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Modelling and implementing circular sawblade stone cutting processes in STEP-NC
Robotics and Computer-Integrated Manufacturing 26 (2010) 602–609
Contents lists available at ScienceDirect
Robotics and Computer-Integrated Manufacturing journal homepage: www.elsevier.com/locate/rcim
Modelling and implementing circular sawblade stone cutting processes in STEP-NC Julio Garrido Campos n, Ricardo Marı´n Martı´n Automation and Systems Engineering Department, University of Vigo, Vigo 36200, Spain
a r t i c l e in f o
a b s t r a c t
Article history: Received 2 December 2009 Received in revised form 2 June 2010 Accepted 24 June 2010
The paper presents a STEP-NC compliant implementation of circular sawblade stone cutting machining processes. Although some stone machining processes has been already covered in the STEP-NC research and standardization initiatives (as for instance stone machining through stone milling machines), there have not been yet, however, any detailed model proposal to cover circular sawblade stone cutting operations. Sawblade cutting technology for stone parts have several specific parameters with no clear equivalent technologies as defined in milling, turning, etc. The paper reviews main characteristics of the circular sawblade stone cutting machining operations, and proposes a STEP-NC extended model based on the selection and definition of new features and on the modelling of these stone cutting operations. The resulting model is the base for the development of the STEP-NC stone cutting CAM and CNC machine. The machine architecture is designed to be able to react to changes in the machining conditions, very common in this technology. The system is based on the definition of features to be communicated to the controller. The controller has the objective of machining the features, and it is able to re planning, on real time, the work to get them despite changing conditions in the stone or in the disc. & 2010 Elsevier Ltd. All rights reserved.
Keywords: STEP-NC Stone cutting Sawblade machining CAM/CNC
1. Introduction Circular sawblades are the most popular cutting mechanism for stone construction parts, such as balusters, columns, mouldings, etc (figure numbers (1)–(3) in Fig. 1) [1]. Processing stone with automated sawblades machines is a multidimensional and complex task in which, factors such as physical material properties, sawblade characteristics, sawing conditions, cooling efficiency, etc., are all interrelated and affect process efficiency and quality. Different conditions during the cutting process such as tool cutting power variations, changes in the stone structure, etc. [2], is a relevant aspect of this technology. From the machine automation point of view and in order to get an optimized process, the control or/and the operator should have the ability to make changes in the middle of the process [3]. Another relevant aspect of the technology is that using discs limit the number of axis which can be moved when they are into the stone (only moves in the cutting plane can be done). To make complex
n Corresponding author at: E.T.S. Ingenieros Industriales, Campus LagoasMarcosende, Universidad de Vigo, 36200 Vigo, Spain. Tel.: + 34 986 812610; fax: + 34 986 814014. E-mail addresses: [email protected] (J. Garrido Campos), [email protected] (R. Marı´n Martı´n).
0736-5845/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.rcim.2010.06.027
features such as the arc patterns in Fig. 1, even five-axis movements may be needed and performed. Disc or circular saw stone cutting machines can be very complex, with 5 or more axis (Fig. 2) so as to machine complex parts. In many cases, machining is programmed using the same technology as that for metal CNC machines counterparts, i.e. G&M codes (ISO 6983) [4]. However CAM-CNC interfacing with ISO 6983 standard limits the implementation of online machining process adaptation to changing conditions [5], as this implies a full pre-calculation of the tool paths [6]. Instead, a common approach for stone machines is embedded CAM systems that directly and continuously generate tool-path axis control movements from feature definition while taking into account online parameters and online operator orders. Stone cutting machine software control systems are, in many cases, featurebased, where the controller program offers a range of information such as the feature to be machined, tool types to be used, etc., while machine-specific decisions are left to the CNC and its operator. This is the same view for future STEP-NC controllers [7]. A wide range of CNC manufacturing processes have already been studied by researchers with a view to extend the STEP-NC standard [8]. There have also been research initiatives with regard to stone machining. Stroud [9] addressed stone milling operations and defined new architectural stone features. Part 15 of the ISO 14649 standard—although aimed primarily at wood and glass
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machining conditions. The system is based on the definition of features to be communicated to the controller. The controller has the objective of machining the features, and it is able to re-plan in real time, the work to get them despite changing conditions in the stone or in the disc (Section 5.1).
2. Sawblade stone cutting process
Fig. 1. Stone constructions parts.
Fig. 2. 6 axis stone cutting machine.
