Abstract
The effects of the Engineered Micro-Geometry (EMG) of the carbide teeth of circular saws on their wear rate and resulting sawing variation for 2-time intervals were studied. The objective was to improve the wear resistance of circular saws used during the primary transformation of wood. The tests were carried out under industrial production conditions with two series of circular saws; 1- with up-sharp tips, and 2- with cutting edges honed to adopt a waterfall geometry. The duration of the tests was 255 min and 645 min. Wood studs were sampled to measure sawing variation. Recession on the rake and clearance faces of the tips as well as the width of the wear land were measured. The wear mechanisms of the cutting edges of both types of saws were studied. Chipping and cracking were the two dominant wear mechanisms observed on the up-sharp tips. Saws with waterfall hone tips showed remarkably reduced chipping and cracking. Wear rate of the latter was notably lower than that of saws with up-sharp tips at both periods of sawing. Between-stud, within-stud, and total sawing variations decreased when saws with modified cutting edges were used.
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Improved Life of Circular Saws Used in Primary Wood Processing
Maryam Torkghashghaei,a William Shaffer,b Bruna Ugulino,c Rémi Georges,c Roger E. Hernández,d and Carl Blais a,*
The effects of the Engineered Micro-Geometry (EMG) of the carbide teeth of circular saws on their wear rate and resulting sawing variation for 2-time intervals were studied. The objective was to improve the wear resistance of circular saws used during the primary transformation of wood. The tests were carried out under industrial production conditions with two series of circular saws; 1- with up-sharp tips, and 2- with cutting edges honed to adopt a waterfall geometry. The duration of the tests was 255 min and 645 min. Wood studs were sampled to measure sawing variation. Recession on the rake and clearance faces of the tips as well as the width of the wear land were measured. The wear mechanisms of the cutting edges of both types of saws were studied. Chipping and cracking were the two dominant wear mechanisms observed on the up-sharp tips. Saws with waterfall hone tips showed remarkably reduced chipping and cracking. Wear rate of the latter was notably lower than that of saws with up-sharp tips at both periods of sawing. Between-stud, within-stud, and total sawing variations decreased when saws with modified cutting edges were used.
DOI: 10.15376/biores.18.1.2024-2044
Keywords: EMG; Circular saw; Wear; Chipping; Sawing variation
Contact information: a: Department of mining, metallurgical and materials engineering, Université Laval, Quebec City, Canada; b: Conicity Technology Inc, Turtle Creek, PA, USA; c: FPInnovations, Quebec City, QC, Canada; d: Department of wood and forest sciences, Université Laval, Quebec City, Canada
* Corresponding author: carl.blais@gmn.ulaval.ca
INTRODUCTION
As for many manufacturing sectors, wear control of cutting tools is critical in the wood industry. Indeed, cutting tools suffer damages of fluctuating severity that stem from various tribological phenomena (Naylor and Hackney 2013). These damages contribute to the loss of productivity and product quality (Etele and Magoss 2013). Circular saws are common cutting tools used in the first and second transformation of wood. The performance of saws is mainly related to the wear resistance of their cutting edges. Cutting edges in circular saws used for wood processing are mainly made from tungsten carbide-cobalt (WC-Co) inserts, which are well known for providing superior wear resistance. The level of sharpness of these tips affects the quality of the finished products. Thus, periodic resharpening and/or replacement of carbide tips are required throughout the useful life of circular saws. The amplitude and frequency of such operations have a remarkable effect on production costs, which can be improved by minimizing tool wear (Nasir and Cool 2020). The blunting and wear of cutting tools generally depend on several factors such as tool geometry, cutting parameters (cutting speed, feed speed, etc.), and workpiece conditions (wood species, moisture content, temperature, wood density, knottiness, etc.) (Geetha and Jegatheswaran 2010).
Nordström and Bergström (2001) reported that abrasion, localized corrosive attack, chipping, and cracks took place on the cutting edges of swaged saw teeth used in the primary transformation of wood. Abrasion was the prevailing wear mechanism due to the presence of sand particles. As discussed by Bayoumi et al. (1988) wood species with acidic content tend to cause chemical or electrochemical reactions, which remove the cobalt binder found in the WC-Co cutting tips. Bailey et al. (1983) reported the same results during cutting oak wood.
