A Study on Effect of Part-loading Errors on the Accuracy of Compound Hole Axis

Drilling compound hole is very common material processing operation in engineering industry. In this special machining process before drilling the hole the part is rotated about an auxiliary axis parallel to one of the reference axes. Accuracy of the drilled hole is paramount in several applications such as in cylinder head and some other applications of bone surgery. Inaccuracy in hole axis may result in the loss of engine efficiency in the case of Cylinder Head or unsuccessful bone surgery.

In the previous papers Murty et al. [8], [9] the algorithms developed had restricted application. In the first paper the tool was restricted to move in vertical plane and WY-plane was considered as reference plane. In the second paper the tool movement was unrestricted in space but the WY-plane was considered as reference plane and the skew error was assumed only in WY-plane. However, in the present paper the authors developed a general purpose algorithm for error compensation when the tool movement is unrestricted in space and the part loading errors may include translational errors in 3-Cartesian directions and rotational errors in WY-plane and XY-plane about 2 axes parallel to Cartesian axes. The authors also give trend lines which can be used to sufficient accuracy for computing corrective motions of the machine axes. This paper employs the principles of inverse kinematics wherein a faulty compound-hole angle axis in space caused by the translational and rotational errors at the part loading position is identified with an imaginary true axis in space by enforcing identity through a modified machine axes taking into effect inclination of the drill tool in space. In the absence of any specific application, this algorithm is verified on AutoCAD, a commercial CAD tool and found to be correct as shown in an example problem given at the end. For small values of part-loading errors plot between skew error in XY-plane and angular corrections in part rotation and spindle axis orientation show linear relationship.