processing—defined some stone sawblade operations. The approach is though very general. So far, there is no detailed research into STEP-NC and circular sawblade stone cutting operations. There is no sawblade stone cutting machine implementation related with STEP-NC reported either. This paper proposes a STEP-NC compliant CAM/CNC architecture to automate sawblade stone cutting machines. The paper is organised as follows. Section 2 summarizes certain particularities of the sawblade stone cutting process. Section 3 is a brief introduction to STEP-NC technology, its parts and architecture. Section 4 translates stone cutting technology particularities to a new STEP-NC compliant model for sawblade cutting processes. This extended model is based on the selection and definition of new features and on the modelling of the machining process. The resulting model is the base for the development of the STEP-NC stone cutting CAM and CNC machine (Section 5). The machine architecture is designed to be able to react to changes in the
Sawblade stone cutting machines do vary in terms of mechanical configuration and in terms of control [10]. Simpler machines make single or repetitive cuts, and some are equipped with several blades or several diamond wires to make more than one cut at a time. Other machines are designed and automated to be capable of making complex features such as the arc patterns (figure number (3) in Fig. 1). Some machines have an additional axis to rotate the stock and hence enable turned and indexed parts to be machined (figures (2) and (3) in Fig. 1, respectively). Indexed parts are, in appearance, very similar to revolution parts, but the process is not a turning one. The part cannot rotate when the blade is cutting; rather, the full perimeter is machined through step-bystep rotating movements of the part. Fig. 3 (Type 1) shows a classic sawblade 2-axis lathe machine and its kinematics model chain—adapted from Suh [11] and from Nassehi [12]. The part is fixed on a chuck attached to a spindle and the cutting tool (sawblade) is mounted on a moving bridge with two linear axis movements. Fig. 3 (Type 2) is a classic sawblade stone contour cutting machine. This is a 5-axis machine designed to perform from simple cuts to complex shapes such as mouldings. The simplest configuration has 3-axis movements (where for a fixed X position, cuts may be performed by moving the disc in the Y direction, while the disc also may go into the stone through the Z-axis). A more complex configuration adds 2 additional axes to the sawblade tool support, making it possible to perform oblique cuts (not just cuts in the Y direction, but following X Y linear paths), as well as cuts in planes other than the vertical. However, these 2 additional axes (an option represented as slashed line boxes in Fig. 3 kinematics graph) can only move from 01 to 901 (also represented in Fig. 3). In turning machines, the cutting plane has to be perpendicular to the revolution axis (as the part is revolving), and there is no possibility here of performing cuts perpendicular to any point of a profile, as in operations for plane mouldings (see the difference between perpendicular and parallel cuts in Fig. 8). Moreover, the machine table can have additional axes. For instance, Fig. 3 Type 2 represents a machine with a 2-axis table, one to rotate the stone part and the other to lift it at a specific angle. These movements are also constrained to a specific range (from 01 to 901 and from 01 to 451, respectively); sometimes, in indexed tables, only a set of fixed angles are allowed. The cutting process is a standard sequence of activities applied worldwide and performed in three phases (Fig. 4). In the first phase, cuts are made progressively with appropriate cut depths following the outline or contour of the final part (the desired surface), leaving a fixed distance between cuts. The sawblade makes a complete longitudinal cut and goes downward step by step, making several parallel cuts at different depths (passes). Once it reaches the desired depth, the sawblade disc exits the stone and moves to start another parallel cut at a given distance from the previous cut (Fig. 5). The number of passes to make a cut and the cutting distance between cuts depend on parameters such as stone hardness, feed rate, sawblade speed, etc. A specific distance is maintained between the depth of the cuts and the desired surface in order to avoid scratching the stone when
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Fig. 5. Single cut process.
material between cuts (slides) is removed, a finishing process obtains the final surface. The rough and terraced profile obtained from the previous phase is smoothed through some overlapping cuts. Ultimately, the choice of tools and adequate machining parameters depends on know-how and experience and also on inprocess parameter change decisions. The finishing phase may also be different in revolution operations, as the sawblade may perform a contour movement following the profile in the direction of the revolution axis in order to remove the thin layers of the remaining material.
3. STEP-NC technology parts
Fig. 3. Machine models.
Fig. 4. Stone cutting process.
performing the next phase. Second phase of eliminating the stone between the cuts is traditionally done by hand using tools such as mallets, chisels, etc. This process is improved if cut distance is reduced in the first phase. The stone slides would be thinner and precision higher, although processing would take longer. Once the
Today, a new standard often known as STEP-NC is being developed to provide a data model for a new breed of intelligent CNC Controllers [8]. Two versions of STEP-NC are being developed. The first is the application reference model (ARM), as ISO 14649 [13]. ISO 14649 models are written in EXPRESS language [14]. The second STEP-NC version, the ISO 10303 AP-238 [15], has adopted the ARM models built in ISO 14649 as the ARMs, and it is defined by the AIM (application reference model) to build a common language with other STEP data models. In both versions, designs are stored in ISO 10303-21 physical file format [16]. ISO 14649 details the information requirements that are to be fulfilled. Part 1 is an overview and fundamental principles definition, while Part 10 [17] is for general process data to provide a set of basic capabilities for process planning for machined parts. Besides these two general parts, other parts are dedicated to specific technologies: Part 11 [18] is the process data for milling, Part 12 [19] is the process data for turning. Additional parts define the specific technology tools, as Part 111 for milling tools, Part 121 [20] for turning tools, etc. (Fig. 6). In order to expand the use of STEP-NC a wide range of CNC manufacturing processes have already been addressed by research initiatives and projects. For instance, electro discharge machining (EDM) [9,21] by defining a complete extended model of STEP-NC that became ISO 14649-13; closed loop manufacturing processes for inspection that became a new ISO 14649-16; ISO 14649-15 to define process data for contour cutting of wood and glass, etc. [22]. Research in other machining processes uses the current state of the standard [23], as for instance, dry high-speed milling of marble and industrial ceramic (the LITHO-PRO project), in which the STEP-NC milling model has been used, but defining a new range of process-specific features [9]. Sawblade cutting technology for stone parts have several specific parameters with no clear equivalent in milling, turning technologies, etc. To explicitly consider them in the STEP-NC model, new technology
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Fig. 6. STEP-NC structure (ISO 14649).
entities would have to be added (a new ARM definition from an ISO 14649 perspective for the sawblade stone cutting technology). A new prototype model is presented next.
