Carpentry and Wood Working: Timber, Joints and Tools
Carpentry and wood working tests timber classification and defects, seasoning, the joint used for a given purpose, and the correct saw or plane for a cut. Expect labelling of tools and a short description question on joints.
Timber
Hardwoods (teak, sal, rosewood, mahogany) come from broad-leaved trees and are dense; softwoods (pine, deodar, fir) come from conifers and are lighter and easier to work. Seasoning removes sap and moisture to prevent shrinkage, warp and decay; natural air seasoning is slow, kiln seasoning is fast and controlled. Defects: knots, shakes, checks, splits, twist, cup, bow, decay and insect attack.
Joints
- Butt - simplest and weakest; needs nails or screws.
- Half-lap and cross-lap - give flush surfaces.
- Mortise and tenon - strongest for frames; tenon thickness about one-third of the rail thickness.
- Dovetail - resists pulling apart, used for drawers.
- Rebate, tongue and groove, dowel, bridle - for boards and light frames.
Tools
Rip saw cuts along the grain, cross-cut saw across it, tenon saw for joint shoulders, dovetail saw for fine work. Jack plane for rough flattening, smoothing plane for finishing, rebate plane for shoulders; firmer chisel for paring, mortise chisel for levering out waste. Marking gauge, try square, bevel, and brace with bits for boring.
Worked example
For a rail 60 mm wide and 25 mm thick, tenon thickness = 25/3 ≈ 8 mm. Mortise width 8 mm, length about 2.5 times the thickness (about 20 mm) and depth about 20 mm. Set out the tenon with a marking gauge from one face so the joint stays flush, and cut the mortise first, then fit the tenon to it.
Traps
Using a rip saw for cross-cutting. Driving a screw without a pilot hole in hardwood. Planing against the grain, which tears the surface. Believing knots strengthen timber - they weaken it and deflect the grain.