Abstract:
In the present invention, the breakage of a cutting tool is sensed during a cutting operation of stock on a lathe. A boundary is defined around the stock. The cutting tool is withdrawn stepwise along the same path taken up to the point of breakage. After each step, inquiry is made as to whether the tool has crossed the boundary. When it does, the tool is brought directly to a tool changing station without further stepwise withdrawal.

Description:
This application is a continuation of application Ser. No. 703,954, filed Feb. 21, 1985, now abandoned. 
    
    
     The present invention relates to the automatic replacement of cutting bits in machine tools such as latches, drilling machines, boring machines and milling machines. 
     BACKGROUND OF THE INVENTION 
     When a cutting bit breaks, during a cutting operation, an operator must manually remove the cutting bit and replace it. This requires that an operator be available at the machine tool, and risks damage to the workpiece and machine tool because of error by the operator in choosing a suitable path along which to move the bit away from the workpiece so it can be replaced. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide a new and improved procedure for automatically replacing machine tool cutting bits. 
     SUMMARY OF THE INVENTION 
     In one form of the present invention, the breakage of a cutting tool is sensed during a cutting operation of stock on a lathe. A boundary is defined around the stock. The cutting tool is withdrawn stepwise along the same path taken up to the point of breakage. After each step, inquiry is made as to whether the tool has crossed the boundary. When it does, the tool is brought directly to a tool changing station without further stepwise withdrawal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates stock being cut in a lathe. 
     FIG. 2 illustrates the stock of FIG. 1 in perspective. 
     FIG. 3 illustrates a boundary (i.e., a &#34;safe zone&#34;) defined around the stock of FIG. 1. 
     FIG. 4 is a flow chart which describes one sequence of events in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates in cross section a workpiece 3 to be cut from stock 6 in a lathe (not shown). The workpiece 3 and stock 6 are illustrated in perspective view in FIG. 2. The lathe rotates the workpiece about the spindle centerline (z-axis). The cutting is done by a cutting bit 9 which is moved by a tool holder (also not shown), as is well known in the machine tool art. As FIG. 1 shows, the cutting bit follows dotted path 12 in cutting inner portion of the workpiece 3. If the cutting bit 9 should break when it is positioned at point 15, and if the breakage is detected by breakage sensor 16, the present invention operates as follows. A safe zone 18, shown in FIG. 3, is called into use by the Machine Control Logic (MCL) which is available with General Electric Mark Century 2000 Numerical controls (NC). The safe zone is defined in the part program and represents the minimum cylindrical region which will completely contain the workpiece profile including all of its stock. It can be easily defined again in the part program if desired, after any amount of stock has been removed. The MCL checks the position of the broken bit 9 against the safe zone boundaries, and safely retracts it out of the workpiece to the tool change position, in an automatic manner. The safe zone 18 defines a radial outer boundary X2 and radial inner boundary X1 (X1 and X2 are radial with respect to the workpiece on lathe spindle centerline), and also defines an axial leftmost boundary Z1 and axial rightmost boundary Z2. 
     The MCL allows axial motion of the bit 9 only when it is radially outside boundary X2 or radially inside boundary X1, and axially inside (i.e., to the left) boundary Z1 and axially outside (to the right) boundary Z2. Thus, a collision with the workpiece is prevented by these restrictions. 
     The MCL allows the bit 9 to move within the safe zone boundaries under control of the NC program, unless and until the break sensor 16 detects that the cutting edge of the bit has broken. Then the bit 9 is retracted by the MCL along the identical path 12 defined in the program which it followed in cutting to the point of breakage, point 15. However, the retraction is executed block-by-block, in reverse order. (A block, as is known in the NC machining art, produces a single cutting step, such as a straight line or an arc. A complete cut is produced by a sequence of blocks, each representing a portion of the geometric shape of the complete cut.) 
     After each block of retraction, the MCL tests whether the safe zone boundary has been reached or penetrated. If not, another retraction step (or block) is executed, followed by a repetition of the inquiry as to reaching or penetrating the safe zone. As soon as the safe zone is penetrated (at either the radial safe zone boundary indicated by X1 or X2 or other outer axial safe zone boundary indicated by Z2), the retraction sequence is terminated, and the MCL commands the bit to move directly to the Z-axis home position, along path 24A. Following this Z-axis homing move, and not before it, the MCL then commands a homing move to the X-axis home position, along path 21A, thereby bringing the bit 9 to the home position of origin O. 
     Of course, Z-axis homing cannot be done if only the inner safe zone boundary (indicated by Z1) is penetrated because the bit 9 may then strike the stock (or workpiece 3) during homing. However, the preferred embodiment is concerned with cutting operations in which it would be unlikely that the bit 9 would cross the axial inner safe zone boundary Z1 during retraction. Therefore, in practice, only three safe zone boundaries, namely, X1, X2, and Z2 would probably be tested. 
     After the bit 9 reaches the origin O, the bit is changed, either manually, or automatically, with changing apparatus known in the art. At the time of safe zone boundary crossing, the X and Z coordinates of the cutting bit 9 are stored by the MCL. Following tool bit change, MCL executes the X and Z homing moves in reversed order, and in reversed directions, along paths 21B and 24B, and brings the cutting but 9 to the boundary crossing point. 
     The MCL then moves the bit along the path 12 up to the point 15 of breakage along the same path as originally executed prior to the break. When the point 15 of breakage is reached, the MCL allows the bit to follow its programmed instructions to finish cutting the workpiece 3. 
     A flow chart of one procedure which implements the present invention is given in FIG. 4 and is considered self-explanatory. 
     Numerous modifications and substitutions can be undertaken without departing from the true spirit and scope of the present invention. 
     What is desired to be secured by Letters Patent of the United States is the invention as defined in the following claims.