Connection of NC/CNC manufacturing and reverse engineering.

Monkova, K. ; Monka, P.

1. Introduction

A considerable number of companies in Slovakia produce components on the basis of component drawings which can be drawn manually but nowadays most often by computer technology. The software using 3D technique notably facilitates the work of constructors and designers. At the design stage of a new product, a 3D model is created at first, and then the analyses and simulations of the production process are conducted. After the fault elimination, drawing and technological documentation is completed and only on its basis is the component being produced.

The problems occur when the actual component exists, e.g. in the form of a hand-made model, is complex-shaped and its geometric characteristics are unknown. The production of other (exactly the same) components is in this case problematic, especially where a mass production is concerned. The dimension characteristics are essential in order to create a 3D model, drawing or to produce the component itself. Obtaining further information about the component, processing of this data, e.g. by a computer becomes a necessity, and only on the basis of acquired data is the drawing preparation and direct generation of CL data for the component production on NC machine possible.

2. The problem definition and solution

The example of problems, which originate at the modelling and manufacturing of part with unknown variable radius flow, can be shown on the template for the coiling of stator winding of electromotor (Fig. 1) that existed in the firm as real steel part but the radius flow and dimensions were unknown. It is a real component, which used to be produced abroad in a way that its finite shape underwent a hand grinding into an anti-template but the drawing documentation of the resultant topography was not available. The average delivery time was longer than 3 month.

[FIGURE 1 OMITTED]

It was needed to create 3D model of this part in CAD/CAM system Pro/Engineer according to sponsor requests to obtain the CL data for the NC program generating.

The production of other complex shaped parts according to real existing original without data digitizing is very difficult and often impossible. The data obtaining and its digitization is enabled due to scanning equipment which allows the conversion of real three dimensional objects into a digital form. The principle of the majority of these equipments is based on the scanning the object's surface in its discrete points and it follows that the digitized object is presented on a computer as a large number of points in space, i.e. the so-called point cloud. Scanners differ from one another especially in the way the scanning of the object's surface points is implemented. The scanning equipment can be divided according to whether the scanning technology is contact or a non-contact one. The former concerns 3D scanners and stationary coordinate measurement systems CMM (Control Measuring Machine). This category offers the digitization equipment ranging from 3D desktop equipment to the systems used for measurement of large objects of several meters in size. The latter, non-contact systems of measurement, i.e. scanners, generally operate on laser or optical principle. For the most part, a choice of the scanner type depends on the requirements posed on the accuracy of homogeneity between a real and a digitized model. Further, another important factor when selecting a scanner is the scanning time. The fastest scanners are the laser ones. Also, a significant factor is the size of the scanned component or, possibly the mobility of the scanning equipment. The majority of scanners are limited by the scanning space in which it is possible to scan. 3D scanners are usually constructed to be able to scan objects as large as 50 cm. For more sizeable objects the larger scanning equipments are produced which are then more expensive. (Hrabovsky, 2002)

2.1 NC Program as the Output of Digital Model Created in CAD/CAM System

Apart from hardware devices a substantial role at digitization of 3D objects is played by software equipment. Individual scanning equipments use own software for processing of the scanned data, however these need to be transformed several times and eventually transferred into a neutral format (IGES, STEP,...) which CAD/CAM systems can operate with.

What makes CNC so flexible and productive is the ability to run different and difficult workpiece programs. With the right program, machining is a pushbutton affair. Provided that the proper cutting tools and set up fixtures are available, all CNC machine needs to begin automatically machining a new and different program.

The creation of such new difficult program has taken various approaches. Many CNC machines can be programmed on the shop floor, with the operator entering data at the control panel. This method has been very popular, especially for simpler workpieces. Programs can also be prepared "off-line," away from the machine tool, using computer-aided manufacturing (CAM) software. This method is most often used for more complex workpieces. The latest CAM software for the PC (personal computer) provides many automated features that make NC programming largely a push-button affair, regardless of how simple or complex the workpiece might be.

