GB1599585A

GB1599585A - Numerically-controlled machine tool system

Info

Publication number: GB1599585A
Application number: GB8417/78A
Authority: GB
Original assignee: Fujitsu Fanuc Ltd
Current assignee: Fanuc Corp
Priority date: 1977-03-17
Filing date: 1978-03-02
Publication date: 1981-10-07
Also published as: SE7802295L; JPS53114580A; DE2811069A1; FR2384293B1; SE427702B; JPS6210773B2; IT7821237A0; FR2384293A1; IT1093419B

Description

(54) NUMERICALLY-CONTROLLED MACHINE TOOL SYSTEM (71 We, FUJITSU FANUC LIMITED, a Company organized and existing under the laws of Japan of 5-1, Asahigaoka 3-chome, Hino-shi, Tokyo, Japan, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- The present invention relates generally to a computerized Numerically-Controlled machine tool system, and more particularly to a method for defining the coordinates of each tool used in such a computerized Numerically-Controlled machine tool system.

The computerized Numerically Controlled machine tool system (hereinafter referred to as CNC machine tool system) is very useful for the automatic production of any products with a high degree of accuracy, a high degree of reliability and at a low cost. In the CNC machine tool system, various kinds of tools are mounted on a cross slide and arranged with predetermined pitch therebetween in a direction perpendicular to the axis of a spindle motor used for rotating a workpiece to be cut by a selected tool. Selection of a desired tool is carried out by punching a corresponding tool number on an instruction tape, i.e., a punched tape.The instruction tape, one of the important members in producing the product, is used for inputting a program to the CNC machine tool system which contains information for carrying out sequential production steps and also various kinds of data corresponding to the design of the product.

One of the most important processes of the above-described CNC machine tool system is to define the coordinates of each tool, especially the coordinates of the edge of each tool relative to some fixed point. In conventional CNC machine tool systems the coordinates of the edge of a tool are defined using the so-called G-function of a program which must be carried out each time a particular tool is selected. Stated in the Japanese CNC standards the G-function of a program is identified by a program code "G50". The equivalent program code specified by the EIA (Electronic Industries Association in the U.S.A.) is "G92". The Gfunction is carried out for the purpose of matching an origin of the coordinates in the machine tool with an origin of the coordinates in the program.In conventional CNC machine tools it is necessary to punch both the G-function program code, mentioned above, and the coordinate values of the edge of a tool relative to the origin in the machine tool on the instruction tape each time a particular tool is selected.

Accordingly, it is very troublesome for an operator to set up the program especially when a large number of tools are used in the CNC machine tool system and also when tool changes occur very frequently in the CNC machine tool system.

It is an object of the present invention to provide a method for matching the coordinates of the edge of a selected tool in the machine tool with the coordinates thereof in the program and for facilitating the process of preparing a program therefor.

According to the present invention there is provided a numerically controlled machine tool system comprising a machine tool having a chuck for holding a workpiece, a cross-slide for holding a plurality of tools adapted to be sequentially selected for use upon the workpiece, a numerical control unit for controlling movement of the cross-side-along at least two axes relative to the workpiece, and present-position-coordinate value registers contained in the numerical control unit for storing the present-position-coordinate values relative to a reference point of the edge of whichever tool has been selected for use upon the workpiece, characterised in that the numerical control unit further comprises means for storing predetermined coordinate values representative of the positions of the edges of each of the tools relative to the reference point when the cross-slide is in a predetermined position relative to the chuck, means for determining the difference between the stored predetermined coordinate values of a selected tool and a further tool which is to be selected next, and means operative when said further tool is selected to either update by said difference the content of the present position coordinate value registers or to move the said further tool a distance indicated by said difference, whereby after said further tool is selected the content of the present-position-coordinate registers represent the position of the edge of said further tool relative to the reference point.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic plan view of a conventional CNC machine tool system; Fig. 2 is a partial plan view of the machine tool 11 shown in Fig. 1, which is utilized for explaining the so-called G-function in respect of a first tool; Fig. 3 is a partial plan view of the machine tool 11 shown in Fig. 11, which is utilized for explaining the so-called G-function in respect of a second tool; Fig. 4 depicts a conventional tape format for an instruction tape 23 as shown in Fig. 1; Fig. 5 is a block diagram of a CNC machine tool system, embodying the present invention; Fig. 6 is a plan view of a cross slide with tools, used for explaining the present invention;; Fig. 7 is a plan view of the cross slide with tools, also used for explaining the present invention.

