JP2003170333A - Method of making tool feed path - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make an optimum feed path always allowing machining under substantially fixed cutting resistance for reduction of abrasion and damage of a cutting blade of a tool over an entire feed path of an end mill without requiring highly skilled and complicated work. <P>SOLUTION: The feed path is made by imparting a geometrical constraint so that a maximum chip thickness t<SB>m</SB>before cutting and a cutting arc length L, which are two geometrical variables of a chip part by the end mill tool, always become fixed values over the entire feed path. Accordingly, irrespective of linear machining and inner and outer arc machinings, machining can be carried out with the cutting resistance substantially fixed, and, feed speed of the tool can be kept constant. The tool feed path and an NC program are thus capable of realizing extremely rational machining. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an end mill tool mounted on an NC machine tool, which is used for machining a complicated shape such as a mold part. The present invention relates to a method for creating a tool feed path which can perform tool wear control rationally while reducing wear and damage of a cutting edge, and can perform high-efficiency machining by keeping a tool feed speed constant. . 2. Description of the Related Art A mold for molding an industrial product such as a plastic product is provided by:
It is very often manufactured by cutting a steel material as a raw material by an NC machine tool using an end mill as a tool. In manufacturing such a mold, first, shape data is created by a shape design using a CAD system, and then a process design for machining is performed based on the shape data by a CAM system, and the process is created according to the result of the process design. The NC program is realized by a procedure for causing the NC machine tool to perform a machining operation. However, in the above procedure, the conventional CAM
In the process design in the system, based on the shape data provided from the CAD system, the feed path and its pattern are determined so that the radial depth of cut is kept constant for the selected end mill tool. Shape processing is customarily performed very often.
Therefore, in order to perform actual machining using the created feed path, it is necessary to create an NC program in which machining conditions such as a feed speed are additionally set so as to be appropriate for each feed path having a change in curvature or the like. . [0004] The setting of the processing conditions such as the feed rate in the NC program is generally performed manually by the operator of the NC machine tool according to past experience. When the end mill is used as a tool, the feed path is set as the processing condition. The above feed speed, that is, the feed amount per cutting edge (mm / tooth) converted to the center of the end mill is set for each feed path. However, in setting processing conditions such as a feed rate, it is contradictory to reduce tool wear and prevent tool damage such as chipping to extend tool life, while shortening the processing time as much as possible for rationalization. There is a long-awaited need to satisfy the demands, and creating an NC program that satisfies such demands is a complicated task that requires a great deal of time even for a skilled operator having much experience. In particular, when creating an NC program for machining a die having a complicated machining shape, it is practically impossible to set an optimum feed speed for each feed route over a wide variety of feed routes. At the same time, it is a burdensome operation. [0006] Under such circumstances, when creating an NC program, priority is given to prevention of tool wear and damage, and machining conditions such as feed speed are often set lower so as to be on the safe side. Has a problem that the demand for shortening the processing time for rationalization cannot be satisfied. In order to solve such a problem, CA
Simulation of machining according to the feed path created by the M system is performed.
