Giacomo Goli

Fracture toughness, chip types and the mechanics of cutting wood. A review COST Action E35 2004–2008: Wood machining – micromechanics and fracture

Abstract Historical studies for predicting cutting forces in wood processing are based on the Piispanen/Ernst-Merchant theory employed in metal cutting where the offcut/chip is formed in shear. This analysis has been recently improved to include significant work of surface separation and formation (i.e., the fracture toughness of the workpiece, as well as the shear yield stress and friction). The new theory is applied here to wood cutting experiments. It is well known that chip formation and surface damage depend on grain orientation and chip thickness, but experiments reveal that chip formation alters with cutting speed as well. During the COST E35 action a series of experiments and special devices to orthogonally cut wood at high and low speed have been developed. In this paper, an overview of the cutting devices and the main results are given.

Proposal of a new method for the rapid assessment of wood machinability and cutting tool performance in peripheral milling

The quality evaluation of a wooden surface machined by peripheral milling can be achieved by means of the visual assessment or by surface roughness measurement. The method defined in this paper is a novel alternative to the state-of-the-art material machine-ability evaluation providing objective results acquired by an automatic system. Critical machining conditions and surface defects are stimulated by a purposely-designed circular sample. The approach proposed bases on a multi-sensor technology where a laser triangulation system reconstructs the 3D surface topography and a camera records the grayscale image of the same surface while the sample is rotated around a central axis. The dedicated software allows automated surface reconstruction, quality assessment and the detection of specific defects. The method is rapid and allows an easy comparison of different cutting conditions intended as a tool to rapidly determine optimal solutions.

Cutting forces by Oak and Douglas fir machining

In this work the multi-factor, non-linear dependencies between main (tangential) FC and normal (radial) FN cutting forces upon two machining parameters by up-routing and down-routing wood of Douglas fir (Pseudotsuga Menziesii) and Oak (Quercus petraea) were evaluated. The relationships are graphically illustrated and discussed. The obtained data were compared with cutting forces evolution models according to grain orientation from the literature in order to verify if literature statements or models comply with measured data. Evidence of several contradictions was found relative to results from available literature.

Verifying the operation of an elastic crossbar system applied to a panel painting: the Deposition from the Cross by an anonymous artist from Abruzzo, sixteenth century

This study deals with the post-treatment evaluation of the elastic crossbar system already designed and installed on the recently restored panel painting Deposition from the Cross by an anonymous artist from Abruzzo (sixteenth century). After the restoration, the panel and the crossbars were subjected to mechanical tests to identify their elastic characteristics. Then the panel, equipped with the elastic crossbar system, was subjected for about two months to controlled environmental cycles made up of approximately constant humidity periods. During the two months, the forces exerted by the springs and the deformations of both panel and crossbars were continuously monitored. A mathematical model, calibrated on the specific parameters derived from the analysis of the panel, provided the deformation that the panel would have shown without the crossbars; comparing the model output with the measured data provided a restraining effect (RE) of approximately 7% (RE could range between zero – no restraint – and 100% – total restraint). Future developments of this project will define appropriate procedures to design an elastic crossbar system for a given panel, once the expert judgement of restorers has identified the most desirable RE to be achieved.

Machinability of Minor Wooden Species before and after Modification with Thermo-Vacuum Technology

The influence of the thermal modification process on wood machinability was investigated with four minor species of low economic importance. A set of representative experimental samples was machined to the form of disks with sharp and dull tools. The resulting surface quality was visually evaluated by a team of experts according to the American standard procedure ASTM D-1666-87. The objective quantification of the surface quality was also done by means of a three dimensions (3D) surface scanner for the whole range of grain orientations. Visual assessment and 3D surface analysis showed a good agreement in terms of conclusions. The best quality of the wood surface was obtained when machining thermally modified samples. The positive effect of the material modification was apparent when cutting deodar cedar, black pine and black poplar in unfavorable conditions (i.e., against the grain). The difference was much smaller for an easy-machinability specie such as Italian alder. The use of dull tools resulted in the worst surface quality. Thermal modification has shown a very positive effect when machining with dull tools, leading to a relevant increment of the final surface smoothness.

Simplified Method for the Characterization of Rectangular Straw Bales (RSB) Thermal Conductivity

This research aims to design and implement tools and methods focused at the assessment of the thermal properties of full size Rectangular Straw Bales (RSB) of various nature and origin, because their thermal behaviour is one of the key topics in market development of sustainable building materials. As a first approach a method based on a Hot-Box in agreement with the ASTM C1363 – 11 standard was adopted. This method was found to be difficult for the accurate measurement of energy flows. Instead, a method based on a constant energy input was developed. With this approach the thermal conductivity of a Rectangular Straw-Bale (RSB λ) can be determined by knowing the thermal conductivity of the materials used to build the chamber and the internal and external temperature of the samples and of the chamber. A measurement a metering chamber was built and placed inside a climate chamber, maintained at constant temperature. A known quantity of energy was introduced inside the metering chamber. A series of thermopiles detects the temperature of the internal and external surfaces of the metering chamber and of the specimens allowing to calculate the thermal conductivity of RSB in its natural shape. Different cereal samples were tested. The values were found consistent with those published in scientific literature.

Beech sawn timber for structural use: A case study for mechanical characterization and optimization of the Italian visual strength grading rule

The potential use of beech (Fagus sylvatica Linnaeus) sawn timber for structures has been investigated. Beech stands in transition from coppice to high forest after thinning interventions have been sampled from different Italian sources. A sample of 160 beams of two different cross sections was extracted and tested, according to the normalized procedures for the characterization of wood for structures. The specimens were visually graded considering the strength relevant defects, according to the Italian standard. To determine the mechanical properties of the timber (namely strength and stiffness), four-point bending tests were performed and the main characteristic values were derived. The results of the study reveal that knot ratio was the principal defect that influenced both resistance and yields. The potential strength class D30 was achieved with the resulting characteristic values, although, due to the small number of samples tested, the reached class could not be formally assigned. To improve the effectiveness of the grading rule in the Italian standard, some modifications and a new grade were proposed for the beech sawn timber. Finally the D40 strength class could be theoretically reached, but with reduced yields.

Preliminary tests for mechanical properties of wooden ‘buttons’ used for attaching auxiliary supports behind panel paintings

Abstract When wooden supports of panel paintings have been severely altered or damaged and the original cross beams are missing, new cross beams or other types of auxiliary supports are connected to the panels back-face in order to achieve desired effects including panel strengthening and control of deformations. Some conservation laboratories use small wooden blocks, each glued on the back face of the panel and holding freely the head of a screw connected (often by means of springs) to the auxiliary support; such blocks being called in Italian ‘piedini’ or ‘bottoni’, the English term ‘buttons’ is proposed here. This paper reports research aiming to characterize the mechanical behavior of three types of ‘buttons’ having different geometries and made of various wood types (oak, walnut, lime, beech, and a medium density fiberboard). Short-term mechanical tests (∼1 minute duration) were performed with a universal testing machine by axially pulling out the 4 mm diameter steel screw from the button, glued onto a dummy board. Load and deformation were recorded, and the load–deformation curves were analyzed. The results show that the deformability and the load-carrying capacity of the buttons were influenced primarily by the manufacturing geometry and secondarily by the wood density.