The Diagnostics of Abrasive Tools After Internal Cylindrical Grinding of Hard-to-Cut Materials by Means of a Laser Technique Using Imaging and Analysis of Scattered Light

The formation of smears on the grinding wheel active surface is one of most undesirable phenomena during abrasive machining. Intensive growth of such smears leads to a decrease in the machining capacity of the grinding wheel, which, in turn increases the grinding power and the effects of friction upon the process, reducing efficiency. What is very important in this context is the possibility to inspect the condition of the grinding wheel active surface during the grinding process in order to detect the excessive growth of smears. Such an inspection can be carried out by automatic machine vision systems, or systems using optical measuring methods (especially those from a group of light scattering methods). In the paper is presented and discussed a proposal for optical inspection using laser scatterometry, as well as image processing and analysis techniques. It involves the acquisition of an image of scattered light during the illumination of tested surfaces by a laser light (wavelength λ = 635 nm). The analysis of such an image can enable the procurement of the values of geometric and photometric parameters, which then be correlated with, for example, the selected parameters of surface roughness obtained by other techniques. Experimental investigations were carried out on grinding wheel active surfaces after internal cylindrical grinding of hard-to-cut materials. The discussed methods were confirmed to be useful, which could be an interesting alternative to solving the important problem of diagnostics of abrasive tools in machining processes.


Introduction
The lengthy contact between grinding wheel and workpiece, which takes place in the internal cylindrical grinding processes [1][2][3][4][5], presents difficulties for the effective insertion of cooling fluids into the grinding zone. Also chips and other grinding products are not properly removed from the contact zone. One result of these undesirable conditions is the proliferation of the smearing phenomenon upon the grinding wheel active surfaces. This phenomenon is especially intense in the event of grinding hard-to-cut materials [6][7][8][9][10], such as new variations of alloy steels and alloys on the basis of nickel, cobalt or titanium (INCONEL ® , INCOLOY ® , NIMONIC ® , HASTELLOY ® etc.), which are very ductile, resistant, hard and have small thermal conductivity.
Of great significance to the internal cylindrical grinding of hard-to-cut materials are the possibilities for monitoring and measuring the degree to which smearing occurs upon the grinding wheel active surface. The precise determining of the moment when regeneration is necessary is crucial because it can impact upon the workpiece surface quality and the durability of the grinding wheel. In extreme cases the rapid growth of smears can increase the friction participation and accelerate an excessive rise in temperature. Finally, grinding defects [6,11] could occur upon the workpiece surface, or increasing temperature stresses could result in disruption to the grinding wheel itself.
The assessment of the degree to which smearing effects the grinding wheel active surface can be carried out using various measuring methods, but most often official methods are utilized. From the many groups of optical methods, which can be used in this type of assessment, the most important are methods based on light scattering phenomenon. These methods are generally known as scatterometry. The many significant advantages of scatterometry (non-contact method of measurement, short-time of measurement, high sensitivity and accuracy) can be a good alternative to other methods (pneumatic, ultrasonic). The possibility of integrating this method with image processing and analysis techniques may, additionally, lead to the acquirement of an effective measuring instrument, designed for diagnostics of abrasive tools.

Internal Cylindrical Plunge Grinding
Grinding wheels with the following technical designations underwent examination, 1−35×20×10-SG/F46 K7VTO, with which grinding tests of hard-to-cut materials were carried out in the process of plunge peripheral grinding of internal cylindrical surfaces [1,2]. The method consisted in carrying out a short grinding test, lasting approximately 3 s on a properly shaped grinding wheel, in the plunge grinding kinematics. The test was carried out without the finish grinding and sparking out stages. The grinding wheel carried out its working movement with the given feed speed v fr , after which it was immediately moved away from the processed material. What was characteristic of the method used was the modification of the geometry of the grinding wheel, which consisted of forming a few zones of various diameters (Fig. 1a) on its active surface in the dressing process.
When performing tests using grinding wheels with various diameters, it must be prepared in such a way so as to obtain an even division of the total height of the grinding wheel in the axial direction T to particular working zones  (Fig. 1a). The methodology used also assumed that the total working engagement a e is divided equally amongst subsequent steps of the grinding wheel. Such plunge grinding, results in representing the shape of the grinding wheel on the workpiece (Fig. 1b). Quick backwards movement and omission of sparking out results in an abrupt interruption of the grinding process. Special shaping of the macrogeometry of the grinding wheel [3] makes particular parts of its active surface work at different times and remove different material volumes. In consequence, in particular areas of the grinding wheel active surface, the processes of wearing off of the abrasive grains [11], chip formation and smearing the intergranular free spaces take place with various intensity and can take different forms.

