Chip mark, diagonal grain, torn grain and wane are the defects in timber due to:
Sources of Defects in Timber
Defects in timber are irregularities that can reduce its strength, durability, or appearance. They can be introduced at any stage of the timber's life, from a growing tree to a finished wooden plank. Understanding the source of a defect is key to preventing it.
📝 Detailed Analysis of the Options
(a) Insects
Defects caused by insects include tunnels and holes bored into the wood. Common culprits are termites, beetles, and marine borers. These defects weaken the timber from the inside.
(c) Natural Forces
These defects develop while the tree is growing. They include irregularities like burls (irregular growths), callus (soft tissue covering a wound), coarse grain (wide annual rings), and chemical stains.
(d) Fungi
Fungi cause decay and staining in timber. This includes various forms of rot (wet, dry, brown, white) and stains (blue stain, sap stain) that break down the wood fibres or discolour them.
(b) Conversion
This is the correct answer. Conversion is the process of sawing a log into usable planks, boards, or beams. Defects can be introduced during this stage due to improper sawing or dull tools. The defects listed in the question are classic examples of conversion defects:
- Chip Mark: A mark or depression made by a cutting tool.
- Diagonal Grain: Caused by sawing a log at an angle to its true axis.
- Torn Grain: Occurs when wood fibres are torn out by the saw, leaving a rough surface.
- Wane: The presence of the original rounded surface of the log on a sawn piece of timber.
📊 Summary of Timber Defects by Source
| Source of Defect | Examples |
|---|---|
| Conversion | Chip Mark, Diagonal Grain, Torn Grain, Wane |
| Fungi | Blue Stain, Brown Rot, Dry Rot, Wet Rot, Sap Stain |
| Insects | Beetles, Marine Borers, Termites (tunnels and holes) |
| Natural Forces | Burls, Callus, Chemical Stain, Coarse Grain |
💡 Study Tips
- Conversion = Sawing: Associate "conversion" defects with the process of cutting a log into lumber. Think about what can go wrong during sawing.
- Categorize by Cause: Group defects mentally by their source: Natural (while growing), Conversion (during sawing), Fungi (decay/stain), and Insects (eating the wood).
- Look for Action Words: Terms like "chip," "torn," and "diagonal" suggest an action, like sawing, which points towards conversion as the cause.