This study presents the results of tool wear and surface roughness of wood processed by plain milling. The tests were done on wood samples of pine and black alder grown in Lithuania in order to clarify time-related tool blunting and the aspects of surface formation. The samples were milled along the fiber in the experimental wood cutting stand at two different cutting and feed speeds. The roughness parameter (Rz) of the processed samples was measured in five sectors along and across the fiber using a contact profilometer. Registered values were analyzed by a Gaussian digital filter and evaluated according to relevant statistics seeking to minimize influence of wood anatomy. The obtained results helped to determine distinctions and variations of surface roughness, which strongly depend on the cutting path, rounding radius of the tool’s cutting edge, cutting, and feed speeds while milling pine and black alder.
Tool Wear Evolution and Surface Formation in Milling Various Wood Species
Gintaras Keturakis,a Regita Bendikiene,b,* and Antanas Baltrusaitisa
This study presents the results of tool wear and surface roughness of wood processed by plain milling. The tests were done on wood samples of pine and black alder grown in Lithuania in order to clarify time-related tool blunting and the aspects of surface formation. The samples were milled along the fiber in the experimental wood cutting stand at two different cutting and feed speeds. Roughness parameter Rz of the processed samples was measured in five sectors along and across the fiber using the contact profilometer. Registered values were analyzed by Gaussian digital filter and evaluated according to relevant statistics seeking to minimize influence of wood anatomy. The obtained results helped to determine distinctions and variations of surface roughness witch strongly depend on the cutting path, rounding radius of the tool’s cutting edge, cutting and feed speeds while milling pine and black alder.
Keywords: Wood milling; Tool wear; Surface roughness, Pine wood, Black alder wood
Contact information: a: Department of Materials Engineering, Kaunas University of Technology, Studentu str. 56, LT-51424 Kaunas, Lithuania; b: Department of Productions Engineering, Kaunas University of Technology, Studentu str. 56, LT-51424 Kaunas, Lithuania; *Corresponding author: email@example.com
One of the main criteria to evaluate the quality of the processed surface is its roughness. It determines the further processing and finishing of the surface, appearance and usage possibilities (Richter et al. 1995; Follrich et al. 2010; Ozcan et al. 2012; Kuljich et al. 2013; Pelit et al. 2015). Wood surface assessment is highly subjective from the scientific, technological, or product exploitation points of view; nevertheless, research based quantitative and qualitative surface texture estimation exists over various aspects of surface formation in wood milling. Distinction between anatomic and processing unevenness, which highly vary for different wood physical properties, grain angle and tool nose conditions, remains problematic (Magoss 2008; Kilic et al. 2007; Malkocoglu 2007; Aslan et al. 2008; Kilic 2015).
Nowadays wood milling process testing, modelling and simulation in the works of different authors remains classic and pragmatic, showing that following main factors affect the surface roughness the most: species of wood, mode of surface milling, the rounding radius r of the cutting edge, cutting vc and feed vf speeds (Malkocoglu 2007, Gaff et al. 2015; Kvietkova et al. 2015; Ghosh et al. 2015; Hernández and Cool 2008; Hernández et al. 2014, Škalić et al. 2009, Novák et al. 2011; Usta et al. 2007).
Wood is an anisotropic biocomposite (Niska and Sain 2008; Stokke et al. 2014). The wooden cells that participate in forming the anatomic unevenness of the wood surface are cut or deformed during the mechanical processing (Goli et al. 2001; Magoss 2008). When the machining of the wood is analyzed, the anatomic unevenness of the surface is usually not taken into the account, because in milling it is usually considerably lower if compared to the mechanical ones.
Postulates of the classical wood cutting theory state that the surface roughness increase with the wear of cutting tool (Magoss 2008). The main cause of the tool wear is considered to be mode of the tool nose and front friction to the wood (Beer et al. 2003; Beer et al. 2005; Beer 2005). Therefore, changes in the form of the cutting edge mainly depend on the angular and micro-geometrical parameters of the tool (Porankiewicz 2006; Kowaluk et al. 2009; Azemović et al. 2014).
Wood cutting theories give various models of chip and surface formation. For milling processes following algorithm related to tool wear evolution explains cutting phenomena’s (Csanády and Magoss 2013): when the wood is milled with sharp tool (r < 20 μm), the beginning of the cutting process is considered to be concentrated at the contact of the cutting edge with the minimal contact stress matrix to the wood (Keturakis and Juodeikiene 2007). The cutting edge of the tool cuts the wooden fibers and forms the continuous shavings of the regular form. The quality of the processed surface is of the highest quality and the defects are caused by the unfavorable direction of wooden fibers or microcrushing at the top of the cutting edge (Su et al. 2002; Su et al. 2003).