4. STEP-NC for sawblade stone cutting The new ARM data model proposal is built upon the basic STEP-NC process model (ISO 14649-10) and in accordance with Part 11 for milling and Part 12 for turning. The model defines technology-specific data types representing stone cutting processes following the basic STEP-NC approach of separating geometric and technological information. The extended model uses features already defined in the standard for milling and turning and in other research projects [9] (Fig. 7). In accordance with other technological parts (for example, Part 11 for milling) and with new developments (for example, Part 16 for inspection), the model defines the so-called sawbladecutting_working_steps, including machining operations and features to be machined. The main model entities for the proposal are described in Fig. 7 (features) and in Fig. 8 (operations). More details may be found at Garrido Campos [24]. STEP-NC programs can be described as a set of complex and structured tasks (workingsteps) to machine the features of a workpiece [25]. Figs. 7 and 8 also represent a STEP-NC program organised around a main workplan containing a series of working_steps. Each workingstep applies a specific machining operation to manufacturing features using specific tools, sets of technology parameters and specific strategies. Features specify the information necessary to identify shapes of interest in a mechanical product. These shapes represent volumes of material either removed by machining operations or resulting from a series of machining operations (ISO 14649-1). The sawblade stone cutting basic feature is the cut-out, which may be described with the slot feature already defined in ISO 14649-10 and also in ISO 10303 AP-224 [26], as well as the ISO 14649-12 cut_in, defined as a slot. Two main kinds of complex features are considered: planar features for mouldings and turning features for columns and balusters (Fig. 7). Fig. 8 represents a simplified model for sawblade stone cutting operations and main machining strategies for some of the operations. Surface operations are used to machine features for a fixed stock and also to machine planar faces in an indexed feature. When performing a sawblade_turning_operation, the stock has to rotate, either continuously to obtain revolving
Fig. 7. Sawblade stone cutting ARM general overview (ISO 14649 style): features.
features through revolving_turning_operations, or step-by-step to obtain indexed features through indexing_turning_operations. In both cases – as it is for surface features – roughing and finishing operations are executed. Different strategies may be adopted in each operation. Fig. 8 represents some of the strategies associated with the sawblade_surface_operation, while other strategies (not illustrated) can be defined for turning operations. As with the standard for milling and turning, the model also contains other information definitions such as technology_parameters, approach_retract_strategies, machine_functions. For
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example, a technology entity would include technology parameters as feed-rate value, sawblade speed value, Boolean values to allow or disallow feed-rate override, etc. Finally, information would also be needed on the main tool parameters, e.g. sawblade diameter, maximum allowed blade depth in the stone, etc.
Fig. 8. Sawblade stone cutting ARM general overview (ISO 14649 style): operations.
5. Implementation A first STEP-NC prototype CAM-CNC system was implemented with the extended model described above and based on a commercial 5-axis circular sawblade stone cutting machine (Fig. 2). The original machining system architecture is composed of a proprietary CAD/CAM-embedded system for selecting feature and machine parameters, although profiles for features may be imported from CAD files in DXF format for example. The CAM information is translated to extensible markup language (XML) and communicated to a man machine interface (MMI). Both CAD/ CAM and the MMI run in the same embedded PC under Windows CE, but the first could well be run in another computer as the MMI system can also import the XML files. The MMI adds shop-floor information to the XML file (for example, current sawblade diameter). The resulting file is the input for the machine low-level control module, responsible for axis motion control, alarm management and input/output management. This module runs in a TwinCAT PLC Run-Time system with TwinCAT NC axis control and is programmed in IEC 1131. In the STEP-NC compliant prototype (Fig. 9), a CAM application allows features and operation parameters to be selected and generates a STEP Part 21 file (Fig. 10) following the AP-238 extended model. STEP AP-238 is the STEP (ISO 10303: standard for the exchange of product data models) [27] extension for CNC programming. Unlike ISO 14649, which has separate Parts, AP238 incorporates the equivalent of all the Parts of ISO 14649 (except Part 1) in a single, very large model. Therefore, while a new technology would represent a new part for the ISO 14649 standard, in ISO STEP it would become an AP-238 extension model. AP-238 ARM model information entities are mapped into the STEP-integrated resources resulting an AIM model. The AIM application interpreted model is an EXPRESS model of (exactly) the information in an ARM but encoded in terms of the STEP integrated resources. The encoding is done using mapping tables [28]. The resulting AP-238 file in STEP part 21 format is transformed to a STEP Part 28 format [29] by a STEPPart21-toSTEPPart28 translator and is communicated to the machine. A more detailed description of the implementation may be found in Garrido Campos and Xu [30], where examples of the extended AP-238 ARM model updated with a new type of working steps for sawblade stone cutting are provided. Also, an example of the mapping tables developed to map the extended AP-238 ARM model information entities into the STEP-integrated resources is provided. An example of the translation of a STEPpart 21—program line and its equivalent following the STEP Part 28 specification is also given in [30].
