Improving Surface Roughness of 42CrMo4 Low Alloy Steel Shafts by Applying Varying Feed in the Multi-Pass Slide Burnishing Process

Abstract

Burnishing is a critical surface finishing process that uses a hard tool to apply pressure on the surface, lasting one or multiple passes to plastically deform surface asperities on high-performance alloys like 42CrMo4 steel. But the conventional method fails to efficiently enhance surface properties by selecting specifically the values of feed in all passes that follow the same path, missing uneven asperities. In this study, surfaces are burnished with multiple passes by changing the feed in each pass, hypothesizing that the approach can optimize the plastic deformation mechanism. By applying this approach, the tool’s path is deviated from its previous path to enhance the surface integrity by targeting residual surface anomalies. Controlled four levels of forces (60, 90, 120, 150 N), four feed levels (0.02, 0.08, 0.14, and 0.2 mm/rev), and three levels of passes (2, 3, 4) were applied on the proposed method and the conventional method to evaluate and compare their effects. Experimental results confirmed better surface roughness by the varying feed approach, demonstrating its efficacy in enhancing finishing quality through controlled plastic deformation. The analyzed S_a, S_sk, S_ku, S_k, S_pk, and S_vk showed changed topography by both methods, specifically 0.02 mm/rev feed in the old and feed combination in the feed-varying method, which included 0.02 mm/rev, produced smoother surfaces, but the highest elapsed time. The findings generally highlight the potential of adaptive feed strategies to overcome limitations of conventional burnishing, offering a viable solution for precision finishing of high-performance alloys.

1. Introduction

Surface roughness is a critical factor influencing the functional performance of mechanical components, particularly in applications involving sliding contact, fatigue, and wear resistance. Due to their excellent strength, toughness, and hardenability, low-alloy steels such as 42CrMo4 are widely used in manufacturing components experiencing dynamic loading, such as shafts, gears, and other high-stress components. However, achieving an optimal surface finish on these materials through conventional machining processes remains challenging due to inherent cutting mechanics and tool wear limitations.

Slide burnishing is a chip-less finishing process that enhances surface integrity by plastically deforming the workpiece surface through a hard, polished tool [1]. The applied pressure in connection with the other burnishing parameters displaces the material to achieve the required smoothness and other properties. Unlike traditional grinding or honing, slide burnishing induces compressive residual stresses, improves hardness, and reduces surface roughness without material removal [2,3]. Like the other machining processes, burnishing technological parameters play an important role in achieving the desired product responses [4,5].

The most frequently studied slide burnishing parameters as per [6] were burnishing force (33%), feed rate (26%), burnishing velocity (18%), and number of passes (8%). These include applying different material types, tool shapes and sizes, parameter magnitude changes [7,8], process-assisting technologies [9,10,11], and so on. Since these approaches affect the physical properties (roughness, flatness, and roundness), as well as the mechanical properties like fatigue [12], wear [13], and stresses [14], and the microstructure of the product, the effect of technological parameters on property changes is among the most studied disciplines. They directly affect the interaction between the burnishing tool and the workpiece surface and subsurface. The tool applies pressure on the surface while the feed rate helps it to progress forward when the workpiece rotates at a predefined speed. This process is repeated several times to achieve the required response change, stating the importance of the multi-pass burnishing process. Doing so allows the process to further refine surface quality by progressively smoothing the asperities [15,16].

Dyl et al. [17] studied the impact of changes in burnishing condition and parameters in roughness and hardness performed on corrosion-resistant steel X2CrNiMo17-12-2. They found out that slide burnishing not only improved the roughness but also increased the hardness by aligning the microstructure in the direction of deformation. Zhou et al. [18] compared a single and reciprocating strain path using a multi-ball burnishing tool at the micro-texture level to study the effect of different deformation modes. Their result showed minor differences between the two strain paths, but grains in the subsurface layers followed different grain rotations. The deformation achieved by the burnishing process depends on the magnitude of the burnishing parameters, the tool material, geometry, and the contact between the tool and the surface. The Maximov et al. study on roller and slide roller burnishing confirmed that slide burnishing achieved greater equivalent plastic deformation on the surface and near-subsurface layers due to sliding friction contact [19]. These studies highlight that slide burnishing enhances surface roughness and hardness through plastic deformation and microstructural alignment, with outcomes influenced by burnishing parameters, tool characteristics, and contact dynamics, underscoring the potential of optimized burnishing strategies for improving surface properties.