When milling is done using the dull tool (20 < r < 40 μm), the undesirable process of tool nose bottom sliding on the wooden surface appears (Keturakis and Juodeikiene 2007). During the sliding period the tool’s cutting edge slides along the surface of the wood, deforms and compresses it. Under the influence of the viscous-elastic deformation, the wooden surface amortizes the compressing effect of the tool’s cutting edge. However, the plastic and residual deformations cause the formation of resilient wooden layer in front of the tool that rolls as the wave. When the tensions of this surface layer achieve the critical limit, the fiber disruption process starts. When the fibers are cracked and up-lifted, the tool’s cutting nose effectively reaches the wood. This is the end of the sliding process and beginning of the chip cutting. The quality of the processed surface gets substantially worse compared with sharp tool, the separate splits and rougher stress-recover parts appear (Magoss and Sitkei 2001; Goli et al. 2001).
When milling is done with the blunt tool (r > 40 μm), the irregular cutting process takes place (Keturakis and Juodeikiene 2007). The blade forms the surface not directly and primarily by the tool nose cutting, but mostly through deep stress penetration and fiber compression and disruption. Due to the resilient changes of the sliding period the place of slippage through the fibers extends much more and remote forward from the cutting nose contact zone. This is the reason why the uncontrolled chip splits along the grain appear causing noticeable rougher surface parts. Besides, the resilient density related stress recovery of the machined wood surface up to 0.2 mm or even more starts. The quality of the processed surface does not satisfy acceptable quality criteria (Magoss and Sitkei 2001).
Similar model might be applied to the milling kinematics. When the feed per cutter fz grows from 0.5 to 3.0 mm, the kinematic (processing) unevenness appears and the quality of the surface decreases (Magoss and Sitkei 2001). The waviness of the surface is attributed to the kinematic unevenness. It is formed by the rotating movement of the milling tool. The surface waviness is described by the length and depth of the wave. It is possible to calculate and predict these parameters. The size of the kinematic unevenness depends on the number of the cutters taking part in the cutting process, cutting radius, feed per cutter and cutting speed, but essential is grain direction. The most optimal cutting regime and the best quality of the surface are achieved, when the feed per cutter fz is from 1.0 to 2.0 mm (Magoss and Sitkei 2001; Brown and Parkin 1999; Jackson et al. 2002; Hynek et al. 2004).
According to the results of various tests, when the cutting speed increases, the quality of the surface normally improves. For the cylindrical milling of the wood planes the recommended cutting speed is from 35 to 55 m/s. In this range of the cutting speed the best quality of the surface and the lower numeric values of the cutting forces are achieved. When the cutting speed is further increased (vc > 60 m/s), the cutting force increases and the vibration of cutting tool becomes more active. The tool’s vibration creates additional unevenness that decreases the quality of the processed surface (Magoss 2008; Kvietkova et al. 2015; Gaff et al. 2015).
The angle of fiber’s direction primarily effects the quality of the surface. The surface roughness decreases with the increase of the angle between the fiber’s direction and the vector of cutter’s feed speed. However, the testing results showed that when the moderately blunt tool is used, the surface roughness decreases with the increase of the angle of fiber’s direction up to 30 degrees, and then it starts grow again. When the tests with the blunt cutter were done, the opposite effect was noticed, i.e. the roughness increases when the fiber’s angle makes from 30 to 40 degrees (Goli et al. 2010).
Modern wood milling studies provide more and more knowledge on tool interactions with wood and subsequent surface formation. The objective of study presented is to determine the influence of the cutting path and rounding radius of the cutting edge on the surface roughness, when the wood samples of pine and black alder are milled along the fiber at different cutting and feeding speeds. Special attention is given to the phenomenon emerging during transition from the initial and highly dynamic tool nose wear towards more stable and monotonic abrasion.
The testing samples were made from the wood of pine and black alder grown in Lithuania (Table 1). In total 60 samples were prepared for testing, which length of 1000 mm, width 100 mm, and thickness 45 mm. The average temperature in the testing room was t 18 2 C, while relative air humidity was 60 5 .
Table 1. Physical Characteristics of Wood
The high-speed tool steel (HS 18-0-1) milling knives were used for the tests (Table 2). The chemical composition of the steel HS 18-0-1 (ISO 4957:2003) presented in Table 3. Before the tests, all the knives were sharpened in the same conditions and then the blades were converged.
