Method of operating a machine tool with a sensing probe in order to gather positional data for the calculation of tool offset parameters

#This invention relates to a method of operating a machine tool of the kind comprising a tool spindle having an axis of rotation, means for rotating the spindle through predetermined angles of rotation, a table movable relative to the spindle transversely to said axis, a surface-sensing probe mounted on the spindle, means for outputting a probe signal at an instant of said probe being brought into a sensing relationship with a surface defined on said table, means responsive to the occurrence of a said probe signal for producing an output corresponding to the position of said axis relative to a primary datum which is fixed in relation to the table and a computer connected to read said output.

#The probe is usually stored in a tool magazine in the same way as the rotary cutting tools to be used in the machine. The probe may be mounted on the spindle by an automatic tool change mechanism when required for a measuring operation. Measuring operations are needed before machining to determine the position of a surface to be machined and after machining to check the dimensions of the machined surface.

#The probe senses the surface at a point which lies a small distance away from the spindle axis. This distance is referred to as the probe offset and has to be taken into account during measuring operations. Offsets are not uniform in all directions around said axis. This is partly due to characteristics of the probe itself and may also be due to unavoidable misalignment between the probe and the spindle. Therefore, when the probe has been mounted on the spindle, it is desirable to calibrate the probe in the sense of determining the offsets in selected directions around said axis so that during subsequent measuring of differently orientated surfaces, the different offsets are available for being appropriately taken into account, the spindle itself not being rotated for the purpose of measuring such differently orientated surfaces. It is an object of this invention to provide an automatically operable method of determining the different offsets of the probe in a machine of the kind described.

#An example of the method according to this invention will now be described with reference to the accompanying drawings wherein:

#FIG. 1 is an elevation of a machine tool,

#FIG. 2 is a section on the line II--II in FIG. 1,

#Referring to FIGS. 1 and 2 the machine tool, being a machining centre, comprises a fixed structure or base 10, a work support or table 11 adapted by means of a fixture 15 to support a workpiece 14 in a predetermined datum position thereon. The machine includes a tool support being a spindle 16 supported on the base 10 for continuous rotation about a fixed axis 16a as well as for being indexed about that axis between given angular positions by stepper motor MI having a position sensor 20I whose output is connected to a digital counter 21I. The table 11 is supported on the base 10 by a slide system 18 for linear movements in directions X,Y,Z of the rectangular coordinate system. The movements in the respective directions are effected by motors MX, MY, MZ and are continually measurable relative to the base 10 by position sensors 20X, 20Y, 20Z outputs to digital counters 21X, 21Y, 21Z. Each counter 21 has an output to a respective comparator 22 (FIG. 9) supplied with demand data by a register 23. Each motor M is drivable to a demanded position by a closed loop whose error signal is the difference between the contents of the relevant counter 21 and register 23.

#The workpiece 14 has a datum bore 24 having an axis 24A defining the workpiece datum in the X,Y directions. The datum position of the workpiece 14 on the table 11 is defined by distances 101, 102, 103 between the axis 24A and table datum surfaces TX, TY, TZ. However, since machining operations are carried out by rotary cutting tools (not shown) mounted on the spindle 16 concentric with the axis 16A, and since measuring operations are performed by a probe 25 (FIGS. 1,3) also mounted on the spindle 16, it is necessary to relate table movements for such operations to the axis 16A which thus becomes a fixed or machine datum. The table datum surfaces T are therefore used only for an initial setting up operation for the fixture 15 say, at the beginning of a production run. But the counters 21 would normally remain zeroed on the surfaces T for all subsequent operations. In order to relate the axis 24A to the axis 16A the arrangement must be such that when the counters 21 show the workpiece to be at the datum position, the axes 16A, 24A should coincide. This does not necessarily occur in practice and may, for example, be due to thermal contraction or expansion of the table between the time when the fixture was originally mounted and a later time in the production run. Therefore the axis 24A may have a nominal datum position at which it coincides with the axis 16A and an actual datum position in which the axis 24A is offset from the axis 16. The difference between these two positions is referred to as the "work offset".

#The probe 25 comprises a body 26 (FIG. 3) secured to the spindle 16 and a stylus 27 on the body 26 and having a ball end 28. The probe 25 is said to sense a work surface, i.e. a surface of the workpiece, if responsive to engagement between the ball 28 and the work surface, the probe outputs a step signal 138 in an electric circuit 29 (FIGS. 3,9). The signal 138 is connected to act on the counters 21 to transfer the instantaneous content of the counters to respective registers 33X, 33Y, 33Z. Thus the position of the work surface is determined at the instant of the transfer to the registers 33 or, for practical purposes, at the instant of the signal 138.