School of Agriculture, Food and Ecosystem Sciences - Theses

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Start date: 2000 × End date: 2009 × Type: Masters Research thesis × Author: Blackwell, Philip A. ×

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    Using portable sawmills to produce high value timber from farm trees in the semi-arid zone
    Blackwell, Philip A. (University of Melbourne, 2002)
    There is a perception by some within the sawmilling industry that portable sawmills cannot produce the sawing quality in hard high-density timbers that is required to be merchantable in the manufacturing industry � especially the higher value sectors such as furniture. This study investigated the on-site processing of farm timbers from the semi arid regions of Australia using two different portable sawmills. There were a limited number of plantations with sufficient quantity of suitable saw logs within the low rainfall regions of south-eastern Australia for sawing trials. The only plantations that had sufficient saw logs were of high-density eucalypt species. The four species selected were Brown Mallet (Eucalyptus astringens), Sugar Gum (Eucalyptus cladocalyx), Yellow Gum (Eucalyptus leucoxylon) and Swamp Yate (Eucalyptus occidentalis). The portable sawmills selected for investigation were at the lower end of automation available on portable sawmills. Both mills required an operator to push and pull the power head along the log to produce sawn boards. The mills used in the sawing trials were a single circular blade type sawmill, which was represented by a `LucasMiIl� Model 8' and a horizontal bandsaw type sawmill, which was represented by a `Laidlaw� Farmill'. Initial results showed the selected species produced dimensionally consistent 150 mm wide boards that could be used in the manufacture of furniture or other products. The key criterion for investigation of sawing with both types of portable mill was the straightness of cut as the measure of sawing accuracy. This was measured with a modified straightedge and digital calipers. A total deviation or waviness in the cut of less than 1.5 mm was considered acceptable. An investigation using the single circular blade sawmill sawing with four different width blade tips revealed the 5 and 5.7 mm wide Carbide-tipped blades produced straighter cuts than the 4.5 and 5.4 mm wide Carbide-tipped blades in each of the trial species. The 5.0 mm blade also produced straight cuts with the fastest feed-speed in the majority of species while the 5.7 mm blade was the slowest sawing blade. The use of a 5.0 mm tip width blade on a single circular blade sawmill could be considered as the most versatile blade that is capable of producing more straight cuts and at the fastest feed-speed when sawing high-density hardwood species than other tip width blades. Investigation using the horizontal bandsaw sawmill and varying the blade parameters of hook angle, tooth set and blade tension showed the consistency in obtaining the blade tension was a major factor in accurate sawing. At low blade tension, the straightness of cut was adversely effected. The other variables revealed that at the widest tooth set the deviation along the cut increased with the faster feed-speed. The range of hook angles tested had no effect on cut straightness. It is possible that at higher hook angles problems might occur, as experience within the sawmilling sector suggests, but the range selected in this study, inadvertently, did not extend that far. Investigation of three blade types with different tip construction and at different widths revealed the 32 mm wide Bi-Metal blade was the best general blade over the range of species. Although this blade did not produce the straightest cuts in all the species the cuts were within the accepted tolerance of 1.5 mm. The carbon steel and Carbide-tipped blade did produce the straightest cuts in some species but were outside the acceptable range in other species. A final trial to assess controlled feed-speed required the horizontal bandsaw to be modified with the attachment of a variable power feed unit to the sawing carriage. This modification allowed the feed-speeds to be doubled in most species without any reduction in the straightness of cut. The production of quality timber is technically feasible using portable sawmills if the conditions and parameters of the blade, timber and mill are matched to meet the problems each log will demonstrate during the milling process.