Fig. 9. Implementation architecture.
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Fig. 10 is an extract from a STEP Part 21 format file generated following the new mapping tables, where, for instance, a disc_cutting_workingstep object (number #172) is a machining_workingstep but with a new string description (‘‘disc_cutting’’), and linked with new AIM technology types (object #171 DISC_CUTTING_TYPE_OPERATION) through an intermediate object (#170 MACHINING_OPERATION_RELATIONSHIP). The machine-embedded CAM/MMI system in Fig. 9 first processes the XML Part 28 file, taking advantage of XML file access libraries. A post-processor translates the data into the machine information system, matching feature operation parameters coming from the STEP-NC file to the feature operation parameters of the machine. Although the system looks like a classical two stages architecture [31] (the first is the CAM programming stage; the
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second is the actual machine stage carried out in a CNC machine tool), much of the intelligence is on the CNC side (Fig. 11 right). The prototype STEP-NC control architecture has the CAMembedded STEP-NC controller structure defined by Zhang [7] (represented in Fig. 9 central part). In Zhang’s architecture, the controller is a combination of a STEP-NC code interpreter, a basic CAM and a NC controller—the interpreter translates the physical file into internal data format and the CAM makes decisions on machine-specific details and generates low-level control commands that are executed by the NC control. The high-level (Windows CE planning process) does not communicate the lowlevel (the CNC controller), the trajectories to make a feature, but it communicates the geometric data of them. It is the CNC system itself that translates the ‘‘high-level program’’ to a ‘‘low-level program’’ where the tool movements sequence is specified. Tasks 2, 3, 4 and 5 of Fig. 11 (right) are performed by the control system, while in a more traditional architecture (Fig. 11 left), only task 4 (and also task 5 if changes are not too big) relays on the low control level. Fig. 11 (right) shows how the embedded CAM takes into account changing conditions. The embedded CAM calculates the tool path for each sawblade pass. If there is a change in some machining parameter so it implies a different tool path—as for instance a change in the distance between cuts—it will be translated to path calculation of the next tool passes.
5.1. An example
Fig. 10. STEP part 21 example.
Fig. 12 is an example of online tool path re-calculation to compensate a change in the machining conditions. It represents the sequential steps—from the design to the machining— performed by the machine system of Fig. 9 with the control structure of Fig. 11 to machine a moulding. Fig. 12 (a) is the feature geometric information—a linear moulding—coming from the design system (activity ‘‘1’’ in Fig. 11 performed by Fig. 9 CAD/ CAM proprietary system). This information is post-processed
Fig. 11. Control architecture.
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Fig. 12. Example of changing manufacturing conditions.
(activity 2 in Fig. 11) to plan the work (activity 3 in Fig. 11) and to decide tool paths, tool speeds and feed rates (Fig. 12b). The tool path trajectory (with a specific feed rate, step length to go downward in the stone and distance between cuts) is planned depending on parameters such as the stone hardness. Finally, the machine control low level algorithms perform the final run-time motion control (activity 4 in Fig. 1). Cutting the stone becomes harder if that hardness changes— increases—in some part of the stone, or if the disc losses its cutting efficiency. The machine may be able to automatically detect the new conditions through several variables (motor current, actual feed speed from the encoders, etc.) (step 5 in Fig. 11). To overcome the cutting difficulties it would be necessary to change the disc speed (easy to do), but also to recalculate the tool path (for instance, with a narrower distance between the cuts). The example illustrates a cutting condition change in the middle of the process: for instance, a change in the disc cutting power (Fig. 12c). In the example, the machine operator realizes of the new situation and, based on his experience, decides not only slow down the process speed rate, but also to change the number of vertical cuts, as well as to reduce the distance between tool passes. As result, the planning algorithm recalculates the tool path from the current tool position and generates a new ‘‘program’’ for the remaining work (step d in Fig. 12).
6. Conclusions The aim of the paper is not to provide a definitive work on STEP-NC sawblade stone cutting, but rather to present one possible approach. The new extended STEP-NC model for sawblade stone machining presented in this paper is a prototype example. To explicitly include it in the STEP-NC standard, the extended model would need to be approved by the ISO SC1 committee to become a new ISO 14649-## part, and ISO SC4
committee would have to expand the AP-238 ARM, AIM and mapping tables. However, much more work needs to be done. To reach this point, all CNC automated processes in a stone processing plant would need to fit in STEP-NC. Also, more feedback from other stone cutting systems would need to be considered. From the machine implementation point of view, it was not difficult to convert a commercial feature-oriented sawblade machine—working with an embedded CAM proprietary system—to an open STEP-NC-compliant machine, as the existing feature control algorithms in the original machine can be used. The machine’s original control system supports online dynamic process planning based on CAM data (geometric information about features and machining parameters), and real-time machining process conditions. While the latter is directly obtained by the controller through its sensors, the former has to be communicated from the design phase. Algorithms to plan the work (toolpath calculations) and algorithms to manage the work (motion order sequencing and dispatcher) reside on the CNC controller side to be dynamic and to allow on-process work. The STEP-NC compliant machine maintains the same behaviour as the STEP-NC CAM data file that contains similar feature and technology parameters, which with a feedback input from the machine may continuously process the tool paths to make the part (the feature).
