Functional and Operational Requirements to Be Fulﬁlled by Conical Picks Regarding Their Wear Rate and Investment Costs

: User’s choices of conical picks currently involve specifying their material and geometric parameters (functional requirements), incorporating the place and conditions of their work (opera-tional requirements). The selection is usually made based on solely one criterion, which is the price. Thus, at the stage of both purchase and operation, the quality of picks, as well as their matching for a speciﬁc machine, are not assessed. The problem of deﬁning whether the producer has fulﬁlled the user’s requirements arises only when the user questions the picks’ quality. Analysis of this problem has resulted in developing assumptions, concepts and research procedures based on the cutting process requirements. The procedure allows conducting tests to determine the geometric parameters of a pick, the type of material of the pick body and WC-Co insert, as well as the pick wear rate (intensity). The C 2 index describes the wear rate (intensity)—the smaller its value, the slower the pick’s wear. Laboratory tests were carried out at the AGH University of Science and Technology in Krak ó w, Poland. Following the developed method and procedure, the quality of picks was precisely and unambiguously assessed. The C 2 index, apart from testing the quality of picks, was also used to forecast their wear. Based on the C 2 index, a method is proposed to estimate the wear rate of conical picks provided by different manufacturers and determine the acceptable unit price and operating costs. Thus, it is possible and reasonable to precisely deﬁne the investment requirements and appropriately select the pick. Relevant tests were carried out for eight different types of conical picks used in roadheaders, longwall shearers and shaft-boring roadheaders.


Introduction
Mining minerals consists of separating their parts from the whole using well-known mining methods, appropriate technology and machines. These methods can be divided into mechanical methods and blasting [1][2][3]. Milling, ploughing or drilling are widely used to extract minerals in underground mines [4][5][6][7][8][9][10], whereas digging and picking are open-pit technologies [11]. It is strictly borehole technologies that rely only on drilling, but in multi-step bits, conical picks are also used [6,12].
Machines and devices realising these processes are dedicated to each method and technology of exploitation (underground, open-pit, borehole). In underground mining, these are longwall shearers [3,10,13], roadheaders [2,[5][6][7]14] and shaft shearers [15] or ploughs [9,10], whereas in road construction and open-pit mining, these are road milling machines, excavators and rippers [11]. The machines are equipped with cutting elements or units called cutting heads. The largest group of cutting heads are the milling heads ( Figure 1) equipped with cutting picks, i.e., radial, tangent and conical picks ( Figure 2). The latter are mainly applied in the mining of natural and artificial minerals [16][17][18][19]. They are widely used not only in underground or open-pit mining, but also in the construction industry.
are widely used not only in underground or open-pit mining, but also in the construction industry.  [13,14,20]. Figure 1. Cutting heads of: (a) a longwall shearer; (b) a roadheader; and (c) a road milling machine [13,14,20]. Conical picks are used in cutting heads due to their excellent durability, greater than radial or tangent picks. They have a specific structure, where the pick body is a solid of revolution, consisting of working and handle parts [21][22][23]. The working part is equipped with a WC-Co insert, which is a pick tip [24][25][26][27]. On the other hand, the pick holder's Conical picks are used in cutting heads due to their excellent durability, greater than radial or tangent picks. They have a specific structure, where the pick body is a solid of revolution, consisting of working and handle parts [21][22][23]. The working part is equipped with a WC-Co insert, which is a pick tip [24][25][26][27]. On the other hand, the pick holder's gripping part is equipped with a spring-ring, a spring sleeve or a Hert ring [23]. This prevents the pick from falling out of the holder during cutting.
