Abstract
Proper valorization of the sawing wastes in industrial sawmills is a permanent issue with strong economic and environmental stakes. Most industrial sawmills are equipped with chipper-canter heads reducing the outer part of the logs into chips used in the pulp and paper industry. Optimization in canter use would increase the acceptable proportion of exploitable chips for this industry. With chipper-canters, the cutting direction varies along the cut. This study investigates the impact of the angle formed between the cutting direction and the grain direction on the required cutting force and the chips’ geometry. Orthogonal cutting is conducted to simulate the chipper-canter machining operation on green beech. To lower the cutting forces when machining, aiming for a cutting direction as parallel as possible to the wood fiber is necessary. However, if this angle is too low, the chips’ generated geometries prevent them from a proper valorization of this resource. A compromise with grain direction between 50° to 70° both limits the cutting forces and improves the steadiness of the chip fragmentation.
Download PDF
Full Article
Effect of Grain Direction on Cutting Forces and Chip Geometry during Green Beech Wood Machining
Rémi Curti,* Bertrand Marcon, Louis Denaud, and Robert Collet
Proper valorization of the sawing wastes in industrial sawmills is a permanent issue with strong economic and environmental stakes. Most industrial sawmills are equipped with chipper-canter heads reducing the outer part of the logs into chips used in the pulp and paper industry. Optimization in canter use would increase the acceptable proportion of exploitable chips for this industry. With chipper-canters, the cutting direction varies along the cut. This study investigates the impact of the angle formed between the cutting direction and the grain direction on the required cutting force and the chips’ geometry. Orthogonal cutting is conducted to simulate the chipper-canter machining operation on green beech. To lower the cutting forces when machining, aiming for a cutting direction as parallel as possible to the wood fiber is necessary. However, if this angle is too low, the chips’ generated geometries prevent them from a proper valorization of this resource. A compromise with grain direction between 50° to 70° both limits the cutting forces and improves the steadiness of the chip fragmentation.
Keywords: Green wood; Orthogonal cutting; Cutting forces; Chip fragmentation; Ultra-fast imaging; Wood chips
Contact information: LaBoMaP (EA 3633), Arts et Métiers, Rue Porte de Paris, 71250 Cluny, France;
* Corresponding author: remi.curti@ensam.eu
INTRODUCTION
Among the products of French sawmills—barks, slabs, edgings, sawdust, and chips—chips have the most consistent added value. In 2015, 2,370 kilotons (16%) of the 14,800 kilotons (dry, debarked) of wood gathered and commercialized in France ended up in chips and slabs sold by sawmills to papermakers (COPACEL 2016; FCBA 2017). Despite this large figure, sawmills have difficulty selling their chips to paper producers. The chips are often rejected due to their geometry and especially their non-compliant thickness. As a consequence, they are burnt to produce energy, which is a poor economic exploitation. To promote their waste valorization, industrial sawmills must obtain new tools to predict chip characteristics, such as their geometry. Chip thickness, which is the most difficult dimension to calibrate, is also a highly regarded quality criterion in the paper industry supply (Akhtaruzzaman and Virkola 1979; Brännvall 2017). In sawmills, most chips are produced by chipper-canters. Chipper-canters are large conical mills on which several knives are clamped. They transform logs into cants, shredding the slabs (cylindrical parts of the logs) into chips. The geometry of the chips generated and the cutting forces induced by the process vary according to multiple parameters.
The first parameters to impact the two aforementioned physical quantities are material-related: the wood species (Hernandez and Quirion 1995), its density (Eyma 2002), its moisture content, and its temperature. However, Hernández et al. (2014) showed that above 0 °C, the moisture content of the wood does not significantly impact the mechanical properties inducing the chip fragmentation mechanisms. The type of wood (reaction, tension, compression, early, or late) might also have an influence, but this has not been determined.
The second parameter set to modify the geometry of the chips and the cutting forces is process-based. These parameters are feed per tooth (Twaddle 1997; Felber and Lackner 2005; Curti et al. 2017), which can be assimilated to chip length when the cutting direction is perpendicular to the fiber, cutting speed (Hernandez and Boulanger 1997; Laganière 2004), cutting direction (Kuljich et al. 2013), and feed rate (Laganière 2004).
