Patent ID: 12228906

DETAILED DESCRIPTION OF THE INVENTION

Please refer toFIGS.1-3.FIG.1shows a flow chart of a calibration method for machine tools according to an embodiment of the present invention.FIG.2shows a schematic diagram of a machine tool to which the calibration method according to the embodiment of the present invention can be applied.FIG.3is a schematic diagram of a first machining mode in the calibration method according to the embodiment of the present invention.

The calibration method of the present invention can be implemented through a calibration system including a workpiece11, a measuring element12and a processing control element (not shown in the figure, for example, it may be a programmable controller). For example, the machine tool to which the calibration method according to the embodiment of the present invention can be applied may be a common multi-axis machining machine in the mechanical field, such as a five-axis machine tool shown inFIG.2that has three linear axes (that is, a X-axis, a Y-axis and a Z-axis commonly known in the mechanical field) and two rotation axes (that is, any two axes of a A-axis, a B-axis, and a C-axis commonly known in the mechanical field), where the A-axis, B-axis and C-axis are the rotation axes for corresponding to the X-axis, Y axis and Z-axis, respectively.

In the step S110, provide the workpiece11on the machine tool. For example, fix the workpiece11on a stage P of the machine tool. First, measure the rotation axis parallel to a main shaft of the machine tool (take the C-axis as an example to describe the embodiment). The main shaft of the machine tool described here refers to the shaft on which a cutter of the machine tool is installed and which drives the cutter.

In the step S120, rotate the workpiece11around a first rotation axis (C-axis) of the machine tool and processing the workpiece11by a first machining mode, wherein the first rotation axis (C-axis) is parallel to the main shaft of the machine tool. As shown inFIG.3, the upper left half ofFIG.3is a schematic front view of the relative position of the workpiece11and a cutter K for processing, that is, the viewing angle is along the Y-direction. The lower left half ofFIG.3shows a schematic side view of the relative position of the workpiece11and the cutter K for processing, that is, the viewing angle is along the X-direction. The right half ofFIG.3shows a schematic top view of the stage P in a plurality of rotation states, that is, the viewing angle is along the Z-direction.

To be more specific, the workpiece11is first cut at a machining position F to form a groove with a depth F1. Then, the stage P of the machine tool rotates 90 degrees (counterclockwise), driving the workpiece11to rotate from the machining position F to a machining position G, while the horizontal position of the cutter K of the machine tool is raised at the same time. Also, the cutter K also follows the workpiece11with a path of five-axis simultaneous machining to the machining position G and cuts the workpiece11at the raised horizontal position. If the rotation center of the first rotation axis (C axis) of the machine tool has an offset error, the cutter K will cut the workpiece11to form a platform structure (corresponding to the dotted circle at the machining position G) having a depth G1.

Under the same operation mode, the stage P of the machine tool rotates 90 degrees (counterclockwise), driving the workpiece11to rotate from the machining position G to a machining position H, while the horizontal position of the cutter K of the machine tool is raised at the same time. Also, the cutter K also follows the workpiece11with a path of five-axis simultaneous machining to the machining position H and cuts the workpiece11at the raised horizontal position. If there is an offset error in the rotation center of the first rotation axis (C axis) of the machine tool, the cutter K will cut the workpiece11to form a platform structure (corresponding to the dotted circle at the machining position H) having a depth H1.

Under the same operation mode, the stage P of the machine tool rotates 90 degrees (counterclockwise), driving the workpiece11to rotate from the machining position H to a machining position I, while the horizontal position of the cutter K of the machine tool is raised at the same time. Also, the cutter K also follows the workpiece11with a path of five-axis simultaneous machining to the machining position I and cuts the workpiece11at the raised horizontal position. If there is an offset error in the rotation center of the first rotation axis (C axis) of the machine tool but the offset error of the machining position I is not greater than the offset errors of the machining position G and the machining position H, there is not an apparent platform structure (but the actual position corresponds to the dotted circle of the machining position I) having a depth formed on the workpiece11at the machining position I after the first machining mode is performed. In short, the dotted circle is the offset error under dynamic conditions, so that after the actual cutting, the segment or dimensional error is caused. If there is no dynamic offset error, the holes after the four-time cutting will be located at the same machining position F, and there will be no segment or dimensional error on the finished product of workpiece.

