Impact of Sycamore Maple Thermal Treatment on a Granulometric Composition of Chips Obtained due to Processing on a CNC Machining Mentre

This article deals with a granulometric composition of chips from the milling process of native and thermal treatment maple cuttings on a 5-axial Computer numerical control (CNC) machining center SCM Tech Z5. The aim of this article was to determine the changes in the granulometric composition of chips due to the thermal treatment of wood and to assess the potential risk of the creation of harmful dust fractions. Cuttings were milled with a shank cutter with exchangeable razor blades at feed speed vf = 1 ÷ 5 m·min−1 and material removal e = 3 mm. The thermal treatment in order to modify the color of the maple wood was done with water vapour at temperatures of tI = 112.5 ± 2.5 ◦C for a period of τ = 5.5 h (Mode I), tII = 127.5 ± 2.5 ◦C for a period of τ = 6.5 h (Mode II), and tIII = 137.5 ± 2.5 ◦C for a period of τ = 7.5 h (Mode III). The granulometric composition of the chips was detected by sifting. A granulometric analysis of the chips provided that more than 2/3 of the produced chips are a coarse fraction consisting of flat chips with dimensions over 1 mm. Dust fractions smaller than 500 μm form isometric grains, i.e., chips having approximately the same size in all three dimensions. Inhalable dust particles, smaller than 125 μm, do not exceed a 2.5% share. The granulometric analysis of chips shows that the thermal treatment of maple wood does not create respirable fractions, and therefore, the thermal treatment of the wood does not have a negative impact on the living and working environments.


Introduction
Wood is a complex and natural material that consists basically of cellulose (40-45%), hemicelluloses (20-30%), and lignin (25-35%). Cellulose represents the crystalline area of the wood, whereas the structures of hemicelluloses and lignin make up the amorphous area. The main mechanical functions of hemicelluloses and lignin are to buttress the cellulose fibrils [1].
Wood materials can be degraded by attacks by numerous biological species and by environmental conditions, so they are protected by the use of various treatment processes. One of these processes is the thermal treatment of wood materials [2].
CNC technologies have become an integral part of the woodworking industry. The CNC machining centers are among the most widely used ones. The 5-axial versions of these centers are trendy, as they, in addition to the basic movements in the X, Y, and Z axes, also allow rotational movements in the B axis (rotational movement around the Y axis) and C axis (rotational movement around the Z axis). The 5-axial CNC machining centers are universal machines that allow machining of the workpiece within its five base areas and their combinations (the 6th area is reserved for the attachment of the workpiece during the machining process). The versatility of the 5-axial CNC machining centers has opened the issue of exhausting the chips from the working environment. Suction bins are used to exhaust the chips; the bins press against the workpiece surface and exhaust the sawdust into the air-conditioning system. Suction bins in the 5-axial version of the machines have taken on significant dimensions, and their volume reaches 0.125-0.350 m 3 (there is a significant pressure loss created). Furthermore, the outlet of the suction bin is axial considering the technical feasibility point of view (while the movement of the produced chips is radial in most cases). The problem arises when the workpiece side edges are machined by the tools that are producing bigger chips. The separated chips are formed into a rotating air stream during the milling. The mixture of rotating air and chips has so much energy that its significant part is not carried by the exhausted air; it hits the walls of the suction bins and then falls into the workspace of the CNC machine so that the exhaust system cannot remove these chips. The practice is that the worker, after the final machining of the workpiece, "blows" out by the compressed air which causes secondary pollution of the working environment around the CNC machine [17,18].
From a physiological point of view and according to the conventions in the sense of the international harmonization (USA: ACGIH and EPA; Europe: ISO, CEN, and BMRC) the dust fractions smaller than 100 µm are divided as follows: breathable (inhalable) mass fraction < 100 µm, thoracal 5-10 µm, tracheobronchial (respirable mass fraction) 2.5-5 µm, and high respirable mass fraction < 2.5 µm.
The main aim of this article is to determine the granulometric composition of the chips from machining thermally untreated maple wood and thermally treated maple wood on a 5-axial CNC machining center SCM TECH Z5 in real conditions in practice. The second aim of this article is to prove (verify) the influence of the thermal treatment of maple wood on the formation and production of the harmful dust fractions.

Material
In this experiment, natural and thermal treatment furniture cuttings with the following parameters were used: wood-sycamore maple (Acer pseudoplatanus L.)-thickness of 25 mm (±0.5 mm), width of 80 mm (±0.5mm), length of 500 mm (±1 mm), and humidity at 10% (±2%). The thermal treatment of maple wood with saturated steam for the purpose of staining the total volume of wood into the pale-brown, brown, and brown-red colorations was performed by the modes in Table 1.

