Put another way – how efficient is it to cut squares and rectangles out of a generally circular, tapered log with defects and irregularities such as butt flare, fluting and nodal swelling? In general, most timber manufacturers in NZ are doing well if they get a yield (conversion) above 60%, which sounds like a lot of waste. But let’s consider:
- The first step is to take off the bark – which most sawmills pay for as the logs are sold by the tonne.
- You then need to take off the outside curved slab of timber to produce a flat, straight, opening face – by definition a board needs to have flat faces. So the slab goes to chip.
- Depending on log length and where in the tree the log comes from, it can have a lot of taper (ie the small-end diameter is considerably smaller than the large end diameter and as builders generally want their timber straight and true, the outside of the log when cut straight produces shorter boards as the saw runs out of log. It also just so happens that the outside wood has the superior structural properties. (So next time you buy timber remember that the “shorts” could well have the superior timber.)
- Each tree has individual characteristics. Therefore, so do the logs! Defects need to be docked out as they arise.
- Unfortunately, in Radiata pine the central 10-12 growth rings are generally the poorest quality timber with lower density and are therefore prone to movement during drying (but they are full length boards), so a reasonable proportion of the full-length log centre ends up as low grade arising products that generate no profit and may not cover the cost of the fibre and the processing
- A saw generates sawdust – the ‘kerf’ is the width of the sawcut and the associated volume loss that occurs from sawing.
- Over-cut or aim size is the actual size the timber is cut to which is greater than the tally size to allow for sawing variation and drying and gauging etc. Green sawn timber is therefore slightly larger than the ‘advertised size’, which again uses up fibre.
A typical structural sawlog will end up producing something like:
Sawn timber 58-65%
Bark 3-4%
Sawdust 6-10%
Chip 28-35%
To make it more difficult, from a 6.0m log a timber manufacturer will only end up with 60-70% of the resulting products in 6.0m lengths and a third of this might be in lower grade arisings.
However, there is often very little of the log that goes to waste.
- The bark is generally on-sold to be processed for landscaping and garden use.
- The chip is sold for processing in the pulp and paper industry
- The sawdust along with the dry shavings from the planer are often used onsite for boiler fuel or sold for fuel elsewhere. Some mills are now investing in the production of fuel pellets.
The moral of this story?
Timber manufacturing is not as simple as it seems.
As an extractive primary industry there is considerable investment and skill required to survive the boom-bust cycles.
One man’s waste is another man’s feedstock.
Have a look at structural timber in packs shorter than 6.0m – you may find that the quality surprises you.
.png)