It is obvious that the burnishing speed and feed rate decide the production rate of the process and the quality of the surface. Considering surface quality, Korzynski et al. [20] reported 0.05 μm–0.18 μm S_a reduction of 30 mm diameter 42CrMo4 shaft burnished by 0.068–0.102 mm/rev feed and 150 N–250 N force with tool diameters 4, 6, and 8 mm. Another similar study in [21] applied force, speed, and feed on the range of 30 N–130 N, 180 rpm–360 rpm, and 0.032 mm/rev–0.094 mm/rev consecutively to slide burnish the same material using two different tools. Ra was reduced to 0.137 μm–0.225 μm by the diamond tool and 0.092 μm–0.265 μm from the initial turning of 0.26 μm. Different burnishing parameters produced reduced roughness, but they all involved changing the technological parameters’ magnitudes and tool material type. Nguyen T. et al. [22] concluded that low speed and/or feed is recommended for achieving a smooth surface, though this comes with a tradeoff in production reduction. Although slide burnishing has significant importance for surface integrity improvement, there is a notable lack of sufficient studies addressing the production rate of the process.

One author introduced a novel tool feed mechanism [23] that achieved remarkable roughness reduction compared to the old tool feed approach. In continuation of this research, the same approach is applied in the current study for different material and product surfaces (cylindrical), as testing the approach in different situations is good for the scope. Different materials have different plastic flow behavior [24] that requires the proper selection of technological parameters for various product response changes. And it requires additional study when it is performed with other machine types, since the technological mechanism to apply the parameter is different.

Despite these advancements, there remains a gap in understanding the precise influence of varying feed rates in multi-pass slide burnishing, especially for 42CrMo4 steel shafts. Most existing studies focus on fixed feed conditions, leaving room for further optimization of the process. This study aims to investigate how systematically varying the feed rate in a multi-pass slide burnishing process affects the surface roughness of 42CrMo4 steel shafts, providing insights into parameter selection for industrial applications. By addressing this gap, the present work seeks to contribute to the broader understanding of burnishing finishing techniques and their role in improving the functional performance of critical mechanical components.

2. Materials and Methods

The workpieces were prepared for the proposed burnishing method through a turning finishing process, which applied the same parameters to all, resulting in a common initial surface. Two burnishing process methods were conducted, and their 3D roughness values were measured and analyzed for comparison of modifications. A flowchart of the complete procedure is shown in Figure 1 for easy understanding of the experiment steps.

2.1. Machined Workpiece and Conditions of Finish Turning

The tested workpieces were Chromium Molybdenum low alloy steel 42CrMo4, delivered with a hardness of 410 HV10 and 65 mm in diameter. Its high tensile strength, toughness, fatigue resistance, hardenability, and other properties make it suitable for dynamic loading critical components in automotive, aerospace, machinery, heavy equipment, and other industries [25,26]. The workpieces were prepared with two burnishing areas (each 10 mm wide), as illustrated in Figure 2, and sufficient space for its holding by the lathe machine chuck.

The initial surface preparation of the workpieces was performed using a turning process, with the goal of achieving a high-quality surface finish. The turning operation was carried out at a cutting speed of 120 m/min, with a feed rate of 0.1 mm/rev and a shallow depth of cut of 0.1 mm. This combination of low feed, minimal depth of cut, and high cutting speed was selected to minimize surface roughness and achieve a smoother finish. The machining was conducted on an OPTimum OPTiturn S600 CNC lathe (manufactured by Optimum Maschinen Germany GmbH, Hallstadt, Germany), which features a heavy cast iron bed for enhanced stability during high-speed cutting. A SANDVIK DCLNL 2525M tool (Sandvik Coromant, Stockholm, Sweden) holder fitted with a CNGA120412 finishing insert (SUMITOMO ELECTRIC, Itami, Japan) was used for precision turning. Due to the workpiece’s higher hardness, dry turning [27] was employed to avoid potential complications associated with cutting fluids while maintaining machining efficiency. Furthermore, this choice applies environmentally friendly machining conditions, which is already an advantage of burnishing to the grinding procedure.

2.2. Burnishing Process and Experimental Setup

A sliding diamond ball 4.4 mm in diameter was applied to burnish the surface. The burnishing tool was equipped with a force sensor to control the force as shown in Figure 3. Keeping the other burnishing parameters constant for all the surfaces and methods, tool feed, burnishing force, and number of passes were changed based on the experimental plan presented in

Table 1. Experimental plan of both burnishing methods.