The amount of chip removed by the feed per tooth (in the case of the straight type end mills of the present invention, the interference between the workpiece and the tool caused by the movement of the feed per tooth of the end mill tool is rotated). The cutting speed (specific cutting resistance) is calculated by calculating the feed speed in each feed path so that the area of the projected portion when projected onto the processing surface along the axis is always constant, and creating an NC program. K (represented by the volume of chips to be removed)), and there is a method of streamlining machining while reducing and preventing wear and damage of tools. (Republished patent WO98 / 1982
No. 2) However, the method of setting a constant feed rate for each feed path is the same even in a method of stabilizing the cutting resistance by fixing the volume of the chips to be removed. . Therefore, as in the case of the manual operation, a unique problem occurs when a different feed speed is set for each path. That is, in the case of end mill cutting using a tool path including an inner arc portion having a small radius ratio (work material radius / tool radius), even if a constant feed force can be achieved for individual tool paths by providing different feed rates. ,
In the transition section to which the tool path connects, there is a problem that a rapid change in local cutting resistance may occur depending on geometric conditions and acceleration / deceleration conditions of the feed rate. For example, as shown in FIG. 1, when continuous machining is performed in the direction indicated by an arrow by an end mill E from a straight line to an inner arc portion having a small radius ratio, a different feed speed is set for each feed path and cutting is performed. Even if the resistance can be made constant, the geometrical cutting arc length of the end mill and the like at the connection part of the tool path (corresponding to the case where the center of the end mill E passes between the P points 1 and 2 in FIG. 1). Changes can cause a rapid increase in cutting force. Even if the cutting force can be made constant for each individual tool path, such local and sudden changes in cutting force at the transition of the tool path, especially the generation of excessive cutting force, can cause tool wear or damage. It is not preferable because it leads to Secondly, in a method for stabilizing the cutting resistance by making the volume of the chip to be removed constant, a chip removed by a feed amount per tooth at an arbitrary tool position is used. There is a problem of calculation load when calculating the volume of waste. Regarding this, as shown in the geometric relationship of the end face cutting of the work material by the end mill in FIG. 2, a projected portion of the chip removed by the feed amount per tooth (shown by a dark hatched portion in FIG. 2). Since is not a simple shape, trying to obtain an accurate area analytically increases the computational load and causes inefficiency in creating an NC program. This situation becomes more serious if the chip becomes a three-dimensional problem. Currently, as this area calculation method, a method is used in which a digitized grid is provided on the processing plane and the area where the projected portion of the chip overlaps is counted to approximately obtain the area, but the calculation efficiency is reduced. In order to improve the size, one side of the grid must be larger than a certain limit value. As a result, there is a problem that the error greatly affects the calculation accuracy of the cutting force. Thirdly, in the method for making the cutting resistance constant by making the volume of the chip to be removed constant, the method is based on the premise that the volume of the chip to be removed and the generated cutting It takes advantage of the fact that resistance is proportional to this, but this is not accurate. That is, according to the latest research results, the cutting force can be expressed by a second-order polynomial by two geometric variables represented by the projected portion of the chip, that is, the maximum chip thickness before cutting and the cutting arc length. You can (Fig. 3)
As shown in the figure, the contour line (solid line) of the cutting force and the contour line (dashed line) of the chip area removed by one cutting edge are:
It can be seen that the assumption that the volume of the chips to be removed and the generated cutting force are in a proportional relationship is inaccurate because they are not parallel but cross. This becomes more remarkable when the object of the end mill cutting is an inner arc portion having a small radius ratio. In Table 1 below, the maximum chip thickness tm and the cutting arc length L in the contour diagram shown in FIG.
Condition when normalized as 2 and X1 = 0, X2 = 0
2 shows the standard cutting conditions. [Table 1] Of course, based on the relationship of (generated cutting resistance = specific cutting resistance K × removed chip volume), it is also possible to use the specific cutting resistance K by changing it in each case and improving accuracy. Although it is possible, it is not rational to prepare the specific cutting resistance K for all cases because the database becomes heavy and the processing becomes complicated. In addition to the above-described method of properly creating an NC program from the feed path created by the CAM system, in order to solve the above-described problem, machining is performed according to a previously created NC program. While
An NC machine tool that detects a cutting resistance actually applied to a tool and performs machining while performing feedback correction of a numerical control command of machining conditions based on the detection result has been studied. In addition, republished patent WO 98/19822
Japanese Patent Application Laid-Open Publication No. H10-209555 detects the cutting force actually applied to a tool, simulates the machining state on the feed path ahead of the current state based on the detection result, and numerically controls the machining conditions based on this result. There is disclosed an NC machine tool that performs machining while correcting a command in a feed-forward manner. However, in the former device which performs feedback control based on the detection result of the cutting force, there is a possibility that the correction of the machining condition may be delayed when a sudden change in the cutting force occurs. Is large, excessive correction is performed in accordance with the fluctuation, and there is a problem that it is difficult to properly set a control gain to be performed to avoid such correction. On the other hand, in the latter device which performs feedforward control based on the detection result of the cutting force, the cutting device is used for detecting cutting force fluctuation due to disturbance factors such as winding of cuttings around a tool and local fluctuation of material characteristics. When the simulation is performed accordingly, there is a possibility that erroneous correction of the processing conditions may be made based on this result, and if the control gain is made small to solve this problem, when the normal cutting force fluctuation occurs Correction of machining conditions may be delayed, making it difficult to properly set the control gain. In addition, of course, both of the above have a problem in that additional equipment for detecting and controlling the cutting force is newly required for the NC machine tool. The present invention has been made in view of such circumstances, and in a CAM system or the like, two geometrical variables, which are indicated by a projected portion of a chip portion on a processing plane by an end mill tool, which is a maximum variable before cutting, is shown. The chip thickness and the cutting arc length are given a geometric constraint so as to always have a constant value regardless of the location in each part of the feed path,
By adopting a method of creating a feed route, there is an excellent feature that all the problems of the existing technology described above can be solved only by setting a route based on a geometric calculation. And by this method, the wear control of the tool can be rationally performed while reducing the wear and damage of the cutting edge of the tool, and in addition, the feed speed of the tool can be kept constant.
