FORM 2 THE PATENTS ACT, 197 0 (39 of 1970) & THE PATENTS RULES, 2 003 COMPLETE SPECIFICATION [See section 10, Rule 13] TOOL-PATH GENERATION APPARATUS AND METHOD; MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANIZED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 100-8310 JAPAN THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. DESCRIPTION Field [0001] The present invention relates to a tool-path generation apparatus and a tool-path generation method that can reduce a machining time and extend the lifetime of a tool by combining a pocket part that is defined by an entire machining-area shape defined on a two-dimensional plane and a depth with a spiral path and a trochoidal path, Background [0002] As a tool-path generation apparatus for machining a concave part that is defined by an entire machining-area shape defined on a two-dimensional plane and a depth, that is, a pocket part, a tool-path generation apparatus that generates a spiral machining path for the largest circle part in the entire machining-area shape and automatically generates a trochoidal machining path in which a machining path and a non-machining path are repeated for a part of the entire machining-area shape other than the largest circle has been conventionally known (see, for example, Patent literature 1). [0003] The tool-path generation apparatus described above can suppress a machining load on a tool, and thus has an advantage such that high efficient machining that effectively uses a cutting edge length of a tool can be performed. Particularly in the spiral path, a machining state is maintained and thus machining is performed at a higher efficiency in the spiral path than that in the trochoidal path in which a machining state and a non-machining state are repeated. Citation List Patent Literature [0004] Patent Literature 1: Japanese Patent Application Laid-open No. 2002-283118 Summary Technical Problem [0005] However, in the conventional technique described above, an efficient spiral path is applied to only the largest circle part in the entire machining-area shape, and thus there is a problem that efficiency improvement by automatically applying a plurality of spiral paths according to the entire machining-area shape cannot be realized. [0006] The present invention has been achieved in view of the above problems, and an object of the invention is to provide a tool-path generation apparatus and a tool-path generation method that can automatically generate a plurality of spiral tool paths according to an entire machining-area shape. Solution to Problem [0007] To solve the above described problem, the present invention includes a tool-path generation apparatus that generates a tool path for forming a concave part that is defined by an entire machining-area shape and a depth into a machining material. The tool-path generation apparatus includes: a reference-circle generation unit that extracts a plurality of circular areas that satisfy a preset condition from the entire machining-area shape; a first machining-path generation unit that generates a first tool path for machining the circular areas extracted by the reference-circle generation unit or an area that includes a circumference of the circular areas by using a spiral path and a machining-area shape after spiral machining in which a machining area by the first tool path is removed from the entire machining-area shape; and a second machining-path generation unit that generates a second tool path for machining the machining-area shape after spiral machining. Advantageous Effects of Invention [0008] The tool-path generation apparatus and the tool-path generation method according to the present invention can automatically generate a plurality of spiral tool paths according to an entire machining-area shape, and thus the machining efficiency can be improved. Brief Description of Drawings [0009] FIG. 1 shows a configuration of a tool-path generation apparatus according to an embodiment of the present invention. FIG. 2 is a flowchart of a flow of an operation of the tool-path generation apparatus according to the embodiment. FIG. 3 shows an example of an entire machining-area shape. FIG. 4 shows an example of a medial axis obtained by medial axis transform. FIG. 5 shows an example of an inscribed circle that is an extraction candidate. FIG. 6 shows an example of circle data to be extracted. FIG. 7 shows an example of a hole machining path. FIG. 8 shows generation of spiral machining. FIG. 9 shows an example of an area shape that is a machining target in a trochoidal machining. FIG. 10 shows an example of a machining path for a trochoidal machining. FIG. 11 shows an example of a tool path as an output result. FIG. 12 shows an example of a tool path generated by a tool-path generation apparatus disclosed in Patent Literature 1. FIG. 13 shows an example of a case of extracting a circle that is not tangent to the contour of an entire machining-area shape at two points. Description of Embodiments [0010] Exemplary embodiments of a tool-path generation apparatus and a tool-path generation method according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. [0011] Embodiment. FIG. 1 shows a configuration of a tool-path generation apparatus according to an embodiment of the present invention.- A tool-path generation apparatus 50 according to the present embodiment includes a machining-area-shape input unit 1, a machining-condition input unit 2, a tool-path generation unit 3, a machining-area-shape storage unit 20, and a machining-condition storage unit 21. [0012] The machining-area-shape input unit 1 receives an external input of an entire machining-area shape data that defines the shape of the entire machining-area and stores the received data in the machining-area-shape storage unit 20. [0013] The machining-condition input unit 2 receives external inputs of data such as: the depth of a machining part; a machining method of a hole where spiral machining starts; the radius of a hole; a machining time per hole; the diameter of an end mill tool used for helical machining, spiral machining, and trochoidal machining; parameters for generating a spiral machining path and a trochoidal machining path; a feed speed in a path where machining is performed in a spiral machining path or in a trochoidal machining path; a feed