GB2099536A

GB2099536A - Guard system

Info

Publication number: GB2099536A
Application number: GB8213732A
Authority: GB
Original assignee: Associated Steels & Tools Co L
Current assignee: Associated Steels & Tools Co L
Priority date: 1981-05-12
Filing date: 1982-05-12
Publication date: 1982-12-08
Also published as: GB2099536B

Abstract

A guard system for a crankshaft driven machine tool such as a press brake includes a photo electric curtain 22 which observes a danger zone of the machine. If broken during the cycle of operation of the machine contacts 14 in the clutch control circuit will be opened to disengage the clutch. If the curtain is broken during the normal period required for the machine to come to rest at the end of a working stroke, an alarm 43 is activated and the control system of the machine then requires to be reset before a further working stroke of the machine can be effected. <IMAGE>

Description

SPECIFICATION Guard system This invention relates to guard systems for crankshaft driven machine tools more particularly but not exclusively press brakes.

A known form of guard system for a press brake comprises a so-called photo-electric curtain which essentially comprises one or more photo-electric devices and the associated light sources. The photo-electric device or devices is/are associated with the clutch control circuit of the machine and observe the danger zone of the machine whereby if intrusion through the curtain takes place the clutch will be disengaged and the machine will stop. Press brakes in the past have been used almost exclusively for bending sheet metal, an operation which requires the operator to hold the sheet.

It has been the practice to allow the photoelectric curtain to operate only during what is considered the dangerous part of the machine cycle that is to say during the period from top dead centre to the point at which there is a very small gap (6 mm) between the tools. At this point the protection offered by the curtain is overridden to allow movement of the sheet of material in the danger zone, it being appreciated that the hands of the operator do not have to enter the danger zone.

There is however an increasing use of press brakes to process small components by fitting the press brakes with tools. Such a use of the machine often requires that the hands of the operator should enter the danger zone each cycle of the machine for the purpose of loading and unloading the component. It has been suggested that the guard system for such a modified press brake should be effective throughout 3600 of rotation of the crankshaft of the machine. A photo-electric curtain in its simple form cannot provide this protection since it is only capable of providing a stop signal to the clutch control circuit of the machine. Assuming therefore that the clutch is disengaged immediately upon intrusion into the curtain, the machine will not stop immediately. It has been estimated that some 200 ms is required for the machine to stop which represents about 400 of crankshaft rotation.During this period the machine is in effect unguarded and if the operator moves his or her hands into the danger zone there is the possibility of injury and particularly this is the case if for some reason the machine performs a repeat stroke.

The object of the invention is to provide a guard system for a machine tool in a simple and convenient form.

According to the invention a guard system for a crankshaft driven machine tool comprises a photoelectric curtain positioned in use to observe the danger zone of the machine tool, switch means operated by the curtain whereby if intrusion of the curtain occurs during a cycle of operation of the machine tool, the machine tool will be brought to rest and additional means operated by said curtain and operable to initiate an alarm and/or require resetting of the control circuits of the machine if intrusion of said curtain takes place whilst the machine is being brought to rest towards the end of a cycle of operation.

An example of a guard system in accordance with the invention will now be described with reference to the accompanying drawings; in which Figure 1 is a diagram showing the position of a switch operated by the crankshaft of the machine tool; and Figure 2 is a circuit diagram of the system.

Indicated at 9 in Figure 1 is the crankshaft of the machine tool having a cam 10 mounted thereon. Located at a position about 400 before top dead centre is a switch housing 8 which accommodates a pair of switches 11, 1 2.

Switch 11 is normally closed but is opened momentarily by the action of the cam 10 whereas switch 12 is normally open but is closed momentarily by the action of the cam.

The switch 11 is connected in a series circuit including a normally open pair of relay contacts 13 a further normally open pair of relay contacts 14, the normally closed contacts 1 5 of a foot pedal control and a relay coil 1 6. In parallel with the contacts 1 5 are a pair of normally open contacts 1 7 which are associated with the relay coil 1 6 and associated with this relay are a further pair of normally open contacts 1 8 which are connected in a series circuit including a solenoid 19 of the clutch control and a normally open pair of contacts 20 associated with the foot pedal.The contacts 14 are associated with a relay having a winding 21 which is supplied with electric current by means of a photo-electric curtain generally indicated at 22 and including a photo-electric device or devices 23 and a light source or sources 24.

The machine tool also includes a main motor control circuit including an electromagnetic contactor having an operating winding 35 one end of which is connected to a supply line 36 to which the relay winding 1 6 and clutch solenoid 1 9 are also connected. In series with the operating winding 35 is a normally open pair of relay contacts 37 associated with the relay winding 32.

The main motor control circuit also includes a motor start switch 38 and an emergency stop switch 39 connected in series and acting to connect the contacts 37 to a further supply line 40. In parallel with the switch 38 is a normally open pair of contacts 41 forming part of the contactor. The switch 11 is also connected to the supply line 40 by way of the switches 38 and 39.

One terminal of the switch 1 2 is connected to the supply line 40 and in parallel with this switch is a normally open pair of relay contacts, the relay having an operating winding 26 one terminal of which is connected to the supply line 36 and the other terminal of which is connected by a pair of timer contacts 27 to the switch 1 2. The actuating winding 28 of the timer is connected in parallel with the contacts 27 and the winding 26.

