Source: https://patents.google.com/patent/WO2002023293A1/en
Timestamp: 2018-10-20 19:58:10
Document Index: 12949164

Matched Legal Cases: ['art 5', 'art 5', 'art 5', 'art 5', 'art 5', 'art 5', 'art 5', 'art 1']

WO2002023293A1 - A method and control system for generating machine tool control data - Google Patents
A method and control system for generating machine tool control data Download PDF
WO2002023293A1
WO2002023293A1 PCT/GB2001/003869 GB0103869W WO0223293A1 WO 2002023293 A1 WO2002023293 A1 WO 2002023293A1 GB 0103869 W GB0103869 W GB 0103869W WO 0223293 A1 WO0223293 A1 WO 0223293A1
PCT/GB2001/003869
Stephen Paul Porter
Christopher Norman Renwick Wilson
A control system (41) comprises a control program (42) which accesses a stored database (43) holding a predefined set of rules for performing predetermined machine tool operations, from which rules computer instructions can be derived for controlling an NC machine tool. A solid model (44) of a component to be manufactured is input to the control program (42). The solid model(44) is in the form of a computer model programmed using the International Geometry Export Standard (IGES). In operation, the control program (42) performs a feature recognition operation on the IGES data to identify geometric features present in the solid model (44). Having identified the geometric features present, the control program (42) uses the set of rules in the database (43) to generate control data in the form of a composite set of computer instructions for working the features of the component represented in the solid model (44).
A Method and Control System for Generating Machine Tool
The task of programming modern NC machine tools is a highly skilled and time consuming task. In situations where the machining head of an NC machine has many axes of freedom in which to move, and the geometry of the underlying workpiece is complex, the amount of data required to be programmed at each machining location, and between machining location, is considerable. Taking the example of the wing assembly mentioned above, a typical NC programming operation involves some on-line digitising. In other words, the NC machine is taken out of its production routine in order to record the location of rivet holes from a 'master' part which has been built manually. The process involves the programmer driving the machine over each location and centralising the machine head using a camera. The co-ordinates of each location are stored as raw NC data. This data then has to be edited manually to add machine cycle commands for installing the rivets, and for controlling the intermediate movements of the machine head.
Since the programming task is highly complex, NC programming personnel must be very skilled which implies long periods of specialised training. If a company wishes to use a new NC tooling system, they will have to spend a considerable amount of time training new programming personnel. Even with specialised training, however, the programming proce'ss is prone to human error which can be costly in terms of both time and expense.
According to a first aspect of the present invention, there is provided a method of generating control data for a numerically-controlled (NC) machine tool, the method comprising: providing a predefined set of rules for performing predetermined machine tool operations, from which rules computer instructions can be derived for controlling the NC machine tool; providing a computer model representing a component to be worked by the machine tool, the computer model including information relating to geometric features of the component; automatically identifying, from the computer model, geometric features of the component, and identifying one or more of the predetermined machine tool operations suitable for working the identified features of the component; and using the set of rules associated with the identified machine tool operations to generate control data in the form of a composite set of computer instructions for working the component represented in the computer model. Using this method, a computer model of a component which contains information relating to particular geometric features can be analysed against a predefined set of rules for performing predetermined machine tool operations. The predefined set of rules effectively comprises a 'knowledge' database relating to particular machine tool strategies and the programming instructions required to put these strategies into effect. By identifying the features described in the component model, determining which machine tool operations are suitable for working those features, and then using the rules associated with those machine tool operations to select a tooling operation and to generate a suitable set of composite computer instructions representative of an overall tool operating strategy, these composite computer instructions can then be made available to an NC machine tool quickly and efficiently. The set of rules can be universal, i.e. they can relate to a large number of machine tool operations for many different NC machine tool types, or they can simply relate to more specialised types of machine tool operation. Provided these rules comprise the information necessary for automatically generating computer instructions from a computer model, the need for expensive and time consuming programming is greatly reduced. The system is less prone to human error, since the code output from the system relates to that which is input, namely the computer model. Provided that the computer model has been verified prior to being interrogated, then very few, if any, errors should result.