Finally, the stage P of the machine tool rotates 90 degrees (counterclockwise), driving the workpiece11to rotate from the machining position I to return to the initial machining position F to complete the first machining mode. In short, the first machining mode is: cutting the workpiece11by the cutter K of the machine tool, wherein each time the stage P of the machine tool rotates the workpiece11by an angle (90 degrees), raises the horizontal position of the cutter K and making the cutter K follows the workpiece11with a path of five-axis simultaneous machining, and perform the cutting at the raised horizontal position. It should be noted that the coordinates of X′-Y′, X′-Z′ and Y′-Z′ shown inFIG.3are the workpiece coordinates of the workpiece11, and the workpiece coordinate of X′-Y′ is rotated according to the rotation of the workpiece11to correspond to the machine coordinate of the stage P of machine tool after the completion of the first machining mode, so as to facilitate the subsequent measurement and calculation about the linear axis.

Next, In the step S130, measure a first dimensional error of a shape of the workpiece11along directions of a first linear axis (X-axis) and a second linear axis (Y-axis) of the machine tool, wherein the first dimensional error of the shape includes a first segment difference DM1and a second segment difference DM2. The first linear axis (X-axis) and second linear axis (Y-axis) are perpendicular to the first rotation axis (C-axis). Please refer toFIG.4.FIG.4shows a schematic diagram of a measuring element12that measures the first dimensional error of a shape of the workpiece11along directions of the first linear axis (X-axis) and the second linear axis (Y-axis).

First, the measuring element12can be fixed on an end of the main shaft of the machine tool. For example, the measuring element12is a dial indicator. As shown in the upper half ofFIG.4, the measuring element12contacts with the workpiece11processed by the first machining mode along the direction of the first linear axis (X-axis), wherein the workpiece11is processed to form a segment difference structure between a surface S1(a side wall being cut out at the machining position F) and a surface S2(a side wall being cut out at the machining position H). The measuring element12is configured to measure the first segment difference DM1between the surface S1and the surface S2farthest in the segment structure along the direction of the first linear axis (X-axis). Since the movement of the measuring element12measuring from the surface S1to the surface S2is toward the positive X-direction, so the first segment difference DM1is taken as a positive value. InFIG.4, since the surface S1′ formed after the workpiece11is processed and opposite to the surface S1is a flat sidewall, there is no segment difference structure formed on the side of the surface S1′. However, if the surface S1′ is not a flat sidewall but has a segment difference structure, the measuring element12needs to measure a segment difference between the surface S1′ and a surface farthest from the surface S1′ along the direction of the first linear axis (X-axis). In this condition, the measuring element12will perform measuring from the surface S1toward the negative X-direction, so the measured segment difference needs to be taken as a negative value. In other words, the first segment difference DM1is the segment difference between the original machining position F and the machining positions G, H, and I in the direction of the first linear axis (X-axis). The actual method is to measure the maximum offset value of a single side wall in the X-axis direction.

After measuring the first segment difference DM1along the direction of the first linear axis (X-axis), the measuring element12continues to contact with the workpiece11processed by the first processing mode along the direction of the second linear axis (Y-axis). As shown in the lower half ofFIG.4, the workpiece11is processed to form a segment difference structure between a surface S3(the sidewall being cut out at the machining position F) and a surface S4(the sidewall being cut out at the machining position H), the measuring element12is configured to measure the second segment difference DM2between the surface S3and the surface S4which are farthest apart in the segment difference structure along the direction of the second linear axis (Y-axis). Since the movement of the measuring element12measuring from the surface S3to the surface S4is toward the negative Y-direction, so the second segment difference DM2is taken as a negative value. InFIG.4, since the surface S3′ formed after the workpiece11is processed and opposite to the surface S3is a flat sidewall, there is no segment difference structure formed on the side of the surface S3′. However, if the surface S3′ is not a flat sidewall but has a segment difference structure, the measuring element12needs to measure a segment difference between the surface S3′ and a surface farthest from the surface S3′ along the direction of the second linear axis (Y-axis). In this condition, the measuring element12will perform measuring from the surface S3′ toward the positive Y-direction, so the measured segment difference needs to be taken as a positive value. Therefore, the first dimensional error including at least the first segment difference DM1and the second segment difference DM2can be obtained. In other words, the second segment difference DM2is the segment difference between the original machining position F and the machining positions G, H and I in the direction of the second linear axis (Y-axis). The actual method is to measure the maximum offset value of a single side wall in the Y-axis direction.

