冰箱接水盒產(chǎn)品造型與模具設(shè)計【說明書+CAD+UG】,說明書+CAD+UG,冰箱接水盒產(chǎn)品造型與模具設(shè)計【說明書+CAD+UG】,冰箱,接水盒,產(chǎn)品,造型,模具設(shè)計,說明書,仿單,cad,ug arXiv:1003.5062v1 physics.gen-ph 26 Mar 2010 Automatic polishing process of plastic injection molds on a 5-axis milling center Journal of Materials Processing Technology Xavier Pessoles, Christophe Tournier* LURPA, ENS Cachan, 61 av du pdt Wilson, 94230 Cachan, France christophe.tournierlurpa.ens-cachan.fr, Tel : 33 147 402 996, Fax : 33 147 402 211 Abstract The plastic injection mold manufacturing process includes polishing operations when surface roughness is critical or mirror eect is required to produce transparent parts. This polishing operation is mainly carried out manually by skilled workers of subcontractor companies. In thispaper, we proposeanautomatic polishing technique ona 5-axismilling center in order to use the same means of production from machining to polishing and reduce the costs. We develop special algorithms to compute 5-axis cutter locations on free-form cavities in order to imitate the skills of the workers. These are based on both lling curves and trochoidal curves. The polishing force is ensured by the compliance of the passive tool itself and set-up by calibration between displacement and force based on a force sensor. The compliance of the tool helps to avoid kinematical error eects on the partduring 5-axistoolmovements. Theeectiveness ofthemethodintermsofthesurface roughness quality and the simplicity of implementation is shown through experiments on a 5-axis machining center with a rotary and tilt table. Keywords Automatic Polishing, 5-axis milling center, mirror eect, surface roughness, Hilberts curves, trochoidal curves 1Geometric parameters C E (X E ,Y E ,Z E ) tool extremity point (u,v) coordinates in the parametric space Trochoidal curve parameters s curvilinear abscissa C(s) parametric equation of the guiding curve P(s) parametric equation of the trochoide curve n(s) normal vector of the guiding curve p step of the trochoid D tr diameter of the usefull circle to construct the trochoid A amplitude of the trochoid Step step between two loops of trochoide Technological parameters D tool diameter D eff eective diameter of the tool during polishing E amplitude of the envelope of the polishing strip e displacement induced by the compression of the tool tilt angle of the tool axis u(i,j,k) tool axis f tangent vector of the guide curve C c point onto the trochoidal curve Machining parameters N spindle speed V c cutting speed V f feed speed f z feed per cutting edge a p cutting depth a t working engagement T machining time 2Surface roughness parameters Ra arithmetic average deviation of the surface (2D) Sa arithmetical mean height of the surface (3D) Sq root-mean-square deviation of the surface Ssk skewness of topography height distribution Sku kurtosis of topography height distribution 31 Introduction The development of High Speed Machining (HSM) has dramatically modied the or- ganization of plastic injection molds and tooling manufacturers. HSM in particular has made it possible to reduce mold manufacturing cycle times by replacing spark machining in many cases. In spite of these evolutions, HSM is not enable to remove the polishing operations from the process. In this paper, we deal with the realization of surfaces with high quality of surface nishing and mirror eect behavior. This means that the part must be perfectly smooth and re ective, without stripes. Such a quality is for example necessary on injection plastic mold cavities in order to obtain perfectly smooth or com- pletely transparent plastic parts. From an economic point of view, polishing is a long and tiresome process requiring much experience. As this process is expensive in terms of price and downtime of the mold, automatic polishing has been developed. Our objective is to use the same means of production from machining to polishing, leading to cost reduction. The aim of the paper is thus to propose a method of automatic polishing on a 5-axis machine tool. Literature provides various automated polishing experiments. Usually, the polishing is carried out by an anthropomorphic robot, 1. Anthropomorphic robots are used for two main reasons. First, their number of axes enables them to have an easy access to any area of complex form. Second, it is possible to attach a great variety of tools and particularly spindles equipped with polishing force control mechanisms. Automatic polishing studies have been also carried out on 3 or 5-axis NC milling machine with