The picks are installed in pick holders, furnished on the cutting head's lateral surface, and together form a system of picks. The rotary mounting of the picks in the holders enables their stochastic rotation and symmetrical wear [17,18,21] (Figure 3a,c). This process occurs if certain conditions related to the cutting angles are obtained [3,[28][29][30][31][32]. Otherwise, the picks are blocked in the holders, and their wear is asymmetrical (Figure 3b). There is also the risk of the tip falling out (Figure 3d). This is why the selecting process of the design parameters of the picks and holders is so crucial, considering the geometrical and kinematic parameters of the cutterhead and the cutting machine [2,3,28]. Concurrently, mining and geological conditions must be considered, particularly the mineral's properties when being extracted (cutting resistance, abrasivity) [33][34][35][36][37]. Hence, the user should apply conical picks of the quality required for the premised cutting efficiency and costs [29]. with the tip falling out [37].
The required quality of conical picks is related to their appropriate construction (cutting angles, dimensions), material parameters of the pick body (chemical composition, hardness, microstructure, steel grade), and the pick tip (dimensions, HV30 hardness, density, chemical composition-determination of the mass fraction of carbide phase (%WC), matrix (%Co) and WC grain size).
A better quality of pick increases the price. This translates into costs involved in purchasing picks and the costs associated with their replacements (operational costs) [29]. Therefore, the quality of picks should be relevant to the required design, material parameters and the rate of their wear. This becomes very important when a user is buying picks from various manufacturers and at different prices. Therefore, the user firstly should pinpoint their requirements and expectations concerning the design and material parameters of the picks. Secondly, a control method should be defined. Then, a decision, as to which picks meet the user's requirements and expectations, is required, with a price adequate to its quality and the assumed cutting efficiency. Hence, a method of determining the quality of conical picks and a procedure for their selection should be applied [28,29].
Based on this methodology and the procedures contained therein, different manufacturers' conical picks are tested to compare and assess the picks themselves. This is especially advisable when placing new conical pick designs or material solutions on the market. The required quality of conical picks is related to their appropriate construction (cutting angles, dimensions), material parameters of the pick body (chemical composition, hardness, microstructure, steel grade), and the pick tip (dimensions, HV30 hardness, density, chemical composition-determination of the mass fraction of carbide phase (%WC), matrix (%Co) and WC grain size).
A better quality of pick increases the price. This translates into costs involved in purchasing picks and the costs associated with their replacements (operational costs) [29]. Therefore, the quality of picks should be relevant to the required design, material parameters and the rate of their wear. This becomes very important when a user is buying picks from various manufacturers and at different prices. Therefore, the user firstly should pinpoint their requirements and expectations concerning the design and material parameters of the picks. Secondly, a control method should be defined. Then, a decision, as to which picks meet the user's requirements and expectations, is required, with a price adequate to its quality and the assumed cutting efficiency. Hence, a method of determining the quality of conical picks and a procedure for their selection should be applied [28,29]. Based on this methodology and the procedures contained therein, different manufacturers' conical picks are tested to compare and assess the picks themselves. This is especially advisable when placing new conical pick designs or material solutions on the market.
For some time now, pick manufacturers have been using various forms of protection of the working part of the pick's surface [22,33,36] (Figure 4). This is often performed by hardfacing with various materials. Their hardness is higher than the native material used to make the body of the pick. In consequence, the abrasion resistance of the working part is better. However, this study revealed that the durability of picks with protected surfaces was not significantly increased compared to standard picks [37]. Hence, based on the research conducted thus far, it has been concluded that investigations need to be continued to find the most effective and cheapest protection and the technology for its implementation.
Energies 2021, 14, x FOR PEER REVIEW 5 of 20 picks were tested according to the methodology described below, and the procedures contained therein (a measurement of constructional and material parameters, wear rate) to assess their quality and wear rate (durability).

