JP4001290B2 - Precision machining method - Google Patents

Description

  The present invention relates to a precision machining method, and more particularly to a precision machining method for precisely grinding a conductive workpiece such as a mold.

  In ultra-precision optical parts such as cameras and video equipment, aspherical lenses having features such as small size and light weight are becoming popular. There is a strong demand for high accuracy of glass mold lenses, and high shape accuracy is required for lens molds when processing and manufacturing lenses. Cemented carbide is often used for mold materials because of shape accuracy, good surface roughness, and reduced thermal effects. It has become mainstream.

As an example of a mold processing method for an axially symmetric aspherical lens, a cross that processes a Soroban tool from the outer periphery of the rotating workpiece to the center according to the NC program using the positional relationship between the tool and the workpiece. A grinding method is used. In this method, the tool diameter and the tool center axis position set at the time of machining setup are raised as one of the major error factors affecting the machining accuracy. The tool center axis is a rotation axis when the tool rotates around the outer periphery of the workpiece, and the tool diameter is a rotation radius when the tool rotates around the outer periphery of the workpiece. The machining setup is a state in which the tool diameter and the tool center axis position are adjusted before actual grinding. If a shape error occurs, the machining efficiency decreases with error analysis and rework.

There are the following methods for measuring the tool diameter and the tool center axis position.

  The main measuring methods currently used include a method in which a pin gauge is statically brought into contact with a tool and a method in which measurement is performed in a non-contact manner by image processing using an image sensor.

FIG. 9A shows a contact measurement method using a pin gauge as an example of a method for measuring a stationary tool shape. In this method, a pin gauge whose diameter is accurately measured is secured to the workpiece axis, and a stationary tool is contacted from both ends, so that the tool diameter and tool relative to the workpiece axis rotation center can be obtained. This is a method of measuring the central axis position. In the method using a pin gauge, since high-precision pins can be obtained relatively easily, measurement at a micrometer unit is possible at a low cost.

FIG. 9B shows a method using an image sensor as an example of a method for measuring a rotating tool shape. In recent years, high resolution has been achieved with the increase in accuracy of image sensors. Since this method is a non-contact method, measurement is possible even when the tool is rotating, and a resolution of about 2 to 3 μm can be obtained with an image sensor of 1 million pixels.
JP-A-10-006191 Japanese Patent Laid-Open No. 06-210476

  FIG. 9A shows a contact-type measuring method using a pin gauge. This method is inexpensive and has high accuracy, but human error is likely to occur, and measurement by a skilled engineer is required. In addition, the centrifugal force and the deviation of the rotation center axis when the tool axis is rotated cannot be measured, and setup and machining with a margin are required. For this reason, the number of times of shape measurement and correction grinding increases, so that there is a problem that processing efficiency is lowered.

  FIG. 9B shows a method using an image sensor. Since this method is a non-contact method, it can measure even when the tool is rotating. However, since the camera installation state and measurement position are the curvature part of the tool, it is difficult to determine the focal position. As an error, there is a problem that sufficient measurement accuracy cannot be obtained.

  The present invention has been made to solve the above problems, and in a system for grinding a work surface such as a die, the shape of a rotating tool can be measured with high accuracy in a short time, and is inexpensive. An object is to provide a precision machining method provided with a highly reliable measurement method.

In order to achieve the above object, the conductive workpiece is held on the work spindle via an insulator, and the conductive tool is held on the tool spindle. By supplying high-frequency current to a closed circuit composed of the workpiece and the tool, the induction current is excited, and the capacitance generated between the tool and the workpiece is measured in a non-contact manner. Find the error in the distance between the tool and the workpiece with high accuracy. It involves geometric calculation of the error based on the arithmetic means obtains the respective error by separating the error of the tool diameter and the tool central axis position, to accurately correct the tool radius and the tool central axis position Features. The time required for this correction is as short as about several tens of seconds, and the processing efficiency can be greatly improved. In addition, since the measurement method is simple, it is possible to perform measurement with low cost and high reliability.

