Experimental Examination and Modification of Chip Suction System in Circular Sawing Machine

The article presents the results of experimental examination of the wood chip suction system in the existing sliding table saw before and after its modifi cation. The studies focused on the extraction hood of the mentioned system. The methodical experimental research of the pressure distribution inside the hood during wood chip removal for the selected rotational speed of saw blades of 3500 and 6000 min-1 with a diameter of 300 mm and 450 mm were carried out. The analysis of the results allowed estimating the areas with insuffi cient vacuum pressure hindering the organized transport of wood chips in the sliding table saw. That pressure was the main factor infl uencing the decision to adjust the hood construction. To achieve the effi cient performance, several changes in the hood geometry were implemented. The results obtained from the experiments were used at the stage of shape modifi cation of the extraction hood. As a result, a new design of the chip suction system was obtained, vastly improving the chip extraction from the tool.


INTRODUCTION 1. UVOD
Currently, design development of woodworking machines, introduction of new technologies, and above of all the machining and feed speed result in the need to provide more effective wood waste (chips) removing systems.A modern wood cutting machine, which operates without a properly designed suction system loses immediately its performance and service life (Barański et al., 2016).Woodworking machines and cutting parameters as well as wood material properties strongly determine the particle size distribution of chipped wood.
In the technological processes of machine wood chipping, a by-product is also formed besides the main product.These are chips whose shape, size and amount mainly depend on the form, physical and mechanical properties of sawed wood as well as on the shape, dimensions, type of machine, sharpness of the cutting blade, and technical and technological conditions of the sawing process (Wasielewski, 1999 Particles of wood, which are formed in individual processes of chipping and machining, are called "bulk wood substance" (Dzurenda, 2007).Thus, wood industry workers exposed to airborne wood dust particles in the surrounding air of the workplace may face different occupational health hazards (Kohler, 1995).Actually, the nature of the production and properties of chips require their continuous removal from the place where they are formed.As far as sanding dust is concerned, it is removed by means of an air-technical device -suction system.To develop such an appropriate suction system, it is important to know the size and shape of bulk particles, which are the basic data for characterizing the bulk material.The above characteristics affect the physical and mechanical properties of the bulk material (bulk density, bulk angle, tilt angle, aerodynamic properties of particles in the piping of the suction system) and conditions of separation or fi ltration in the separating device (Dzurenda, 2007).Also, those characteristics strongly affect the service life of the equipment in the workplace, where dust is generated as well as transportation equipment and fi ltering elements and, last but not least, the safety of the working environment.
Many authors investigate the possibility of reducing the (airborne) particles by controlling the machining parameters and by varying the cutting speed, feed speed, tool type and tool size, cutting angles, number of blades and processed material (Fujimoto and Takano, 2003;Hemmilä et al., 2003).According to the results of these researches, the average chip thickness is one of the most important parameters of wood machining (Wieloch and Osajda, 2007).Furthermore, the sanding process, as a major source of airborne dust, was investigated by Rogoziński and Dolny, 2004; Očkajová and Beljaková, 2004;Beljaková and Očkajová, 2007;Rončka and Očkajová, 2007).
Depending on the machine type and the shape and size of its dust zone, serious problems may arise concerning the effective discharge of dust through a suction system during certain sanding positions.The machine type and method of processing signifi cantly infl uence the increase of dust concentration in the air (Kos et al., 2004).It is very diffi cult to remove dust when the working zone is large and when the tool operates at relatively high velocity.The dispersion of chips in different directions in the space of the treatment zone is very unfavorable in this respect.When the movement direction of the chips created during machining does not coincide with that of the air fl ow created by an extraction system, many chips are still not removed and can become dispersed in the air surrounding the machine.This takes places during sawing when the whole tool goes into the material piece.For this reason, there are problems with the direct removal of chips from the working zone and working tools.The dispersion of chips in all directions also occurs due to the high-speed rotation of those tools.
Wood working enterprises, as sources of air pollution, emit into the air wood dust classifi ed as solid pollutants.Clean Air for Europe is the initiative taken by the EU Commission, by which one of its main aims set in 2002 was: "To reach such a quality of the environment where the level of pollutants coming from human activities does not cause any signifi cant impacts and risks for human health" (EU Decision, 2002).
In this paper, experimental investigations of the wood chip removing system for sliding table saw were performed.The aim of the study was to examine chip suction system in the existing machine, commercially available on the market.Special attention was focused on its extraction hood without infl uence of fan parameters.The changes in a hood design have been proposed, causing higher effi ciency of the chip removing system, which was confi rmed by the results of experimental research.