Feed-Varying Burnishing Method				Old Burnishing Method
Force [N]	Pass	Sur.No.	Feed Combination [mm/rev]	Force [N]	Pass	Sur.No.	Feed [mm/rev]
F	2	N1	f1 (0.02, 0.08)	F	2	O1	0.02
		N2	f2 (0.02, 0.14)			O2	0.08
		N3	f3 (0.02, 0.2)			O3	0.14
		N4	f4 (0.08, 0.14)			O4	0.2
		N5	f5 (0.08, 0.2)		3	O5	0.02
		N6	f6 (0.14, 0.2)			O6	0.08
	3	N7	f7 (0.02, 0.08, 0.14)			O7	0.14
		N8	f8 (0.02, 0.08, 0.2)			O8	0.2
		N9	f9 (0.02, 0.14, 0.2)		4	O9	0.02
		N10	f10 (0.08, 0.14, 0.2)			O10	0.08
	4	N11	f11 (0.02, 0.08, 0.14, 0.2)			O11	0.14
						O12	0.2

. The proposed feed-varying approach changes the feed after each pass repetition, which is only applicable in the multi-pass case. For this reason, 2, 3, and 4 passes, 60 N, 90 N, 120 N, and 150 N forces, as well as 0.02 mm/rev, 0.08 mm/rev, 0.14 mm/rev, and 0.2 mm/rev feeds were applied for both burnishing methods. For each force and pass category, all possible feed combinations were tried and their equivalent old burnishing method feeds were tested for comparison. Since it is difficult to represent feed combinations with their exact value during graphical presentation of measured values, f1 to f11 (hereafter representing the feed combination of the feed-varying burnishing method) symbols were used as presented in Table 1. In addition, N1–N44 for the feed-varying method and O1 to O48 for the workpieces’ surface naming method were used. The table presents an experimental plan for one force (F) only, since the same feed combination and number of passes apply for all forces used. A larger-to-smaller feed application procedure was followed to smooth tool marks created by the higher feeds and remove leftover asperities through the smaller feed overlapping tool path. For example, if f1 is selected, 0.08 mm/rev is applied first, then 0.02 mm/rev. The technological parameters in this experiment were selected based on the authors’ previous work [23] and literature [4].

Surface roughness was evaluated before and after burnishing to compare surface quality improvements, using the AltiSurf 520 device with a CL2 confocal chromatic sensor and MG140 magnifier. Measurements followed the ISO 25178-2:2021 [28] standard, with a 4 mm sampling length and 0.8 mm cut-off, 8 mm × 8 mm area was scanned and analyzed via Altimap v6.2 software after cleaning the 42CrMo4 surface of dust, lubricant, and debris.

The burnishing process is known for deforming surface asperities, and as a result, roughness values and their characteristics are changed. In this study, we analyzed arithmetic mean height (S_a), skewness (S_sk), kurtosis (S_ku), core roughness depth (S_k), reduced peak height (S_pk), and reduced valley depth (S_vk). By examining the changes in these measured values of the workpieces, the 3D roughness height and functional characteristics that change when the two burnishing processes are applied to the surfaces can be understood. The elapsed time is calculated for further understanding.

3. Result and Discussion

The measured results of surfaces burnished by the proposed and old methods showed various changes in the selected responses, ranging from improvement to worsening, from different functional property perspectives. S_a, S_pk, and S_vk values decreased by both burnishing processes compared to the initial turned stage, while S_sk, S_ku, and S_k experienced both increasing and decreasing scenarios when the burnishing parameters changed. The visible cutting tool patterns were destroyed to a certain extent, as shown in Figure 4. They possess roughness values of turned (S_a = 0.82 um, S_pk = 1.49 um, and S_vk = 0.78 um), old method (S_a = 0.45 um, S_pk = 0.46 um, and S_vk = 0.25 um), and feed-varying method (S_a = 0.21 um, S_pk = 0.43 um, and S_vk = 0.19 um). This indicates that the varying feed burnishing method flattens surface asperities better than the old burnishing method.

For all force categories, values are compared when the number of passes and feed changes. For presentation ease, line graphs are used for two and 3threepasses, and bar graphs are used for four passes. All measured and analyzed response parameters are presented in Appendix A.

