X-axis accuracy in two axis machine tools

Details X-axis accuracy in two axis machine tools X-axis accuracy in two axis machine tools

1996-01-29

2001-05-08

Rose, Robert A. (Department: 3723)

Abrading

Precision device or process - or with condition responsive...

With indicating

C451S049000

Reexamination Certificate

active

06227940

ABSTRACT:

FIELD OF INVENTION
This invention concerns machine tools particularly apparatus for grinding piece parts and other components to a very high accuracy, particularly but not exclusively cylindrical components such as spindles and shafts.
BACKGROUND TO THE INVENTION
Due to forces exerted on a workpiece, the worktable and workpiece supporting structures of such machines, distortions and misalignments occur. These result in inaccuracies in the dimensions and shape of the final machined article. Where the dimensional errors are of the same order as the accuracy to which the article is to be machined it is impossible reliably to machine the article. Distortions misalignments and other error producing effects which can arise and tend to be cumulative must therefore be reduced to a level which is such that the magnitude of the cummulative error in the final article is much less than the error which can be permitted therein.
One of the primary controlling parameters in a machining operation is the accuracy with which the workpiece position relative to a machining tool is known and it is an object of the present invention to provide a system by which the position o the worktable and the workpiece thereon can be accurately determined relative to a machining tool so that if the latter can be moved in a precise manner to engage the workpiece, material can be removed therefrom to a known depth. When applied to a machine tool incorporating features such as are described in our copending applications (our references C421.01/L and C422.01/L to C427.01/L) a highly accurate machine tool can be obtained.
The invention is of particular relevance to a cylindrical grinding machine.
PRIOR ART
From Japanese patent publication No. JP-A-2160457 and No. JP-A-54102671 are known machine tools having straight edges and sensor means cooperating therewith, aimed at compensating for errors in accuracy in the path of movement of a machine drive.
SUMMARY OF THE INVENTION.
According to one aspect of the invention there is provided a machine tool comprising:
(a) a machine frame;
(b) a worktable assembly slidably carried by said machine frame for reciprocal movement along a work axis and which is configures to carry a workpiece whereby the worekpiece is movable along the work axis;
(c) a worktool assembly slidably carried by said machine frame for reciprocal movement along a worktool axis at a predetermine angle with respect to said work axis and so as to intersect same, with the worktool movable along the worktool axis;
(d) a reference straight edge carried by said worktable assembly and having a reference surface;
(e) reference sensing means carried by said machine frame for coaction with said reference straight edge reference surface, to measure reference spacing between said reference surface and said reference sensing means and to provide a reference output indictive of said reference spacing; and
(f) control means to facilitate movement of said worktool toward and away from said work axis which is responsive to said reference output to further facilitate worktool movement towards said work axis, where the reference spacing is measured at height above the worktable which is commensurate with the height of the work axis.
Advantageously a point in the reference surface at which the reference spacing is measured, the work axis and the worktool axis all lie in the same plane.
Typically said worktool assembly includes a worktool carriage movably disposed on a slideway carried by said machine frame for movement along said worktool axis and worktool carriage position sensing means providing an output signal indicative of the position of said worktool carriage to said control means to further facilitate movement of said worktool towards said work axis.
The worktool carriage position sensing means is preferably located at the same height as the said point in the reference surface at which the reference spacing measurement occurs.
Preferably the worktool carriage position sensing means and the reference spacing measuring point are aligned wish the worktool axis.
The invention is of particular application to a machine in which the worktool is a grinding wheel rotatable about a wheel axis and the reference spacing measurement point, workpiece axis worktool axis and the wheel axis all lie in the same plane.
The machine is preferably mounted so that the said one plane horizontal.
Conveniently said control means includes a look-up table storing information relative to initial dispositions of said referee surface and said reference sensing means and to provide initial disposition output indicative thereof to modify s reference output to further facilitate worktool movement towards said work axis.
Preferably said look-up table initial disposition output indicative of the flatness of said reference surface.
Conveniently said reference sensing means includes a fixed electrode that forms with said reference surface a capacitance gauge.
Preferably a second capacitance gauge is disposed close to the first capacitance gauge and a capacitance bridge is provided including the first and second capacitance gauges, to provide a capacitance variation signal.
Preferably said worktable assembly includes a work carriage movably disposed on slideways carried by said machine frame for movement along said work axis, and said worktool assembly carries a worktool, a working face of which is positionable thereby with respect to said work axis; said reference spacing being indicative of variations in spacing between said working face of said worktool and said work axis due to shifting of said wore carriage with respect to said work face of said work tool.
Conveniently said worktool carriage position sensing mean includes a grating disposed on said worktool carriage in worktool imaginary reference line that intersects work when carried by said worktable assembly.
A rigid cover is to advantage mounted on the worktable to protect the reference sensing means, the cover being spaced from the latter.
A second rigid cover is preferably mounted on the worktool carriage, spaced from the position sensing means to protect the latter.
Prefarably there is provided a cover assembly for said slideways mounted to said machine frame movable relative to and responsive to movement of said work carriage but not connected to said work carriage.
The cover assembly preferably includes a first cover port mounted to said machine frame to one side of said work carriage and a second cover portion mounted to said machine frame to the other side of said work carriage.
The invention is of particular application to a machine in which the tool is a grinding wheel.
According to another aspect of the present invention a machine tool comprises a bed, a worktable movable relative to the bed along a linear path, a tool movable to engage a workpiece on the worktable, tool drive means for advancing and retracting the tool along a tool path generally orthogonal to the said linear path, worktable drive means for effecting the movement of the worktable relative to the bed along the linear path and therefore relative to the tool, a reference straight edge carried by the worktable, reference sensing means fixed on the machine bed and stationary relative to the straight edge and spaced therefrom by a nominal distance generally at the level of the path of movement of the workpiece, and cooperating therewith to produce an electrical signal and circuit means associated therewith to generate from said electrical signal an error signal indicative of any departure of the distance between the straight edge and the sensing means from the nominal spacing therebetween, memory means for storing a calibration of the flatness of the straight edge for points along its length, electrical signal generating mean for generating a target position signal for the tool along the X-axis (the X-axis target signal), tool position sensing means for generating an electrical signal indicative of the actual position of the tool along the X-axis (the feedback signal), further electrical circuit means responsive to the ele

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