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Contents lists available at ScienceDirect
Robotics and Computer-Integrated Manufacturing journal homepage: www.elsevier.com/locate/rcim
Modelling and implementing circular sawblade stone cutting processes in STEP-NC Julio Garrido Campos n, Ricardo Marı´n Martı´n Automation and Systems Engineering Department, University of Vigo, Vigo 36200, Spain
a r t i c l e in f o
a b s t r a c t
Article history: Received 2 December 2009 Received in revised form 2 June 2010 Accepted 24 June 2010
The paper presents a STEP-NC compliant implementation of circular sawblade stone cutting machining processes. Although some stone machining processes has been already covered in the STEP-NC research and standardization initiatives (as for instance stone machining through stone milling machines), there have not been yet, however, any detailed model proposal to cover circular sawblade stone cutting operations. Sawblade cutting technology for stone parts have several specific parameters with no clear equivalent technologies as defined in milling, turning, etc. The paper reviews main characteristics of the circular sawblade stone cutting machining operations, and proposes a STEP-NC extended model based on the selection and definition of new features and on the modelling of these stone cutting operations. The resulting model is the base for the development of the STEP-NC stone cutting CAM and CNC machine. The machine architecture is designed to be able to react to changes in the machining conditions, very common in this technology. The system is based on the definition of features to be communicated to the controller. The controller has the objective of machining the features, and it is able to re planning, on real time, the work to get them despite changing conditions in the stone or in the disc. & 2010 Elsevier Ltd. All rights reserved.
Keywords: STEP-NC Stone cutting Sawblade machining CAM/CNC
1. Introduction Circular sawblades are the most popular cutting mechanism for stone construction parts, such as balusters, columns, mouldings, etc (figure numbers (1)–(3) in Fig. 1) [1]. Processing stone with automated sawblades machines is a multidimensional and complex task in which, factors such as physical material properties, sawblade characteristics, sawing conditions, cooling efficiency, etc., are all interrelated and affect process efficiency and quality. Different conditions during the cutting process such as tool cutting power variations, changes in the stone structure, etc. [2], is a relevant aspect of this technology. From the machine automation point of view and in order to get an optimized process, the control or/and the operator should have the ability to make changes in the middle of the process [3]. Another relevant aspect of the technology is that using discs limit the number of axis which can be moved when they are into the stone (only moves in the cutting plane can be done). To make complex
n Corresponding author at: E.T.S. Ingenieros Industriales, Campus LagoasMarcosende, Universidad de Vigo, 36200 Vigo, Spain. Tel.: + 34 986 812610; fax: + 34 986 814014. E-mail addresses: [email protected] (J. Garrido Campos), [email protected] (R. Marı´n Martı´n).
0736-5845/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.rcim.2010.06.027
features such as the arc patterns in Fig. 1, even five-axis movements may be needed and performed. Disc or circular saw stone cutting machines can be very complex, with 5 or more axis (Fig. 2) so as to machine complex parts. In many cases, machining is programmed using the same technology as that for metal CNC machines counterparts, i.e. G&M codes (ISO 6983) [4]. However CAM-CNC interfacing with ISO 6983 standard limits the implementation of online machining process adaptation to changing conditions [5], as this implies a full pre-calculation of the tool paths [6]. Instead, a common approach for stone machines is embedded CAM systems that directly and continuously generate tool-path axis control movements from feature definition while taking into account online parameters and online operator orders. Stone cutting machine software control systems are, in many cases, featurebased, where the controller program offers a range of information such as the feature to be machined, tool types to be used, etc., while machine-specific decisions are left to the CNC and its operator. This is the same view for future STEP-NC controllers [7]. A wide range of CNC manufacturing processes have already been studied by researchers with a view to extend the STEP-NC standard [8]. There have also been research initiatives with regard to stone machining. Stroud [9] addressed stone milling operations and defined new architectural stone features. Part 15 of the ISO 14649 standard—although aimed primarily at wood and glass
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machining conditions. The system is based on the definition of features to be communicated to the controller. The controller has the objective of machining the features, and it is able to re-plan in real time, the work to get them despite changing conditions in the stone or in the disc (Section 5.1).
2. Sawblade stone cutting process
Fig. 1. Stone constructions parts.
Fig. 2. 6 axis stone cutting machine.
