Sawmilling Softwoods - The Issues
|Even softwoods can have tensions. This Cryptomeria japonica (Japanese cedar) board has split through the pith as a result of opposing tensions (see arrows). Boards with pith can be unstable resulting in cupping or splitting.|
"Softwoods" are sometimes called needle-leaved trees, conifers or gymnosperms. These include cypress, macrocarpa, redwood, cedar and totara.
Softwoods are considered easier to saw than hardwoods because they are generally faster cutting and require fewer saw sharpenings. Also the sawing techniques generally do not need to consider board movement off the log (i.e. resulting crook and bow) and thus are faster and easier.
Drying of softwoods is generally fairly straight forward and shrinkage
Tangental shrinkage is greater than radial is fairly low. Wide boards can be cut without the level of degrade from cupping that is prevalent with drying wide hardwood boards. The sawing orientation of the board (i.e. from flatsawn to quartersawn) is not an important factor in seasoned product degrade. However orientation does affect figure and grain patterns, a consideration when marketing the resulting timber.
Softwood is usually cut green to nominal (or "call") sizes.
|Call sizes (mm)||25||40||50||75||100||125||150||200||225||250||300|
|Actual dressed dry size (mm)||19||35||45||70||90||115||140||180||205||230||280|
Movement in service is fairly low for most softwoods. Quartersawn material does tend to be more stable but the additional level of stability offered is not usually a consideration when specifying product. Boards containing pith are inevitably the most unstable, followed by wide flatsawn boards.
Multi-blade (dimension sawmills) and swing-blade circular sawmills can be inexpensive and productive portable sawmills. Bandsaws, however, because of their thin kerf are able to convert up to 20% more of the log into lumber than circular saws. Swing-blade sawmills (e.g. Lucas, Peterson) cannot cut wide boards or large dimension stock and Multi-blade sawmills (e.g. Mahoe, Rimu) cannot cut wide (bark to bark) slabs from logs. Portable bandsaws such as the Woodmizer are very versatile and can cut everything from large beams to wide slabs with very little waste due to the thin-kerf band. Portable swingblades and dimension mills are good for milling large logs into standard sized boards and without having to move them.
There are many sawing options to convert logs into lumber and it is advised to talk with suppliers about what options are appropriate.
Lawsons and Leyland cypress should have the sap and heart segregated when fresh as its harder to differentiate heart and sap in the seasoned state. Lawsons cypress has a fairly wide sap band.
Macrocarpa and lusitanica
Macrocarpa and lusitanica have few problems with sawing.
Farm-totara can have tension in the logs and sawing should aim for straight boards.
Redwood has a large sap band and sawing patterns, if targeting heart, should aim to segregate the more valuable heart from the sap. Paper wasps can chew the wood as its seasoning
Western red cedar is prone to collapse and internal checking as it dries. Always saw to the final dimension as the option of ripping seasoned larger stock into smaller boards will result in extensive surface checking on the ripped face. Western red cedar should also be sawn oversized to allow for the high shrinkages and collapse. If possible it should be quartersawn.
Paper wasps can chew the wood as its seasoning.
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