specially designed tool to manage polishing force 2 as well as on parallel robots 3. Indeed, the polishing force is a key parameter of the process. The abrasion rate in- creases when the polishing pressure increases 4. But as mentioned in 3 the contact pressure depends on the polishing force and also on the geometrical variations of the part. An adequate polishing force facilitates the removal of cusps and stripes left on the part during milling or previous polishing operations. Nevertheless, the contact stress has to be as constant as possible to avoid over-polishing and respect form deviation tolerances. Many authors have thus chosen to develop abrasive systems allowing a dynamic manage- ment of the polishing force. In 5, Nagata et al. use an impedance model following force control to adjust the contact force between the part and the sanding tool. In 6, Ryuh et al. have developed a passive tool, using a pneumatic cylinder to provide compliance and 4constant contact pressure between the surface and the part. A passive mechanism is also used in 7. The contact force is given by the compressive force of a spring coil. In order to carry out an automatic polishing, it is important to use adapted tool trajectories. According to 8, polishing paths should be multidirectional rather than mo- notonic, in order to cover uniformly the mold surface and to produce fewer undulation errors. Moreover, the multidirectional polishing path is close to what is made manually. If we observe manual polishers, we can notice that they go back on surface areas accor- ding to various patterns such as trochoidal polishing paths (or cycloidal weaving paths 8 (g 1). Therefore, it could be protable to follow such a process in order to obtain the required part quality. For instance, some papers use fractal trajectories like the Peano Curve fractal9, which is anexample of a space-lling curve, rather than sweepings along parallel planes 10. Elementary pattern Elementary pattern Multidirectional polishing Multidirectional polishing Figure 1 Manual polishing patterns This brief review of the literature shows that there is no major diculty in using a 5- axis machine for automatic polishing with a passive tool. This paper aims at showing the feasibility of automaticpolishing using 5-axismachine toolsandproposing some polishing strategies.Intherstsection,weexposehowautomaticpolishingispossibleusinga5-axis HSM center. In particular, we present the characteristics of the passive and exible tools used. A specic attention is paid to the correlation between the imposed displacement of the tool and the resulting polishing force. Once the feasibility of 5-axis automatic poli- shing is proved, the various dedicated polishing strategies we have developed are detailed in section 2. These strategies are for the most part issued from previous experiences as for fractal tool trajectories coming from robotized polishing or cycloidal weaving paths 5representative of manual polishing. In section 3, the eciency of our approach is tested using various test part surfaces. All the parts are milled then polished on the same pro- ductionmeans:a5-axisMikronUCP710millingcentre. Intheliterature,theeectiveness of polishing is evaluated using the arithmetic roughness Ra 2. However, as it is a 2D parameter, this criterion is not really suited to re ect correctly the 3D polished surface quality. We thus suggest qualifying the nish quality of the polished surface through 3D parameters. This point is discussed in the last section as well as the comparison of the surface roughness obtained using automatic polishing with that obtained using manual polishing, a point hardly addressed in the literature. 3D surface roughness measurements are performed using non-contact measuring systems. 2 Experimental Procedure 2.1 Characteristics of the tools As said previously, our purpose is to develop a very simple and protable system. Therefore, the tools used are the same than those used in manual polishing. The poli- shing plan is divided into two steps, pre-polishing and nishing polishing. Pre-polishing is performed with abrasive discs mounted on a suitable support. The abrasive particle size is determined by the Federation of European Producers of Abrasives standard (FEPA). This support is a deformable part made in an elastomer material xed on a steel shaft that allows mounting in the spindle. We thus deal with a passive tool. Hence, we do not have a force feedback control but a position one. We have studied the relationship between the de ection of the disc support and the polishing force applied to the part. To establish this relationship, we use a Quartz force sensor Kistler 9011A mounted on a specially designed part-holder. The sensor is connected to a charger meter Kistler 5015 itself connected to the computer through a data-collection device Vernier LabPro to save the data. The experimental system is depicted in gure 2. In addition, the used sensor is a dynamic sensor. The eort must therefore change over time otherwise there would be a driftof themeasure. To do so,themovement imposedonthetoolover time isa triangular signal. 