Machine
The experiment was realized on a 5-axial CNC machining center SCM Tech Z5 ( Figure 1) from the manufacturer SCM Group, Rimini, Italy.

Machine
The experiment was realized on a 5-axial CNC machining center SCM Tech Z5 ( Figure 1) from the manufacturer SCM Group, Rimini, Italy. Figure 1. The CNC machining center SCM Tech Z5. CNC-Computer numerical control-it is a commonly used abbreviation, has already been used in the text before.

Tool
The single-shank cutter, type designation KARNED 4451, by the manufacturer Karned Tools Ltd., Prague, Czech Republic was used during the experiment (the tool was chosen with the assumption that it creates a chip of maximal dimensions copying the shape of the cut off chip). The reversible knife HW 49.5/9/1.5 from cemented carbide T10MG is fitted in the shank cutter.

Milling Process
The workpiece is milled within the following conditions: material removal e = 3 mm (the amount of the removal is based on standard inputs of the processed materials), cutter speed n = 20,000 min −1 (this is the value recommended by the tool manufacturer), and feed speed vf = 1, 2, 3, 4, and 5 m·min −1 (maximal feed speed recommended by a manufacturer is 5 m·min −1 , but the operator adjusts/reduces the value to the level of 1 m·min −1 according to the local conditions of the milling process and the required quality of the created surface). For each combination of parameters, a milling process is performed on at least 6 samples until 3 × 50 g of chips have been taken.

Chips Removal
The selected methods of chip removal reflect the requirements that it removes actual created chips and not only the chips that are possible to be removed by the suction device. The main exhaustion of the machine was off, and the collecting bin of the machine was lifted over the working space during the experiment. The main exhaustion of the machine was replaced by the preparation (Figure 2). The intake vacuum of the chips from the workspace was created by a mobile suction device OP 1500 from the Proma SK company, Zvolen, Slovakia (with a suction capacity 1020 m 3 ·hr −1 , maximal intake vacuum 1400 Pa, and suction pipe diameter 110 mm). Exhausted chips were collected on the cloth filter Hyundai VCP 200 by Hyundai Mobis Co. Ltd., South Korea (filter class G3: STN EN 779/1822). The chips collected on the cloth filter were dropped into the collecting bin, and when it reached the weight m = 50 g, it was taken up for further analysis. The intake port for sawdust Figure 1. The CNC machining center SCM Tech Z5. CNC-Computer numerical control-it is a commonly used abbreviation, has already been used in the text before.

Tool
The single-shank cutter, type designation KARNED 4451, by the manufacturer Karned Tools Ltd., Prague, Czech Republic was used during the experiment (the tool was chosen with the assumption that it creates a chip of maximal dimensions copying the shape of the cut off chip). The reversible knife HW 49.5/9/1.5 from cemented carbide T10MG is fitted in the shank cutter.

Milling Process
The workpiece is milled within the following conditions: material removal e = 3 mm (the amount of the removal is based on standard inputs of the processed materials), cutter speed n = 20,000 min −1 (this is the value recommended by the tool manufacturer), and feed speed v f = 1, 2, 3, 4, and 5 m·min −1 (maximal feed speed recommended by a manufacturer is 5 m·min −1 , but the operator adjusts/reduces the value to the level of 1 m·min −1 according to the local conditions of the milling process and the required quality of the created surface). For each combination of parameters, a milling process is performed on at least 6 samples until 3 × 50 g of chips have been taken.

Chips Removal
The selected methods of chip removal reflect the requirements that it removes actual created chips and not only the chips that are possible to be removed by the suction device. The main exhaustion of the machine was off, and the collecting bin of the machine was lifted over the working space during the experiment. The main exhaustion of the machine was replaced by the preparation (Figure 2). The intake vacuum of the chips from the workspace was created by a mobile suction device OP 1500 from the Proma SK company, Zvolen, Slovakia (with a suction capacity 1020 m 3 ·hr −1 , maximal intake vacuum 1400 Pa, and suction pipe diameter 110 mm). Exhausted chips were collected on the cloth filter Hyundai VCP 200 by Hyundai Mobis Co. Ltd., Yongin-Shi, South Korea (filter class G3: STN EN 779/1822). The chips collected on the cloth filter were dropped into the collecting bin, and when it reached the weight m = 50 g, it was taken up for further analysis. The intake port for sawdust exhausting was located at the site of the resulting direction of the sawdust stream. There were 3 samples of 50 g taken for each combination of parameters.
Sustainability 2019, 11, x FOR PEER REVIEW 4 of 10 exhausting was located at the site of the resulting direction of the sawdust stream. There were 3 samples of 50 g taken for each combination of parameters.