MATERIJALI I METODE
The sampling of pressure measurements was carried out during circular saw operation without machining.Table 1 shows the parameters of saw blades used during experiments.
The measurements were performed using the wide and narrow cover mounted in the hood of the suction-chip removing system and the saw blades of different diameter and rotational speed.For the saw blade No. 1, two of its maximum and minimum rotational speeds of 3500 min -1 and 6000 min -1 , and for saw blade No. 2, its nominal rotational speed of 3500 min -1 were considered, respectively.Variable fan speed was achieved by changing fan motor frequency.The relative static pressure was measured using a digital multifunction measuring instrument.The temperature, humidity and air fl ow meter Testo 480 (TESTO SE & Co., Germany) was applied.The results were averaged for 10 seconds at each measuring point.Measuring range was from -10 to +10 kPa, resolution 0.01 kPa and accuracy ±0.3 Pa +1 % of the value measured in the lower range.Velocity at the entrance to the hood, as well as to the bottom shelter, was measured using the same instrument, with a hot wire probe.Measuring range was 0-20 m/s, resolution 0.01 m/s and accuracy ±0.03 m/s + 5 % of the measured value.
Experimental tests consisted of measuring the relative static pressure distribution zero-referenced against ambient air pressure at several points on the wide and narrow cover of suction-chip extraction hood (points "A"-"I" on the upper part and "a"-"i" on the side part of the wide cover and points "b*"-"h*" on the side of narrow cover).Locations of all measuring points are shown in Fig. 1 and Fig. 2.During preliminary measurements, more points were considered.However, it was found that there was a small pressure difference between the points situated close to each other.
The obtained results were analyzed and verifi ed by the experiments performed using different shapes of the upper hood, Fig. 3.

REZULTATI I RASPRAVA
Results of relative static pressure measurements for the wide and narrow cover mounted in the hood of the existing system, further referred to as "base case", showed that at most of the measurement points there was a vacuum pressure, as expected.However, some  can be seen by increasing saw blade diameter (Fig. 5).
Adverse effect occurs at all critical points.As can be noticed, when the saw blade operated at the rotational speed of 3500 min -1 , there was a small vacuum pressure in locations B, b and b*.The value of relative static pressure ranged between -9 Pa and -10 Pa.On the other hand, when the saw blade speed was 6000 min -1 , the value of relative static pressure changed signifi canlty and even slight gauge pressure was measured on a narrow cover of the hood (location b*).For the wide cover of the hood, there was still vacuum pressure, but its value was lower than that for blade rotational speed of 3500 min -1 .
Changing saw blade diameter from 300 mm to 450 mm caused the transition from vacuum pressure of 9-10 Pa to gauge pressure of 1 Pa or 4 Pa for locations "B" and "b", respectively, and 0 pressure for location "b*".It is caused by higher linear velocities, occurred during the operation of larger saw blades.
In order to determine the effect of suction, fan speed on relative static pressure at critical locations of fan motor frequencies were changed ±20 %, from the initial value of 50 Hz without modifi cation of the suction system.The measurement results for saw blade No. 1 (300 mm) and for fan motor frequencies of 40, 50 and 60 Hz are shown in Fig. 6.In case of reduced fan motor speed, a very large decrease in vacuum pressure and even the transition to gauge pressure was observed.The value of relative static pressure in locations "B", "b" and "b*" for that case was from +14 Pa to -3 Pa.This can be explained by the infl uence of the main air fl ow in the hood, the scoring saw and the main saw blade rotational speed.On the other hand, increasing fan speed improved pressure distribution in all chosen measurement points and in consequence the performance of the chip removing system was improved.
Velocities in the outlet channel of the hood for both the wide and narrow cover were also measured.The results are shown in Fig. 7 and Fig. 8 for different saw blade diameters, rotational speeds and fan motor frequencies.During the operation of fan motor with the frequency of 50 Hz for saw blade diameter 300 mm at rotational speed of 3000 min -1 and 6000 min -1 , the velocity differences are small.Even for saw blade di-

