TECHNICAL FIELD The present invention relates to a method for compensation of tool wear in orbital drilling and a system for orbital drilling comprising a computer including a computer program for carrying out the method. The present invention also relates to a computer programme and a computer programme product for performing the method steps.

BACKGROUND ART

Orbital drilling is based on machining the material both axially and radially by rotating the cutting tool about its own axis as well as eccentrically about a principal axis while feeding the tool through the material. The general principles in orbital drilling, are for instance disclosed in US-A-5,641 ,252 and EP-B1 -1102653.

A reliable and repeatable hole quality is essential, especially in automatic drilling applications with robots or gantries. Tool wear has always been a big problem when drilling in abrasive materials like Carbon Fibre Reinforced Plastics (CFRP), CFRP / Titanium and CFRP / Alloy stacks and other complex materials. In some cases just a few holes can be drilled before the cutter has to be scraped or re-ground. Different type of coating and Poly crystalline Diamond (PCD) type of cutters has improved the lifetime but still the process is very expensive and time consuming. Especially in complex stacks with material combinations like CFRP/Ti or CFRP/AI, the process parameters will be a large compromise and even more reduce the lifetime of the cutters. The object of the present invention is to provide a method for tool wear compensation in orbital drilling, which overcomes the disadvantages of prior art. A further object is to provide an alternative solution in view of the state of the art and an improved method for obtaining increased bore hole quality, with correct hole diameter within narrow tolerances, and a longer tool life.

SUMMARY OF THE INVENTION

The present invention relates to a method for compensation of tool wear in orbital drilling, comprising the following steps: providing a receipt for the orbital drilling in a predetermined material, including predetermined drilling process parameters, wherein one parameter is a desired hole diameter, to be used in the drilling operation; collecting parameters of the tool; and, calculating a first tool wear value based on the receipt and the collected tool parameters. The method of the present invention is characterized in that it further comprises: collecting additional parameters of the material to be drilled; calculating said first tool wear value, based also on the material parameters, in addition to the receipt and the collected parameters; receiving an offset compensation value based on the calculated first tool wear value; and drilling of the hole diameter in the material, by compensation for the offset compensation value, based on said calculations, such that the diameter of the hole is dynamically adjusted during the orbital drilling operation in order to obtain the desired diameter of the hole.

According to the solution of the present invention, it was realized that the orbital drilling technique can minimise the problems as mentioned in the background art, by successively adjusting the orbital drilling diameter such that a hole with correct diameter within narrow tolerance can be drilled even with a worn tool. Thereby, the tool life can also be increased and it is possible to compensate for behaviour when drilling in different materials and combinations of materials, including complex materials. Thus, the method according to the present invention combines the unique feature to dynamically adjust the drilling diameter in very small steps in orbital drilling, by predicting and compensating for the tool wear in different materials. With this method a large number of successive holes can be drilled without changing the cutting tool. With other words, an advantage is that a method for continuous adjustment in operation during drilling of a plurality of holes in sequence can be provided, taking in consideration variables such as drilling in different materials and combinations of materials, varying thicknesses, as well as different diameters of the holes.

The present invention also relates to a system for orbital drilling comprising a computer including a computer program for carrying out the method according to the present invention, in which a software algorithm provides said calculations. The software algorithm makes it possible to predict and compensate for the tool wear in different materials. With the method according to the present invention a large number of holes can be drilled without changing the cutting tool, and a predicable and uniform result can be obtained.

The present invention also relates to a computer programme and a computer programme product for performing the method steps according to the present invention.

By the term "hole" is meant forming of an opening or recess in the material by the orbital drilling that results in a hole configuration or geometry. Thus, the hole is not limited to a circular hole but can be of any shape, such as triangular or polygonal shaped. The hole can be a through hole or a blind hole. Hence, by the term hole "diameter" is meant any distance straight across the opening that forms the hole and not only the largest opened distance cross the hole. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will hereinafter be described with reference to an embodiment of the invention and the enclosed figures, where

Fig. 1 shows an example of a recipe view in a user interface for programming,

Fig. 2 shows a typical drilling curve in CFRP according to prior art, without compensation,

Fig. 3 shows hole roundness, according to prior art, without compensation,

Fig. 4 shows drilling results with compensation for tool wear according to an embodiment of the present invention,

