Influence of the V-shaped rubber knife of natural rubber trees on the quality of cut wood

In this study, to investigate the influencing factors between the rubber tapping knife and the cutting surface quality during the tapping process of natural rubber trees, as well as the cutting quality of wood using horizontal push-type knives, a simulated rubber tapping experiment was conducted, specifically involving the use of a rubber tapping knife to cut rubber wood blocks. During the experiment, the V-angle of the rubber knife and the feed speed were varied, and the variations in cutting force and cutting distance were monitored in real time. The surface flatness of the processed rubber wood blocks was then tested, followed by a latex flow experiment. After the experiments, the wear condition of the cutter was measured, and an analysis was performed to evaluate the relationship between five evaluation indices: maximum cutting force, energy consumption, chip morphology, cutting surface quality, and latex flow velocity, with the V-angle of the rubber tapping knife and the feed speed. The test results showed that the 83° knife achieved the best cutting surface quality, while the 90° knife had the least wear. Based on the comprehensive evaluation of cutting performance, the 83° knife with a tapping speed of 90 mm/min produced in the best overall evaluation of cutting surface quality and knife wear.

As a significant economic crop, natural rubber trees are predominantly cultivated in tropical and subtropical regions. The majority of plantations are located in countries and regions, such as India, Indonesia, Malaysia, Thailand, Vietnam, and the provinces of Hainan, Yunnan, and Guangdong in China [1, 2]. Natural latex, which is produced by rubber trees, is utilized extensively as an essential industrial raw material and strategic resource [3]. It is employed in a multitude of fields, including aerospace, national defense, transportation, medicine, energy, daily life, and other areas [4]. Natural latex is obtained primarily through the regular tapping of the outer bark of the rubber tree, which destroys the milk ducts in the rubber bark, allowing the latex to flow out [5, 6]. This process results in the bark of the rubber tree being tapped as the sole means of obtaining natural latex [7,8,9]. The current rubber tapping technology indicates that the typical lifespan of natural rubber trees is approximately 30 years [10]. The external surface of the rubber tree bark is composed of five distinct layers: rough bark, outer sand bark, inner sand bark, yellow bark, and water bladder bark. It is prohibited to tap the water bladder bark. The yellow bark area, which contains numerous mature latex tubes, is the primary site of latex production [11]. The history of large-scale rubber tree planting extends over a period of more than a century. The most prevalent rubber tapping technology currently in use involves rubber farmers holding rubber tapping knives to tap the rubber. The traditional rubber tapping knives are generally V-groove knives or jabong knives [12]. In the V-groove knife, the longer of the two wings is designated as the long blade, while the shorter one is referred to as the short blade. In recent years, a novel rubber tapping technology has emerged, whereby rubber is tapped by intelligent rubber tapping equipment. This intelligent rubber tapping equipment is divided into two categories: fixed rubber tapping machines and self-propelled rubber tapping robots. The three primary tapping methods entail the bark of the rubber tree being cut by the blade to facilitate the flow of latex, thereby completing the rubber tapping process. The essence of rubber tapping can be summarized as the cutting of wood with a push knife.

The current mainstream wood cutting methods can be broadly classified into four categories: sawing, planing, milling, and turning [13,14,15,16]. The simulated rubber tapping experiment studied in this paper is analogous to the single-layer board cutting in the cutting process. In addition, the commonly used cutting methods for single-layer board cutting mainly involve sawing, milling and laser cutting. Since this study is mainly based on the relevant evaluation and analysis of cutting tools and cutting surface quality, the applications of sawing and milling in single-layer board cutting are mainly considered. As sawing is generally used in the primary processing stage of wood raw materials and is less applied to the cutting of single-layer boards, attention is also paid to the sawing of logs. In terms of sawing logs, it was studied by Richard Kminiak [17] that when spruce, beech and oak were sawed by a sliding miter saw with manual saw blade feeding using saw blades with 24, 40 or 60 teeth and a special saw blade with 24 teeth, respectively, it was found that the surface roughness decreased almost proportionally with the increase in the number of teeth, and both the uniformity of sawn wood and the density affected the surface roughness of the obtained surface. Moreover, the denser and more uniform the wood was, the smaller the surface roughness obtained by cutting would be. In terms of sawing particleboards, it was demonstrated by Bartosz Pałubicki [18] that when circular saws were used to cut particleboards, there was a positive correlation between the feed speed and the feed force. At higher feed speeds, the cutting tools became blunt more quickly. With the increase in the feed speed, the proportion of sawdust particles was increasing, and there was a negative correlation between the feed speed and the size of sawdust particles. In terms of milling single-layer boards, it was pointed out by Kidung Tirtayasa Putra Pangestu [19] that during the milling process of wood materials such as laminated veneer lumber (LVL), differences in milling processes and feed speeds would lead to differences in tool wear, chip shape and surface roughness. It was studied by Pangestu, Kidung Tirtayasa Putra [20] that when tungsten carbide coated with AlCrN, TiN and TiAlN was used to cut laminated veneer lumber (LVL), compared with uncoated tungsten carbide cutting tools, coated cemented carbide cutting tools had better wear resistance and lower noise levels when cutting laminated veneer lumber (LVL).