By embodying the steps of the method in a computer application, it will be appreciated that a complete 'black box' approach can be adopted. Such a black box approach has the advantage of enabling design modifications to be embodied quickly and efficiently without necessarily taking the machine tool offline. By simply changing the appropriate part of the computer model and applying the method again, a new set of data is obtained and the output can be analysed. The training of specialised personnel is also reduced. It will be appreciated that references to 'geometric features' refer to features or characteristics of shape or configuration which are identifiable on the component. Typically, these features are three-dimensional in nature. Examples include bores, wells, fillets and ribs and other protrusions. The distance between particular features may also be regarded as a geometric feature of a component. It will also be appreciated that the term 'worked' covers manufacture of a component, i.e. by forming or shaping, for instance by milling, operating on a component using the NC tool, e.g by riveting or impacting by the NC tool.
Preferably, in the step of providing the computer model, a three- dimensional solid computer model specifying geometric features of the component is provided. This solid computer model can be acquired from a CAD application, such as Intelligent CAD (ICAD), or a text-based application such as the International Geometry Export Standard (IGES). It will be understood that the computer model may not specifically state or show particular geometric features, it being more likely that the geometric features will be defined in terms of the data forming the model. Indeed, in the case of the text-based IGES application, components are defined in the IGES language from which a 'mesh' representation can be formed. In the step of identifying the geometric features of the component to be worked, the mesh representation will be reconstructed and used in the identifying process.
Figure 1 is a cross-sectional view of a wing skin and stringer assembly and part of a riveting tool.
Figure 2 is a cross-section of the wing skin and stringer assembly of Figure 1 , including ICAD attributes;
Figure 3 is an attribute table detailing certain attributes, as shown in Figure 2.
Figure 4 is a plan view of an interspar rib component for manufacture using a set of control data generated from a control system in accordance with the invention;
Figure 5 is a block diagram of a control system used in manufacture of the component of Figure 4;
Figures 6 to 9 are computer graphical representations of a billet of material at successive stages in the machining of the component of Figure 4, the representations being generated in a simulation application using the NC data from the control system of Figure 5;
Figure 10 is a screen capture of a Graphical User Interface (GUI) for monitoring a machine tool operation simulation using NC control data generated by the control system of Figure 5; and
Figure 11 is a further graphical representation generated by the simulation application using the NC control data generated by the control system of Figure 5.
A preferred method according to the present invention is described with reference to Figure 1. Referring to Figure 1 , a wing skin 1 and stringer assembly 3 is shown, positioned over a lower anvil part 5 of a so-called Low Voltage Electromagnetic Riveting (LVER) tool. The LVER tool is well-known in the aerospace industry and is commonly used to fasten wing skins to an underlying stringer assembly on the basis of NC programming instructions. In terms of size, the LVER tool is very large and is capable of attaching many thousands of fasteners to secure aircraft wing components together. Accordingly, the time and expense required in programming a control system of the tool is also large.
As shown in Figure 1 , the stringer assembly 3 comprises a stringer flange 7 for forming an attachment interface with the wing skin 1. Extending perpendicularly from the stringer flange 7 is a stringer web 9 which terminates at a distal edge with a stringer crown 11 , extending perpendicularly at one side thereof. The space between the stringer crown 11 and a bottom face of the stringer flange 7 is referred to as the stringer throat 13. In Figure 1 , two fastener locations 15 are shown for attaching the wing skin 1 to the stringer flange 7. The distance from the edge of the stringer flange 7 to the centre of each fastener location 15 is referred to as the edge distance 17.
The lower anvil part 5 of the LVER tool includes a tracer mechanism 18 which forms a contact for controlling the distance of a fastener from an edge of the stringer web 9 and the placement of the lower anvil within the stringer throat 13. The tracer mechanism 18 also acts as a checking mechanism in order to take account of any build up of manufacturing tolerances on the assembled components. A so-called 'flying height' 19 is also defined as the distance between the lower anvil part 5 and the stringer throat 13. It will be clear from Figure 1 that there is restricted space in which the lower anvil part 5 has to be positioned in certain areas of the overall wing structure.
In conventional methods of generating NC control data for such LVER machine tools, extensive manual calculation and data input is required. This is mainly due to the presentation of design data in the form of paper drawings. Typically, the distance of a fastener from the edge of a stringer is critical to the programming operation and this positional value is generally shown on the paper drawings of the stringer assembly. However, since, in operation, the mechanical tracer 18 touches the stringer web 9 (not the stringer edge) the value for setting the tracer has to be manually calculated from the following equation: Tracer Offset = Flange Width/2 - (Edge Distance + Web Thickness/2)
It will also be appreciated that, in order for the lower anvil part 5 (in this case, on the so-called 'stringer side' of the stringer assembly) to be driven to each fastener location 15, a dogleg motion is required in order to avoid collision of the anvil part with the stringer crown 11. Knowledge of the stringer crown widths and web dimensions combined with the lower anvil part 5 sizes is required to provide for clearance without hitting the back of the lower anvil part 5 on the adjacent stringer assembly 3. Again, conventional techniques require manual calculation and programming of control system software.
