There are many modern defi nitions of grinding. All of them defi ne the process as an energy transfer from a drive unit into a workpiece surface layer, while abrasive

grits fi xed by the bond serve as energy transfer channels. A part of that energy dissipates into the bond and leads to wearing and structural changes. Some of it ends up as swarf and a large portion is transferred to the surface of a workpiece causing fl aws and defects.

High material removal rate is considered a necessary requisite for high cost effi ciency of grinding. Technical effectiveness of grinding is defi ned by three interconnected parameters, namely the removal rate, the tool life and the qualitative characteristics of a machined surface.

A necessity in high material removal rate requires faster transfer of large quantities of energy into a workpiece surface layer. This obviously results in more mechanical strain in the contact zone that leads to plastic deformation and chip formation along with high heat release. High level of cutting forces and heat in the grinding zone are in opposition to high tool durability, because both factors increase bond wear, fracture and fallout of abrasive grits. On the other hand, the utilization of a more durable tool under fi xed material removal rate leads to the increase of temperature and cutting forces.

In turn, the minimization of the structural changes in a workpiece surface under constant machining by a tool requires lower speed of material removal rate. Otherwise the increase of temperature and grinding forces will lead to burning, unwanted structural changes, residual stress and poor surface micro-geometry.

Under traditional approaches, improvements to the three main parameters needed for an effective grinding process require three separate and practically incompatible solutions. This situation is unavoidable when almost all energy that is transferred by the abrasive grits along with chip formation is spent on plastic deformation with applicable heat release. Note that the thermal factor contributes faster than the material removal rate if we increase cutting depth for example. This energy dissipation in the cutting zone depends on positive feedback meaning that higher temperatures and cutting forces lead to higher bond deformability and larger contact zone with the machined surface. An increase in immediate contact cross section in the tool/workpiece pair stimulates an increase in temperatures and forces that leads to further bond deformability and so on.

If we possess positive feedback, an increase in thermomechanical strains as a result of higher material removal rate can be slowed down at the expense of lower tool life. But this solution is often impossible to implement fully not only because of an increase in tool costs as they deteriorate faster, but because of lower workpiece accuracy caused by the lower dimensional durability of a cutting tool profi le. Another obvious solution of decreasing the thermomechanical intensity of grinding under higher material removal rates is an increase in quantities of cooling liquids used, especially those that are highly physically/chemically active in the cutting zone. This is problematic due to ecological concerns, and this problem will remain unsolvable even further as time passes. Aside from ecology, during the process of production grinding, cooling liquid does not penetrate into the cutting zone, and while it is a useful and a desired factor, it does not bring a radical qualitative effect on processes in the instant cross section of contact.

More obvious methods of solving these contradictory problems of increasing grinding effi ciency as a whole are limited to trivial optimization. Study of new horizons where a signifi cant increase

in effi ciency of machining is possible requires an alternative approach. Further development and utilization of adaptive abrasive composites may be considered as a solution to current technological demands made for modern grinding. Adaptive behaviour includes self-tuning of the structure and qualities of an abrasive material, including the binder and abrasive grits, primarily diamond or CBN, to varying conditions in the work zone. Whereas traditional abrasive composites demonstrate positive feedback between deformability and thermomechanical cutting factors, adaptive composites demonstrate inverse feedback. This means that in certain temperature ranges and cutting forces, the tool material responds by decreasing its deformation values instead of increasing them.

This type of adaptive behaviour in a tool surface layer of a certain depth, means that if we increase the external action, we will get a reversible increase in bond rigidity as well as adhesive interaction of the binder with the abrasive grits. This process occurs with a certain speed and magnitude of change of physical characteristics of the composite parameters. Discussion of microscopic molecular aspects of structural transitions in developed resin and hybrid organic/non organic composites is outside the scope of this article.