It is evident that the method of pushing and cutting the rubber tree with a push-type knife, employed during rubber tapping, exhibits distinctive characteristics and fundamentally differs from the prevailing traditional wood cutting techniques. The conventional approach to cutting wood with a knife is to employ a rotating knife to cut stationary wood or a stationary knife to cut rotating wood. Conversely, the knife utilized in tapping is a V-shaped knife that employs a near-horizontal pushing motion to cut stationary wood. This technique is unique to rubber tapping. Consequently, there exist substantial disparities between the two methods in terms of cutting techniques and objectives, which serve to highlight the uniqueness of horizontal push knives in wood cutting. As a consequence, no researchers have yet examined the factors and evaluation metrics associated with horizontal push knife wood cutting, including cutting force, cutting energy loss, cutting surface quality, and knife wear. In light of the aforementioned circumstances, this paper represents the inaugural study on cutting rubber wood with a horizontal push knife. It offers fundamental insights into this research domain.

The current state of research into intelligent rubber tapping equipment is characterized by slow progress and the presence of significant shortcomings that are challenging to address. A rubber plantation may contain hundreds of rubber trees, and the cost of installing a fixed tapping machine on each tree is considerable. The complex contour curve of the rubber tree trunk presents a challenge in fitting with a rubber tapping machine [21], and rubber plantations are predominantly located in hilly or hillside areas [22, 23]. Consequently, self-propelled rubber tapping robots are still in the research and development stage. In recent years, the development of intelligent rubber tapping equipment has been marked by a continuous stream of related machine and equipment innovations. The current focus of intelligent rubber tapping equipment research is on image recognition, navigation, path identification, and overall machine optimization. However, these developments have not yet addressed the V-shaped end of the machine. A detailed examination of the cutting process of a rubber tree with a knife blade. The push-type rubber tapping knife cutting wood, the subject of this article, allows for the determination of the role of various influencing factors of the knife during rubber tapping. Furthermore, it provides supplementary effects and guidance for future research on cutting rubber trees with the end blade of intelligent rubber tapping equipment, as well as guidance for the development of intelligent rubber tapping equipment. The development has significant reference value.

In conclusion, while the utilization of a push-type rubber tapping knife remains the primary method for tapping rubber, the characteristics of the blade and the quality of tapping are of particular importance for achieving superior quality latex and augmenting its production. Given the complex environment of rubber forests, which are typically situated in mountainous or hilly regions, the growth of rubber tree trunks is uneven and not an ideal cylindrical shape. Even on the same rubber tree trunk, there are inconsistencies in the thickness of the bark and the circumference of the tree [24]. This paper introduces a novel rubber tapping simulation experiment, depicted in Fig. 1a, which represents a significant advancement in the field. By magnifying the cutting edge of the rubber tapping knife, the arcuate cutting surface can be perceived as a plane. Subsequently, a small wooden block crafted from rubber wood serves as a representation of the rubber tree bark, simulating the process of the rubber tapping knife cutting the bark. For the first time, the flatness of the cutting surface and the wear of the blade are regarded as pivotal factors influencing the quality of rubber tapping. The flatness of the cutting surface is a definitive factor in latex yield and flow, and thus a decisive factor affecting the quality of rubber tapping. Consequently, rubber tapping is conducted by adjusting the angle between the two wings of the V-shaped knife (the standard angle between the two wings of the V-shaped knife in China’s current push-type rubber tapping knife is 80°) and the feed speed [25]. This can mitigate the impact of external factors on rubber tapping, reduce the impact of errors resulting from variations in rubber tree girth, enhance the efficiency and reproducibility of rubber tapping simulation experiments, and facilitate the control of changes in variables. Once the rubber tapping simulation experiment has been completed, a latex flow experiment is conducted on the obtained cutting surface to ascertain whether there is a correlation between the flow speed and the cutting surface. Wu [26] identified a novel evaluation metric for cutting quality, which incorporates the influence of cutting force on the cutting surface. Kminiak’s [27] research demonstrated that distinct cutting conditions exert disparate influences on the surface quality of medium-density fiberboard (MDF). Darmawan's [28] research demonstrated that spiral edge milling cutters with varying helix angles exhibit disparate effects on the surface quality of wood milling. This article presents a significant contribution to the field of rubber tapping knives, particularly in relation to the impact of different edge angles on cutting performance. Zhu [29] demonstrated that there are discernible variations in the morphology of chips generated under disparate feed speeds. Bendikiene [30] observed that the wear of wood milling knives differs at different angles.

