In summary, materials that exhibit extreme brittle fracture and good thermal conductivity reduce thermal loading of the wheel bond and of the diamond grit. Higher grit concentration may be expected to give benefits if these conditions are met due to the material or to other process parameters.
Dressing Strategy and Bond Specification
The decisive factors for grinding behavior include not only grit type, grit size, and concentration, but also specification of the bond system and conditioning of the grinding layer. The interplay between wear process on the grid and on the bond determines grinding behavior and the working result. Evidence is shown in the example of creep-feed grinding HPSN with various diamond wheels (Figure 4.22). The least sensitive reaction is that of vitrified-bond diamond wheels to different dressing conditions. The vitrified bond has brittle facture behavior, and the bond webs are weakened by the existence of pores in the cross-section. This situation results in low grinding forces during grinding because wear causes the diamond grits to break out of the bond at an early stage without forming large wear flats, resulting in low normal force and high wheel wear. Metal bond wheels in particular react strongly to differences in dressing. Normal forces are significantly reduced by wheel dressing, and also in wheel wear, but not to the same extent. Therefore, even with metal bond wheels, the use of suitable dressing conditions can produce process parameters comparable to those of vitrified bond and resin bond wheels. Knowledge of dressing behavior, the use of suitable dressing units, and the appropriate dressing strategy is necessary.
Figure 4.22.
Effects of diamond wheel type on surface quality in creep-feed grinding of HPSN [15]
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Figure 4.23 shows a summary of the various study results on the influence of wheel composition on process parameters and working results.
Figure 4.23.
Influence of wheel composition on process parameters [15]
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Bending Fracture Strength
The mechanical and thermal load on the surface zone is influenced by wheel composition, including grit type, grit size, concentration, and bond. These factors overlaid by changes in wear condition of the active cutting edges over time and by the number of active cutting edges.
An example of the results obtained in creep-feed grinding of HPSN is shown in Figure 4.24. The use of sharp-edged, friable diamonds in phenolic resin bond with a concentration of C50 evidently produced considerable damage in the surface layer. The characteristic bending fracture strength have the lowest values at = 618 N/mm2. A less friable diamond grit type produces a considerable increase in characteristic bending fracture strength by the formation of new sharp cutting edges and the establishment of inherent compressive stresses. The Weibull modulus also increases. A more blocky diamond causes a decrease in bending fracture strength, and the low Weibull modulus levels also show greater scatter in the strength data. High individual grit contact forces and hence higher thermal loads and increased wheel wear now lead in turn to greater damage to the subsurface zone. An increase to concentration C100 with the use of sharp-edged, splintery grit types likewise fails to produce an increase in strength values due to increased wheel wear.
Figure 4.24.
Influence of wheel composition on the surface zone
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The conditions shown in Figure 4.24 also suggest that the influence of the bond can be largely eliminated by using suitable dressing procedures, so that strength behavior is mainly determined by the characteristics of the diamond grit, even where different bonds are used. Wheels D64/phenolic resin and D126/ductile bronze were dressed continuously during the grinding process, using different dressing material removal rates adapted to the bond and the grit size. So, the same percentage protrusions were generated on both grinding wheel types, which made it possible to avoid thermal overload even with blocky diamonds in a ductile bronze bond. The working result is largely determined by the wear characteristic of the diamonds and by the chip formation process. The attainable bending fracture strengths are comparable to those achieved with phenolic resin bond wheels of the same grit size and with blocky diamonds. In proper continuous dressing, the working result is mainly determined by the diamond grit type.
Note that different bond systems move the position of minimum damage to different grit sizes [15]. An increase in concentration from C50 to C100 can likewise produce an increase in bending fracture strength. Chip thickness per individual grit is smaller and, provided that thermal surface damage can be prevented by adequate cooling/lubrication, higher grit concentrations also permit higher bending facture strengths and smaller scatter range of the strength data. In principle, the same can also be achieved by a reduction in grit size from D126 to D64 (Figure 4.24). The conditions shown in this figure are characteristic of grinding with HPSN and have also been demonstrated for grinding with zircon oxide [15].
4.3. Conditioning of grinding wheels
Eckart Uhlmann and Nikolas Schröer
Introduction
Grinding is a temporal transient process due to the simultaneous appearance of wear on the abrasive grain and the bond. This leads to changes in the micro- and macrogeometries of the grinding wheel, caused by the different wear behaviors: complete or partial grain breakout, microcrystalline splinter, and plateau formation (Figure 4.25) [16]. Therefore conditioning of the grinding wheel is necessary in order to restore the original state or generate a certain condition for the topography on the wheel.
Figure 4.25.
Wear behavior of grinding wheels [16]
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The structure of the cutting area changes due to grinding wheel wear and thus has different impacts on the grinding process. The macroscopic wear of the grinding wheel can lead to profile deficiency and dimensional faults of the workpiece. Microscopic wear, on the other hand, influences the grinding forces, surface roughness, and the peripheral thermal damage of the workpiece [17]. Grinding machines are therefore equipped with additional conditioning devices in order to achieve a regeneration of the grinding ability. InTable 4.1 the classification and goals of grinding wheel conditioning are shown.
Table 4.1.
Classification and goals of grinding wheel conditioning [18]
Conditioning
________________________________________
Trueing and dressing
________________________________________ Cleaning
Profiling Sharpening
Goal:
Generation of proportion and shape Goal:
Generation of cutting area structure Goal:
Removal of chip, grain, and bonding residue
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Conditioning is divided into dressing/truing and cleaning, and dressing is subdivided into profiling and sharpening. The purpose of profiling is to achieve dimensional and shape accuracy of the grinding wheel profile and macrogeometry, whereas sharpening provides the generation of a microgeometric abrasive grain structure with the desired grain protrusion by affecting only the bond. Clogging and contamination of the grinding wheel surface caused by workpiece particles can be removed by cleaning. Generally it can be differentiated between several kinds of contaminations: thread chips, grain coatings, and layer chips. Thread chips accumulate sporadically at the chip space, whereas grain coatings only occur on single grains or groups of grains. Layer chips cover larger areas of the grinding wheel’s surface. Such clogging can be removed by sharpening blocks, redressing, or the use of cleaning nozzles.
Grinding wheels with superhard abrasive grains, such as diamond or CBN, usually require an additional sharpening process after profiling in order to set back the bond. This process is necessary to generate enough chip space for the transport of chips and cooling lubricant. Superabrasive products can be sharpened with loose or bonded sharpening tools. In most cases, sharpening with bonded abrasive grains is carried out by sharpening blocks made of corundum or silicon carbide. During this so-called block sharpening, the grinding wheel is fed radially into the sharpening block. In addition to bonded abrasive grains, a long-chipping steel block can be used with identical kinematics, depending on the bonding system. Sharpening with loose abrasive grains is carried out with jet sharpening in which a mixture of blasting material, air, and carrier medium affects the grinding wheel’s surface under high pressure. Blasting material can be made of quartz, silicon carbide, or corundum sand. Other sharpening processes are electrical discharge machining (EDM) and electrolysis (SCM).
Characteristics of the Dressing Process
During the dressing process, numerous factors influence the macro and micro structures of a grinding wheel.Figure 4.26 gives an overview of the influencing variables. These variables are divided into the following sections: type of dressing tool, dressing tool specification, dressing kinematics, and dressing system.
Figure 4.26.
Characteristics of the dressing process