When we talk about adaptive control in machining

operations, we usually mean electronic monitoring

systems based on sensor readings that enable the

machining parameters (feeds, cutting speeds, depth

of cut, etc) to be automatically varied to provide

optimum effi ciency. This article by N. Ignatov and

N. Tikhon, however, discusses adaptive control from

within the tool material itself, in this particular case,

CBN and diamond grinding wheels. As a category of

tool materials, adaptive composites (abrasive grit and

bond together), demonstrate a new comprehension

of adaptability, in that the composite serves both as a

sensor and as a work medium, changing its properties

according to conditions in the cutting zone. As a result,

the authors claim that it becomes possible to combine

a high rate of material removal whilst maintaining

the maximum surface integrity of the workpiece.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Adaptive abrasive composites look to the future

Grinding

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(DOUBLE PA

At Aignesco Inc we utilise modifi ed epoxy oligomers, oligoimides, other heterocyclic compounds and hetero-organic oligomers, comprising ultra-dispersed powders of metals and non-metals with a modifi ed surface structure.

We have developed a two-level system (Scheme 1):

Initial rigidity ⇔ Increased rigidity

As well as a three level system for more degrees of freedom (Scheme 2):

Lower rigidityInitial rigidity ⇔ Increased rigidity

⇔⇔

When it comes to mesoscopic properties of adaptive abrasive composites responsible for the behaviour of abrasive grits, this needs to be discussed and clarifi ed more thoroughly as it is a new type of tool material. Dynamic and force aspects of the interaction of the abrasive grits fi xed in a binder material with the machined surfaces are very dependent on the strength of fi xation and strength of retention of the grits by the binder. Adaptive composites are able to self-tune these parameters in response to changes in temperature, vibration intensities and shear strains in the cutting zone.

For a three-level system, the general system of adaptive behaviour of grains is as follows. Grits that are in contact with the machined material are entrenching into the material and at the same time, they deform the bond. Energy dissipation of this deformation within the binder causes a structural transition of the adaptive composite into a more rigid state. Deformation quickly slows down (within limits of a cycle of contact for a given group of grits). For grits with the most protrusion, the stress is larger, and this forces the bond into a less rigid state (Scheme 2).

Grits that protrude will sink into the bond, until the energy dissipated into the contact micro area of the composite material will not become equal to that of the other grits. The main consequence for the cutting profi le of the composite will be leveling out of all grits that are in contact to the same height above the bond. During that process, if volumes

of the bond that are being transformed overlap, abrasive grits on the work surface become “aware” of each other’s condition and as a result, they create a self regulated system, working as a single unit.

Experimental test of this system in the immediate contact area of a tool with a workpiece is hardly possible at the present moment. However, model experiments confi rm this system. Because we decided not to touch physiochemical mechanisms of adaptive behaviour of bonds in this article as well as the molecular processes, we will concentrate on the behaviour of grits that protrude during contact with the machined surface, the working behaviour of the tools and the properties of the machined surfaces.

Direct experimental study of contact behaviour of abrasive grits system is possible only under static conditions. A schematic of the experiments carried out for this study is shown on Fig 1. In real conditions, the role of the shear component of contact strains is important. To imitate that, contact pairs (the abrasive composite or tool in contact with the counterbody or workpiece) were subject to ultra sound vibrations parallel to the contact surface.

Two samples were tested, and in both cases we used monocrystalline sapphire with a 5 mm thickness, one side polished to a Ra 2.2 nm. The contact volume between the pairs, given the roughness of their surfaces and the protrusion of the abrasive grits, was fi lled with a special gel that contained a component that is able to become luminescent under pressure. Study of the distribution of real areas of contact of the abrasive composite and counterbody under different pressures, as well as identifi cation of components of the contact area of the grits, caused by plastic or elastic interaction with the machined material, was done using a photo detector. A signal from the photo detector was passed into an image analysis system that was used to identify areas of real contact according to luminescence levels. Luminescence intensity in this scenario is directly connected to the amount of energy dissipated on contact areas under pressure and ultra sound vibrations (the latter mimics the shear factor). In turn, this energy is directly connected with the rigidity of the bond in the abrasive composite.

Experiments allowed us to directly witness reversible structural changes in adaptive abrasive composites engaged in contact with the counterbody surface. Changes initiated by increasing ultra sound vibrations under fi xed or non fi xed pressure

was detected according to the changes in luminescence intensity. Change time for different adaptive bonds was 10-5 – 10-3 sec (Figs 2 and 3)

Distribution of points of contact by luminescence intensity for a traditional polymer bond containing no abrasive grits and for an adaptive bond is shown in Fig 4. The effi ciency of energy transfer is proportional to luminescence intensity.