processing—defined some stone sawblade operations. The approach is though very general. So far, there is no detailed research into STEP-NC and circular sawblade stone cutting operations. There is no sawblade stone cutting machine implementation related with STEP-NC reported either. This paper proposes a STEP-NC compliant CAM/CNC architecture to automate sawblade stone cutting machines. The paper is organised as follows. Section 2 summarizes certain particularities of the sawblade stone cutting process. Section 3 is a brief introduction to STEP-NC technology, its parts and architecture. Section 4 translates stone cutting technology particularities to a new STEP-NC compliant model for sawblade cutting processes. This extended model is based on the selection and definition of new features and on the modelling of the machining process. The resulting model is the base for the development of the STEP-NC stone cutting CAM and CNC machine (Section 5). The machine architecture is designed to be able to react to changes in the
Sawblade stone cutting machines do vary in terms of mechanical configuration and in terms of control [10]. Simpler machines make single or repetitive cuts, and some are equipped with several blades or several diamond wires to make more than one cut at a time. Other machines are designed and automated to be capable of making complex features such as the arc patterns (figure number (3) in Fig. 1). Some machines have an additional axis to rotate the stock and hence enable turned and indexed parts to be machined (figures (2) and (3) in Fig. 1, respectively). Indexed parts are, in appearance, very similar to revolution parts, but the process is not a turning one. The part cannot rotate when the blade is cutting; rather, the full perimeter is machined through step-bystep rotating movements of the part. Fig. 3 (Type 1) shows a classic sawblade 2-axis lathe machine and its kinematics model chain—adapted from Suh [11] and from Nassehi [12]. The part is fixed on a chuck attached to a spindle and the cutting tool (sawblade) is mounted on a moving bridge with two linear axis movements. Fig. 3 (Type 2) is a classic sawblade stone contour cutting machine. This is a 5-axis machine designed to perform from simple cuts to complex shapes such as mouldings. The simplest configuration has 3-axis movements (where for a fixed X position, cuts may be performed by moving the disc in the Y direction, while the disc also may go into the stone through the Z-axis). A more complex configuration adds 2 additional axes to the sawblade tool support, making it possible to perform oblique cuts (not just cuts in the Y direction, but following X Y linear paths), as well as cuts in planes other than the vertical. However, these 2 additional axes (an option represented as slashed line boxes in Fig. 3 kinematics graph) can only move from 01 to 901 (also represented in Fig. 3). In turning machines, the cutting plane has to be perpendicular to the revolution axis (as the part is revolving), and there is no possibility here of performing cuts perpendicular to any point of a profile, as in operations for plane mouldings (see the difference between perpendicular and parallel cuts in Fig. 8). Moreover, the machine table can have additional axes. For instance, Fig. 3 Type 2 represents a machine with a 2-axis table, one to rotate the stone part and the other to lift it at a specific angle. These movements are also constrained to a specific range (from 01 to 901 and from 01 to 451, respectively); sometimes, in indexed tables, only a set of fixed angles are allowed. The cutting process is a standard sequence of activities applied worldwide and performed in three phases (Fig. 4). In the first phase, cuts are made progressively with appropriate cut depths following the outline or contour of the final part (the desired surface), leaving a fixed distance between cuts. The sawblade makes a complete longitudinal cut and goes downward step by step, making several parallel cuts at different depths (passes). Once it reaches the desired depth, the sawblade disc exits the stone and moves to start another parallel cut at a given distance from the previous cut (Fig. 5). The number of passes to make a cut and the cutting distance between cuts depend on parameters such as stone hardness, feed rate, sawblade speed, etc. A specific distance is maintained between the depth of the cuts and the desired surface in order to avoid scratching the stone when
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Fig. 5. Single cut process.
material between cuts (slides) is removed, a finishing process obtains the final surface. The rough and terraced profile obtained from the previous phase is smoothed through some overlapping cuts. Ultimately, the choice of tools and adequate machining parameters depends on know-how and experience and also on inprocess parameter change decisions. The finishing phase may also be different in revolution operations, as the sawblade may perform a contour movement following the profile in the direction of the revolution axis in order to remove the thin layers of the remaining material.
3. STEP-NC technology parts
Fig. 3. Machine models.
Fig. 4. Stone cutting process.
performing the next phase. Second phase of eliminating the stone between the cuts is traditionally done by hand using tools such as mallets, chisels, etc. This process is improved if cut distance is reduced in the first phase. The stone slides would be thinner and precision higher, although processing would take longer. Once the
Today, a new standard often known as STEP-NC is being developed to provide a data model for a new breed of intelligent CNC Controllers [8]. Two versions of STEP-NC are being developed. The first is the application reference model (ARM), as ISO 14649 [13]. ISO 14649 models are written in EXPRESS language [14]. The second STEP-NC version, the ISO 10303 AP-238 [15], has adopted the ARM models built in ISO 14649 as the ARMs, and it is defined by the AIM (application reference model) to build a common language with other STEP data models. In both versions, designs are stored in ISO 10303-21 physical file format [16]. ISO 14649 details the information requirements that are to be fulfilled. Part 1 is an overview and fundamental principles definition, while Part 10 [17] is for general process data to provide a set of basic capabilities for process planning for machined parts. Besides these two general parts, other parts are dedicated to specific technologies: Part 11 [18] is the process data for milling, Part 12 [19] is the process data for turning. Additional parts define the specific technology tools, as Part 111 for milling tools, Part 121 [20] for turning tools, etc. (Fig. 6). In order to expand the use of STEP-NC a wide range of CNC manufacturing processes have already been addressed by research initiatives and projects. For instance, electro discharge machining (EDM) [9,21] by defining a complete extended model of STEP-NC that became ISO 14649-13; closed loop manufacturing processes for inspection that became a new ISO 14649-16; ISO 14649-15 to define process data for contour cutting of wood and glass, etc. [22]. Research in other machining processes uses the current state of the standard [23], as for instance, dry high-speed milling of marble and industrial ceramic (the LITHO-PRO project), in which the STEP-NC milling model has been used, but defining a new range of process-specific features [9]. Sawblade cutting technology for stone parts have several specific parameters with no clear equivalent in milling, turning technologies, etc. To explicitly consider them in the STEP-NC model, new technology
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Fig. 6. STEP-NC structure (ISO 14649).
entities would have to be added (a new ARM definition from an ISO 14649 perspective for the sawblade stone cutting technology). A new prototype model is presented next.