Mechanism of the Smear Forming
The phenomenon of smearing the grinding wheel active surface with chips from the machined material and other grinding products (crumbled abrasive grains, bond) takes place as a result of excessive temperature increase in the area of contact of the grinding wheel and the machined material. Quite often such temperature growth is caused by increasing friction forces in the grinding process [6,11]. This is true mainly for machined materials with poor heat conductivity. Smears are formed in areas of contact between the grinding wheel active surface and the workpiece, such as vertexes of the abrasive grains. An example of smearing upon the microvertexes of a SG microcrystalline sintered corundum abrasive grain is presented in Fig. 2.
Under the exceptionally difficult working conditions of the grinding wheel active surface, smears of significant surface area can be formed, which would include both grains and intergranular free spaces (Fig. 3). The reason for their formation is most frequently excessive material removal rate, wearing off of the grinding wheel, as well as too few intergranular free spaces, which do not guarantee effective transportation of the grinding products beyond the processing area and do not provide efficient supply of the cooling liquid to the grinding zone.
This phenomenon can occur with higher intensity in grinding processes, in which the path of contact of the grinding wheel with the material is longer, as in the case of internal cylindrical grinding, for example. What plays a significant role in the prevention of smearing on the grinding wheel active surface is the type, outlay and method of application of the cooling liquid.
Smears upon the grinding wheel active surface lead to excessive temperature increase in the area of contact of the grinding wheel with the machined material, which can cause the appearance of grinding defects, such as grinding burns or microfractures of the top layer of the machined surface. The decrease in the number of cutting apexes, caused by the smears, deteriorates the cutting capacity of the grinding wheel and may have an influence on the effectiveness of the machining process, as well as the attainment of surface roughness and dimensional-shaped precision. As the smears cause significant temperature growth, mainly in the grinding wheel, in the case of grinding wheels with ceramic bond to the destruction of the tool, caused by the uneven distribution of the thermal stress in the grinding wheel's mass.

Light Scattering and Light-Scattering-Based Methods
The surface roughness assessment and detection of surface defects using scattered light is based on the observation of light reflected in a different way from a smooth surface and a rough one. These cases are shown in Fig. 4.
The mathematical description of the above-detailed cases is given in principle light scattering theories-Kirchhoff's diffraction theory and Rayleigh-Rice's perturbation theory.
The wide review of these theories was presented in Refs. [12,13]. Many optical measuring methods are based on light scattering phenomenon. The most dynamically developed for many years are the following methods: reflectometry [14], total integrated scattering (TIS) [15], angle-resolved scattering/differential scattering (ARS/DS) [16] and optical Fourier transform (OFT) [17]. These methods often use laser radiation (visible light or infrared ranges from 0.75 to 15 µm) and are thus defined by one general term-laser scatterometry [18][19][20].
Laser scatterometry allows for the assessment of very small surface irregularities that are distributed over, e.g., precision optical elements [21], silicon wafers [22] and thin films [23]. The height of these irregularities ranges from 0.1 nm to several nanometers. The great advantages of these methods are their high sensitivity, the fact that they offer a non-contact form of measurement, short-time of surface roughness assessment and the possibilities for detection of surface defects.
The measurements which are realized by laser scatterometry often relate to the assessment of machined surfaces made of metal or its alloys, carried out in high-efficiency machining processes such as grinding [24], polishing [25], lapping [26], microfinishing [27]. In this case, the height of irregularities could reach a somewhat higher value, even close to several micrometers. Much less frequently, in the literature, instances can be found which use these methods to assess surfaces made of plastics and ceramics. From this last material abrasive tools are commonly produced. They are difficult to assess by laser scatterometry methods. It depends mainly from constructions of grinding wheels (stochastically packed grains bonded by binder). This highly heterogeneous structure, without visible directivity of machined grooves, is causing the increase of the light scattering phenomenon. It visually manifests in creating on observation plan an irregular shape of an image of scattered light (Fig. 5f). This image is totally different from the images created for typical machined surfaces which are shown in Fig. 5.
To improve the accuracy and efficiency of the assessment of surface roughness and detection of surface defects, laser scatterometry can be supported, in many cases, by image analysis techniques [27]. It concerns both types of measurement, those carried out in static conditions and those carried out in motion [28]. Image analysis, realized by use of specialist software, can present many possibilities to carry out complicated multi-criterion quantitative and qualitative anal-ysis, in manual or automatic mode, measuring the geometrical quantities of elements in image and the visualization of measurement data.