To achieve this objective, the current study conducted a simulation experiment on the tapping of rubber using three different types of tapping knives with varying blade angles. Taking the blade angle and feed speed as experimental variables, this research examined: (1) their impact on cutting forces and the energy consumed during the cutting process; (2) the effect on surface quality; (3) the influence on chip morphology; (4) the effect on knife wear; and (5) also included the flow rate of latex as an evaluation criterion. Ultimately, the optimal tapping conditions identified resulted in the most optimal surface flatness and knife wear of the cutting surface.

Rubberwood block material and tapping knife characteristics

The objects of this study are the customized special rubber cutting knives produced by Qianjin Tool Manufacturing Factory and the wooden blocks made from natural rubber wood. The knives are classified into three types according to the included angles of the V-groove knives, namely, the 80° knife, the 83° knife, and the 90° knife, as illustrated in Fig. 1. The included angle between the two blades of the push-type rubber tapping knife is not a clearance angle in the strict sense. Generally speaking, there is no absolutely fixed standard value for the clearance angle of the push-type rubber tapping knife, but it usually falls within the range of 5° to 15°. In this study, it is set at 10°. This setting is based on the consideration of ensuring both the smoothness of the knife when cutting into the bark, thus reducing the cutting resistance, and ensuring that the cutting edges have sufficient strength and stability to prevent issues such as edge chipping or excessive wear during the rubber tapping process. The knives are manufactured from Cr12MoV high-carbon tool steel, with a hardness of 60 HRC. The characteristics of the knives are listed in Table 1. The knives are used as received, without requiring any mechanical treatment. The width of the short blade is 15 mm, the width of the long blade is 30 mm. Both blades have a length of 50 mm, the handle of the knife has a length of 50 mm, the thickness of both blades is 2 mm, and the cutting-edge angle of the blades is 25°, as shown in Fig. 1b. The wooden blocks are cut from rubber wood, with specific dimensions of 20 ×ばつ 30 ×ばつ 40 mm, a moisture content of 6%, and a density of 0.85 g/cm³. The wood fibers in the wooden blocks are oriented vertically downward, as indicated by the yellow arrow in Fig. 1b.

Cutting experiments and mechanical properties

In the cutting experiment, the clamping device of a universal testing machine was used to fix the shank part of the knife. The knife vertically descended to cut out a long and thin wooden block, that is, a rubber wooden block with a cutting thickness of 2 mm, a width of 5 mm, and a length of 40 mm. The cutting is made in the direction of the fibers of the rubberwood block, as shown by the yellow arrow in Fig. 1b. During the simulation test stage, the relationship between the cutting tool and the quality of the cutting surface was the main focus of the research. The experimental model can be simplified by cutting parallel to the wood fiber direction, and the interference caused by the complex stress distributions and fiber fracture patterns during vertical cutting can be excluded initially. The tensile and tearing effects of the wood fibers under different tool angles and cutting speeds as the tool advances along the fiber direction were intended to be determined. However, during vertical cutting, these factors would be intertwined with other factors, such as the transverse fiber cutting force, which is unfavorable for the observation of a single factor. Through the cutting simulation test parallel to the wood fiber direction, a basic theoretical and mathematical model of the cutting process can be constructed. Although actual rubber tapping is carried out perpendicular to the wood fiber direction, these basic theories can be used as a comparative reference.

Malkocoglu's [31] research indicates that there are significant differences in the machining performance and surface flatness of wood under different cutting conditions. Accordingly, the cutting experiment was conducted under the following conditions, as outlined in Table 2. Each knife angle was tested at five different speeds, with each speed being tested three times. Since the standard for the angle between the two wings of the V-shaped groove of the push-type tapping knife currently in use in China is 80°. During the actual rubber tapping process, when the V-shaped angle of the tapping knife is 80° or larger, after the rubber bark is cut, the shape of the cut formed is conducive to the natural flow of latex. If the angle is too small, the cut may be partially closed or form a shape that is unfavorable for the convergence of latex. However, an angle of 80° can provide a relatively open and regularly shaped channel, ensuring that the latex can flow out smoothly, thus improving the efficiency of latex collection. Moreover, if the angle is too small, the latex is prone to remain at the edge of the cut or in the internal folds. This part of the residual latex may solidify in the subsequent process, blocking the cut and affecting the latex yield in the next tapping. With an angle of 80°, the cut is relatively flat, and the latex can be collected relatively completely during the flowing process, reducing the waste of latex. Furthermore, the V-shaped angle of the mainstream tapping knives on the market at present is 83°, and 90° is the limiting angle that conforms to the mechanical laws. When the angle is greater than 90°, it is difficult to accurately control the depth of cutting the rubber bark, which will damage the trunk and affect the latex yield. Therefore, these three groups of angles are selected as variables to conduct the cutting experiment. The universal testing machine utilized was a computerized electronic universal testing machine from Shenzhen Sansi Testing Machine Co., Ltd. This universal testing machine can achieve speed adjustment from 0 to 500 mm/min, and its built-in mechanical sensor can measure the force used in cutting at any time, as well as the relationship between displacement and force during the experiment. The surface wear of the knife was captured using an industrial camera (6000W pixels, optical size 1/2.33 inch). The macroscopical morphology of the cut surface was captured using Nikon RS-N3, while the microscopical surface morphology was analyzed by a laser scanning confocal microscope (CLSM, VK-X237.5 K) from Keyence (Hong Kong) Co., Ltd., and a field emission scanning electron microscope (SEM, S-4800) from Japan.