4. STEP-NC for sawblade stone cutting The new ARM data model proposal is built upon the basic STEP-NC process model (ISO 14649-10) and in accordance with Part 11 for milling and Part 12 for turning. The model defines technology-specific data types representing stone cutting processes following the basic STEP-NC approach of separating geometric and technological information. The extended model uses features already defined in the standard for milling and turning and in other research projects [9] (Fig. 7). In accordance with other technological parts (for example, Part 11 for milling) and with new developments (for example, Part 16 for inspection), the model defines the so-called sawbladecutting_working_steps, including machining operations and features to be machined. The main model entities for the proposal are described in Fig. 7 (features) and in Fig. 8 (operations). More details may be found at Garrido Campos [24]. STEP-NC programs can be described as a set of complex and structured tasks (workingsteps) to machine the features of a workpiece [25]. Figs. 7 and 8 also represent a STEP-NC program organised around a main workplan containing a series of working_steps. Each workingstep applies a specific machining operation to manufacturing features using specific tools, sets of technology parameters and specific strategies. Features specify the information necessary to identify shapes of interest in a mechanical product. These shapes represent volumes of material either removed by machining operations or resulting from a series of machining operations (ISO 14649-1). The sawblade stone cutting basic feature is the cut-out, which may be described with the slot feature already defined in ISO 14649-10 and also in ISO 10303 AP-224 [26], as well as the ISO 14649-12 cut_in, defined as a slot. Two main kinds of complex features are considered: planar features for mouldings and turning features for columns and balusters (Fig. 7). Fig. 8 represents a simplified model for sawblade stone cutting operations and main machining strategies for some of the operations. Surface operations are used to machine features for a fixed stock and also to machine planar faces in an indexed feature. When performing a sawblade_turning_operation, the stock has to rotate, either continuously to obtain revolving
Fig. 7. Sawblade stone cutting ARM general overview (ISO 14649 style): features.
features through revolving_turning_operations, or step-by-step to obtain indexed features through indexing_turning_operations. In both cases – as it is for surface features – roughing and finishing operations are executed. Different strategies may be adopted in each operation. Fig. 8 represents some of the strategies associated with the sawblade_surface_operation, while other strategies (not illustrated) can be defined for turning operations. As with the standard for milling and turning, the model also contains other information definitions such as technology_parameters, approach_retract_strategies, machine_functions. For
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example, a technology entity would include technology parameters as feed-rate value, sawblade speed value, Boolean values to allow or disallow feed-rate override, etc. Finally, information would also be needed on the main tool parameters, e.g. sawblade diameter, maximum allowed blade depth in the stone, etc.
Fig. 8. Sawblade stone cutting ARM general overview (ISO 14649 style): operations.
5. Implementation A first STEP-NC prototype CAM-CNC system was implemented with the extended model described above and based on a commercial 5-axis circular sawblade stone cutting machine (Fig. 2). The original machining system architecture is composed of a proprietary CAD/CAM-embedded system for selecting feature and machine parameters, although profiles for features may be imported from CAD files in DXF format for example. The CAM information is translated to extensible markup language (XML) and communicated to a man machine interface (MMI). Both CAD/ CAM and the MMI run in the same embedded PC under Windows CE, but the first could well be run in another computer as the MMI system can also import the XML files. The MMI adds shop-floor information to the XML file (for example, current sawblade diameter). The resulting file is the input for the machine low-level control module, responsible for axis motion control, alarm management and input/output management. This module runs in a TwinCAT PLC Run-Time system with TwinCAT NC axis control and is programmed in IEC 1131. In the STEP-NC compliant prototype (Fig. 9), a CAM application allows features and operation parameters to be selected and generates a STEP Part 21 file (Fig. 10) following the AP-238 extended model. STEP AP-238 is the STEP (ISO 10303: standard for the exchange of product data models) [27] extension for CNC programming. Unlike ISO 14649, which has separate Parts, AP238 incorporates the equivalent of all the Parts of ISO 14649 (except Part 1) in a single, very large model. Therefore, while a new technology would represent a new part for the ISO 14649 standard, in ISO STEP it would become an AP-238 extension model. AP-238 ARM model information entities are mapped into the STEP-integrated resources resulting an AIM model. The AIM application interpreted model is an EXPRESS model of (exactly) the information in an ARM but encoded in terms of the STEP integrated resources. The encoding is done using mapping tables [28]. The resulting AP-238 file in STEP part 21 format is transformed to a STEP Part 28 format [29] by a STEPPart21-toSTEPPart28 translator and is communicated to the machine. A more detailed description of the implementation may be found in Garrido Campos and Xu [30], where examples of the extended AP-238 ARM model updated with a new type of working steps for sawblade stone cutting are provided. Also, an example of the mapping tables developed to map the extended AP-238 ARM model information entities into the STEP-integrated resources is provided. An example of the translation of a STEPpart 21—program line and its equivalent following the STEP Part 28 specification is also given in [30].