Test Procedure for Conical Picks
The problem mentioned above related to assessing the quality of conical picks has been solved by developing a method and an appropriate procedure for experimentation [28,29,36,37]. For the contemporary work of conical picks, the following elements have been considered: • The measurement of geometric parameters of the whole pick; • The measurement of geometric parameters and determining the shape of the pick tip in the form of a WC-Co insert; • Testing of pick body material parameters; • Testing of pick tip material parameters; • Determination of the pick wear rate in laboratory conditions. Therefore, the tests were carried out in three steps. The first step included measuring the geometric parameters; the second step was material testing; and the third step was measuring the wear rate of conical picks at a laboratory test stand.
The measuring of geometric parameters was conducted first for the whole pick. Next, (c) pick with a ring made of WC-Co; (d) pick with two rings made of WC-Co; (e) pick with a lowered body and WC-Co insert in the CAP type [28].
For this purpose, the researchers obtained eight different types of conical picks used on the cutterheads of roadheaders, longwall shearers and shaft-boring roadheaders. These picks were tested according to the methodology described below, and the procedures contained therein (a measurement of constructional and material parameters, wear rate) to assess their quality and wear rate (durability).

Test Procedure for Conical Picks
The problem mentioned above related to assessing the quality of conical picks has been solved by developing a method and an appropriate procedure for experimentation [28,29,36,37]. For the contemporary work of conical picks, the following elements have been considered: Therefore, the tests were carried out in three steps. The first step included measuring the geometric parameters; the second step was material testing; and the third step was measuring the wear rate of conical picks at a laboratory test stand.
The measuring of geometric parameters was conducted first for the whole pick. Next, the pick tip, in the form of a WC-Co insert fixed in the pick body by hard soldering, was obtained in the machining process. Then, it was possible to test the geometric parameters of the WC-Co insert.
The most essential linear and angular dimensions of a conical pick are ( Figure 5): •  . Two-and single-stage conical pick with its measured design parameters [28,29].
The linear dimensions were measured using an altimeter (maximum permissible error ±40 μm) and a calliper (maximum error ±30 μm), while the angles were measured with a protractor characterised by a maximum permissible error of ±2°). Figure 6 presents picks on this particular measuring base, equipped with an altimeter, calliper and protractor.
The next part of the research procedure was material testing, which requires separating the tip from the steel body. Consequently, separate tests should be carried out for two different materials (Figure 7). . Two-and single-stage conical pick with its measured design parameters [28,29].
The linear dimensions were measured using an altimeter (maximum permissible error ±40 µm) and a calliper (maximum error ±30 µm), while the angles were measured with a protractor characterised by a maximum permissible error of ±2 • ). Figure 6 presents picks on this particular measuring base, equipped with an altimeter, calliper and protractor.  The chemical composition of the pick body material was analysed by the spark method using a Foundry Master device (Figure 8a), whereas the hardness measurement was carried out using the Rockwell method (Figure 8b), following the standard [38], on samples taken from the working part and the gripping part.
WC-Co inserts (pick tips) have a particular chemical composition and mechanical properties primary for the cutting process [24][25][26][27]. However, measuring all the parameters of the WC-Co insert leads to a significant increase in research costs while being relatively insignificant for the valuation of the insert suitability for mining a specific rock. The next part of the research procedure was material testing, which requires separating the tip from the steel body. Consequently, separate tests should be carried out for two different materials (Figure 7).  The chemical composition of the pick body material was analysed by the spark method using a Foundry Master device (Figure 8a), whereas the hardness measurement was carried out using the Rockwell method (Figure 8b), following the standard [38], on samples taken from the working part and the gripping part.
WC-Co inserts (pick tips) have a particular chemical composition and mechanical properties primary for the cutting process [24][25][26][27]. However, measuring all the parameters of the WC-Co insert leads to a significant increase in research costs while being relatively insignificant for the valuation of the insert suitability for mining a specific rock. The chemical composition of the pick body material was analysed by the spark method using a Foundry Master device (Figure 8a), whereas the hardness measurement was carried out using the Rockwell method (Figure 8b), following the standard [38], on samples taken from the working part and the gripping part. Therefore, considering the costs of the test and their subsequent application, it is recommended to perform a quantitative analysis of density and hardness for a mentioned WC-Co insert.
Quantitative analysis of the chemical composition of the WC-Co insert was performed by the X-ray method, using a Mini Pal4 EDXRF Analyser (Energy Dispersive Xray Fluorescence), produced by Malvern Panalytical (Malvern, UK). The specific density was determined by the hydrostatic weighing method [39], and hardness by the Vickers method [40], using an HPO-250 hardness tester ( Figure 8b).
Quantitative description of the WC-Co insert microstructure in the cobalt matrix was performed using the Met-Ilo automated image analysis computer software [41]. The microstructure images for analysis were captured with a DM 4000 light microscope, produced by Leica (Wetzlar, Germany), at 200-and 500-times magnification.
Measurement of the wear rate of conical picks leads to determining their durability. It must always be conducted under the same conditions so that the results are reliable, reproducible and comparable. This enables assessment of the pick durability, but it also allows comparing picks from different manufacturers.
To evaluate the conical picks' durability, their wear rate index was adopted for tests, as described by the C2 index, which is defined as the total loss of mass of the pick or picks concerning the volume of the mined rock obtained (Equation (1)).