When the applications became more complicated and especially when new programs were required on a regular basis, the writing of programs manually became much more difficult. To simplify the programming process, CAM system can be used. It is a software program that runs on a computer (commonly a PC) that helps the CNC programmer/ machinist/manufacturing engineer to program from the drawings and with the whole programming process. The making of drawings, and programming parts from drawings, was (and still is) time consuming and subject to a lot of human error. Someone got the bright idea to eliminate this to-and-from drawing step, and integrated CAD/CAM was born. Integrating computer-aided design with computer-aided manufacturing (CAD/CAM) system produces quicker and more efficient manufacturing processes. This compatibility of CAD/CAM systems eliminates the need for redefining the work piece configuration to the CAM system.

Most computer numerical control (CNC) machines are programmed in the ISO 6983 "G and M code" language. Programs are typically generated by computer-aided manufacturing (CAM) systems that use computer aided design (CAD) information. However, ISO 6983 limits program portability for three reasons:

* First, the language focuses on programming the tool centre path with respect to machine axes, rather than the machining process with respect to the part.

* Second, the standard defines the syntax of program statements, but

in most cases leaves the semantics ambiguous.

* Third, vendors usually supplement the language with extensions that are not covered in the limited scope of ISO 6983.

Fig.2 shows how design data is communicated to manufacturing in current practice. Design creates the specification for a product as a 3D model. Detailing decides the manufacturing requirements for the product by making a drawing. Path planning generates tools paths. Manufacturing controls production. The job of design is performed using a CAD (Computer Aided Design) system, the job of detailing is performed using a drawing CADD (Computer Aided Design Drafting) system, the job of path planning is performed using a CAM (Computer Aided Manufacturing) system, and job of manufacturing is controlled using a CNC system. In many cases the CAD, CADD and CAM functions are combined into a single integrated CAD/CAM system but in all cases the CNC function is performed by a separate system.

[FIGURE 2 OMITTED]

Data flow in mechanical engineering:

* First the information about a product is imported into the CAM system. Usually 3D CAD model is imported.

* In mechanical engineering CAM is used to calculate toolpath to cut material. The CNC programmer just specifies the machining operations and the CAM system creates the toolpath, usually written in CL data (Cutter Location data) file.

* Calculated toolpath is imported to the postprocessor which converts the CL data to the NC program--the specific machine codes that are required to operate numerically controlled machine tools. Machine codes vary by machine tool. The output from a postprocessor should be usable in the controller without further modification.

* NC program written in a notation called G-code is exported to the NC machine and the manufacturing process can begin.

One of the most important links in the CNC machining process is postprocessor. The company can have best programming system, the best DNC system, and the most accurate machines, but without dependable postprocessors the code getting to its machine could cause problems. Most machines require some tweaking to the postprocessor to make it to produce code to user's likings and programming habits. Most CAM software comes with built in postprocessor; however there are many stands alone postprocessor. It is the last software link between an ideal CAD model and a "real" machined part. Some problems on the subject of the postprocessors are: (Monkova et al., 2007)

* Absence in a CAD/CAM system of the ready postprocessor for specific "machine tool /machine control" combination and also far too high cost of new postprocessor development.

* As a rule, it is very difficult and often impossible, to modify the postprocessor available in the CAD/CAM system to the specific "machine tool / machine control" combination. In general, the technologist is forced to permanently correct wrong NC program file with the text processor.

* Usually generalized postprocessors of majority of the CAD/CAM systems are very difficult to adapt to NC/CNC equipment, made in 70-80 years.

* Each new CAD/CAM system requires an individual postprocessor for the same NC equipment, so the company has to pay twice for the same stuff.

* Creating a custom postprocessor for each unique machine is often difficult and costly.

One of the manners how to simplify the data flow and data translation at the manufacturing is the introducing of STEP NC into production.

