Process for qualifying accuracy of a numerically controlled machining system

A method for assessing accuracy of positioning of a multi-axis numerically controlled machine and for diagnosing and correcting sources of errors in positioning, the machine being of the type having a tool-holding spindle mounted on a spindle-holding body, the body being rotatably mounted in a head which is rotatably mounted on a ram that is movable along each of three mutually orthogonal translational axes, the method comprising:

• (a) mounting a measuring probe in the spindle such that, when the body is in a home position thereof, a longitudinal axis of the probe is colinearly aligned with a first rotational axis of the machine about which the head rotates with respect to the ram, and such that the probe has a predetermined length between a tip of the probe and a second rotational axis about which the body rotates with respect to the head;
• (b) checking accuracy of the machine along each of the three translational axes by placing the body and head in fixed rotational positions, causing the machine to move the ram along each of the three translational axes such that the probe tip is brought into contact with a fixed monument, comparing positions of the monument measured by the probe with previously taught positions so as to determine whether there is a shift of the machine or the monument along any of the three translational axes, and correcting any such shift that exceeds a maximum allowable shift;
• and
• (c) checking accuracy of rotational positioning of the body of the machine by positioning the head in a position in which the second rotational axis is parallel with one of the translational axes, rotating the body about the second rotational axis and detecting whether the tip of the probe undergoes movement along said one translational axis, said movement indicating inaccuracy in rotational positioning of the head, and correcting any such inaccuracy in rotational positioning that exceeds a maximum allowable inaccuracy.

A method for assessing accuracy of positioning of a multi-axis numerically controlled machine and of a holding fixture used in conjunction with the machine for holding a workpiece for machining, the machine being of the type having a tool-holding spindle mounted on a ram that is movable on ways along each of three mutually orthogonal translational axes, the fixture being of the type having a plurality of movable holding members variably positionable along at least one direction for engaging the workpiece and having a numerical control unit operable to control positioning of the holding members and to internally determine a position of each holding member along said at least one direction, the method comprising:

• mounting a measuring probe in the spindle of the machine, the probe being operable to cause the machine to determine a position of a tip of the probe relative to a frame of reference of the machine when the tip makes contact with an object;
• placing each of a plurality of the holding members of the fixture in a fixed position along said at least one direction in which the holding members are movable, the numerical control unit of the holding fixture determining a position of each holding member along said at least one direction;
• moving the machine to each of said plurality of the holding members such that the probe tip contacts the holding member and the machine determines a position of the holding member along at least one axis of the machine's frame of reference;
• and
• comparing the machine-determined position of each holding member with a position of the holding member determined within the holding fixture.

A method of detecting and diagnosing potential sources of errors in positioning of a machining system having a multi-axis numerically controlled machine and a flexible holding fixture for holding workpieces to be machined, the machine having a tool-holding spindle mounted on a wrist of the machine that has two rotatable joints defining two perpendicular rotation axes, the machine including linear ways permitting the wrist to be translated along each of at least two mutually orthogonal translation axes, the holding fixture having a plurality of holding members that are movable relative to the machine and having a control unit operable to control positioning of the holding members, the method comprising:

• (a) performing a probe check to assess accuracy in positioning of the machine along each of the translation axes by:
• mounting a measuring probe in the spindle such that a longitudinal axis of the probe has a predetermined orientation with respect to the rotation axes of the machine, the probe being operable to cause the machine to determine a position of a tip of the probe when the tip contacts an object;
• moving the machine along each translation axis while the other translation axes and the rotation axes are fixed so as to cause the probe tip to contact a corresponding fixed reference surface defined for each translation axis such that the machine determines a position of the reference surface;
• determining a difference between the machine-determined position of each reference surface and a previously taught position for the reference surface;
• and
• when any of the differences exceeds a maximum allowable difference, checking each of a plurality of possible causes that include shifting of the reference surface and shifting of the machine along the translation axes, and correcting any such shift that is unacceptably large;
• and
• (b) performing a probe check of the holding fixture to assess accuracy in positioning of the holding members by:
• causing the holding fixture to position at least some of the holding members such that a predefined surface of each said holding member is located in a predetermined position along one of the translation axes of the machine as determined within the control unit of the fixture;
• moving the machine along said one translation axis while the other translation axes and the rotation axes are fixed so as to cause the probe tip to contact said predefined surface of each of a plurality of the holding members such that the machine determines a position of said predefined surface along said one translation axis;
• calculating a discrepancy between the machine-determined position of said predefined surface and the position determined within the control unit of the fixture for each holding member contacted by the probe;
• and
• accounting for the discrepancies within the control unit of the fixture such that the control unit adjusts the positioning of the holding members by amounts corresponding to the discrepancies.