Machining and Other Material RemovingOperations

Previous Page Limitations on cold-hobbing result from the mechanical properties of hob and blank and therefore the size of a cavity 2.4 Machining and Other Material Removing Operations 2.4.1 Machining Production Methods Machining production methods may be divided into processes with geometrically defined cutter (turning, milling, drilling, sawing) and geometrically undefined cutter (grinding, honing, lapping) The machinery, frequently special equipment, has to finish the object to the extent that only little postoperation, mostly manual in nature (polishing, lapping, and finishing), is left Modern tooling machines for mold making generally feature multiaxial CNC controls and highly accurate positioning systems The result is higher accuracy and greater efficiency against rejects The result of a survey [2.45] shows NC machining as having just a 25% share compared to 75% for the copying technique, but this does not hold true for modern tool shops and the fabrication of large molds Nowadays, heat-treated workpieces may be finished to final strength by milling (e.g Rm up to 2000 MPa) Various operations, e.g cavity sinking by EDM, can be replaced by complete milling operations and the process chain thus shortened Furthermore, the thermal damage to the outer zone that would otherwise result from erosion does not occur Hard milling can be used both with conventional cutting-tool materials, such as hard metals, and with cubic boron nitride (CBN) For plastic injection molds, hard metals or coated hard metals should prove to be optimum cutting-tool materials Machining frees existing residual stresses This can cause distortion either immediately or during later heat treatment It is advisable, therefore, to relieve stresses by annealing after roughing Any occurring distortion can be compensated by ensuing finishing, which usually does not generate any further stresses After heat treatment, the machined inserts are smoothed, ground and polished to obtain a good surface quality, because the surface conditions of a cavity are, in the end, responsible for the surface quality of a molding and its ease of release Defects in the surface of the cavity are reproduced to different extents depending on the molding material and processing conditions Deviations from the ideal geometrical contour of the cavity surface, such as ripples and roughness, diminish the appearance in particular and form "undercuts", which increase the necessary release forces There are three milling variants: - three-axis milling, - three-plus-two-axis milling and - five-axis milling (simultaneous) Competition has recently developed between high-speed cutting (HSC) and simultaneous five-axis milling HSC is characterized by high cutting speeds and high spindle rotation speeds Steel materials with hardness values of up to 62 HRC can also be machined with contemporary standard HSC millers [2.46] HSC machining can be carried out as a complete machining so that the process steps of electrode manufacturing and eroding can be dispensed with completely In addition, better surface quality is often achieved, and this allows drastic reduction in manual postmachining [2.47] For the production of injection and die-casting molds, a combination of milling and eroding may also be performed The amount of milling should be maximized since the machining times are shorter on account of higher removal capability However, very complex contours, filigree geometries and deep cavities can be produced by subsequent spark-erosive machining Often, field electrodes are used [2.48] The electrode can, in turn, be made from graphite or copper by HSC (for details of the production method for micro cavities, see Sections 20.1.2-20.1.2.6) 2.4.2 S u r f a c e T r e a t m e n t (Finishing) In many cases, and by no means exclusively for the production of optical articles, the condition of the cavity surface (porosity, ripples, roughness) is crucial to the quality of the final product This has a decisive effect on the time needed for mold making and thus on the costs of the mold Moreover, the ease with which the molding can be released and deposits from thermosets and rubber are affected Mirror-finish surfaces require the greatest amount of polishing and facilitate demolding As opposed to these are untreated cavity surfaces for the production of moldings which not have to meet optical requirements Here release properties are the criterion governing the condition of the cavity surface This also applies to textured surfaces The texture determines the ease of demolding and calls for more draft than for polished molds if the texture forms "undercuts", as when grooves run across the direction of demolding Some polishing procedures will now be presented below 2.4.2.1 Grinding and Polishing (Manual or Assisted) After the cavity has been completed by turning, milling, EDM, etc., the surfaces generally have to be smoothened by grinding and polishing until the desired surface quality of the moldings is obtained and release is easy Even nowadays, this is still mainly done manually, supported by electrically or pneumatically powered equipment or with ultrasonics [2.49-2.51] The sequence of operations, coarse and precision grinding and polishing, are presented in detail in Figure 2.22 Coarse grinding produces a blank-metal, geometrically correct surface with a roughness of Ra < um, which can be finished in precision-grinding step or immediate polishing [2.52] Careful work and observance of some basic rules can yield a surface quality with roughness heights of 0.001 to 0.01 um (see Table 2.1) after polishing A precondition for this, of course, is steels that are free from inclusions and have a uniform fine-grained structure, such as remelted steels (Section 1.1.9) A disadvantage of manual finishing