The concept with the using of STEP NC is simple. It enables a product model database to serve as direct input to a CNC machine tool. No separate files of tool paths. No G or M codes. No post processors. This is a radically different approach to CNC programming. STEP NC is an extension to STEP, the STandard for the Exchange of Product model data. STEP is the international standard that specifies a neutral data format for digital information about a product. STEP allows this data to be shared and exchanged among different and otherwise incompatible computer platforms. STEP NC standardizes how information about CNC machining can be added to parts represented in the STEP product model.

STEP NC defines a CNC part program as a series of operations that remove material defined by features. The features supported include holes, slots, pockets and volumes defined by 3D surfaces. Each operation contributes to the manufacture of a feature by defining the volume of material to be removed, the tolerances, the type of tool required and some basic characteristics such as whether this is a roughing or finishing operation. The operations are then sequenced into a work plan that converts the stock into the final part. The work plan may be sophisticated and include conditional operations that depend on the results of probing operations, and it may be divided into sub-plans to be executed concurrently on machines that have multiple cutting heads. In the new method enterprises can continue to use their existing systems for CAD, CADD and CAM, but the end result is sent to the CNC as a STEP NC AP-238 file instead of an RS274D file. Fig. 3 shows the modified data flow with the using of STEP NC. (ISO/DIS 14649-1, 2002)

[FIGURE 3 OMITTED]

STEP NC allows a complete database of machining information to be built around the digital product model and ultimately makes it possible for this enhanced product model to serve as machine tool input. This database is structured such that part features are linked to specific "working steps," generic descriptions of various machining operations. STEP NC working steps are roughly equivalent to machining commands formatted as traditional M and G codes. With the concept of "working steps" in place, the manufacturing process becomes streamlined. Now, a machine tool can receive a file with STEP NC data, "know" what it means, and proceed milling the piece without any more instructions. There will be no more programming the machine tool for each individual piece. Moreover, the benefit of the standard goes further. With a set of standard "working steps" in place, all manufacturers will be able to share information reliably and instantaneously. STEP NC converted CAD file that is completed on the east coast can be sent over the internet to a machine shop on the west coast and they can immediately start milling the part.

Machine tools with PC-based open architecture control systems may be able to install this software to upgrade to STEP NC compatibility rather effectively. The conventional input/output (I/O) structure and the servo system of the CNC machine do not need to be modified under STEP NC. The project that developed STEP NC has estimated that it can reduce the time required to program a CNC by about 35%, reduce the number of drawings that have to be sent from design to manufacturing by about 75% and decrease the time required to machine parts on CNC tools by about 50% for small to mid-sized job lots. (ISO/DIS 14649-1, 2002)

On the basis of listed above it is visible that for the manufacturing of complex shaped parts by means of CNC machines is useful to create 3D model serving for CL data generation.

2.2 Template for the coiling of stator winding of electromotor

One of the modes how to acquire the dimensions and in the conditions of Faculty of Manufacturing Technologies of the Technical University in Kosice, with the seat in Presov, was the using of 3D laser scanner with the Dr. PICZA3 software.

This scanner that integrates hardware/software system is an ideal 3D capture solution for all popular CAD/CAM and animation applications. It allows designers to capture complex data for hand-held consumer products, blister package design, handsculpted characters for feature animation, and face models for anaplastologists. It makes it incredibly fast and easy to generate precise 3D models, it will save designers hours of manual reverse engineering work. It uses an advanced non-contact laser sensor to quickly generate precise models. The combination of precision laser optics and motion control within a rigid enclosure lets the LPX 250 produce high quality scans with minimal surface noise. Final version of a 3D model created without geometric and drawing definition is on the Fig. 4.

