System for monitoring workpiece and tool carriages movement using a two-axis scale attached to a carriage cooperating with a reading head attached to the other carriage

A machine comprising a movable workpiece carriage for carrying a workpiece to be machined and a movable tool carriage for carrying a tool for removing material from the workpiece, a two-axis scale means mounted on one of the carriages so as to move therewith, a reading head adapted to cooperate with the scale means attached to the other said carriage, and signal processing means for receiving signals from the reading head indicative of the position and/or movement of one carriage relative to the other, and generating positional data about one or both of the carriages to assist in controlling the movement of one or both carriages to effect a machining operation, and in which the machine tool includes a further reading head also adapted to cooperate with the scale means, and mounted on the same carriage as the reading head but at a different position from that at which the head is mounted, and in which the signal processing means is receptive of signals from both heads and provides a first output signal indicative of the linear movement of one carriage relative to the other, and a second output signal indicative of any difference between the positional signals derived from the two heads.

A method of reducing or eliminating machine induced errors which affect the orthogonality of the axes of a first and a second carriage of a machine tool the carriages being moveable substantially orthogonally with respect to one another, each carriage carrying a respective one of a workpiece and tool for machining the workpiece, comprising the steps of attaching a two-axis scale means to one carriage so as to move therewith, attaching a reading head which cooperates with the scale means to the other carriage to move therewith, the reading head being disposed in substantially the same plane as the two-axis scale means, deriving positional data from the reading head and scale means for the first carriage, exerting a force on at least one of the two carriages to effect a movement thereof relative to the machine, and deriving a value for the linear displacement of the first carriage relative to the second by determining the difference between the positional data at the beginning, and at the end, of the carriage movement.

A machine tool comprising a moveable workpiece carriage for carrying a workpiece to be machined by rotation about its axis and a tool carriage for carrying tool for removing material from the workpiece, the two carriages being moveable in orthogonal directions, two-axis scale means mounted on one of the carriages so as to move therewith, reading head means adapted to cooperate with the scale means attached to the other carriage, and signal processing means for receiving signals from the reading head means indicative of the position and/or movement of one carriage relative to the other, and for generating positional data about one or both of the carriages to assist in controlling the movement of one or both carriages to effect a machining operation, the reading head means being disposed in substantially the same plane as the scale means but spaced apart a minimal distance on opposite sides respectively of the workpiece axis such that parallax errors as minimised.

A method of machining so as to remove material from a rotating workpiece using a tool which may be static or rotating, the method comprising the steps of moving the tool so as to engage the surface of the workpiece to remove material therefrom, determining the position of the tool carriage using a two-axis scale measuring system comprising a reading head mounted to one of the workpiece carriage or tool carriage and two-axis scale means mounted to the other, positioned close to the working region, and controlling the advance of the tool, and therefore the extent of the machining, using positional data from the reading head, and further including the steps of determining the tool carriage position by means of a second reading head also cooperating with said two-axis scale means, and mounted to the same carriage as the first mentioned reading head, and measuring any distortion and utilizing any measured divergence between scale readings and predicted scale readings following any given linear carriage movement of to enable a correction to be made in the linear carriage movement and thereby compensate for any distortion due to machining forces.

In a machining process in which a tool is progressively moved by means of discrete movement instructions relative to a rotating workpiece so as to remove material from a rotating, surface thereof, first tool position defining signals are obtained from a first reading head which is attached to and movable with the tool support and which cooperates with two-axes scale means attached to the workpiece support for movement parallel to the axis of rotation of the workpiece as the workpiece support is moved;

• and second tool position defining signals are obtained from a second reading head also attached to and movable with the tool support and cooperating with the two-axis scale means;
• and error signals indicative of any positional error of the tool are obtained by determining the values of the signals which should be obtained from the second reading head for a given linear movement of the tool from the first position to a second position using tool position defining signals obtained from the first reading head, and making a comparison therewith of the actual second tool position defining signals obtained from the second reading head.