Fig. 5 shows tool wear algorithm elements in the user interface database according to an embodiment of the present invention,

Fig. 6 shows tool wear as a function of cutting length according to an example of the present invention, Fig. 7 shows compensation as a function of tool wear according to an example of the present invention,

Fig. 8 shows tool wear calculation according to an example of the present invention,

Fig. 9 shows compensation calculation for hole number two according to an example of the present invention,

Fig. 10 shows the resulting diameter in CFRP and Al, according to an example of the present invention,

Fig. 11 shows the resulting diameter with compensation in booth CFRP and

Al, according to an example of the present invention, Fig. 12 illustrates the method according to an embodiment of the present invention in a block diagram,

Fig. 13 shows an orbital drilling apparatus according to an embodiment of the present invention, and

Fig. 14 illustrated a diagram of one embodiment of an apparatus according to an embodiment of the present invention. DETAILED DESCRIPTION

The method for compensation of tool wear in orbital drilling and a system for orbital drilling comprising a computer including a computer program for carrying out the method according to an embodiment of the present invention will now be described by way of example only. The disclosure is not intended to limit the scope of the enclosed claims in any way.

Orbital drilling is characterized by a tool diameter that is less than the diameter of the resulting hole; a tool cutting edge that is intermittently in contact with the hole edge; small chip formation; and a low thrust force. These characteristics offer certain benefits. For example small chips in combination with a tool diameter less than the hole diameter allows for efficient chip removal using vacuum. Efficient chip removal, in turn, prevents heat build-up and eliminates the risk for matrix melting in composite materials and heat affected zones in metals. In addition, it eliminates the risk for chip induced damage and makes cleaning of structures obsolete.

An intermittent cutting edge contact with the part, allows for efficient cooling and makes dry drilling possible. It increases the tool life in dry drilling. Dry drilling is highly desirable as it reduces cost and improves the environment. In some applications, minimal quantity lubrication (MQL) is required to reduce friction. A low thrust force allows for burr-less drilling in metals and delamination-free drilling in laminated composite material. It minimizes the risk for part deflection when drilling in thin structures, and it facilitates automation using light equipment such as industrial robots, which are force- sensitive.

The eccentricity, e, or off-set, is an adjustable parameter. By adjusting the off-set, a tool of one diameter can be used to drill several holes of different diameters. In the more advanced machines this offset adjustment can be dynamically modified during the drilling in very small steps (1 -2 μm). This is achieved by a dual eccentric mechanism controlled by two servo motors. The speed of the servo motors will determine the "orbital speed" and the phase angle between the motors will determine the current offset (diameter).

Fig. 1 shows an example of a recipe view from a user interface, such as Orbital Manager. The Orbital Manager is the user interface (HMI) for programming the different drilling recipes. The Orbital Manager includes a database for storing all current and historical data regarding, drilling recipes, cutting tools, machine setup, drilling statistics etc.

The example below in Fig. 2 shows a typical behavior when drilling a 14,3 mm (9/16 inch) hole in an abrasive CFRP without compensation; the offset is constant during the drilling sequence.

In the example, the cutting tool used is Braced PCD, 3-flute Kennametal special orbital cutting tool, with diameter 10mm. The process parameters are: Spindle speed: 24700 rpm, orbital speed: 300 rpm, and feed rate: 100 mm/min.

As shown in the diagram in Fig. 2, the average diameter, defined as the mean value of all measured points for each hole, gets about 60 μm smaller during the drilled series of 300 holes. This is the actual wear of the cutting tool during the drilled series of holes.

To be able to calculate a reliable and predictable compensation algorithm when producing circular holes, it is very essential that the roundness or the cylindricity of the holes stays in a tight limit during the whole series. The roundness is defined as the difference between the max and the min value of all measurements for each hole measured with a split ball gauge for every 22,5 degree and for every 10mm depth. The new developed machine, End- effector ED-100, together with the orbital cutting tools has made it possible to achieve a unify roundness or cylindricity throughout the whole series, which is disclosed in Fig. 3.

The same example as above but with an implemented tool wear compensation algorithm is disclosed in Fig. 4.

