Moving on from his last article, which saw him discussing methods of harvesting, Peter Bishop now makes the natural leap to looking at the processes used for converting logs to lumber.
‘Conversion’ is the phrase used when turning roundish logs into usable, flat pieces of timber. Here we’ll take a look at what happens after the tree trunk has been felled, the equipment used and the best cuts.
The primary objective of timber conversion is to get as much usable timber, planks, boards and scantlings from the log. The amount produced is called the ‘yield’. In high production mills, mainly softwood mills, this can be as high as 80%. In a temperate hardwood mill the yield will probably be no more than 50% at best with tropical timbers dropping somewhere in between. A secondary consideration, especially with hardwoods, is to produce material of the best quality regardless of yield. By this we mean stuff that might be sawn to produce highly decorative boards, such as quartersawn figured oak.
Most softwood is produced with the yield factor in mind, with the possible exception of some top quality North American species. Everything is used. The bark is removed for fibre extraction or simply broken up for garden mulch. Unusable, small pieces of slab wood are chipped and collected along with the sawdust produced to go into particle board manufacture and pulp for paper.
For furniture and cabinet work hardwoods are of more interest, especially temperate species from North America and Europe. Because of this most of what is discussed here will relate to them. However, all conversion techniques are universally the same, whether for hardwoods or softwoods.
The head-rig saw
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