1818_玩具汽車的逆向造型研究

1818_玩具汽車的逆向造型研究,玩具,汽車,逆向,造型,研究,鉆研 1Enhancing moulds manufacturing by means of reverse engineeringL. Iuliano & P. MinetolaReceived: 5 March 2008 / Accepted: 1 September 2008 /Published online: 23 September 2008# Springer-Verlag London Limited 2008Int J Adv Manuf Technol (2009) 43:551–562DOI 10.1007/s00170-008-1739-3L. Iuliano : P. Minetola (*)Department of Manufacturing Systems and Economics,Politecnico di Torino,Corso Duca degli Abruzzi,24-10129, Torino, Italye-mail: paolo.minetola@polito.itL. Iulianoe-mail: luca.iuliano@polito.itAbstract :Modern CAD/CAM techniques together with five-axis high-speed milling allow to reduce moulds manufacturing time and costs.Nevertheless, in order to put a mould into use,operations of manual finishing and fitting are still always required. Such operations, performed manually by mould makers, modify the milled surfaces of moulds. Reverse engineering techniques can be employed in quality control to evaluate dimensions and geometrical tolerances on moulds after machining and fitting. Changes in the product’s shape are sometimes decided after a mould has already been machined. In such cases, if possible, the mould maker directly modifies the mould. Thus the final real geometry of the mould does not reflect the one of the original CAD model. The aim of this paper is to point out the benefits of non-contact qualitycontrol and to illustrate a procedure, based on reverse engineering techniques, to reconstruct and update the mathematical model of the mould after it has been polished and fit. The procedure was tested on a mould for the production of a plastic camera body that was previously inspected by means of a structured light scanner2Keywords :CAD model . Reverse engineering . Mould . Surfaces reconstruction . Computer aided inspection1 IntroductionProduction tools (better known as moulds) are a essential element in the manufacturing of wide consumption goods.The design and the fabrication of moulds are a very long and expensive step in the developmen of a new product.For such reason, any effort improving moulds’manufacturing helps to reduce times and costs.Nowadays, modern CAD/CAM technologies an fiveaxis high-speed milling [1] allow:& A fast design of moulds starting from the 3D CAD model of the piece;& The construction of electrodes for EDM, to overcome problems related with geometries tha are not machinable by milling tools;& The generation of the NC tool path, using strategies developed to reduce machining times [2];& The achievement of high material removal rates in rough machining;& Better dimensional tolerances and low roughness in finishing [3, 4]No matter how far the boundaries of the cutting technology have been pushed, before putting a mould into use, the following operations are stil always required:& Manual finishing, to obtain a smooth surface by removing marks of milling and EDM processes;& Fitting, to ensure the mould correct closureOn one hand, polishing and fitting assure the moulding of the piece, but on the other one, the modify the cavity surface in a way that is mathematically non-definable.Therefore, the final real geometry of moulds does not reflect the one of the original CAD model (ideal geometry). Today CAD software resources do not allow to eliminate this incongruity.Another aspect, that is often disregarded, is the introduction of small changes on moulds. After finishing, moulds are sometimes modified to reflect last-minute modifications in product shape. In such cases, the mould maker directlymodifies the mould without updating the CAD model. Consequently, there may be 3problems:& During the assembly of the group to which the moulded part belongs;& When moulds have to be replaced because of the wear or a fracture.The lack of alignment between the CAD model and the mould real geometry increases re-machining costs and times of product development and launch.Reverse engineering techniques offer new opportunities to mould makers to overcome such problems. In recent years, non-contact digitising devices have been improved mainly in terms of accuracy and number of points measured per single scan. High scanning speeds allow the user to retrieve hundred thousands (or even millions) points on the mould surface in a few seconds.The information contained in high-density point clouds can be employed for both mould’s quality control and CAD model updating [5, 6].Coloured deviation maps, displayed as a result of the comparison between CAD data and point cloud data, provide a complete analysis of tolerances and deviations.That goes much beyond what can be done measuring some scattered points during the point-wise inspection of the tool on a Coordinate Measuring Machine (CMM) [7, 8].Overall and local deviation on the cavity surface can be evaluated to identify machining errors and to investigate their causes. Considerations on coloured deviation maps of the mould cavity surface may also be useful to reduce the set-up time before putting the