An object of the present invention is to provide a tool feed path and an NC program capable of realizing extremely efficient machining. According to the method of the present invention, a plurality of cutting blades are provided on the outer periphery, and the three-dimensional outer peripheral envelope of the cutting blade formed by rotation has a substantially cylindrical shape. Creates a program for NC machining in which an end mill tool is moved along a feed path set on a machining surface substantially perpendicular to the rotation axis, and the workpiece is machined into a predetermined shape by the outer peripheral blade. In this method, when an interference portion between a workpiece and a tool caused by a unit amount of movement of an end mill tool is projected on a machining surface along a rotation axis, two geometric variables indicated by the projection portion are cut. Creating a feed path or a feed path pattern by giving a geometric constraint so that the previous maximum chip thickness and cutting arc length always have a constant value regardless of location in each part of the feed path. It is characterized by. In the present invention, in the process design by the CAM system, when determining the feed path of each part of the end mill, the maximum cutting before cutting described above is performed based on the geometrical relationship with the corresponding machining surface. The feed path is calculated and determined by giving a geometric constraint such that the waste thickness and the cutting arc length always have a constant value. As will be described later, it is proved that the feed speed of each part feed path is constant as a result of applying the above-described geometric constraint without changing the part feed path. In addition, the feed speed here is a feed amount (mm / blade) per one cutting edge converted into the center of the end mill, as described above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing the embodiments. FIG. 2 shows the geometric relationship of each variable in the end face cutting of the inner circular arc portion of the work material by the end mill tool, which is the principle of the method of creating a tool feed path according to the present invention (hereinafter referred to as the present invention method). In FIG. 2, the chip thickness tm is the maximum chip thickness expected to be cut by each of the plurality of cutting blades arranged in the circumferential direction of the end mill E, and the cutting arc length L
Is the length of the portion where the outer peripheral edge of the end mill E during cutting contacts the machined surface of the workpiece 1. R is the radius of the inner arc of the work surface after machining, r is the radius of the end mill E, fz is the feed amount per cutting edge (mm / blade), and Feeding speed of end mill E (mm /
min) by the number of rotations and the number of cutting edges arranged on the circumference. Further, fze is the feed amount (mm / blade) at the edge of the end mill per one cutting edge, and αen is the cutting engagement angle, specifically, a portion where the peripheral surface of the end mill E being cut is in contact with the workpiece 1. Is the central angle of the plane in the processing plane. The following equations (1) to
The relationship shown in (5) is established. Equation (1) is based on the cosine theorem. ## EQU1 ## FIG. 4 is a perspective view showing a state of machining by an end mill tool in that case. The end mill E shown in FIG. 4 has a columnar shape with a flattened tip, extends radially from the center of the tip face, and has a plurality of cuts spirally provided on the outer peripheral face near the end face. It is a straight type end mill with a blade. The processing of the workpiece 1 by these means is performed by rotating the end mill E as indicated by the arrow in each figure and moving the end mill E in the direction indicated by the arrow. In addition, Ad indicates the axial depth of cut, Fx, Fy
Indicates the components of the cutting force applied to the end mill E in the feed direction and the direction perpendicular thereto. If the cutting force of the bottom blade is negligible compared to the cutting force of the side blade, the cutting forces Fx and Fy applied to the end mill E used for machining are considered.
