Method and machine for removing a coating off a coated sheet of glass

Abstract

 A method and machine (1) for removing a coating (3) off a coated sheet of glass (2), whereby the coating (3) is removed off at least one portion (A1)(B1) of the sheet of glass (2) by a grinding tool (6a), which is fed along the sheet of glass (2) and is dressed/replaced on the basis of a response signal emitted by a coating material detecting device (10) which, in use, supplies a test signal to the portion (A1)(B1) of the sheet of glass (2) machined by the grinding tool (6a).

Description

[0001] The present invention relates to a method of removing a coating off a coated sheet of glass.

[0002] As is known, a coated sheet of glass is obtained by depositing on an extensive surface of an ordinary sheet of glass a coating material selected from a number of coating materials having different characteristics, such as electric conductivity, degree of reflection or refraction, etc.

[0003] When cutting and/or gluing coated sheets of glass, the coating material along the cutting or gluing lines must be removed. This is done using grinding wheels, which are selected according to the type of coating material for removal, and which, after a certain length of time, must be dressed to maintain constant wheel efficiency and finished product quality. At present, determining when the wheel needs dressing or replacing depends on the operator who, after grinding or parting the sheet of glass, inspects the ground portions of the sheet for any traces of coating material, and, when this is found to be present in excessive quantity, proceeds to dress the wheel.

[0004] Though widely adopted, the above wheel dressing method is unsatisfactory by depending exclusively on the skill, experience, and degree of concentration of the operator. That is, any error on the part of the operator, or even simply a heavy work load, may result in neglect to dress or change the wheel when required, thus resulting inevitably in rejection of the sheet of glass, which in most cases is already partly processed, and in rejection costs which at times can be considerable.

[0005] It is an object of the present invention to provide a method of removing a coating off a coated sheet of glass, designed to provide a straightforward, low-cost solution to the aforementioned drawbacks.

[0006] According to the present invention, there is provided a method of removing a coating off a coated sheet of glass; the method comprising the steps of machining, by means of a grinding tool, at least one portion of the sheet of glass to remove the relative said coating at least partly; determining a limit wear condition of the grinding tool; and commanding dressing/replacement of said grinding tool; and being characterized in that determining the limit wear condition and commanding dressing/replacement of the grinding tool comprise the steps of supplying a test signal to said portion machined by said grinding tool; picking up a response signal in response to said test signal; and transferring said grinding tool to a tool dressing/disposal station in the presence of said response signal.

[0007] The present invention also relates to a machine for removing a coating off a coated sheet of glass.

[0008] According to the present invention, there is provided a machine for removing a coating off a coated sheet of glass; the machine comprising a grinding tool; and actuating means for controlling said grinding tool to move the grinding tool over at least one portion of said sheet to remove the coating off said portion; and being characterized by comprising detecting means for determining a limit wear condition of the grinding tool; and control means for commanding dressing/replacement of said grinding tool on the basis of a signal received by said detecting means.

[0009] A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a plan view, substantially in block form, of a preferred embodiment of a machine for machining coated sheets of glass in accordance with the present invention;

Figure 2 shows a circuit diagram of a detail of the Figure 1 machine.

[0010] Number 1 in Figure 1 indicates as a whole a machine for machining a coated sheet of glass of the type comprising a sheet of glass 2, and a coating 3 (Figure 2) deposited on an extensive surface 2a of sheet 2 and selectable from a number of coating materials of different characteristics. Machine 1 comprises a frame 4 defining a supporting surface for the sheet of glass 2 and fitted with a slide 5. Slide 5 is moved over the sheet of glass 2 in two perpendicular directions by respective known actuating units (not shown), and supports a grinding head 6, a grinding wheel 6a of which removes coating 3 along predefined paths A and B to remove portions of coating 3 off respective portions A1, B1 of sheet 2.

[0011] Wheel 6a forms part of a number of different grinding wheels housed in a wheel store 7 and moved between head 6 and store 7 by a known automatic wheel-change device 8 located, in the example described, adjacent to a wheel dressing station 9 to transfer some of wheels 6a between store 7 and station 9, and adjacent to a wheel disposal station 9a for worn-out wheels.

