In plywood, three or more veneers in odd numbers are placed one above the other with the direction of grains of successive layers at ______ to each other.
🪵 The Engineering Behind Plywood: Cross-Graining
Plywood is an engineered wood product made by gluing together thin layers of wood veneer. The key to its strength and stability lies in a principle called cross-graining. Natural wood is very strong along its grain but weak across it. It also tends to warp, swell, and shrink with changes in moisture.
By layering the veneers with their grains oriented at an angle to each other, plywood overcomes these natural weaknesses. This creates a composite material that is strong, stable, and resistant to splitting in all directions.
📝 Detailed Analysis of the Options
(a) 60⁰, (b) 45⁰, and (d) 30⁰
While it is theoretically possible to orient veneers at these angles, it is not the standard practice and would not produce the desired properties of conventional plywood. These orientations would create an unbalanced panel and would not effectively distribute strength along the primary length and width axes of the board.
(c) 90⁰
This is the correct answer. The fundamental principle of plywood manufacturing is to place the grain of each successive layer at a right angle (90°) to the previous one. This cross-graining technique achieves several critical goals:
- Distributes Strength: It makes the panel nearly as strong across the grain as it is along the grain, overcoming the main weakness of solid wood.
- Improves Stability: It restricts the natural tendency of wood to swell, shrink, or warp with changes in humidity. The layers essentially hold each other in place.
- Resists Splitting: It is very difficult to split plywood because a crack would have to fight against the grain of alternating layers.
📊 Plywood vs. Solid Wood
| Property | Solid Wood | Plywood |
|---|---|---|
| Strength Direction | Strong along the grain, weak across it | Strong in all directions (uniform) |
| Stability (Warping/Shrinking) | Prone to movement | Highly stable and resistant |
| Splitting Resistance | Splits easily along the grain | Very high resistance to splitting |
💡 Study Tips
- Plywood = 90 Degrees: This is the most fundamental rule to remember. The strength of plywood comes from orienting the grain of adjacent layers at right angles.
- Odd Number of Plies: Plywood is almost always made with an odd number of layers (3, 5, 7, etc.). This creates a balanced panel where the grain of the top and bottom faces runs in the same direction, preventing warping.
- "Cross" means 90°: Think of the term "cross-graining." In this context, "cross" implies a perpendicular or 90-degree intersection.