JP2850344B2 - Mold manufacturing method - Google Patents

Description

The present invention relates to a computer-aided mold design and manufacturing method.

[Summary of the Invention]

3 of the shape model of the mold designed by computer
This is a manufacturing method in which a high-precision mold capable of correcting a directional dimensional error of a molded product caused by molding by performing three-axis independent expansion / contraction correction on dimensional data is obtained.

[Conventional technology]

CAD / CAM that uses a computer to design a shape model with a free-form surface using three-dimensional data, and generates an NC program (tool path data) from the shape model to automatically process products or dies with NC machine tools. The system has been put to practical use.

When NC processing is performed on a plastic mold or an alloy mold based on a designed shape model, it is necessary to consider expansion and contraction of the mold material and the mold due to heating and cooling. For this reason, the CAD / CAM system is provided with a function of multiplying three-dimensional data of a designed shape model by a predetermined expansion / contraction ratio to compensate for a dimensional error of a molded product.

The dimensional error of a molded product caused by the expansion and contraction of a mold material or a mold due to heat generally occurs at the same expansion and contraction rate in the three axes of x, y and z. Therefore, in the conventional CAD / CAM system, uniform correction is performed on the designed shape model in three axes.

[Problems to be solved by the invention]

However, the expansion and contraction ratio may be different in each of the three axes. For example, when a molded product 10 that is flat in the z-axis direction and is not uniform in thickness as shown in FIG. 10 is molded with engineering plastic or the like, the thin portion 10a is first hardened in the mold, and the thickness is reduced. The thick portion 10b gradually hardens after the molded article 10 is discharged from the mold. For this reason, the molded article 10 having such a shape is likely to have a dimensional error mainly in the z-axis direction due to shrinkage due to cooling.

As another problem, as shown in FIG.
In the case where the mold 12 is formed by electric discharge machining, assuming that the zigzag direction is the digging direction, the convex mold 11 is vibrated in the xy direction with a small amplitude of about 0.2 mm in order to provide a discharge gap during machining. Therefore, the dimension of the finished concave mold 12 is 0.2 m only in the xy directions.
It is expanding by m.

For this reason, in order to process the concave mold 12 having the correct dimensions as indicated by the dotted line 12a, the xy dimensions of the convex mold 11 must be set in advance as indicated by the dotted line 11a.
It needs to be made small by 0.2mm. Therefore, in this case as well, it is necessary to correct the dimension only in the direction excluding the z-axis among the three axes with respect to the designed model of the convex 11.

The present invention has been made in view of the above-described problem, and has as its object to enable fine correction of a model shape independently for each axis.

[Means for solving the problem]

The mold manufacturing method according to the present invention includes the steps of: setting a three-axis orthogonal expansion / contraction ratio α, β, γ for a free-form surface given by connecting a number of patches S (u, v) formed of parametric surfaces, Performing a correction of 1 + α, 1 + β, and 1 + γ on each of the three-axis data of each control point P _ij of each patch, with respect to a directional dimensional error of a molded product caused by the molding. A modified three-dimensional model is obtained, and numerically controlled machining data of a molding die is generated based on the three-dimensional model.

Another feature of the present invention is that the three-dimensional data on the offset plane F offset from the cutting edge of the tool whose cutting target is the free-form surface to the center of the tool is 1 + α, 1 + β,
+ Gamma correction, and generating, on the offset plane, numerically controlled machining data of a molding die in which a directional dimensional error of a molded product caused by molding is corrected. I do.

[Action]

By manufacturing a mold that expands and contracts at a specified ratio independently in three axial directions, even if there is directionality in the dimensional error of the molded product due to heat shrinkage, etc., it is possible to correct the error reliably, and a highly accurate molded product can be produced. can get.

〔Example〕

FIG. 2 shows the overall configuration of the CAD / CAM system of the embodiment. In FIG. 2, a free-form surface generating unit 1 generates shape data of a geometric model representing a three-dimensional free-form surface of an object based on an input operation of an operator in a portion corresponding to CAD, and accumulates the data in a file. The target object is a machined article or a mold.

The created shape data is converted into machining data, that is, an NC program for determining a moving path of the cutting tool in the free-form surface cutting tool path generating unit 2. The machining data is dropped on a floppy disk and the NC milling machine 3 (NC
Automatic processing is performed by mounting a floppy disk on a milling machine or a machining center.

