Patent Publication Number: US-2005119781-A1

Title: Detecting breakages in machine tools and the like

Description:
FIELD OF THE INVENTION  
      This invention concerns the detection of an object movable along a path to carry out a process on a workpiece, particularly but not exclusively for detecting cutter breakage in computer numerical control (CNC) machine tools intended to run unmanned.  
     DESCRIPTION OF THE RELATED ART  
      Modern machine tools are very versatile. A CNC lathe, for instance, can be provided with a range of cutters programmed to engage a workpiece successively to produce a complex shape. This versatility has facilitated efficient production management methods such as zero-stock, just-in-time and kanban manufacturing. However these methods have in turn given rise to great variability in throughput. With demand for a particular machine varying by 200% or more, the provision of capacity to cover peak demand results in an unacceptable surplus when demand is low. Unmanned operation, where a machine tool runs automatically without a machinist present, has been introduced to deal with this.  
      Unmanned operation creates a further problem: there is no machinist present to change a cutter which breaks, which inevitably happens from time to time, especially with workpieces of materials such as high alloy steels which are difficult to machine. And as a result, if the machine tool is left unmanned for a long period such as overnight, a great deal of its output may be scrap and other damage may be caused by its continuing to run with a broken cutter.  
      To counter this, various proposals have been made to monitor the machine tool and stop it automatically if a cutter breaks. Most commonly in these prior proposals, an example of which is described in U.S. Pat. No. 5,342,275, some kind of sensor is intermittently moved to engage the cutter and stop the machine tool if it does not engage the cutter. The drawbacks in this approach are (a) the time required for the sensor to complete its monitoring cycle, which may be as long as 20 s with some arrangements (which means 20 s of lost production capacity for every tool monitored per cycle), (b) the need to set the sensor accurately, and possibly reset it every time a cutter is changed, and (c) at least for a CNC machine tool, the need to program the movement of the sensor if production versatility is not to be compromised. Other approaches include monitoring the machining load (as proposed in British Patent GB 2 244 610. for example) and monitoring machining vibration (as in U.S. Pat. No. 4,724,524). However each of these approaches requires, as well as customised setting, complex processing likely to be expensive.  
      It is an object of the present invention to provide a means of detecting cutter breakage which is simple, inexpensive and as versatile as the machine to which it is applied.  
     BRIEF SUMMARY OF THE INVENTION  
      Thus according to the invention there is provided a control system for apparatus in which an object is movable along a path to carry out a process on a workpiece, the apparatus having start means and stop means, which control system comprises a fixed sensor engageable by the object at a position on said path where the object comes to rest and check means operatively associated with the sensor and arranged so that the object stays at its rest position unless the object engages the sensor thereat  
      Preferably the stop means is actuated automatically to stop the apparatus when the object comes to its rest position and the check means then automatically actuates the start means only if the object has engaged the sensor. The start means and the stop means may each be manually operable.  
      The system preferably includes programming means whereby the path of the object may be set.  
      The system may include signal means operable automatically if the object has not engaged the sensor at the rest position.  
      The sensor may comprise a probe operatively connected to a switch.  
      For application to apparatus having a process cyclical over a set period, the system may include shut-down means operable to stop said apparatus automatically if said object does not engage the sensor after a time lapse longer than said period but shorter than two said periods The shut-down means may also be operable to stop the apparatus in response to a receipt of a fault signal from equipment associated with said apparatus. The system may include means for varying the time lapse and may also include means for alternatively enabling and disabling the shutdown means.  
