Effect of average uncut chip thickness on the colour of beech wood (Fagus sylvatica L.) during the face milling proces

Abstract

This study aimed to establish quantitative relationships between the average uncut chip thickness and the colour change of beech wood (Fagus sylvatica L.) during face milling. A comprehensive analysis of colour change was conducted based on measurements of chromaticity parameters, lightness, total colour difference, and hue angle variation. It was concluded that the average uncut chip thickness was a key factor in determining the surface’s visual quality. It was found that an increase in average uncut chip thickness (0.13 to 0.38 mm) correlated with a decrease in total colour difference and an increase in lightness, bringing the machined surface’s colour closer to the original wood. This effect was attributed to the minimisation of local thermal effects through optimizing the cutting process. This avoids thermally sensitive chemical reactions that cause colour changes. Results from various statistical analyses showed significant differences in total colour differences across average uncut chip thicknesses of 0.13, 0.25, and 0.38 mm.

Full Article

Effect of Average Uncut Chip Thickness on the Colour of Beech Wood (Fagus sylvatica L.) During the Face Milling Process

Zoya Karatkevich  ,^a Aleksandra Suchta  ,^a Kacper Broclawik,^b

Tomasz Ochrymiuk  ,^c and Kazimierz A. Orlowski  ,^a,*

This study aimed to establish quantitative relationships between the average uncut chip thickness and the colour change of beech wood (Fagus sylvatica L.) during face milling. A comprehensive analysis of colour change was conducted based on measurements of chromaticity parameters, lightness, total colour difference, and hue angle variation. It was concluded that the average uncut chip thickness was a key factor in determining the surface’s visual quality. It was found that an increase in average uncut chip thickness (0.13 to 0.38 mm) correlated with a decrease in total colour difference and an increase in lightness, bringing the machined surface’s colour closer to the original wood. This effect was attributed to the minimisation of local thermal effects through optimizing the cutting process. This avoids thermally sensitive chemical reactions that cause colour changes. Results from various statistical analyses showed significant differences in total colour differences across average uncut chip thicknesses of 0.13, 0.25, and 0.38 mm.

DOI: 10.15376/biores.21.3.6335-6349

Keywords: Beech wood; Face milling; Average uncut chip thickness; Colour changes; CIELab system

Contact information: a: Institute of Manufacturing and Materials Technology, Faculty of Mechanical Engineering and Ship Technology, Gdańsk University of Technology, 11/12 G. Narutowicza Street, 80-233 Gdańsk, Poland; b: Laboratory of Manufacturing Technology and Reverse Engineering, Faculty of Mechanical Engineering and Ship Technology, Gdańsk University of Technology, 11/12 G. Narutowicza Street, 80-233 Gdańsk, Poland; c: Institute of Fluid-Flow Machinery Polish Academy of Sciences, 14 Fiszera Street, 80-231 Gdansk, Poland; *Corresponding author kazimierz.orlowski@pg.edu.pl

Graphical Abstract

INTRODUCTION

Beech wood (Fagus sylvatica L.) is widely and continuously distributed from southern Scandinavia to the Mediterranean, and in parts of Western and Central Europe, it is the most common hardwood species (EUFORGEN n.d.; Caudullo et al. 2017). In addition to spruce and pine, beech wood is widely used in the furniture industry as solid wood and as plywood or veneer. Pramreiter and Grabner (2023) noted the potential use of beech wood for structural applications.

Beech wood is prone to moisture absorption (Ilek et al. 2019; Glass and Zelinka 2021). Moreover, it cracks rapidly (Oltean et al. 2007; Fu et al. 2023) and deforms during drying (Yin and Liu 2021). Steaming beech wood helps relieve internal stresses, which improves its bending properties and machinability (Dudiak et al. 2024).

Beech is classified as a species with false heartwood (facultative coloured heartwood), which is formed due to pathological processes such as air penetration and oxidative reactions in the stem. The false heartwood may take various shapes, including round, star-shaped, tongue-shaped, and irregular forms. Its colour ranges from pale reddish to vivid reddish-brown, while the surrounding sapwood is typically pale yellow to pinkish (Knoke 2002). After steaming, the sapwood acquires a uniform reddish-brown colour, resulting in a more homogeneous appearance (Wernsdörfer et al. 2005). Growth rings are distinguishable to the naked eye on all cross-sections. The wood surface generally lacks gloss and has few distinctive textural features.

Colour is one of the key aesthetic qualities influencing consumer choice of wood products. The visual aspect plays a decisive role. Sedliačiková and Moresová (2022) noted that consumers prefer reddish-brown shades of beech wood when selecting furniture. Moderate use of wood in interior design positively affects visual attention and psychological perception (Li et al. 2021; Lipovac and Burnard 2021).