The free-form surface generating unit 1 and the free-form surface cutting tool path generating unit 2 are computers, and an input device 4 such as a keyboard or a digitizer for a user interface.
And a display device 5 such as a CRT.

The principle of the method of the present invention is shown in the three-dimensional view of the main part in FIG. 1, and the procedure for correcting the expansion / contraction ratio for the designed shape model is shown in FIG.

First, one of the curved surfaces is selected in step S1. In this example, the curved surface of the molded article 10 is designed as a Bezier curved surface which is an aggregate of quadrilateral surface elements (patches) as shown in FIG. Each patch is Is a bicubic parametric surface using parameters u, v (0 to 1) represented by the following equation, and 16 control point vectors P _ij
(I, j = 0 to 3).

Next, the expansion / contraction directions x, y, and z are selected in step S2, and the expansion / contraction rates α, β, and γ are input in step S3. In the example of FIG. 1, the expansion and contraction directions are three-axis directions of x, y, and z, and the expansion and contraction rate is z.
The axis value γ is large, for example, 0.015, and the other α-axis and α-axis values α, β are relatively small values, such as 0.007 and 0.008, respectively. Note that “all directions” may be provided as a support for the expansion / contraction direction.

Next, in step S4, the expansion and contraction ratios α, β, and γ are set for the three axis components of the 16 control point vectors P _ij (i, j = 0 to 3) that define the individual patches S (u, v). Is corrected. The corrected values of the control point vector are (1 + α) P _ijx , (1 + β) P _ijy , and (1 + γ) P _ijz . That is, as shown in FIG. 1, the original patch S (u,
For v), the correction δ in three axial directions determined by α, β, and γ, (1 + α) S _x (u, v) (1 + β) S _y (u, v) (1 + γ) S _z (u, v) A new patch S '(u, v) indicated by the dotted line is obtained. The curved surface based on the corrected patch is displayed on the display in step S5 in the form of a segment model of the patch boundary line or the like.

Therefore, with respect to the curved surface of the shape model represented by the set of modified patches, if the tool path for concave shape is formed by the tool path generating unit 2 for free-form surface cutting in FIG. In this case, a mold that has been expanded and contracted at individual ratios α, β, and γ can be processed. The dimensional error of the three axes of a molded product molded using this mold is extremely small.

Next, FIG. 5 is a schematic diagram showing another expansion / contraction rate correction method. This method corrects the expansion / contraction ratio for an offset curved surface obtained by offsetting the shape model by the tool shape. The offset plane is a plane which is offset from the surface S of the shape model by the radius r of the ball end mill BM which is a cutting tool of the NC milling machine 3 as shown in FIG. 6, and the tool path is generated on this offset plane. .

As shown in FIG. 7, the patch S (u, v) a number of sample points K _i on set in a grid pattern, determine the offset vector F _i of length r in the normal direction of the curved surface at each sample point ,
The offset plane F is represented by a polyhedron defined by an end point Q _i of each vector F _i .

Correction stretch ratio is, 1 + alpha relative to 3-axis component of the offset vector F _i, 1 + β, is performed by multiplication of 1 + gamma. As a result, as shown in FIG. 5, the correction vector δ (αx _i , βy _i , γz _i ) is added to the vector F _i , and the end point is Q _i ′.
Shift to The tool path is set on a polyhedron composed of triangular or square elements formed by connecting the end points Q _i ′. Also in this example, the correction coefficients α, β,
By individually specifying γ, the expansion and contraction ratio can be corrected independently for each axis.

Next, FIG. 8 shows a method of performing expansion / contraction correction on the three-axis data of the tool path generated by the tool path generator 2 in FIG. For example, when processing a hemispherical convex mold 11 as shown in FIG. 11, a large number of offset vectors _Fi are first set on the curved surface of the designed three-dimensional model of the convex mold 11, and the offset plane F is determined by the coordinates of the end point. And set the tool path TP on this offset plane. Since the tool path TP is generated as coordinate data of the three-axis milling machine 3, the data (x _i , y _i , z _i ) are subjected to α, β, γ expansion and contraction correction to obtain a corrected tool. Milling is performed along the path TP '(dotted line).