The high abrasive wear resistance of cemented carbide tools can be attributed to their high hardness (Prakash 1995). However, abrasion, crack, chipping, and fracture can occur, and these wear mechanisms may simultaneously lead to the gradual and/or sudden failure of carbide tools when facing severe loads (Sugihara et al. 1979). Some of these mechanisms may play a dominant role, depending on the cutting conditions (Ekevad et al. 2012). The sawing process entails intense contact between the cutting edges of the saw and the wood, which leads to increased temperature and stress in primary shear zone (Nordström and Bergström 2001). Additionally, the wear level of the cutting edges has a decisive influence on wood surface finish as well as its geometrical conformance (Ghosh et al. 2015). Therefore, optimization of the cutting edge is an important issue that involves the generation of a controlled geometry to minimize the impact of edge defects and micro-chipping that are inherent in all cutting tools (Zhuang et al. 2021).
Ventura et al. (2017) demonstrated that properly applied edge preparation improves the performance of cutting tools during machining of metals by retarding the onset or reducing the rate of chipping and abrasive wear on their clearance face. In metal cutting, typical cutting edge profiles applied by tool manufacturers are rounded (radius or waterfall hone shapes), chamfered, or a combination of the two (Kandráč et al. 2013).
For example, Ventura et al. (2015) surveyed the influence of different form factors of chamfers on tool wear performance of CBN tools in interrupted hard turning of metals. They found that a lower form factor resulted in a larger contact length between tool and workpiece which increased the force acting on the tool.
The cutting edges of circular saws used in North American sawmills are typically up-sharp. The lower toughness of up-sharp cutting edges is the root cause of the failure and excessive wear of saws. Several studies have been conducted regarding edge preparation of tools in the metal industry but not in the industry of wood transformation. Therefore, the present work summarizes studies on the effect of Engineered Micro-Geometry applied on the carbide tips of circular saws used in the first transformation of wood with the intent of maximizing their tool life. Comparison of sawing variations and prevailing wear mechanisms of circular saws with up-sharp tips and saws with waterfall hone tips are presented and discussed.
EXPERIMENTAL
The circular saws used in this study consisted of a steel body where carbide tips were brazed on the teeth of the saws. The saw specifications are presented in Table 1.
Table 1. Circular Saw Specifications
Following the brazing operations, the carbide tips of each saw were ground by a grinding machine (Vollmer) at the manufacturer. Therefore, after grinding all the faces, the cutting edge of the tips is defined as up-sharp geometry as presented in Fig. 1a. Three brand new commercial circular saws had the geometry of their tips modified at Conicity Technology, PA, USA. The modification was performed by honing the cutting edge of each tip using abrasive filament brushes made of nylon bristles loaded with micrometric natural diamond particles. The prepared cutting edge had waterfall hone profile as illustrated in Fig. 1b.
The dimensions of these profiles on the clearance face (Sα) and rake face (Sγ) are very important. A form factor K = 1 (K = Sγ / Sα) defines a symmetrical cutting edge microgeometry, whereas the slope tendency of honing towards rake face (K > 1) or clearance face (K < 1) are asymmetrical microgeometries (Denkena et al. 2011). For metals, the advantages of an asymmetrical honed cutting edge (waterfall hone shape) over a symmetrical one (radius hone shape) in minimizing tool wear were reported in several studies. The new profile developed on the carbide tips of the saws used in this study measured 20 microns on the rake face of the tip (Sγ) and 10 microns on the clearance face (Sα). The geometry of this profile was selected based on previous work performed internally at Conicity Technology.
Fig. 1. Schematic representation of the (a) up-sharp cutting edge and (b) prepared cutting edge
To perform wood cutting tests in an industrial mill, six new commercial circular saws were sent to a partnering sawmill located in the Province of Quebec, Canada. Three saws had prepared cutting edges, while the remaining three had up-sharp tips. The industrial wood cutting tests lasted for two different periods of time i.e., 255 and 645 min. All saws were mounted on the vertical arbor of the gang saw system (USNR VSS) as shown in Fig. 2. The tests were carried out by sawing cants of black spruce and balsam fir. These types of softwood are currently processed together and are used in similar situations at sawmills across Eastern Canada as they have similar properties.
Fig. 2. Image of the arbor of the guided gang saw system used in this study
The first phase of the test involved utilizing the saws in industrial conditions for 255 min of sawing. The rotational speed of the saws and the cutting speed were 2600 rpm and 83 m/s respectively. Owing to that, the feed rates were 145 and 164 m/min for the studs with a width of 152.4 and 101.6 mm, respectively. The feed per tooth was 1.33 and 1.50 mm for the two sizes of studs mentioned. At the start of the test (during 15 min of sawing), 10 studs that were cut between saws 2 and 3 were sampled as well as 10 studs cut between saws 4 and 5. This condition represents a wear level at 0 min for these saws. At the end of the test i.e., after 255 min of sawing, studs originating from the same two locations were collected for a total of 20 samples, and this latter condition corresponds to the wear level after 255 min for these saws. The collected studs had a length of 3.65 m, a width of 152.4 mm and a thickness of 50 mm. All 40 specimens along with the six saws were then transferred to Université Laval to measure sawing variations and tool wear.