The last main impacting parameters are related to the tool. The tool rake angle was studied by Kuljich et al. (2013), and the wear of the tool was studied by Ghosh et al. (2015). Nati et al. (2010) also showed that in the case of industrial chippers the tool wear increases the proportion of large chips. Material and/or type of coating of the knife or its clearance angle have not been investigated yet, even though these parameters have been widely studied for other purposes like tool life duration. Others parameters exist, but their significance appears to be very low or situational.
Among the aforementioned parameters, the angle between the cutting direction and the grain direction (GD) impacts the chip thickness and varies a lot during the cut. This angle changes the cutting mechanism involved in the chip splitting (McKenzie 1960). Focusing on the balsam fir wooden species, Kuljich et al. (2013) highlighted the impact of a 45° variation in the cutting direction on the cutting forces and the surface integrity of the specimen after machining. For this reason, on some chipper-canter lines, the in-feed log conveyor height can be set up to modify the knives’ working angle; this makes it possible to act on the cutting direction (Laganière 2004; Kuljich et al. 2017) by providing an additional way to control the process.
The present work draws attention to this angle’s impact on both the cutting forces and the generated chips’ thickness when machining beech green wood. Beech green wood is a widely sawn species in Western European sawmills. Moreover, it is very homogeneous (toward softwoods) with small growth rings. Those two properties permit investigation of these factors with smaller scaled samples. The cutting experiments were conducted in planing, as a simplified process. In order to highlight cutting mechanisms and properly design the experiments, orthogonal cutting was chosen over canting. This operation also allowed the studied parameters to be isolated; the fiber angle was changed without affecting the other cutting parameters such as the cut section, which is continuously changed during canting. In orthogonal cutting, the cutting direction is invariant, so GD is the variable parameter used to act on the angle formed by those two directions according to the following rule: when the cutting direction is parallel to the fiber, GD = 0°.
EXPERIMENTAL
Materials
Experiments were conducted on green beech specimens. One beech log was flat sawn into 1 m long planks. Every plank was then machined into 10 mm-thick boards. Centered in each board, up to nine 100 × 105 × 10 mm3 specimens were machined with various orientations on a Computer Numerical Control routing machine Record 1 (SCM, Rimini, Italy), resulting in a total of 56 specimens. The moisture content (MC) and specific gravity (SG) of each specimen were computed from Eq. 1 and Eq. 2, respectively,
, (1)
, (2)
where mgreen is the mass of the specimen measured right after the cutting experiment (approximately 1 min), movendry is the mass of the specimen measured after drying in an oven until the difference between two 4-h consecutive measurements is less than 0.5%, Vsaturated is the volume of the specimen saturated by water, and ρwater is the density of water. This last quantity was measured by using Archimedes’ principle, following the recommendations of Williamson and Wiemann (2010). The mean MC of the specimens was 69.44%, with a standard deviation of 13.39%, and SG was equal to 0.52 ± 0.03.
Orthogonal Cutting Set-Up and Instrumentation
Orthogonal cutting experiments were conducted with a 3-axes CNC milling center DMC 85 VL (DMG MORI Aktiengesellschaft, Bielefeld, Germany) in which axes were powered by linear motors, allowing for a maximum cutting speed of 2 m s-1. Since usual chipper-canter cutting speeds are between 40 m s-1 and 60 m s-1, this available speed was fairly low. However, 15 complimentary cutting experiments over one decade of cutting speed (between 0.2 m s-1 and 2 m s-1) were run without significant impact on the measured forces. The specimens were clamped in the spindle head with a self-designed stiff fixture composed of an angle bracket and a vise. The cut was operated by a static single chipper-canter tooth screwed onto a 3-component piezoelectric dynamometer 9255A (KISTLER GROUP, Winterthur, Switzerland) that was clamped itself on the CNC table. The cutting speed (Vc) for all experiments was 2 m s-1, and the chosen uncut chip thickness was h = 10 mm. Considering the specimen thickness (b) of 10 mm, which was also equal to the cutting width of the operation, the cut section was 10 × 10 mm². Using an analogy with milling, the uncut chip thickness corresponds to the feed per tooth, and the cutting width corresponds to the depth of cut. The chips produced by canters present on average a higher section (around 30 × 40 mm², although it can vary greatly from one operation to another), but the motors of the machine did not permit a higher section to be cut without risk of stalling.
The whole cutting process was recorded by an ultra-fast camera FastCAM SA-Z (PHOTRON, Tokyo, Japan) mounted with a telecentric lens and extension tubes to set the desired observation field of 18 × 18 mm². Due to the speed of the experiments, the shutter time was low (1⁄40,000 s in the present case) to access very clear frames with no blur. Due to the short exposition time, the observed area was strongly enlightened by 10 high power LEDs (3200 lumens each), where the light was directed and focused through optical fibers (Fig. 1).