In producing the computer model for this particular case, each fastener location 15 is represented by an individual 'tag' name such that a series of text- files are then created in a design stage, these text files holding the cross- sectional dimensions of the assembled components at each fastener location. Such text files are then made accessible to other programming elements, such as macros and subroutines within the control program. These macros and subroutines define rules for generating NC control data according to the text files. A refinement of this is to utilise non-geometric properties which are available as associated data for entities stored in the CADDS5 software.
In order to generate the text files for interrogation by the control program, the LVER machine tool capabilities have to be well understood and incorporated in the control program. The knowledge of the tool capabilities acquired is used to define computer-based rules for performing different machine tool operations, such as controlling the movement of the machine tool head between different fastener locations and the actual fastening (riveting) operation. The rules also define a desired assembly fastening strategy of the
LVER machine tool. By combining the computer based rules (defining the machine capabilities and strategy) with the computer model of the wing structure, a set of further rules are identified. These further rules take into account each individual skin 1 to stringer configuration 3 at each fastener location 15. For example, since stringer assemblies taper at their ends, there are areas on the wing where no stringer crown 11 and even no stringer web 9 are present, this being reflected by the further rules. These further rules are then embedded in an Intelligent CAD (ICAD) software tool to generate so-called 'attributes' relating to each fastener location.
Thus, by providing a set of operating rules based on knowledge of the different capabilities of the LVER machine tool and a desired fastening strategy, and combining these rules with a computer model of the components to be worked on, a further set of rules are generated to form the set of 'attributes' using the ICAD tool. These attributes are then exported back to the CADDS5 software to form a so-called 'intelligent' CAD model.
In the case where the computer model is a wing assembly for an Airbus A340-600, for example, thirteen attributes for each of the 110,000 fasteners present on the wing are generated from the ICAD tool. Figure 2 shows the skin 1 and stringer assembly 3 with eight of the thirteen attributes illustrated. Figure 3 is a table giving full details of all thirteen attributes. The attributes are essentially generated by forming a virtual line normal to the wing surface at each fastener location giving the length of the assembled stack (see attribute 2 in Figure 3), together with a solid CADDS5 model of the stringer assembly 3. Using this information, an ICAD routine is then used to analyse a cross-section of the stringer assembly 3 at each faster location 15, to measure certain specific dimensions, and then to combine this data with the computer model of the fastener and the hole to generate the attributes.
Of the thirteen attributes generated by ICAD, eight provide measurements calculated from the analysis of the cross-section of the stringer assembly 3. These eight attributes are those shown in Figure 2. The remaining five attributes record the fastener dimensions and their type, together with properties of the holes into which the fasteners are to be placed, i.e. whether or not the hole is cold worked (using a sleeve and pull-through mandrel to strengthen the material microstructure around the hole).
The predefined control rules comprise a series of subroutines (or exceptions) made available for selection based on the identified attributes at each fastener location 15, and, in response their selection, the control program outputs the NC control data, including data relating to machine axis and head control statements. The subroutines have to be robust to react to the differing cross-section profiles so as to be applicable to each different fastening locations 15 over the wing structure. The subroutines also have to be capable of detecting when a particular fastening location 15 does not have any associated attribute data and so reset accordingly. By using dimensions acquired from three-dimensional models of the LVER lower anvil 5, in conjunction with the attribute data for each fastener (or fastener location 15) the subroutines can include formulae which are defined to give clearance values for raising and lowering the stringer side tooling. Additional formulae can be incorporated in the subroutines to enable a tracer offset value to be automatically included for each fastener or fastener location 15. By using knowledge of the lower anvil 5 and stringer dimensions, it is possible to determine where certain types of anvil may, or may not, fit, giving a form of real-time collision checking which greatly reduces the time required for test purposes (sometimes referred to as 'tape try-out').