Experimental Investigations
The main goals of the experimental investigations were: 1. analysis of the possible use of laser scatterometry, and image processing and analysis techniques, in the optical inspection of the condition of the grinding wheel active surface, 2. development of a correct methodology for the inspection of grinding tool surfaces that have visible smears, using images of scattered light, carry out tests on grinding wheels smeared with various types of hard-to-cut material, characterized by different properties. grinding wheel with zone diversified structure 1−35×20 ×10-SG/F46 K 7 V TO (Ra > 3 µm). ||-tested surface is illuminated by laser light beam when plane of incidence is parallel to axis of sample, ⊥-tested surface is illuminated by laser light beam when plane of incidence is perpendicular to axis of sample, • -angle of incidence of the laser light beam, ↑-measurement is carried out in static condition Experimental investigations were realized in five steps, which will be fully described in this section.

Step 1: Abrasive Machining with the Aim of Forming the Smears
The experimental investigations were carried out using three different hard-to-cut materials-Titanium Grade 2 ® , INCO-LOY ® alloy 800HT ® and INCONEL ® alloy 600. A typical 100Cr6 steel was used as a reference material. The characteristics of the tested materials are given in Table 1.
The above-mentioned materials underwent plunge grinding utilizing a grinding wheel with a specifically shaped macrogeometry of the active surface, according to the diagram presented in Fig. 1a. The total working engagement in Zone I was 0.04 mm, in Zone II 0.08 mm and in Zone III 0.12 mm. The machining time resulted from the assumed radial table feed speed v fr = 0.04 mm/s and was, respectively, for the subsequent zones: t I ≈ 1 s, t II ≈ 2 s, t III ≈ 3 s. With such machining parameters, three zones with variable material removal value V w were obtained and its value fluctuated from 42.22 mm 3 for Zone I, to 126.67 mm 3 in Zone III. The internal cylindrical plunge grinding with purposefully applied high material removal rate (Q w ≈ 42 mm 3 /s) soon led to the appearance of smears on the grinding wheel active surface, which were then measured.

Step 2: Acquisition of Images of Scattered Light
Acquisition of images of scattered light were carried out by an experimental setup that included a single semi-conductor laser CPS180 produced by Thorlabs, Inc. (Sweden), which emitted a continuously visible light beam (wavelength λ = 635 nm). The beam was directed at an angle of 45 • and illuminated the surface of the grinding wheel, mounted in a