speed in a path where machining is not performed in a trochoidal machining path; and a feed speed in a path between spiral machining paths, and stores the data in the machining-condition storage unit 21. Examples of the machining method of a hole where spiral machining starts include drill machining by a drill tool and helical machining by an end mill tool. Examples of the parameters for generating a spiral machining path and a trochoidal machining path include a cutting amount in a tool radius direction and a contact angle of a tool with respect to a machining material. [0014] The tool-path generation unit 3 includes a spiral-machining-path reference-circle generation unit 4, a hole-machining-path generation unit 5, a spiral-machining-path generation unit 6, a trochoidal-machining-path generation unit 7, a tool-path output unit 8, a spiral-machining-path reference-circle storage unit 22, a trochoidal-machining-area shape storage unit 23, a tool-path storage unit 24, and a control unit 25. The tool-path generation unit 3 controls the order of performing the respective functional units, thereby generating tool paths for hole machining, spiral machining, and trochoidal machining and outputting the tool paths to outside. [0015] The spiral-machining-path reference-circle generation unit 4 serving as a reference-circle generation unit generates, in response to an execution instruction from the control unit 25, circle data that is the reference of a spiral machining path based on the entire machining-area shape data stored in the machining-area-shape storage unit 20 and the machining condition data stored in the machining-condition storage unit 21, and stores the circle data in the spiral-machining-path reference-circle storage unit 22. [0016] The hole-machining-path generation unit 5 generates, in response to an execution instruction from the control unit 25, machining path data for forming a hole where spiral machining starts based on circle data stored in the spiral-machining-path reference-circle storage unit 22 and machining condition data stored in the machining-condition storage unit 21, and stores the machining path data in the tool-path storage unit 24. [0017] The spiral-machining-path generation unit 6 serving as a first machining-path generation unit generates, in response to an execution instruction from the control unit 25, spiral machining path data as a first tool path based on the entire-machining-area shape data stored in the machining-area-shape storage unit 20, the circle data stored in the spiral-machining-path reference-circle storage unit 22, and the machining condition data stored in the machining-condition storage unit 21, and stores the spiral machining path data in the tool-path storage unit 24. Further, the spiral-machining-path generation unit 6 generates data of a machining-area shape after spiral machining serving as a trochoidal machining target, in which a machining-area shape formed by a generated path is removed from the entire machining-area shape, and stores the data in the trochoidal-machining-area shape storage unit 23. [0018] The trochoidal-machining-path generation unit 7 generates, in response to an execution instruction from the control unit 25, trochoidal machining path data as a second tool path based on the data of a machining-area shape after spiral machining stored in the trochoidal-machining-area shape storage unit 23 and the machining condition data stored in the machining-condition storage unit 21, and stores the data in the tool-path storage unit 24. [0019] The tool-path output unit 8 outputs the machining path data stored in the tool-path storage unit 24 to outside in response to an execution instruction from the control unit 25. [0020] The machining-area-shape storage unit 20 stores therein the entire machining-area shape data inputted to the machining-area-shape input unit 1. [0021] The machining-condition storage unit 21 stores therein the machining condition data inputted to the machining-condition input unit 2. [0022] The spiral-machining-path reference-circle storage unit 22 stores therein the circle data generated in the spiral-machining-path reference-circle generation unit 4. [0023] The trochoidal-machining-area shape storage unit 23 stores therein the data of the machining-area shape after spiral machining generated in the spiral-machining-path generation unit 6. [0024] The tool-path storage unit 24 stores therein the machining path data generated in each of the hole-machining-path generation unit 5, the spiral-machining-path generation unit 6, and the trochoidal-machining-path generation unit 7. [0025] The control unit 25 transmits an execution instruction to each of the spiral-machining-path reference-circle generation unit 4, the hole-machining-path generation unit 5, the spiral-machining-path generation unit 6, the trochoidal-machining-path generation unit 7, and the tool-path output unit 8, thereby controlling the operating order of the respective units. [0026] FIG. 2 is a flowchart of a flow of an operation of the tool-path generation apparatus according to the embodiment. First, data that defines an entire machining-area shape is input from outside to the machining-area-shape input unit 1 and is stored in the machining-area-shape storage unit 20 (Step S201). The data that defines the entire machining-area shape is data such as the type, coordinate, and dimension of a shape element that forms the contour shape of an area. As a method of inputting data from outside to the machining-area-shape input unit 1, an operator can input data by operating a keyboard and the like or a specified part of CAD (Computer Aided Design) data can be converted. [0027] FIG. 3 shows an example of an entire machining-area shape. In the present embodiment, the entire machining-area shape is a shape in which two rectangular areas with rounded corners are connected to each other by a trench area. Data for forming a concave part N of the shape in which two rectangular areas with rounded corners are connected to each other by a trench area in a machining material 40 is stored in the machining-area-shape storage unit 20 as the entire machining-area shape data that defines the entire machining-area shape. It is assumed that the depth of the concave part N is a fixed value. [0028] Next, machining condition data is