mould into use. The results of the inspection extended to the whole figure surface and not limited to some point-wise measures can be analysed to:& Define re-machining strategies, when the real surface of the mould is “higher” than the corresponding one of the CAD model. That means there is some residual material that can still be removed;& Detect unrecoverable errors and investigate their causes, when the real surface of the mould is “l(fā)ower” than the corresponding one of the CAD model. That means no residual material is available for further machining or manual finishing. In such case, the mould cannot be put into use if the errors go beyond given tolerances On the other side, reverse engineering resources can be employed to update the moulds mathematical model through:& Digitisation of male and female mould parts, getting point cloud data as output;4& Reconstruction of cavity surfaces;& Generation of the 3D CAD model of the mould This paper aims to show the advantages of contactless inspection compared to point-wise measurements. After operations of polishing and fitting, the updating of the 3D CAD model of a mould should also become a good practice among mould makers. A procedure to fulfil that task was defined using common reverse engineering techniques and software packages. The proposed procedure was tested and validated on a mould for injection moulding of a plastic camera body.2 Inserts digitising and inspection (CAD model comparison)The inserts of the mould are made of light alloy (Fig. 1).After rough milling on an NC machine, the female figure was finished by EDM of the tiny grooves located on the camera handle. Because of their size, in fact, those grooves cannot be machined by milling.The left half of the male mould cavity was not treated after the finishing milling operation so it still presents evidence of the 45° crossing mill path. The other half of the cavity was instead polished after machining (Fig. 2). Such manual operation was introduced to evaluate if the noncontact scanner is able to discriminate the different finish of the two halves properly. In order to point out the advantages of non-contact inspection, mould inserts were digitised using the optical scanner ATOS Standard produced by GOM GmbH (Fig. 3). Contactless scanning of large moulds or products is less time consuming than contact scanning and undercuts can be easily digitised by changing the relative position of the optical sensor to the part. Moreover, current optical Fig. 1 Photo of the mould inserts (overall dimensions: 200×98× 35 mm) 552 Int J Adv Manuf Technol Another aspect, that is often disregarded, is the introduction of small changes on moulds. After finishing, moulds are sometimes modified to reflect last-minute modifications in product shape. In such cases, the mould maker directly modifies the mould without updating the CAD model. Consequently, there may be problems:& During the assembly of the group to which theMoulded part belongs;& When moulds have to be replaced because of the wear or a fracture The lack of 5alignment between the CAD model and the mould real geometry increases re-machining costs and times of product development and launch. Reverse engineering techniques offer new opportunities to mould makers to overcome such problems. In recent years, non-contact digitising devices have been improved mainly in terms of accuracy and number of points measured per single scan. High scanning speeds allow the userto retrieve hundred thousands (or even millions) points on the mould surface in a few seconds. The information contained in high-density point clouds can be employed for both mould’s quality control and CAD model updating [5, 6].Coloured deviation maps, displayed as a result of the comparison between CAD data and point cloud data, provide a complete analysis of tolerances and deviations. That goes much beyond what can be done measuring some scattered points during the point-wise inspection of the tool on a Coordinate Measuring Machine (CMM) [7, 8]. Overall and local deviation on the cavity surface can be evaluated to identify machining errors and to investigate their causes. Considerations on coloured deviation maps of the mould cavity surface may also be useful to reduce the set-up time before putting the mould into use. The results of the inspection extended to the whole figure surface and not limited to some point-wise measures can be analysed to:& Define re-machining strategies, when the real surface of the mould is “higher” than the corresponding one of the CAD model. That means there is some residual material that can still be removed;& Detect unrecoverable errors and investigate their causes, when the real surface of the mould is “l(fā)ower” than the corresponding one of the CAD model. That means no residual material is available for further machining or manual finishing. In such case, the mould cannot be put into use if the errors go beyond given tolerances. On the other side, reverse