In practice, the system is successfully employed to the extent that 'balancing' of the lower anvil position to 1mm clearance is achievable and continues to run in a production environment. Of course, however, the usual limitations apply as in all CAD based systems, in that all accessible attribute data is at nominal dimension and so care must be taken to account for possible tolerance build-ups during the creation of the subroutines.
In respect. of the present embodiment, the benefits included: (a) a 90% saving in NC programming time, compared with that of manual programming; (b) the facility to incorporate design changes with a 90% time saving compared with the manual programming method; (c) an 85% reduction in 'tape try-out' time; and (d) reducing rework time by over 90%.
A second embodiment of the present invention will now be described with reference to Figures 4 to 11. In this embodiment, a further control system generates NC control data for manufacturing a mechanical component, namely an interspar rib for an aircraft wing, from a blank billet of material. Figure 4 is a diagram showing the interspar rib (in its finished form) . The interspar rib 35 comprises a number of geometric features including stiffener ribs 36, wells 37, pockets 38, feet 39 and holes 40.
Figure 5 is an overview block diagram of the control system 41. The control system 41 comprises a control program 42 which accesses a stored 'knowledge' library or database 43 holding predefined data relating to particular machine tool strategies (explained in more detail below). The knowledge database 43 is programmed using the known programming language LISP. LISP is an object-oriented programming language which is especially useful for defining 'knowledge-based' engineering operations.
In use, a solid model 44 of the component to be manufactured is input to the control program 42. The solid model 44 is a virtual component in the sense that it is a set of stored data and, in this case, is in the form of IGES data. As will be appreciated by those skilled in the art, IGES is a known software standard capable of defining geometric features of mechanical components and objects. It should also be appreciated, however, that other CAD packages can be used to export solid model data for the component. In this case, the IGES data represents a computer model 44 of the interspar rib component 35 shown in Figure 4.
In operation, the control program 42 performs a feature recognition operation on the IGES data to identify, amongst other attributes, the geometric features present in the solid model 44 of the interspar rib component 35. In using the IGES model 44, such features are defined in a numerical form (the numerical model itself usually having been tested and verified) and so the feature recognition operation is not subject to errors often incurred during manual programming. Having identified the features present in the solid model 44, and their relative locations in the model, the control program 42 operates to generate the NC control data by developing a suitable machining strategy for producing the various features making up the overall component 35. This strategy is developed according to the LISP knowledge database 43 mentioned above, the knowledge \ibrary storing a set of rules for performing particular machine too) operations depending on the features identified in the IGES solid model 44, their relative position within the billet of material (i.e. the solid blank of metal from which the interspar rib is produced) and the position of other geometric features relative to each 'subject' feature. Having developed a suitable strategy according to all of the identified features, a complete set of NC control data for producing the component is generated and can be fed to an NC machine tool.
The design feature rules relate to strategies for forming particular geometric features in an appropriate and efficient manner. These rules can be more 'universal' in that they can be used to generate control data for any component having such geometric features, although in the present case, they are only used for forming particular features of the interspar rib 35. The control program 42, having identified a feature which is to be formed in the billet of material, accesses the design feature rules which comprise a set of conditions for determining how (and when) the machine tool will form that feature. Such rules not only have to take into account the size, shape and position of each feature to be formed, but also any other geometric feature which is located adjacent to that feature. In many cases, a feature may not be formed until a different feature is first completed. In effect, the design feature rules are arranged as a hierarchy of conditional rules for determining an appropriate machine tool strategy. The overall set of NC control data generated is based on a combination of the machine feature rules and the design feature rules.
A brief overview of a few main steps involved in deriving the machine tool strategy for the interspar rib of Figure 4 will now be described with reference to Figures 6 to 9 and Figure 11. It will be appreciated that these steps represent a fraction of the stages actually taken by the control program 42, and are intended to be illustrative. It should also be understood that these Figures do not represent the inputted IGES computer model 44, but actually show simulated output of what the NC machining tool produces in accordance with the NC machining instructions being generated by the control program 42. In this case, an application program VeriCut (TM) has been used to produce the simulated output. In practice, the overall machine tool strategy for generating the component is broken down into so-called 'stages' and Operations'. A 'stage' represents work performed on a given face of the billet, whilst an 'operation' is a sequence of work carried out from the introduction of a particular tool up to the point when that tool is changed for another tool. It follows, therefore, that the machine tool strategy can further be defined in terms of the tools required and the description of the tool action within each operation.