Cutting test

In the simulated rubber tapping experiment, each of the three types of cutting tools was reused and cut three times at five different speeds, with each tool making a total of 45 cuts. Since axial cutting causes less wear on the tool than radial cutting, under the same cutting conditions, at the same magnification and shooting distance, the wear of the tool was recorded by an industrial camera (6000W pixels, optical size 1/2.33 inches). The tools were all made of Cr12MoV high-carbon tool steel, and the hardness of the finished tool could reach 60HRC. The cutting surface obtained after the tool cutting symbolizes the rubber tapping surface in actual rubber tapping, and the quality of the cutting surface plays an extremely crucial role in the latex yield and latex flow after rubber tapping. A total of 45 cutting surfaces obtained after the simulated cutting experiment were measured.

Surface quality measurement

Under each combination of V-shaped angle and cutting speed, 15 cutting surfaces that appeared relatively flat intuitively were selected, and a combination of macroscopic and microscopic methods was used to analyze and evaluate the quality of the cutting surface.

At the macro level, Yu Long and colleagues [32] studied and established a finite element model for orthogonal cutting and applied it to simulate the burr formation process. Three typical workpiece materials were investigated, and the simulation results revealed the entire burr formation process. Consequently, the occurrence of burring during the cutting of rubber wood blocks using cutting knives is also a normal phenomenon. Qualitative analysis can be conducted based on the presence of destructive unflattens and the presence of wood wool or burring. Destructive unflattens refers to the stripping or tearing of bundles of wood fibers on the wood surface, which is caused by the inappropriate sharpness of the cutting knife during processing. Wood fibers are single fibers that are still connected to the wood surface at one end, while the other end stands upright or adheres to the surface. Burring refers to bundles or patches of wood fibers that are not completely separated from the wood surface [33].

At the micro level, quantitative analysis can be conducted based on the arithmetic mean height and maximum height of the cutting surface. Kazlauskas and colleagues [34] evaluated the cutting quality by measuring the roughness of the cutting surface after the experiment. In this paper, a laser microscope (CLSM, VK-X237.5 K) from Keyence (Hong Kong) Limited is employed to scan and analyze the cutting surface morphology, which is presented in the form of two-dimensional color gradient changes and three-dimensional stereoscopic displays. In this paper, a combination of these two methods is employed for evaluation purposes.

Flow experiment and experimental equipment

The rubber tapping surface, which is the subject of this study, follows a specific arc, typically half the circumference of the tree trunk, and is inclined at a certain angle with respect to the horizontal plane. To gain a deeper understanding of the phenomenon under investigation, a locally magnified rubber tapping simulation experiment has been employed. Consequently, the magnified cutting surface can be conceptualized as a planar inclined surface, with the angle between the inclined surface and the horizontal plane set at 25 degrees. In this study, the Yang Dao cutting technique, as specified in the Chinese rubber tree tapping technical standard, is employed. This technique involves making an initial cut at the bottom and a concluding cut at the top, resulting in a cutting surface facing upward.

A schematic of the flow experiment is shown in Fig. 1c. The funnel utilized is a long-necked small-bore funnel procured from a commercial source, thereby ensuring uniformity in the latex flow speed. The cutting surfaces obtained from the previous rubber cutting simulation experiment serve as the basis for this part of the experiment. Fifteen cutting surfaces are subjected to the latex flow experiment to further verify whether there is a correlation between the cutting surface quality and the latex flow rate. The cutting surfaces obtained after the simulated rubber tapping experiment represent the actual tapping surfaces in practical rubber tapping work. To investigate whether different knife angles at different speeds would affect the flow rate of latex on the tapping surface, a latex flow experiment was conducted on the cutting surfaces. To ensure a consistent latex flow rate, the rubber wood block is fixed on a 25° inclined plane. The timer is initiated when latex begins to drip from the top of the wood block and ceases when latex reaches the bottom of the cutting surface.

Influence of cutting conditions on cutting force

As shown in Fig. 2A, the relationship between force and displacement of tools with three different angles is presented under the condition of a feed rate of 15 mm/min. As shown in Fig. 2B, the relationship between force and displacement of tools with three different angles is presented under the condition of a feed rate of 37.5 mm/min. As shown in Fig. 2C, the relationship between force and displacement of tools with three different angles is presented under the condition of a feed rate of 60 mm/min. As shown in Fig. 2D, the relationship between force and displacement of tools with three different angles is presented under the condition of a feed rate of 90 mm/min. As shown in Fig. 2E, the relationship between force and displacement of tools with three different angles is presented under the condition of a feed rate of 110 mm/min. In the figure, a, b, and c represent the data of three repeated experiments.