Granulometric Analysis
A granulometric composition of the chips was detected by sifting. For this purpose, a special set of sieves arranged one above the other was used (2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.125 mm, 0.063 mm, 0.032 mm, and the bottom), and the sieves were placed on a vibration stand of the sifting machine Retsch AS 200c from the manufacturer Retsh GmbH, Haan, Germany. The parameters of sifting were the following: the frequency of sifting interruption: 20 seconds, the amplitude of sieves deflection: 2 mm·g −1 , the sifting time: τ = 15 min, and the weighed sample: 50 g. The granulometric composition was obtained by weighing the portions remaining on the sieves after sifting on an electric laboratory scale Radwag 510/C/2 from the manufacturer Radwag Balances and Scales, Radom, Poland and weighed to an accuracy of 0.001 g. The sifting was realized with 3 samples for each combination of parameters.
With the purpose of specifying the information about the size of the smallest particles of fine fraction of the dry sawdust, a microscopic analysis of the granules of fraction of the dry sawdust with a size lower than 125 μm was realised. The proposed analysis of sawdust was carried out by an optical method: the analysis of the picture obtained from the microscope Nikon Optiphot-2 with the objective Nikon 4×. Granules of sawdust were scanned by three low-cost television HITACHI HV-C20 (RGB 752 × 582 pixel) Charge Coupled Device (CCD) cameras with horizontal resolution 700 TV lines and were evaluated by a LUCIA-G 4.0 (Laboratory Universal Computer Image Analysis) software installed on a PC with the processor Pentium 90 (RAM 32 MB) and the graphics card VGA Matrox Magic under the operation system Windows NT 4.0 Workstation. The program of analysis of the picture, LUCIA-G, enables the identification of the individual particles of disintegrated wood material and the quantitative determination of individual particles situated in the analysed picture and of the basic information such as the width and length of particles and the circularity expressing the measure of deviation of the projection of a given chip shape from the projection of the shape of a circle according to the scheme: ψ = (4*π*S)/P 2 (1) where S is the surface of the particle (m 2 ) and P is the perimeter of particle (m).

Granulometric Analysis
A granulometric composition of the chips was detected by sifting. For this purpose, a special set of sieves arranged one above the other was used (2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.125 mm, 0.063 mm, 0.032 mm, and the bottom), and the sieves were placed on a vibration stand of the sifting machine Retsch AS 200c from the manufacturer Retsh GmbH, Haan, Germany. The parameters of sifting were the following: the frequency of sifting interruption: 20 seconds, the amplitude of sieves deflection: 2 mm·g −1 , the sifting time: τ = 15 min, and the weighed sample: 50 g. The granulometric composition was obtained by weighing the portions remaining on the sieves after sifting on an electric laboratory scale Radwag 510/C/2 from the manufacturer Radwag Balances and Scales, Radom, Poland and weighed to an accuracy of 0.001 g. The sifting was realized with 3 samples for each combination of parameters.
With the purpose of specifying the information about the size of the smallest particles of fine fraction of the dry sawdust, a microscopic analysis of the granules of fraction of the dry sawdust with a size lower than 125 µm was realised. The proposed analysis of sawdust was carried out by an optical method: the analysis of the picture obtained from the microscope Nikon Optiphot-2 with the objective Nikon 4×. Granules of sawdust were scanned by three low-cost television HITACHI HV-C20 (RGB 752 × 582 pixel) Charge Coupled Device (CCD) cameras with horizontal resolution 700 TV lines and were evaluated by a LUCIA-G 4.0 (Laboratory Universal Computer Image Analysis) software installed on a PC with the processor Pentium 90 (RAM 32 MB) and the graphics card VGA Matrox Magic under the operation system Windows NT 4.0 Workstation. The program of analysis of the picture, LUCIA-G, enables the identification of the individual particles of disintegrated wood material and the quantitative determination of individual particles situated in the analysed picture and of the basic information such as the width and length of particles and the circularity expressing the measure of deviation of the projection of a given chip shape from the projection of the shape of a circle according to the scheme: ψ = (4*π*S)/P 2 (1) where S is the surface of the particle (m 2 ) and P is the perimeter of particle (m).