In the step S140, calculate a positional error of the first rotation axis (C-axis) according to the first dimensional error of the workpiece11to calibrate a rotation center of the machine tool. That is, calculate the positional error of the first rotation axis (C-axis) according to the first segment difference DM1and the second segment difference DM2. Specifically, the obtained first segment difference DM1and second segment difference DM2can be substituted into the following algebraic formula to obtain the positional error XOC and positional error YOC of the first rotation axis (C-axis), such as: XOC=−(dxmax+dxmin)/2, YOC=−(dymax+dymin)/2, the meaning of the algebraic formula is to obtain the average value of the segment differences caused by an offset of two side walls formed from the processed workpiece in the direction of a single linear axis (X-axis or Y-axis) and then multiply the obtained average values in the direction of the first linear axis by a negative sign (because the positional error XOC and the positional error YOC are opposite to the direction of measurement). The segment differences caused by the offset of two side walls formed from the processed workpiece11in the direction of the X-axis at least include the first segment difference DM1. The segment differences caused by the offset of other two side walls formed from the processed workpiece11in the direction of the Y-axis at least include the second segment difference DM2. Further, dxmaxis substituted into the first segment difference DM1which is a positive value with the largest segment difference in the X-direction, dxminis substituted into zero because the surface S1′ is flat and no segment difference structure is formed; dymaxis substituted into zero because the surface S3′ is flat and no segment difference structure is formed, dyminis substituted into the second segment difference DM2which is a negative value with the smallest segment difference in the Y-direction. The obtained positional errors XOC and YOC are the linear offsets between the actual installation center of the C-axis of the machine tool and the ideal position in the X and Y directions. Therefore, the offset between the positional error XOC and the positional error YOC is reversely compensated by the processing control element. For example, the origin position of the first rotation axis (C-axis) adds/subtracts the positional error XOC and the positional error YOC, thereby calibrating the rotation center of the first rotation axis (C-axis) of the machine tool to an ideal position. Afterwards, the calibration of another different second rotation axis (A-axis) of the machine tool can be performed.

Then, measure the rotation axis perpendicular to a main shaft of the machine tool (take the A-axis as an example to describe the embodiment). In the step S150, rotate the workpiece11around the second rotation axis (A-axis) of the machine tool and process the workpiece11by a second machining mode, wherein the second machining mode is significantly different from the first machining mode, described as follows. Please refer toFIG.5.FIG.5shows a schematic diagram of the second machining mode, wherein the left half ofFIG.5shows a schematic front view of the workpiece11that goes through the second processing mode, and the viewing angle is along the Y-direction. The right half ofFIG.5shows a schematic side view of the workpiece11goes through the second processing mode and the viewing angle is along the X-direction.

As shown inFIG.5, the coordinate of the cutter K of the machine tool can be fixed first, then rotate the workpiece11around the second rotation axis (A-axis). The workpiece11can be milled to form a first processed curved surface E1with the cutter K of the machine tool being in a fixed state. Specifically, the stage P of the machine tool rotates around the second rotation axis (A-axis), driving the workpiece11to swing in a cradle-like manner to move relative to the fixed cutter K, thereby forming the concave first processed curved surface E1of the workpiece11.

After the concave first processed curved surface E1is formed on the workpiece11by milling, make a displacement O between the cutter K of the machine tool and the workpiece11along the direction of the second rotation axis (A-axis), and then mill the workpiece11with the cutter K of the machine tool moving around a circle, wherein a milling depth M2corresponding to the milling with the cutter K moving around a circle is set to be greater than a milling depth M1corresponding to the milling with the cutter being in a fixed state. As such, by moving the cutter K of the machine tool around the virtual rotation center V of the cutter K, a concave second processed curved surface E2is formed on the workpiece11that is in a fixed state, wherein there is a misalignment between the first processed curved surface E1and the second processed curved surface E2due to the displacement O. If the rotation center of the second rotation axis (A-axis) of the cutter K of the machine tool has an offset error, the virtual rotation center V and a real rotation center W are not concentric, and the second processed curved surface E2will not be parallel to the first processed curved surface E1. Thus, it is necessary to measure the offset of the rotation center of the second rotation axis (A-axis) to calibrate. In short, the second machining mode is to rotate the workpiece11around the second rotation axis (A-axis) and then mill the workpiece11to form the first processed curved surface E1with the cutter K of the machine tool being in a fixed state; to make the displacement O between the cutter K and the workpiece11along the direction of the second rotation axis (A-axis) and then mill the workpiece11to form the second processed curved surface E2with the cutter K of the machine tool moving around a circle, wherein the milling depth M2corresponding to the milling with the cutter K moving around a circle is set to be greater than the milling depth M1corresponding to the milling with the cutter K being in a fixed state.FIG.5is a side view of the first processed curved surface E1and the second processed curved surface E2that are projected and superimposed, for the convenience of description.