[0012] The need to dress or replace wheel 6a machining sheet 2 is determined by a detecting device 10, which provides for determining both the presence or absence of coating 3 on work portions A1, B1, and the amount of coating material left on portions A1, B1 after grinding, owing to the reduced efficiency of wheel 6a.

[0013] Detecting device 10 is moved along paths A and B by a respective known actuating unit (not shown) or together with slide 5, and, in the example described, comprises a first and a second brush-type input electrode 10a, 10b which rest on work portions A1, B1; and an output terminal 10c.

[0014] With reference to Figure 2, detecting device 10 comprises an amplifying circuit 11, a resistive element 12, and a constant-voltage source 13. More specifically, amplifying circuit 11 has a first and a second input terminal connected respectively to a first and a second terminal of resistive element 12; and an output terminal connected to the output terminal 10c of detecting device 10. The first terminal of resistive element 12 is also connected to a first terminal of constant-voltage source 13, which has a second terminal connected to first input electrode 10a; and the second terminal of resistive element 12 is connected to second input electrode 10b of detecting device 10.

[0015] Detecting device 10 cooperates with an electronic numeric control unit 14 of machine 1, to which automatic wheel-change device 8, wheel dressing station 9, wheel store 7, and the respective actuating units of slide 5 and detecting device 10 are connected. Control unit 14 comprises a memory 15 in which is stored a limit grinding value corresponding to a minimum permissible grinding condition, i.e. beyond which the wheel working the sheet must be dressed.

[0016] Operation of machine 1 will now be described as of the condition in which a sheet of glass 2 rests on the supporting surface, and in which detecting device 10, wheel 6a and automatic wheel-change device 8 are set to their respective withdrawn rest positions.

[0017] As of the above condition, wheel 6a is brought into contact with sheet 2 and moved along one of paths A, B to remove the respective portion of coating 3 off respective portion A1, B1 of sheet 2. After a predetermined or random grinding period - e.g. after a given number of meters traveled by the slide, or after a portion of the sheet has been machined - and with head 6 moving or stationary, device 10 is moved towards the machined portion A1, B1 of sheet 2 to position electrodes 10a and 10b on and test portion A1, B1. In the absence of any coating material on portion A1, B1, i.e. in the event the wheel is still working efficiently, the two electrodes 10a and 10b are still isolated electrically so that no output signal is emitted by device 10. Conversely, if any traces of coating material remain, an output signal related to the amount of coating material left on portion A1, B1 is emitted. More specifically, in this case, device 10 emits a response signal related to the amplitude of a current I_R; and, by means of a block 14a, control unit 14 compares the amplitude of current I_R with the memorized limit grinding value, and accordingly determines whether or not to activate the wheel-change procedure, i.e. whether the wheel 6a working on the sheet needs dressing or replacing. If so, unit 14 commands automatic wheel-change device 8 to remove the wheel working on sheet 2 and transfer it to dressing station 9 or station 9a; and an identical wheel is then withdrawn from store 7 and fitted to grinding head 6, so that grinding continues while the previous wheel is being dressed.

[0018] Machine 1 described has the following advantages. Firstly, compared with known machines, machine 1 is far more efficient by removing coating 3 in the best possible operating and output conditions at all times. Machine 1, in fact, involves no consent on the part of the operator, and provides for determining, according to a predetermined program, the amount, if any, of coating 3 left on sheet 2 after grinding, and for deciding how much longer the same wheel can be used without compromising subsequent notching or gluing operations. In other words, device 10 enables store 7, device 8, station 9 and unit 14 to dialog directly and so eliminate any possibility of the wrong type of wheel being selected - seeing as unit 14 knows the type of wheel being used and selects the same type when changing the wheel - and any possibility of error in determining when to dress/replace the currently used wheel. This therefore translates into a net reduction in rejects following the grinding stage or, worse still, after the notching and parting stage which normally follows grinding; into maximum exploitation of the grinding wheels; and into longer working life of the notching tools which, as is known, undergo severe and unpredictable wear in the presence of coating material.