Step 3: Computer Image Processing and Analysis
In the last stage of the experimental investigations, images of the scattered light underwent analysis using Image-Pro ® Plus 5.1.0.20. software produced by Media Cybernetics, Inc. (USA). The analysis included calculating the values of some selected geometric and photometric parameters from the acquired images. The characteristics of these parameters are given in Table 2. Before the actual analysis took place, the images were pre-processed. Their resolution was decreased to size 640 × 480 pixels with the aim of accelerating image Table 1 The characteristics of the tested hard-to-cut materials and reference material (each area was 5 × 5 mm in size and contained 25 points at which we were carrying out acquisition of images of scattered light), e corresponding to selected areas images of scattered light obtained for exemplary points 12,9,5 analysis; with use of the software, brightness and contrast were corrected and the images were suitably framed. Then the prepared images were analyzed. For this purpose the software segmented particular images to obtain the outlines necessary for calculating the values of geometric parameters, or utilized tonal correction, to improve brightness and contrast, helping to obtain the correct values of the photometric parameters, amongst other things.
Regardless of the method of initial processing of the images, the above-mentioned parameters were obtained using the Count/Size function. It enabled the authors to perform the quantity analysis on the basis of the automatic counting of all dark objects in all images of scattered light.

Step 4: Measurements of Surface Microtopography
The initial assessment of the results obtained with computer image analysis showed the existence of a couple of exceptionally interesting surface images in each of the zones of the tested surfaces. The authors came to the conclusion that these areas should also be analyzed in relation to the value meaning of the chosen surface microtopography parameters, which would correlate with parameters determined from the images of scattered light. For this purpose, two referential methods were used-optical profilometry and stylus profilometry.
The first method connected directly to the machining tool, making it possible to realize the topographical  Distribution of intensity between the center of an object and the edge of the object measurements of the tested grinding wheels active surface. During these measurements an optical profilometer Talyscan 150, produced by Taylor Hobson Ltd. (UK), was used. This non contact scanning instrument was equipped with a laser measuring head with vertical resolution 1 µm. All microtopographies were registered using laser triangulation for a 19.0 × 10.0 mm surface area. The measurement was made in 201 passes with a sampling step 50 µm in a double-pass mode, i.e., registration of data took place in both directions on the measured surface. In a single pass approximately 1,900 points were registered with the tracing speed V = 5.5 mm/s. The Talyscan 150 was also equipped with dedicated software-Talyscan 1.0. and TalyMap Universal 3.1.0. using Mountain Technology™ provided by Digital-Surf (France). The first of these programs was used for the control of measuring elements and for setting conditions for measurements. The second was mainly used to aid the analysis and visualization of the obtained measurement data. The second method, connected with the machined object, made it possible to obtain parameters for the microtopography of the surface of cylindrical elements made from hard-tocut materials, which had underwent plunge grinding. In this case, the Hommel-Tester T8000 produced by Hommelwerke GmbH (Germany) was used. The stylus profilometer, was equipped with a TKL100 pick-up with diamond stylus tip (tip radius r = 2.5 µm). Hommel-Tester T8000 used traverse unit Waveline™ 60 Basic (tracing length l = 60 mm), which was mounted on measuring column Wavelift™ 400 M (max. traverse l max = 400 mm). The column was mounted on a granite plate Wavesystem™ 780. In the plate was a groove for mounting a motorized positioning stage (Y-Positioner) equipped with a stepper motor, intended to carry out precise movements in the y-axis. All microtopographies in this case were registered for a 17.0 × 5.0 mm surface area. The measurement was made in 251 passes with a sampling step 20 µm in a single-pass mode, i.e., registration of data took place only in one direction on the measured surface. In a single pass approximately 9,581 points were registered with the tracing speed V = 0.15 mm/s. The stylus profilometer used a similar software package as an optical profilometer. Turbo Roughness for Windows 3.1 was used for the control of measuring elements as well as for setting conditions for measurements. For analysis and visualization of the obtained measurement data HommelMap Basic 3.1.0, using Mountain Technol-ogy™ provided by DigitalSurf (France), was used. During both type of measurements (stylus and optical) the surface of machining tool and the surface machined object was dry.
From a number of determined geometrical structure of surface (GSS) parameters only those which showed the highest sensitivity to changes in the surface features, caused by smears, were used. These were the four parameters: arithmetic mean high Sa, total height of the surface St, kernel roughness depth Sk and developed interfacial area ratio Sdr. The characteristics of these parameters are given in Table 3.

