After the headrig: edging, trimming, sorting and uptime

Once the log is broken down, flitches and wany boards go on to the edger, then the trimmer, then the sorting line. Sawing decisions are worth little if these stations waste the value that the headrig created.

Edging and trimming: where value is won or lost

An edger removes wane (the rounded, bark-covered edge) to produce square-edged boards; a trimmer cuts them to length and removes end defects. Saw edgers are more common than chipping edgers; chipping edgers turn outer waste into chips, while saw edgings break easily and are harder to convey (North American trade column, 2005).

In hardwoods the decision is mainly about grade. US research reports that about 20% of hardwood lumber needs edging and nearly all needs trimming, that human operators reached only 62 to 78% of the optimal value in edging and trimming rough green hardwood, and that a prototype scanning system recovered 99% of maximal value in 92% of boards (Schmoldt et al. 2001, USA). Earlier studies suggested a potential to raise lumber value by over 20% through better edging and trimming decisions. In the yield article, a computerised edging case was estimated at 12.9% above operator decisions (Steele 1984, USA, old data).

Practitioner claims in trade columns (for example trimmer speeds of 30 to 40 or 90 pieces per minute, or an 80% cut in sawing variation by climb-cut edgers) come without stated conditions and are unverified; do not use them for planning.

Sorting

Sorting does not cut the wood, but it decides whether the sawing becomes a saleable product. OSU Extension (USA, 2017):

• The green chain (hand-pull sorting) is the classic method for small mills handling unseasoned lumber; it stays popular where labour is affordable.
• Drop sorters (sling or bin types) suit mid-size and large mills, with photocells directing boards into bins; large ones can have 70 or more bins.
• Sling sorters need less capital; bin sorters have lower operating costs because slings never need replacing.
• Tray sorters are mainly used at planer mills because boards slide instead of dropping, protecting planed surfaces.

The articles give no throughput figures for sorters, so their capacity cannot be compared here.

Sawing accuracy: a worked example

A study of a band saw on wawa (Triplochiton scleroxylon) at a Kumasi mill, Ghana (KNUST thesis, 2002) found mean board thickness of 42.42 mm against a 41.00 mm target. The excess is 42.42 - 41.00 = 1.42 mm, and 1.42 / 41.00 = about 3.5% oversize. The thesis converted this into 3.46 m³ lost per 100 m³ of lumber. The main driver of thickness variation was excessive kerf width, not the band saw as such; setting, tooth geometry and maintenance matter.

Blade care and downtime

A band saw works properly only when it is correctly tensioned and flat; the Australian training unit for wide band saw blades lists checking for lumps and ridges, controlling tension and back-gauging the blade length. Reported frame saw blade working times in the Polish paper range from 3 to 4 hours for hollow and swage blades up to 16 to 20 hours for carbide.

Downtime is where theoretical capacity meets reality. In a single small Estonian mill observed for 11 days (a bachelor's thesis, 2019), saw blade problems took 4.0% of working time in winter and 0.8% in spring, preparatory work 16.8% and 22.9%, and breaks 22.6% and 12.1%. This shows categories of loss, not a benchmark: log handling and preparation can consume more time than blade problems. A US study (Wiedenbeck and Blackwell, 2003) argued the logic of costing every minute: at that time, operating cost for hardwood mills was 4 to 25 US dollars per operating minute.

Checklist: a weekly line review

• Measure board thickness at several points on sample boards; compare mean with target.
• Log blade changes and the reasons for them.
• Record stops by category: blade, preparation, breaks, log handling, waiting for logs.
• Check whether the optimiser or an operator decides edging and trimming.
• Find the bottleneck station: faster sawing is wasted if the sorter limits output.

Key points

• Edging and trimming decisions can swing board value by a fifth or more; scanners close much of the gap between operator and optimum (USA studies).
• Sorting converts sawing into saleable product: green chain for small mills, drop sorters for larger ones.
• Mean thickness above target is lost wood: 42.42 mm against 41.00 mm is about 3.5% oversize.
• Track downtime by category; log handling and preparation can matter more than blade problems.
• Unsupported trade-column figures are not planning data.