3.1. Study of the Arithmetic Mean Height

Considering a commutative property and attempting the feed from bigger to smaller value, two-pass burnishing has six combinations with its four feed levels. This reduces to four when the number of passes increases to three, and only one combination when four passes are applied. All 92 burnished surfaces’ (44 feed-varying and 48 old methods) measured roughness S_a values decreased from the initial 0.82 μm.

Comparing both methods based on force and number of passes, Figure 5a,b and Figure 6 show the measured values of S_a. The circular symbol is assigned to the old method, and the square symbol to the feed-varying method, and they are consistent for all line graphs. During two passes, the varying method modified the roughness up to f4 feed, except for 150 N force when f1 and f4 feed were used. Applying f5 to f6 feeds, 90 N and 120 N forces, a lower value of S_a than the old method when f5 feed was used and a higher value when f6 feed was used were achieved. Feeds from f1-f3 for all force levels produced lower roughness than feeds from f4 to f6 due to the inclusion of the smallest feed, which is 0.02 mm/rev, in the combination. The highest feed (0.2 mm/rev) inclusion on f6 and the lowest feed in f1 showed the highest and lowest roughness due to the contact between the tool and surface variation and overlapped path differences. Lower feed produces more overlapping paths than the other higher feeds, allowing the tool to deform the surface asperities by creating more contact. This behavior was observed in the old method, except for the 0.2 mm/rev feed. Close-range values in both methods were observed when 0.02 mm/rev and 0.02 mm/rev and f1 and f4 feed were used, indicating that the feed-varying method produces no difference. Values were also settled to a close range when f6 was used, irrespective of the force magnitude. From the possible six combinations among the four feeds, those smaller feeds performed well compared to their counterpart old method and the other feed combinations. For example, f6 includes neither 0.02 mm/rev nor 0.08 mm/rev, and as a result, it gave a rougher surface in all force categories. Among the forces categorized feed-varying burnishing methods, 120 N, 60 N, 90 N, and 150 N forces created smoother surfaces consecutively when f1, f2, and f3 feed were applied. After that, the trend was not clear to conclude. Although we considered a common initial surface with an average value, it is not possible to maintain such an ideal surface in reality without the presence of outliers.

Increasing the number of passes to three (Figure 5b) also showed a similar trend that depicts close range at f7 and f10 of the feed-varying and 0.02 mm/rev and 0.2 mm/rev feed usage. In these specific feeds, the ranks achieved by the forces are not stable. For example, the 60 N force of the old method was rougher than the feed-varying method when a lower feed was used, ending with smoother roughness when a higher feed was used. In the intermediate feeds (f8 and f9 vs. 0.08 mm/rev and 0.14 mm/rev feed), a clear distinction was observed that the feed-varying method performed better for all forces except for 150 N (f9 and 0.14 mm/rev feed). Similar to the case with two passes, three passes, 60 N and 150 N force approaches showed unstable results compared to the other two forces, as an under- and overloading effect can occur on the surface. These causes left the surface rough or cracked and with material pileups.

Surfaces burnished with four passes are presented in Figure 6 using a bar graph. There is one surface burnished with four feed combinations for four counterpart surfaces in the force category, which is difficult to represent using a line graph like in the case of two and three passes. These four combined feeds performed better than the equivalent old method, except during 60 N and 90 N force burnishing when a lower feed was used. For all forces, surfaces burnished by the feed-varying method scored from 0.28 μm to 0.41 μm, indicating that the force magnitude was minimal. For the old method, except 0.02 mm/rev feed, unpredictable roughness was observed irrespective of reducing it compared to the initial roughness. Achieving smoothness better or closer to the old method when low feed was used gives an advantage of a quicker process.

3.2. Analysis of the Skewness

From Figure 7, we can observe that the skewness values of all surfaces were reduced from 0.40 to a range from 0.38 to −0.44 by the feed-varying method and from 0.36 to −0.20 by the old method when two passes were used. Shifting the skewness toward plane level by most of the forces was achieved by the varying feed method, with the exception of 150 N, which shifted the roughness to negative skewness. The old method with the same force also performed similarly with a smaller magnitude. The 3D view of the sample surfaces depicted in Figure 4 shows these changes from peak-dominated turned surface (S_a 0.82 μm, S_pk 1.49 um, and S_vk 0.78 μm) to partially destroyed tool marks by the old method (S_a 0.45 μm, S_pk 0.46 μm, and S_vk 0.25 μm), and valley-dominated surface by the proposed method (S_a 0.21 μm, S_pk 0.43 μm, and S_vk 0.19 μm). Compared to other force levels, 150 N of the varying feed method showed reduced skewness. Except the 60 N force, all feed-varying force levels showed stable dominance when f1, f2, and f3 feeds were used. The other feeds showed no clear trend related to the old method feeds and among themselves, but showed increased values except for 120 N.

