Numerically controlled curved surface machining unit

Details Numerically controlled curved surface machining unit Numerically controlled curved surface machining unit

2003-05-14

2004-01-06

Picard, Leo (Department: 2125)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C700S187000, C318S568150, C345S442000, C345S420000

Reexamination Certificate

active

06675061

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a numerically controlled curved surface machining unit; and, more particularly, the invention relates to a unit that provides improved machining surface roughness and accuracy and makes it possible to achieve high-speed machining.
In conventional numerically controlled (NC) curved surface machining, since a workpiece is machined by linear approximation, as shown in FIG.
2
(
a
), the machining surface roughness is poor and a number of hand-finishing steps are needed. Besides, as shown in FIG.
2
(
b
), since the average feed rate decreases as a result of acceleration or deceleration during positioning, a long machining time is needed and, what is worse, there is a problem in that a vast amount of NC data at shorter pitches are required to improve the machining surface accuracy.
Japanese Application Patent Laid-Open Publication No. HEI 9-114512 (1997) proposes a method of performing curved surface machining using a NC machine tool for the purpose of improving the machining surface accuracy and decreasing the NC data volume. Even with this conventional method, however, there is still a problem of poor machining surface roughness due to linear approximation and lower average feed rate due to acceleration or deceleration during positioning.
In order to solve these problems, interpolation by a NURBS curve, as shown in
FIG. 3
, has been proposed. A NURBS (Non-Uniform Rational B-Spline) curve is a kind of B-spline curve that is expressed by a rational expression wherein the pitches of the nodal points constituting the curve are not uniform. It is a characteristic feature that the NURBS curve uses a rational expression in defining a curve, while other curves use a polynomial.
By controlling these, a curve can be locally transformed without difficulty. Besides, it becomes possible to uniformly handle shapes, such as a cylinder, cone, s sphere, hyperbola, ellipse, and parabola, that cannot be expressed accurately by other curves.
In
FIG. 3
, the NURBS curve defines a curve by the control point Pi, weights wi, and knot vector xi; where k is degree, Pi is control point, wi is weights, xi is knot (xi≦xi+1), [x
0
, x
1
, . . . , xm](m=n+k) is knot vector, and t is spline parameter.
When a B-spline basis function N(t) is expressed by a de Boor-Cox recursive expression, Expressions 1 and 2 are obtained. The NURBS curve P (t) for interpolation results in Expression 3.
N
i
,
l
⁡
(
t
)
=
{
1
⁢
(
x
i
≤
x
i
+
1
)
0
⁢
(
t
<
xi
,
x
i
+
1
≤
t
)
Expression
⁢
 
⁢
1
N
i
,
k
⁡
(
t
)
=
(
t
-
x
i
)
⁢
N
i
,
k
-
1
⁡
(
t
)
x
i
+
k
-
1
-
X
i
+
(
x
i
+
k
-
t
)
⁢
N
i
+
1
,
k
-
1
⁡
(
t
)
x
i
+
k
-
X
i
+
1
Expression
⁢
 
