Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
2002-04-25
2004-02-03
Picard, Leo (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C451S048000, C409S066000
Reexamination Certificate
active
06687566
ABSTRACT:
This application claims priority on Japanese Patent Application No. 2001-131833 filed on Apr. 27, 2001 the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of machining a screw-shaped portion or screw groove in a workpiece, particularly, when a female screw groove is ground by means of a machine tool, such as a grinder.
2. Description of the Related Art
Various methods for machining a female (or internal) screw thread-groove by grinding are, as schematically shown in
FIGS. 12A-C
, already known. In the method shown in
FIG. 12A
, an internal grinding wheel spindle
101
is tilted relative to a workpiece
102
at the same angle as the lead angle &agr; of the screw groove while the workpiece is machined with a grinding wheel
103
mounted on one end of a grinding wheel quill
104
. According to this method, the grinding wheel
103
is shaped by a form rotary dresser that has an identical contour to that of the female thread-groove to be machined.
In the method shown in
FIG. 12B
, the internal grinding wheel spindle
101
is tilted relative to the workpiece
102
at an angle, &xgr;, which is smaller than the lead angle &agr; of the screw thread while the workpiece is machined with a grinding wheel
103
. In this case, the grinding wheel
103
is machined with a form rotary dresser whose shape corresponds to the shape of the female groove to be machined. More particularly, after the shape of the female thread-groove is determined, numerous form rotary dressers are machined until one with an optimal shape is obtained through corrections and adjustments.
The method illustrated in
FIG. 12C
is disclosed in Japanese Published Examined Patent Application No. 8-11329, in which the internal grinding wheel spindle
101
is disposed in parallel to the axis of the workpiece
102
during the machining of the desired female groove. According to this method, the shape of the grinding wheel
103
is first calculated from data representing specifications of the female thread-groove. The grinding wheel
103
is then machined by a single point dresser under synchronous numerical control on the basis of data representing the calculated grinding wheel shape. When the lead angle &agr; is large or when deep grinding is required, this method has the advantage over the one shown in
FIG. 12A
of being capable of avoiding interference between the grinding wheel quill
104
and the female screw groove.
While achieving their intended objectives, the foregoing conventional methods suffer from a number of deficiencies that reduce their utility. For example, the method of
FIG. 12A
, which is very commonly practiced, has an inherent problem in that a female screw groove cannot be machined if the lead angle &agr; of the screw thread is very large or if deep grooving is required, as the wheel quill
104
interferes with the thread-groove, thus limiting the machinable ranges.
The method of
FIG. 12B
is often employed if the lead angle &agr; of the female screw thread is relatively large with respect to the hole diameter bored in the workpiece. In contrast to the method of
FIG. 12A
, this particular method reduces the interference between the grinding wheel quill
104
and the hole in the workpiece, thereby enabling machining of female screws with larger lead angles &agr; and greater depths. However, machining female screws by this method proves a time-consuming process as much trial and error is required to determine a suitable shape for the grinding wheel.
If the third known method (
FIG. 12C
) is employed to grind a female screw with a large lead angle &agr;, considerable interference occurs between the female screw groove and the grinding wheel
103
, requiring reduction in the diameter of the grinding wheel and thus the diameter of the wheel quill
104
.
SUMMARY OF THE INVENTION
In view of the above-identified problems, an important object of the present invention is to provide a method of correctly shaping an internal grinding wheel without trial and error and machining a female screw in a workpiece, while the axis of the wheel spindle tilted relative to the axis of the workpiece at an angle smaller than the lead angle of the screw thread.
Another important object of the present invention is to provide a method of grinding a female screw with an internal grinding wheel spindle machined without trial and error as above.
Another important object of the present invention is to provide a method of dressing, without trial and error, an internal grinding wheel suitable for machining a female screw groove in a workpiece while the axis of the wheel spindle tilted relative to the axis of the workpiece at an angle smaller than the lead angle of the screw thread to be machined.
The above objects and other related objects are realized by the invention, which provides a method of machining a female screw groove in a hole in a female screw workpiece extending along a z-axis with an internal grinding wheel coupled to a spindle having an axis, with the axis of the spindle being tilted relative to the z-axis at an angle smaller than the lead angle of the female screw thread, whereby relative rotation is caused between the workpiece and the grinding wheel while relative movement is caused between the grinding wheel and the workpiece along the z-axis and along an x-axis normal to the z-axis. The method comprises the steps of: (A) providing data representing specifications of the female screw and the tilt angle of the axis of the wheel spindle relative to the z-axis; (B) obtaining data on dressing the grinding wheel from the data representing the specifications of the female screw and the tilt angle of the wheel spindle axis; (C) dressing the grinding wheel on the basis of the obtained dressing data by simultaneous triaxial control of relative movement between the grinding wheel and a single point dresser along the z-axis of the workpiece and the x-axis, and rotation of a tip of the single point dresser about a b-axis in the plane defined by the x- and z-axes; and (D) machining the female screw groove with the dressed grinding wheel.
According to the conventional method whereby the spindle axis is maintained parallel to the axis of the workpiece, the diameter of the grinding wheel must be decreased as the lead angle of the female screw thread increases, which in turn reduces the diameter of the wheel spindle, thus sacrificing the rigidity of the spindle. According to the method of the invention, however, the spindle of the internal grinding wheel is disposed at a smaller angle relative to the axis of the spindle than the lead angle of the female screw thread throughout the machining operation, such that a relatively large-diameter grinding wheel with an accordingly large-diameter spindle can be employed so as to prevent reduction in the spindle rigidity resulting from a small spindle diameter.
According to one aspect of the present invention, the grinding wheel has a width along the axis of the wheel spindle and the workpiece is machined to a female ball screw with a Gothic arc profile, and the specifications of the female screw includes data representing: the lead of the female screw; the radius of the Gothic arc; a Gothic arc offset value; the diameter of the female ball screw as measured between the centers of diametrically opposing balls; and the diameter of the cross-section of the grinding wheel located at the center of the wheel's width and oriented normal to the wheel spindle axis.
According to another aspect of the present invention, the grinding wheel has a diameter that changes along the axis of the wheel spindle, and additionally, the step of obtaining data for dressing the grinding wheel includes the steps of:
a) calculating the distance from the center of a ball to a bottom of the screw groove and the coordinates of the center of the grinding wheel on a cross-section normal to the wheel spindle axis;
b) determining the polar coordinates of the locus of the female screw groove on a cross-section normal
Nishimoto Takashi
Terasaki Fumitoshi
Cabrera Zoila
Foley & Hoag LLP
Okuma Corporation
Picard Leo
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