In ultra-precision grinding, there are many factors that affect machining efficiency and machining accuracy, such as machining setup, grinding wheel setting status, machining conditions, etc. In addition, it is performed to obtain high shape accuracy {machining-measurement-machining}. In this work cycle, many databases including experience values are required. Among them, the tool shape error in the setup stage before machining, that is, the error of the tool diameter and the tool center axis position has a great influence on the machining efficiency despite the non-machining process, and human error is the main, Efforts to suppress this are essential. In addition, since the tool is rotating at the time of setup before machining and deforms under the influence of centrifugal force or the like, it is necessary to measure the error of the tool diameter and the tool center axis position with high accuracy.

According to the present invention, the tool diameter and tool center axis position of a rotating tool can be measured with high accuracy, and the measurement time can be corrected manually or automatically in a very short time of 10 tens of seconds. Processing efficiency can be greatly improved. In addition, the method of measuring the error of the tool diameter and the tool center axis position by measuring the capacitance between the tool and the workpiece does not require a new measuring device, and is inexpensive and highly reliable. ing.

  Here, the present invention will be specifically described with reference to the drawings, taking an ultra-precision aspheric grinding machine as an example.

  FIG. 1 shows a schematic configuration of an ultra-precise aspheric grinder based on the present invention.

  In this apparatus, each axis of XYZ has a detection and moving resolution of 1 nm. The XZ axis is arranged as a horizontal axis, and the Y axis installed on the X axis table moves in the vertical direction.

  The work spindle 2 is mounted on the Z-axis, and the tool spindle 5 is mounted on the Y-axis table, and is installed on the machine body 1. Both spindles are supported by air bearings.

A work spindle 2 was attached via an insulating material 4 using a planar shape or a convex shape (R = 10 mm) made of ultrafine cemented carbide as the workpiece 3 .

As the tool 6, a conductive resinoid bond grindstone ( _Φ 70) was used. The abrasive and grain size is diamond # 3000. The tip of the grindstone was on-machine truing into an arc shape (R = 5 mm). During the measurement, the grinding wheel shaft was rotated and the grinding fluid was supplied as in actual machining.

The distance between the tool 6 and the workpiece 3 was measured by the change in the induced current induced in the closed circuit 7 composed of the machine body 1 -the tool 6 -the workpiece 3. The induction current was performed by supplying a high frequency current from the transmitter 8 to the ring 9b attached to the closed circuit 7. The excited induced current was detected by a ring 9 a attached to the closed circuit 7 and recorded by a recorder 11. Since noise from the machine main body 1 or the like is mixed in the detected signal, the signal is output through the band-pass filter 10 in the excitation frequency band. When the electrostatic capacity of each part was measured, the distance between the spindle bearings was 7.37 nF, and the tool 6 and the workpiece 3 (interval of about 5 μm) were 0.42 nF. Therefore, the excitation frequency is set to 1 MHz in order to reduce the impedance in the closed circuit 7.

2 is recorded in the recorder 11 in the case where the workpiece 3 is moved along the Z axis and then moved away in the direction opposite to that at the time of approach by numerical command while rotating the tool axis. Output value. 2A shows the response when the tool axis is moved in steps of 3 μm and FIG. 2B shows the step of 1 μm.

  As shown in FIG. 2A, the output value increases almost inversely as the distance between the tool 6 and the workpiece 3 decreases. At the point where the distance between the tool 6 and the workpiece 3 is the narrowest, the output value is also the maximum, and the step change at the time of approach and separation has relativity, and this confirms reproducibility.

  Further, even in the step feed of 1 μm shown in FIG. 2B, a step-like step change can be confirmed, and it can be seen that sufficient responsiveness and reproducibility are obtained during measurement.

  FIG. 3 shows an output value when the tool 6 is brought close to the workpiece 3 in a step of 0.1 μm from the position where the interval is narrowest in FIG. Although an abrupt change is observed in the output value, this indicates that the tool 6 and the workpiece 3 are in contact with each other, and there is a feature that the moment of contact can be detected.