⁢
2
P
⁡
(
t
)
⁢
∑
i
=
0
n
⁢
 
⁢
N
i
,
k
⁡
(
t
)
⁢
w
i
⁢
zP
i
∑
i
=
0
n
⁢
 
⁢
N
i
,
k
⁡
(
t
)
⁢
w
i
Expression
⁢
 
⁢
3
(
X
k−1
≦t≦X
m−k+1
)
A NURBS interpolation instruction is outputted in the following format. G05P10000; (high-accuracy continuous tool path control mode ON)
. . .
G06.2
[P_]K_X_Y_Z_&agr;_&bgr;_[R-][F_];
K_X_Y_Z_&agr;_&bgr;_[R_];
K_X_Y_Z_&agr;_&bgr;_[R_];
K_X_Y_Z_&agr;_&bgr;_[R_];
. . .
K_X_Y_Z_&agr;_&bgr;_[R_];
K_;
. . .
K_;
G01 . . .
. . .
G05PO; (high-accuracy continuous tool path control mode OFF)
where;
G06.2: NURBS interpolation mode ON
P: Degree of NURBS curve
K_X_Y_Z_&agr;_&bgr;_: Control point (&agr;, &bgr;: Rotary axis instruction)
R: Weights
K: Knot
F: Feed rate
In NURBS interpolation machining, since a curve can be smoothly machined, as shown in FIG.
2
(
c
), less hand-finishing steps are needed. In addition, since acceleration and deceleration during positioning becomes smooth and the average feed rate increases, as shown in FIG.
2
(
d
), the machining time can be shortened and high-speed machining becomes possible. Further, it is said to be advantageous that, since the control points for the NURBS interpolation can be set effectively, the required NC data volume can be less.
With the conventional NURBS (non-uniform rational B-spline) interpolation method, simultaneous machining on three linear axes has been possible for the purpose of machining a mold. On pages 12-17 of “Machines and Tools” (February 1998 issue), is an article entitled “High-Speed High-Accuracy Machining by NURBS Interpolation and Smooth Interpolation”, a machining method with an enhanced function up to simultaneous 5-axis machining including two rotary axes is described for high-efficiency machining of a turbine blade, hydraulic turbine impeller, or the like.
On pages 8-9 of “Mold Engineering” (July 1998 issue), in an article entitled “Generation of High-Quality Machining Surface by Additional Axis NURBS Interpolation Machining”, a method of machining a turbine blade under control of software using a simultaneous 5-axis NURBS interpolation function is described. Since the chord length between the knot vectors is uniform in this machining method, there is a problem in that less consideration is given to the control of a curve using the knot vector, which is a characteristic feature of a NURBS curve.
According to this article, the control points on a NURBS curve calculated on a workpiece coordinate system are transformed into a machine coordinate system in accordance with the tool axis vector, and the result is employed as the control points for 5-axis NURBS interpolation without any compensation and the same knot vector as used on a workpiece coordinate system is applied to the 5-axis NURBS interpolation.
Generally speaking, however, there is no guarantee of achieving a smooth curve even if the same knot vector is employed after the coordinate transformation. For this reason, wind or warp is likely to be caused on a machining surface.
In addition, the method described in this article handles ball end mill machining where the offset of a contact point between the tool control point and the curved surface is small, and it does not handle radius end mill machining where the offset of a contact point between the tool control point and the curved surface is big. Because of this, there remains a high possibility that wind or warp is caused depending upon the extent of the offset.
SUMMARY OF THE INVENTION
Under these circumstances, it has been desired to develop a method for calculating the knot vector and control point applicable to smooth machining of a curved surface by 5-axis NURBS interpolation using the NC data that is converted from a NURBS curve calculated on a workpiece coordinate system into a machine coordinate system in accordance with the inclination of the tool axis vector.
The object of the present invention is to provide a numerically controlled curved surface machining unit which, by moving a tool smoothly along a NURBS curve, makes it possible not only to improve the machining surface roughness and machining surface accuracy but to achieve high-speed machining so as to be able to eliminate hand finishing and reduce the number of machining steps drastically.
In order to solve the above problems, the present invention proposes a numerically controlled curved surface machining unit equipped with three linear axes and, at least, one rotary axis, including a simultaneous multiple-axis control NC machine, which is numerically controlled by a numerical control unit (NC controller) with a numerical control NURBS (non-uniform rational B-spline) interpolation function. This machining unit is provided with a means for reading the tool control point vector data and tool axis vector data, that is calculated along the tool path on a workpiece coordinate system with defined curved shape by a host computer, as cutter location (CL) data and converting the CL data into a position vector of the three linear axes and a rotation angle on a machine coordinate system so as to operate the simultaneous multiple-axis control NC machine in accordance with the machine configuration of the NC machine; a means for cal

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