This setup allowed for each cutting experiment the following to be recorded: 6 force signals (resultants and momentum in all 3 directions of space) and 1,400 images (one image each 5 × 10-5 s, also equivalent to one image every 0.1 mm of specimen displacement). In addition, the position of the machine head in the cutting direction was directly recorded with an incremental coder and used to trigger the imager when the specimen passed through the camera’s field of observation. All signals were synchronized thanks to acquisition cards mounted on a chassis cDAQ9188 (National Instruments, Austin, USA). The experimental conditions are summarized in Table 1.
Fig. 1. Experimental setup and instrumentation
Table 1. Experimental Parameters
Experimental Plan: Study Area and Grain Directions
To quantify the impact of cutting angle on the chip thickness and the cutting forces, specimens presenting 11 different targeted grain directions (GDth) from 10° to 110° by 10° steps (6 repetitions) were machined. The first eight of them, from 40° to 110°, were tested with six repetitions. The three other targeted grain directions (10°, 20°, and 30°) were tested with fewer repetitions (respectively 2, 3, and 3) because they are unusually encountered in chipper-canter lines. From 10° to 90°, the cut was along the grain, and from 90° to 110°, the cut was against the grain. The total cutting length was 105 mm (corresponding to the specimen’s length), but the studied length for the subsequent analysis was reduced to an almost central 55 mm length. The 30 last millimeters of the specimen can tend to be torn when the forces are high due to the lack of boundary conditions, and the 20 first millimeters of entry in the specimen are required to reach a steady-state in the cutting forces and chip fragmentation mechanisms. Thus, the entry and the exit of the specimen were not considered in the study in order to focus on the steadiest zone far from the boundaries (Fig. 2).
Because the wood fibers are not straight, even in clear wood, their orientation can vary. The theoretical orientation obtained when machining specimens must be corrected. To minimize the time the specimens are manipulated and to preserve their moisture content (green state), methods such as laser propagation (Nyström 2003; Simonaho et al. 2004; Daval et al. 2015) were avoided. The measurement of the real grain direction was done “online” on the images recorded during the cut under and above 5 mm of the tool edge, as it was the most solicited area. The method developed to do so is presented in the next subsection. The measurement of the chip thickness was also realized on the images containing the recorded 55 mm-long studied area. The specimen covered 0.1 mm between each image, so the measurement error due to this span was also equal to 0.1 mm.
Fig. 2. Study area and example of used data to compute grain direction and cutting forces for one trial
Grain Direction Measurement
The grain direction assessment was inspired by the method developed by Ehrhart et al. (2017) in which the innate texture of the beech is used to estimate GD. In this study, pictures were treated using Matlab R2012a (MathWorks, Natick, MA, USA) to detect medullar rays. Images (6 at once, to observe the whole study area) were firstly automatically thresholded into several levels (Fig. 3a and 3b); they were then binarized, conserving only the (darkest) level corresponding to the well-marked medullar rays (Fig. 3c). Medullar rays’ contours were drawn and fulfilled to create identified clusters (Fig. 3d). The two main directions of the clusters were determined using a principal components analysis. The eigenvectors of the covariance matrix and their corresponding eigenvalues were calculated. The first eigenvector of each cluster indicates the direction it is elongated the most while the second is its orthogonal vector. Based on the hypothesis that the medullar rays are aligned with the fibers, each eigenvector becomes an indicator of the grain direction. In addition, small contours with internal surfaces lower than 0.03 mm² and those with a ratio between the two eigenvalues that is too low (˂ 15) were removed. These two conditions prevented very small medullar rays, knots, or areas where humidity and color gradients were strong to be considered as medullar rays. This strategy increased the robustness of the image processing and focused on long medullar rays with an angle well defined. The average direction of the remaining clusters were processed and finally averaged to compute the mean grain direction ( 
) in the specimen.
The absolute necessity of this grain direction correction to increase the accuracy of the study was highlighted and supported by the important differences between GDth and (Fig. 4a). The errors and relative errors (Fig. 4b) were well centered, with mean error = 0.1° and mean relative error = 1.8%, which was mainly due to the presence of one single outlier with GDth = 10° and = 17°, which produced an important 60% relative error. For 23 specimens, the relative error overcame ± 5%.