1. Field of the Invention
This invention relates to a method and an apparatus for truing a grinding wheel, and more particularly, to a method and an apparatus for truing a grinding wheel having a non-straight cylindrical grinding surface.
2. Prior Art of the Invention
In a numerical controlled grinding machine, a grinding wheel having a stepped peripheral shape is used for grinding a workpiece having a stepped outer surface, namely, a workpiece having plural cylindrical outer surfaces and plural shoulder end surfaces adjacent to the respective cylindrical outer surfaces. Although such grinding wheel is manufactured to have a desired stepped peripheral shape, the grinding wheel has an initial shape slightly different from the desired shape because of inaccuracy of the manufacturing process. Further, there is a case where a user wants to slightly change the peripheral shape of the grinding wheel before using the grinding wheel.
Accordingly, it is necessary to true the grinding wheels before using them, so that the grinding wheels have desired peripheral shapes.
An apparatus which can be used for the above-mentioned truing operation is disclosed in the U.S. Pat. No. 4,899,718 which was assigned to the assignee of this application. The apparatus is provided with a contact detection sensor such as an AE (acoustic emission) sensor for outputting a contact signal when a truing tool contacts a grinding wheel. In dressing operation, the truing tool is moved relative to the grinding wheel so that the truing tool contacts each of the two inclined surface portions of the grinding wheel, and then the position of the truing tool is detected when the truing tool contact the respective surface portions. Based on these positions thus detected, it is judged which surface portion has a larger removal amount to be removed by truing operation, compared to the other. An initial truing start position of the truing tool is determined based on the radial position of the surface portion which has a larger removal amount, and the truing tool is then traversed along the outer surface of the grinding wheel while following a template. Ascertainment is made as to whether the contact sensor outputs a contact signal continuously during the traverse fed. If the continuous issue of the contact signal is not ascertained, then the truing tool is infed a predetermined amount against the grinding wheel before it is traversed. The infeed and the traverse feed of the truing tool are repeated until the continuous issuance of the contact signal during each traverse feed movement is finally ascertained.
The above-mentioned truing apparatus has a disadvantage that the mechanical structure thereof is complex, because it needs a template and a stylus, and that the template must be changed when the grinding wheel is trued in different shapes.
Further, in the conventional apparatus, it is required that the AE sensor outputs the contact signal continuously during the time period when the truing tool contacts the grinding wheel. However, it is difficult to obtain a stable contact signal from the AE sensor, because the AE sensor detects a sound wave which is generated when the truing tool contacts the grinding wheel. The level of the sound wave changes depending on the shape of a surface portion which the truing tool contacts, namely, depending on whether the surface portion has a straight shape or a curved shape. Further, the level of the sound wave changes depending on the amount of coolant supplied to the outer surface of the grinding wheel during truing operation. Therefore, it is difficult to judge whether or not the truing tool continuously contacts the grinding wheel, even though the detection sensitivity of the AE sensor is adjusted. This causes inaccuracy of truing operation.