6. Performance characteristics of wood Flashcards
What are the different stock forms of wood?
- rough sawn
- planed square edge (PSE)
- planed all round (PAR)
- natural wood
- manufactured boards
- mouldings
What is the rough sawn stock form?
- wood comes direct from seasoning
- has rough surfaces
- will have nominal sizes rather than accurate finished sizes
- cheapest to buy
What is the PSE stock form?
- wood only has one edge that is planed accurately
- the other edges are rough sawn
- planing removes about 3mm from the original nominal size
What is the PAR stock form?
- wood has sides and edges that are all planed square, straight and level
- smooth finish
- ready to use
- 3mm smaller all round than original nominal size
- most expensive to buy
What is the natural wood stock form?
- comes from trees
- available at only the maximum width of the tree
- natural planks would need to be joined to create a larger surface
- planks should be joined with the end grain going in opposite directions to minimise distortion
What is the manufactured board stock form?
- man-made
- produced in large sheets typically 1220mm x 2440mm
- wide surfaces can be produced from one sheet without the need for joining processes
What is the mouldings stock form?
- pre-made mouldings (e.g. skirting boards) are readily available in standard lengths up to 4 metres
What are the characteristics of wood?
- natural material
- will naturally biodegrade if left in landfill
- wood fibres grow along the length of the tree or branch (grain)
- wood expands and shrinks with changes in humidity
- strong in the direction of the grain and weaker across the grain
- wood is converted (cut from tree into useable pieces) and then seasoned (air or kiln) to remove excess moisture before use
What is air seasoning?
- traditional
- inexpensive
- wood is stacked in a shelter outside, protected from rain
- air circulates between planks to remove excess moisture
- slow process (up to a year)
- the wood is used for outdoor products
What is kiln seasoning?
- expensive
- uses controlled temperature and humidity levels
- wood is stacked on trolleys
- initially steamy in the kiln, then becomes hotter and drier
- quick process (up to a few weeks)
- wood is used for indoor products
Why is wood seasoned?
- gives greater immunity from decay
- increased resistance to rot
- increases strength and stability
- helps preservatives to penetrate
- makes wood less corrosive to metals
- helps prevent defects (twisting, warping, cracks, cupping, blowing)
How can the time taken to season wood be reduced?
- by felling trees in the winter
- grow less
- have less sap
- contain least moisture
What does the toxicity of wood depend on?
- the species
What is hardwood?
- wood from a deciduous tree
What is softwood?
- wood from a coniferous tree
What do both hardwood and softwood dusts have under the Control of Substances Hazardous to Health (COSHH) regulations?
- workplace exposure limits (WELs) of 5mg per cubic metre
What is hardwood dust listed as?
- carcinogenic
How can an employer ensure the WEL is not exceeded?
- by providing PPE
- make sure extraction and ventilation systems are used
What are hardwoods?
(examples)
- oak
- ash
- mahogany
- teak
- birch
- beech
What are the properties of oak?
- hard
- tough
- attractive grain
- good weather resistance
- contains tannic acid (corrodes steel screws or fixings)
What are the uses of oak?
- furniture
- flooring
- boat building
- cladding
- interior and exterior joinery
What are the properties of ash?
- tough
- attractive open grain pattern, making it more flexible
What are the uses of ash?
- tool handles
- ladders
- sports goods
- laminating
