performance of wood Flashcards
How does grain affect a woods workability?
- can increase the price
- tight and fine grains make them easy to turn [beech]
How is wood structured?
- wood fibres are hollow straw-like cells held by lignin [tracheid]
- tracheid carry water and nutrients from the roots to the leaves
how is the grain distinguished?
- direction of the rays
- wood splits in the direction of the rays
- planning with the grain avoids the wood tearing
why is cutting across the grain easier than along it?
Fibers are shorter and hold together yet are easier to overcome so let go sooner
because wood splits along the grain so the wood would continue to split after it was cut
what is the definition of Anisotropic?
the grain giving wood different physical properties in different directions
-natural wood is stronger along the grain not across it
are manufactured boards anisotropic?
no
what defects can occur to wood?
- Knotts can fall out as wood shrinks
- machining can leave marks of the wooden surface
- damage can occur from insects, rot and disease
what term best describes waters sponge-like qualities?
hygroscopic [absorbs moisture which causes swelling]
what prevents timber distorting?
seasoning
what causes wood to distort?
[evaporation]
- water evaporates from freshly felled timber causing shrinkage
- if it evaporates too quickly or unevenly it can; twist, warp, bow, cup or split
the process of seasoning - air dried and kiln dried
air dried
- stored undercover and left outside to acclimatize naturally
Kiln dried
- uses steam to artificially dry the wood which speeds up the process
why is wood seasoned?
To make wood stable to prevent warpage, shrinkage and splitting. This makes it less likely to change during manufacture
why is outdoor seasoned timber best suited for external projects?
it was acclimatized to outdoor conditions so is stable in that environment
what are the differences between kiln drying and air drying wood ?
- air drying is naturally done with limited control over the process
- it takes longer than a kiln
what properties does seasoned timber have?
- stronger [stable]
- less likely to decay/rot
- less likely to corrode ferrous metals
- absorbs preservatives better
what are the 3 shrinkages and why does it occur?
- wood swells and shrinks with the seasons due to moisture levels
- diamond shape [square sections]
- cup [u shaped]
- uniform shrinkage
where does shrinkage occur?
- most around the year rings
- radial shrinkage between 2-8%
- less shrinkage along the length
advantages and disadvantages of joining wood
- woods with high oil contents are hard to glue [teak]
- acidic woods can corrode metal fixtures
- ash and beech glue well as they have a big cell structure
what is forming?
additive and wasting processes along with manipulation
what are wasting processes?
carving, turning, milling and routing
what is steam bending?
uses heat and moisture to make wood pliable
[open pore structures are easier to steam]
steam bending process
- heat is applied to make the wood bend without breaking
- held in a former to cool and dry in shape
- open pore structures are better to steam bend like ash and beech
- steam bending maintains the longitudinal strength of the wood
machining qualities of wood
- natural wood is difficult to machine as it is hard and tough
- density of wood is less predictable
- oil can clog up tools
- manufactured boards have uniform quality and density so they are predictable to work with
what is laminating?
combines wood and adhesives for different purposes
why is the laminating process used?
- used to create curves using areoply and flexiply
- used to get larger wider wooden sections
- melamine formaldehyde or wood veneer can be added to enhance surface properties
how do insects affect wood and how can it be prevented?
- insects are attracted to the starch and sugars in wood
- insects lay larvae inside the wood which hatch and burrow out
- dry, varnished or painted wood with high oil content reduces the risk
fungal attacks
- damp or poorly ventilated conditions allow fungi to attack
- fungal spores attach to the wood and feed of its nutrients that causes rot
- dry rot makes the timber weak, dry and brittle
- wet rot allows fungi to be seen
- dry wood is better protected
how is wood made moisture resistant?
- oily woods [teak and redwood] can endure wet conditions
- marine ply is made from water- resistant hardwood veneers and waterproof adhesive to withstand wet conditions
- paint, varnish and oil can be pressure treated to increase moisture resistance
what conditions do toxic trees cause?
wood dust and sap can cause; irritation to the skin, respiratory problems, nausea and affect vision
-cedar can cause migraines, stomach problems and effect the nervous system
what regulations are in place by Control of Substances Hazardous to Health (COSHH) for toxic woods?
- wood dust is limited to 5mg per cubic metre
extraction and ventilation systems
-PPE must be provided
-beech and bamboo are food safe and used for kitchen utensils and toys
what are grain patterns?
how the wood is cut up
what is warpage?
wood has swelled and moved
What woods are toxic?
akasha wood, oak, seeder, teflon, eucaliptus
What are machining qualities?
how easy the wood is to work with
what 2 factors cause wood to split?
heat and movement
what is the end grain of wood?
pattern at the end of a piece of wood
how are knots formed ?
branches snap so resin fills the whole and seals it off to form the knot
Is wood weak or strong if the grain is in different directions?
weak due to different forces
Why is kiln dried wood better for indoors?
there is absolutely no water inside
