GB2334226A

GB2334226A - Overload detection

Info

Publication number: GB2334226A
Application number: GB9803175A
Authority: GB
Inventors: Colin Andrew Miles; Simon James Albert Hewson
Original assignee: TDT TECHNOLOGY Ltd
Current assignee: TDT TECHNOLOGY Ltd
Priority date: 1998-02-16
Filing date: 1998-02-16
Publication date: 1999-08-18
Anticipated expiration: 2018-02-16
Also published as: GB9803175D0; GB2334226B

Abstract

A method and apparatus arranged to detect the mechanical overload of a machine tool, for example, a CNC lathe, comprises the use of one or more force measurement elements 12 for detection of surface distortion of said machine tool. Control means 16 receives information regarding the surface distortion and controls the power supply to the machine tool such that the machine tool is placed in an inoperative state when a threshold value of distortion is exceeded. The threshold value can be set by pre-loading the element 12 by setting screw 28.

Description

CRASH DETECTION APPARATUS This invention relates to an apparatus for detecting mechanical overload of a machine tool, for example, a CNC lathe or machine centre.

Many known machine tools have incorporated into different parts of the machine one or more monitoring systems for checking and preferably, correcting a particular function, for example, the wear of a tool or the efficiency of a machining process. One advantage of using such monitoring systems is to improve the accuracy and/or performance of the machine tool or machining centre.

There is a tendency for some known monitoring systems to incorporate many different measurements which results in a complex monitoring system which can itself be subject to reliability problems. There are also many types of sensors used with monitoring systems, including spindle power sensors, feed force sensors, piezo electric force sensors and magnetic inductive force sensors. Each type of sensor is generally used for a particular application, depending upon the attributes of that sensor. The use of more complex amplifiers and electronic processing systems to monitor an array of different attributes is considered by those in the machine tool industry to be the way forward.

Most known monitoring systems (particularly single axis or three axis force sensors) are required to be fitted by the machine manufacturer during construction of the machine. Thus, it is normal for machine modifications to be made. This can mean machining major parts of the construction to accept the sensors and then engineering a complicated interface between the systems.

However, there are problems associated with such use. For example, the devices according to the prior art require complex circuitry having a relatively large number of components such as visual output displays. Such devices are relatively expensive to manufacture.

Additionally, the known devices can also suffer from reliability problems, typically damage or false alarms due to the extreme and/or hazardous conditions that electronic circuitry has to operate in.

The inventor has identified another problem of the prior art in particular relating to the detection of mechanical overload caused by the condition of a cutting tool for example, an unintentional collision between the machining tool and work piece caused by machine malfunction or operator error. This type of mechanical overload is often referred to as a "crash". It is important for the machining centre or lathe to be disconnected from a power source as quickly as possible if a machine overloads, by, for example, the breaking of the cutting tool. However, those known monitoring systems that detect mechanical overload are required to process information at a very fast rate to compensate for this complexity in order to disconnect the power source. It is necessary to use a more costly processing capacity.

By contrast, the present inventor has, by going in a direction contrary to conventional thought, designed an apparatus which can be used to detect mechanical overload which has proved successfiil in confidential trials even though it has dispensed with features previously considered essential, such as to provide an apparatus which is accurate, reliable and fast to respond to external forces due to the position of the apparatus in the machine tool. Further, it is simple to replace if damaged. Beneficially the device of the invention is inexpensive to buy and easy to install.

Therefore, the present invention and its preferred embodiments seek to overcome or at least mitigate the problems associated with the prior art.

According to one aspect of the invention there comprises an apparatus arranged to detect the mechanical overload of a machine tool, for example, a CNC lathe, which apparatus comprises one or more force measurement elements for detection of surface distortion of said machine tool, and control means comprising means for receiving information regarding the surface distortion which control means controls the power supply to the machine tool such that the machine tool is placed in an inoperative state when a threshold value of distortion is exceeded.

According to an optional feature of this aspect of the invention, said force measurement element is fitted to an outer surface of said machine tool.

According to another optional feature of this aspect of the invention, said force measurement element is provided with adjustment means to adjust the measurement sensitivity of said element.

According to a yet further optional feature of this aspect of the invention, there further comprises a signal processing means adapted to receive a signal generated by said force measurement element and to generate a signal to said control means, when the threshold value is exceeded.

According to another further optional feature of this aspect of the invention, the signal processing means comprises a manual input means for inputting said threshold value.

According to a further optional feature of this aspect of the invention, the force measuring element is a piezo-electric transducer.

According to another aspect of the invention, there comprises a method of monitoring of machine tools, comprising the steps:- (i) monitoring the surface distortion of said machine tool during processing of a work piece; (ii) generating a signal when a threshold of distortion is exceeded; (iii) causing the signal to be received by a control means and (iv) controlling the power supply to the machine tool, thereby to disconnect the power supply to the machine when said threshold is exceeded. Optionally, the monitoring of surface distortion involves measuring the forces transmitted through the machine tool.

Embodiments of the invention will now be described by way of example only, by reference to the accompanying drawings, in which: FIGURE 1 illustrates a simplified layout of a first embodiment the invention mounted to a machine tool.

FIGURE 2 illustrates a simplified layout of a second embodiment of the invention.

Referring to Figures 1 and 2, there is shown an apparatus 10 arranged to detect the mechanical overload of a machine tool, for example, a CNC lathe or machine centre. The apparatus comprises one or more force measurement elements 12 for detection of surface distortion of the machine tool and control means 14 comprises means 16 for receiving information regarding the surface distortion which control means controls the power supply to the machine tool such that the machine tool is placed in an inoperative state when a threshold value of distortion is exceeded.