Applying three passes also decreased the skewness, except in two cases when a 60 N force with a f9 feed and a 120 N force with a 0.14 mm/rev feed were applied. In addition, 90 N and 150 N produced negatively skewed surfaces while f7–f9 feeds were applied. On the contrary, 60 N force with both methods (except 0.2 mm/rev feed of the old method) kept the positive skewness. Another important observation is the dominance of the old method over the feed-varying method when a 0.2 mm/rev feed was used for all force levels. It achieved valley-dominated surfaces, including the most skewed surface, when 150 N was applied.

The feed-varying method managed to deform the surfaces until they were negatively skewed by applying four passes for all face levels, as shown in Figure 8, while the old method showed no clear trend as in the other passes, when the feeds were increased. Surfaces burnished by 90 N when the old method was applied, which showed a positively skewed roughness with a decreasing skewness reduction. These unpredictable trends could be caused by other burnishing technological parameters and material behavior that need additional study.

3.3. Investigation of the Kurtosis

The coexistence of peak and valley due to the turning tool marks’ expectation resulted in a 2.50 kurtosis value that later changed to peak- and valley-dominant surfaces due to the burnishing methods. The initial f1 and 0.02 mm/rev feeds from both methods showed in Figure 9 produced roughness closer to the reference line (S_ku = 3) except 60 N of the feed-varying and 120 N of the old method. Further changing the feeds revealed features that alternated between increasing and decreasing values, either with force or through the feed. All old methods increased when a 0.2 mm/rev feed was applied, with a similar trend of the feed-varying method when f6 was applied. Further, 60 N and 150 N of the feed-varying method and 60 N of the old method experienced decreasing values for all selected feeds. Generally, with the peak deforming mechanism of the burnishing process, kurtosis was expected to be reduced as a result. Both methods achieved this by either bringing the kurtosis closer to the reference or further flattening it, except in some cases. When feed rates of 0.08 mm/rev and 0.14 mm/rev were applied, the old method performed better for loads of 60 N, 90 N, and 150 N.

Performing additional repetitions, called the number of passes, is intended to deform the peaks further if the previous passes missed them. This gives the surface more smoothness, which is needed for some critical components for property modifications. A three-pass application modified the surface’s kurtosis to stay around the reference line, which indicates that peaks and valleys equally exist. Figure 9b shows values of both methods settled in a close range when f7 vs. 0.02 mm/rev and f10 vs. 0.2 mm/rev feed rates were selected. The old method achieved lower kurtosis in almost all feed and force levels, interpreted as a smoother surface with valley dominance. A clear difference between the two methods is observed when f8 and f9 as well as 0.08 mm/rev and 0.14 mm/rev feeds with 60 Na and 90 N force are selected compared to the other feed and force combinations.

Kurtosis was distributed in the range of 1.71 to 3.27 (if averaged 2.56) as depicted in Figure 10 when burnished by both methods and four passes for all force and feed levels. Feed-varying burnishing method performed closer to or lower than the old method to achieve a flatter peak and valley distribution. Except for 120 N force, the old method with 0.08 mm/rev and 0.14 mm/rev feeds yielded surfaces with kurtosis less than that of the initial turned surface. The proposed method almost produced similar kurtosis levels regardless of the selected forces. This indicates that all applied forces were enough to create the required material deformation, but the feed was responsible for their distribution.

Generally, the kurtosis levels of the initial surfaces were kept similar by three and four passes. But the usage of two passes was a little bit complicated when the feeds and forces were changing. Peaks and valleys created by the turning tool were smoothed by keeping their balanced distribution.

The main purpose of applying the burnishing process after various machining processes is to reduce surface roughness as it affects different properties of the surface, for example, its fatigue life [29]. Height parameters in our case, S_a and S_sk, decreased in both methods, indicating a smoother surface, which is good for increasing the fatigue life, friction reduction, corrosion resistance, and other properties. S_ku values showed increasing and decreasing at some times, similar to the initial value, depending on the selection of burnishing parameters. As a higher S_ku value is good for lubricant and debris collection from the tribological perspective, proper parameter selection is required for the desired values.