In the step S160, measures a second dimensional error of a shape of the workpiece11along the direction of a third linear axis (Z-axis) of the machine tool, wherein the third linear axis (Z-axis) is perpendicular to the second rotation axis (A-axis), the second dimensional error of the shape includes a first depth difference, a second depth difference and a third depth difference. Please refer toFIGS.6A-6C.FIGS.6A-6Cshow schematic diagrams of the measuring element12measuring the second dimensional error of the shape of the workpiece11along the direction of the third linear axis (Z-axis). Wherein,FIG.6Ashows that the measuring element12measures the first processed curved surface E1and the second processed curved surface E2of the workpiece11at a first rotation angle θ1along the direction of the third linear axis (Z-axis);FIG.6Bshows that the measuring element12measures the first processed curved surface E1and the second processed curved surface E2of the workpiece11at a second rotation angle θ2along the direction of the third linear axis (Z-axis).FIG.6Cshows that the measuring element12measures the first processed curved surface E1and the second processed curved surface E2of the workpiece11along the direction of the third linear axis (Z-axis) at a third rotation angle θ3.

To be more specific, the measuring element12can be fixed to an end of the main shaft of the machine tool. As shown inFIG.6A, the workpiece11is rotated around the second rotation axis (A-axis) by the first rotation angle θ1, where the so-called rotation angle is the angle relative to a horizontal line L with the stage P rotating along the second rotation axis (A-axis), TakingFIG.6Aas an example, the first rotation angle θ1is, but not limited to, 0 degree. As shown in the upper half ofFIG.6A, the measuring element12contacts with the workpiece11at the first rotation angle θ1along the direction of the third linear axis (Z-axis) and measures a read value of the depth of the first processed curved surface E1. As shown in the lower half ofFIG.6A, the measuring element12contacts with the workpiece11at the first rotation angle θ1along the direction of the third linear axis (Z-axis) and measures a read value of the depth of the second processed curved surface E2. As shown inFIG.6B, the workpiece11is rotated around the second rotation axis (A-axis) by a second rotation angle θ2, in this condition, the second rotation angle θ2is an acute angle relative to the horizontal line L with the stage P rotating around the second rotation axis (A-axis) counterclockwise. As shown in the upper half ofFIG.6B, the measuring element12contacts with the workpiece11at the second rotation angle θ2along the direction of the third linear axis (Z-axis) and measures the depth of the first processed curved surface E1. Then, as shown in the lower half ofFIG.6B, the measuring element12contacts with the workpiece11at the second rotation angle θ2along the direction of the third linear axis (Z-axis) and measures a read value of the depth of the second processed curved surface E2. As shown inFIG.6C, the workpiece11is rotated by a third rotation angle θ3with the second rotation axis (A-axis), in this condition, the third rotation angle θ3is an acute angle relative to the horizontal line L with the stage P rotating around the second rotation axis (A-axis) clockwise. As shown in the upper half ofFIG.6C, the measuring element12contacts with the workpiece11at the third rotation angle θ3along the direction of the third linear axis (Z-axis) and measures a read value of the depth of the first processed curved surface E1. As shown in the lower half ofFIG.6C, the measuring element12contacts with the workpiece11at the second rotation angle θ2along the direction of the third linear axis (Z-axis) and measures a read value of the depth of the second processed curved surface E2.