In the illustrated embodiments, the force measuring element 12 is a transducer which is fitted directly onto the outer surface of the machine casting (or fabrication) 18, for example, at the rear of the indexing turret 20 of the CNC lathe, sandwiched between the bracket fitted to the cross slide 22 and the main indexing body. It is preferred that the force measuring element 12 is maintained in direct contact to the surface of the casting (or turret) and that it is fitted in the plane of the applied stress S from the turret or work piece. In this embodiment, the transducer is a piezo electric element, although it is envisaged that other direct contact force measuring elements can be used without departing from the scope of the invention. For most uses the transducer would have a maximum safe working load of 350kg. v As shown in Figure 1, the transducer is pre-loaded to a required torque setting, for example 5Nm, by means of a setting screw 28 or other suitable adjustment means. Of course, the use of such adjustment means is optional depending upon the siting of the transducer.

Turning again to Figure 1, the force measuring element 12 produces a signal, for example a voltage output, which is substantially proportional to the surface distortion of the machine casting 18 caused by the machining cycle. The signal should be continuous and diagnostic means is preferably provided to check the signal is being generated by the sensor. For example, an LED can be used to indicate that the signal is being received by the control means.

Acoustic emission and vibration or other transient frequencies are preferably removed by suitable filtering, although in trials it has been found that the signal generated by the sensor is not significantly distorted by these transient frequencies, so in some embodiments, the filtering circuitry is removed.

The output voltage is transmitted to the control means 14 where the values are compared to a threshold value which is commonly pre-set. In this embodiment, a signal processor unit 24 is incorporated into the control means for receiving and comparing the output voltage to the threshold value. Typically, the threshold value is set at 50% to 70% over the most stringent cutting forces. In one class of embodiments, a range setting facility or manual input means (not shown) isEprovided to vary the threshold value, so that it can be altered to a required value in situ.

A machine control unit 26 forming part of the control means is connected to the signal processor unit 24 and this adapted to receive a signal generated from the signal processor unit (or control means) in the event that a mechanical overload occurs. In other embodiments the signal generated by the signal processor can be in a digital form.

During the normal machining cycle, the forces applied to the machine casting are below the critical threshold so no signal is transmitted from the signal processor unit to the machine control unit.

In the event that a mechanical overload occurs, for example, a machine crash, an immediate exaggerated surface distortion occurs which is picked up by the transducer 12. Thus, the transducer is caused to produce a high voltage output which is transmitted to the signal processor 24. This value is compared with the threshold value and where the mechanical overload has occurred, a signal is generated to the machine control unit 26 which in turn, disconnects the power supply to the machine tool, thereby placing it in an inoperative state: commonly, the control unit would be incorporated into the emergency stop circuitry, or in appropriate circumstances, a feed hold/spindle stop circuit.

The second embodiment illustrated in Figure 2 is substantially identical to the first embodiment and so only those differences are described. Thus, a force measuring element 12 is connected to the machine casting or fabrication 18 by a fixing bracket 30. The adjustment means comprises a setting screw 32 mounted in a locking nut 34.

It will be appreciated that the simplicity of such a system produces an accurate and reliable monitoring device. The minimum amount of information transmitted between signal processor and the machine control unit provides a fast response time thereby minimising the unwanted damage to the machine tool.

A further advantage of using a direct contact sensor is that unlike contactless sensors for example magnetic inductor sensors, it is largely unaffected by coolant or material ingress which would adversely affect the signal or introduce a reliability problem.

It is envisaged that the present invention in its preferred embodiment can be fitted to a machine during its construction or supplied as a module to be fitted to a machine on a retrofit basis. Ideally, the present invention would be suited for machines with up to 60kw spindle power.

The preferred embodiment described herein is shown as part of a machine for CNC lathe although the invention is not limited to machine tools of this type. As will be recognised by those skilled in the art, the invention may be used or incorporated into any machining process where it is required to detect mechanical overload of that machine.

Modifications may be incorporated without departing from the scale of the present invention as defined in the accompanying claims.

Claims

CLAIMS 1. An apparatus arranged to detect the mechanical overload of a machine tool, for example, a CNC lathe, which apparatus comprises one or more force measurement elements for detection of surface distortion of said machine tool, and control means comprising means for receiving information regarding the surface distortion which control means controls the power supply to the machine tool such that the machine tool is placed in an inoperative state when a threshold value of surface distortion is exceeded.
2. An apparatus as claimed in claim 1 wherein said force measurement element is in direct contact with an outer surface of said machine tool.
3. An apparatus as claimed in claim 1 or claim 2 wherein said force measurement element is provided with adjustment means to adjust the measurement sensitivity of said element.
4. An apparatus as claimed in any of claims 1 to 3 further comprising a signal processing means adapted to receive a signal generated by said force measurement element and to generate a signal to said control means, when the threshold value is exceeded.
5. An apparatus as claimed in claim 4, wherein the signal processing means comprises a manual input means for inputting said threshold value.
6. An apparatus as claimed in any preceding claim wherein the force measuring element is a piezo-electric transducer.
7. A method of monitoring of machine tools, comprising the steps: (i) monitoring the surface distortion of said machine tool during processing of a work piece; (ii) generating a signal when a threshold of distortion is exceeded; (iii) causing the signal being received by a control means and (iv) controlling the power supply to the machine tool, thereby to disconnect the power supply to the machine when said threshold is exceeded.
8. The method of claim 8 wherein the monitoring of surface distortion involves measuring the forces transmitted through the machine tool.
9. Use of an apparatus of any of claims 1 to 7 to regulate the power to a machine tool thereby to stop the machine when the threshold of distortion is exceeded.
10. An apparatus substantially as hereinbefore described by reference to or as illustrated in Figure 1 or Figure 2.