= ∆ • [-]
(1) where C2 is the pick wear rate index based on mass, Δm is the loss of pick mass during tests (body with the tip) [g], m is the pick mass before tests [g], Vw is the sample standard volume [m 3 ], and Vun is the volume of sample mined during tests [m 3 ].
The definitions of the above parameters and the requirements for testing the wear rate of conical picks necessitate the use of the following test methodology: • Cutting an artificial cement-sand sample with almost-isotropic properties; • Mining by cutting in laboratory conditions; • Measurement pick's mass before and after the cutting process; • Measurement of the volume of rock cut. WC-Co inserts (pick tips) have a particular chemical composition and mechanical properties primary for the cutting process [24][25][26][27]. However, measuring all the parameters of the WC-Co insert leads to a significant increase in research costs while being relatively insignificant for the valuation of the insert suitability for mining a specific rock. Therefore, considering the costs of the test and their subsequent application, it is recommended to perform a quantitative analysis of density and hardness for a mentioned WC-Co insert.
Quantitative analysis of the chemical composition of the WC-Co insert was performed by the X-ray method, using a Mini Pal4 EDXRF Analyser (Energy Dispersive X-ray Fluorescence), produced by Malvern Panalytical (Malvern, UK). The specific density was determined by the hydrostatic weighing method [39], and hardness by the Vickers method [40], using an HPO-250 hardness tester ( Figure 8b).
Quantitative description of the WC-Co insert microstructure in the cobalt matrix was performed using the Met-Ilo automated image analysis computer software [41]. The microstructure images for analysis were captured with a DM 4000 light microscope, produced by Leica (Wetzlar, Germany), at 200-and 500-times magnification.
Measurement of the wear rate of conical picks leads to determining their durability. It must always be conducted under the same conditions so that the results are reliable, reproducible and comparable. This enables assessment of the pick durability, but it also allows comparing picks from different manufacturers.
To evaluate the conical picks' durability, their wear rate index was adopted for tests, as described by the C2 index, which is defined as the total loss of mass of the pick or picks concerning the volume of the mined rock obtained (Equation (1)). Laboratory tests need to be conducted in a unique test stand that meets the adopted test methodology requirements. Therefore, the tests were carried out at a unique laboratory test stand for testing the cutting process with single cutting tools or heads belonging to the AGH University of Science and Technology, Kraków, Poland ( Figure 9). in the horizontal plane, allowed the cutting process within a particular area. The hydraulic advance mechanism enabled the base to be moved together with the rock block. The engine revolutions were regulated by the control system located in the control cabinet, while a hydraulic power unit regulated the value of advance rate mining and sumping.
A sample could be placed on the advance mechanism base. In the case of testing the wear rate of conical picks, it is advisable to cut artificial rock (cement-sand sample), which has strong abrasive properties. The test stand was equipped with a measuring system: comprising a torque meter, pressure transducers, distance transducers, and measuring computer. These measured the load of the cutting element, the speed and pressure in the advance system, and, consequently, determined the resistance or energy consumption of the cutting process.  The test stand consists of two units: a cutting head drive and a block advance system intended for mining. The working element is a unique drum with commercial holders of single-stage and two-stage conical picks.
The rotations of the head and the rectilinear, reciprocating motion of the rock sample in the horizontal plane, allowed the cutting process within a particular area. The hydraulic advance mechanism enabled the base to be moved together with the rock block. The engine revolutions were regulated by the control system located in the control cabinet, while a hydraulic power unit regulated the value of advance rate mining and sumping.
A sample could be placed on the advance mechanism base. In the case of testing the wear rate of conical picks, it is advisable to cut artificial rock (cement-sand sample), which has strong abrasive properties. The test stand was equipped with a measuring system: comprising a torque meter, pressure transducers, distance transducers, and measuring computer. These measured the load of the cutting element, the speed and pressure in the advance system, and, consequently, determined the resistance or energy consumption of the cutting process. Four picks mounted in holders of the testing drum were used ( Figure 10) to calculate their wear rate. Before testing, each pick had to be marked to enable accurate identification after the tests (position on the test drum).
Four picks mounted in holders of the testing drum were used ( Figure 10) to calculate their wear rate. Before testing, each pick had to be marked to enable accurate identification after the tests (position on the test drum).
Picks were weighed both before and after tests with a certificated laboratory scale (verification scale interval 1 g). The obtained weight loss and the excavated rock sample volume were used to calculate the C2 index. It should be emphasised that the smaller the value of the C2 index, the smaller the pick wear. In this case, laboratory tests are particularly recommended, but most importantly, they need to be carried out at a unique test stand that meets the adopted test methodology requirements. Figure 10. Diagram of the applied pick system with the marking of picks' numbers [21,28,29,37].

Price of the Conical Pick and Its Wear Rate versus Cutting Efficiency
The previously described method and its implementation allow for checking the geometric and material parameters of the conical pick body and the tip, as well as the wear rate index, C2.
Therefore, it is reasonable to establish the correlation of the C2 index with the number of worn picks and the cutting efficiency. The dependency (1) shows that the higher the C2 index value, the faster the pick wears out; thus, it has a lower quality. Using these picks on the cutting drum results in faster wear and a decrease in cutting efficiency. This is caused by a more frequent change of picks and downtime of the mining machine (milling machine). In [29], dependency (2) was given, which indicated when slower wearing and more expensive picks could be used without increasing the purchase costs of picks (3). This condition makes it possible to set the maximum price of a better-quality pick, which results in savings in terms of investment and operation (the cost of purchasing picks, transport, and replacement).
Assuming that the price of a lower-quality pick Cn2 is equal to PLN 80 (approx. EUR 20) per piece and the C22:C21 ratio is equal to 1.5, the price for a better-quality pick Cn1(C21) cannot exceed PLN 120 (approx. EUR 30). However, the C22:C21 ratio may assume different values depending on the C2 index values for individual picks obtained through empirical tests. The ratio is always higher than one when C22 is higher than C21.
Picks were weighed both before and after tests with a certificated laboratory scale (verification scale interval 1 g). The obtained weight loss and the excavated rock sample volume were used to calculate the C2 index. It should be emphasised that the smaller the value of the C2 index, the smaller the pick wear. In this case, laboratory tests are particularly recommended, but most importantly, they need to be carried out at a unique test stand that meets the adopted test methodology requirements.