      The invention is particularly applicable to apparatus being a machine tool wherein said object is a cutter thereof, In this application, in contrast to previous proposals in which a moving sensor is used for detecting cutter breakage in the present invention the cutter (which is necessarily movable for machining) is brought to a fixed sensor. Those skilled in the science will appreciate that this arrangement, as well as saving the cost and complexity of moving the sensor is particularly suitable in CNC or other programmable machine tools, because these allow the path of the cutter to be readily varied. Thus, within the parameters of a given machine, any cutter can be brought to the sensor, wherever positioned. In practice, of course, it is preferred to position the sensor so that the path of the cutter is as close as possible to its machining movement, whereby the time lost in checking the cutter is minimised. For machine tool application, the preferred arrangement is that the programming means (which may be computer numerical control) brings the cutter to a haft at said rest position and, if the cutter does not engage the sensor thereat, the check means does not operate the start means and the cutter remains at the rest position. It will be noted that the control system thus operate independently of the programming means and without requiring any modification of its program.  
      A machine tool embodying the invention may include a spindle operative to rotate the workpiece about a longitudinal axis, the path of the cutter having forward and rearward longitudinal end points. In such an arrangement the sensor is preferably located at the forward end point of the path of the cutter. The path of the cutter may lie substantially in a plane containing said longitudinal axis, with the sensor located laterally of said longitudinal axis and in said plane. The machine tool may include a turret for carrying said cutter and other cutters wherein the sensor is so located as to be clear of the turret and the other cutters (being in the same plane as the cutter in use).  
      The machine tool may be arranged for operation under a CNC system including a macro operative (a) to check the CNC programming relative to the sensor, (b) to provide information to an operator and (c) automatically to control movement of the cutter to the sensor. A macro is a programming subsystem that automates frequently used sequences, and those skilled in the science will appreciate that in the present invention it can provide automatic checking of the CNC programming format and information concerning safe usage of the control system, provide messages for operator information and guidance, and ensure that the relevant part of a cutter is automatically presented to the sensor without operator calculation or risk of collision—irrespective of the program work shift, tool offsets, tool length, tool type, tool projection or tool size. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The invention will now be described by way of example only with reference to the accompanying drawing, which illustrates the application of the invention to a computer numerical control lathe and in which  
       FIG. 1  shows schematically a machined product with a profile produced by the lathe;  
       FIG. 2  shows schematically how the lathe used to produce the product of  FIG. 1  may be arranged to detect cutter breakage;  
       FIG. 3  is a schematic illustration of the overall control system of the arrangement of  FIG. 2 ;  
       FIGS. 4 and 5  illustrate a preferred position of a sensor in a lathe. 
    
    
     WRITTEN DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION  
      Referring first to  FIG. 1 , this shows a product  10  with an external profile which may be produced by a lathe. In such a lathe, as shown in  FIG. 2 , a longitudinally extending bar  12  is rotated about a longitudinal axis  14  (commonly called the Z axis) and a cutter  16  is moved along a path, indicated in brokene lines at  18 , to machine the bar  12 . Thus, to machine the rotating bar  12 , the cutter  16  moves generally in the direction of arrow A as well as towards and away from the Z axis  14 .  
      The profile of the product  10  is simple, and other, more complex profiles may be produces by a lathe. However those skilled in the science will appreciate that even a simple profile may be difficult to produce with certain materials such as high alloy steel, which require large machining forces and creates swarf that tends to interfere with the machining process. With these materials, cutters like the cutter  16  are often broken.  
      Breakage of the cutter  16  is a particular problem if the machine tool is unmanned because, at the least, it means that bars  12  subsequently loaded into the tool (eg by an automatic bar feeder) do not receive the intended profile. And damage may be caused, for instance if the cutter breaks in mid-process and swarf is wrapped around the rotating bar  12 .  
      To detect cutter breakage, and stop the lathe if this occurs, a sensor  20  is positioned along the path  18  of the cutter  16 . The sensor  20  comprises a microswitch  22  operatively connected to a probe  24  extending across the path  18  of the cutter  16 . The lathe is programmed to bring the cutter  16  to rest at the position of the sensor  20 . If, but only if, the cutter  16  engages the probe  24  in this rest position, then the microswitch  22  closes to re-energise the drive to the cutter  16 , which then returns to its original position to begin another cycle.  