[0019] Moreover, device 10 provides for exploiting the full potential of robot-operated wheel-change devices, which in some cases are unwarranted by depending on the operator decision-making element still involved.

[0020] Clearly, changes may be made to machine 1 as described herein without, however, departing from the scope of the present invention.

[0021] In particular, detecting device 10 may be of a different type from the one indicated by way of example, and, more specifically, may be designed according to the specific characteristics of the coating material. For example, device 10 may be optical to determine the refraction index or any other optical characteristic of coating 3. Moreover, machine 1 may have no wheel-change device 8, and device 10 may emit an alarm signal when the amount of coating material left on sheet 2 after grinding exceeds a given reference value.

Claims

1. A method of removing a coating (3) off a coated sheet (2) of glass; the method comprising the steps of machining, by means of a grinding tool (6a), at least one portion (A1) (B1) of the sheet (2) of glass to remove the relative said coating (3) at least partly; determining a limit wear condition of the grinding tool (6a); and commanding dressing/replacement of said grinding tool (6a); and being characterized in that determining the limit wear condition and commanding dressing/replacement of the grinding tool comprise the steps of supplying a test signal to said portion (A1)(B1) machined by said grinding tool (6a); picking up a response signal in response to said test signal; and transferring said grinding tool (6a) to a tool dressing/disposal station (9)(9a) in the presence of said response signal.

2. A method as claimed in Claim 1, characterized in that determining the limit wear condition and commanding dressing/replacement comprise the further steps of memorizing a limit grinding value corresponding to a minimum permissible grinding condition; and comparing a quantity of said response signal with the memorized said limit value.

3. A method as claimed in Claim 1 or 2, characterized in that said response signal is picked up during the removal of said coating (3) off said portion (A1)(B1).

4. A method as claimed in Claim 1 or 2, characterized in that said response signal is picked up after removal of said coating (3) off said portion (A1)(B1).

5. A method as claimed in any one of the foregoing Claims, characterized in that determining the limit wear condition and commanding dressing/replacement of the grinding tool (6a) comprise the steps of moving electrode means (10a)(10b) of an electric device (10) towards the machined portion (A1) (B1); bringing said electrode means (10a) (10b) into contact with said portion (A1)(B1); and supplying the test signal to said sheet (2) by means of said electrode means (10a) (10b).

6. A method as claimed in Claim 5, characterized in that said grinding tool (6a) is transferred to said dressing/disposal station (9)(9a) by sending the response signal to an electronic control unit (14) which emits a dressing/replacement control signal on receiving said response signal.

7. A machine (1) for removing a coating (3) off a coated sheet (2) of glass; the machine (1) comprising a grinding tool (6a); and actuating means (5, 14) for controlling said grinding tool (6a) to move the grinding tool (6a) over at least one portion (A1)(B1) of said sheet (2) to remove the coating off said portion (A1)(B1); and being characterized by comprising detecting means (10, 14) for determining a limit wear condition of the grinding tool (6a); and control means (8, 14) for commanding dressing/replacement of said grinding tool (6a) on the basis of a signal received by said detecting means (10, 14).

8. A machine as claimed in Claim 7, characterized in that said detecting means (10, 14) comprise emitting means (10) for supplying a test signal to said portion of the sheet of glass machined by said grinding tool; and receiving means (14) for receiving a response signal in response to said test signal.

9. A machine as claimed in Claim 8, characterized in that said control means (8, 14) comprise memory means (15) for memorizing a limit grinding value corresponding to a minimum permissible grinding condition; and comparing means (14a) for comparing said response signal with the memorized said limit value.

10. A machine as claimed in Claim 8 or 9, characterized in that said control means (8, 14) also comprise transfer means (8) for transferring said grinding tool (6a) to a tool-dressing station (9); and operating means (14) for operating said transfer means (8); said operating means receiving said response signal, and sending an output signal to said transfer means (8) on receiving said response signal.

Drawing