With an increase of contact pressure, the integral intensity of luminescence increased, but the distribution becomes wider. The whole range of intensity connected with contact zones that exist under minimal contact pressure fi ts into the area of contact with increased contact pressure. The contact

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• • •Fig 1 Detection of machining parameters at the contact zone between an abrasive composite and a counterbody (workpiece)

• • •Fig 2 Reversible contact luminescence changes as indicators of the rigidity of the abrasive composite bond for (a) a two-level adaptive bond and (b) a three-level adaptive bond

• • •Fig 3 Kinetics of structural changes for different adaptive bonds (1, 2)

Grinding

DIAMOND TOOLING JOURNAL 1·1100 00

zone of the bond with the machined material increases by growing and merging of initial areas according to the principle of positive feedback between energy dissipated in the contact zone and deformation of the composite. During that process, the speed of growth of the contact zone on a non-adaptive polymer bond with the counterbody surpasses growth in luminescence intensities.

For the adaptive polymer bond, above a certain threshold pressure, there occurs a sharp change in shape of distribution of luminescence of contact areas. Such deep changes are possible only with a complete rearrangement of morphology of contact zones under inverse feedback between contact strain and composite deformability.

Intensity of luminescence increases much faster than the contact zone.

In both cases, the amount of energy transferred by the composite into the counterbody surface layer increases with an increase of contact loading. However for non-adaptive bonds, that occurs mainly by a rapid increase in a total area of contact and little increase in the amount of energy per area unit.

For adaptive composites on the other hand we have a slow increase in the contact area but fast increase in energy per area unit (Fig 5). Principle changes in morphology of contact and speed of energy transfer over a single contact section create conditions for qualitative changes in structure, micro-

geometry and performance attributes of surfaces generated with adaptive abrasive composites.

For composites containing abrasive grits, under low pressure, when the grits are in elastic (without penetration) contact with the counterbody, a second level of luminescence is observed. A third level appears at pressures causing a plastic contact when grains penetrate the counterbody. Typical distribution of contact areas connected to the bond as well as zones of elastic and plastic deformation of the counterbody by abrasive grits are shown in Fig 6.

The dependence of areas of elastic grit contact (correlates with the deforming action of a tool) and plastic grit contact (correlates with cutting ability of a tool) on pressure is shown in Fig 7. For both, adaptive and traditional composites, the area of contact of abrasive grits increases as we increase pressure. Increase in a dynamic factor by increasing amplitude of ultra sound vibrations achieves the same result under constant pressure.

The proportion of contact area for grits where we observed plastic contact with the machined surface and when in dynamic conditions chips are separately generated, was 10-25%. For the composite that is unable to adapt its behaviour, the speed of an increase of cutting and deformation areas contact for grits decreases with an increase of pressure. For adaptive composites, the speed of an increase of grits in deformation (elastic contact) also decreases, but cutting ability (plastic contact) increases. It appears that in both types of composites there are reversible and irreversible changes in grit orientation occurring. Probably this affects those grits, whose attack angle is close to 90° (Fig 6).

Even though the total areas of contact of abrasive grit systems for non-adaptive and adaptive composites are close, the structure of the contact sections has principal differences. For traditional and for adaptive composites, grits with purely plastic contact with a counterbody are absent. But for traditional composite, grits with purely elastic type of

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• • •Fig 7 Infl uence of contact pressure on areas of elastic (a) and plastic (b) contact of abrasive grits with a counterbody (monocrystal sapphire) for adaptive (1) and non-adaptive (2) composites [S is the ratio of real contact area (mm2) to nominal contact area (cm2)]

• • •Fig 4 Distribution of contact point area by effi ciency of energy transfer between polymeric bond and counterbody (monocrystal sapphire sample) (a) traditional bond and (b) adaptive bond at different contact pressures (1 - P1 = 0.3 MPa; 2 - P2 = 1.0 MPa)

• • •Fig 6 Three levels of contact luminescence intensities in an abrasive composite/counterbody pair

AGE SPREAD) Grinding

DIAMOND TOOLING JOURNAL 1·1100 00

contact (deformative grits) comprise 50 - 80% of the total amount of grits, whereas for the adaptive composite this proportion was no more than 10-15% (Fig 8). Grits with elastic contact protect the composite from wear, but increase heat generation and damage to a machined surface.