In the first operation, i.e. stage #1 , operation #1 , the billet is located on a table of the machine tool and is either clamped or held on a vacuum chuck.
The purpose of the operation is to prepare a first side (side one) of the billet by removing the so-called 'dead-zone' of unwanted stock to create a flat reference surface. Accordingly, the control program 42 generates control data for performing this task, including generating the necessary data for selecting and picking up a milling tool and for controlling its speed of milling. In operation #2, central tooling holes and clamping holes will be drilled, and so the operation for picking-up the drilling tool and the required operating speed and toolpath is generated. These holes are used to secure the billet firmly to the machine tool fixture/bed. Figure 6 shows a billet 45 with the clamping holes 46 and tooling holes 47 included. Lifting holes 48 are also provided. Control data for a number of further operations are generated by the control program 42. These operations are performed once the billet 45 is secured to the machine tool fixture/bed and are generated using the design feature rules, which, as explained above, relate to the particular geometric features which make up the interspar rib 35 shown in Figure 4.
As would have been clear from Figure 4, the control program devises a machine tool strategy for producing a number of complex geometrical features, e.g. "holes" 40, "feet" 39, "pockets" 38, "stiffeners" 36". Figures 7 and 8 represent intermediate stages of the formation of one side of the interspar rib 35. In analysing the computer model 44 of the interspar rib 35, the identified geometric features are compared with the design feature rules predefined in the LISP knowledge database 43. These design feature rules effectively comprise a hierarchy of conditional rules for determining which machine tool operations are suitable for producing or operating on a particular feature, and additionally, how each feature should be produced or operated on, conditional on other factors. These other factors may include the position of adjacent features.
To illustrate the principle of the design feature rules, the top level rules define the actual feature, i.e. "holes", "apertures", "feet", "pockets", "stiffeners", "walls" etc. Associated with each feature is one or more tooling types e.g. drilling, milling etc. which can be used to produce that feature. Thus, when a well is identified in the model, the control program accesses the LISP knowledge database and identifies the tooling types which can be used to produce the well. Sub-rules are provided which define conditions for determining which tool to use, i.e. milling tool if the well is wide and shallow, or drilling tool if the well is relatively narrow and deep. Other more complex sub- rules are provided in order to determine the time (in the course of the overall strategy) at which each machine tool operation will be performed. These sub- rules generally relate to the location of other features. For example, the IGES computer model may require a hole to be provided at the bottom of a deep well. Thus, a sub-rule may be provided which stipulates that, in such circumstances, the well is to be milled prior to the drilling of the hole in the bottom of that well. It follows that a whole 'tree' of sub-rules with many levels can be predefined in the LISP knowledge database, stipulating many such conditional statements for generating a suitable strategy.
Having generated control data for machining the interspar rib 35 up to the stage shown in Figure 8, data for performing 'finishing' operations is generated. Figure 9 shows such a finishing operation being performed on the walls of a pocket 38 using tool 46.
Having generated control data for finishing side one of the interspar rib
35, the machine feature rules will stipulate that the billet 45 is to be flipped so that work can be performed in Stage #2, i.e. on side two of the interspar rib 35. As with side one, the design feature rules are then used to generate a suitable machine tool strategy for producing the geometric features of side two.
Figure 10 is a screen capture showing a Graphical User Interface (GUI) for use in monitoring the progress of a simulation. Here, an intermediate stage of the control program operation is shown. As can be seen from the right-hand part of the screen capture, the machine and design feature rules (at least at the top level) are shown, which rules can be expanded to show the underlying sub- rule hierarchy.
In the final stage, rib feet are formed on the edges of the interspar rib, as shown in Figure 11.
In conclusion, both embodiments provide a system which receives a computer model of a component to be manufactured, and from which a set of NC control data suitable for producing, or working features represented in that computer model. Such a 'black box' system readily responds to changes in input and facilitates rapid implementation of design changes.
2. A method according to claim 1 , wherein, in the step of providing the computer model, a three-dimensional solid model specifying geometric features of the component is provided.
PCT/GB2001/003869 2000-09-14 2001-08-31 A method and control system for generating machine tool control data WO2002023293A1 (en)
WO2002023293A1 true true WO2002023293A1 (en) 2002-03-21
WO1997034734A1 (en) * 1996-03-22 1997-09-25 The Boeing Company Determinant wing assembly
Schofield et al. 1998 Open architecture controllers for machine tools, part 1: design principles
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