  FIG. 4 shows the positional relationship between the master ball and the tool when measurement is performed using a convex master ball. The master ball is a reference sphere for measuring the tool center axis position and the tool diameter, and has a shape accuracy of about 0.2 μm.

The measurement flow is shown below.
(1) Using a grindstone as the tool 6, with the grindstone stopped, the grindstone and the master ball are brought into static contact, and the set values of the grindstone center position (tool center axis position) and grindstone diameter (tool diameter) are set . decide.
(2) Next, in order to match the atmosphere during processing, the grindstone is rotated while supplying the working fluid, and a constant distance from the master ball is maintained based on the grindstone center position and grindstone diameter obtained in (1). However, memory operation is performed by circular interpolation. Note that the current position and output value of the machine during circular interpolation are recorded simultaneously.
(3) If the set values of the wheel center position and the wheel diameter are accurate, the interval is kept constant and the output value becomes constant, but in reality, the output value does not become constant due to these two error factors.
(4) Measurement is performed until the output value becomes constant while performing memory operation by circular interpolation again based on the grindstone center position and the grindstone diameter calculated and corrected from the measured value.
By the above operation, the tool diameter and the tool center axis position before machining can be set accurately.

Figure 5 (A) to the tool diameter R ₂ is smaller than the set value, exhibited tool path when there is an error in the further tool center axis position in FIG. 5 (B). In the figure, R ₁ is a master ball diameter, and R ₂ is a tool trajectory diameter (a trajectory diameter of a tool tip close to the master ball). _{Further, A (X 1, Z 1} ) is an arbitrary point of the master ball diameter _{R 1, X 1} is X coordinate, _{Z 1} master ball diameter _{R 1} is Z-coordinate of the master ball diameter _{R 1.} On the other _{hand, B (X 2, Z 2} ) is an arbitrary point of the tool path diameter _{R 2} of the tool tip draws, X-coordinate of _{X 2} are the tool path diameter _{R 2, Z 2} is Z-coordinate of the tool path diameter _{R 2} It is.

Since the master ball is always held at the center of rotation of the workpiece axis, the surface shape (X ₁ , Z ₁ ) is defined as follows.

これに対し、工具径に誤差がある場合はＺ軸方向に誤差量ΔＲをシフトさせた円弧軌道と考えることができ、工具先端位置（Ｘ_２，Ｚ_２）は次式で示される。

On the other hand, when there is an error in the tool diameter, it can be considered as an arc trajectory in which the error amount ΔR is shifted in the Z-axis direction, and the tool tip position (X ₂ , Z ₂ ) is expressed by the following equation.

From the three-square theorem, Z ₂ The above equation holds.

From Equation 2 and Equation 3, X ₂ The above equation can be derived.

工具径Ｒ _２ はマスターボール径Ｒ _１ と間隔ｄと誤差ΔＲの和で表されるため、Ｒ _２ について上式を導くことができる。上式を任意θに沿って計算すると、工具径が設定値よりも小さい場合は、θが小さくなるほど、すなわちマスターボールの頂点に近づくほど間隔ｄが小さくなる。つまり中央部では周辺部よりも大きな出力が得られることになる。

Since the tool diameter R ₂ is expressed by the sum of the master ball diameter R ₁ , the distance d, and the error ΔR, the above equation can be derived for R ₂ . When the above equation is calculated along an arbitrary θ, when the tool diameter is smaller than the set value, the interval d decreases as θ decreases, that is, as the apex of the master ball is approached. That is, a larger output can be obtained at the central portion than at the peripheral portion.

Similarly, when there is a tool center axis error as shown in FIG. 5B, it can be considered as an arc trajectory shifted by an error amount ΔX in the X-axis direction, and the tool, that is, the tip position (X _{2) of the} tool. , Z ₂ ) is given by:

From the three-square theorem, Z ₂ The above equation holds.