3.4. Study of Core Roughness Depth

In addition to height parameters like arithmetic average roughness, studying parameters that reveal the functional performance of the surface is important. These parameters help to understand the initial contact characteristics and whether stress concentration peaks are available. After a run-in, load-carrying roughs, lubrication, and debris entrapment valleys are also tribological information interpreted by them. The studied core roughness depth, reduced peak height, and reduced valley depth underwent different changes when the burnishing technologies changed.

When discussing core roughness depth based on the three changing burnishing parameters, like in the other burnishing responses, it is easy to see the effects of each. Applying two passes by the two methods reduced the S_k values in combination with all forces when 0.02 mm/rev and f1 feeds were used; see Figure 11a. Further changing the feeds and forces increased it, except in the feed-varying method when f1, f2, and f3 feeds were used, from what was achieved by the lowest feeds, in some cases, even more than the turned value, which is 1.91 μm. This indicates that higher peaks and deeper valleys were reduced, and as a result, the core roughness depth was decreased. In most cases, the feed-varying method achieved a lower S_k value, making it favorable for producing smoother surfaces if care is taken in feed selection. Moderate forces (90 N and 120 N) offered lower S_k in the feed-varying method, but no clear differences were observed in the old method.

Similar to two passes, three passes of the feed-varying method yielded a lower S_k value, as demonstrated in Figure 11b. This was realized by all feeds, and forces to reduce the 1.91 um turned value to a region of 1.77 um after burnished by 150 N force and f9 feed and to 0.53 um after burnished by 120 N force and f7 feed. Increasing the feeds from 0.02 mm/rev to 0.14 mm/rev and changing the other feeds from f7 to f19 increased the core roughness depth, excluding 60 N (when feeds were f8 and f9) and 90 N (when the feed was f9). Furthermore, three passes and the final feeds of both methods delivered very close S_k. In this study, the entangled lines representing burnishing force explain that the core roughness depth does not favor force magnitude, showing few trends for specific feeds that suggest additional study. These all indicate that when the burnishing tool passes over the surface repeatedly (three times), changing the feed when varying feed is used sends the peaks to the valley to achieve a smoother surface. When the feed increases (realized by introducing higher feed in the combination in the feed-varying method), a rougher surface is created, producing higher peaks and deeper valleys, which is the reason for higher S_k.

As discussed in the other subsections of this study, in most cases, the old method provided similar or lower values than the feed-varying method when a 0.02 mm/rev feed was used. This characteristic was observed for S_k, too, when four passes were applied in all force levels as demonstrated in Figure 12. All force levels by the feed-varying method produced surfaces with reduced S_k values distributed from 0.89 μm to 1.33 μm, applying the same feed combination ordered similarly. Increasing the number of passes gave the tool more chances to repeatedly attempt to deform the surface asperities to produce a smoother surface plastically. Increasing the feed in the old burnishing method approach resulted in a rougher surface compared to the level achieved with a feed of 0.02 mm/rev.

3.5. Analysis of Reduced Peak Height

Reduced peak height is the roughness part above the core roughness depth after the roughness zone is truncated in three parts. It receives the initial contact and load between the fitting surfaces and is removed during the run-in. As demonstrated in Figure 13a, both methods and all technological parameters applied in the experiment significantly reduced the S_pk value, indicating that stress concentration peaks were plastically deformed and moved to the valleys. All feeds and two passes of both methods performed similarly, decreasing it from 1.49 μm to a range of 0.16 μm by f3 feed with 150 N and 0.14 mm/rev feed with 60 N to 0.76 μm by 0.2 mm/rev feed with 120 N. Excluding the extreme values, almost all feeds and forces delivered reduced peak height around 0.3 μm with no preference for their levels.

The three-pass experiment depicted in Figure 13b shows a significant reduction in the first two feeds of both methods to three to nine times, considering all force levels. In this specific feed and force levels, S_pk converged to a very close range and increased when they were further changed. Reaching its maximum value during f9 and 0.14 mm/rev feeds burnishing, it was decreased when the final feeds were applied, excluding 60 N and 90 N forces. Compared to two passes, three passes with both methods showed moderately shaped trends in increasing and decreasing S_pk values, except in some cases. And in most cases, the feed-varying method performed better than the old method.