engineering resources can be employed to update the moulds mathematical model through:& Digitisation of male and female mould parts, getting point cloud data as output;& Reconstruction of cavity surfaces;& Generation of the 3D CAD model of the mould.This paper aims to show the advantages of contactless inspection compared to point-6wise measurements. After operations of polishing and fitting, the updating of the 3D CAD model of a mould should also become a good practice among mould makers. A procedure to fulfil that task was defined using common reverse engineering techniques and software packages. The proposed procedure was tested and validated on a mould for injection moulding of plastic camera body.2 Inserts digitising and inspection (CAD model comparison)The inserts of the mould are made of light alloy (Fig. 1).After rough milling on an NC machine, the female figure was finished by EDM of the tiny grooves located on the camera handle. Because of their size, in fact, those grooves cannot be machined by milling. The left half of the male mould cavity was not treated after the finishing milling operation so I still presents evidence of the 45° crossing mill path. The other half of the cavity was instead polished after machining (Fig. 2). Such manual operation was introduced to evaluate if the noncontact scanner is able to discriminate the different finish of the two halves properly. In order to point out the advantages of non-contact inspection, mould inserts were digitised using the optical scanner ATOS Standard produced by GOM GmbH (Fig. 3). Contactless scanning of large moulds or products is less time consuming than contact scanning and undercuts can be easily digitised by changing the relative position of the optical sensor to the part. Moreover, current optical Fig. 1 Photo of the mould inserts (overall dimensions: 200×98×35 mm) 552 Int J Adv Manuf Technol (2009 43:551–562 scanners’ accuracy and resolution allow a good definition of part edges as a great number of points are measured on them. The selected scanning device exploits stereoscopic vision as it has two Sony XC75 built-in CCD cameras (resolution of 768 × 572 pixel/8 bit), which store images of the light fringes projected on the scanned object. The projector, placed in the centre of the sensor, projects a sequence of ten slides: four interference patterns (phaseshift technique [9]) followed by six Gray coded [10, 11] binary images. The six-bit code allows distinguishing between 26 = 64 columns in the field of view. Considering the overall size of mould inserts, the scanner was calibrated on a working area of 160 × 200 mm at a distance of 600 mm. The accuracy of the scanner in such configuration is 0.06 mm as declared on the device datasheet. Before digitising, a thin layer of white opaque powder was sprayed 7on the inserts in order to avoid light reflection problems typical of metallic surfaces. Furthermore, reference points are applied on the inserts sticking adhesive targets (i.e. markers) on areas that contain less detail. Multiple scans are automatically registered in one point cloud, as the scanning software recognises the fixed reference grid created by markers. There is a lack of data in the areas covered by markers, but GOM’s software allows to complete the virtual model of the object by closing such holes. The registration errors shown in Table 1 were computed by GOM software during the alignment procedure based on the reference grid of markers.Rapidform software by Inus Technology Inc. was employed for the mould inspection. The result of the comparison between the triangulated point cloud model and the original CAD one were computed by the software and are shown in Table 1 in terms of average distance, standard deviation and maximum distance.2.1 Female mould inspectionThe digitising of the female mould cavity has required twelve scans. The comparison between the digitised data and the original CAD model shows a maximum deviation of 0.84 mm on the left superior pocket. Such value was verified by point-wise measurements on a CMM: measuring the depth of that pocket, the real deviation resulted of 0.85 mm.In the superior half of the female cavity, the deviation is about ?0.30 mm, while the error is positive (about 0.20 mm) in the other half. The deviation map with relative errorsFig. 2 Different finish of the male mould surfaceFig. 