2.1 Material
Eucalyptus globoidea boards originated from 31 trees felled at age 28-years old on Banks Peninsula (43°47'32.0"S; 172°50'20.8"E), New Zealand as part of a taper and volume study (Boczniewicz et al. 2022). Logs were then live sawn into 30 mm thick boards using a mobile horizontal band saw and subsequently ripped into 110 mm wide heartwood boards without pith. The boards were air-dried indoors for three years. The least defective, and most approximately flat-sawn boards were selected from the available material. Deviating from ASTM D1666-17, specimens with live knots not larger than 15 mm in width were included due to material constraint. As a control, thirteen flat-sawn, kiln-dried, rough-sawn, clears grade Pinus radiata boards (5,200 mm x 150 mm x 25 mm) were purchased locally.
From the rough sawn board samples for planing and sanding; grooving, edging, boring, and mortising; and turning; as well as density and moisture content determination were prepared according to ASTM D1666-17 and equilibrated to ~ 12% MC in a climate-controlled room at 65% relative humidity and 20°C.
2.2 Grading
The ASTM D1666-17 standard distinguishes five visually assigned grades for each machined surface. To ensure consistency only one person graded all samples. The grades were interpreted as follows:
Grade 1: defect-free pieces, where no fixing (sanding) would be required to finish the product to a high standard.
Grade 2: defects which could be remediated by light sanding.
Grade 3: remediation would require significant sanding or chiselling, resulting in slightly rounded edges or altered dimensions.
Grade 4: as Grade 3, except remediation will result in deep-rounded edges or visibly altered dimensions.
Grade 5: no remediation of the defect possible to achieve a high-quality surface finish.
Machining defects associated with knots were excluded from the surface quality analysis, in particularly for sanding and planing as wood near knots is at higher risk of torn grain and fuzzy grain (Öhman et al. 2016). The worst defect determined the grade of a piece. The machine score for a tool was defined as the percentage of the combined Grade 1 and 2 or Grades 1–3 as defined in (ASTM D1666-17).
2.3 Planing
Equilibrised boards were planed in random order regardless of species to remove the effect of tool wear with an SCM S630 thicknesser operated at 4500 RPM (revolutions per minute) with two new high-speed steel knives installed at a cutting angle of 30°. The feeding speed was 11.25 m/min, resulting in 0.8 cuts/mm. Samples were planed in two passes, with a first pass taking off 1 mm to remove artefacts of previous machining and a second pass taking off a further 1.6 mm of the same face. The surface quality was measured after the second pass.
2.4 Sanding
After planing, the samples were sanded using a Hafco Woodmaster DS-25 Dual Drum Sander fitted with 80-grit and 120-grit aluminium oxide paper. The feed speed was 6.1 m/min and 1.2 mm was sanded off one face for assessment. Each species was machined independently on fresh sandpaper.
2.5 Boring
A Dyco 12 mm bench drill press was used in conjunction with a 25 mm Bosch Forstner drill bit featuring a high-speed steel cutting edge to assess boring. The targeted feed speed was 0.5 m/min. The drill was run at 1200 RPM, deviating from the standard-specified 3600 RPM, as the samples burned at the higher speed. All samples were randomised. Two holes were bored into each sample and the order was reversed in the second run to remove the effect of tool wear.
For all boreholes three data points were graded independently: entry, the edge where the tool entered the wood; hole, the plane inside the borehole; and exit, the edge where the tool exited the wood. The grade of a data point on a board was the worst grade of the two holes per board.
2.6 Shaping
An Ascent Pro CNC machine was used with three router bits featuring high-speed steel cutting edges: a 6.4 mm straight bit, a 10 mm rebating bit, and an 8 mm rounding bit. The CNC machine was run with a feed speed of 4240 mm/min at 10,000 RPM. The samples were machined in random order to reduce to effect of tool wear. First, a J-shaped groove running parallel and perpendicular to the grain was cut with the 6.4 mm straight bit, followed by a rebate along the edge of the specimen, first parallel, then perpendicular, using the 10 mm rebating bit. Finally, the lower step of the rebate was rounded with the 8 mm rounding bit.
Four data points were graded for each groove: Side-grain inside the groove; the corner inside the groove; the end-grain inside the groove; and the edge where the tool exited the sample. Three data points were graded for each edge: the side-grain surface, the corner, and the end-grain surface.
2.7 Mortising
A Luxcut TM-3VSL turret mill was used in conjunction with a 13 mm custom-made high-speed cutting steel mortising chisel and a single spiral auger centre. The targeted feed speed was 0.23 m/min. Holes were mortised at 1200 RPM as the machine could not perform this task safely at the (ASTM D1666-17) specified 3600 RPM. The samples mortised in a randomised order and the two mortised holes per sample were machined in reversed order to minimise the effect of tool wear.
Four data points that were graded for each mortise hole independently: entry, the edge where the tool entered the wood; side-grain, the side-grain plane inside the borehole; end-grain, the end-grain plane inside the borehole; and exit: the edge where the tool exited the wood. The grade of a data point on a board was the worst grade of the two holes per board.
2.8 Turning
A Luxcut Z330X1000 lathe was used for turning in conjunction with a custom-made knife profiled as specified in (ASTM D1666-17) featuring a high-speed steel cutting edge. The targeted feed speed was 0.02 m/min. The lathe was run at 2000 RPM. Samples were randomised to reduce the effect of tool wear.
Four data points were graded independently per sample, at the angled, flat, concave, and convex part.
2.9 Data analysis
Regression modelling and t-tests were performed with R (R Core Team 2022).