In the example of FIG. 8, the expansion and contraction ratio γ in the z-axis direction is zero, and the other two
Correction is performed using the expansion and contraction ratios α and β of the axes x and y to obtain the target object which is transformed into a somewhat oblong ellipsoid. By making the expansion and contraction ratios α and β negative, a vertical ellipsoid like a convex 11a in FIG. 11 can be formed.

FIG. 9 is a diagram showing a method of generating a tool path. First, as shown in FIG. 9A, each side of a quadrilateral formed by the end point Q _{i of the} offset vector and a constant plane y parallel to the xz plane. Intersection C ₁ with _i ,
Ask for C ₂ …. As shown in FIG. 9 (B), the constant plane y _i
The line connecting the upper points C ₁ , C ₂ ... Is the tool path, and each point is represented by three-axis coordinate values (x ₁ , y _i , z ₁ ) (x ₂ , y _i , z ₂ ). It is fixed.

As shown in FIG. 9 (C), the x-axis coordinate is increased with the tool movement in the x direction while the y _i value (i = 1, 2,...) Is fixed,
The tool position is controlled so that the z coordinate values z ₁ , z _2, ... corresponding to the x coordinate values x ₁ , x ₂ ,. Each time one of y _i is completed, the tool is moved step by minute width Δ,
By repeating this, curved surface cutting is performed.

The method shown in FIG. 8 is for correcting the expansion / contraction ratios α, β, and γ to the data of the point sequence C ₁ , C _2, ... Constituting the tool path TP in FIG.

〔The invention's effect〕

As described above, the present invention is characterized in that three-dimensional independent expansion / contraction correction is performed on three-dimensional data of a shape model of a model designed with the aid of a computer. Therefore, according to the present invention, due to the heat shrinkage or the coefficient of thermal expansion of the molding material, the mold, etc., a directional dimensional error in the molded product is generated due to the shape of the molded product, the characteristics of the molded material, and the like. Even in this case, a high-precision mold capable of correcting an error can be obtained by correcting the expansion and contraction rate for each of the three axes.

[Brief description of the drawings]

FIG. 1 shows 3 for a patch (pixel) constituting a free-form surface.
FIG. 2 is a block diagram of a CAD / CAM system to which the present invention is applied, and FIG.
FIG. 4 is a flowchart of expansion / contraction correction, FIG. 4 is a three-dimensional diagram showing control points of a Bezier surface representing a patch, and FIG. 5 is a three-dimensional line showing a method of performing expansion / contraction correction on an offset vector for obtaining a tool path. FIG. 6, FIG. 6 is a sectional view showing an offset vector of the ball end mill, FIG. 7 is a three-dimensional diagram showing an offset plane formed on the patch, FIG.
FIG. 9 is a cross-sectional view showing a method for performing expansion / contraction correction on a tool path, FIGS. 9A to 9C are diagrams showing a procedure of tool path generation, and FIGS.
FIG. 10 is a cross-sectional view showing a cross-sectional shape of a molded article, and FIG. 11 is a cross-sectional view showing electric discharge machining. In addition, in the code | symbol used for drawing, 1 ... Free-form surface generation process part 2 ... Tool path generation part for free-form surface cutting 3 ... NC milling machine 4 ... Input device 5 ... Display device 10 ... Molded product 11 ... ... convex 12 ... concave S (u, v) ... patch BM ... hole end mill P _ij ... control point F ... offset plane F _i ... offset vector TP ... tool path.

Claims (2)

(57) [Claims] 1. A step of setting expansion and contraction ratios α, β, and γ in three orthogonal axes for a free-form surface given by connecting a number of patches composed of parametric surfaces, and a step of setting three control points of each patch. 1 for each axis data
And performing a correction of + α, 1 + β, and 1 + γ to obtain a three-dimensional model in which a directional dimensional error of a molded product caused by mold forming is corrected, and based on the three-dimensional model, A mold manufacturing method characterized by generating numerical control processing data of a molding die. 2. A step of setting expansion and contraction ratios α, β, and γ in three orthogonal axes for a free-form surface given by connecting a number of patches composed of parametric surfaces, and a tool having the free-form surface as a cutting target. Correcting 1 + α, 1 + β, 1 + γ to the three-axis data on the offset plane offset from the cutting edge of the tool to the center of the tool. And a method for producing numerically controlled machining data of a molding die in which the modified data is generated on the offset surface.