Types of T-Joints and Lap Joints

The joint for the job

To serve their purposes efficiently, wood joints must be marked out and cut accurately and put together with the correct adhesives and fastenings.

Difficulty in cutting accurately enough to make close-fitting joints can be overcome by using a sawing jig. With some jigs the angle of the saw is changed: with others the line of sawing is constant and the wood itself is fixed at an angle to that line.

This post deals with the several variations of basic joints, from the simplest to the more difficult. T-joints, for example, start with simple nailed joints and finish with mortise and tenon and dovetail joints.

Joints can be divided into six groups:

T-joints: one piece is joined at right angles to the face or edge of another, forming a T-shape.

L-joints: two pieces are joined to form a corner.

X-joints: the pieces cross over or fix into each other to form a cross.

Edge-to-edge joints: edges are joined to produce wider surfaces.

Lengthening joints: two pieces are joined end-to-end.

Three-way joints: three pieces of wood are joined, eg. a chair leg and rails.

Nailed T-joints

Simple nailed joints are quite good enough for light frames where the sides meet the cross-pieces flat on.

Cut the cross-pieces dead square, otherwise the joint cannot form a true right angle.

Drive the nails in from the outside whenever you can — this is easier than nailing from the inside. Use three nails: hammer the middle one in first to hold the wood firm, then drive in the other two on either side of it, sloping them inwards at 20° to 30°. These nails form a dovetail which will lock the pieces of wood together.

Nailing from the inside of a frame needs more care because hammering tends to knock the cross-piece out of line. Prevent this by hammering from both sides alternately and re-aligning the work as the nail points begin to bite into the side piece.

Drive the nails in line with the grain, but stagger them a little to avoid splitting.

Punch the heads under for extra tightness: Fill the holes with stopping or putty before painting.

Use oval wire-nails on wood up to 32 mm. (1-1/4 in.) thick, cut brads on thicker sections. The length should be at least twice the thickness of the side piece.

Nailed T-joints are only as strong as the nails that hold them together, but the strength and rigidity of a frame increases considerably if you cover it with hardboard or plywood.

Do not use nails on dry hardwood framing, as you may split the timber. Instead, make halving or tenon joints.

Using brackets and fasteners

Metal brackets are a useful rough-and-ready aid to making flat T-joints. There are two types: one is a strip of drilled metal bent into a simple L-shape for fitting into corners; the other is in the shape of a T, and screws flat on to the work.

Use them whenever the appearance and thickness of the bracket do not matter, as in light framing which needs a little more strength than nailed joints provide.

Obviously, the stronger the bracket the stronger the joint, but the brackets themselves will bend easily unless you use one on each side of the joint.

The screws should fit the drilled holes snugly. Drill pilot holes in the wood to prevent splitting. Drive the screws home flush with the bracket top.

A far quicker way to make T-joints is with corrugated metal fasteners which are hammered straight into the work. The fasteners are sharpened on one edge. They are best used on light indoor work and in box-making.

Make sure that the joint is as tight as possible by putting the frame in a sash-cramp or pushing it against a fixed block before you drive the fasteners home.

Position the fasteners well in from the edges of the cross-piece to prevent splits. Tap the fasteners gently until they are going in evenly, then hit them centrally until they are flush with the surface.

Overlap joint using supporting blocks

The overlap T-joint combines simplicity and strength for general-purpose jobs such as framing, fencing, shelving racks and lightweight gate construction.

It can be screwed, nailed, or bolted and, for strongest results, should also be glued.

To make a screwed overlap joint, cramp both pieces of wood together with a G-cramp while drilling holes.

Drill a clearance hole through the top piece and a pilot hole in the lower piece with a twist bit small enough to allow the screw thread to bite firmly.

Countersink the holes in the top piece, coat both inner surfaces evenly with glue, fit the pieces together, and screw.

A simple glued and screwed T-joint, using supporting blocks, makes effective shelving units or bookcases.

Glue and screw blocks to each side of the casing, and then glue and screw the shelving to the blocks. On heavier shelving, — say, over 200 mm. (8 in.) wide — use the housing joint.

A firmly fixed support block restricts shrinkage or expansion across the grain, which can be considerable.

Shelf-support blocks can also be screwed or masonry-nailed direct to the walls of alcoves.

To make a neat job, chamfer the exposed edges of the support blocks, or chamfer their front corners on the underside.

 

Full lap and half lap T-joints

These joints are far stronger and neater than the simple overlap construction. Use them for fitting cross-rails flush into frames which are to be covered in with panelling.

[youtube: http://www.youtube.com/watch?v=bXJNxUNudKE]

In a full lap joint the side rail is cut out to accommodate the whole of the cross-rail.

To make it, mark the exact shape of the cut-out on both faces of the side rail and across its top edge.

Cut out the waste with a tenon saw and chisel, paring away gradually from each side until the base of the cut-out is level.

Check the fit, glue all mating surfaces, and complete the joint by pinning or screwing.

In a half lap or T-halving joint, the cross-rail and the side rail are both cut away to give a flush fit when they are mated.

Mark the width of the cross-rail across the face of the side rail and half-way down both edges.

On the back of the cross-rail, mark a shoulder line across at a distance from the end a little greater than the width of the side rail.

Continue the line half-way across the edges. Set a gauge to half the thickness of the wood and gauge lines from the face of both pieces.

Saw a centre slot in the cross-rail, skimming the gauge line on the waste side. Remove the waste block by cutting across the shoulder line.

Saw just inside the lines marking the side rail cut-out. Saw an extra cut in the centre to make waste removal easier. Pare away the waste from both sides to complete the cut-out, check for fit, fix and trim.

27. June 2011 by admin
Categories: Joints, Woodworking | Tags: , | Comments Off on Types of T-Joints and Lap Joints

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