Price of the Conical Pick and Its Wear Rate versus Cutting Efficiency
The previously described method and its implementation allow for checking the geometric and material parameters of the conical pick body and the tip, as well as the wear rate index, C2.
Therefore, it is reasonable to establish the correlation of the C2 index with the number of worn picks and the cutting efficiency. The dependency (1) shows that the higher the C2 index value, the faster the pick wears out; thus, it has a lower quality. Using these picks on the cutting drum results in faster wear and a decrease in cutting efficiency. This is caused by a more frequent change of picks and downtime of the mining machine (milling machine). In [29], dependency (2) was given, which indicated when slower wearing and more expensive picks could be used without increasing the purchase costs of picks (3). This condition makes it possible to set the maximum price of a better-quality pick, which results in savings in terms of investment and operation (the cost of purchasing picks, transport, and replacement).
Assuming that the price of a lower-quality pick C n2 is equal to PLN 80 (approx. EUR 20) per piece and the C2 2 :C2 1 ratio is equal to 1.5, the price for a better-quality pick C n1 (C2 1 ) cannot exceed PLN 120 (approx. EUR 30). However, the C2 2 :C2 1 ratio may assume different values depending on the C2 index values for individual picks obtained through empirical tests. The ratio is always higher than one when C2 2 is higher than C2 1 . Similarly, from Equations (4) and (5), it is possible to determine the maximum price of a better-quality pick concerning the possible decrease in the cutting efficiency for lowerquality and cheaper picks.
where ∆V u is the volume of mined rock [m 3 ], γ w is the density of mined rock [kg/m 3 ], C jur is the unit price of mined rock in relation to its volume [PLN/m 3 ], C jum is the unit price of mined rock in relation to its mass [PLN/kg], and n is the number of picks to be replaced after one cutting cycle [pcs]. Bearing in mind the above dependencies, in most cases, better-quality picks should be used [29].

Research Material
Five pieces of each type of conical pick were assigned for testing. One pick was intended for material tests, and the other four were for measuring the design parameters and the wear rate. The pick's bodies were made of various grades of steel and reinforced with various cemented carbide (WC-Co) inserts. The surface of the working part of the pick body was protected with various padding welds-reinforced picks (Figure 11b-f)-or was not protected-standard picks (Figure 11a,c,g,h). The following picks were used for testing:
Energies 2021, 14, x FOR PEER REVIEW 11 Similarly, from Equations (4) and (5), it is possible to determine the maximum p of a better-quality pick concerning the possible decrease in the cutting efficiency for low quality and cheaper picks.
where ∆Vu is the volume of mined rock [m 3 ], γw is the density of mined rock [kg/m 3 ], is the unit price of mined rock in relation to its volume [PLN/m 3 ], Cjum is the unit pric mined rock in relation to its mass [PLN/kg], and n is the number of picks to be repla after one cutting cycle [pcs]. Bearing in mind the above dependencies, in most cases, better-quality picks sho be used [29].

Research Material
Five pieces of each type of conical pick were assigned for testing. One pick was tended for material tests, and the other four were for measuring the design parame and the wear rate. The pick's bodies were made of various grades of steel and reinfor with various cemented carbide (WC-Co) inserts. The surface of the working part of pick body was protected with various padding welds-reinforced picks (Figure 11bor was not protected-standard picks (Figure 11a,c,g,h). The following picks were u for testing:

Conical Pick Testing Results and Discussion
The selected conical picks were tested in accordance with the previously descri procedure. The obtained results allowed for evaluating the picks and formulating con sions regarding their design and material parameters as well as the wear rates.

Determination of Design Parameters
The parameters obtained from the measurements of construction dimensions' a age values are summarised for each pick in Table 1. The designations of individual rameters in the table are consistent with the designations in Figure 6. Measurement of dimensions of the tips in the form of sintered carbide (WC-Co) inserts was possible o for one of the five picks, which was intended for material testing. For this purpose, insert was extruded from the pick body (Figure 12). At the same time, the body of same pick was subjected to material tests.
The results of measurements of conical picks' construction dimensions indicate the obtained cutting angles are correct. Therefore, the rate of the picks' wear can be tes at a laboratory test stand. However, notably, the tips of two picks for shaft-boring ro headers had a different shape (Figure 12d,f). In both cases, they were mushroom-shap unlike the others, which were cylindrical-conical.