The tests were done in the stand for wood cutting, specially arranged on the base of thickness planer (SR3-6). The samples were processed according to the scheme of the longitudinal milling, when the directions of the cutting speed vc and feed speed vf vectors are opposite. The conditions of milling tests are presented in the Table 4. Two knives were fastened in the cylindrical head of the knives, but only one took part in the cutting process. The second was used for balancing compensation.
Table 2. Specifications of Milling Tool
Table 3. Chemical Composition of HS 18-0-1 Steel
Table 4. Milling Test Conditions
The thickness of the shaving a (mm) was changed indirectly, through the feeding per cutter fz 0.50 and 1.00 mm. The samples were processed at two cutting speeds vc 22 and 40 m/s.
The main characteristic for describing the wear of milling tool was selected to be the rounding radius r, μm of the cutting edge. The factual values of rounding radius were determined using the method of lead imprints (Miklaszewski et al. 2000) and optical microscope (Nikon Eclipse E200) with digital video camera (Lumenera Infinity 1).
The values of the rounding radius r of the cutting edge were measured at the following intervals of the effective (real) tool nose cutting path L: 0; 50; 100; 150; 200; 400; 800 and 1600 m. The measurements were repeated five times in each interval of the path. The received results were measured and processed using the personal computer and software (Infinity Analyze Release 5.0.2). The received results were processed using the methods of mathematical statistics and error of radius measurement was 2 m.
The parameter of the processed surface roughness Rz (m) was measured by contact stylus profilometer (Mahr MarSurf PS1), the radius of its diamond tip was 2 μm, measurement angle 90º, and measurement length was 17.5 mm. The surface unevenness was measured in the same intervals of cutting path L: 50; 100; 150; 200; 400; 800 and 1600 m. Five sectors were selected in one sample (17.5×17.5 mm), and their roughness was measured along and across the fiber. In total 280 measurements were performed through the testing series. All the measurement results were processed by Gaussian digital filter and roughness measurement error did not exceeded ± 10 %.
RESULTS AND DISCUSSION
The performed tests allowed to determine the influence of the cutting path and reached rounding radius of the cutting edge on the surface roughness, when the wood samples of pine and black alder are milled along the fiber at different cutting and feed speeds.
While analyzing the received results on tool’s wear (Table 1), it was determined that the tool gets worn the most intensively in the first wear stage until the limit of cutting path L = 400 m. In the first stage of wear the species of wood does not have any significant influence on the wear intensiveness. The difference between the values of the rounding radius of the cutting edge in case of milling pine and black alder samples was from 5 to 7 %. In this stage the wear of tool is expressed by mechanical crumbling and deburring of sanding scarfs on cutting edge; wood species or density are not decisive or does not affect the blunting process. When the cutting path reaches the distance of 400 m, the tool’s wear gradually passes to the stage of monotonic wear. In this stage the intensity of rounding radius growth is reduced, the further wear of tool is even. The microgeometry of cutting edge changes because of temperature effect and just negligibly because of abrasive wear (Porankiewicz 2006; Pamfilov et al. 2014). The tool’s wear was observed until the cutting path of 1600 m.
When the influence of the wood species on tool’s wear was analyzed, it was noticed that the tool gets worn more intensively when black alder is milled, although the density of black alder wood is lower if compared to the pine wood (Table 1). However, the exception was also noticed: when the feed per cutter is fz = 0.5 mm, and the cutting speed is increased from 22 to 40 m/s, the tool’s wear is lower if compared to the pine wood milling results. When the cutting speed increases, the large volume of the shavings lose contact with the processed surface due to the pine’s tendency to split under the influence of lower cutting forces. The shaving is formed easer; the true cutting length of cutter’s contact with the wood are reduced when compared to the milling of black alder which tendency to split less.
While analyzing the influence of feed per cutter (shaving thickness) on the rounding radius of the cutting edge, it was determined that when the feed per cutter grows from 0.5 to 1.0 mm, the intensity of the tool’s wear gets lower. When the feed per cutter is fz = 0.5 mm, the most intensive wear of the tool takes place when the black alder is milled. When the pine wood is milled, the tool worn down in 4 % lower if compared to black alder.