6Figure 2 Experimental set-up In order to ensure the evacuation of micro chips during the polishing and guarantee a nonzero abrasion speed at the contact between the part and the tool, the tool axis u is tilted relatively to the normal vector to the polished surface n and to the feed direction f. The tilt angle (gure 3) is dened as follows : n u q f v Ce Cc Workpiece C L Figure 3 Tool axis tilting 7u = cosn+sinf (1) Polishing tests have been conducted considering three dierent tilt angles (5,10,15) between the tool axis and the normal vector to the surface in the feed direction. The correlation between the tool de ection and the polishing force is shown in gure 4. Polishing force 0 2 4 6 8 10 12 14 16 18 0,00 0,15 0,30 0,45 0,60 0,75 0,90 1,05 1,20 Displacement (mm) Force (N) 5 inclination angle 10 inclination angle 15 inclination angle Figure 4 Polishing forces vs displacement The green curve (5 deg) is interrupted because the abrasive disks unstick when the tool de ection is too large. In this conguration, the tilt angle is too low and the body of the disk support, which is more rigid, comes in contact with the workpiece, which deteriorates and unsticks the disk. With a 10 or 15 degrees tilt angle, this phenomenon appears for a higher value of tool de ection, outside the graph. However, low tilt angle congurations allow faster tool movements since the rotation axes of the 5-axis machine tool are less prompted 11. Furthermore, it has been showed that trochoidal tool paths require a dynamic machine tool to respect the programmed feedrate 12. Then in si- multaneous 5-axis congurations, polishing time will be greater with low tilt angles. In addition,the exibility of thetoolwillhelp to reduce oravoid 5-axiskinematic errors 13. Indeed, interfences between the tool and the part could happen because of great tool axis 8orientation evolutions between two succesive tool positions. Therefore, the disc support de ection would avoid the alteration of the mold surface. If one considers the law of Preston 14, the material removal rate h in polishing is proportional to the average pressure of contact, P, and to the tool velocity relative to the workpiece, V : h =K P PV (2) where K P is a constant ( m 2 s N ) including all other parameters (part material, abrasive, lubrication,etc.).Hence,inordertoreachanadequatecontactpressure,wemustincrease the tool de ection and consequently we raise the shear stress and the disk unsticks. From a kinematical behavior point of view, low rotational axes movements lead to decrease the polishing time. So we must use a rather low tilt angle (5-10 degrees) and a quite high tool de ection to ensure a satisfactory rate of material removal. 2.2 5-axis polishing tool path planning To generate the polishing tool path, the classical description of the tool path in 5- axis milling with a at end cutter is used. This leads to dene the trajectory of the tool extremitypointC E aswellastheorientationofthetoolaxisu(i,j,k)alongthetoolpath. With regards to polishing strategy, we use trochoidal tool paths in order to imitate the movements imparted by theworkers tothe spindle. To avoid marks orspecic patternson thepart,we choose to generatetrochoidal toolpathonfractalcurves inorder to cover the surface in a multidirectionnal manner. We use more particularly Hilberts curves which are a special case of the Peanos curve. These curves are used in machining as they have the advantage of covering the entire surface on which they have been generated 15. We will develop below the description of the Hilberts curve which is used as a guide curve for the trochoidal curve then we will examine the trochoidal curve itself. 2.2.1 Hilberts curve denition The use of fractal trajectories presents two major interests. The rst one is that tool paths do not follow specic directions which guarantees an uniform polishing. The second one is linked to the tool path programming. Indeed, tool paths are computed in the 9