3 3D scanner ATOS Standard and female mouldwith reference point markers Intscanners’ accuracy and resolution allow a good definition of part edges as a great number of points are measured on them.The selected scan ning device exploits stereoscopic vision as it has two Sony XC75 built-in CCD cameras (resolution of 768 × 572 pixel/8 bit), which store images of the light fringes projected on the scanned object. The projector, placed in the centre of the sensor, projects a sequence of ten slides: four interference patterns (phaseshift technique [9]) followed by six Gray coded [10, 11] binary images. The six-bit 8code allows distinguishing between 26 = 64 columns in the field of view. Considering the overall size of mould inserts, the scanner was calibrated on a working area of 160 × 200 mm at a distance of 600 mm. The accuracy of the scanner in such configuration is 0.06 mm as declared on the device datasheet.Before digitising, a thin layer of white opaque powder was sprayed on the inserts in order to avoid light reflection problems typical of metallic surfaces. Furthermore, reference points are applied on the inserts sticking adhesive targets (i.e. markers) on areas that contain less detail.Multiple scans are automatically registered in one point cloud, as the scanning software recognises the fixed reference grid created by markers. There is a lack of data in the areas covered by markers, but GOM’s software allows to complete the virtual model of the object by closing such holes. The registration errors shownin Table 1 were computed by GOM software during the alignment procedure based on the reference grid of markers.Rapidform software by Inus Technology Inc. was employed for the mould inspection. The result of the comparison between the triangulated point cloud model and the original CAD one were computed by the software and are shown in Table 1 in terms of average distance, standard deviation and maximum distance.(Fig. 4) allows to clearly notice the relief of the inferior half of the cavity, whereas the mould parting plane is almost aligned correctly with the CAD data.The deviation different sign of the two halvessuggests that the EDM operation was not done correctly. The yellow line in the middle of the deviation map can be thought as an imaginary axis around which the cavity surface may be rotated (in the direction shown by the arrows) to coincide again with the original CAD model.Probably during EDM, the electrode motion was not perfectly perpendicular to the parting plane of the female mould. The little inclination to the normal might be due to an incorrect mounting of the mould on the EDM machine.For this reason, the electrode machined the piece eroding too much material in the superior half of the female insert.The inclination is calculated considering the normal of the parting plane on the digitised data and on the CAD model: the angle between them measures 0.63°.2.2 Male mould inspection9Nine scans were necessary to completely digitise the male mould insert. In the comparison between the scan data and the original CAD model, the maximum error is localised on the superior right prominence of the cavity (Fig. 5).In that zone, there is a graze caused by the fall of the milling tool during a tool change.In the CAD data, the bottom boundaries of the handle tiny grooves have sharp edges. The male mould insert was machined by a ball-nose finish cutter having a diameter of 4 mm and the CNC mill path followed crossed 45° diagonal trajectories in parallel planes. Such finishing operation does not allow to create sharp edges on the grooves’ bottom.Hence, the scan data show a great deviation on the grooves of the handle. Figure 2 allows to observe the graze and the marks left by the 45° diagonal trajectories of the milling tool.Moreover, the deviation map of the male mould insert exhibits a difference between the left half and the right one. That is the evidence of the manual finish of the right half.The diversity is exalted by limiting the representation scale up to an error of 0.20 mm(Fig. 5). The polishing operation has removed a very thin layer of material, resulting in a deviation of less than 0.10 mm. In order to confirm such result obtained by contactless inspection of the whole male insert, the difference between the polished half of the cavity and the other one was checked by means of a CMM. Two zones were selected on the superior surface of the male cavity, one on each half.The first one, labelled A, refers to the half that was not manually finished, so it lays on the left part of male insert.The other one, labelled B, refers to the polished half (Fig. 6). Traditional point-wise measurements were repeated on the two zones by means of Renishaw TP-20 probe mounted on a coordinate measuring machine DEA Global Image model 07.07.07. After the alignment with the CAD model coordinate system, 83 points were inspected on zone A and 106 points on zone B. CMM inspection’s result are reported in the second column of Table 2 and 3 in terms of average distance, standard deviation and maximum distance between measured points and the corresponding ones on the mould CAD model.10To further compare CMM inspection results with the ones of non-contact scanning, two limited sets of points, corresponding to zones A and B, were isolated from the whole scan data of the male insert. The results of the comparison of those two sets with the matching points of mould CAD model are shown in the last column of Tables 2 and 3. As a matter of fact, such results are very similar to those obtained by traditional quality control, but are re