processes is that they are personnel-intensive and that they not guarantee reproducible removal Machine-assisted removal with geometric undefined cutter (grinding, honing, lapping) has nonetheless been unable to make a breakthrough These techniques have major kinematic and technologicial restrictions in the case of complex, 3D contours Some of the fully-automatic polishing processes presented here have also exhibited considerable shortcomings For this reason, they are almost exclusively used in Milling Turning EDM Roughing Coarse Grain size No Fine Fining Coarse Grain size No Polishing with diamona paste Coarse Grain size 45jjm Fine Fine Figure 2.22 Steps of the mechanical surface treatment [2.52] combination with manual mechanical polishing methods They are presented here briefly, for the sake of completeness 2.4.2.2 Vibratory Grinding Vibratory or slide grinding is an alternative to the conventional rotary barrel process The workpieces are placed in a container which is subsequently filled with a mixture of granulated zinc, water, alumina as polishing medium, and a wetting agent or anti-rust compound until the pieces are completely covered Then the container is set into vibrating motion This presses and thoroughly mixes the mixture against the walls of the molds Thus, a kind of wiping action occurs that smooths the walls A distinct disadvantage of this technique is pronounced abrasion of protruding edges These have to be covered for protection [2.53] Limitations on this process are imposed by the size and weight of the molds 2.4.2.3 Sand Blasting (Jet Lapping) Sand blasting is of the best known and most common procedures For mold making, it is modified such that the blasting medium is a water-air mixture containing fine glass beads Mold surfaces are treated with this mixture under a pressure of 500 to 1000 kPa This levels out any unevenness, such as grooves The attainable surface quality is not comparable to that of surfaces treated mechanically The roughness height is about um [2.53] The application of this technique appears to make sense only for flat parts Disadvantages are non-reproducible removal and relatively low dimensional stability 2.4.2.4 Pressure Lapping This process is a variant of jet lapping and also known as "extrude-honing" It is limited to the treatment of openings As the name indicates, it has found special significance in the fabrication of profile-extrusion tools where arbitrarily shaped openings with the lowest of cross sections have to be polished The procedure uses applications a pasty polishing compound of variable viscosity that contains silicon carbide, boron carbide or diamond grits of various sizes depending on the dimension of the opening The compound is moved back and forth and average roughness heights of Ra = 0.05 um are achieved in no time [2.54 to 2.56] The process is done automatically and requires only a short set-up time 2.4.2.5 Electrochemical Polishing With electrochemical polishing, or electro-polishing in short, the top layers of a workpiece are removed [2.57] The process is based on anodic metal machining and therefore qualifies as a "cold" process Thus, the workpiece does not become thermally stressed; see also Section 2.6 The process works without contact between workpiece and mold, so no mechanical loading occurs Since removal only occurs at the workpiece, the workpiece is subjected to virtually no abrasion [2.58] Through the removal of material, leveling of the surface of the workpiece occurs High dimensional and molding accuracies, as well as good surface properties, can be achieved by electrochemical polishing The aim is often to remove impurities introduced into the outer surface layer during preceding machining processes Further advantages of the operation are reproducible removal and the resultant high degree of automatability [2.58] Defects in the steel, such as inclusions and pores, are exposed Therefore, the materials to be electrochemically polished must be of high purity Various steels, especially the usual carbon steels, cannot be optimally electrochemically polished [2.53] 2.4.2.6 Electric-Discharge Polishing Electric-discharge polishing is not essentially a new or independent procedure It is an extension of electric-discharge machining (Section 2.5.1) and immediately follows erosive fine finishing Thus, erosion and polishing are done on the same equipment using the set-up Consequently, to an extent depending on the level of surface finish required, it can replace time-consuming and costly manual postmachining In electric-discharge polishing, the discharge energies are very much reduced, e.g through lower discharge currents, relative to electric-discharge fine finishing As a result, removal rates are low and so electric-discharge polishing is also a time-consuming finishing process Because electric discharge polishing works on the principle of removal by heat, thermal damage is done to the outer zone The outer zone can be minimized but it can never be removed completely The structure of surfaces after electric-discharge polishing characterized is by rows of adjoining and superimposed discharge craters similar to that of electric-discharge Table 2.1 Steps for grinding and polishing operations [2.52] Roughing Grain size no 180 - Grinding operations must not develop so much heat that structure and hardness of the material are affected Therefore it is important to select the correct grinding wheel and appropriate cooling - Only clean wheels and stones which are not clogged should be used - The workpiece has to be carefully cleaned after each application of a compound, before the next compound is applied - If the operation is done by hand, a change of direction is essential to avoid unevenness or scratches - One should work with one grain size in one direction, then with the next size in an angle