Conical Pick Testing Results and Discussion
The selected conical picks were tested in accordance with the previously described procedure. The obtained results allowed for evaluating the picks and formulating conclusions regarding their design and material parameters as well as the wear rates.

Determination of Design Parameters
The parameters obtained from the measurements of construction dimensions' average values are summarised for each pick in Table 1. The designations of individual parameters in the table are consistent with the designations in Figure 6. Measurement of the dimensions of the tips in the form of sintered carbide (WC-Co) inserts was possible only for one of the five picks, which was intended for material testing. For this purpose, the insert was extruded from the pick body (Figure 12). At the same time, the body of the same pick was subjected to material tests.
The results of measurements of conical picks' construction dimensions indicate that the obtained cutting angles are correct. Therefore, the rate of the picks' wear can be tested at a laboratory test stand. However, notably, the tips of two picks for shaft-boring roadheaders had a different shape (Figure 12d,f). In both cases, they were mushroom-shaped, unlike the others, which were cylindrical-conical. same pick was subjected to material tests. The results of measurements of conical picks' construction dimensions indicate that the obtained cutting angles are correct. Therefore, the rate of the picks' wear can be tested at a laboratory test stand. However, notably, the tips of two picks for shaft-boring roadheaders had a different shape (Figure 12d,f). In both cases, they were mushroom-shaped, unlike the others, which were cylindrical-conical.

Determination of Material Parameters
As mentioned before, one pick from each batch of picks was intended for material testing. For this purpose, samples from the steel body of each pick were obtained in the process of machining (Figure 13a,b), and tips in the form of sintered carbide inserts ( Figure  13c) were obtained.

Determination of Material Parameters
As mentioned before, one pick from each batch of picks was intended for material testing. For this purpose, samples from the steel body of each pick were obtained in the process of machining (Figure 13a,b), and tips in the form of sintered carbide inserts ( Figure 13c) were obtained.

Determination of Material Parameters
As mentioned before, one pick from each batch of picks was intended for material testing. For this purpose, samples from the steel body of each pick were obtained in the process of machining (Figure 13a,b), and tips in the form of sintered carbide inserts ( Figure  13c The results of measurements of the tested picks' material parameters have been provided in tables, i.e., Tables 2-6, where the results of measurements for the pick used in roadheaders are presented (Figure 11a). The tables contain information on the hardness of the working and gripping parts, the chemical composition of the steel comprising the tested pick's body, as well as the hardness of the WC-Co insert and the value of its density. Based on this, the grade of steel used to make the bodies of the tested picks [42] and the types of carbides used to make their tips were determined. The type (symbol) of sintered carbide was determined using the PN-88/H-89500 polish standard [43], classifying sintered carbides based on WC and Co percentages, density, HV30 hardness and other mechanical parameters, and also indicates the place of their application. In the case of picks reinforced with padding weld, their hardness was additionally measured (Figure 14). The results of material tests for individual picks are given in Table 7. Table 2. Results of measurements of HRC hardness of the gripping part of the pick for roadheaders (Figure 11a).  Table 3. Results of measurements of HRC hardness of the working part of the pick for roadheaders (Figure 11a). Measurement Site Number   1  2  3  4   1  1  35  35  35  35  2  3  36  34  36  35  3  10  35  30  37 36 Table 4. Chemical composition of the material (steel) of the conical pick for roadheaders (Figure 11a). The chemical elements, the content of which is underestimated or overestimated in relation to the standard, are marked in red. Table 5. Results of measurements of HV30 hardness of the sintered carbide insert of the pick for roadheaders (Figure 12a). Average  1  2  3  4  5  6  7  8  9  10   1  1076  1034  1041  1075  1111  1071  1057  1081  1125  1135  1081   Table 6. Results of measurements of the density of the sintered carbide insert of the pick for roadheaders (Figure 12a).    The test results contained therein show that, with the exception of picks 5 and 6 whose bodies were made of manganese alloy steel (36Mn6), the remaining picks were made of chrome-manganese (100CrMn6) steel. The hardness of the working part of the picks was mostly above 40 HRC; its value was lower-38 HRC only in the case of pick No. 8. A similar situation was observed in the case of the tested picks' gripping parts. The hardness values of the gripping parts of picks 1 and 2 (35 HRC) were lower than that of the working parts (over 40 HRC), which is an advantage because the pick-holder sockets are not knocked out. On the other hand, the values of padding welds hardness were much higher, exceeding 60 HRC.

