1. Field of the Invention
The invention relates to a method for creating profile data used to process a non-circular workpiece. More specifically, the invention relates to a method according to which profile data is created based on lift data and corrections are made to the profile data, the profile data being used to control the reciprocation position of a tool in synchronization with rotation of a non-circular workpiece, for example, a cam, about its rotation axis when the workpiece undergoes a grinding process.
2. Description of the Related Art
According to an existing technology, when a non-circular workpiece, for example, a cam, is processed by a grinding machine that includes a rotating grinding wheel, lift data used to identify the shape of the workpiece is converted into profile data that indicates a control amount which is used by a numerical control unit for the grinding machine, and the workpiece is processed based on the profile data. Next, lift data will be described with reference to FIG. 4A. The lift data is used to identify the shape of a non-circular workpiece. In the lift data, a lift amount (L) is set (obtained) based on a rotation angle of the workpiece (lift data rotation angle α) (detailed description will be provided later). As shown in the left view in FIG. 4A, when the lift data rotation angle α is 0 degrees (α=0°), a contact point SP(0) between a cam W and a tappet face TM is positioned right under a rotation axis Pw of the cam W, and (α=0°, lift amount L(0)) is the lift data. When the cam W at a portion at the contact point SP(0) with the tappet face TM is ground by a grinding wheel T, the grinding wheel T is at the position shown in the left view in FIG. 4B. A spindle rotation angle (θ(α) at this position is θ(0). In this case, an inter-axis distance D(θ(0)) from the rotation axis Pw of the cam W (that is, the rotation axis of a spindle) to a grinding wheel rotation axis Pt is equal to the sum of a radius Rw of a circular portion of the cam W and a grinding wheel radius Rt. (Spindle rotation angle θ(0), inter-axis distance D(θ(0))) is the profile data that is obtained by converting the lift data (α=0°, lift amount L(0)).
As shown in the right view in FIG. 4A, when the lift data rotation angle α is 135 degrees (α=135°), the contact point SP(135) between the cam W and the tappet face TM is apart from the position right under the rotation axis Pw of the cam W, and (α=135°, lift amount L(135)) is the lift data. When the cam W at a portion at the contact point SP(135) with the tappet face TM is ground by the grinding wheel T, the grinding wheel T is at the position shown in the right view in FIG. 4B. The spindle rotation angle θ(α) at this position is θ(135). The spindle rotation angle θ(135) is a value that is larger than 135°, as shown in the right view in FIG. 4B, and is calculated based on the lift data and the grinding wheel radius Rt. The inter-axis distance D(θ(135)) is a value that is larger than the sum of the radius Rw of the circular portion of the cam W and the grinding wheel radius Rt as shown in the right view in FIG. 4B, and is calculated based on the lift data and the grinding wheel radius Rt. (Spindle rotation angle θ(135), inter-axis distance D(θ(135))) is the profile data that is obtained by converting the lift data (α=135°, lift amount L(135)).
However, various errors may be caused when the lift data is converted into the profile data. In such a case, corrections need to be made. For example, Japanese Patent Application Publication No. 1-206406 (JP-A-1-206406) describes a numerical control unit for processing a non-circular workpiece. The numerical control unit obtains corrected lift data by smoothing lift data based on a permissible error for the shape of a finished product, and converts the corrected lift data into profile data. In this way, an error in the shape of the finished product falls within a permissible error range and the workpiece is processed at a higher speed and within a relatively short time. Japanese Patent Application Publication No. 9-160623 (JP-A-9-160623) describes a numerical control unit for processing a workpiece. The numerical control unit creates the profile data in a manner similar to that described in JP-A-1-206406. However, the numerical control unit described in JP-A-9-160623 designates a range in which lift data is smoothed. In this way, the time required to calculate the corrected lift data is reduced, which makes it possible to process a workpiece in a shorter time.
According to the technologies described in JP-A-1-206406 and JP-A-9-160623, the lift data is corrected and the corrected lift data is converted into the profile data. However, the profile data obtained by converting the corrected lift data are not smoothly continuous, in some cases. For example, if the lift data arranged in the order of the lift data rotation angle are sequentially converted into the profile data, the manner in which the spindle rotation angle is changed may be reversed at part of profile data. If the workpiece is processed by the grinding machine without making any corrections, a portion of the cam W, which is within the range in which the spindle rotational direction is reversed, is ground each time the spindle rotational direction is reversed. This may affect the accuracy of the shape of the finished product. Even if the profile points, each of which indicates the tool reciprocation position that corresponds to the spindle rotation angle obtained based on the profiled data, are connected to each other with a smooth line in order to correct the profile data, a wave may be formed in the interpolation curve. This may affect the processing accuracy. The profile point at which the manner in which the spindle rotation angle is reversed may be just deleted. However, the line that connects the profile points to each other does not extend smoothly and a wave (step in the inter-axis distance) may be formed. This may affect the processing accuracy.