In each force category and four passes shown in Figure 14, the feed-varying method scored similar S_pk values at least with surfaces burnished by the old method using 0.02 mm/rev and 0.08 mm/rev feeds. Compared to the initially turned surface, the deviation among the mentioned surfaces was slight, indicating that the employed feeds had little effect. But when 0.14 mm/rev and 0.2 mm/rev feed were used, a pronounced difference was observed, illustrating the creation of higher peaks.

3.6. Investigation of Reduced Valley Depth

A crucial characteristic of a surface that determines its tribological behavior is its lubrication retention capacity. Different machining processes give different textures, further measured by their height, distributions, and directionalities. Reduced valley depth is one measure of these that defines the depth of the valleys below the core roughness. Deeper valley denotes higher lubrication and debris entrapment. Figure 15a shows S_vk values of the two burnishing methods when applying two passes. Both methods decreased the reduced valley depth value from 0.78 μm, which is its initial value after turning. Other than 120 N and 150 N forces of the feed-varying method, giving different trends when f4, f5, and f6 feeds (120 N) and f6 (150 N) feed were used, they showed stable results. Especially 60 N and 90 N forces produced similar results, irrespective of feed changes, which are shallow valleys. Generally, valley depth was decreased by these burnishing processes, producing low lubrication and debris holding capacity surfaces.

Increasing the number of passes to three, as demonstrated in Figure 15b, also decreased S_vk values in every case. As with S_pk values for the same passes, all forces and the first two feeds for both methods produced similar reduced valley roughness surfaces, ranging from 0.14 μm to 0.4 μm depth. Further changing the feeds increased the S_vk values except for the 90 N force of the feed-varying method, indicating the production of deeper valleys. This is achieved when rougher surface is realized by increased feeds in the old method and the availability of higher feeds in the combination of the feed-varying method. Like the other response parameters, force level created no clear trends.

The effect of further increasing the passes to four produced higher depth when 60 N and 90 N forces were utilized, using the feed-varying method, compared to the old method, except in the 0.2 mm/rev feed case; see Figure 16. The old method produced higher S_vk values when 120 N and 150 N forces were applied, with some exceptions. All forces delivered similar values with little deviations when feeds were changed, suggesting their constrained significance.

The studied functional parameters S_k, S_pk, and S_vk decreased especially when 0.02 rev/min feed was applied in all force categories and methods. This decrease stayed for all burnishing parameter levels in some instances but increased from the achieved minimum level when the burnishing parameters changed (increased feed). Core roughness depth, peak height, and valley depth give additional information on the characteristics of the produced surface. Higher peaks mean the surface is prone to friction as the stress is concentrated on the tips of the peaks. On the contrary, higher S_k means a higher load carrying section of the roughness. A higher S_vk value is preferred when the contacting surfaces require lubrication for movement ease.

The results of this study demonstrate that both the feed-varying and conventional slide burnishing methods significantly reduce roughness height and core roughness parameters on 42CrMo4 steel. The feed-varying method achieving lower or comparable roughness values, particularly at feed rates that included 0.02 mm/rev. Lower roughness compared to the turning process indicates smoother surface that can contribute the fatigue life and other properties of the surface. Reduced core roughness parameters show the reduction in friction-inducing peaks, lubrication, debris retention valley, and load-carrying core roughness. This improvement is attributed to plastic deformation induced by burnishing, which smooths the surface by altering its roughness characteristics. The feed-varying method’s success likely stems from its ability to modify the tool–workpiece interaction. This is realized by changing its path from the previous pass’s path, which helps it to deform the asperities from another angle, supporting the hypothesis that feed variation optimizes surface smoothing. This phenomenon can be observed in Figure 17, taken by Zeiss Stereo Discovery V8 microscope (Carl Zeiss Microscopy GmbH, Jena, Germany) with 80 M, showing clear turning tool and burnishing tool marks. The tool marks, which are a source of roughness, are reduced on the surface burnished by the feed-varying method (Figure 17c). Compared to the author’s prior study on a flat surface [23], these findings extend the understanding of burnishing by introducing feed variation as a novel approach for 42CrMo4, a material widely used in high-strength applications. The feed-varying method’s ability to match the conventional method’s performance at 0.02 mm/rev while reducing processing time highlights its potential to enhance manufacturing efficiency in industries like automotive and aerospace, where smoother surfaces improve component performance. However, the lack of clear distinctions among the selected technological parameters levels, especially burnishing force, suggests that further research is needed to fully understand the method’s effects. Future studies should explore other materials, a wider range of burnishing parameters, and additional surface properties like hardness, residual stress, wear resistance, and others to broaden the method’s applicability. This study underscores the potential of feed-varying burnishing to produce smoother surfaces with minimal time compared to the lowest feed rate of conventional method, paving the way for advancements in precision machining.