No
Measurements of the hardness and density of the WC-Co inserts allowed for determining their grade. B23 cemented carbide inserts are recommended for cross-bits and drills (picks 6,8). The properties of these carbides were intermediate, between those of B2 and B45 carbides. B40 carbides are intended for reinforcing the tips of radial tools and rotary drilling tools (drill bits, multi-step bits, picks 3 and 8), whereas B45 carbides are used to reinforce the tips of conical picks, cross-bits and drills (picks 1, 2). G15 (pick 5) and G30 (pick 7) carbides are recommended for insert bits and discs [17,19,34,44].
For WC-Co cemented carbide inserts, quantitative analysis of the WC carbide phase was also performed. For this purpose, the test specimens were prepared by the traditional method of grinding and polishing metallographic specimens using polish suspensions with a diamond grain, with coarseness values of 3 μm, 1 μm and 0.25 μm. The prepared   The test results contained therein show that, with the exception of picks 5 and 6 whose bodies were made of manganese alloy steel (36Mn6), the remaining picks were made of chrome-manganese (100CrMn6) steel. The hardness of the working part of the picks was mostly above 40 HRC; its value was lower-38 HRC only in the case of pick No. 8. A similar situation was observed in the case of the tested picks' gripping parts. The hardness values of the gripping parts of picks 1 and 2 (35 HRC) were lower than that of the working parts (over 40 HRC), which is an advantage because the pick-holder sockets are not knocked out. On the other hand, the values of padding welds hardness were much higher, exceeding 60 HRC.
Measurements of the hardness and density of the WC-Co inserts allowed for determining their grade. B23 cemented carbide inserts are recommended for cross-bits and drills (picks 6,8). The properties of these carbides were intermediate, between those of B2 and B45 carbides. B40 carbides are intended for reinforcing the tips of radial tools and rotary drilling tools (drill bits, multi-step bits, picks 3 and 8), whereas B45 carbides are used to reinforce the tips of conical picks, cross-bits and drills (picks 1, 2). G15 (pick 5) and G30 (pick 7) carbides are recommended for insert bits and discs [17,19,34,44].
For WC-Co cemented carbide inserts, quantitative analysis of the WC carbide phase was also performed. For this purpose, the test specimens were prepared by the traditional method of grinding and polishing metallographic specimens using polish suspensions with a diamond grain, with coarseness values of 3 µm, 1 µm and 0.25 µm. The prepared specimens were subjected to chemical etching in order to reveal the boundaries of WC carbide grains (Figure 15) [45]. carbide grains (Figure 15) [45].
WC grains were measured for all tested WC-Co inserts. The average WC particle size ranged from 4.78 μm to 6.15 μm, and the cobalt content from 8.4% to 9.5%.
Analytical and empirical studies have shown that the best properties of WC-Co inserts for mining tools are 9% cobalt content (hardness, impact toughness). The size of the WC grains is also important. Based on the literature [24] and our previous work, the average grain size should be above 3 μm.

Determination of the Rate (Intensity) of Wear
To determine the rate (intensity) of wear of the tested picks, first, the values of their mass before and after the test were determined. Next, the cutting parameters (advance speed, revolutions) were set, and efforts were made to ensure the same conditions for each set of picks. On the same sand-cement sample with uniaxial compressive strength equal to 35 MPa, all picks were tested. The results of the tests and the determined values of the C2 index for individual picks are summarised in Table 8. Based on these results, a table of C2 index values was prepared depending on the pick's location in the test drum ( Figure  10, Table 8). Similarly, Figure 16 shows the C2 index point values for individual picks as a function of the test drum's location.  WC grains were measured for all tested WC-Co inserts. The average WC particle size ranged from 4.78 µm to 6.15 µm, and the cobalt content from 8.4% to 9.5%.
Analytical and empirical studies have shown that the best properties of WC-Co inserts for mining tools are 9% cobalt content (hardness, impact toughness). The size of the WC grains is also important. Based on the literature [24] and our previous work, the average grain size should be above 3 µm.

