Machining error correction method adapted for numerically...

Details Machining error correction method adapted for numerically... Machining error correction method adapted for numerically...

2003-04-03

2004-05-04

Picard, Leo (Department: 2125)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C700S164000

Reexamination Certificate

active

06732009

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a numerically controlled machine tool, such as an NC milling machine or an NC grinding machine, which performs machining of a work by moving a tool based on a programmed movement amount, and in particular to a machining error correction method for eliminating an error in machining of a work resulting from an error in movement of a tool.
Conventionally, as a machine tool that performs machining of a work using various kinds of tools, there has been known a so-called numerically controlled (NC) machine tool that automatically machines a work by controlling movement of the tools in accordance with a numeric data inputted in advance.
FIG. 10
shows an example of a vertical NC grinding machine that performs grinding machining on works W using a grinding wheel T. This grinding machine is provided with a work table
101
for fixing the works W on a base
100
, a gate-shaped column
102
is provided so as to stand on the base
100
while straddling the work table
101
, and the work table
101
is adapted so as to be capable of moving on the base
100
in an X-axis direction (right-left direction on the paper plane of FIG.
10
). Also, the grinding wheel T that performs the grinding machining on the works W is held by a spindle
103
and this spindle
103
, is held by a wheel spindle stock
104
functioning as a tool moving unit that is capable of moving in a U-axis direction (top-bottom direction) and a Y-axis direction (depth direction on the paper plane of FIG.
10
). Further, this wheel spindle stock
104
is mounted on one of side surfaces of the column
102
.
The movements of the wheel spindle stock
104
and the work table
101
are controlled by a numerical control unit composed of a computer. A tool path, a machining speed, and the like are programmed in advance in this numerical control unit. In general, the tool path is shown by a coordinate system where one point within a moving range of the wheel spindle stock is set as a program origin, and is defined using distances from the program origin in the U-axis direction and the Y-axis direction. Accordingly, when machining is started, the wheel spindle stock
104
and the work table
101
are driven in accordance with an instruction from the numerical control unit. Therefore, the grinding wheel T held by the spindle
103
is moved from the program origin to a predetermined position at a predetermined speed and the work table
101
is also moved, so that the works W are machined.
Essentially, in this NC machine tool, if the wheel spindle stock and the work table are moved as instructed by inputs into the numerical control unit, it is possible to machine the works with a high degree of precision without causing any dimension error. If the temperature in a plant changes, however, the lead of a ball screw that is responsible for the movement of the wheel spindle stock fluctuates, and the column and base supporting the wheel spindle stock also expand or shrink, albeit only slightly. Also, the spindle holding the grinding wheel rotates at a high speed, so that in the case where machining is continuously performed, this results in a situation where its main axis expands in an axis direction due to heat generated by a motor or a bearing. Consequently, even if the wheel spindle stock is moved from the program origin by an instructed movement amount, there occurs a situation where the position of the tool with reference to the works slightly differs from an intended position, which means that it is difficult to machine the works to predetermined dimensions with precision.
In view of this problem, in order to machine a work with a high degree of precision by eliminating environmental fluctuation factors such as those described above, a sizing device has conventionally been used in combination with numeral control. This sizing device compares a machined work with a master piece that has been machined in advance with a high degree of precision, and checks the machined dimensions of the work. For this purpose, the sizing device includes a probe
106
that is brought into contact with corresponding portions of the master piece and the work. In the grinding machine shown in
FIG. 10
, a sizing device
105
is attached to a side surface of the column
102
on a side opposite to the wheel spindle stock
104
, and the above-mentioned probe
106
is held by the column
102
through a moving unit
107
that is similar to the wheel spindle stock
104
. On the other hand, a master piece
108
is fixed at a position that is virtually the same as that of the works W on the work table
101
. During machining using this sizing device
105
, first, the probe
106
is brought into contact with the master piece
108
and a position of this contact is stored in a memory of the numerical control unit. Next, the probe
106
is brought into contact with each work W that has been machined, and a position of this contact is compared with the contact position of the master piece
108
stored in the memory. A difference found as a result of this comparison is a machining error of the work W with reference to the master piece
108
. Therefore, if the machining of the work W is performed by moving the wheel spindle stock
104
by the detected difference, it is possible to machine the work W with the same degree of precision as the master piece
108
.
However, a long period of time is required to measure the machining accuracy of each work using the sizing device, which becomes a factor for reduction in production efficiency. Also, due to the cost of the sizing device itself, the necessity to produce a master piece with a high degree of precision in advance, and the like, there is a problem in that the introduction cost of the sizing device itself also rises.
Also, in the case of the grinding machine that uses the grinding wheel, it is required to modify the shape of the grinding wheel T using a dresser
109
and to perform generation of an abrasive grain cutting edge each time a predetermined number of works have been machined. In addition, the position accuracy of the grinding wheel T with reference to the dresser
109
is of extreme importance. In the case where the position accuracy of the grinding wheel T with reference to the dresser
109
is low, this results in a situation where the dresser
109
that is essentially provided to modify the shape of a grindstone actually destroys the grindstone shape, which becomes a factor for a degradation in the machining accuracy of the work W.
As can been seen from the above description, the above-described sizing device is designed to enhance the accuracy in machining of a work through relative comparison of the work with the master piece, which means that the sizing device does not guarantee the positional accuracy of movement of the grinding wheel itself and serves no effect with respect to a relation between the dresser and the grinding wheel. Accordingly, in the case of a grinding machine, there is a problem in that even in the case where this grinding machine is equipped with a sizing device, the accuracy in machining of a work tends to be degraded.
Also, in order to shorten a machining time taken to perform grinding machining and to realize a reduction in production cost, it is desired to use a cubic boron nitride grindstone (also referred to herein as a CBN grindstone) that uses abrasive grains such as alumina-based grains that are harder than conventional abrasive grains. If the accuracy in modifying the shape of a grindstone by a dresser is poor, however, this results in a situation where more abrasive grains are scraped off by a single dressing operation. Therefore, there occurs a problem in that the life span of the grindstone is shortened and it becomes impossible to introduce the CBN grindstone that is superior in machining efficiency but is high-priced.
OBJECT AND SUMMARY OF THE INVENTION
The present invention has been made in the light of the problems described above, and an object of the present invention is to provide a ma

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