With the method according to the present invention a large number of holes can be drilled without changing the cutting tool, and a Cpk value of more than 2,5 can be achieved. Cpk is a measure of the actual process capability. It is usually required according to the state of the art that the Cpk > 1 ,33 in production. With an automatic compensation, according to the present invention, the variation in average diameter stays within a tolerance of 7 μm. If the accepted tolerance band is 55 μm the Cpk value will be 2,974.

Each cutting tool has a number of different defined parameters, e.g. diameter, length, materials. For each tool there is a "current wear" factor. This factor is the current wear of the tool, which then is used for calculating the compensation needed to achieve a correct diameter in the reference material (e.g. CFRP).

With reference to fig. 5, each material has two predefined functions c(w) and w(cl). These functions are used to calculate the offset compensation, given the current wear and to calculate the new wear that will occur after the given operation.

Input to the function, c(w) is the current wear factor (w) and output is the needed compensation for the material given. For each material there is a unique relation between the current wear factor and the compensation needed.

Input to this function is the cutting length (cl) in the given material during the operation and output is the wear factor (w). The cutting length is defined as the helix (in meter) described by the orbital motion of the cutting edge through the material. To calculate a new wear factor the function is used in the reverse order, first the current wear factor is used to find the corresponding cutting length and then new cutting length is added to the current length and a new wear factor is calculated.

Before a recipe can be executed, all recipe points have to be analyzed and compensated for the tool wear Data for the first recipe point (diameter, feed length, orbital speed etc.) are downloaded. Material data (c(w) and w(cl)) for the recipe point are downloaded.

The compensation needed is calculated from the above data. A new wear factor for the next recipe point is calculated. Data for next recipe point is downloaded and the compensation is calculated. This will be repeated until all recipe points are calculated.

In this example, two holes are to be drilled, diameter 10,000 mm in a single 30mm thick CFRP stack. The cutting tool is new and therefore the initial tool wear is zero.

If a two step process is used, orbital speed 200 rpm, feed rate 100-200 mm/min a cutting length per hole about 5 m will be obtained. With the used cutting tool the tool wear are approximately 0,3 μm in CFRP.

The wear function and compensation function are assumed to be simple linear functions as follows disclosed in connection with Figs. 6 and 7.

For the first hole there is no need for any compensation (tool wear zero) in this example. Turning now to Fig. 8, the new wear factor is calculated from the cutting length after the first hole. With a cutting length of 5 m, a new tool wear factor of 1 ,50 μm is received. This value is stored in the database for used cutting tool. For the second hole the compensation is calculated from the current tool wear factor (1 ,5 μm) as input in the compensation diagram in Fig. 9. In this simple example, a compensation value of 1 ,50 μm then received. The offset has to be increased by 1 ,50 μm to get a correct diameter of 10,000 mm.

Fig. 2 shows that the compensation curve is normally a more complex function. The tool wear is much larger on the first 10 - 30 holes and then a more linear wear appears until the tool wear affects the roundness of the holes, which results in that the tool can not be used for more holes.

All these factors are stored in the Orbital Manager database and used to optimize the drilling process and lifetime of the cutting tool.

When drilling in more complex stacks as CFRP/AI, both the wear factor and compensation curve are totally different in the two materials. The wear in Aluminum is very small but the necessary compensation could be rather large. The Orbital Manager and the compensation algorithm makes it possible to create a recipe that dynamically adjust the offset and other process parameters during the drilling process to compensate both for the different material characteristics and tool wear. The diagrams below in Figs. 11 and 12 show the result without and with tool wear compensation in a CFRP/AI stack.

After a large number of practical drilling trials with the ED-100 end effector in different stacks, with and without the compensation algorithm implemented, a predicable and uniform result with very good Cpk values has been achieved. It has been demonstrated that the combination of Orbital Drilling and tool wear compensation makes it possible to drill a large number of holes with tight tolerance and to significantly increase the lifetime of the cutting tool. This is also possible in complex stacks like CFRP/AI and CFRP/Ti without any additional reaming operation. In operation, the method and the system, respectively, according to an embodiment of the present invention, is characterised by the following:

A receipt (see Fig. 1 ) for the orbital drilling in a predetermined material (4) is provided (see Fig. 5), including predetermined drilling process parameters, wherein one parameter is a desired hole diameter, to be used in the drilling operation. Parameters (6) of the tool are collected. A first tool wear value (8) is calculated based on the receipt and the collected tool parameters (see Fig. 8). The first tool wear value (8) is calculated, based also on the material parameters (4), in addition to the receipt (Fig. 1 ) and the collected parameters. An offset compensation value (10) is received (see Fig. 9), based on the calculated first tool wear value (8). The hole is drilled with the desired hole diameter in the material, by compensation for the offset compensation value (10), based on said calculations. Thereby, the diameter of the hole is dynamically adjusted during the orbital drilling operation in order to obtain the desired diameter of the hole.