Various factors affect wood’s colour changes (Mai and Militz 2023). Color deviations are also caused by certain special features (e.g., knots, compression wood, red heart, foxiness). In softwoods, the color differences between clear wood and knots are usually greater than in hardwoods (Niemz et al. 2023). Therefore, detection in hardwoods is usually more difficult. For example, S. Sandoval-Torres et al. (2010) note that hot air thermal treatment leads to wood darkening. Esteves et al. (2008) established that the visual effect of autoclave treatment with saturated steam at 190 to 210 °C causes a reduction in wood lightness, which increases with the treatment’s duration and temperature. In the study by Konopka et al. (2021), colour changes in steamed wood during drying were found to be greater than those in untreated wood. However, the depth of the affected colour layer was minor. Conversely, Tolvaj et al. (2009) demonstrated the possibility of homogenising beech wood with both white and red heartwood. It was also noted that the initial moisture content of the wood does not affect the colour change if it is above the fibre saturation point.

Wood is also subject to constant atmospheric exposure, which contributes to colour change. Kržišnik et al. (2018) conducted a comprehensive analysis of both biological and climatic factors influencing wood colour and observed its cyclical variation. Unlike ultraviolet ageing, artificial ageing caused by temperature changes occurs volumetrically, leading to varying degrees of colour change depending on the wood species. This is associated with the content of hemicellulose, lignin, and extractives, as concluded by Liu et al. (2017). Colour change under light irradiation is explained by lignin decomposition, with coniferous species exhibiting greater colour change than deciduous species, as confirmed by Mitsui (2004). Oltean et al. (2008) noted that softwood species discolour faster than hardwood species under simulated indoor sunlight exposure.

Another factor affecting wood colour change is the chemical reactions from exposure to salts and acids. The intensity of colour change strongly depends on the amount of extractives, the structure of lignin, and the type of diazonium salt used for staining (Ortlieb et al. 2025). Škėma et al. (2023) showed that the intensity of colour change correlates with the concentration of iron ions in the wood, and can be used as an indicator of the depth and intensity of hydrothermal processes.

Mechanical processing also plays a significant role in wood colour change. Colour transformation is caused by changes in surface roughness (Wood 2011; Davim et al. 2009), which alter the angle of light refraction and scattering (Liu et al. 2023). Cutting forces lead to surface heating, resulting in changes in the wood colour (Chuchała et al. 2022). The application of lubricants and cooling fluids causes substantial changes in the colour of the machined wood surface, which gradually diminish over time but remain even after 24 h (Chuchala et al. 2024). Moreover, colour change in wood can serve as an indirect indicator of changes in its elastic and strength properties (Nguyen et al. 2019). Thus, wood colour can be considered an aesthetic attribute and a functional indicator.

The verification of wood colour depends on industrial and production requirements. For example, the use of the CIELab system in accordance with ISO 11664-2 (2022) and ISO 11664-4 (2019) standards allows colour changes to be determined for different processing types (Barański et al. 2020). For colour-based automated selection of beech wood, Wang et al. (2021) proposed a machine vision technology combined with an unsupervised learning technique that reduced data processing time and computational complexity. In turn, modelling approaches evaluate the influence of various external factors on wood colour. For instance, Schnabel et al. (2009) proposed a locally weighted regression method that visualized surface ageing kinetics and statistically justified comparisons of different technological treatments or operating conditions.

In addition to natural effects, the influence of anthropogenic factors has also been studied. Li et al. (2018) proposed using response surface methodology (RSM) to model and establish relationships between colour changes and the parameters of laser modification of wood. Due to the high variability of wood colour and other factors, artificial neural networks have been used. For example, Van Nguyen et al. (2018) used the support vector machine model with improved particle swarm optimization as studied by Li et al. (2023). Liang et al. (2025) used various models with optimisation techniques, such as the extreme learning algorithms with hybrid kernels (IZOA-DHKELM). Such approaches enable improved accuracy in predicting wood colour characteristics. The integration of intelligent systems into the research process accounts for nonlinear dependencies between technological parameters and colour, reduces the number of experimental studies required, and facilitates quality control.

Despite numerous studies addressing wood colour change under thermal, climatic, and chemical influences, the effect of machining parameters has remained insufficiently investigated. Average uncut chip thickness, h_m, is a cutting parameter describing feed movement and cross-section of the removed cutting layer. It is one of the main parameters that affects the cutting force values in machining processes and can be related to the progression of thermally sensitive chemical reactions in the wood’s surface layers.

The available literature provides limited quantitative data regarding the average chip thickness and changes in the colour parameters of beech wood during face milling. Therefore, the present study aimed to establish quantitative relationships between the average uncut chip thickness (h_m) and changes in the colour parameters (lightness L*, chromaticity parameters a* and b*, total colour difference ΔE*, and hue angle variation h*) of beech wood (Fagus sylvatica L.) during face milling.

EXPERIMENTAL

Materials

The materials used were beech wood (Fagus sylvatica L.) samples harvested from the Pomeranian region in Poland. The samples were selected from the sapwood zone without visible signs of false heartwood. Sample 6, in which false heartwood was identified during processing, was excluded from the colour change analysis. The specimens were sawn using a table-circular-sawing-machine into dimensions of 120 mm in length and a square cross-section of 50 mm × 50 mm. The samples were stored under laboratory conditions with a moisture content of MC = 8 ± 1.2%. The oven dry density of the samples was ρ = 690.9 ± 30.1 kg·m^-3, determined by the oven-dry method. Storage conditions prevented any sunlight from affecting the sample preparation materials. The samples were numbered from 1 to 10, and the wood annual rings orientation was determined. This allowed the appropriate number of surfaces (A, B, C), where the machining direction was either tangential to the fibres (denoted T) or radial (denoted R). Figure 1 shows the samples with the radial and tangential fibre directions relative to the milled surfaces.