Fig. 1 is a schematic plan view of a conventional CNC machine tool system. In Fig. 1, the CNC machine tool system 10 comprises a machine tool 11 and a numerical control unit (CNC) 12 which contains a computer therein. The machine tool 11 cuts a workpiece 20 under the control of an instruction signal provided from the numerical control unit 12 via signal lines 13. In the machine tool 11, a table 14 is located on a bed 15 on the machine tool 11. The table 14 is slidable on the bed 15 along a Z-axis (see arrow Z) by means of a lead screw 16 and a ball nut (not shown). The lead screw 16 is driven by a servomotor 17Z (SZ). A cross slide 18 is located on tne table 14. The cross slide 18 is slidable on the table 14 along an X-axis (see arrow X) by means of a lead screw (not shown) and a ball nut (not shown). The lead screw is driven by a servomotor 17X (SX).

On the cross slide 18, four tools 19-1, 19-2, 19-3 and 19-4 are mounted and arranged with a predetermined pitch therebetween along the X-axis. The workpiece 20, to be cut by one of the tools 19-1 through 19-4, is rigidly gripped by a chuck 21. The chuck21 and also the workpiece 20 are rotated by a spindle motor 22 (SPM). The horizontal axes of the workpiece 20, the chuck 21 and the spindle motor 22 are aligned to be parallel with the axis of each of the tools 191 through 19-4.

The numerical control unit 12 receives instruction information from an instruction tape 23 and accordingly provides respective control signals to the servomotor 17Z, the servomotor 17X and the spindle motor 22 via the signal lines 13. The control signals to the servomotors 17Z and 17X are derived from control pulses by means not shown.

The conventional method of matching the coordinates of the edge of a selected tool in the machine tool 11 with the coordinates of the edge of the tool defined in the program is as follows. Referring to Fig. 2, when a tool numbered No. 1, and corresponding to the tool 19-1, is selected, information concerning the difference in distance between a reference point PO and the edge 25 of the tool 19-1 is formed on the instruction tape 23 (Fig. 1). This difference is measured, before the operation of the CNC machine tool system, as the number of control pulses, for example, X=30,000, Z=40,000, required to reset the cross slide 18 to its origin from reference point PO.

The reference point PO is set at the central base point of the chuck 21 which grips the workpiece 20, and corresponds to the origin of the coordinates in the program.

The program for processing the workpiece is next arranged on the instruction tape. That is, the following two data blocks (1) and (2) are formed on the instruction tape, (1) T1* (2) G50X60000Z40000* wherein the code T1 indicates that the tool No. 1 is being selected, the code * indicates the end of a data block, the code G50 indicates that the above-mentioned Gfunction must be carried out, the code X60000 corresponds to twice the number of control pulses required to move the edge of tool 19-1 to reference point PO along axis X and the code Z40000 corresponds to the number of control pulses required to move the edge of tool 19-1 to reference point PO along axis z.

When the numerical control unit 12 (Fig. 1) receives information regarding the abovementioned data block (1) from the instruction tape, the unit 12 will acknowledge that the tool corresponding to tool No. 1 has been selected. Next, the above numbers 60000 and 40000 are stored in a present - position - coordinate - value register (not shown in Fig. 1 but shown in Fig. 5). In the following third data block (3) of the instruction tape, the so-called absolute instructions contained, for example, (3) GOOX15000Z18000* wherein the code GOO indicates an instruction for a positioning control, and the code X15000Z 18000 indicates a position corresponding to the values of the coordinates (X, Z) relative to the origin PO to which the edge of the tool No. 1 is to be transferred.