Is approximately proportional to the length of the cutting depth Ad in the axial direction, and the predicted value (time average value) of the cutting force Fxy given as the resultant force is a processing condition that the cutting depth Ad is constant. Below, the size is controlled by the maximum chip thickness before cutting (hereinafter simply referred to as chip thickness) shown as tm in FIG. 2 and the cutting arc length also shown as L. Because, in the end milling of a high-hardness material in the die machining, the chip thickness becomes extremely small, and the cutting conditions are far away from the region usually used in the conventional cutting. This is because the mathematical model used for estimating the grinding resistance at the time of the grinding process should be used because it is closer to the region of the grinding process. Therefore, in the present invention, the chip thickness tm and the cutting arc length L are selected as variables governing the cutting resistance. The fact that the magnitude of the cutting resistance value is determined by the chip thickness tm and the cutting arc length L described above is based on the fact that the side surface shape of the work material to be cut is as shown in FIG. It has been experimentally found that the same applies to the inner arc portion, the outer arc portion, and the straight line as shown. (This is clear from the experimental results as shown in FIG. 8 described later.) That is, the chip thickness tm targeted by the present invention.
And the cutting arc length L are constrained to be constant, thereby making it possible to stabilize the cutting resistance value only by giving geometric processing conditions. Then, assuming that the chip thickness tm and the cutting arc length L are constant values, and substituting the equations (2) to (5) into the equation (1), the following equation (6) is obtained. . ## EQU2 ## Therefore, by giving a geometrical constraint such that the chip thickness tm and the cutting arc length L are always constant, the feed rate (feed rate at the center of the end mill) is always constant for each feed path. Was proved. That is, in this case, the feed speed fz is constant regardless of the radius R of the work material. Similarly to the inner arc portion shown in FIG. 2, similarly to the outer arc portion, a geometric relational expression equivalent to the equations (1) to (5) is established, and the equation (1) is obtained from the same geometric constraint. 6) is established. In the case of linear cutting, the geometrical relational expression becomes simpler, corresponding to the case where the work material radius R shown in FIG. 2 is infinite, as shown in the following expressions (7) to (9). Become. (Equation 3) Of course, the feed path of the tool on the machining surface is
A circle, an ellipse, a parabola, a hyperbola, etc., can be constituted by an appropriate combination of curves having various forms. For the process design by NC control, these are approximated by a single arc or a combination of a plurality of arcs, If necessary,
Since it is usual means to approximate and correspond to a combination of a single arc and a straight line or a combination of a plurality of arcs and a straight line, analysis of only the straight line and the inner and outer arcs is sufficient. FIG. 5 is a flowchart of a method of calculating and determining a feed route using the method of the present invention, which is performed in a CAM system or the like. As an example, a description will be given of a case in which a feed path for roughly removing a certain area from the inside using an inner arc cutting is created. First, the values (= constant value) of the chip thickness tm and the cutting arc length L as the reference are determined. This is based on the standard processing conditions for a straight line,
This is performed by a method such as calculation using (9). At the same time, the end mill radius r and the cutting participation angle αen are also given as constant values (step 1). Next, an inner arc radius R0 of the work material at the start of the cutting process is given (step 2). Then, the calculation is started by setting the radius of the inner circular arc after processing in the first pass for cutting the inner circular arc as R1 (step 3). That is, the difference between R1 and R0 is the radial cutting depth Rd of the end mill by the first pass. First, assuming that R1 is an unknown number, equation (4) gives R
1 and R1 is obtained (step 4,
5) Then, from the difference between R1 and R0, the radial depth of cut Rd1 of the end mill by the first pass is obtained (step 6). As a result, the feed path (first path) at the center of the end mill can be easily determined. Further, the inner arc radius after machining in the first pass is replaced with R1 as an initial inner arc radius R0 using R1 as a new work surface (step 7). The radial depth of cut Rdi is sequentially and repeatedly determined to determine the feed path (the i-th pass) at the center of the end mill (step 8). This is repeated until the radius of the inner circular arc after processing becomes larger than a desired value (step 9). As described above,
According to the method of this flowchart, as a result, a feed path that is sequentially enlarged and processed from the inside to the outside can be obtained. In the above description, an example of enlarging the area by the inner arc cutting is used. However, the present invention can be applied to the outer arc cutting from the outside to the inside in the opposite direction. The pattern of the feed path of the tool according to the present technique can have various valuations depending on the target cutting area. For example, when cutting inward from the end face of the work material, an NC program can be created by an enlarged path pattern for each layer in a semicircular shape as shown in FIG. The cutting area can be expanded in the direction of the solid arrow. In the case where the region is enlarged from the center of the work material to the outside, the calculated inner arc radius Ri and the calculated radial cut amount Rd are used.