Step 5: Acquisition of Microscopic Images
The last step of the investigation was the acquisition of the micrography of selected areas of the grinding wheel active surface, with and without visible smears. In this case a scanning electron microscope JSM5500LV, produced by JEOL Ltd. (Japan), was used. Samples were placed in the microscope chamber without any special preparation. The images were obtained with 50×, 100×, 500× magnification with an accelerating voltage of 15-20 kV.

Results and Discussion
As a result of a number of operations described in Sect. 3, very extensive research material was obtained. It comprised results obtained from: 1. computer analysis of the images of scattered light, 2. optical measurements of selected areas of the grinding wheel active surface, 3. stylus measurements of selected areas of the grinding wheel active surface, 4. visual analysis of the micrographs.
All of the above will be discussed in detail in this section.
The computer image analysis was carried out in the Image-Pro ® Plus 5.1.0.20 environment and consisted of defining the value of the set of geometrical and photometrical parameters given in Table 3. The authors notice that the set of the proposed parameters does not fully reflect the characteristics of the surfaces under investigation. The parameters have successfully been used in assessing the surfaces made from plastics, metals and metal alloys. However, bearing in mind the totally different characteristics of the abrasive tool's surface (especially its considerable heterogeneity), it was necessary to find in both of the groups those parameters which would make it possible to find out more about the surface being assessed. After a detailed analysis, one parameter of both the groups was proposed. The parameters proposed were: 1. geometrical parameter: A n -area (of the bright regions of an image of scattered light), 2. photometrical parameter: I -integrated optical density-IOD (of the bright regions of an image of scattered light).
The above-mentioned parameters turned out to be key for computer analysis of the images of scattered light and were clear referents for other determined parameters. Due to the above-mentioned reasons, the authors decided to graphically present results of the measurements, using only these two key parameters. Figure 8 presents the results of computer image analysis-calculated average values of areas of the bright regions of an image of scattered light for the sample number. In each column of the graphs, the values of standard deviation σ are also indicated. Additionally, the average values of the area of the image of scattered light are given in table below.
On the basis of the diagram above it may be assumed that the average values of areas of the bright regions of an image of scattered light for hard-to-cut materials did not differ in a substantial way. The highest values (from the range of 119,491-124,020 pixels) were visible in Zone I, where smears with the machined material were scarce or not present at all. In Zone II, in which there appeared medium size smears with the machined material, the values were slightly lower (from the range of 98,295-104,044 pixels). The values were lowest (from the range of 73,066-90,698 pixels) for Zone III, with many smears on the machined material. It may therefore be assumed that regardless of the type of hard-tocut material, the differences in the values obtained were not significant. This means that the type of material had practically no influence on a major augmentation or reduction   . 10 The methodology of calculating the selected parameters of the GSS of the grinding wheel with shaped zones of various diameter, on the basis of a surface after grinding Titanium Grade 2 ® of these values. Figure 8 also presents the average values of areas of the bright regions of an image of scattered light, acquired for the grinding wheel active surface after grinding the reference material, which, in this case, was 100Cr6 steel. The values were slightly higher because the grinding wheel was substantially less smeared. Its surface had practically no large and medium sized smears. Figure 9 presents the results of computer image analysis-calculated average values of IOD of the bright regions of an images of scattered light for the sample number. In each column of the graphs, the values of standard deviation σ are also indicated. Additionally, the average values of IOD of the bright regions of an images of scattered light are given in the table below.
The averaged values of IOD of the bright regions of an image of scattered light gave similar results as the aforementioned graph presented in Fig. 8. Here, too, the levels of different average values of IOD of the bright regions of an image of scattered light, for three hard-to-cut materials, were similar. Moreover, the average values of IOD of the  Fig. 8. A certain correlation may then be observed between these two parameters of key importance for this analysis, which can only be an evidence of the fact that they were chosen appropriately.