In the numerical control unit 12 (Fig. 1), an incremental instruction value V, is calculated by subtracting the present position value Vp from the absolute instruction value Va, that is, V1=y-V1, The value Vp corresponds to the above number 60000, and the value Vb corresponds to the above number 15000, with respect to the X-axis. Similarly, with respect to the Zaxis, the values Vp and Va correspond respectively to the above numbers 40000 and 18000. Both the incremental instruction value Vi with respect to the Xaxis and the incremental instruction value V, with respect to the Z-axis are applied to an interpolator (not shown in Fig. 1 but shown in Fig. 5) in the numerical control unit 12 (Fig. 1).The interpolator provides control signals to the servomotors 17X and 17Z in accordance with the incremental instructions vi which cause the edge of tool 9 to be moved to the absolute position.

Similarly, if the tool 19-2 shown in Fig. 3 is specified, a program for processing the workpiece 20 is formed on the instruction tape. That is, the following two data blocks (1) and (2) are formed on the instruction tape.

(1) T2* (2) G50X14000Z20000* wherein the code T2 indicates that the tool No. 2 is being specified. The codes G50, X14000, Z20000 and * have already been explained hereinbefore.

As mentioned above, in the prior art, a data block indicating the above program codes that is [G50X . . . Z . . .*] has to be formed on the instruction tape every time a tool is selected to indicate the position of the tool relative to a fixed reference point on the machine tool. An example of the tape format of the instruction tape is as shown in Fig. 4. As is apparent from the tape format of Fig.

4, information indicated on data blocks represented by reference numerals 31, 32, 33, 34, 35 etc. must be formed on the instruction tape 23 each time a tool is selected. As a result, it is very troublesome for an operator to set up such a program, which must include the data blocks of G50 . . . Z . . .* formed on the instruction tape each time a tool change occurs.

However, in embodimemts of the present invention since all the data blocks of [GSOX. . .Z. . .*] can be omitted from a program, it becomes much easier for an operator to set up a required program. The data blocks of [G50X... ... .*j are omitted from the program, by storing in a programme unit (explained hereinafter) the coordinates (X, Z) of all the tools which coordinates indicate the differences between the edges of respective tools and the origin (referred to PO in Figs. 2 our 39 of the coordinates in the program.When a tool number (see T1, T2, T4, etc., in Fig. 4) is selected by the instruction tape 23 (Figs. 1 and 4), the corresponding coordinates (X, Z) are read out from the memory unit and the corresponding coordiantes (X, Z) are then set in a present - position coordinate - value register of the numerical control unit.

Fig. 5 is a block diagram of the CNC machine tool system 40 embodying the present invention, in which the prior art data blocks [G50X...Z...*] are omitted from the instruction tape. In Fig. 5, it should be noted that only members used for carrying out the method of the present invention are shown and that conventional members are not shown therein. The CNC machine tool system 40 embodying the present invention is comprised of a numerically controlled unit 41 which controls a machine tool 11 (MT) similar or identical to that of Fig. 1 by means of servomotors 17X and 17Z. The servomotor 17X is rotated forward and backward by means of a servocontrol circuit 42X (SCX).

The servomotor 17Z is rotated forward and backward by means of a servo-control circuit 42Z (SCZ). The numerically controlled unit 41 is comprised of a central processing unit 43 (CPU), a memory unit 44, a tape reader 45 (TR), a tape reader control circuit 46 (TRC), a manual data input device 47 (MDI), a digital data input device 48 (DI), an interpolator 49 (INT) and the above-mentioned servo-control circuits 42X and 42Z. The central processing unit 43, the memory unit 44, the tape reader control circuit 46, the manual data input device 47, the digital data input device 48 and the interpolator 49 are all interconnected by means of address and data buses 50 (ADB).

