Machining properties of wood : Tool wear, cutting force and tensioning of blades
Abstract: Cutting processes, in general, and wood cutting processes, in particular, are complex to explain and describe with many influencing factors. Wood, in contrast to man-made fabricated materials, is not a homogenous and distinct material, but a multifaceted and nonhomogeneous biological material. A fundamental understanding of wood cutting processes and the machining properties of wood can be obtained by investigating the interaction of wood properties, cutting tools and machining parameters. Such an understanding provides possibilities for improving product quality, increasing production efficiency, or otherwise improving the machining processes. The aim of this thesis was to find ways of improving the machining properties of some wood species, focusing on tool wear, cutting forces and the tensioning of circular sawblades. The studied wood species were five Mozambican tropical species, namely: Swartzia Madagascariensis (ironwood); Pseudolachnostylis maprounaefolia (ntholo); Sterculia appendiculata (metil); Acacia nigrescens (namuno); Pericopsis angolensis (muanga) and two main Swedish wood species: Pinus sylvestris L. (Scots pine); and Picea abies (L.) Karst. (Norway spruce). A series of experimental tests were conducted to determine the suitability of different cutting tool materials when machining these wood species. Machining tropical hardwood and Swedish frozen wood under winter conditions is still a challenge when it comes to the choice of which cutting tool material to use. Tool wear was used as a criterion to evaluate the performance of the cutting tool materials. Additionally, the relationship between tool wear and some chemical and physical properties for Mozambican tropical wood species was analysed. Different wear mechanisms were identified using a scanning electron microscope. It was concluded that tool hardness alone was not the only factor affecting tool wear; a certain amount of tool toughness was also needed to obtain low tool wear. The predominant wearing mechanisms for the tropical wood species tested were abrasion and edge-chipping. Furthermore, tropical hardwood species were subjected to cutting force tests. A standard single saw tooth, mounted on a piezoelectric load cell, was used to evaluate cutting forces. The theoretical approach used for the prediction of the main cutting forces was based on surface response methodology. Among the studied variables, chip thickness and cutting direction had the greatest effect on the main cutting force level, while wood density, moisture and rake angle had the least effect.Power consumption using double arbour circular saw machines was investigated. The experiments were carried out, under normal production, in two Swedish sawmills. The climb-sawing model in both sawmills was able to estimate the power consumption better than the counter-sawing model. Climb-sawing had higher power consumption than counter-sawing. The lowest power consumption was found using a higher overlap between circular sawblades. Finally, experimental and theoretical models to improve circular sawblade dynamic lateral stability were developed. Different methods for monitoring flatness and tensioning in circular sawblades for wood cutting were discussed. Additionally, the effects of the magnitude of the roller load, number of grooves and groove positions were tested. The roll-tensioning effects were evaluated by measuring the shift in natural frequencies of several vibration modes. Natural frequencies obtained with the finite element method were in good agreement with the experimental test results. The magnitude of the roller load, number of grooves, and groove positions all affected the natural frequencies.
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