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Kminiak, R., Gašparík, M., and Kvietková, M. (2015). "The dependence of surface quality on tool wear of circular saw blades during transversal sawing of beech wood," BioRes. 10(4), 7123-7135.

Abstract

The article deals with the influence of the tool wear of several circular saw blades, each with different numbers of teeth (24, 40, and 60), on the quality of a machined wood surface. The surface quality was evaluated based on the surface roughness, which was represented by the surface arithmetical mean deviation Ra. To achieve the conditions of manual sawing, the saw blade was shifted into the cut with a constant feed force of 15 N. The results showed that the 40-tooth saw blade obtained the most suitable results; it exhibited the longest sawn distance and reached moderately good values of tool wear. The average values of surface roughness ranged from 3.9 to 14.5 µm, and the saw blade wear increased proportionally with sawn distance. The tool wear of the saw blade had no unambiguous effect; increase in tool wear did not lead to a deterioration of surface quality.


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The Dependence of Surface Quality on Tool Wear of Circular Saw Blades during Transversal Sawing of Beech Wood

Richard Kminiak, Miroslav Gašparík,* and Monika Kvietková

The article deals with the influence of the tool wear of several circular saw blades, each with different numbers of teeth (24, 40, and 60), on the quality of a machined wood surface. The surface quality was evaluated based on the surface roughness, which was represented by the surface arithmetical mean deviation Ra. To achieve the conditions of manual sawing, the saw blade was shifted into the cut with a constant feed force of 15 N. The results showed that the 40-tooth saw blade obtained the most suitable results; it exhibited the longest sawn distance and reached moderately good values of tool wear. The average values of surface roughness ranged from 3.9 to 14.5 µm, and the saw blade wear increased proportionally with sawn distance. The tool wear of the saw blade had no unambiguous effect; increase in tool wear did not lead to a deterioration of surface quality.

Keywords: Tool wear; Surface roughness; Transversal sawing; Surface quality; Feed force; Beech

Contact information: Department of Wood Processing, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 – Suchdol, 16521 Czech Republic;

* Corresponding author: gathiss@gmail.com

INTRODUCTION

The transversal sawing of wood is an operation that relates to almost every machined wood product. During transverse cutting, the quality of the sawn surface depends primarily on the surface roughness after cutting and not least also on the characteristics of the tool.

Every process causes a disruption in the material’s initial properties and leaves certain irregularities on the machined surface. These irregularities are represented by microscopic (surface roughness, waviness) and macroscopic (scratches, corrugations, hollows, furrows, thorn fibers) changes on the machined surface (Siklienka and Kminiak 2013; Kminiak 2014). The quality of the machined wood surface depends primarily on the suitability of machining procedures used; machining parameters, such as feed speed, feed force, and cutting speed; as well as the suitable technical treatment of tools, such as regular sharpening and maintenance. Similarly Malkoçoğlu (2007) found that the machining conditions are directly related to defects created during the sawing (fuzzy grain, torn grain, raised grain). A number of authors, such as Örs et al. (1991), Efe et al. (2007), Budakçi et al. (2011), and Kvietková et al. (2015), have dealt with the influence of various technological parameters (cutting speed, feed speed, number of teeth) on the final surface quality of wood.

Tooth profile is directly responsible for the final quality because of its shape and geometry affects the individual wood elements during cutting. The tooth profile must conform to the desired utilization of a circular saw and to the quality of machined surface (Kováč and Mikleš 2010). If the outer lateral edge of the set teeth is chamfered, then the bite profile is reduced while the surface quality becomes improved because of less damage to the wood fibers (cleaner cuts) (McKenzie 2000). Conversely, the chamfering of the inner lateral edge of the set teeth increases the bite profile. This can cause an increase in cutting forces and deterioration in the quality of machined surface (Naylor and Hackney 2013).

The properties of the wood are another factor affecting surface quality. The surface roughness is mostly influenced by wood structure, which includes direction and angle of fibers, spring and summer wood, and knots, but also physical properties such as moisture and density. The density affects almost all the mechanical properties of wood (Gaff 2014; Gaff and Matlák 2014), and thus also the machining processes.

An important factor affecting the machined surface quality is tool wear. Tool wear can be described as a gradual and small but sufficient change in wedge geometry during machining (cutting) by which the tool loses its sharpness and consequently its ability to cut effectively. Tool wear may exist as accidental wear or as normal (mechanical) wear. Accidental wear occurs when, during cutting, the tensile strength in the tooth reaches a higher value than the fracture strength of the material from which it is made. Mechanical wear can be characterized by the removal of metal particles from teeth by the friction between tool and the wood (Cosmin et al. 2009). Generally, tool wear is used to assess tool performance because of its direct influence on surface quality, cutting force, power consumption, etc.(Cristóvão 2013). In general, the higher the wear of tool, the worse surface quality of machined material. Increasing the blunting of tool leaves bigger and bigger tracks at the surface of the wood because it disrupts the wood fibers and raises them. Thus, tool wear increases the surface roughness, and thus the surface quality deterioration occurs.

The aim of this work was to determine the influence of number of saw blade teeth as well as sawn distance (sawn running meters) on the wear of saw blades and surface roughness of beech wood during transversal cutting at a constant cutting speed (vc = 62 m/s).

EXPERIMENTAL

Materials

Forty-five-year-old European beech (Fagus sylvatica L.) trees, which grew in the Zvolen basin, were used for the experiment. Suitable zones were cut from the trunk at a height of 1.5 m, measured from the stump. The flat-sawn samples (wood fibers angle 3 = 90° ÷ 80°) had dimensions of 25 × 150 × 1000 mm. Clear samples were conditioned in a conditioning room (relative humidity (ϕ) = 65 ± 5% and temperature (t) = 20 ± 2 °C) for more than four months to achieve their equilibrium moisture content (EMC) of 12%. The average oven-dry density of beech wood was 668 kg/m3. Whole experiment contained 200 samples.

Methods

Sliding mitre saw

The cutting for the experiment was carried out using a GCM 10S Professional (Robert Bosch GmbH, Germany) sliding mitre saw. The mitre saw parameters are listed in Table 1.

Table 1. Mitre Saw Parameters

Saw blade

Three “PREMIUM” saw blades (EXTOL, Czech Republic) with sintered carbide tips having 24, 40, and 60 teeth, respectively, were selected for the experiment (Fig. 1). The saw blades had identical diameters (D = 250 mm), identical tool thicknesses (b = 3.2 mm), identical angle geometries (clearance angle α = 15°, wedge angle β = 60°, rake angle γ = 15°), and alternating set teeth.

Fig. 1. Saw blades with a) 24 teeth, b) 40 teeth, and c) 60 teeth

Transversal sawing

Many similar experiments have used a constant feed speed, vf. However, for the present experiment, the use of a feed force, Fp, is typical. Feed force was substituted for feed speed due to the manual feeding. Because the requirements for constant experimental conditions, such as a constant feed force, could not be met during manual feeding, the feed force was simulated in an experimental stand.

Fig

Fig. 2. Sequential saw blade movement through the sample with manual feeding