Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2000-09-08
2002-08-20
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S600000, C318S632000
Reexamination Certificate
active
06437534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling the operation of a feed drive system and a spindle of an NC machine tool and, more particularly, to a control method which effectively reduces the wear and abrasion of a drill edge during a drilling with a drill.
2. Description of the Prior Art
In recent years, NC machine tools have been designed so that a spindle and a feed drive system operate at higher speeds and at higher accelerations. It is known that the higher-speed and higher-acceleration operation extends the drill life. This is supposedly because the drill of such a high-speed and high-acceleration machine tool is subjected to a slow feed process for a shorter period of time when retracted opposite to a drilling feed direction, and a drill edge is kept in friction contact with the bottom of a drilled hole for a shorter period of time than in a conventional-speed NC machine tool. This will be explained in detail with reference to FIG.
14
.
FIG. 14
shows velocity fluctuations in the feed rate of the drill in the vicinity of the bottom of the hole in the high-speed and high-acceleration machine tool (solid line) and in the conventional machine tool (broken line) Provided that the drill edge is in friction contact with the bottom of the hole when the feed rate is within a range of ±2 m/min, the friction contact lasts for about 0.13 second in the case of the high-speed and high-acceleration machine tool and for 0.26 second in the case of the conventional machine tool, as shown in FIG.
14
. In the conventional machine tool, the period during which the drill edge is kept in friction contact with the bottom of the hole is longer by 0.13 second than in the high-speed and high-acceleration machine tool, so that the drill life is short.
The operations of the spindle and the feed drive system are typically controlled by a controller as shown in FIG.
15
. As shown, the controller
100
includes a machining program storing section
101
, a program analyzing section
102
, a command generating and distributing section
103
, a feed drive controlling section
104
, and a spindle controlling section
105
. The machining program storing section
101
stores a machining program preliminarily created. The program analyzing section
102
analyzes the machining program stored in the machining program storing section
101
to pick out commands concerning the rotation of the spindle, and the feed rate and feed position of the feed drive system from the machining program, and then sends a command signal concerning the rotation of the spindle to the spindle drive controlling section
105
and command signals indicative of the feed rate and feed position of the feed drive system
106
to the command generating and distributing section
103
.
The spindle drive controlling section
105
controls a spindle drive system
107
according to the received command signal for driving thereof. The command generating and distributing section
103
determines target feed positions at regularly spaced time points for the operation of the feed drive system
106
on the basis of the received command signals and a predetermined time constant to generate operation command signals indicative of the respective target feed positions, and then transmits the operation command signals one after another to the feed drive controlling section
104
. The feed drive controlling section
104
generates a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system
106
from a received operation command signal by a position loop gain K
p
. Then, the feed drive controlling section
104
generates an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system
106
from the generated velocity command signal by a velocity loop gain K
v
. The feed drive controlling section
104
further generates an output by multiplying a deviation of a present drive electric current signal fed back from the feed drive system
106
from the generated electric current command signal by an electric current loop gain K
I
, and then transmits the output as a drive command signal to the feed drive system
106
. The operation of the feed drive system
106
is controlled on the basis of the received drive command signal. Although the single feed drive system is shown in
FIG. 15
, machine tools such as machining centers generally have a plurality of feed drive systems
106
, and the command generating and distributing section
103
and the feed drive controlling section
104
are provided for each of the plurality of feed drive systems
106
.
The NC machine tool (e.g., machining center) is adapted to perform a variety of machining operations such as end milling, boring, reaming and milling. In particular, the end milling is generally employed for contouring control, so that it is important to precisely control the respective feed drive systems
106
in order to achieve the contour of great precision. Even if the respective feed drive systems
106
are simultaneously driven, parameters including the position loop gains K
p
, the feed velocity loop gains K
v
and the cutting feed time constants for the respective feed drive systems are set at the same levels in order to prevent reduction in the precision of the contouring control. In the aforesaid high-speed and high-acceleration machine tool, the machining operations are each performed at a spindle rotation speed of 20,000 to 30,000 m
−1
or greater and at a feed rate of 10 to 20 m/min or greater, so that the time constant is set at such a level that the feed drive systems
106
are each driven at an acceleration of lower than 0.1 G during the machining operation.
In the conventional machine tool, the parameters for the feed drive systems
106
are thus set mainly for the contouring control. Therefore, the parameter settings are not necessarily optimized for the drilling in order to reduce the stagnant time of the drill in the bottom of the drilled hole for reduction of the wear and abrasion of the drill and for extension of the drill life.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a control method for an NC machine tool, which ensures a high precision of the contouring control, and allows for extension of the drill life, and reduction in machining time.
In accordance with the present invention to achieve the aforesaid object, the present control method for an NC machine tool comprises generating an operation command signal on the basis of a machining program and a time constant, generating a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system from the generated operation command signal by a position loop gain, generating an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system from the generated velocity command signal by a velocity loop gain, and controlling a drive motor of the feed drive system on the basis of the generated electric current command signal, wherein a machining mode prescribed in the machining program is determined and, if the machining mode is a drilling mode, a predetermined operation modification value is added to the operation command signal to generate the velocity command signal and then the electric current command signal, and the drive motor of the feed drive system is driven and controlled on the basis of the generated electric current signal, when the feed drive system is driven to be retracted opposite to a drilling feed direction.
With this arrangement, where the machining mode is the drilling mode, the predetermined operation modification value is added to the generated operation command signal and the generation of the velocity command signal is based on the resulting operation command signal when the feed drive system is driven to be retracted opposite to the drilling feed directi
Fujishima Makoto
Kakino Yoshiaki
Nakagawa Hideo
Otsubo Hisashi
Takeshita Torao
Kakino Yoshiaki
Ro Bentsu
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