Minor cutting edge angles of sawing teeth: effect on cutting forces in wood

The Swedish sawmill industry is developing cutting tools that produce fewer by-products such as sawdust and that produce a higher volume of sawn timber. More efficient tools for bandsawing of timber could result in economic benefits and environmental gains as the volume of end products that can bind carbon dioxide for a longer time. Wasielewski et al. (2012) state that the kerf width as well as the position and sawing accuracy can increase the volume yield of a sawmill. They found that it is possible to reduce the sawdust (waste) by 16% when altering a circular saw blades geometry to have a higher static and dynamic stiffness. In bandsawing, the saw kerf width, for example, can be reduced by decreasing the major cutting edge width which can result in significant improvements in the sawing yield (Orlowski 2007; Orlowski et al. 2009; Li et al. 2018) but also give lateral stability problems to the tool and its cutting teeth. It is therefore very important to study the forces involved with cutting when altering the tooth geometry.

The cutting forces that act on sawing teeth indicate the sawing tool performance. The conventional cutting forces on a cutting tooth are defined by the cutting force (responsible for removing material), the feeding force (also called the normal force), and the lateral force. In the 1950s, Kivimaa (1950) carried out the first cutting force measurements and many since have linked the cutting forces to the fracture mechanics of wood during chip formation and flow (Orlowski et al. 2013; Hlásková et al. 2015; Williams et al. 2010). Parameters such as the wood conditions and feeding conditions were tested in addition to tooth geometry parameters. There are three edges on a cutting tooth that cut for the bandsawing of logs: the major cutting edge (\(S_t\)) and the two minor cutting edges (\(S'_t\)) as shown in Fig. 1. Kivimaa (1950) and other researchers after him have mainly studied the cutting forces on the major cutting edge, but when the saw kerf width is reduced, force contributions from the minor cutting edges and minor first flanks \(\left( A'_{\alpha 1}\right)\) of the cutting teeth start to play a more important role (Li et al. 2018; Meulenberg et al. 2022). The interaction between the tooth and the workpiece determines the stability of sawing, and as cutting with the minor cutting edge starts to play a more crucial role in thin kerf bandsawing, it is essential to study the effect of the tooth minor cutting edge geometry, such as the minor cutting edge angle, \(\kappa '_r\) (also called the radial clearance angle), and minor cutting edge clearance angle, \(\alpha '_p\) (also called the tangential clearance angle or tool back clearance angle), on the cutting process. These angles can be seen in Fig. 2.

Faces and cutting edges of a single cutting tooth as defined by ISO 3002:1 (1982)

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Typical geometry of a single cutting tooth indicating the minor cutting edge angle and minor cutting edge clearance angles as defined by ISO 3002:1 (1982)

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Sawing teeth have a side clearance (distance between the major cutting edge width and band thickness) which avoids contact between the back of the band and the wood as the cutting teeth create kerfs. In addition, this side clearance should be sufficient to allow for the resharpening of the cutting teeth. The clearance angle leaves room for elastic spring-back of the wood and avoids contact with the major first flank. This clearance angle directly affects the cutting forces. A clearance angle of \(12^{\circ }\) is commonly used in Swedish sawmills, and researchers have found that the cutting force is at its lowest for clearance angles between \(10^{\circ }\) and \(15^{\circ }\) (Kivimaa 1950; Ekevad et al. 2019; Schmidt et al. 2019). The \(\kappa '_r\) and \(\alpha '_p\) angles of a sawing tooth minimize contact between the minor first flanks of the tooth and wood. \(\kappa '_r\) is largely responsible for the timber surface quality after sawing and determines the minor cutting edge geometry. Li et al. (2017) showed that a smaller \(\kappa '_r\) and longer minor cutting edges result in lower surface roughness. A low \(\kappa '_r\) might, however, result in higher cutting forces due to increased friction forces, and therefore vibrations as the contact area between the tooth and wood is increased. Wood has a significant degree of elastic spring-back which can cause contact and friction forces on the major and minor first flanks of the teeth.