[FIGURE 4 OMITTED]

Created 3D model was compared by geometry with the scanned shape in Pro/Engineer. The spaces between measured points were 1 mm and the tolerance was 0.1 mm. It can be said that 98 % of the surfaces were inside tolerance. On the basis of created 3D model it was made the new physical part by Rapid Prototyping method. The cast plastic part was compared with the original real part by means of 3D measuring equipment and then it was used in real situation. (Dobransky & Mihalcova, 2008). It was possible to allege that the model corresponds to the real steel part in required accuracy and so CL data were generated. The obtained data were transformed and applied as NC program for controller of concrete CNC machine in company. After the creating of 3D model and after the generating of NC program, the terms of delivery were shortened about 98 % (from 180 days on 2-5 days), the number of stored templates decrease about 50 % and the price of the parts made in Slovakia derogated about 60 % compared with original foreign supplier.

3. Conclusion

The automation of the manufacturing is one of the main goals in present days, what is enabled by the quickly development of information technology and by the sequential application of computer aid into all areas of the production.

Within the automation the NC program creation and the complex manufacturing with the utilization of CNC machines are needful for the every plant that wants to be a success with its products on the market today. The generating of CL data as the output of CAM software enables to create NC program for selected control system very simply by postprocessor. Nowadays, the virtual 3D models are used in the majority of mechanical engineering companies in all production stages from proposal to delivery. Their advantages are undeniable. By means of a 3D model it is possible to create a component drawing in 2D format in a relatively short time, to define mass and geometric characteristics of the real component and the whole structure, to carry out various types of analyses, to simulate the process of machining with the output of CL data which define the position of the tool when machining and the cutting conditions, to translate these data to NC program for a particular control system of the selected NC (CNC) machine (Valicek et al., 2009).

The creation of a 3D model also allows to predict conflicting situations not only at the machining of individual components but also at their putting together to assemblies and subassemblies resulting in the reduction of the preparation time, expenses and the quality enhancement of production. One modelled component can thus become the basis for its simple modification and subsequent production of other, type similar components, e.g. within a group technology.

DOI: 10.2507/daaam.scibook.2009.85

4. References

Dobransky, J. & Mihalcova, J. (2008). Measuring of dimensional accuracy of plastic product manufactured by injection moulding, Mechanics, Tribology, Machine Manufacturing Technology, Vol. 22, serie c (2008), p. 153-158, ISSN 12243264

Hrabovsky M. (2002). Optical methods in experimental mechanics, Proceedings of 40th International conference experimental stress analysis, Czech Republic, June 2002, Prague

Monkova, K., Hatala, M. & Cep, R. (2007). Step NC in relation to the manufacturing data flow, Proceedings of scientific works of VSB TU--Mechanical Engineering Part, Vol. 53, no. 1, p. 135-139, ISSN 1210-0471

Valicek, J., Hloch, S. & Kozak, D. (2009). Surface geometric parameters proposal for the advanced control of abrasive waterjet technology. In: The International Journal of Advanced Manufacturing Technology, Vol. 41, no. 3-4, p. 323-328, ISSN 0268-3768

*** http://www.steptools.com/library/stepnc/tech_resources/ FDIS_Feb2002_ISO14649, International Organization for Standardization--ISO/DIS 14649-1 Industrial automation systems and integration--Physical device control, (2002), Accessed on: 2002-09-28

This Publication has to be referred as: Monkova, K[atarina] & Monka, P[eter] (2009). connection of Nc/cNc Manufacturing and Reverse Engineering, chapter 85 in DAAAM International Scientific Book 2009, pp. 881-888, B. Katalinic (Ed.), Published by DAAAM International, ISBN 978-3-901509-69-8, ISSN 1726-9687, Vienna, Austria

Authors' data: Doc. Eng. PhD. Monkova, K[atarina]; Doc. Eng. PhD. Monka, P[eter], Technical University in Kosice, Faculty of Manufacturing Technologies with seat in Presov, Sturova 31, 080 01, Presov, Slovakia, monkova.katarina@apeiron.eu, direct@apeiron.eu