The memory unit 44 is comprised of a first memory area 44-1 (CPM), a second memory area 44-2 (RA) and a third memory area 44-3 (CDA). The first memory area 44 1 stores a control program. The second memory area 44-2 stores the tool number, the absolute instruction value, the present position coordinate values of the edge of a selected tool and the incremental instruction value. The third memory area 44-3 stores data indicating the relationships between respective tool numbers and the coordinates (X, Z) of the corresponding tools.More specifically, the second memory area 44-2 comprises registers, RT, RXA, RZA, RXa, RZa, RXI and RZI, wherein the register RT stores a selected tool number, the register RXA stores the absolute instruction value (XA) with respect to the X-axis of the selected tool the register RZA stores the absolute instruction value (ZA) with respect to the Z-axis of the selected tool, the register RX. stores the value of the coordinate (X3 of the edge of the tool, the register RZa stores the value of the coordinate (Za) of the edge of the tool, the register RXI stores the incremental instruction value (XI) with respect to the Xaxis and the register RZI stores the incremental instruction value (ZI) with respect to the Z axis. More specifically, the third memory area 44-3 is divided into areas RT1 through RTN.The divided areas RT1 through RTN correspond respectively to the tool numbers, No. 1 through No. N, where n is the number of tools used in the machine tool 11. Each of the divided areas RTI through RTN is comprised of two registers, one of which stores the values of the coordinates along the X axis of the edge of each of the tools to be selected with respect to the X axis when the tool is in a known position such as Xl, X2 . . . Xn, and the other register stores the values of the coordinates along the Z axis thereof, such as the values Z1, Z2. . .Zn.

The manual data input device 47 is used to input data indicating both the tool numbers and the values of the predetermined coordinates (X, Z) of the edges of each of the tools which correspond to the tool numbers, respectively. The values of the coordinates (X, Z) usually vary among factories which utilize the CNC machine tool system. In such cases, the tool numbers are manually set by using a number switch 47-1. An address switch 47-2 is used to select either the X-axis (CX) or the Z-axis (CZ). A data key 47-3 is used to input the value of the coordinate X and also to input the value of the coordinate Z of the edge of each tool. A transfer key 474 is used to transfer data, which have been collected by means of the above members 47-1 through 47-4, to the address and data buses 50.A first display 47-5 confirms the input value of the coordinate X and also the input value of the coordinate Z. A second display 47-6 confirms the specified tool number.

The digital data input device 48 has a mode selection switch 48-1. Either a "manual data input" (MDI) mode or an "instruction tape control" (TAPE) mode can be selected by setting the switch 48-1 to the terminal MDI or to the terminal TAPE.

An interpolator 49 is comprised of a first register RXII and a second register RZII.

The first register RXII stores the incremental instruction value (XI) with respect to the X-axis. The second register RZII stores the incremental instruction value (ZI) with respect to the Z-axis. Then, in a known manner, the interpolator 49 distributes control pulses to the servocontrol circuit 42-X and 42-Z, in accordance with the stored incremental instruction values (XI, ZI) corresponding to the X-axis and the Z-axis respectively. Thus the table 14 (Fig. 1) and the cross slide 18 (Fig. 1) in the machine tool 11 are moved to a desired position in accordance with the design of the products.

The operation of the CNC machine tool system shown in Fig. 5 is as follows.

(1) Storage of both the tool number and the values of the predetermined coordinates (X, Z) of the edges of each of the tools corresponding to said tool numbers when the cross slide is reset to its origin.

(a) The mode selection switch 48-1 in the digital data input device 48 is first set to the "manual data input" (MD-I) mode. Then, the control program for the MDI mode is accessed from the first memory area 44-1 in the memory unit 44.

(b) In the manual data input device 47, the tool number is specified by pushing the number switch 47-1. The tool number may be expressed by, for example, two digits (dt, d2) shown in the switch 47-1. The specified tool number is displayed for confirmation on the second display 47-6.

(c) The X-axis is specified by setting the address switch 47-2 to the terminal CX.

(d) The value of the coordinate X of the edge of the tool corresponding to the tool specified in the above step (b) is inputted by pushing the digit indication (0 through 9) marked on each of the data keys 47-3. These digits are displayed for confirmation on the first display 47-5.

(e) After confirmation of the digits displayed on displays 47-5 and 47-6 is completed, the transfer key 47-4 is pushed.

Then, the value of the coordinate X of the edge of the tool corresponding to the specified tool number is transferred to the third memory area 44-3 in the memory 44 and stored therein. If a tool No. 1 is specified and the value of the coordinate X is Xl, the value Xl is stored in one of the registers comprising register RTI (refer to the third memory area 44-3 in Fig. 5).

(f) The Z-axis is specified by setting the address switch 47-2 to the terminal CZ, which setting is similar to that of the above step (c).