From Equation 7 and Equation 8, X ₂ The above equation can be derived.

工具径Ｒ _２ はマスターボール径Ｒ _１ と間隔ｄの和で表されるため、上式を導くことができる。この場合、測定開始点と終了点で間隔ｄが異なり、測定開始点と終了点では、不均衡な出力結果が得られる。

Since the tool diameter R ₂ is represented by the sum of the master ball diameter R ₁ and spacing d, it can be derived the above equation. In this case, the interval d is different between the measurement start point and the end point, and an unbalanced output result is obtained at the measurement start point and the end point .

Actually, a shape error occurs due to the sum of the two factors described above. By measuring the output difference between the peripheral part and the central part of the master ball and the output difference between the measurement start point and the end point , the tool diameter and the tool center axis are measured. It can be broken down into position errors.

FIG. 6 shows a method for correcting the tool diameter and the tool center axis position using this method.

FIG. 6A shows an output example when circular interpolation is performed from the set value using the tool center axis position and tool diameter obtained by bringing the tool 6 and the workpiece 3 into contact with each other statically. Measurement distance 24.6 mm, shaft feed speed measurement time for the 120 mm / min as possible out be performed in a short time as about 12 seconds. In FIG. 6A, the output value decreases as the measurement time elapses, and the output value is minimized at a position where the elapsed time is about 8 seconds. Since the distance between the tool and the master ball is inversely proportional to the output value, the distance on the arc interpolation start side is narrow. This indicates that there is an error in the tool center axis position, and the actual tool diameter is larger than the set value.

First , the error of the tool center axis position is corrected. FIG. 6B shows an output value when the tool center axis position is moved by 5 μm in the X axis plus direction with respect to the set value . As compared with FIG. 6A, the output of the measurement start point and the end point shows an opposite tendency. In FIG. 6B, the output difference between the measurement start point and the end point is 15 mV.

FIG. 7 shows the relationship between the distance between the tool and the master ball and the output. If it is considered that almost linearity is obtained within the measurement range, 9 mV is obtained at intervals of 1 μm. Therefore, an output difference of 15 mV has an error of 1.6 μm between the measurement start point and the end point .

Error of 1.6μm from the tool central axis error characteristics of FIG. 8 (A), since the tool center axis error is equivalent to the output difference when the 3 [mu] m, a tool center axis position 6 of the (B) The position is set to -3 μm from the state. Note that FIG. 8 applies the above-described calculation formula, and the relationship between the error of the master ball center and peripheral parts and the tool radius error obtained from the output difference when an error occurs in the tool radius and the tool center axis position (see FIG. 8). 8 (A)), and a characteristic diagram showing the relationship between the measurement start point and end point errors and the tool center axis error (FIG. 8B).

FIG. 6C shows the output when the tool diameter is +40 μm with respect to the set value after correcting the tool center axis position. Since the tool center axis position is corrected, the values of the measurement start point and end point are substantially equal. FIG. 6C shows that the value near the center is increased by about 7 mV compared to the left and right, and the set value is larger than the actual tool diameter. From FIG. 7, the output of 7 mV corresponds to 0.8 μm.

Moreover, the tool diameter error characteristics of FIG. 8 (B), it can be seen the tool diameter error is 23 .mu.m. Therefore, a correction of −23 μm is performed on the set value of the tool diameter in FIG. FIG. 6D shows an output value when circular interpolation is performed again after correction. In FIG. 6 (D), a substantially uniform output is obtained, and it is considered that the center and radius of the arc movement coincide with the tool center axis position and the tool diameter, respectively. In addition, although a protrusion-like change is seen in FIG. 6, it is noise during measurement and does not affect the measurement accuracy.