Fig. 1. Sample designation indicating the orientation relatively to the fibres tangential T, or radial R direction of processing

Machine Tools, Cutting Tools, and Cutting Parameters

The experimental study was carried out on a 5-axis machining centre AX320 (Pinnacle Machine Tool Co., Ltd., Taiwan). The common experimentation method was conducted using OFAT (one-factor-at-a-time) experimentation (Astakhov 2012). The analysed face milling process was performed with three average uncut chip thicknesses (h_m): 0.13, 0.25, and 0.38 mm for surfaces A, B, and C. These values were determined based on the selected feed per tooth values f_z (0.2, 0.35, 0.5 mm) and other geometric parameters (Eq. 1, Fig. 2). The feed per tooth values in the analysis case were equal to the maximum uncut chip thickness values (Fig. 2), calculated as follows,

 (1)

where κ_r is the tool cutting edge angle (90°), a_e is the width of cut (50 mm), D is the cutting tool diameter (50 mm), and φ_s is the – wrap angle (180°).

Fig. 2. Face milling process with basic cutting parameters, where v_c – cutting speed, v_f – feed speed, n – spindle speed, f_z – feed per tooth, h – maximum uncut chip thickness, h_m – average uncut chip thickness, D – tool diameter, φ_s – wrap angle, φ_m – angle of the average chip thickness position, a_e – width of cut

The cutting speed v_c was stable for each machining test at 25 m·s⁻¹. The depth of cut a_p remained the same throughout the study at 1 mm.

Analysed Colour Parameters

One of the main problems of X, Y, and Z systems is that color differences are not perceptually equidistant. Hence, the CIE (International Commission on Illumination) developed a color system with equidistant color differences (Van Acker et al. 2023). To measure the colour parameters (L*, a*, b*) used in the three-dimensional CIELab system recommended by the CIE (ISO 11664-4 2019; Barański et al. 2020; ISO 11664-2 2022), a handheld CR-10 spectrometer from Konica Minolta was employed. The parameters L*, a*, and b* represent the coordinates of the three-dimensional colourimetric space. The L* parameter corresponds to the lightness axis (0 – black, 100 – white), a* to the red-green axis (positive values indicate red, negative values indicate green), and b* to the blue-yellow axis (positive values indicate yellow, negative values indicate blue). These parameters were measured directly before and after machining the samples.

The total colour difference was determined using the ΔE* parameter (Eq. 2) (Van Acker et al. 2023). The hue angle h* was defined as the polar angle derived from the position of the colour point in the a*/b* plane.

 (2)

The hue angle change h* was calculated as the difference between the hue before and after machining (Eq. 3).

 (3)

Colour changes were evaluated according to the scale proposed by Cividini et al. (2007). Measurements were conducted at five points on each of the surfaces A, B, and C in the central part of the sample to avoid the influence of edge colour variations.

Statistical Analysis

The significance of the effect of the depth of cut on colour change was determined through the statistical analysis of the colour measurements data of beech wood samples before and after milling.

Normality of the data distribution for each depth level was assessed using the Shapiro–Wilk test (Ghasemi and Zahediasl 2012; Mishra et al. 2019). After confirming normality, potential outliers were identified using the Grubbs test (Grubbs 1969). To evaluate differences in mean values, a one-way analysis of variance (ANOVA) was performed. Additionally, the Wilcoxon test with Bonferroni correction (Wilcoxon 1945; Conover 1999) and the non-parametric Friedman test were performed (Hazra and Gogtay 2016).

RESULTS AND DISCUSSION

The machined surfaces of beech wood samples during face milling with three different feed per tooth are shown in Fig. 3. All analysed surfaces were noticeably darkened due to the phenomenon “burning” of wood. As the average uncut chip thickness increases, the “burning” effect decreases. In addition, there were darker areas around the edges of the samples and lighter central areas (Fig. 3). This was related to the variation in the average uncut chip thickness resulting from the kinematics of face milling (Fig. 2). On the cutting tool axis position in feed direction (centre of sample) was the maximum uncut chip thickness h value. These values decreased to almost zero as they approached the sample edge. It was observed that small values of uncut chip thickness result in a “burning” phenomenon presenting as very dark, sometimes almost black, surfaces.

In sample 8 (Fig. 3a), a brightened area of irregular shape was visible. This was caused by a hidden internal crack in the sample, which was not visible during sample preparation but caused the material to vibrate during milling. As a result of these vibrations, the cross-section of the cutting layer changed, and the phenomenon’s effects were visible. Additionally, false heartwood was identified in sample 6, which noticeably distorted the colour of the analysed surfaces. Both samples were excluded from the colour change analysis.