Fig. 9. Implementation architecture.
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Fig. 10 is an extract from a STEP Part 21 format file generated following the new mapping tables, where, for instance, a disc_cutting_workingstep object (number #172) is a machining_workingstep but with a new string description (‘‘disc_cutting’’), and linked with new AIM technology types (object #171 DISC_CUTTING_TYPE_OPERATION) through an intermediate object (#170 MACHINING_OPERATION_RELATIONSHIP). The machine-embedded CAM/MMI system in Fig. 9 first processes the XML Part 28 file, taking advantage of XML file access libraries. A post-processor translates the data into the machine information system, matching feature operation parameters coming from the STEP-NC file to the feature operation parameters of the machine. Although the system looks like a classical two stages architecture [31] (the first is the CAM programming stage; the
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second is the actual machine stage carried out in a CNC machine tool), much of the intelligence is on the CNC side (Fig. 11 right). The prototype STEP-NC control architecture has the CAMembedded STEP-NC controller structure defined by Zhang [7] (represented in Fig. 9 central part). In Zhang’s architecture, the controller is a combination of a STEP-NC code interpreter, a basic CAM and a NC controller—the interpreter translates the physical file into internal data format and the CAM makes decisions on machine-specific details and generates low-level control commands that are executed by the NC control. The high-level (Windows CE planning process) does not communicate the lowlevel (the CNC controller), the trajectories to make a feature, but it communicates the geometric data of them. It is the CNC system itself that translates the ‘‘high-level program’’ to a ‘‘low-level program’’ where the tool movements sequence is specified. Tasks 2, 3, 4 and 5 of Fig. 11 (right) are performed by the control system, while in a more traditional architecture (Fig. 11 left), only task 4 (and also task 5 if changes are not too big) relays on the low control level. Fig. 11 (right) shows how the embedded CAM takes into account changing conditions. The embedded CAM calculates the tool path for each sawblade pass. If there is a change in some machining parameter so it implies a different tool path—as for instance a change in the distance between cuts—it will be translated to path calculation of the next tool passes.
5.1. An example
Fig. 10. STEP part 21 example.
Fig. 12 is an example of online tool path re-calculation to compensate a change in the machining conditions. It represents the sequential steps—from the design to the machining— performed by the machine system of Fig. 9 with the control structure of Fig. 11 to machine a moulding. Fig. 12 (a) is the feature geometric information—a linear moulding—coming from the design system (activity ‘‘1’’ in Fig. 11 performed by Fig. 9 CAD/ CAM proprietary system). This information is post-processed
Fig. 11. Control architecture.
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Fig. 12. Example of changing manufacturing conditions.
(activity 2 in Fig. 11) to plan the work (activity 3 in Fig. 11) and to decide tool paths, tool speeds and feed rates (Fig. 12b). The tool path trajectory (with a specific feed rate, step length to go downward in the stone and distance between cuts) is planned depending on parameters such as the stone hardness. Finally, the machine control low level algorithms perform the final run-time motion control (activity 4 in Fig. 1). Cutting the stone becomes harder if that hardness changes— increases—in some part of the stone, or if the disc losses its cutting efficiency. The machine may be able to automatically detect the new conditions through several variables (motor current, actual feed speed from the encoders, etc.) (step 5 in Fig. 11). To overcome the cutting difficulties it would be necessary to change the disc speed (easy to do), but also to recalculate the tool path (for instance, with a narrower distance between the cuts). The example illustrates a cutting condition change in the middle of the process: for instance, a change in the disc cutting power (Fig. 12c). In the example, the machine operator realizes of the new situation and, based on his experience, decides not only slow down the process speed rate, but also to change the number of vertical cuts, as well as to reduce the distance between tool passes. As result, the planning algorithm recalculates the tool path from the current tool position and generates a new ‘‘program’’ for the remaining work (step d in Fig. 12).
6. Conclusions The aim of the paper is not to provide a definitive work on STEP-NC sawblade stone cutting, but rather to present one possible approach. The new extended STEP-NC model for sawblade stone machining presented in this paper is a prototype example. To explicitly include it in the STEP-NC standard, the extended model would need to be approved by the ISO SC1 committee to become a new ISO 14649-## part, and ISO SC4
committee would have to expand the AP-238 ARM, AIM and mapping tables. However, much more work needs to be done. To reach this point, all CNC automated processes in a stone processing plant would need to fit in STEP-NC. Also, more feedback from other stone cutting systems would need to be considered. From the machine implementation point of view, it was not difficult to convert a commercial feature-oriented sawblade machine—working with an embedded CAM proprietary system—to an open STEP-NC-compliant machine, as the existing feature control algorithms in the original machine can be used. The machine’s original control system supports online dynamic process planning based on CAM data (geometric information about features and machining parameters), and real-time machining process conditions. While the latter is directly obtained by the controller through its sensors, the former has to be communicated from the design phase. Algorithms to plan the work (toolpath calculations) and algorithms to manage the work (motion order sequencing and dispatcher) reside on the CNC controller side to be dynamic and to allow on-process work. The STEP-NC compliant machine maintains the same behaviour as the STEP-NC CAM data file that contains similar feature and technology parameters, which with a feedback input from the machine may continuously process the tool paths to make the part (the feature).
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