Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
1998-07-07
2001-10-02
Lee, Thomas (Department: 2182)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C451S008000, C700S178000, C700S159000, C700S192000
Reexamination Certificate
active
06298278
ABSTRACT:
FIELD OF INVENTION
This invention concerns methods and apparatus for monitoring the movement of workpiece and tool carriage movement in machine tools, and machine tools, when fitted with such improved monitoring systems. By machine tool is meant any machine in which a tool is used to remove material from a workpiece and wherein either the tool and workpiece are moved relative to one another during the machining operation typically involving rotation and also linear displacement of one or both.
BACKGROUND TO THE INVENTION
Currently the means of obtaining carriage positional feedback in a two-axis machine such as a lathe, has involved fitting a linear position sensing device between each moving axis and the base of the machine. Thus the relationship of position between a tool and a workpiece is derived from two sets of feedback information, namely
1. a point on the workpiece carriage which is monitored for change of position with respect to the base, and
2. a point on the tool carriage which is monitored for change in position with respect to the base.
However the position on the base which is checked against the point on the tool carriage is not the same as the point on the base which is used as the reference for monitoring the position of the point on the workpiece carriage. As a consequence errors can arise in the computation of the position of the tool relative to the workpiece at the point of engagement due to the following possibilities:
A) variation in relationship between the machining point and the reference point of the work-feed measuring system,
B) similar variations in the relationship between the machining point and the tool feed measuring system, and
C) any static or dynamic changes in the relationship between the two points on the base to which the two axis measuring systems are attached.
It is an object of the present invention to provide an improved method of measuring the position of workpiece carriage and tool carriage so as to reduce the error in the computation of the tool/workpiece point of engagement and to provide a machine capable of high precision machining.
It is also an object of the invention to provide a design of machine which does not require the same degree of build accuracy as is necessary using conventional carriage monitoring systems and should therefore be considerably cheaper to build.
Although the invention may be applied to machine tools in which the workpiece carriage and/or the tool carriage move through considerable distances during workpiece machining, the invention is in fact of primary application to machine tools having a small working volume, in which both the workpiece carriage and the tool carriage only move through relatively small distances during the machining operation.
SUMMARY OF THE INVENTION
A method of determining the linear movement of a first carriage relative to a second carriage in a machine tool, wherein the two carriages respectively carry a workpiece and a tool for machining the workpiece, and wherein two-axis scale means is attached to one carriage so as to move therewith, and a reading head which cooperates with the scale means is attached to the other carriage to move therewith, wherein the method comprises the steps of: deriving positional data from the reading head and scale means for the one carriage, exerting a force on one or the other or both of the two carriages to effect a movement thereof relative to the machine, deriving a value for the linear displacement of the one carriage relative to the other by determining the difference between the positional data at the beginning and that at the end of the carriage movement.
The invention also provides a further method as aforesaid if a second reading head is attached to the said other of the two carriages at a position thereon separate from the position of attachment of the first reading head measured parallel to the axis of movement of the said other carriage, the further method comprising the steps of deriving second positional data relating to the said one carriage from the second reading head, and computing an error value from any difference between the displacement values derived from the positional data from the two separate reading heads, and correcting the linear displacement value obtained from the first reading head by using the said computed error value.
The preferred development thus enables any angular deflection or any distortion of one carriage relative to the machine or to the other to be detected and compensated by comparing the position determinable from the data derived from the second reading head with that determinable from the data derived from the first reading head during or at the end of a carriage movement. By moving a carriage in incremental steps and adjusting the position value at the end of each step, so very accurate positioning of a carriage can be achieved.
Differences in the position data can be attributed inter alia to the angular displacement of one carriage relative to the other and/or non-orthogonality of the two carriage axes, and although any such difference may be small in a well designed and constructed machine, cumulative machining errors will arise if the errors are not corrected and the invention provides for this by enabling the positional data obtained from the first reading head to be corrected by using the difference values obtained by using signals from the two reading heads.
The invention also lies in a machine tool wherein a two-axis scale means is mounted on the tool carriage or the workpiece carriage thereof so as to move therewith, and a reading head adapted to cooperate with the scale means is attached to the other said carriage, at a point which is close to the machining point, and signal processing means receives signals from the reading head indicative of the position and/or movement of one carriage relative to the other, 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.
A machine tool as aforesaid preferably includes a second reading head, also adapted to cooperate with the scale means, and mounted to the same carriage as the first reading head but at a different position, (measured parallel to the carriage axis) from that at which the first head is mounted, and 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 (and therefore indicative of inter alia any non-linear or non-orthogonal movement of one carriage relative to the other).
Where two reading heads are provided, the invention thus also includes the step of adjusting the position of the workpiece or tool carriage, or both, to compensate for any difference in the positional data obtained from the reading heads consequent on any unwanted movement of one carriage relative to the machine and/or the other.
Typically the reading head of the first measuring system is attached to the underside of the tool carriage close to the machining point and a grating therefor is mounted in a similar manner to the workpiece carriage.
The scale means may comprise a single relatively large area two-axis scale, or two smaller two-axis scales arranged in substantially the same plane.
In a preferred embodiment two separate two-axis scales are attached to the one carriage, the two scales being arranged in substantially the same plane and as close as possible to the point of engagement between the tool and the workpiece, and two reading heads for cooperating with the two scales are carried by the other of the two carriages. If the two carriage axes of movement are orthogonal and no distortion occurs between one carriage axis and the other, then the change in X and Z coordinates derived from the two reading heads for any relative movement as between one carriage and the other will be the same. If however, any distortion arises
Cao Chun
Lee Thomas
Lee Mann Smith McWilliams Sweeney & Ohlson
Unova U.K. Limited
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