  After determining the tool center axis position and the tool diameter using the master ball in this way, the workpiece to which the rough machining has been completed is started instead of the master ball. As a method of correcting the shape error by obtaining the error of the tool diameter and the tool center axis position from the error with respect to the set value of the tool path, the tool center axis position is obtained. After the correction, the method of correcting the tool radius error by obtaining the tool radius error and repeating the operation of correcting the set value of the tool radius has been described. Therefore, it is possible to adopt a method of analytically obtaining and correcting simultaneously. Furthermore, the correction of the tool shape error is either manual or automatic. In the machining, it is also possible to quickly set the incision or the like by grasping the shape of the workpiece after the rough machining is finished by using this method.

  Although the superprecision aspherical surface grinding has been described above, it is apparent that the present invention can be applied to grinding any curved surface. For example, it can be applied to cutting, laser processing, electron beam processing, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an embodiment of the present invention and is a schematic view of an ultra-precision aspheric grinding machine. FIG. 5 is a characteristic diagram illustrating an output value when the tool is moved along the Z axis according to the embodiment of the present invention. FIG. 3 is a characteristic diagram showing an embodiment of the present invention and showing an output value when the interval is further reduced by a step of 0.1 μm at the position where the interval is the narrowest in FIG. 2. The embodiment of the present invention is shown, and is a plan view showing a positional relationship between a tool and a workpiece when a master ball is attached instead of the workpiece. FIG. 9 is a plan view showing a tool locus when the tool diameter is smaller than a set value and there is an error in the tool center axis position according to the embodiment of the present invention. It is a characteristic view which shows the method of correct | amending the tool diameter and tool center axis position by this invention. It is a characteristic view which shows the space | interval between the tool-master balls by this invention, and the relationship of an output. It is a characteristic view which shows the calculation result by the computing equation when the error arises in the tool diameter and the tool center axis position according to the present invention. It is explanatory drawing which shows the conventional method of measuring by making a pin gauge contact a tool statically, and the method of non-contacting by image processing using an image sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Machine main body 2 Work spindle 3 Work material 4 Insulation material 5 Tool spindle 6 Tool 7 Closed circuit 8 Oscillator 9a Ring coil a
9b Ring coil b
10 Band pass filter 11 Recorder

Claims (4)

A workpiece gripped by an insulator on the workpiece spindle;
A tool made of a conductive material held by a tool spindle;
A closed circuit formed between the workpiece and the tool;
A ring coil for supplying a high-frequency current from an oscillator to the closed circuit;
A ring coil for detecting an induced current generated in the closed circuit ,
The tool of the tool spindle with respect to a set value based on the induced current detected by the ring coil that detects an induced current by moving the arc around the workpiece without contacting the tool while rotating the tool spindle. An error due to the centrifugal force of the center axis position and the tool diameter is obtained by calculation means, and the work surface is ground by correcting the tool diameter and the tool center axis position of the tool spindle so as to eliminate the error. Machining method. In the calculation means, the tool diameter error ΔR is set to X ₂ ² + (Z ₂ + ΔR) ² = R ₂ ² , R ₂ = R ₁ + d + ΔR.
Where (X ₂ , Z ₂ ): tool tip position, R ₁ : master ball diameter,
R ₂ : Tool path diameter, d: Obtained by intervals, and corrects the tool diameter of the tool spindle manually or automatically,
The error ΔX of the tool center axis position is set to (X ₂ −ΔX) ² + Z ₂ ² = R ₂ ² , R ₂ = R ₁ + d
Where (X ₂ , Z ₂ ): tool tip position, R ₁ : master ball diameter,
_2. The precision machining method according to claim 1, wherein R _{2 is a} tool trajectory diameter, d is a distance obtained by an interval, and the center axis position of the tool spindle is corrected manually or automatically.   3. The calculation means according to claim 2, wherein the calculation means obtains a tool center axis position error ΔR and a tool diameter error ΔX simultaneously, and corrects the tool center axis position and the tool diameter of the tool spindle manually or automatically. Precision machining method.   4. The precision machining method according to claim 1, wherein the arithmetic means performs correction while rotating the tool spindle while grinding the work surface.

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