Cutting by the minor cutting edges occurs in a different direction than cutting by the major cutting edge. For example, when the major cutting edge is cutting in the \(90^{\circ }\)–\(90^{\circ }\) direction, the minor cutting edges cut in the \(0^{\circ }\)–\(90^{\circ }\) direction. The cutting direction can significantly affect the cutting forces (Kivimaa 1950; Axelsson et al. 1993; Lhate et al. 2019; Kuljich et al. 2013; Moradpour et al. 2013; Krenke et al. 2018; Li et al. 2018; Ekevad et al. 2019). Li et al. (2018) considered the resultant force of the major cutting edge as well as the contribution from minor cutting edge forces by creating open and closed cuts into birch wood. Tests were performed with single cutting teeth at low cutting speeds with a \(3^{\circ }\) minor cutting edge clearance angle and minor cutting edge angles of \(0^{\circ }\), \(1.5^{\circ }\) and \(3^{\circ }\). The authors concluded that a larger minor cutting edge angle decreases the main and minor cutting edge forces as there is less cutting of the “elastic spring-back wood” by the minor cutting edges.

The Scandinavian species Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) are processed in all environmental conditions, even when temperatures drop well below zero. Sawmills, therefore, process frozen logs with different moisture contents, which affects the cutting behaviour significantly (Kivimaa 1950; Axelsson et al. 1993; Vazquez-Cooz and Meyer 2006; Meulenberg et al. 2021). When high moisture content wood freezes, the free water in the cell lumina freezes and expands, which exerts a compressive force on the cell walls, making it harder to cut as the cell walls are more rigid (Hernández et al. 2014). Furthermore, when freezing, the moisture in the cell walls can migrate out and condense onto the cell walls as ice crystals. This then acts as additional reinforcement, as well as drying the cell wall out, which increases the mechanical properties of the wood, making it harder to cut (Kubler et al. 2007; Shmulsky and Shvets 2006). It has been found that frozen high moisture content wood results in high cutting forces (Axelsson et al. 1993; Vazquez-Cooz and Meyer 2006; Orlowski et al. 2009; Meulenberg et al. 2021) and in negative feeding forces (Meulenberg et al. 2021).

This paper describes results from cutting and feeding force measurements on sawing teeth with varying \(\kappa '_r\) and \(\alpha '_p\) angles. The sawing process aimed at was bandsawing. Tests were conducted in non-frozen and frozen wood where both the heartwood and sapwood of Norway spruce and Scots pine were tested. To the author’s knowledge, such measurements are not reported earlier in the literature. Thus, this research can provide insight into the most suited combinations of tooth parameters to develop narrow kerf cutting tools with stable behaviour when bandsawing timber with varying properties.

Learn from a saw filer what working in a saw mill is like

Description of the career of a saw filer.

sawing machine, device for cutting up bars of material or for cutting out shapes in plates of raw material. The cutting tools of sawing machines may be thin metallic disks with teeth on their edges, thin metal blades or flexible bands with teeth on one edge, or thin grinding wheels. The tools may use any of three actions in sawing: true cutting, grinding, or friction-created melting.

The power hacksaw machine provides a vise for clamping the work and means for reciprocating a U-shaped frame on which is mounted a straight steel hacksaw blade that cuts when moving in one direction only. The saw presses down on the work during the cutting stroke but is raised clear of the work during the return stroke.

The band saw employs an endless flexible steel band with teeth on one edge; the band is carried on two large-diameter rotating wheels mounted on parallel axes some distance apart. Band saws that cut vertically are particularly suitable for cutting out shapes in thin, flat plates from workpieces that lie on horizontal tables.

Cold-sawing machines with toothed disk cutters are used extensively in steel-rolling mills and in places where large quantities of bars are cut. A V-shaped clamping vise enables bundles of bars to be clamped and cut at one time.

Friction-sawing machines are used largely for cutting off steel structural shapes such as I beams, channels, and angles. The cutting wheels, with or without teeth, rotate at such high speeds that the heat from the friction of contact is sufficient to remove the metal by melting it. Abrasive cutoff saws, thin rubber or Bakelite-bonded abrasive wheels that are operated at high peripheral speeds, are particularly suitable for cutting off thin tubes and hardened steel bars.