Preferably, the method comprises drilling of at least two holes, whereby the method further comprises: calculating a second tool wear value based on the first tool wear value; receiving an offset compensation value based on the calculated first tool wear value; and drilling of the second hole in the material, by compensation for the offset compensation value, based on said calculations.

According to a preferred embodiment of the present invention, the receipt includes additional predetermined drilling process parameters such as the number of holes to drill, cutting length, feed length, spindle speed, orbital speed, feed rate and bore depth.

The parameters of the tool can suitably include the original tool diameter and a current tool wear factor. Suitably the parameters of the material includes the offset compensation, that is calculated based on current wear and the new wear of the tool that will occur after the drilling operation.

According to an embodiment of the present invention, the method comprises drilling of a hole in at least two layers of different material, whereby the method further comprises: calculating a second tool wear value based on the received first tool wear value in the first layer material; receiving a new offset compensation value based on the calculated second tool wear value; and drilling in the second layer, by compensation for the new offset compensation value, based on said calculations. In that respect, the materials of the layers have different characteristics. The materials of the layers may have different wear influence on the tool. One of the layers can consist of a complex material and the other layer consists of a metal. The complex material can be carbon fibre reinforced plastics. The metal can be titanium, aluminum or a metal alloy.

In Fig. 12, the method according to the present invention, for compensation of tool wear in orbital drilling, is disclosed in a block diagram. A receipt for the orbital drilling in a predetermined material is provided (A). In this step (A), predetermined drilling process parameters are included, wherein one parameter is the tool offset to achieve a desired hole diameter, to be used in the drilling operation. In another step (B) parameters of the tool are collected. In a subsequent step (C), a first tool wear value based on the receipt (A) and the collected tool parameters (B) is calculated. The method further comprises calculation of said first tool wear value, based also on the material parameters (D), in addition to the receipt (A) and the collected parameters (B). In a subsequent step (E), an offset compensation value is received, based on the calculated first tool wear value (C). Next (F), the hole with desired hole diameter is drilled in the material, by compensation for the offset compensation value (E), based on said calculations, such that the diameter of the hole is dynamically adjusted during the orbital drilling operation in order to obtain the desired diameter of the hole.

The drilling can be carried out by a control unit. The control unit receives information about the receipt, the collected tool and material parameters, and the offset compensation value, whereby the control unit uses the information for running the drilling operation. The control unit is preferably run by a computer, having a software algorithm adapted for providing said calculations.

The method for compensation of tool wear can be performed in an orbital drilling apparatus, operating according to the method as mentioned herein. Fig. 13 shows the orbital drilling apparatus (11 ). The orbital drilling apparatus (11 ) comprises a cutting tool (12) for drilling a hole (13) in a material (14), comprising suitably of two layers (as shown in Fig. 13) or more layers. Further, the orbital drilling apparatus comprises a control unit (15). The cutting tool (12) has a cutting tool axis (16). The cutting tool is rotated about its own axis as well as eccentrically about a principal axis (17) of the orbital drilling apparatus (11 ). The control unit receives information about the receipt, the collected tool and material parameters, and the offset compensation value, whereby the control unit uses the information for running the drilling operation. The control unit is preferably run by a computer (18), having a computer program (P) including a software algorithm adapted for providing said calculations.

The present invention also relates to a computer programme and a computer programme product for performing the method steps. The computer programme comprises a programme code for performing the method steps according to the present invention as mentioned herein, when said computer programme is run on a computer. The computer programme product comprises a program code stored on a, by a computer readable, media for performing the method steps according to the present invention as mentioned herein, when said computer programme is run on the computer. Alternatively, the computer programme product is directly storable in an internal memory into a computer, comprising a computer programme for performing the method steps according to the present invention, when said computer programme is run on the computer.