The received results are interesting for interpretation the postulates of classical wood cutting theory. With increase of feed per cutter, the shaving’s length increases a bit, as well as true contact of cutting edge with wood. Wherefore because of increment of shaving thickness the intensity of tool wear should grow, and the tendency of dry wood splitting should increase due to real friction and wear of cutting edge. Such effects depend on wood materials and machining factors, and usually do not completely follow mathematical equations and models.
Results presented in Fig. 1 demonstrate existence of clear regularity and distinctive character of transition between initial and monotonic tool nose wear. Certainly, individual cases for different wood specie differ, but wear modes received suggest possibilities to create reliable modeling explaining initial and monotonic tool nose wear phenomena’s. Such a mechanical-tribological tool nose wear mathematical model is the objective of ongoing specialized theoretical and experimental research of the authors of this paper.
Fig. 1. Impact of cutting path L on the rounding radius r of the cutting edge: a – when vc = 22 m/s; b – when vc = 40 m/s.
While analyzing the results of surface roughness (Figs. 2 and 3), it was noticed that the surface of pine wood was smoother than that of black alder. This regularity does not change when the surface roughness is measured along and across the wood fiber. The tests confirm the theory that in case of higher density of the wood and lower width of rings (Table 1), the smoother surface is received. Although the density of pine wood is higher only by 9 %, but the received surface roughness Rz along the fiber in average by 11 % lower, and across the fiber – by 14 %.
It is evident that the increase of the cutting path and rounding radius of tool’s cutting edge causes the degradation of the milled surface quality. This tendency was noticed in all the cases of cutting speed and feed per cutter. The highest surface roughness is achieved while milling up to 200 m of the cutting path. From 200 to 800 m limit the surface roughness along and across the fiber increases gradually. As expected, the highest numeric values of the surface roughness Rz were achieved with cutting path of 1600 m.
Fig. 2. Cutting and feed speed effects on surface roughness Rz along the fiber: a – when vc = 22 m/s; b – when vc = 40 m/s.
When the feed per cutter is increased, the surface roughness also increases. The best quality of the surface was reached when the feed per cutter was fz = 0.5 mm, when fz = 1.0 mm lead to lower quality; it does not depend on the species of wood and cutting speed. The lowest increase of numeric values of surface roughness Rz was noticed while processing pine wood.
When the cutting speed is increased from 22 to 40 m/s, the roughness of pine wood is decreased on average by 12 % and that of black alder – by 7 %. This surface roughness declining tendency remained in various segments of cutting path when the cutting speed was increased. It is possible to state that with increase of cutting path the wear of the tool gets more intensive but with different dynamics of blunting. When the wood is milled with blunt tool, the irregular milling process takes place, during which the top layers of the fibers on the newly formed surface are deformed and compressed with different intensity and in different depth.
Fig. 3. Cutting and feed speed effects on surface roughness Rz across the fiber: a – when vc = 22 m/s; b – when vc = 40 m/s.
Contrary the above presented hypothesis to explain and model tool nose wear phenomena’s, variation of surface roughness results (Fig. 3) show more tendencies than theoretical regularities. However, some stochastic roughness behavior at the initial cutting path up to 400 m and following monotonous surface formation was noticed. This fact confirms presumptions on extreme importance of enhancing knowledge on tool wear and surface formation stages and dynamics to facilitate evolution of tools, creation eco- and energy-effective cutting processes and highly productive wood machining.
- The milling tool worn down the most intensively when the first 400 m of cutting path was milled. After 400 m of cutting path the milling tool’s wear gradually passes to the phase of monotonic wear.
- The tool worn down more intensively when black alder was milled; wear of tool by 6 % lower while milling of pine wood.
- With increase of feed and cutting speeds, the intensity of the wear of milling tool was reduced.
- The rounding radius of the milling tool’s cutting edge has the highest influence on the roughness of the milled surface along and across the fiber. The best quality of the surface was reached when sharp (r ≤ 13 μm) milling tool was used in the segment up to 400 m of the cutting path. When the rounding radius of the tool’s cutting edge had been increasing, the quality of the processed surface had decreased.
- When the feed per cutter had been increasing, the quality of the processed surface had decreased. The best quality of the surface was reached when the feed per cutter was fz = 0.5 mm, it does not depend on the species of wood and cutting speed.
- When the cutting speed was increased, the quality of the processed surface had increased. The best quality of the surface was reached when the cutting speed was vc = 40 m/s.
- The surface quality of pine wood milled under the same conditions is higher than that of black alder. The surface roughness Rz of pine wood along the fiber is lower in average by 11 %, and across the fiber – by 14 %.
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