(g) The value of the coordinate Z of the tool corresponding to the tool number specified in the above step (b) is inputted by pushing the digit indication marked on each of the data keys 47-3, and the digits thereof are displayed, for confirmation, on the first display 47-5 similar to above step (d).

(h) After the digits are confirmed by the displays 47-5 and 47-6, the transfer key 47-4 is pushed. Thereafter, the value of the coordinate Z of the edge of the tool corresponding to the specified tool number is transferred to the third memory 44-3 and stored therein, similar to the above step (e).

If the value of the coordinate Z is Zl, the value Zl is stored in the other one of the registers comprising register RTI.

The tool numbers and the coordinates (X, Z) of the edges of the remaining tools are also stored in the third memory area 44-3 by following steps which are similar to the above steps (a) through (h).

(2) Execution of the program provided by the instruction tape 23.

(I) The mode selection switch 48-1 in the digital data input device 48 is next set to the "instruction tape control" (TAPE) mode.

Then, the control program for the TAPE mode is accessed from the first memory area 44-1 of the memory unit 44.

(II) The CPU 43 instructs the tape reader control circuit 46 to start actuating the tape reader 45. Thereafter, actuating the tape reader 45 carries out the operation of reading out the data recorded on the instruction tape 23. The tape format of the read-out data is similar to the conventional tape format shown in Fig. 4; however, the conventional data blocks 31, 32, 33, 34, 35, etc., are omitted from the tape format of the embodiments of the present invention. The operations corresponding to these blocks 31, 32, 33, 34, 35, etc., are all replaced by the initial set-up of data carried out by following the above steps (a) through (h) to derive the predetermined coordinates.

(III) The conventional tool selection instruction code, for example Tl* is read out from the instruction tape. The code Tl selects a tool No. 1. This tool number is transferred to the register RT in the second memory area 44-2 and stored therein. Since tool No. 1 is selected, the address in the memory unit 44-3 which stores the value of the predetermined coordinates (i.e., (Xl, Zl)) of the edge of the tool corresponding to tool No. 1, is instructed from the CPU 43 in accordance with the control program for the TAPE mode. The values of the coordinates (Xl, Zl) are read out sequentially from the memory area 44-3 corresponding to the instructed address.The read-out values of the coordinates (Xl, Zl) are transferred to the registers RXa and RZas respectively, and stored therein. Accordingly, the present position coordinate values (X, Z) in the registers RXa and RZa are replaced by the coordinate values (Xl, Zl). At this stage, the process for defining the coordinates (X, Z) of the edge of the tool No. 1 is completed.

(IV) The following absolute instruction values corresponding to tool No. 1 are read out from the instruction tape. The read-out absolute instruction value, XA, corresponding to the X-axis and the read out absolute instruction value, ZA, corresponding to the Z-axis are respectively, stored in the registers RXA and RZA in the second memory area 44-2.

(V) The incremental instruction value XI is derived frdm the following equation: XAXaXI by means of a known arithmetic circuit (not shown). The value X8 is the present position coordinate value which was stored previously in the register RXa during the above step (III). At this stage, the value X, corresponds to Xl. The derived incremental instruction value XI is then stored in the register RXI in the second memory area 44 2.

Similarly, the incremental instruction value ZI is derived from the following equation: ZAZa=ZI by means of the known arithmetic circuit.

.The value Za is the same as the present position coordinate value which was stored previously in the register RZa during the above step (III). At this stage, the value Z.

corresponds to ZI.

(VI) The incremental instruction values XI and ZI are respectively transferred to the registers RXII and RZII of the interpolator (INT) 49 and stored therein.

(VII) The interpolator 49 distributes control pulses to the servo-control circuits 42X and 42Z, lespectively, in accordance with the incremental instruction values XI and ZI. Accordingly, the specified tool is moved along the desired route in the machine tool 11, by means of the servomotors 17X and 17Z, the table 14 (Fig. 1) and the cross slide 18 (Fig. 1).

In this step (VII), each time a control pulse is applied to the servo-control circuit 42X or 42Z, the present position coordinate values X. and Za in the register RXa and RZa are updated by adding the value + 1 or -1 thereto in accordance with the direction moved by the tool.