As can be seen from Fig. 2, it can be observed that the maximum cutting force occurs at the initial moment of cutting. Furthermore, under the same feed speed, the maximum cutting force appears during the cutting process of the 80° knife, followed by the 83° knife, and finally the 90° knife. In addition, it can be observed that during the middle phase of cutting, there are intermittent spikes in cutting force, approaching the initial maximum cutting force. There may be three scenarios. The first scenario involves the initial cutting process, where the cutter follows the direction of the wood fibers. This often results in splintering of the portion of the wood block being cut, causing a small central section to escape actual cutting by the blade edge. Subsequently, when the cutter encounters the unsplintered part, the cutting process resumes. The second scenario arises when encountering knots or hard patches within the wood block, which require greater cutting force to penetrate. Finally, the third scenario relates to the cutting forces required due to the varying contact areas during the cutting process. Specifically, an 80° cutter exhibits the largest contact area during cutting, necessitating more cutting force, whereas a 90° cutter, with the smallest contact area, requires the least amount of cutting force. Contact area refers to the area of the two blades of the V-shaped knife, one blade is parallel to the surface of the wood block, and the other blade is in contact with the interior of the wood block when cutting the wood block. According to the Pythagorean theorem, in a right triangle, the hypotenuse is always longer than the right Angle, and when the length of the right Angle is fixed, the smaller the acute Angle adjacent to it, the longer the hypotenuse. Therefore, the larger the contact area.

The influence of cutting conditions (V-groove angle of the cutting edge and feed speed) on the cutting force indicates that the maximum cutting force always occurs at the initial moment of cutting, as shown in Fig. 2. Since each cutting condition was repeated three times, the average value of the maximum cutting force under each condition was taken for horizontal comparison, as shown in Fig. 3a. It can be concluded that the 90° knife requires the smallest maximum cutting force during cutting, and that the maximum cutting force increases with the increase in speed. Nevertheless, the maximum cutting force required by the 80° and 83° knives is largely comparable. Both the 80° and 83° knives exhibit a similar trend in the change of maximum cutting force at the speeds of 15, 37.5, 60, and 90 mm/min, with the maximum cutting force of the 80° knife being greater than that of the 83° knife. At a speed of 110 mm/min, the maximum cutting force of the 80° knife undergoes a sudden decline, while the maximum cutting force of the 83° knife experiences a sudden increase, exceeding that of the 80° knife. The maximum cutting forces for all three angles did not exceed 350 N, and this difference in the actual tapping process is almost negligible. In conclusion, as the speed continues to increase, the maximum cutting forces of the three knives exhibit a tendency to increase. However, it is recommended that a V-groove angle of 90° be selected for the tapping knife, as the maximum cutting force of the 90° knife is smaller than that of the 80° and 83° knives. Based on the data obtained from the experiment, a variance analysis was conducted on the evaluation indicator, which was the maximum cutting force in a single cutting operation. The results of the variance analysis for the maximum cutting force are shown in Table 3. It can be seen from the table that the P value of the regression term "included angle of the cutting edge" is less than 0.01, indicating that it has an extremely significant impact on the maximum cutting force. Meanwhile, the P value of the regression term "cutting speed" is less than 0.05, suggesting that it also has a significant impact on the maximum cutting force.

The area enclosed by the force–displacement graph represents the consumed energy. Therefore, the average energy consumption under each condition is taken for horizontal comparison. As illustrated in Fig. 3b, the energy consumed by the 83° knife at a speed of 15 mm/min is significantly higher than that of the other two knives, with an energy consumption of 12.88 J. At speeds of 37.5, 60, and 90 mm/min, the energy consumed by the three knives with different angles decreases as the speed increases. This phenomenon may be attributed to the fact that an increase in speed leads to an increase in the cutting force of the blade, which enables faster cutting of the wood block, thereby reducing energy loss. Nevertheless, at a speed of 110 mm/min, the energy consumed by the 80° knife exhibits a slight increase. The rate of decrease in energy consumed by the 83° knife begins to slow down, while the rate of decrease in energy consumed by the 90° knife remains constant. The energy consumed by the 80° knife during cutting at speeds of 15, 37.5, 90, and 110 mm/min is the largest among the three knives, but the excess compared to the other two can be considered negligible. In conclusion, the trend of the curves indicates that the energy consumed by cutting with the three knives with different angles decreases as the speed increases. As the speed continues to increase, the energy consumed by cutting tends to converge, remaining between 0 and 4 J. A variance analysis was conducted on the evaluation indicator, namely, the energy loss in a single cutting operation, based on the obtained data. The results of the variance analysis for the energy loss are shown in Table 4. It can be seen from the table that the P values of both the regression term "included angle of the cutting edge" and the regression term "cutting speed" are less than 0.01, indicating that both of them have an extremely significant impact on the energy loss.