A manually operable reset switch forms part of the system and includes series connected normally open and normally closed switches 29, 30 respectively. In parallel with the switch 29 is a pair of normally open relay contacts 31 which form part of a relay having an operating winding 32 connected by way of normally open contacts 33 in series with the switch 29 between the supply line 36 and 40. The pairs of contacts 13, 37 also form part of the relay having the operating winding 32. A pair of normally closed contacts 34 connected in series with the switch 30 and the winding 32 form part of the relay of which the winding 21 is part. Finally a pair of normally closed contacts 42 connected in series with a warning device between the supply lines 36, 40 are provided the contacts 42 forming part of the relay of which the winding 32 is part.

In operation, before the driving motor of the machine tool can be started the contacts 37 must be closed and this is achieved by operating the reset switch which if there is no intrusion of the photo-electric curtain 22, will energise the winding 32. The contacts 31 will close to provide a hold circuit for the winding 32 and the contacts 42 will open to de-energise the warning device.

With the contacts 37 closed operation of the switch 38 will energise the contactor winding 35 and the contacts 41 provide the holding facility enabling the switch 38 to be released.

The crank of the machine prior to starting the working cycle will be at top dead centre and hence the switch 11 will be closed as also will be the contacts 13 and 14. As a result the winding 16 will be energised and the contacts 1 7 and 18 closed. Depression of the foot pedal control will close the contacts 20 to energise the solenoid 19 and engage the clutch of the machine tool. The foot control must remain depressed to complete the working cycle. During the cycle, if intrusion of the curtain 22 occurs, even momentarily, the contacts 14 will be opened and the winding 1 6 de-energised thereby opening the contacts 17 and 1 8 to disengage the clutch. Once the winding 1 6 has been de-energised it cannot be re-energised except by releasing and re-operating the foot control.When the crankshaft has rotated through about 3200 the switch 11 is momentarily opened and the switch 1 2 momentarily closed. Opening of the switch 11 will have the effect of de-energising the winding 1 6 and hence dis-engagement of the clutch. A further stroke of the machine can only be obtained if the foot pedal is released and reoperated.

When during correct operation of the machine the switch 12 is closed, the winding 26 will be energised to close the contacts 25 to start the timer. The timer will remain operative after the switch 12 has re-opened. The fact that the winding 26 is energised means that the contacts 34 are open. If no intrusion of the curtain 22 takes place the winding will remain energised and at the end of the time period of the timer the contacts 27 will be opened to de-energise the winding 26. The time period of the timer is chosen to be slightly longer than the time required for the machine tool to come to rest say about 300 ms.

If during the period after closure of switch 12 intrusion of the curtain takes place, contacts 33 will be opened to de-energise the winding 32. This will have a number of effects the first of which is to allow the contacts 42 to close to actuate the warning device 43. A further effect will be to allow the contacts 1 3 to open to de-energise the winding 1 6 in the event that this has not already taken place due to opening of the contacts 1 4.

With the winding 1 6 de-energised the clutch will be disengaged. A further effect is to de-energise the contactor winding 35 which opens the holding contacts 41 and isolates the drive motor of the machine. In order to restart the machine the starting sequence as described must take place i.e. operation of the reset switch, followed by operation of the switch 28.

As mentioned the period of timer is about 300 ms. This is made up by 200 ms for the crank to stop and a further 1 00 ms for the operator to visually establish that the machine is at rest.

After the period of 300 ms has lapsed the winding 26 is de-energised so that the contacts 34 reclose. It is then possible if a new cycle is not started, for intrusion of the curtain 22 to take place without the warning device being operated and without the contactor 35 being de-energised.

The operator can therefore place his hands in the danger zone to extract a workpiece.

By the arrangement described the photoelectric curtain is operative through the full 3600.

of crankshaft rotation. The operator will quickly learn by virtue of the need to go through the starting procedure not to intrude into the curtain during the final 400 of movement of the crankshaft.

Claims

1. A guard system for a crankshaft driven machine tool comprises a photo-electric curtain positioned in use to observe the danger zone of the machine tool, switch means operated by the curtain whereby if intrusion of the curtain occurs during a cycle of operation of the machine tool, the machine tool will be brought to rest and additional means operated by said curtain and operable to initiate and alarm and/or require resetting of the control circuits of the machine if intrusion of said curtain takes place whilst the machine is being brought to rest towards the end of a cycle of operation.

2. A guard system according to claim 1 in which said additional means includes a timer which is actuated substantially at the time the crankshaft of the machine tool is disconnected from its driving means towards the end of the cycle of operation, said timer having an active period which is slightly longer than the time required for the crankshaft to be brought to rest, said timer effecting a control over a control circuit for the alarm device, whereby if intrusion of the curtain occurs during the active period of the timer, the alarm device will be operated but at the end of said active period with the machine at rest, intrusion of the curtain will not result in operation of the warning device.

3. A system according to claim 2, in which said control circuit includes a self-hold feature which if intrusion of the curtain occurs during said active period, will be broken whereby the control circuit must be reset before further operation of the machine tool can take place.

4. A system according to claim 3 in which said control circuit in the event of intrusion of the curtain during said active period of the timer will operate to effect stopping of the drive motor of the machine tool, whereby in order to restart the machine tool it is necessary to run through the normal starting sequence of the machine tool.

5. A guard system for a crankshaft driven machine tool comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.