While analyzing the graphs above a certain tendency may be noticed in the shaping of the values of the parameters described, if one differentiates between the zones of the grinding wheel active surface after grinding the next hard-to-cut material. In every case the values tend to decrease in comparison with the values obtained for the reference material (100Cr6 steel). Relative reduction of the parameter values for different zones of the grinding wheel active surface in percentage terms, are presented in Table 4.
The decreasing values of the assessed parameters for different zones of the grinding wheel active surface and the next hard-to-cut material mean that the optical method used was characterized by high sensitivity. Laser scatterometry is a method sensitive enough to allow for detection of grinding wheel smears of even a relatively small area. Among its Fig. 12 Listing of the changes in values of the GSS parameters for 3 zones of the grinding wheel active surface after grinding Titanium Grade 2 ® , INCONEL ® alloy 600, INCOLOY ® alloy 800HT ® and 100Cr6 steel: a arithmetic mean height Sa, b kernel roughness depth Sk, c developed interfacial area ratio Sdr other advantages are: non-contact measurement, short time of assessment, a high degree of precision, and relatively simple measurement apparatus. An additional argument for choosing this method is the possibility of supporting it with the image processing and analysis techniques. This increases it's potential future applications, especially regarding the processing, analysis and visualization of the measuring data in the form of acquired static digital images. The method described can be an alternative to the other methods used in grinding tools' diagnostics.
Using the optical profilometry described earlier in Sect. 3, measurements were made of some surfaces of the grinding wheel active surface with visible smears. The results of these measurements were then analyzed using the TalyMap Universal 3.1.0 software. Table 3 describes the characteristics of calculated selected parameters of the GSS chosen for the analysis of the measured microtopographies. First, a measurement was taken of the area, being a fragment of the grinding wheel active surface (size 19×5 mm), and then it was divided and leveled into the areas of 4 × 4 mm, corresponding to the 3 different working zones. On the basis of the topographies thus prepared, surface parameters were then calculated. The methodology of proceeding and calculating selected parameters of the GSS of the Fig. 13 The methodology of calculating selected GSS parameters of workpiece surface ground using a grinding wheel with zones of various diameter, on the basis of the surface of Titanium Grade 2 ® grinding wheel are shown in Fig. 10. Additionally there was shown an example of surface microtopographies for three different working zones of the grinding wheel after grinding Titanium Grade 2 ® . In Fig. 11 were shown the microgeometric views of different areas of the grinding wheel active surface, together with the values of the GSS parameters, calculated after machining four machined materials considered in the investigation.
The values of an arithmetic mean height Sa in different zones of the grinding wheel active surface are presented as a graph in Fig. 12a. The values did not vary to any great extent and their dispersion was never higher than 30 %. The calculated values of this parameter are mainly the result of the open structure of the grinding wheels used in the experimental investigations.
If one compares the values of kernel roughness depth Sk for different zones of the grinding wheel active surface (Fig. 12b), the influence of the smears upon the active surface, in different zones of the grinding wheel, is clearly visible. As the time and depth of grinding increases, so does the phenomenon of microsmearing of the abrasive grains and smearing of the intergranular free spaces, which reflect the changes in the value of kernel roughness depth Sk. The value of this parameter may also be influenced by the dullness of the abrasive grain's vertex. A substantial number of smears in Zone III of the measured grinding wheel's fragment are visible after grinding Titanium Grade 2 ® and INCOLOY ® alloy 800HT ® . In both the cases the value of the parameter for Zone III was c.a. 40 % lower in comparison to the value in Zone I.
The level of the Sdr parameter's values, which describes the degree of surface development for different functional zones of the grinding wheel active surface, has been much lower for the surface of the grinding wheel after grinding hard-to-cut materials, in comparison to the results after grinding 100Cr6 steel (Fig. 12c). This is further evidence of the parameter's sensitivity to smears upon the grinding wheel active surface, as (according to visual analysis of the grinding wheels under investigation) they did not appear in the case of grinding 100Cr6 steel.