From the relationship of 1, an NC program by a continuous spiral machining path from the center to the outside as shown in FIG. 7 may be created. As a result, as shown by the arrows in the figure, enlargement processing is performed in a spiral path by an end mill inward and outward. As an embodiment based on the method according to the present invention,
FIG. 8 shows the measured cutting resistance values when the end mill cutting was performed at a constant feed rate by the NC program using the continuous spiral processing path from the center to the outside shown in FIG. 7 described above. The values of the reference chip thickness tm and the cutting arc length L and the cutting conditions conform to the standard cutting conditions shown in Table 1. According to the figure, although the feed path 2 is formed by a series of work surfaces having different arc radii, it is clear that the cutting resistance is almost constant over the entire feed path 2. According to the latest research results, if the chip thickness tm and the cutting arc length L are made equal to make the cutting resistance constant, regardless of the feed path pattern such as linear cutting or arc cutting. It has been clarified that a substantially constant cutting distance can be obtained for the tool life. Therefore, by using the tool feed path created by the method according to the present invention, it is possible to rationally estimate the tool life in actual die machining from the experimental tool life value. That is, as described above, the feed speed of the tool determined on the basis of the method according to the present invention is such that the feed speed of each feed route is always a constant value, and By properly maintaining the cutting force applied to the end mill, tool wear can be rationally managed while reducing wear and damage to the cutting edge of the tool, and the feed rate of the tool is kept constant. This enables high-efficiency processing. As described in detail above, in the method of creating a tool feed path according to the present invention, when determining the feed path of each part of the end mill in the process design by the CAM system, the corresponding machined surface is determined. The feed path is calculated and determined based on the geometric relationship between the feed path by giving the above-mentioned maximum chip thickness before cutting and the cutting arc length a geometrical constraint that always has a constant value. In addition, since the feed speed of each feed path at that time is always a constant value, the cutting resistance applied to the end mill is appropriately maintained over the entire feed path, thereby reducing wear and damage of the cutting edge of the tool. It requires a high level of skill and complicated work for the tool feed path that can perform tool wear management rationally while reducing the tool feed rate and that enables high-efficiency machining with the tool feed speed kept constant. It can be created without having to do so.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan cross-sectional view showing a state of processing when performing continuous processing from a straight line to an inner circular arc portion by an end mill. FIG. 2 is a cross-sectional plan view showing a geometric relationship of variables representing processing conditions when an end mill is performed on an inner circular arc portion of a work material by an end mill. FIG. 3 is a contour diagram of a cutting force prediction formula expressed by a chip thickness tm and a cutting arc length L, and a contour diagram of a chip volume per one blade to be removed. FIG. 4 is a perspective view showing a state of processing by a straight type end mill. FIG. 5 is a flowchart of a method for determining a tool feed path based on the technique of the present invention. FIG. 6 is an explanatory diagram of a feed path of a tool for enlarged cutting by semicircular arc cutting based on the method of the present invention. FIG. 7 is an explanatory diagram of a feed path of a tool by continuous spiral machining for enlarging and cutting an area from a center portion of a work material to an outer side based on the method of the present invention. FIG. 8 is a diagram showing actual measurement results of cutting resistance when cutting is performed along a tool feed path by a spiral processing path created based on the method of the present invention. [Description of Signs] 1 Workpiece 2 Feeding path

Claims (1)

Claims: 1. An end mill tool having a plurality of cutting blades on its outer periphery and having a three-dimensional outer peripheral envelope of a cutting blade formed by rotation having a substantially cylindrical shape. In a method of creating a program for NC machining in which a workpiece is machined into a predetermined shape by an outer peripheral side blade by moving the workpiece along a feed path set on a vertical machining surface, a unit amount of an end mill tool is used. When the interference between the workpiece and the tool caused by the movement of the tool is projected on the machining surface along the rotation axis,
Regarding two of the maximum chip thickness before cutting, which is a geometric variable indicated by the projected portion, and the cutting arc length of the tool interference portion, the constant value is always obtained regardless of the location in each part of the feed path. Forming a feed path or a pattern of the feed path by giving a geometric constraint to the tool.