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13 diseases


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13 diseases

  2. 2. Chapter 13 Diseases and Defects
  3. 3. Defects  Defects in timber can occur 1. naturally (during the growth of a tree) or 2. artificially (as a result of incorrect stacking or seasoning)  Some defects can be decorative and very pleasing to the eye, so these will be kept  Others may cause problems during the manufacture of a project, such defects must be overcome
  4. 4. Natural Defects Occur during the growth of a tree – Knots – Resin pockets – Shakes – Waney Edge
  5. 5. Knots: These are formed from small branches which are cut off the tree or fall off. They reduce timber strength and make it hard to work Resin pocket: • Small cavities in the wood which are full of resin. Exist deep in the wood, especially in softwoods. • − Live knot − Dead knot(loose) • Cracks caused by high wind or extremes in temperature Reduce the strength of the timber
  6. 6. Shakes − Radial shakes − Tangential shakes
  7. 7. Shakes  Shakes are splits in the end grain of wood  Occur along either: rings Ray lines Annual  Caused by tension forces which build up as the tree is growing. When it is felled, or during seasoning weaker points break and the wood splits causing shakes
  8. 8. Waney Edge Occurs during the conversion process Occurs when the bark is left on the edge of the plank
  9. 9. Waney edge
  10. 10. Artificial Defects Occur as a result of incorrect stacking or inappropriate drying schedule, stresses will be created in the wood that will cause – Cupping – Bowing – Twisting / Warping – End Splitting – Case Hardening – Honeycomb Checks
  11. 11. Cupping Caused by differential shrinkage Planks will cup away from the pith Annual rings appear to try to straighten out
  12. 12. Bowing Takes the form of a bend along the length of the piece Can be as a result of improper stacking – stickers too far apart
  13. 13. Twisting Occurs when opposite corners move in a similar direction and the plank loses its flatness as a result
  14. 14. End Splitting Caused if the ends of the boards dry out too quickly due to exposure to the sun or heat Occurs during Air Seasoning Can be prevented by sealing or covering the end grain of the planks of wood
  15. 15. Artificial defects – seasoning Case hardening − Moisture is trapped in the centre if surfaces dry too quickly − Boards bend when tension is released
  16. 16. Fungal life cycle
  17. 17. Types of rot  Wet rot − Occurs outside − Doors and window frames − White residue  Dry rot − Under wood floors − Musty smell − Poor ventilation − Charred wood appearance
  18. 18. Dry rot  Treatment − All infected wood removed and 500mm beyond − All blockwork and wood treated with fungicide − The cause of infection must be found and fixed − All new timber must be treated with preservative
  19. 19. Most common wood boring insect The furniture beetle (woodworm)
  20. 20. Insect attack Insect life cycle  Eggs are laid in wood  The larva (worm) hatches and begins eating the wood  When grown, the larva makes a cocoon (pupa) and changes into the adult beetle  Adult emerges at the surface
  21. 21. Quiz
  22. 22. Name the defects shown in the diagrams − Cupping − Warping/twisting − Heart shake
  23. 23.  The life cycle of a wood boring insect is shown. At which stage does the most damage occur?  Most damage occurs while the insect is at the larva stage  The larva bores tunnels under the surface of the wood
  24. 24.  List the conditions necessary for a fungal attack to occur in wood  Conditions – A moisture content above 20% – A supply of food (wood) – Oxygen
  25. 25. Questions  1 What is the difference between natural and artificial defects in timber.  2 Give an example using a Sketch of both a radial shake and a tangential shake.  3 Describe using sketches what is meant by bowing, cupping and twisting in planks.  4 Why does case hardening occur?