Influence of cutting conditions on the quality of the cut surface

The cutting surfaces of the 80° cutting tool at five speeds are shown in Fig. 4, and the influence of the rubber tree texture on the cutting surface is not considered. When the tool is used for cutting at speeds of 15, 37.5, and 60 mm/min, a large number of burrs appear, and when the speeds are 90 and 110 mm/min, wood wool appears. In the actual rubber tapping process, a large number of burrs may damage the latex ducts in the rubber bark or cause the rubber bark to develop bark necrosis, resulting in a reduction in latex yield, indicating that the surface quality obtained by cutting at these five speeds does not meet the requirements of rubber tapping. The cutting surfaces of the 83° cutting tool at five speeds are shown in Fig. 4. When cutting at speeds of 15, 37.5, 90, and 110 mm/min, the surfaces are relatively flat, without obvious burrs or wood wool. When the speed is 60 mm/min, only a small number of negligible burrs appear on the inner edge of the cutting surface. It indicates that the surface quality obtained by cutting at these five speeds meets the requirements of rubber tapping. The cutting surfaces of the 90° cutting tool at five speeds are shown in Fig. 4. When cutting at speeds of 15 and 37.5 mm/min, the surfaces are relatively flat, without obvious burrs or wood wool. When cutting at speeds of 60, 90, and 110 mm/min, burrs appear, and when the speed is 110 mm/min, the cutting surface cracks. It indicates that the surface quality obtained by cutting at low speeds meets the requirements of rubber tapping, that at medium speeds does not, and that at high speeds has the problem of cutting surface cracking.

The surface microtopography of the 80° cutting tool after cutting at five speeds (Fig. 5) is as follows: at a speed of 15 mm/min, the surface has good flatness, with only a local abnormal protrusion in the upper right corner; at 37.5 mm/min, the surface has a slightly larger undulation; at 60 mm/min, the lower half of the surface has a large undulation, with a concave in the middle and a relatively flat upper half; at 90 mm/min, the whole surface is relatively flat; at 110 mm/min, there is a significant local abnormal protrusion in the lower half of the region, and there are occasional local abnormal protrusions in the remaining part (possibly due to incomplete focusing during instrument scanning).

The surface microtopography of the 83° cutting tool after cutting at five speeds (Fig. 5) is as follows: at a speed of 15 mm/min, the middle part of the surface is concave, and there are obvious protrusions in the upper and lower half parts; at 37.5 mm/min, the whole surface is relatively flat, with only a concave in the upper left region; at 60 mm/min, the right half of the surface is significantly lower than the left half; at 90 mm/min, the overall undulation is not obvious; at 110 mm/min, the surface has good flatness and is overall flat.

The surface microtopography of the 90° cutting tool after cutting at five speeds (Fig. 5) is as follows: at a speed of 15 mm/min, the surface has a large undulation, and there is an obvious concave in part of the upper half region; at 37.5 mm/min, the left half of the surface is flat and the right half has an obvious undulation; at 60 mm/min, the lower left region is low and flat and the upper right region has an obvious undulation; at 90 mm/min, there are too many local protrusions on the whole surface; at 110 mm/min, the middle and most regions of the surface have a severe concave (due to the cracking of the cutting surface).

Given that the images are unable to provide a clear indication of the differences in flatness between different cutting planes, the arithmetic mean height (Sa) and maximum height (Sz) of each cutting plane have been compiled. Figure 6a presents the arithmetic mean height (Sa) of the cutting planes. The data indicates that there is no significant difference in the arithmetic mean height of the planes obtained by cutting with 80°, 83°, and 90° knives at speeds of 15, 37.5, 60, and 90 mm/min. However, the arithmetic mean height of the cutting surface obtained with the 83° knife exhibits a decreasing trend as the speed increases. The arithmetic mean height of the planes obtained by cutting with 80° and 90° knives at a speed of 110 mm/min is almost identical and significantly higher than that of the 83° knife.

The maximum height (Sz) of the cutting planes is illustrated in Fig. 6b. The maximum heights of the cutting planes obtained using knives with angles of 80°, 83°, and 90° at speeds ranging from 15 to 37.5 mm/min all exhibit a decreasing trend. Furthermore, the cutting surface of the 80° knife exhibits a greater maximum height than that of the 83° knife, and the 83° knife has a greater maximum height than the 90° knife. When cutting at speeds ranging from 37.5 to 60 mm/min, the maximum height of the cutting surface of the 80° and 90° knives exhibits a slight upward trend, whereas the maximum height of the 83° knife's cutting surface remains in a downward trend. In contrast to the findings at 60 mm/min, the situation at 90 mm/min is reversed. During cutting at speeds ranging from 90 to 110 mm/min, the maximum heights of the cutting surfaces of the 80° and 83° knives exhibit a pronounced downward trend, with the final values approaching each other. Nevertheless, the maximum height of the cutting surface of the 90° knife exhibits a pronounced upward trend.

