This invention relates to a cyclic or step feed drilling machine in which one drilling process is carried out, instead of being performed in a single stroke, by gradually deepening a bore-depth by means of repeating reciprocative movement of a tool for a plurality of times before completing the drilling process.
More particularly it relates to a cyclic drilling machine in which magnitude of a load applied on the tool is detected so that the tool is, when the detected load grows larger to a certain value, retreated backwards once for being advanced again.
As a prior art belonging to this category, a technological idea disclosed in the Japanese patent publication TOKU-K0-No. SHO 53(1978)-40745 can be cited, in which magnitude of a load applied on the tool is detected or measured by the fluctuation or variation of load current observed in a tool driving motor, and the tool, i.e. drill in this instance, is returned backwards once in response to an indication of reaching of the load current to a predetermined value.
In this kind of device a load current of a motor at a non-load status is memorized once as a minimum value (Io) in a memory circuit, and a load current of a motor corresponding to a cutting (drilling) torque within the maximum allowable limit, wherein the drill can not be broken, is set in advance on the other hand as a current set value ({ Is). Sum of the both values (Is (=Io+.DELTA.Is)) is regarded as a reference value. Comparing of this reference value (Is) with a load current (I) of a motor which is detected in the course of a cutting is executed in a comparing circuit or comparator, so that the drill in advancing movement may be instantly returned backwards as soon as the load current (I) reaches the reference value (Is), that is when a cutting torque of the maximum allowable limit is applied on the drill. When a certain predetermined period of time has elapsed after the motor was placed under a non-load condition due to the return of the drill, the then non-load current value is memorized in the memory circuit in place of the previous minimum value as a new minimum value (Io). The drill is advanced again so as to resume the cutting of a workpiece which has been halfway suspended. The then load current value (I) of the motor is compared in the comparator with the reference value (Is). When the drill receives a cutting torque of the maximum allowable limit again it is instantly returned backwards similarly to the previous instance. This type of operation is repeated until a bore of a predetermined depth is formed in the workpiece, so that the drill may be prevented from damage or breakage.
The cutting process is however finished in general within 1-3 minutes after a workpiece is placed on the drilling machine, and the motor is stopped between the finish of cutting on one workpiece and the beginning of cutting on another workpiece. In other words, the cutting is carried out in the prior art while the motor (in this instance AC motor is used, but DC motor is also permissible) is under an unstable status, as shown in FIG. 1, wherein the winding current gradually decreases from a large initial current value to a stable current value. It means that the aforesaid minimum value (Io) is set while the winding current is unstable in the course of falling as shown in FIG. 2 and the reference value (Is), on which the cutting process depends, is set according to this minimum value (Io). Comparison of the minimum value (Io) with a current value (Io1), which would have been attained if the minimum value had been set when the winding current reached the reference value (Is), will show that the current value (Io1) is smaller than the minimum value (Io) by an amount of (.DELTA.I.alpha.).
For preventing the drill from the breakage, the reference value (Is) must be made, therefore, smaller by the difference (.DELTA.I.alpha.) of the minimum value (Io) minus the current value (Io1), i.e., to (Iss (=Is-.DELTA.I.alpha.)). It signifies that for making the reference value (Is) smaller by .DELTA.I.alpha., the aforementioned current set value (.DELTA.Is) must be set smaller by that much. In practical machining lines, such as a machining factory, wherein power source is variable in voltage even when a motor itself is in a stable status, the winding current thereof varies a great detal so that mistaking of this variation for that owing to load variation will cause an error operation. External noise will also cause the error operation. Reducing of the current set value (.DELTA.Is) is therefore envisaged with a limit. The step feed type cutting (drilling) with a thin drill which requires setting of a small current set value (.DELTA.Is) is practically next to impossible.
Comparison of a characteristic curve Ip1 of the winding current in the initial stage of the motor driving starting from a sleeping status with those curves Ip2 and Ip3 in the initial stage of the motor driving at a repeatedly operated status will show that the time required for decreasing value is longer in the curve Ip1 than in the others Ip2, Ip3, due to variation in the motor itself such as internal resistance in the motor. And the difference (I.alpha.) is maintained in a larger state, in other words, a status where the difference (.DELTA.I.alpha.) is large is maintained for a long period of time in the drilling of the workpiece in the initial operation period. It means that the time with a large difference (.DELTA.I.alpha.) continues for a comparatively longer period, particularly in a colder region for quite a long time, before the motor enters a stabilized operation wherein the machining can be stably performed. It naturally deteriorates the operational efficiency, to the great disadvantage.
In the conventional devices variation of the load current, i.e. variation of the winding current of the motor which is caused by the load variation is the detection object. It is quite difficult to get true variation of the cutting torque in this method, because the variation obtained herewith is nothing but a sum of the true variation value of the cutting torque plus a torque produced in the transmission system from the motor to the drill. And the torque produced in the transmission system is variable in itself during operation thereof. It is, therefore, problematical in this connection whether the variation of the winding current can be truly regarded as the variation of the cutting torque.
Besides, the technology disclosed in the Japanese patent publication TOKU-SO-No. SHO 54(1979)-23158 attempts to raise in put impedance by means of an emitter follower with FET (field-effect-transistor) so as to minimize leakage of electric charge which is charged in a capacitor for the purpose of preventing the timewise variation of the minimum value (Io) in a memory circuit storing the minimum value (Io). It is however practically very difficult to prevent the leakage in the capacitor, so exact maintenance of the minimum value (Io) and subsequent setting as well as maintaining of an exact reference value (Io) is next to impossible, to the great problem.