The distribution of grits in plastic contact with the counterbody, in terms of contact area and penetration depth, are also principally different for adaptive and traditional composites (Figs 9 and 10).

During an abrasive tool operation, grits that are in contact with the machined workpiece material act as a system of point sources of heat and shear deformation, leading to chip separation. Using identical abrasive grits and with negligible differences in physical mechanical properties of the composites, the adaptation ability can qualitatively change the behaviour of this

system. The dynamics of stress distribution in the contact zone of normal and adaptive composites with a counterbody in a single cycle of loading is shown in Fig 11.

Duration of the loading cycle with a force of 0.7 MPa was 10-2 sec. Kinetic curves for luminescence intensity in the contact zone of the abrasive composite with the counterbody refl ect a rearrangement of stresses and deformations, according to structural changes in the composite and contact interactions. For the adaptive composite, a quick growth of luminescence levels is seen, due to increased rigidity of its surface layer as a result of a structural transition initiated by the load. This increase in rigidity provides quick and effi cient energy transfer, connected with the load, through the grit system and contact areas of the bond, and into the surface layer of the counterbody. During that process, it is clear that the main role of energy transfer is done by the abrasive grits. In this case, the role of the bond is secondary.

The quick completion of structural changes in the composite layer as a result of the loading cycle corresponds to a quick setting of the residual stress in the machined surface layer. Other than tensiometric measurements, optical measurements were also used to ascertain the spread of stress over the depths of the monocrystalline sapphire that was in

contact with abrasive composites. For adaptive composites, in all experiments there was a distinct localization of an area of maximum strain (Fig 12). Strain maximum accurately corresponded to an average depth of indentations made by grit penetration. We bear in mind that in real dynamic contact in the cutting zone, maximum strain corresponds to maximum temperature softening of a machined material. Coincidence of the depth of localization of this area with the depths of intrusion of abrasive grits demonstrates one of the main principles of infl uence of adaptivity on the behaviour of an abrasive composite.

When it comes to a traditional composite, its curve of kinetics of luminescence intensity has a low-pitched maximum and a tail that goes over the limit of load duration. Because of that, the setting of an equilibrium in the contact zone occurs slowly. The speed of energy transfer from an outside source over an abrasive composite and into the surface layer of the counterbody was calculated by the tangent of a slope ratio of the start of the curve σ(τ). The result was that with practically equal microhardness and Young’s modulus, the adaptive composite transfers energy 10-100 times faster than the traditional one.

The role of a bond in energy transfer to a counterbody for traditional composites is signifi cant. At least it is equal to that of grits, but often even more signifi cant. Strain maximum in that case is usually closer to the surface of a counterbody than when an adaptive composite is used. For the latter, the maximum remains stable in wide load range, and only total strain changes. During contact with a traditional composite, the

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• • •Fig 8 Relationship between pressure and common amount of grits per unit of work area of composite (1, 3), including those with elastic interaction with a machined surface (2, 4), [1 and 2 – traditional composite; 3 and 4 – adaptive composite]

• • •Fig 9 Distribution of number of grits with plastic contact (N) by penetration depths into a counterbody (h) for adaptive (1) and non adaptive (2) composites

• • •Fig 13 Relationship between number of grits (N) and area of contact points with a counterbody (S) when grinding with (a) an adaptive composite and (b) a traditional composite (1 - P1 = 0.3 MPa; 2 - P2 = 1.0 MPa)

• • •Fig 10 Distribution of number of grits (with plastic contact N) by area of contact with a counterbody (S) for adaptive (1) and non adaptive (2) composites

• • •Fig 12 Distribution of stress over depth of penetration (h) of the monocrystal sample in contact with traditional (2) and adaptive (1) composites

• • •Fig 11 Dynamics of stress redistribution in the contact zone between an abrasive composite and a counterbody during a process of load increasing: (1) trajectory of load, (2) changes in contact luminescence for an adaptive composite, (3) changes in contact luminescence for a traditional (non adaptive) composite

(DOUBLE PAGrinding

DIAMOND TOOLING JOURNAL 1·1100 00

location of a maximum and a shape of profi le can greatly change under load changes. The principal difference to an adaptive composite is the absence of correlation between maximum strain in the surface layer of a counterbody with the depth of indentations made by the protruding abrasive grits.

