In the classical approach, energetic effects (cutting forces and cutting power) of wood sawing process are generally calculated on the basis of the specific cutting resistance, which is in the case of wood cutting the function of more or less important factors. On the other hand, cutting forces (or power-more interesting from energetic point of view) could be considered from a point of view of modern fracture mechanics. Cutting forces may be employed to determine not only toughness but also shear yield strength, which are then applied in the models. Furthermore, forecasting of the shear plane angle for the cutting models, which include fracture toughness in addition to plasticity and friction, broadens possibilities of energetic effects modelling of the sawing process even for small values of the uncut chip. Mentioned models are useful for estimation of energetic effects of sawing of every kinematics. However, for band saws and circular sawing machines, the chip acceleration power variation as a function of mass flow and tool velocity ought to be included in analysis of sawing at larger cutting speeds.
Several properties of wood including the cutting power requirements can be correlated to wood density. Therefore, according to the literature, the cutting power requirements (and/or cutting forces) could be computed as a function of the wood specific gravity. This research shows that such an approach, based solely on specific gravity, may be considered a rather rough and imperfect estimate of cutting power. Samples of Scots pine (Pinus sylvestris L.) wood from different provinces in Poland with varying densities were machined on a sash gang saw. The average cutting force versus average wood density (estimated with the standard gravimetric method) was calculated, and the local cutting forces correlated to the local wood density. The average values of the cutting forces measured at selected points along the sample's length were calculated by linear regression to the X-ray absorbance (density) estimated by means of Xray radiography.
In this paper, absolute and density normalized cutting model parameters of natural and impregnated Scots pine (Pinus sylvestris L.) are shown and a method for the calculation of their corresponding material properties in the principal material directions of wood is presented. The parameters were determined from measurements of cutting power on a sash gang saw, and are in detail the fracture toughness and the shear yield strength of wood. The cutting model used for fitting the data and calculating the parameters is based on a minimum energy criterion originally developed to describe an orthogonal single tooth cutting process where the chip of an isotropic material is built by shear. The effects of impregnation on wood are clearly visible in cutting power and model parameters, where for large chips less power is required compared to natural wood. Impregnated pine wood shows a reduced value of shear yield strength compared to natural pine whilst the correlated fracture toughness increased. The observed behavior might be explained by a lower moisture content of the impregnated pine compared to natural wood. Orthotropic fracture toughness and shear yield strength constants of natural and impregnated Scots pine might be used to predict cutting power for other cutting geometries and processes like circular sawing.
In the classical approaches, used in Central Europe in practice, cutting forces and cutting power in sawing processes of timber are commonly computed by means of the specific cutting resistance kc. It needs to be highlighted that accessible sources in handbooks and the scientific literature do not provide any data about wood provenance, nor about cutting conditions, in which cutting resistance has been empirically determined. In the analyses of sawing processes, the use of a model with elements of fracture mechanics involved is an alternative way. In this work, predictions of the newly developed model (FRAC_MOD) for the circular sawing machine are presented. Thanks to this modern approach, it was possible to reveal the usefulness of the FRAC_MOD, using experimental results data on fracture toughness and shear yield stresses of both Polish pine (Pinus sylvestris L.) and Czech beech wood (Fagus sylvatica L.). The achieved results were compared to the forecasted values obtained with classical models (CLAS_PL and CLAS_CZ), which are commonly applied in Central European sawmills. The carried out analyses allowed us to discover undesired effects in the form of underestimation of cutting power when applying the CLAS_PL and CLAS_CZ models. For that reason, the FRAC_MOD cutting model could be suggested for the prediction of energetic effects in cases of dynamical analyses and even unsteady cases.
The modern wood converting processes consists of several stages and material drying belongs to the most influencing future performances of products. The procedure of drying wood is usually realized between subsequent sawing operations, affecting significantly cutting conditions and general properties of material. An alternative methodology for determination of mechanical properties (fracture toughness and shear yield stress) based on cutting process analysis is presented here. Two wood species (pine and beech) representing soft and hard woods were investigated with respect to four diverse drying methods used in industry. Fracture toughness and shear yield stress were determined directly from the cutting power signal that was recorded while frame sawing. An original procedure for compensation of the wood density variation is proposed to generalize mechanical properties of wood and allow direct comparison between species and drying methods. Noticeable differences of fracture toughness and shear yield stress values were found among all drying techniques and for both species, but only for beech wood the differences were statistically significant. These observations provide a new highlight on the understanding of the effect of thermo-hydro modification of wood on mechanical performance of structures. It can be also highly useful to optimize woodworking machines by properly adjusting cutting power requirements.
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