input from outside to the machining-condition input unit 2 and is stored in the machining-condition storage unit 21 (Step S202). The machining condition data is input from outside by an operator operating a keyboard and the like or by a host system (such as a CAM (Computer Aided Manufacturing) device and a numerical controller). [0029] The tool-path generation unit 3 generates circle data that is the reference of a spiral machining path in the spiral-machining-path reference-circle generation unit 4 and stores the data in the spiral-machining-path reference-circle storage unit 22 (Step S203). [0030] As a method of generating circle data, for example, medial axis transform that is generally known can be used. By medial axis transform, a medial axis in which the center points of inscribed circles that are tangent to a given contour shape at two or more points gathers and the radius of the inscribed circle at each point on a center line can be obtained. FIG. 4 shows an example of a medial axis obtained by medial axis transform. In FIG. 4, a medial axis MA obtained by performing medial axis transform on the machining-area-shape data of the concave part N shown in FIG. 3 is shown. A point on the medial axis MA indicates a position where the radius of an inscribed circle is increased or reduced, that is, the radius of the inscribed circle becomes the maximum value or the minimum value. The center of the inscribed circle with the largest radius that is explained later is at a position where the radius of the inscribed circle becomes the maximum value or a position where the radius of the inscribed circle becomes the minimum value. [0031] At Step S203, circle data is extracted according to the following procedures. (a) An inscribed circle with the largest radius is extracted as a first inscribed circle from a plurality of inscribed circles based on information (specifically, a medial axis and the radius of an inscribed circle) obtained by medial axis transform. (b) Among from a second inscribed circle that is tangent to the contour of the entire machining-area shape at three or more points and a third inscribed circle that does not overlap with the second inscribed circle and is tangent to the contour of the entire machining-area shape at two points, an inscribed circle with the largest radius in inscribed circles that has a radius larger than a predetermined value and does not overlap with the extracted first, second, and third inscribed circles is extracted. (c) As a result of (b) described above, if there is no inscribed circle to be extracted, the extraction process ends, and if there is an inscribed circle to be extracted, the process returns to the procedure (b). [0032] In the procedure (b) described above, an inscribed circle that is tangent to the contour of the entire machining-area shape at three or more points is set as an extraction candidate. This is because the inscribed circle that is tangent to the contour of the entire machining-area shape at three or more points may become an inscribed circle with the largest radius locally. Further, an inscribed circle that does not overlap with an inscribed circle that is tangent to the contour of the entire machining-area shape at three or more points and that is tangent to the contour of the entire machining-area shape at two points is set as an extraction candidate. This is because there is a sufficient space between inscribed circles that are tangent to the contour of the entire machining-area shape at three or more points and by applying spiral machining to an inscribed circle in the space, the efficiency can be improved. FIG. 5 shows an example of an inscribed circle that is an extraction candidate. In FIG. 5, the entire machining-area shape is a concave part that is an elongated hole. As shown in FIG. 5, if there is a sufficient space between inscribed circles C4 and C5 that are tangent to the contour of the entire machining-area shape, an inscribed circle C6 in the space is set as an extraction candidate. [0033] In the procedure (b) described above, an inscribed circle to be extracted is limited to an inscribed circle that has a radius larger than a predetermined value. This is because, to secure a spiral machining allowance, the radius of an inscribed circle needs to be larger, to some extent, than that of a hole from which a spiral machining starts. For example, the predetermined value is calculated as follows by using a radius RH of a hole and a diameter DEM of an end mill tool stored in the machining-condition storage unit 21. [0034] predetermined value =RH+KxDEM •••(1) [0035] In the above formula (1), K is a constant larger than 0. If K is set to be large, the lower limit value of the radius of an inscribed circle to be extracted becomes large. Therefore, spiral machining can be performed only on an area that has a certain size and the effect of efficiency improvement by spiral machining can be increased. However, if K is too large, the number of inscribed circles that are extracted as candidates is reduced and the effect of efficiency improvement by spiral machining is reduced. Therefore, it is desirable to appropriately set K according to the entire machining-area shape and machining conditions. [0036] In the procedure (b) described above, an inscribed circle to be extracted is limited to an inscribed circle that does not overlap with inscribed circles that have already been extracted. This is because of the following reason. If spiral machining areas overlap with each other, machining is not performed during a subsequent tool movement, resulting in a decrease in the efficiency. However, even if an inscribed circle slightly overlaps with the already extracted inscribed circle, it is presumed that the effect of efficiency improvement by spiral machining is increased. Therefore, whether there is an overlap may be determined by the following condition formula.. [0037] Assuming that the position of the center of an extracted inscribed circle is denoted as PE, the radius of an extracted inscribed circle is denoted as RE, the position of the center of an extraction-candidate inscribed circle is denoted as PC, the radius of an extraction-candidate inscribed circle is denoted as RC, and RE>RC, if the following formula (2) is satisfied, it is determined that there is no overlap. [0038] RE+RC-H