Interesting information about features of the interaction of abrasive composites with a counterbody as a “static model” of a machined workpiece was obtained by measuring the infl uence of pressure on the shape of the density of distribution of abrasive grits by contact points (Fig 13). This analysis demonstrates qualitative differences in the behaviour of adaptive and non-adaptive materials. For traditional composites, areas of contact points form a wide massif. With increased load, the areas of contact for all grits increases, as well as the width of distribution, but any correlation in values of additional area is absent. For abrasive grits in an adaptive composite, the distribution of areas of contact with a counterbody was much narrower in all cases. With increased loads, areas of additional contact defi nitely correlate between each other and the width of distribution practically does not change.

These results show that grits that stand out of an adaptive composite during contact with a counterbody create an interconnected system that reacts to changes in an environment as a single unit. This is their primary difference from grits in traditional composites that poorly interact and affect machined material linearly and additively in character.

The process of grinding is always connected with oscillations of all parameters of contact of the abrasive grits with the machined surface of a workpiece. If grits create a single interconnected system, these oscillations have a tendency for coherence. This can lead to different phenomena, similar to non linear effects in optics. These can be seen as “self-focusing” of shear strains on the protrusion depth of the grit system into the machined material. However, even if the individual grit particles form a cutting profi le with minimum height differences (for example in precision electroplated tools), but grits do not “feel” each other and not form a single system, self-focusing does not occur because of coherence absence.

Tests of grinding wheels, made with adaptive composites, were made compared to commercially available traditional resin bonds. In all cases cooling liquid was applied at 4 bar pressure. In the experiments sapphire was used as the counterbody (Fig 14).

Grinding ratios for adaptive and non-adaptive composites for the applied cutting

depths were very close (10-15% difference). However, the surface micro-geometry of the monocrystalline sapphire formed with test tools showed signifi cant differences.

With an almost equal Ra value (0.36 µm for non-adaptive and 0.38 µm for adaptive composite), the maximum deviation of the profi le Rmax formed with the adaptive composite was 3.5-4.5 times lower. The surface of roughness bearing face on an 0.8 µm level for the sample machined with the adaptive composite was 0.60, whereas non-adaptive composite’s result was 0.11. The signifi cance of these results for processing of items like substrates for photoelectric cells or joint implants components is obvious. In the fi rst case, the most important consequence of using an adaptive composite will be less time spent on the polishing of workpieces. In the second case this advantage will be supported by increased frictional characteristics and the life duration of the part.

In other tests, the tools made with Aignescotec adaptive composites were used for grinding cemented carbides and hardened steel. Samples of hard alloy H10F were subject to face grinding by a 4A2 D64 wheel using coolant. The main recorded parameters were grinding ratio, surface fi nish (Rmax), bearing face area and mean radius of asperities (summits) (Fig 15). All parameters were recorded with cutting speeds of 20 m/s and 60 m/s. When it comes to grinding ratios (Fig 15a), the tool with an adaptive composite showed more stable behaviour at 20m/s with increased cutting depth. However an increase of speed to 60m/s almost nullifi ed any differences between the adaptive and non adaptive

tools. In general, we posses numerous comparison results for tool resilience for different composites, where only the highest quality commercially available samples were used. In many cases adaptive composites showed increased resilience. However, whereas high resilience of grinding wheels is always desirable and very important in some cases, it is never the most important factor in gauging tool effectiveness.

The example where compared wheels have very close resilience at all ranges of cutting depths used, allows us to fully value the effect of adaptability on the structure and micro-geometry of the surface layer of the machined workpiece. On Figs 15b, c and d we can see the effect of grinding depth on the (Rmax) value, the bearing face area and the mean radius of asperities. when using adaptive and non-adaptive composites. With a 20 m/s speed, the Aignescotec composite has a clear advantage. The surface created by Aignescotec demonstrates structural differences that will provide greater

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• • •Fig 14 Relationship between depth of cut and surface roughness (a) and contact surface area (b) of monocrystalline sapphire when ground with adaptive (1) and traditional (2) composites

• • •Fig 15 Infl uence of cutting depth on grinding ratio (a) surface fi nish [Rmax] (b), bearing face area (c) and mean radius of asperities (summits) (d) for adaptive (1) and traditional (2) abrasive composites under different cutting speeds

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DIAMOND TOOLING JOURNAL 1·1100 00

wear resistance, better fatigue strength, better frictional characteristics, and as for the sharpened edge cutters – increased sharpness and absence of spalls.