of 30 to 45 ° until the surface does not exhibit anymore traces of the previous direction The same procedure has to be repeated with the following grain size Fining Grain size 200-600 - Only clean and unclogged tools should be used Steps for manual polishing of fixed workpiece: - Add ample coolant to prevent heating of the surface and to flush chips - Workpiece has to be carefully cleaned A pin-head-size amount of diamond paste is applied with a polishing stick of desired hardness and moved back and forth until cutting starts Then thinner is added and polishing continued until all marks from previous operation have disappeared - Grain size of tools depends on previous roughing and intended polishing - With every change of grain size, workpiece and hands have to be cleaned to prevent larger grains interfering with finer size - This procedure becomes even more important with decreasing grain size - Careful cleaning of workpiece and hands Then one uses either a polishing tool of the same hardness with a finer paste or a softer tool with the same paste and works in an angle of 30 to 45° to the preceding direction Thus the end of each step can be easily recognized - Pressure should be distributed uniformly when working manually Scratches and cold-deformed layers from the preceding grain size have to be removed before switching to the next size - One continues with these operations until the desired result is obtained Large, plane faces should not be worked on with abrasive paper Abrasive strones reduce the danger of creating waviness - When working the inside of an object the speed has to be reduced with increasing hole size - After traces have disappeared, continue each operation for the same time to make sure that the cold-deformed layer is removed Steps for manual polishing of rotating workpieces: - The polishing stick is moved back and forth to remove chips from the hole Special adjustable tools for polishing bores are available For polishing the outside of cylindrical workpieces special lap rings can be employed R a 0.1 to urn Ra urn Finishing Diamond flour or paste, 0.1-180 jum Ra 0.001 to 0.1 urn machining Here, however, they are shallow, largely circular and all of about equal size The surface roughness of so polished molds is about Ra = 0.1 to 0.3 um with a diameter of the discharge craters of about 10 um These patterns are in the range of finely ground surfaces and meet the requirements of mold making in many cases Thus, it is possible to forgo manual polishing, which is difficult with complex geometries [2.57, 2.60] The necessary time is 15 to 30 min/cm2, the exact pattern depending on shape and size Hence, electric-discharge machining allows molds to be machined completely in one set-up by means of roughing, prefinishing, fine finishing and polishing However, the workable area is limited in this process Furthermore, electric-discharge polishing is very time-consuming On account of the thermal removal principle of electric-discharge machining, a thermally damaged outer zone always remains on the workpiece This can be minimized by electric-discharge polishing, but can never be removed completely 2.5 Electric-Discharge Forming Processes Modern mold making would be inconceivable without electric-discharge equipment With its help, complicated geometric shapes, the smallest of internal radii and deep grooves can be achieved in one working step in annealed, tempered and hardened steel with virtually no distortion [2.58, 2.61] The process is contactless, i.e there is a gap between the tool and the workpiece Material removal is heat-based, requiring electric discharges to occur between tool and workpiece electrode [2.58] (For method of producing microcavities, see Section 20.1.2-20.1.2.6) 2.5.1 Electric-Discharge M a c h i n i n g ( E D M ) Electric-discharge machining is a reproducing forming process, which uses the material removing effect of short, successive electric discharges in a dielectric fluid Hydrocarbons are the standard dielectric, although water-based media containing dissolved organic compounds may be used The tool electrode is generally produced as the shaping electrode and is hobbed into the workpiece, to reproduce the contour [2.58] With each consecutive impulse, a low volume of material of the workpiece and the electrode is heated up to the melting or evaporation temperature and blasted from the working area by electrical and mechanical forces Through judicious selection of the process parameters, far greater removal can be made to occur at the workpiece than at the tool, allowing the process to be economically viable The relative abrasion, i.e., removal at the tool in relation to removal at the workpiece, can be reduced to values below 0.1% [2.48,2.58] This creates craters in both electrodes, the size of which are related to the energy of the spark Thus, a distinction is drawn between roughing (high impulse energy) and planing The multitude of discharge craters gives the surface a distinctive structure, a certain roughness and a characteristic mat appearance without directed marks from machining The debris is flushed out of the spark gap and deposited in the container Flushing can be designed as a purely movement-related operation This type of flushing is very easy to realize since only the tool electrode, together with the sleeve, has to lift up a short distance This lifting movement causes the dielectric in the gap to be changed Admittedly, this variant is only really adequate for flat cavities For complex contours, pressure or suction flushing by the workpiece or tool electrodes would need to be Principle of process Dielectric fluid Dielectric fluid Tool supply Servo control D C Generator Electric spark Medium: Workpiece: Wear: Roughing: Finishing: Dielectric fluid (Paraffin) Duplicating electrode subject to occurring wear Copper