With reference to Figure 14, a diagram of one embodiment of an apparatus 500 is shown. The above-mentioned control unit 18 may include the apparatus 500. The apparatus 500 comprises a non-volatile memory 520, a data processing device 510 and a read/write memory 550. The non-volatile memory 520 has a first memory portion 530 wherein a computer program, such as an operating system, is stored for controlling the function of the apparatus 500. Further, the apparatus 500 comprises a bus controller, a serial communication port, l/O-means, an A/D-converter, a time date entry and transmission unit, an event counter and an interrupt controller (not shown). The non-volatile memory 520 also has a second memory portion 540.

The computer program P comprising routines for carrying out the inventive method according to different aspects of the invention is provided. The program P may be stored in an executable manner or in a compressed state in a memory 560 and/or in the read/write memory 550.

The data processing device 500 may be, for example, a microprocessor.

When it is described that the data processing device 510 performs a certain function it should be understood that the data processing device 510 performs a certain part of the program P which is stored in the memory 560, or a certain part of the program P which is stored in the read/write memory 550. The data processing device 510 may communicate with a data port 590 by means of a data bus 515. The non-volatile memory 520 is adapted for communication with the data processing device 510 via a data bus 512. The separate memory 560 is adapted to communicate with the data processing device 510 via data bus 511. The read/write memory 550 is adapted to communicate with the data processing device 510 via a data bus 514.

When data is received on the data port 590 it is temporarily stored in the second memory portion 540. When the received input data has been temporarily stored, the data processing device 510 is set up to perform execution of code in a manner described above. According to one embodiment, data received on the data port 590 comprises information about the receipt, the tool 12, parameters of the material 14, and a compensation offset value. This information can be used by the apparatus 500 so as to control a drilling operation according to the inventive method.

The apparatus 500 is arranged to control operation of the tool 12 by means of control signals generated by the data processing device 51 O.The control signals are transmitted via the data port 599 to a dedicated drilling machine.

Parts of the methods described herein can be performed by the apparatus 500 by means of the data processing device 510 running the program stored in the memory 560 or read/write memory 550. When the apparatus 500 runs the program, parts of herein described methods are executed.

A method for drilling a hole in a material by means of a tool, comprising the steps of:

- determining a first tool wear value being associated with said tool; - determining an offset compensation value on the basis of said tool wear value; and - drilling said hole paying regard to the offset compensation value so as to achieve a desired diameter of said hole.

An aspect of the invention relates to a computer programme comprising a programme code for performing the steps of:

- determining a first tool wear value being associated with the cutting tool;

- determining an offset compensation value on the basis of said tool wear value; and

- controlling drilling of said hole paying regard to the offset compensation value so as to achieve a desired diameter of said hole, when said computer programme is run on a computer.

An aspect of the invention relates to a computer programme product comprising a program code stored on a, by a computer readable, media for performing steps of:

- determining a first tool wear value being associated with the cutting tool;

- determining an offset compensation value on the basis of said tool wear value; and

- controlling drilling of said hole paying regard to the offset compensation value so as to achieve a desired diameter of said hole, when said computer programme is run on a computer.

An aspect of the invention relates to a computer programme product directly storable in an internal memory into a computer, comprising a computer programme for performing the steps of:

- determining a first tool wear value being associated with the cutting tool;

- determining an offset compensation value on the basis of said tool wear value; and

- controlling drilling of said hole paying regard to the offset compensation value so as to achieve a desired diameter of said hole, when said computer programme is run on a computer. An aspect of the invention relates to a computer readable medium having embodied thereon a computer program for processing by a computer program comprising: a first code segment for determining a first tool wear value being associated with the cutting tool; a second code segment for determining an offset compensation value on the basis of said tool wear value. a third code segment for controlling drilling of said hole paying regard to the offset compensation value so as to achieve a desired diameter of said hole.

Said medium may be a propagated signal. Said propagated signal may be a carrier wave.

By the expression that "the diameter of the hole dynamically adjusted during the orbital drilling operation", in order to obtain the desired diameter of the hole, it is preferably meant that the offset compensation adjustment is carried out before a new hole is drilled in a new material. In that respect, the word "during" refers to the whole drilling operation of a plurality of holes in series.