A large number of parameters have been reported in the literature regarding means of characterizing tool wear and bluntness of the cutting edge including cutting edge recession, recession on the rake and clearance faces, nose width, and edge radius or cutting edge rounding (Geetha and Jegatheswaran 2010; Sheikh-Ahmad and Bailey 1999a).
For saws 1, 2, 5, and 6, quantitative characterization of wear was performed using a Reichert Jung 580 optical microscope, and the images were analyzed using the software Image J 1.53e. As presented in Fig. 3, parameters used to quantify the wear of the tips were the change in recession on the rake face (RR) and clearance face (RC) of all 42 tips, as well as the change in the nose width (NW) representing the width of the wear land (Cristóvão 2013). Moreover, tips from saws 3 and 4 located at the center of the arbor were randomly selected and extracted to characterize their wear in scanning electron microscopy, FEI Inspect S50. Wear was analyzed on the primary cutting edges, rake and clearance faces of the tips. It represented the level of wear after 255 min of sawing at the sawmill.
Fig. 3. Parameters used to measure tips wear (Cristóvão 2013).
Sawing variations were characterized by measuring the thickness of each stud with a caliper having a precision ± 0.02 mm at ten evenly spaced locations along each specimen. Since the surfaces that are closer to the drive shaft of the arbor typically show less deviation, it was decided to measure the sawing deviation on the edge of the studs that were furthest from the drive shaft, as in Fig. 4.
Fig. 4. Measurements of stud thickness at the edge furthest from the drive shaft
Once the measurements on the studs and wear characterization from the first test were completed, saws 1, 2, 5, and 6 were returned to the mill for the second leg of the study. They were mounted on the arbor gang saw system as described in Fig. 2 and were used during a complete work shift of 645 min. Upon completing the tests at the mill, the saws were returned to Université Laval to characterize the wear of the tips in optical and scanning electron microscopy. Wear characterization of the tips after 900 min (15 h) of utilization followed the same procedure as the one described above i.e., after 255 min of use.
RESULTS AND DISCUSSION
Wear Rate of Tips
A comparison of wear between the two types of circular saws (saws 1, 2 with waterfall hone tips vs. saws 5, 6 with up-sharp tips) vs. working time (0 min, 255 min, and 900 min) is shown in Fig. 5. The recession of the rake and clearance faces and the width of the wear land of all saws that were measured before the industrial tests (0 min) are presented in Table 2.
The recession on the rake and clearance faces, and the width of the wear land of circular saws with waterfall hone tips were considerably lower than those of the circular saws with up-sharp tips after 900 min of sawing at the sawmill. For the saws with waterfall hone tips, the initial values at time 0 min presented in Table 2, is due to the modification of their cutting edges according to Fig. 1b.
Table 2. Initial Measurements at 0 min on the Clearance Face, Rake Face, and the Width of the Wear Land of All Saws
Table 3 presents the average reduction in wear brought about by using saws with waterfall hone tips (saws 1 and 2) compared to saws with up-sharp tips (saws 5 and 6).
The rate of wear on the rake and clearance faces as well as the rate of increase of the wear land are presented in Table 4. As expected, for all saws, the wear rate was more important at the beginning of the test, which corresponds to the break-in period. As the working time increased, the wear rates decreased, reaching values 2 to 3 times lower than rates that prevailed at the beginning of the test. Similarly, the wear rates of the saws with up-sharp tips were 2 to 3 times higher than those of the saws with waterfall hone tips.
Table 3. Average Reduction in Wear after 255 and 900 min of Sawing While Using Saws with Waterfall Hone Tips Compared to Saws with Up-sharp Tips
Table 4. Rate of Wear of Saws 1, 2, 5, and 6 as a Function of Working Time
Fig. 5. Average wear of circular saws vs. working time: (a) recession on the clearance face,
(b) recession on the rake face, and (c) width of the wear land
The results of tips characterization by optical microscopy after 255 and 900 min of sawing are shown in Figs. 6 through 9. The symmetric geometry of the up-sharp tips (Fig. 6a) was converted into an asymmetric one having a slope toward the clearance face (Fig. 6b) or rake face (Fig. 6c). This wear profile was rarely observed in the case of waterfall hone tips (Fig. 7b, c). Most of the up-sharp tips had small and large chippings at their corners after 900 min of sawing (Fig. 8a-c). On the contrary, waterfall hone tips showed notably less chipping (Fig. 9a-c).