Results
The obtained data were subjected to a statistical analysis which did not confirm the dependence of the granulometric composition of the chips on the monitored parameters (degree of thermal treatment and strength of the feed force). The basic evaluation of the granulometric composition of the chips was done as a mixture of the chips from several feed speeds because there is a permanent change in the feed speed in the milling process at the CNC machining center.
The mixture of chips produced by the milling process of thermal treatment maple wood by Mode II can be characterized as follows (Table 2 and Figure 3): • the majority share (66.64 ÷ 96.27 % * 1 ) consists of fractions of over 2 mm, • the particle size <500 µm (dust particles) does not exceed 27.05 % (3.40 ÷ 27.05 % * 1 ), • the particle size <100 µm (breathable dust fraction) does not exceed 0.85 % (0.12 ÷ 0.85 % * 1 ), • the particle size <10 µm (thoracal and tracheobronchial dust fraction) was not proven.   The mixture of chips produced by the milling process of thermal treatment maple wood by Mode III can be characterized as follows (Table 2 and Figure 3):
(* 1 Depending on the feed speed) The average values of the granulometric composition of the chips for all combinations of parameters are given in Table 2 with graphical representation presented in Figure 3.
The individual granulometric stages for all combinations of the examined parameters can be characterized in the following summary.
Fraction chips with dimensions over 2mm belong to a category of flat chips, i.e., the length and width of the chip significantly exceed its thickness. The chip has the shape of a cut off layer of milled wood. At lower feed speeds, the chips are without breaks. With the increasing feed speed, the incidence of breaks increases. Fraction chips with the grain from 2 mm to 500 µm belong to a category of fibrous chips, i.e., chips with a significant extension in one direction. Fractions smaller than 500 µm can be characterized as isometric chips, i.e., chips having approximately the same size in all three dimensions. The inhalable dust fraction of particles smaller than 125 µm represents the 0.6 % share of natural, 0.71 % share of Mode I, 0.58 %, share of Mode II, and 0.57 % share of Mode III. The CNC machining center does not produce respirable fractions smaller than 10 µm.

Evaluations of the Thermal Treatment Effects
Based on the available literature reviews, we have expected the statistically significant effect of the thermal modification on the granulometric chips´composition, namely reducing the share in fractions over 1 mm and increasing the share in fractions below 1 mm; however this assumption has not been confirmed. An explanation of this effect is provided by a chemical analysis of the maple wood sawdust which shows that in the case of maple wood, comparing to, for example, beech wood, there are only minimal changes in the chemical structure of the wood. The most significant differences are in the hemicelluloses content. The content of hemicelluloses in thermally treatment wood species point to their different contents in maple and oak wood and also to the different degrading intensities (Figure 4). Both wood species show the greatest decrease in hemicelluloses in Mode III. While in maple wood, this difference was 6.4% and represented 19% of the original wood, in oak wood the difference was 22.9%, which represented a decrease of up to 75.6%. For both wood species, a similar course of lignin and polysaccharides ratio were also recorded [34].

Evaluations of the Thermal Treatment Effects
Based on the available literature reviews, we have expected the statistically significant effect of the thermal modification on the granulometric chips´ composition, namely reducing the share in fractions over 1 mm and increasing the share in fractions below 1 mm; however this assumption has not been confirmed. An explanation of this effect is provided by a chemical analysis of the maple wood sawdust which shows that in the case of maple wood, comparing to, for example, beech wood, there are only minimal changes in the chemical structure of the wood. The most significant differences are in the hemicelluloses content. The content of hemicelluloses in thermally treatment wood species point to their different contents in maple and oak wood and also to the different degrading intensities (Figure 4). Both wood species show the greatest decrease in hemicelluloses in Mode III. While in maple wood, this difference was 6.4% and represented 19% of the original wood, in oak wood the difference was 22.9%, which represented a decrease of up to 75.6%. For both wood species, a similar course of lignin and polysaccharides ratio were also recorded [34].

Evaluation of the Feed Speed Effects
The increase of feed speed is expected to cause a spall of chips, which is inconsistent with the statistical evaluation. For this reason, we realized an individual statistical evaluation of the obtained data for each granulometric stage. In the individual evaluations, the statistical dependence of the percentage representation of the fraction on the feed speed was proven. The change of feed speed caused the percentages of the individual fractions to regroup. This rearrangement essentially meant decreasing the percentage share of fractions over 2 mm and increasing the share of fractions below 1 mm and conversely. The increase was evident within the fractions 500 and 250 μm. Changes in the fractions < 125 μm were only minimal (max 1.8%). Significant finding is that even in one of the cases under consideration, there was no formation of chips < 32 μm.

Evaluation of the Feed Speed Effects
The increase of feed speed is expected to cause a spall of chips, which is inconsistent with the statistical evaluation. For this reason, we realized an individual statistical evaluation of the obtained data for each granulometric stage. In the individual evaluations, the statistical dependence of the percentage representation of the fraction on the feed speed was proven. The change of feed speed caused the percentages of the individual fractions to regroup. This rearrangement essentially meant decreasing the percentage share of fractions over 2 mm and increasing the share of fractions below 1 mm and conversely. The increase was evident within the fractions 500 and 250 µm. Changes in the