SELECTION OF GRINDING WHEELS FOR THE MACHINING OF PRECISION PARTS WITH INCREASED SURFACE HARDNESS BY LASER

ABSTRACT

The surface hardness of precision parts subjected to laser surface hardening falls within the range of  MPa. The surface phases formed consist of chromium carbides (Cr23C6, Cr3C2, Cr7C3) and chromium nitrides (Cr2N). A literature review reveals that the abrasive, diamond, and electro-diamond machining of laser-hardened precision parts has not been sufficiently studied. To assess the feasibility of machining such parts and to evaluate the effectiveness of various abrasive materials, different grinding wheels were utilized. Analysis indicates that the required machining productivity and surface roughness can be achieved using an AC6-100/80 diamond wheel with 100% concentration. After laser surface hardening, plungers exhibited deformations within the ranges of 5–16 µm for bending (P), 2–8 µm for ovality (Q), and 4–12 µm for conicity (K). During a 4-second grinding process, the ovality of the plunger did not exceed 2 µm, the taper was reduced to 3 µm, and bending was entirely eliminated. The material removal during machining ranged from 25 to 35 µm. The obtained experimental results closely align with theoretical studies, demonstrating high accuracy.

АННОТАЦИЯ

Твердость поверхности прецизионных деталей, подвергшихся лазерному упрочнению, находится в диапазоне   МПа. Формируемые поверхностные фазы состоят из карбидов хрома (Cr23C6, Cr3C2, Cr7C3) и нитридов хрома (Cr2N). Обзор литературы показывает, что абразивная, алмазная и электроалмазная обработка лазерно-упрочнённых прецизионных деталей изучена недостаточно. Для оценки возможности обработки таких деталей и эффективности различных абразивных материалов были использованы разные шлифовальные круги. Анализ показал, что требуемая производительность обработки и шероховатость поверхности могут быть достигнуты при использовании алмазного круга AC6-100/80 с 100% концентрацией. После лазерного упрочнения плунжеры имели деформации в пределах 5–16 мкм по изгибу (P), 2–8 мкм по овальности (Q) и 4–12 мкм по конусности (K). В процессе 4-секундного шлифования овальность плунжера не превышала 2 мкм, конусность снижалась до 3 мкм, а изгиб полностью устранялся. Слой снимаемого материала при обработке составлял 25–35 мкм. Полученные экспериментальные результаты с высокой точностью совпадают с теоретическими исследованиями.

Keywords: laser, grinding, concentration, diamond, surface roughness.

Ключевые слова: лазер, шлифование, концентрация, алмаз, шероховатость поверхности.

1. Introduction

Laser surface hardening was carried out using the methodology described in the literature [1-3]. The surface hardness of precision parts subjected to laser hardening falls within the range of  MPa, and the surface phases formed consist of chromium carbides (Cr23C6, Cr3C2, Cr7C3) and chromium nitrides (Cr2N). Machining hardened surfaces using white electrocorundum wheels, which are employed in existing factory technologies, presents significant challenges.

A literature review reveals that the abrasive, diamond, and electro-diamond machining of laser-hardened precision parts has not been sufficiently studied [4-8]. In some studies, the use of diamond wheels for machining hardened parts is not recommended [9]. The main reason for such proposals is the unevenness of reinforced surfaces, the calcification of circles, as well as their leveling. These factors lead to increased diamond consumption and make the use of diamond wheels economically inefficient. Additionally, existing research indicates that various studies have explored the application of electro-diamond machining for precision parts of machines and equipment after surface hardening [10-12]. However, the capability of grinding wheels to machine laser-hardened precision parts of machines and equipment has not been thoroughly investigated. In the grinding process of surface-hardened parts, the geometric wear of the grinding wheel does not meet technical requirements, resulting in significant drawbacks such as poor surface quality after machining.

The purpose of the work: The aim of this study is to select appropriate grinding wheels for the mechanical machining of precision parts of machines and equipment that have undergone laser surface hardening.

2. Methodology

To assess the feasibility of machining laser-hardened precision parts of machines and equipment and evaluate the effectiveness of various abrasive materials, the following grinding wheels were used: white electrocorundum – ПП 350×50×127 24А СМ1 К6; chrome-titanium electrocorundum – ПП 350×50×127 92А СМ1 К6; green silicon carbide – ПП 350×50×127 03C СМ1 К6; synthetic diamond – 1A1 350x50x127x5 AC6 100/80 MB1, K50, 100, 150%; and elbor – 350x50x127x5 ЛО 10 CM1 KБ100%.

The grinding process experiments were conducted at the "Climate Science-Production Facility" on a 3B182 model universal centerless grinding machine, using the following grinding conditions: wheel rotation speed – 1890 min^-1; workpiece speed – 32 min^-1; cross-feed rate (s) – 0.36 mm/min; grinding time – 5 minutes.

The results of the experiments, as shown in Figure 1, indicate that during grinding with 24A16CM1K6, 63CM16K6, and 92A16CM1K6 abrasive wheels, the productivity of the process is 48 mg/min, 408 mg/min, and 450 mg/min, respectively, while the specific consumption of abrasives is 376 mg/cm², 1538 mg/cm², and 1468 mg/cm². The surface roughness of the processed parts is within the range of 0.42–0.31 µm. Based on this data, it is evident that the geometric wear of the grinding wheel during the mechanical processing of surface-hardened parts does not meet the technical requirements, and the surface quality after machining is poor. The productivity of the grinding process with diamond wheels (Figure 1) is 558 mg/min, with a specific consumption of 5.2 mg/cm², which is 307, 334, and 320 times lower than the specific consumption of white electrocorundum, carbide green silicon carbide, and chrome-titanium electrocorundum, respectively. The surface roughness achieved is R_a = 0.21 µm. Grinding with elbor wheels does not offer any significant advantages compared to diamond wheels, as the productivity of the process decreases, and the specific consumption of elbor increases by 2.5 times. However, the surface roughness achieved after machining is 0.17 µm, which is 2 times lower than with abrasive wheels and 1.2 times lower than with diamond wheels.