Fig. 6. Optical micrographs of randomly selected up-sharp tips at 60X magnification: (a) non-used tip, (b and c) after 255 min of sawing
Fig. 7. Optical micrographs of randomly selected waterfall hone tips at 60X magnification: (a) non-used tip, (b and c) after 255 min of sawing
Fig. 8. (a-c) Optical micrographs of randomly selected up-sharp tips after 900 min of sawing at 60X magnification
Fig. 9 (a-c) Optical micrographs of randomly selected waterfall hone tips after 900 min of sawing at 60X magnification
Wear Characterization
Analyses of the wear patterns of the tips revealed that their primary cutting edges were no longer straight after the industrial test at the sawmill. Representative SEM micrographs of the cutting edge after 255 and 900 min of working time are presented in Figs. 10 and 11. A comparison of Figs. 10a and b shows that the modified cutting edge was notably less damaged than that of unmodified tips after 255 min of sawing. This difference is even more obvious when comparing the tips tested for 900 min of sawing (Fig. 11a, b).
Fig. 10. Typical aspect of the cutting edge of carbide tips (clearance face view) after 255 min of sawing at 70X magnification: (a) with up-sharp tips, and (b) with waterfall hone tips
Fig. 11. Typical aspect of the cutting edge of carbide tips (clearance face view) after 900 min of sawing at 70X magnification: (a) with up-sharp tips, and (b) with waterfall hone tips
The SEM micrographs presented in Fig. 12 exhibit different manifestations of wear on the primary cutting edges of the up-sharp tips after 255 min of sawing. Cracking, chipping, and abrasive wear appear to be the main wear mechanisms responsible for the degradation of the cutting edges as seen in Fig. 12a-e. Crack formation is most likely the first mechanism to take place, leading to chipping as cracks grow and merge. Cracking and chipping appear to be more prevalent at the corners of the tips as local tool breakage can be easily observed in those locations. Corner wear or rounding edges were also common on the cutting edge of the carbide tips. The central sections of the cutting edges indicate that chipping and abrasion were the principal wear mechanisms in those locations. Plastic deformation was seen on the primary cutting edges of some tips, as shown in Fig. 12f.
Fig. 12. SEM micrographs of the cutting edges of the up-sharp tips (clearance face view) after 255 min of sawing at 50X and 100X magnifications: (a, b) crack and chipping, (c) abrasion and microchipping, (d, e) microchipping and corner wear, and (f) plastic deformation
Fig. 13. SEM images of the cutting edges of the up-sharp tips (clearance face view) after 900 min of sawing at 50X-500X magnifications: (a, b) crack and chipping, (c) abrasion, (d) crack, (e) chipping, and (f) breakage
Typical SEM micrographs of unmodified tips used during 900 min of sawing are shown in Fig. 13. The wear mechanisms are identical to those observed for the same tips after 255 min (Fig. 12). Cracking, chipping, and abrasive wear were the principal wear mechanisms involved as seen in Fig. 13a-e. Fig. 13d shows a crack parallel to the cutting edge that is the product of impact wear followed by a chip on the primary cutting edge of the tip. Edge chipping was the dominant failure mode after 900 min of sawing (Fig. 13e). The corners of the tips (Fig. 13f) sustained substantial local breakage, once again highlighting the weakness in the strength of this area of the cutting tools.
Typical SEM micrographs of modified tips are presented in Fig. 14. By comparing the micrographs of Fig. 14 with those of Fig. 12 and 13, the wear manifestations for the waterfall hone tips were notably less severe than that of the up-sharp tips. EMG edge preparation considerably reduced the tendency of the carbide tips to chip. For modified edges, the main wear mechanism involved was abrasive wear after 900 min of sawing (Fig. 14f). However, the abraded areas are less compared to those observed for unmodified edges (Fig. 12c, 13c).
Crack formation and propagation were markedly reduced, resulting in a much lower volume fraction of chipping of the primary cutting edges. Furthermore, corner strength was distinctly increased by the honing process to a point where none of the tips characterized showed localized corner breakage after 900 min of sawing. The latter observation certainly constitutes the most important contribution of edge preparation to the improvement of the wear resistance of circular saws used in the wood industry.