Influence of cutting conditions on knife wear

The areas enclosed by the red rectangles in Fig. 7 represent the wear areas of the knives, and the selected areas are consistent, each with an area of 360 mm². By comparing the areas lost due to wear on the wood blocks in the selected regions, as shown in Fig. 7d, it can be concluded that the 80° knife had the largest wear area with a loss of 81.5312 mm². The 83° knife showed a wear area in the middle with a loss of 64.8932 mm². Finally, the 90° knife had the smallest wear area with a loss of 40.4512 mm². Specifically, the 83° knife's wear area was 60.42% larger than that of the 90° knife, while the 80° knife's wear area was 101.55% larger than that of the 90° knife.

Influence of cutting conditions on chip formation under axial cutting

Wood fibers are mechanical tissues composed of lignified and thickened cell walls and fiber cells with fine slit-like pits [35]. Radial cutting interrupts the wood fibers, requiring greater force to sever the thickened cell walls within the wood fibers; in contrast, axial cutting (or with-the-grain cutting), which proceeds along the grain direction of the wood, results in smoother cut surfaces with minimal cross-sectional deformation. As demonstrated by Alen Ibrišević [36], during the cutting of three types of wood, cutting perpendicular to the grain (radial cutting) led to an increase in the roughness value of the cutting surface compared to cutting parallel to the grain (with-the-grain cutting). Consequently, this experiment employs axial cutting to achieve better surface flatness and minimal cutting force. The SEM image of the cutting surface under axial cutting is shown in Fig. 8a, b. The interior of rubberwood comprises wood fibers and anatomical pores, which are naturally formed to facilitate water and nutrient transport during tree growth. Figure 8a reveals that some wood fibers are intact in length, while others are incomplete, potentially due to the blade's dullness during axial cutting or its failure to cut vertically.

In the rubber tapping work in actual rubber plantations, specific requirements also exist for the bark consumption amount in rubber tapping, which is generally required to be within 0.5–2.4 mm. The bark consumption amount refers to the thickness of the bark removed from the surface of the rubber tree trunk during each rubber tapping operation. During rubber tapping, the latex ducts in the bark need to be cut open to allow the latex to flow out. If the bark consumption amount is less than 0.5 mm, it may not be possible to fully cut open a sufficient number of latex ducts, resulting in a reduction in latex yield. On the other hand, if the bark consumption amount exceeds 2.4 mm, excessive damage will be caused to the bark of the rubber tree, the self-repair ability of the tree body will be challenged, the risk of the tree body being infected by diseases will be increased, the healthy growth of the rubber tree will be affected, thereby shortening the latex-producing lifespan of the rubber tree and ultimately reducing its economic efficiency.

Based on field investigations and interviews with experienced rubber farmers, this article determines that the bark consumption for cutting is 2 mm, the cutting thickness is 2 mm. The chip morphologies obtained using three types of cutting knives at five different speeds are shown in Fig. 8c. Where a, b, c, d, and e, respectively, represent the chips obtained by cutting at five speeds of 15, 37.5, 60, 90, and 110 mm/min. It can be observed from the figures that most of the chips are arcuate in shape, consistent with the chip morphology formed during axial cutting. During the cutting process, the blade enters the gaps between the aligned wood fibers, and as the cutting progresses, the wood fibers are compressed, causing the cut portion to bend outward. However, a small portion of the chips may show cracks at one-third of the arc, but they remain connected to the overall chip and do not completely split. The chips obtained using the 90° knife at a speed of 110 mm/min have a long stripe-like appearance, similar to the shape of the part to be removed. This is due to the fact that at this speed, the 90° knife is subject to chipping, which is unacceptable in the actual rubber tapping process.

Influence of cutting conditions on the flow velocity of latex

Under consistent conditions, rubber wood blocks were placed on a 25° inclined plane and the latex flow time was recorded for comparison, as shown in Fig. 9.

At a speed of 15 mm/min, the latex flow rate on the cutting surface of the 80° knife was the lowest, followed by the 90° knife, and the highest flow rate was observed on the cutting surface of the 83° knife. At 37.5 mm/min, the latex flow rates on the cutting surfaces of the 80° and 90° knives increased, but the latex flow rate of the 80° knife remained the lowest, while the latex flow rate on the cutting surface of the 83° knife decreased slightly, similar to that of the 80° knife. At 60 mm/min, the latex flow rates on the cutting surfaces of all three angles increased, which may be related to the better surface smoothness. At 90 mm/min, the latex flow rates on the cutting surfaces of the 80° and 83° knives increased to a peak, with the 83° knife exhibiting the highest latex flow rate of 1.068 mm/s, while the latex flow rate on the cutting surface of the 90° knife remained largely unchanged. At 110 mm/min, the latex flow rates on the cutting surfaces of all three angles decreased significantly.