Therefore, record the above-mentioned read values to calculate the depth difference between the two curved surfaces (namely, the first processed curved surface E1and the second processed curved surface E2) formed from the workpiece11by the milling at the same rotation angle. Specifically, the first depth difference between the depth of the first processed curved surface E1and the depth of the second processed curved surface E2at the first rotation angle θ1can be calculated by the processing control element. Also, the second depth difference between the depth of the first processed curved surface E1and the depth of the second processed curved surface E2at the second rotation angle θ2can be calculated by the processing control element. Besides, the third depth difference between the depth of the first processed curved surface E1and the second processed curved surface E2at the third rotation angle θ3can be calculated by the processing control element. Therefore, a second dimensional error including at least the first depth difference, the second depth difference and the third depth difference can be obtained.

In the step S170, the positional error of the second rotation axis (A-axis) is calculated according to the second dimensional error to calibrate the rotation center of the machine tool. Specifically, firstly obtain coordinates of a first contact point, a second contact point and a third contact point between the measuring element12and the first processed curved surface E1of the workpiece11being at the first rotation angle θ1, the second rotation angle θ2and the third rotation angle θ3respectively. The coordinates of the first contact point, the second contact point and the third contact point between the first processed curved surface E1by the first milling and the measuring unit12can be directly obtained by the processing control unit of the machine tool. Then, the processing control element is configured to calculate a coordinate of center of circle corresponding to the first processed curved surface E1according to the first contact point, the second contact point and the third contact point. The circle formula can be obtained by substituting the coordinates of the first, second and third contact points into the general formula of a circle: y2+z2+a1y+b1z+c1=0, and then constants a1, b1and c1can be known. Then, the general formula of the circle can be converted into (y−d1)2+(z−e1)2=r12to obtain the Z-Y coordinates (d1, e1) of the center of the circle corresponding to the first processed curved surface E1. The processing control element is further configured to calculate a coordinate of a fourth contact point between the measuring element12and the second processed curved surface E2of the workpiece11being at the first rotation angle θ1according to the coordinate of the first contact point and the first depth difference. The processing control element is further configured to calculate a coordinate of a fifth contact point between the measuring element12and the second processed curved surface E2of the workpiece being at the second rotation angle θ2according to the coordinate of the second contact point and the second depth difference. The processing control element is further configured to calculate a coordinate of a sixth contact point between the measuring element12and the second processed curved surface E2of the workpiece11being at the third rotation angle θ3according to the coordinate of the third contact point and the third depth difference. The coordinates of the fourth, fifth and sixth contact points can be obtained respectively based on the first, second and third contact points plus the trigonometric functions (sine, cosine) of the first, second and third depth differences with the first rotation angle θ1, the second rotation angle θ2and the third rotation angle θ3and then by converting the Z′-Y′ coordinate system shown inFIG.6A-6Cinto the components of the Z-Y coordinate system shown inFIG.6A-6C. By substituting the coordinates of the fourth, fifth and sixth contact points into the general formula of the circle: y2+z2+a2y+b2z+c2=0, the constants of a2, b2, c2can be known. After knowing the general formula of the circle, convert it to (y−d2)2+(z−e2)2=r22to obtain the Z-Y coordinates (d2, e2) of the center of the circle corresponding to the second processed curved surface E2. Then, just compare the Z-Y coordinate of the center of the circle corresponding to the first processed curved surface E1, such as (d1, e1), and the Z-Y coordinate of the center of the circle corresponding to the second processed curved surface E2, such as (d2, e2), so as to calculate the Z-axis difference (d2minus d1) and Y-axis difference (e2minus e1). The Z-axis difference and the Y-axis difference correspond to the positional error YOA and the positional error ZOA of the second rotation axis (A-axis). The obtained positional error YOA and positional error ZOA are the linear offsets between the actual installation center of the A-axis of the machine tool and the ideal position in the Y and Z directions. Therefore, the offsets between the positional error YOA and the positional error ZOA can be reversely compensated by the processing control unit. For example, the origin (namely, the virtual rotation center V or the real rotation center W) of the second rotation axis (A axis) adds/subtracts the positional error YOA and the positional error ZOA, thereby calibrating the rotation center of the second rotation axis (A axis) of the machine tool to an ideal position.

The present invention provides a calibration method and calibration system for machine tools, which can achieve that there is no need to use expensive instruments or special jigs/fixtures during the process of measuring the error of the rotation center and that only the simplest dial indicator is needed to measure the workpiece and then use the processing control element to obtain the rotation center error with uncomplicated mathematics, thereby perform calibration.

While the invention has been described by way of example and in terms of the preferred embodiment(s), it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.