      It will be noted that the movement of the cutter  16  is not materially different from that required for the machining process, so the time lost in checking the cutter  16  is minimal. Tests show that checking a cutter by the arrangement of  FIG. 2  can be done in as little as 2 s, compared with perhaps 20 s if a sensor has to be moved to the cutter. Also, the sensor  20  is fixed and the CNC system of the lathe itself is used to move the cutter  16  (to the sensor  20 ) in the same way as it is moved for machining, so there is no need to modify or even interface with the CNC system.  
      It may be noted at this point that the microswitch  22  may be replaced by, for instance, a solid state proximity switch or possibly a camera or other means to generate a signal when the cutter  16  comes to the sensor  20 .  
      The control mechanism is illustrated in  FIG. 3 . The sensor  20  is operatively connected to a cycle start button  26  by way of a programmable logic controller  28 . If the sensor  20  is not engaged by the cutter  16  ( FIG. 2 ) when this comes to its rest position, the start button  26  is not operated, and the lathe remains still. It will be noted that the circuitry is independent of the CNC system, and is separately powered from a 0-24V supply.  
       FIG. 3  also shows a shut-down control  30 , which is alternatively enabled and disabled (conveniently by means of a switch not shown readily accessible to an attendant). The shut-down  30  control is enabled when the machine tool is left unmanned for a prolonged period, eg overnight. When enabled, the shut-down control  30  stops the machine tool automatically if either (a) there is a lapse of time longer than a process cycle without the sensor  20  being engaged by the cutter  16  or (b) the shut-down control receives a fault signal from a bar feeder associated with the machine tool.  
      Referring now to  FIGS. 4 and 5 , these illustrate a preferred position of a sensor.  FIG. 4  is a schematic end view of a lathe embodying the invention and  FIG. 5  is a side view corresponding to  FIG. 4 . The same reference numerals are used in both figures.  
      A workpiece  40  is mounted in a spindle  42  for rotation about the Z axis  44 . A cutter  46  is mounted in a turret  48  which also carries a number of other cutters  50 . As will be well known to those skilled in the science, the turret is operated under CNC to move the cutter  46  along a path in a plane  52  containing the Z axis  44  and the X axis of the lathe. This path has a forward end point  54  and a rearward end point  56  respectively marginally beyond the opposite longitudinal ends of the length L of the workpiece  40  to be machined. A sensor  58  is located at the forward end point of the path of the cutter  46 , laterally of the Z axis  44  (ie displaced from the Z axis along the X axis). The cutter  46  is detected by the sensor  58  at the forward endpoint of its path, but the turret  48  and the other tools  50  remain clear of the sensor  58 .  
      The lathe is operated under a CNC system including a macro which provides automatic checking of the CNC programming format and information concerning safe usage of the control system, provides messages for operator information and guidance, and ensures that the relevant part of the cutter  46  is automatically presented to the sensor  58  without operator calculation or risk of collision—irrespective of the program work shift, tool offsets, tool length, tool type, tool projection or tool size. As and when the turret  48  is turned for use of one of the other cutters  50 , this other cutter is likely to have a position in the plane  52  and/or a projection from the turret  48  different from that of the preceding cutter  46 , in which case the macro automatically repositions the new cutter  50  to be correctly positioned for engagement with the sensor  58 .  
      Those skilled in the science will appreciate that the sensor  58 , being in the same plane as the cutter  46 , is so located that none of the other cutters  50  on the turret  48  can collide with it. Further, irrespective of workpiece size, or the employment or otherwise of a tailstock, the position of the sensor  58  is such that it will correctly monitor any cutter  50  which is used.  
      The invention is not limited to lathes, or indeed to machine tools, and those skilled in the science will appreciate the benefits of the invention when applied to a wide range of automated equipment operated unmanned. Briefly summarised, these are improved performance (including evidence of adequate process control during unmanned operation), increased efficiency and avoidance of equipment damage.