Thus, after laser surface hardening, it can be recommended to use diamond grinding for the mechanical processing of precision parts of machines and equipment.

Figure 1. Productivity and specific consumption of abrasives during the grinding process of laser-hardened precision parts, and the surface roughness achieved with different abrasive wheels

The effect of the grit size of the grinding wheels on process productivity (W), specific diamond consumption (q), and surface roughness (Ra) is shown in Figure 2. In all cases, the diamond concentration ranged from 50% to 150%, and the grit size varied from 80/83 to 160/125. The processing conditions were: wheel rotation speed nş=1890 min^-1; workpiece rotation speed na=32 min^-1; t=5. Figure 2 shows that with the increase in the grit size of the grinding wheel to 80/63, 100/80, 125/100, and 160/125, the process productivity increases by 2.4 times, surface roughness increases by 2.3 times, and the specific diamond consumption decreases by 5.2 times. Based on the manufacturer's requirements, the desired surface roughness after grinding is Ra = 0.4–0.2 µm, and the geometric precision should be within 0.002–0.003 µm. These requirements are met by grinding with wheels having a grit size of 100/80. The study of the effect of diamond powder concentration on the quality parameters of the grinding process was conducted with AC6-100/80-MB1 diamond wheels at 50%, 100%, and 150% concentration. The processing conditions were: n_s=1890 min-1; n_a=32 min^-1; S=0,4 mm/min. The results in Figure 2 show that as the diamond concentration increases, process productivity and surface roughness increase, while the specific consumption decreases.

Figure 2. The dependencies of process productivity, specific diamond consumption, and surface roughness on grit size and diamond concentration

The analysis shows that the required productivity and surface roughness of the grinding process can be achieved with an AC6-100/80 diamond wheel with 100% concentration. A decrease in concentration reduces the process productivity and surface roughness, while an increase slightly improves productivity and surface quality but raises the specific consumption of diamonds.

The AC6-100/80 MB1-100% diamond wheel was used to evaluate the effect of grinding conditions on the process quality indicators. The processing time was 5 seconds, the rotation speed of the grinding wheel was n_s=1890 min^-1 (fixed due to machine design limitations), and the longitudinal feed was incrementally varied at 0.1, 0.2, 0.3, 0.4, and 0.6 mm/min. The rotation speed of the driving wheel was n_a=15, 30, 45, 60 min^-1.

Figure 3. Effect of the driving wheel's rotation speed on process productivity, diamond consumption, and surface roughness

During the investigation of the rotation speed of the driving wheel, the longitudinal feed was kept constant at S_v = 0.36 mm/min. Figure 3 shows the effect of the rotation speed of the driving wheel on process productivity (W), specific diamond consumption (q), and surface roughness (R_a). The figure reveals that increasing the rotation speed of the driving wheel from 20 min^-1 to 50 min^-1 increases process productivity. At this point, the minimal diamond consumption is observed at a rotation speed of 50 rpm. A decrease in the rotation speed to 20 min^-1 results in a 1.6 times increase in diamond consumption. The application of higher rotation speeds increases diamond consumption. The rotation speed of the driving wheel has a minimal effect on surface roughness. During the change of the rotation speed from 20 min^-1 to 80 min^-1, surface roughness increases from 0.16 µm to 0.26 µm. Thus, after laser surface hardening, it is recommended to use a rotation speed of 50 min^-1 for processing precision parts.

Figure 4. Effect of longitudinal feed on process productivity, diamond consumption, and surface roughness, showing that an optimal feed of 0.42 mm/min meets surface roughness requirements

During the investigation of the effect of longitudinal feed (s), the rotation speed of the driving wheel was kept constant. Changing the longitudinal feed from 0.12 mm/min to 0.60 mm/min resulted in a proportional increase in process productivity (Figure 4). During this process, the specific consumption of diamonds (q) initially decreased to a certain extent, then sharply increased, and the surface roughness (R_a) first decreased, reaching its minimum value when s=0.42 mm/min, and then increased. The data indicates that a longitudinal feed of s=0.42 mm/min meets the surface roughness requirements of the manufacturing plant

After laser surface hardening, the required polishing time to restore the geometric shapes of precision parts and reduce their surface roughness to the level recommended by the manufacturer has been determined. Figure 4 shows the relationship between polishing time (τ), the thickness of the removed layer (Δc), and geometric parameters. After laser surface hardening, plungers had deviations of 5 to 16 μm (P), ovalities of 2 to 8 μm (Q), and conicity of 4 to 12 μm (K). From Figure 4, it can be seen that after 4 seconds of polishing, the plunger's ovality did not exceed 2 μm, the conicity reached up to 3 μm, and the deviation was completely eliminated. The thickness of the removed material during this process ranged from 25 to 35 μm. The experimental results obtained correspond with the theoretical research with very high accuracy.

Conclusion

The conducted experiments recommend using the AC6-100/80 MB1 polishing wheel for grinding. In this case, the optimal polishing time should be 4-5 seconds, and the material removal depth should be 25...30 μm.

The research results suggest that after laser surface hardening, it is advisable to use the AC6-100/80 MB1-100% grinding wheel for the mechanical processing of precision parts. The following grinding conditions should be applied: the rotation speed of the driving wheel n_a=50 min-1, feed rate s=0.36 mm/min, and manufacturing time t=4 seconds. It is also recommended to perform the initial and final finishing of precision parts with KT-10/7 30% paste.

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