With a 60 m/sec speed, the differences in the Rmax value are nullifi ed, but the key advantages of the surfaces created by Aignescotec remain stable . This conclusion is confi rmed by data on the residual stress in ground samples of cemented carbide and the structural changes in the tungsten carbide crystalline lattice, caused by grinding (Figs 16 and 17).

Compression stresses caused by the adaptive composite increase the mechanical durability and life of a workpiece, whereas

stability in areas of coherent-scattering regions provides minimum lattice defects. Macroscopic evidence of these structural differences is demonstrated by the dry friction quotient of a ground cemented carbide/aluminium alloy pair (Fig 18).

Hard alloy cutters and mills sharpened using grinding wheels with adaptive composites demonstrate service lives 2-3.5 times greater when used on steel and cast irons, than those sharpened with non adaptive counterparts with similar grinding ratio (Fig 19).

Regularities of the grinding ratio, surface fi nish, bearing face area, mean radius of asperities and residual stresses in a workpiece such as hardened steel when ground using CBN tools with adaptive composites and non-adaptive composites are very similar to results obtained when grinding of hard alloys with diamond wheels. This proves that this behaviour is not connected with a selection on the tool/material pair, but shows deep differences in the physicochemical mechanisms of the contact behaviour of adaptive and traditional abrasive bonds.

The infl uence of adaptive behaviour of abrasive composites on the surface structure of machined steel components is very signifi cant. In Fig 20 we can see the relationship between cutting depth and the dislocation density in the surface layer of rods made of hardened steel ground by an abrasive wheel with adaptive and non adaptive composites (grit: CBN B76). With negligible differences in the grinding ratio

(10% higher for the Aignescotec composite) and a cutting depth of 0.06 mm, the adaptive composite ensured material removal with a much lower concentration of defects in the created surfaces layers of the workpieces. With high homogeneity of workpiece structure and high quality of surface layer formed by non-adaptive composite, the adaptive composite introduced several positive advantages.

Fig 21 demonstrates a reduction by 10 times the previous amount in the number of ground workpieces whose durability is less than 90% of that specifi ed. The average service life of these workpieces will be higher compared to those machined by non-adaptives. With large-scale production this in itself will be a noticeable advantage. Also, the chance of a sudden destruction during forced operation or because of fatigue defects decreases, in this case by an order of magnitude. This may constitute a prominent competitive advantage for heavy loaded components like shafts, gearwheels, bearings and turbine blades.

Fig 22 shows general tendencies in the morphology changes in the machined surfaces of a workpiece when grinding with adaptive and non-adaptive composites.

We are not talking about dramatic, visually obvious profi le charts, but about stable, qualitative tendencies leading to an increase in bearing capacity and contact rigidity, decreased unsoundness and improvement of frictional parameters. Full implementation of the potential abilities of adaptive composites assumes not just minimal surface defects but improvement of whole complex of qualities compared to the unchanged machined material. In many cases this leads to noticeable differences in operational performances.

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• • •Fig 16 Distribution of residual stress in samples of hard alloy: (1) initial sample, (2) sample ground with a traditional abrasive composite, (3) sample ground with an adaptive abrasive composite

• • •Fig 17 Structural changes made in a tungsten carbide crystalline lattice made by grinding with an adaptive abrasive composite (1) a traditional abrasive composite (2) [abrasive grit D 64]

• • •Fig 18 Dry friction quotient of a cemented carbide/aluminium alloy pair ground by adaptive composite (a) and non-adaptive composite (b)

• • •Fig 19 Wear of hard alloy cutter over fl ank surface during steel machining when cutter was sharpened by an adaptive composite wheel (1) and a traditional composite wheel (2)

• • •Fig 20 Relationship between cutting depth and density of dislocations in a steel rod surface ground by an adaptive abrasive composite (1) and a traditional composite wheel (2)

• • •Fig 21 Strength distribution of steel rods ground by adaptive (1) and traditional (2) abrasive composites (N = number of rods, σ = strength of rod)

• • •Fig 22 Differences in surface morphology formed by traditional (a) and adaptive (b) abrasive composites

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DIAMOND TOOLING JOURNAL 1·1100 00

In Fig 23 differences in noise levels made by cogwheel gears ground by adaptive and non-adaptive composites are shown.

