based on a difference between a command speed and an actual speed.

For monitoring the cutting conditions, there has been proposed a device for controlling the machining condition (Japanese Patent Laid-Open (KOKAI) No. 15060/1987). In this device, a load cell is interposed between a chuck and a mounting surface therefor. The load cell detects the thrust force applied to the drill and delivers a voltage corresponding to the thrust. When the voltage exceeds a predetermined value, the terminating point of the drilling operation is detected without requiring any other positioning.

However, even if the drill feeding speed is controlled according to the method disclosed in Japanese Patent Laid-Open (KOKAI) No. 274843/1986, and the completion of the drilling of one material is determined by the device disclosed in Patent Laid-Open 15060/1987, the process becomes too complicated because multiple drilling conditions equal to that of the number of materials must be set beforehand. Furthermore, the feed speed and the rotational speed of the drill must be varied in accordance with the thickness of each material.

In addition, the drilling condition even for the same material must be varied according to the thrust force applied to the drill for preventing the boundary layer from peeling and for improving the quality of the drill hole. Optimum control of the drill is difficult under such conditions. The known method does not provide such operations.

An object of the present invention is to provide a method and an apparatus for controlling drilling operations, wherein the difficulties of the conventional method are eliminated without using complicated processes.

Another object of the invention is to provide a method and an apparatus for controlling drilling operations wherein the setting of drilling conditions for a workpiece made of composite materials is not required, and each material of the workpiece is drilled depending on the thrust force detected through the operation of drilling the material.

These and other objects of the present invention are achieved by a method of controlling drilling operations, comprising the steps of initiating the drilling operation for a material at a feed speed and a rotational speed of a drill; detecting the feed speed and the rotational speed of the drill and thrust force applied to the drill; and controlling the feed speed and the rotational speed of the drill based on the detected thrust force so as to maintain an optimum drilling condition.

According to the present invention, there is also provided an apparatus comprising: a feed sensor for detecting the feed speed of a drill; a speed sensor for detecting the rotational speed of the drill; a thrust sensor for detecting thrust force applied to the drill while the material is drilled; comparison means for comparing outputs of said sensors with optimum values; correction means for correcting the feeding speed and the rotational speed according to the results of the comparison; and a control circuit for controlling the feed speed and the rotational speed of the drill according to the output from the correction means.

A preferred embodiment of the present invention will become understood from the following detailed description referring to the accompanying drawings.

FIG. 1 is a block diagram showing an apparatus for controlling drilling operations according to the present invention; and

FIG. 2 is a flow chart showing the steps of the method for controlling drilling operations according to the present invention.

Referring to the block diagram of FIG. 1 showing the general arrangement of the apparatus for controlling drilling operations of the present invention, a drill 1 is fixedly mounted on a drilling unit 2. The drill 1 is rotated by the drilling unit 2 and is shiftable toward and away from a workpiece 3, which is a composite workpiece made of a plurality of materials 3a and 3b having different machining properties.

A control device 4 is connected to the drill unit 2 for controlling the rotational speed and the feed speed of the drill 1. The control device 4 comprises a rotational speed controller 4a and a feed speed controller 4b. A feed speed sensor 5 and a rotational speed sensor 6 are connected to the drill unit 2 for detecting the feed speed and the rotational speed of the drill 1, respectively.

Furthermore, a thrust sensor 8 such as a load cell is coupled to the drill unit 2 for detecting axial thrust force applied to the drill 1 when the drill 1 is forced into the workpiece 3. Outputs of the feed speed sensor 5, the rotational speed sensor 6 and the thrust sensor 8 are supplied to a CPU 7 via data converters 9a, 9b, and 9c, such as A-D converters, respectively. In the CPU 7, there is stored or placed in memory beforehand optimum relationships of the values for the thrust force, the feed speed and the rotational speed of the drill 1 for respective materials of the workpiece. The CPU 7 functions to compare the output data obtained from the sensors 5 and 6 with the optimum relationship for the thrust force, the feed speed, and the rotational speed of the drill 1, and delivers an output to the correction circuit 10. The correction circuit 10 corrects the feed speed and the rotational speed depending on the output of the CPU 7, and delivers control signals to the rotational speed controller 4a and the feed speed controller 4b of the control device 4. The control device 4 controls the rotational speed and the feed speed of the drill 1 depending on the control signals.

FIG. 2 is a flow chart illustrating the method for controlling the drilling operation according to the present invention.

At the start of the control, the thrust force applied to the drill 1 is set at zero before drilling. In step 1, the rotational speed and the feed speed are set for the first material of the workpiece. In step 2, the thrust force applied to the drill 1 due to the drilling of the first material is detected by the thrust sensor 8. In step 4, the rotational speed and the feed speed (condition (A)) of the drill 1 detected by the sensors 6 and 5 are compared with optimum rotational speed and optimum feed speed (optimum condition (B)) of the drill 1, which are determined depending on the detected thrust force in step 3. When it is determined in step 4 that the condition (A) is at least substantially equal to the optimum condition (B), the drilling operation is carried out in step 6 with the initially set speeds being maintained. However, at step 4 when it is determined that the condition (A) is not substantially equal to the condition (B), the rotational speed and the feed speed are corrected in step 5 to the optimum condition (B) at step 6 in a manner of conventional feed-back control, and the process returns to step 2 where the thrust force is detected. Then, the cycle (steps 2.fwdarw.3.fwdarw.4.fwdarw.5.fwdarw.2) repeats until condition (A) becomes close to condition (B), whether or not an adaptive control is obtained is checked at step 7, and when the result is YES, the drilling operation for the materials is terminated at step 8, whereas the process returns to step 2 when the result is NO at step 4. Thereafter, the drilling operation is carried out in the same manner for the second material of the workpiece.

Although in the method described above, the rotational speed and feed speed are set at step 1, it will be apparent that the rotational speed and the feed speed set for the previously drilled material may be used for these values.

Further, although the CPU 7 are used for storing in memory the optimum conditions and comparing the outputs of the sensors 5 and 6 with the optimum conditions set based on the output of the sensor 8, it will be understood that any other circuit adapted for accomplishing these functions may also be used instead of the CPU 7.

While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

According to the present invention, the optimum which the best feed speed and rotational speed for respective materials are taken by detecting the thrust force generated by the drilling operation. Therefore, accuracy and quality of the drilled holes are improved and usable life of the drill is increased. As it is not required to previously set the drilling condition according to thickness of the materials, the operation is facilitated and made more rapid.