The highest difference between Zone I (Sdr = 274 %) and III (Sdr = 160 %), namely that of 70 %, was observed in the case of the grinding wheel active surface after grinding Titanium Grade 2 ® . Such a great change in the value was caused by extensive smearing of intergranular free spaces, with smears clearly visible in the obtained microtopographies (Fig. 11) and in the micrograph visible in Fig. 3. For the remaining machined materials, the values of the Sdr parameter for each zone didn't show differences higher than that of 30 %.
In the case of the microtopography of the surface elements, analogical proceedings were used after grinding in order to calculate the parameters describing the GSS in three different machining zones (Fig. 13). The only difference was the fact that in the beginning a microtopography of 18 × 5 mm was measured for the machined surfaces. Figure 14 shows the microtopographical images of different zones of the workpiece surfaces, and the calculated values of selected parameters of the surface's geometrical structure, taken int account during the investigation of the materials.
In the case of most of the investigated materials there is a visible increase in the value of the GSS parameters for subsequent grinding zones (Fig. 15). It means that as the grinding depth increases, so do the cross-sections of the layers ground Fig. 15 The changes in calculated selected GSS parameters for three zones of the surface of Titanium Grade 2 ® , INCONEL ® alloy 600, INCOLOY ® alloy 800HT ® and 100Cr6 steel: a arithmetic mean height Sa, b total height of the surface St, c kernel roughness depth Sk, d developed interfacial area ratio Sdr with single abrasive grains which shape the surface of higher roughness.
The values of selected parameters calculated for the surface of Titanium Grade 2 ® after grinding with machining zone III of the grinding wheel active surface are characteristically high. This results from the fact that, in the case of titanium, changing the depth and time of grinding had a much higher influence on the structure of the machined surface. This was probably influenced by the increasing temperature in the contact zone between the grinding wheel and the machined material.
The highest roughness of the machined surface of titanium in Zone III did not only become apparent in the case of the Sdr value of surface development (Fig. 15d). The highest Sdr values were calculated for the surface of Zone III of INCONEL ® alloy 600 and Zones II and III of INCOLOY ® alloy 800HT ® .
The calculated values of selected GSS parameters of the machined element clearly indicates the significant influence of the grinding parameters, here together with the condition of grinding wheel active surface, and especially the degree to which smearing is present.
The data presented here clearly indicate a correlation in the calculated values of parameters describing the acquired images of scattered light and the parameters describing the geometrical structure of both the grinding wheel active surface and the surface of the machined elements.
Parameter analysis of the results of the measurement techniques adopted here, together with visual analysis of the micrographs have shown a higher degree of smearing upon the grinding tools active surface after grinding the hard-to-cut materials. Among the samples chosen for the investigation, those which showed the largest areas of smears were made of Titanium Grade 2 ® -a fact additionally supported by the presented measurement results.

Conclusion
The results obtained from these investigations allow for a statement that the methods proposed in the article could be used for the purposes of assessing the condition of the grinding wheel active surface. This pertains to both its structure and the nonspecific elements which appeared on its surface as a result of different harmful phenomena accompanying the machining process. One of these nonspecific elements, described in this paper, is smearing. The intensive growth of smears leads to a decrease of the machining capacity of the grinding wheel, a reduction in its efficiency, increases in its grinding power and the participation of friction in the process. It also results in increasing the temperature in the contact zone between the grinding wheel and the workpiece, which intensifies the appearance of such grinding defects as grinding burns. In this context, proper diagnostics of grinding tools turns out to be a problem of relatively high importance. As the research results presented here have shown, the methods the authors used may be an interesting way of solving this problem. They may also be an alternative to the methods already in use. Laser scatterometry is characterized by high sensitivity, which allows for precise location and identification of smears. Supporting it with computer image processing and analysis techniques makes it possible to run an advanced image analysis in order to obtain more detailed information about the condition of the surface under investigation. The considerable usefulness of the methods used may allow for their future application in low-cost industrial, or laboratory, measuring apparatus with the characteristic improvements of non-contact measuring, short-time measurements and high accuracy.