Signifi cant changes in the structure of machined surfaces formed by adaptive composites are based on greater energy transfer into the surface layer of a machined material compared to non-adaptive alternatives. At a fi rst glance, this contradicts the concept of damage minimization in machined surfaces. However, experimental data has proven a decrease in such defects when adaptive composites are being used, largely because these not only provide a more intense input of energy through an adaptive system of abrasive grits, but also create effective channels for non thermal dissipation of that energy. Experiments to determine electron emission intensities from the surface of a metal just after contact with a grinding wheel showed that for adaptive composites this amount is 3-10 times greater. However, the temperature of the surface of a workpiece under similar material removal rates and grinding ratios is 1.1-1.2 times lower.

Whereas the defectiveness of surface layers generated by adaptive composites was always lower in our experiments, chip defectiveness acted in a reverse proportion. For chips formed by grinding steel with CBN wheels, dislocation density during adaptive composite use was much greater at all grinding depths than if we used a non-adaptive composite (Fig 24).

This proves the assumption about the localization of the shear zone on the depth

of penetration of abrasive grits when using an adaptive composite. During that process excess energy is used to generate extremely strong chips with minimal damage to the machined surface.

In the swarf created during grinding, we can fi nd many more middle sized fragments of diamond and CBN if we use adaptive composites, than if we use traditional composites (Fig 25).

It seems that wearing of abrasive grits in an adaptive system resembles electrospark erosion. There is constant and stable (in time) separation of fragments, with constant self replication of the jagged cutting profi le of each separate grit (Fig 26).

In general, the grits in traditional composites work as independent micro-cutters, and wear out in a cycle of “blunting-spalling”. The steady wearing out of abrasive grits in an adaptive system can happen more slowly than the blunting-spalling wear pattern associated with a traditional bond, but can also happen faster or at the same speed. The more active destruction of equal in size, shape and durability grits in an Aignescotec bonded wheel compared to a non-adaptive composite, also presents a channel of dissipation of excess energy that is transferred by the composite into the surface layer of a machined workpiece. Other than that, it provides adaptive abrasive composites with a better ratio between the removal rate of machined material on one hand, and the created surface parameters on the other.

Adaptive composites, as a category of tool materials, demonstrate a new comprehension of adaptiveness. We are not talking about adaptive control based on sensor readings, but about self-tuning

of bond material properties that retain an abrasive grit. The molecular structure of a composite serves both as a sensor and as a work medium, changing its properties according to conditions in the cutting zone. Consequences of that behaviour for the cutting profi le of an abrasive wheel and for the mechanics of cutting are based on the creation of instant contact point of a wheel with a component of an adaptive grit system, that affects the machined material in coordination. Leveling of cutting forces and depths of protrusion for all grits in a system provides a great increase in energy transfer speed into the machined surface compared to non-adaptive counterparts. Probable consequences of that will be localization of the zone of maximum shear and maximum heating in a machined material to a depth close to the protrusion depth of the abrasive grit. As a result, it becomes possible to combine high rate of material removal with minimal damage to the surface layer of a workpiece and formation of such a structure and micro-geometry of the surface area that cannot be achieved through the use of high quality traditional composites.

Although innovative CBN tools with Aignescotec adaptive bonds have been developed and approved of by the industry, further evolution of adaptive abrasives suggests a steady introduction and intensive industrial utilization in a wider spectrum of modern technological operations. •

• AuthorsNikolay Ignatov and Nikolay Tikhon work for Aignesco Abrasive Systems Co., Toronto, Canada.

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• • •Fig 25 Distribution of abrasive grit fragments fractured during machining with adaptive and traditional composites in identical conditions

• • •Fig 26 Typical trench profi le made by an abrasive grit embedded in adaptive (a) and non adaptive (b) composites used to grind the surface of a hard alloys

• • •Fig 23 Noise levels of cogwheel transmissions machined by adaptive (1) and traditional (2) abrasive composites (N = number of cogwheel pairs, D = noise level)

• • •Fig 24 Relationship between cutting depth and dislocation densities in swarf during the grinding of steel by adaptive (1) and traditional (2) abrasives

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