Figure 5
Relative static pressure as a function of saw blade diameter at different locations for wide and narrow cover of the hood; saw blade rotational speed 3500 min -1 , fan motor frequency 50 Hz Slika 5. Relativni statički tlak kao funkcija promjera lista pile na različitim mjestima na širokom i uskom poklopcu usisnog ušća; frekvencija vrtnje lista pile 3500 min -1 , frekvencija motora ventilatora 50 Hz Figure 6 Relative static pressure as a function of fan motor frequency at different locations for wide and narrow cover of the hood; saw blade No. 1 (300 mm), rotational speed 6000 min -1 Slika 6. Relativni statički tlak kao funkcija frekvencije motora ventilatora na različitim mjestima na širokom i uskom poklopcu usisnog ušća; list pile br. 1 (300 mm), frekvencija vrtnje lista pile 6000 min -1 ameter 450 mm, the velocity only slightly increases.Although these differences are due to the diameter and rotational speed of the saw blade, the fan motor frequency is still the most infl uencing factor.After changing frequency to 60 Hz, the value of velocity in the outlet channel of the hood signifi cantly increased.Velocity improved by about 4 m/s in each case after frequency was increased by 20 %.However, the obtained velocity was still insuffi cient according to literature (Dzurenda, 2007).
In order to improve the performance of chip removing system without interfering with the fan structure, several modifi cations were carried out.The lower shelter shape was modifi ed and changed to obtain smaller volume and, as a result, more effi cient air fl ow and chip removal were achieved.Its leakages were minimized in critical places according to technological possibilities.The upper hood shape was also changed.The examples of its modifi cations are presented in Fig. 3b and 3c.In this element, the outlet channel angle and its inside passage for air fl ow were modifi ed.
The results of pressure distribution after those modifi cations are shown for both wide and narrow cover in Fig. 9-11 as "modifi ed" and "fi nal" cases.
The results of measurement presented above show that blade shape, blade rotational speed and fan It can be emphasized that higher vacuum pressure in the modifi ed and fi nal hood shape was achieved in comparison with "base case", except for points "A" and "a" for modifi ed case.When wide cover was used, gauge pressure was observed for "base case" and "modifi ed case" in this location.For "fi nal case", a little vacuum pressure was obtained in front of the hood.In the rest of the hood, the satisfactory pressure distribution was noticed without modifi cation of the fan structure.Its fi nal shape and inside construction is presented in Fig. 12.

ZAKLJUČAK
The existing chip removing system in the analyzed woodworking machine did not provide satisfactory chip extraction from the working area.The experiments proved that, in the area around the tool, insuffi cient vacuum pressure could hinder the organized transport of wood wastes.After several changes in the system, especially in the upper hood, all parts of the suction system were optimized and modifi ed, except for the fan structure, which provided effi cient perfor- In order to achieve the most desirable results, however, it is strongly recommended that changes be made in the fan design or its motor resolution be optimized regardless of other modifi cations.

Acknowledgement -Zahvala
It is kindly acknowledged that this work has been carried out within the framework of the project POIR.01.01.01-00-05888/15, which has been fi nancially supported by the European Regional Development Fund.The authors would also like to acknowledge the company REMA S. A. in Reszel (Poland), which is the benefi ciary of the project, for helping out in the experimental part of this work.The data included in this paper was partially presented during the 23 rd International Wood Machining Seminar, Warsaw, Poland.It could be emphasised that the sliding table saw Fx550, in which the presented solution of the suction system is applied, was awarded with the Gold Medal at the International Fair DREMA in Poznan, Poland in 2017.

Figure 7 7 .
Figure 7Velocity in outlet channel as a function of fan motor frequency at different saw blade diameters and rotational speed for the wide cover Slika 7. Brzina u izlaznom kanalu kao funkcija frekvencije motora ventilatora pri različitim promjerima i brzinama vrtnje lista pile za široki poklopac

Figure 8 8 .
Figure 8 Velocity in outlet channel as a function of fan motor frequency at different saw blade diameters and rotational speed for the narrow cover Slika 8. Brzina u izlaznom kanalu kao funkcija frekvencije motora ventilatora pri različitim promjerima i brzinama vrtnje lista pile za uski poklopac