Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2000-09-08
2002-02-05
Ip, Paul (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S570000, C318S571000, 36
Reexamination Certificate
active
06344724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a numerical control apparatus for an NC machine tool detecting a cutting load on the basis of an electric current for driving that is output to a feed drive system and/or an electric current for driving that is output to a spindle drive system, controlling an operation of the feed drive system to be retracted when the detected cutting load exceeds a specified reference value.
2. Related Art Statement
When drilling a deep hole in a work piece by making use of a drill and the like, a drilled hole is stuffed with chips as shown in
FIG. 6
when chips are not finely exhausted from the hole, a cutting load rapidly increases as a result, consequently a tool is broken. Here in
FIG. 6
, a torque computed from the electric current for driving supplied to the spindle drive system is grasped as such cutting load.
Conventionally for removing above-mentioned problems, a retracting operation (or a pecking operation) is applied to the feed drive system before the cutting load will increase too much to break the tool, for exhausting the chips from the drilled hole through the retracting operation of the feed drive system so as to reduce the cutting load. Following three embodiments have been well known to apply the pecking operation to the feed drive system.
In a first embodiment provided with a process section for executing such pecking operation, a pecking process is sure to be executed non-selectively according to a pecking execution code commanded in a machining program. Here, the pecking process is certainly executed irrespective of the actual cutting load or even when the cutting load has not yet reached a load (an overload) which sure needs the pecking operation.
In a second embodiment also provided with the same process section for executing the pecking operation, the pecking process is started according to an automatic pecking execution code commanded in the machining program, for making the feed drive system execute the pecking operation only when the cutting load detected by a load detector installed to a tool holder has reached the overload.
In a third embodiment also provided with the same process section for executing the pecking operation, the pecking process is started according to the automatic pecking execution code commanded in the machining program, on the other hand, the cutting load is detected on the basis of the electric current for driving that is output to the feed drive system of the machine tool and/or the electric current for driving that is output to the spindle drive system, for letting the feed drive system execute the pecking operation only when the detected cutting load exceeds a specified reference value and reaches the overload.
However, in above-mentioned three embodiments, there have been following problems described below. That is to say, in the first pecking process according to the first embodiment, the pecking operation is sure to be executed once the pecking execution code is commanded in the machining program even when the actual cutting load has not yet reached the overload or in a condition not needing the pecking process, therefore, an unnecessary air-cutting time (non-cutting process time) increases, consequently making the process itself inefficient. Furthermore, unnecessary retracting operations and retracting amounts with considering safe first lead to an inefficient process as a result because the actual cutting load has not been detected.
In addition, in the second pecking process according to the second embodiment, the cutting load is detected by the load detector installed to the tool holder for executing the pecking process only when the actual cutting load has reached the overload, therefore an effective process is possible, on the other hand, the pecking process itself needs a large cost because of necessity of a special tool holder and an electric circuit.
Furthermore, since the cutting load fluctuates according to a feed rate of the feed drive system and a wear and abrasion of a tool, it is impossible to uniformly set a reference value for judging the above-mentioned overload, whereby a problem happens such that an experiential skill is necessary to set the optimal reference value. One embodiment is shown in
FIG. 7
, wherein the cutting load increases according to the feed rate. In
FIG. 7
, with the use of a coated carbide solid drill with an oil hole (MDW085MHK available from Sumitomo Denko Co., Ltd.) having a diameter of 8.5 mm, a work piece of FC250 was drilled at a cutting speed of 150 m/min for formation of holes each having a depth of 26.5 mm. In addition, in
FIG. 7
, a first peak indicates the cutting load at the feed rate of 0.2 mm/rev, a second peak indicates same at 0.25 mm/rev, a third peak indicates same at 0.3 mm/rev, a fourth peak indicates same at 0.35 mm/rev, and a fifth peak indicates same at 0.4 mm/rev respectively.
As has been described above, the above-mentioned reference value has to be respectively set according to each feed rate of the feed drive system. Furthermore, the setting of the reference value is so complicated with considering the wear and abrasion of the tool. This can be the same in the third pecking process according to the third embodiment wherein the reference value cannot be set uniformly, either.
SUMMARY OF THE INVENTION
For solving the above noted problems in the pecking process, the present invention has its principal object the provision of a numerical control apparatus which can judge the overload with using a specified reference value without any affection of the wear and abrasion of the tool even in machining wherein the feed rate changes every moment, and can also securely execute the pecking process.
In the present invention, a numerical control apparatus for an NC machine tool comprising a command generating and distributing section to generate an operation command signal according to a machining program, a feed drive controlling section to control an operation of a feed drive system on the basis of the obtained operation command signal, a spindle drive controlling section to control a rotational operation of a spindle drive system according to the machining program, cutting load detecting means to detect a cutting load on the basis of an electric current f or driving that is output to the feed drive system from the feed drive controlling section and/or an electric current for driving that is output to the spindle drive system from the spindle drive controlling section, and pecking operation controlling means to make the feed drive system retracted if the cutting load detected by the cutting load detecting means exceeds a specified reference value during a drilling process, wherein the cutting load detecting means comprise a bite load (a load on cut starting) detecting section to detect the cutting load at an early stage being in a stationary condition after a tool has contacted with a work piece, and a stationary load detecting section to detect the cutting load after the early stage, the numerical control apparatus for an NC machine tool further comprises pecking judgement means to compute a fluctuation load by reducing the cutting load detected by the bite load detecting section from the cutting load detected by the stationary load detecting section for judging that the feed drive system should be retracted when the computed fluctuation load exceeds a specified reference value, wherein the pecking operation controlling means make the feed drive system retracted on the basis of judgement of the pecking judgement means.
In the present invention, the cutting load at the early stage is detected by the bite load detecting section, on the other hand, the cutting load after such early stage is detected by the stationary load detecting section. In addition, in the pecking judgement section, the fluctuation load is computed by reducing the cutting load detected by the bite load detecting section from the cutting load detected by the stationary load detecting section. When the computed fluctuation load
Fujishima Makoto
Kakino Yoshiaki
Nakagawa Hideo
Otsubo Hisashi
Takeshita Torao
Armstrong Westerman Hattori McLeland & Naughton LLP
Ip Paul
Kakino Yoshiaki
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