Comprehensive evaluation of the cutting performance of V-shaped rubber tapping knife

In actual rubber tapping operations in rubber plantations, the quality of the tapping surface and knife wear are critical factors resulting from the combined effect of various tapping variables. Intelligent tapping equipment requires precise utilization of V-shaped tapping knives with appropriate force and angle to perform the tapping task. Therefore, it is necessary to determine the most appropriate angle of the blade and the required force during tapping. In addition, the smoothness of the tapping surface formed by the cutting action of the tapping blade must meet the required standards, and the latex flow rate on the tapping surface should not be too low.

During the simulated rubber tapping experiments, the knife is subjected to repeated cutting operations. During this process, wood fibers are continuously severed, and the overall smoothness of the cutting surface is determined by the degree of flatness achieved as the wood fibers are cut. Unlike actual rubber tapping, where there are variations in the bark of different rubber trees, the dimensions and material specifications of the wood blocks used in the simulation are consistent. Since no researchers have performed similar simulated rubber tapping experiments, this demonstrates the need for such simulations.

From the images of the cutting surfaces, it can be seen that under different knife angles and feed speeds, wood wool and burr can appear on the cutting surfaces, which significantly affects the smoothness of the cutting surfaces. From the images of knife wear after cutting, it is clear that changing the angle has a significant effect on knife wear. In addition, the differences in latex flow velocity also indicate that the flow velocity of different cutting surfaces is significantly different.

Based on the above data analysis, the two factors that have the greatest impact on the quality of the cutting surface and knife wear were identified: (1) blade angle and (2) feed speed. To comprehensively evaluate the cutting performance of the V-shaped cutter, a quantitative analysis of the various influencing factors was performed, and the results are shown in Fig. 10. As can be seen from the figure, the comprehensive cutting performance of the 83° knife is the best; combined with the above analysis, the optimal feed speed is 90 mm/min. Ultimately, according to the experimental data presented in this paper, the best cutting conditions are with an 83° knife and a feed speed of 90 mm/min, which provides the best comprehensive evaluation of cutting surface quality and knife wear.

Under actual rubber tapping conditions, a simulated rubber tapping experiment was conducted to study the cutting surface quality of rubber wood blocks obtained from three different angled knives at five different speeds. The flow velocity of latex on different cutting surfaces was compared. The wear condition of the knives was also investigated and analyzed. Based on the obtained results, the following conclusions can be drawn: the studied knives showed differences in cutting surface quality, latex flow velocity and knife wear. Among the knives studied, the 83° knife achieved the best cutting surface quality, while the 90° knife showed the least wear. On the other hand, the cutting surface obtained at a feed speed of 90 mm/min showed the highest latex flow velocity.

1. 1.

   The maximum cutting forces of the three knives increased synchronously with the increase in speed, with the 90° knife always having the smallest maximum cutting force. However, the energy consumed during cutting decreased as the speed increased, with the minimum occurring at a feed speed of 110 mm/min.

2. 2.

   The cutting surface quality of the 83° knife was the best at all five speeds, with almost no burrs or wood fuzz, and the surface smoothness was also optimal. Its arithmetic mean height and maximum height were the best among the three, reaching 10.598 μm and 98.274 μm at a speed of 110 mm/min.

3. 3.

   Under the same cutting conditions, the 90° knife had the smallest wear area of only 40.4512 mm². The wear area of the 83° knife was 1.6 times larger, and the wear area of the 80° knife was 2.01 times larger than that of the 90° knife.

4. 4.

   With all other factors held constant, the fastest latex flow velocity for all three angled knives occurred at a feed speed of 90 mm/min. The maximum velocity was achieved by the 83° knife, specifically 1.068 mm/s, followed by the 80° knife and finally the 90° knife.

5. 5.

   Based on the experimental data in this paper, the optimal rubber tapping conditions were determined to be the 83° knife and a tapping speed of 90 mm/min, resulting in the best comprehensive evaluation of cutting surface quality and knife wear.

The data sets used or analyzed during the current study are available from thecorresponding author upon reasonable request.

This work was supported by the National Natural Science Foundation of China (52305251). We would like to thank all the reviewers who participated in the review. We also thank the funders of the project. All supports and assistance is sincerely appreciated.

This research was funded by the National Natural Science Foundation of China (52305251).

Authors and Affiliations

1. School of Mechanical and Electrical Engineering, Hainan University, Haikou, 570228, China

   Shaofeng Ru, Yangding Han & Xirui Zhang

Contributions

Conceptualization, S. Ru and Y. Han.; methodology, S. Ru and Y. Han.; formal analysis, S. Ru. and X. Zhang.; investigation, S. Ru. and Y. Han.; resources, S. Ru., YD. Han. and X. Zhang.; writing—original draft preparation, Y. Han.; writing—review and editing, S. Ru. and X. Zhang.; supervision, S. Ru.; project administration, S. Ru. All authors have read and agreed to the published version of the manuscript.

Corresponding author

Correspondence to Shaofeng Ru.

Competing interests

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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