Determination of the Rate (Intensity) of Wear
To determine the rate (intensity) of wear of the tested picks, first, the values of their mass before and after the test were determined. Next, the cutting parameters (advance speed, revolutions) were set, and efforts were made to ensure the same conditions for each set of picks. On the same sand-cement sample with uniaxial compressive strength equal to 35 MPa, all picks were tested. The results of the tests and the determined values of the C2 index for individual picks are summarised in Table 8. Based on these results, a table of C2 index values was prepared depending on the pick's location in the test drum ( Figure 10, Table 8). Similarly, Figure 16 shows the C2 index point values for individual picks as a function of the test drum's location.  The above comparisons clearly show a strong correlation between the pick's wear rate and their position on the test drum. The wear rate was the fastest for picks in position one and two, followed by picks in position four, whereas picks in position three wore the most slowly. It can also be observed that a specific pick wears differently depending on the position on the test drum, as already mentioned, although this wear expressed by C2 index correlated with its quality.
The average values for individual picks are summarised in Table 9 (C2, the presence of padding weld, type of cemented carbide insert, and the hardness of the working part).
It can be observed that the tested picks considerably differed in their wear rate, which is puzzling and does not depend or depends only to a minimal extent on the protective layer of the working part (padding weld). The lowest wear (C2 index) was observed in the case of the pick in position 1 ( Table 9). The next pick in the table had a 30% higher C2 index value. In contrast, the C2 index value of the third pick was twice as high. The C2 index values for the fourth and fifth pick were almost four times higher. The C2 index value for the sixth pick was six times higher, and for the seventh and eighth pick, this rose to eight times higher. It is also worth noting that the padding weld and carbide grade do not determine the wear rate of the pick, as with the hardness of the working part.
Generally, it is possible to use the C2 index to assess the pick's wear resistance. The lower the C2 index value, the longer the pick should cut while meeting the cutting process requirements. As a result, it is possible to forecast the value of picks' wear and, consequently, their purchase cost. In this case, it is easy to determine the maximum price of a pick with a lower C2 index value by referring it to the price of picks with a higher C2 index value. It is enough to multiply the prices of the picks by the ratio of their C2 index values. The above comparisons clearly show a strong correlation between the pick's wear rate and their position on the test drum. The wear rate was the fastest for picks in position one and two, followed by picks in position four, whereas picks in position three wore the most slowly. It can also be observed that a specific pick wears differently depending on the position on the test drum, as already mentioned, although this wear expressed by C2 index correlated with its quality.
The average values for individual picks are summarised in Table 9 (C2, the presence of padding weld, type of cemented carbide insert, and the hardness of the working part).  It can be observed that the tested picks considerably differed in their wear rate, which is puzzling and does not depend or depends only to a minimal extent on the protective layer of the working part (padding weld). The lowest wear (C2 index) was observed in the case of the pick in position 1 ( Table 9). The next pick in the table had a 30% higher C2 index value. In contrast, the C2 index value of the third pick was twice as high. The C2 index values for the fourth and fifth pick were almost four times higher. The C2 index value for the sixth pick was six times higher, and for the seventh and eighth pick, this rose to eight times higher. It is also worth noting that the padding weld and carbide grade do not determine the wear rate of the pick, as with the hardness of the working part.
Generally, it is possible to use the C2 index to assess the pick's wear resistance. The lower the C2 index value, the longer the pick should cut while meeting the cutting process requirements. As a result, it is possible to forecast the value of picks' wear and, consequently, their purchase cost. In this case, it is easy to determine the maximum price of a pick with a lower C2 index value by referring it to the price of picks with a higher C2 index value. It is enough to multiply the prices of the picks by the ratio of their C2 index values.

Conclusions
In the presented method of assessing the quality of conical picks, a three-stage testing process was adopted, consisting of measuring geometric and material parameters and the wear rate, expressed by the C2 index. The tests carried out on eight types of conical picks from different manufacturers have shown that their geometric and material parameters are in accordance with the manufacturers' declarations and users' requirements. However, based on these data, it is only possible to assess the correctness of their manufacture, not their quality. Only when a complete procedure is carried out can the quality of the pick be assessed.
This strategy was confirmed by comparing two picks for shaft-boring roadheaders (Figures 11d and 12e). Both picks were made of the same 36Mn5 steel, their working part hardness was the same, and both were welded, although the pick for the shaft-boring roadheader in Figure 11d had a C2 index value as much as eight times greater than the pick for shaft-boring roadheaders in Figure 11e, which means that its quality is eight times lower.
It follows that particular attention should be paid to the C2 index because, as a result of this research, it does illustrate the wear rate of the pick and thus its durability (quality). However, carrying out a complete procedure also allows conclusions in the future and indicates further research directions to improve the quality of conical picks. For example, it was noticed that further attention should be paid to the correlations between the hardness of the working part and padding weld and the value of the C2 index.
Determining the C2 index, it can also be observed that the position of the pick on the test drum and, presumably, the sequence of entering the cut have a significant influence on the pick wear process. Moreover, the C2 index enables defining various causes related to the value and the price of the picks, as well as the costs of their replacement and reduced cutting efficiency. In most cases, it has been established that picks of better quality (the lowest value of the C2 index) should be used regardless of the price. However, there are cases where a pick's choice depends on its price and losses in cutting.
The C2 index is the parameter that best describes the quality of the pick; therefore, it is recommended that the proposed method and procedure should be used to evaluate picks both when placing orders and to file complaints, but also in other cases, whenever cutting tools need to be selected.