3.7. Comparison of Burnishing Time

One of the main characteristics of the burnishing process is repeatedly attempting the process based on a preselected number of passes. For example, if the previous machining process left surface asperities to be removed by the burnishing process, the responsible person must decide how many passes are required to achieve the required smoothness. So, when the number of passes increases, the time required to finish the whole process is extended. We present elapsed time and burnishing rate comparison calculations for both methods. The following equations are used:

$${\mathrm{t}}_{\mathrm{m},\mathrm{n}}=60·{\displaystyle \frac{{\mathrm{L}}_{\mathrm{w}}}{{\mathrm{n}}_{\mathrm{w}}}}·\left[{\displaystyle \frac{1}{{\mathrm{f}}_1}}+{\displaystyle \frac{1}{{\mathrm{f}}_2}}+{\displaystyle \frac{1}{{\mathrm{f}}_3}}+{\displaystyle \frac{1}{{\mathrm{f}}_4}}\right],$$

(1)

$${\mathrm{t}}_{\mathrm{m},\mathrm{o}}=60·{\displaystyle \frac{{\mathrm{L}}_{\mathrm{w}}}{{\mathrm{n}}_{\mathrm{w}}{\mathrm{f}}_1}}·\mathrm{i}$$

(2)

where the machining time of the new (t_m,n) and the old (t_m,o) method are calculated based on the axial length of the burnished surface (L_w), the rotations of the workpiece (n_w), the applied feed in each pass (f₁, f₂, f₃, and f₄), and the number of passes (i).

The elapsed time calculation of the old burnishing method is simply calculating the time for a single pass and multiply it by the number of passes factor (Equation (1)). On the other hand, the feed-varying method time is calculated independently for each pass, as they use different feeds and sum them up (Equation (2)).

Figure 18 demonstrates feed verses time graph comparison between the two methods. The lowest feed of the old method (0.02 mm/rev) and the first three feeds of the feed-varying method, which includes the slowest feed (0.02 mm/rev), have the longest elapsed time; see Figure 18a. Increasing the feeds marked red on the top second x-axis and applying f4, f5, and f6 requires less time to finish. The same is true with three passes up, applying the smallest feed of 0.02 mm/rev (0.02 mm/rev and f7, f8, and f9); see Figure 18b. The f10 feed combination (0.08 mm/rev, 0.14 mm/rev, and 0.2 mm/rev) and 0.2 mm/rev finished the three passes at times close to each other, which also produced similar roughness results. In the case of four passes presented in Figure 18c, the proposed method finished the process faster when the old method applying a 0.02 mm/rev feed. Considering the time taken to complete the process, the feed-varying method is faster only when the old method uses the smallest feed; however, it produces a closer or smoother surface roughness, which can make it a competitive method. In this case, industries can select which method to use, balancing the trade-off between time and surface roughness level.

4. Conclusions

This study aimed to investigate the effect of changing the feed in each pass in the context of the multi-pass slide burnishing method and compare it with the old method of applying the feed. The proposed method achieved improved roughness values when feed combinations with lower feeds were applied. The following conclusions are drawn from the experimental results:

• Both methods decreased height roughness parameters, but the feed-varying method showed better performance, especially when the number of passes was three and four.
• The functional parameters that study the peaks, core, and value heights decreased compared to the initial turning process, with the feed-varying method performing better or closer to the old method in achieving a lower height.
• The smoothest surfaces were produced when 0.02 mm/rev in all force and pass levels was used in both methods upon interpreting the analyzed measured data.
• The core roughness height (S_k) reached its minimum value when using the lowest feed rates in both methods and exhibited a proportional increase with higher feed rates.
• The calculated time revealed that the feed-varying method generally requires more time than the conventional method, except when using the lowest feed rate of 0.02 mm/rev. Notably, the results obtained with 0.02 mm/rev in both the conventional method and feed-varying method (where 0.02 mm/rev was part of the combination) were closely comparable. This demonstrates that the feed-varying method can achieve similar performance while reducing processing time.
• In most studied burnishing response parameters, the technological parameters showed no clear distinctions between the two methods. This requires further study to understand their effect on the roughness properties of the produced surfaces.