(VIII) When it is desired to select a second tool the predetermined coordinate values of the first and second tools previously stored in registers RT 1 and RT2 are subtracted from each other and the difference is added to the present position - coordinate values stored in registers RXa and RZ. The coordinates stored in registers RXa and RZ. express the present position - coordinates of the second tool and as a result the next set of absolute instructions may be entered into registers - RXa and RZa expressed with respect to the origin PO.

In the above steps (a) through (h) and (I) through (VIII), the predetermined coordinate values of the edges of each of the tools are expressed by absolute values with respect to the origin PO (Figs. 2 and 3), for example, by the absolute values: (Xl, Zl) of the tool No. 1 (X2,Z2) of the tool No. 2 (Xn, Zn) of the tool No. n which are stored in the registers RT1, ....... RTN, respectively. However, the predetermined coordinate values of the edges of the tools may be expressed by relative values with respect not to the origin PO (Figs. 2 or 3) but with respect to the position of a reference tool, for example Tool No. 1, the co-ordinates of the position of the reference tool relative to the machine origin also being stored.In this case, the relative values: (0, 0) of the tool No. 1 (X2', Z2') of the tool No. 2 (Xn', Zn') of the tool No. n are stored in the registers RT1, RT2,...

RTN, respectively. If four tools are present, as indicated by 19-1 through 19-4 in Fig. 1, the above relative values are represented by X2, Z; X3, Z3, X4, Z4, as shown in Fig. 6. In this case, if the tool No. 2 is changed from tool No. 48 then the difference value (X4'-X2') and the difference value (Z4'-Z2') are added to the values X. stored in the register RXa and to the values Za stored in the register RZa respectively.The sum -of the stored value Xa and the difference value (X4'-X2') and the sum of the stored value Za and the difference value (Z4'-Z2') express the present position coordinate values of the edge of the tool (No. 4) relative to the machine origin.

In another case with respect to the four tools shown in Fig. 1, the relative values: (0, 0) of the tool No. 1 (X2'. Z2') of the tool No. 2 (X3', Z3') of the tool No. 3 (X4', Z4') of the tool No. 4 are stored in the registers RT1 through RT4, respectively. The values (X2', .......

(X4', Z4') are absolute values with respect to the coordinate value of the reference tool No. 1. In this case, with reference to Fig. 7, if the tool No. 2 is changed to the tool No. 4, the cross slide 18 is moved along the direction indicated by the said arrow 61, so that the edge of the tool No. 4 coincides with the edge of the tool No. 2.

The above movement of the cross slide 18 is caused by applying control pulses to the servo-control circuits 42X and 42Z (Fig. 5). The number of the applied control pulses corresponds to the difference values (X4'-X2') and (Z4'-Z2'). Accordingly, it is not necessary to update the values X. and Z. stored in the present - position - coordinate - value registers RXa and RZ" shown in Fig. 5, due to the initial movement of the cross slide 18 which causes the edge of the tool No. 4 to coincide with the edge of the last tool No.2.

In this case, proper gate circuits should be located at the input terminals of the registers RX. and Rza. Since the gate circuits are closed when movement of the cross slide is being carried out, the difference values (X4'-X2') and (Z4'-Z2') are prevented from being applied to the registers RXa and Rza. Should it not be possible for gate circuits to be located at the input terminals of the registers RX. and RZ.

for some particular reason, the difference values (X4'-X2') and (Z4'-Z2') will be applied to the registers RXa and RZa Accordingly, the present position coordinate values stored in the registers RX. and RZa corresponding to the tool No. 2 will be changed.In this case the stored value should not be changed, instead the actual position of the edge of the tool No. 4 should only be shifted to the last position where the edge of the tool No. 2 was located However, where the difference values (X4'-X2') and (Z4'Z2') are applied to the registers RXa and RZa, respectively it is necessary to subtract the values (X4'X2') and (Z4'-Z2') from the value X. in the register RX. and the value Za in the register RZa respectively, when the edge of the tool No. 4 reaches the last position located on the edge of the tool No. 2, the position where the tool No. 2 finishes its work.

Hereinbefore, only a tool post having tools arranged in the form of a comb is disclosed with reference to Figs. 1, 2, 3, 6 and 7 as an embodiment of the present invention.

However, a conventional turret tool post can also be employed in the machine tool according to the present invention.

As mentioned above, typical and conventional program codes, such as "G50X . . Z . . .*", which must be created every time a tool change occurs, may be omitted from the program of the invention.

As a result it becomes much easier for an operator to set up a program.

WHAT WE CLAIM IS: 1. A numerically controlled machine tool system comprising a machine tool having a chuck for holding a workpiece, a cross-slide for holding a plurality of tools adapted to be sequentially selected for use upon the workpiece, a numerical control unit for controlling movement of the cross-slide along at least two axes relative to the workpiece, and present-positioncoordinate value registers contained in the numerical control unit for storing the present - position - coordinate values relative to reference point of the edge of whichever tool has been selected for use upon the workpiece, characterised in that the numerical control unit further comprises means for storing predetermined coordinate values representative of the positions of the edges of each of the tools relative to the reference point when the cross-slide is in a predetermined position relative to the chuck, means for determining the difference between the stored predetermined coordinate values of a selected tool and a further tool which is to be selected next, and means operative when said further tool is selected to either update by said difference the content of the present - position - coordinate value registers or to move the said further tool a distance indicated by said difference, whereby after said further tool is selected the content of the present position coordinate registers represent the position of the edge of said further tool relative to the reference point.

2. A numerically controlled machine tool system according to claim 1, characterised in that the predetermined coordinate values of the edges of each of the tools are defined with respect to the edge of a selected one of the tools, the coordinates of which are defined relative to the chuck.

3. A numerically controlled machine tool system according to any preceding claim, wherein said operative means moves the said further tool a distance indicated by said difference, characterised in that means are provided to maintain the content of the present - position - coordinate value registers unchanged as the edge of the said further tool is moved by said operative means.

4. A numerically controlled machine tool system according to claim 1, 2 or 3 wherein as the said operative means moves the said further tool by a distance indicated by said difference, the contents of the present position - coordinate value registers are changed by the said difference in the predetermined coordinates between the selected tool and the further tool to be selected, characterised in that means are provided for subtracting the said difference from the contents of the present position - coordinate register when the said further tool has reached the coordinates previously occupied by the said selected tool.

5. A numerically controlled machine tool substantially as hereinbefore described with reference to Figs. 5, 6 and 7 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. position of the edge of the tool No. 4 should only be shifted to the last position where the edge of the tool No. 2 was located However, where the difference values (X4'-X2') and (Z4'Z2') are applied to the registers RXa and RZa, respectively it is necessary to subtract the values (X4'X2') and (Z4'-Z2') from the value X. in the register RX. and the value Za in the register RZa respectively, when the edge of the tool No. 4 reaches the last position located on the edge of the tool No. 2, the position where the tool No. 2 finishes its work. Hereinbefore, only a tool post having tools arranged in the form of a comb is disclosed with reference to Figs. 1, 2, 3, 6 and 7 as an embodiment of the present invention. However, a conventional turret tool post can also be employed in the machine tool according to the present invention. As mentioned above, typical and conventional program codes, such as "G50X . . Z . . .*", which must be created every time a tool change occurs, may be omitted from the program of the invention. As a result it becomes much easier for an operator to set up a program. WHAT WE CLAIM IS:

1. A numerically controlled machine tool system comprising a machine tool having a chuck for holding a workpiece, a cross-slide for holding a plurality of tools adapted to be sequentially selected for use upon the workpiece, a numerical control unit for controlling movement of the cross-slide along at least two axes relative to the workpiece, and present-positioncoordinate value registers contained in the numerical control unit for storing the present - position - coordinate values relative to reference point of the edge of whichever tool has been selected for use upon the workpiece, characterised in that the numerical control unit further comprises means for storing predetermined coordinate values representative of the positions of the edges of each of the tools relative to the reference point when the cross-slide is in a predetermined position relative to the chuck, means for determining the difference between the stored predetermined coordinate values of a selected tool and a further tool which is to be selected next, and means operative when said further tool is selected to either update by said difference the content of the present - position - coordinate value registers or to move the said further tool a distance indicated by said difference, whereby after said further tool is selected the content of the present position coordinate registers represent the position of the edge of said further tool relative to the reference point.