What these are for
When you glue boards edge to edge to make a wider panel, such as for a tabletop or a cabinet side, the biggest practical problem is keeping the faces level while the glue is wet. Clamp pressure tends to make boards slide and ride up against each other. Dowels and biscuits are two common ways to keep adjoining boards lined up.
They are mostly alignment aids. A well-planed edge-to-edge glue joint is typically strong already, because it glues long grain to long grain. In practice, dowels and biscuits help to register the faces and to add a little shear resistance, rather than being the main source of strength.
Round dowels
A dowel is a short round wooden pin, often with grooves or flutes along its length so air and glue can escape. Holes are drilled into the edges of both boards at matching positions, glue goes in, and the pins join the two.
How they are used:
- Mark the position along the edge. Spacing is commonly several inches apart, with a pin near each end, though no exact number suits every panel.
- Drill holes of the same diameter into each edge. A doweling jig clamps to the board and guides the drill bit. A drill press with a fence is another option. The key is that holes on mating edges are the same distance from the face.
- Choose a dowel diameter at about one third to one half of the board thickness, a common rule of thumb. Holes should be slightly deeper than half the dowel length to leave room for glue.
- Add glue in the holes and on the edges, insert the dowels, and clamp.
Dowel center-markers, small pointed pins that transfer hole positions from one board to the other, help if you are marking by hand.
Strengths: Dowels need only a drill and a jig, they are inexpensive, and they add resistance in all directions across the pin.
Weaknesses: The holes must line up accurately. A drill that wanders by a small amount can stop the boards from sitting flush. There is little adjustment once the glue is wet. Dowels also swell with moisture from glue, and cheap dowels can be slightly oversize or irregular. Dry fit first.
Biscuits
A biscuit is a thin, flat, oval-shaped piece of compressed wood, often beech. A biscuit joiner is a small power tool with a spinning blade that cuts a half-moon slot in each edge. When you add water-based glue, the biscuit swells slightly in its slot, tightening the fit.
How they are used:
- Mark the position of each biscuit across both boards, usually with pencil marks that cross the joint.
- Set the joiner’s fence to cut the slot in the center of the thickness, or use the base of the tool on the face for consistent registration.
- Cut a slot at each mark in both boards, with the tool braced firmly.
- Glue the slots and edges, insert the biscuits, and clamp.
The slots are slightly wider than the biscuits, which gives a little sideways play. That play lets you slide the boards to align them before the glue sets, which is the main advantage.
Strengths: Fast, forgiving of small errors along the length of the joint, and less fussy than dowels to line up.
Weaknesses: Needs a dedicated tool; biscuits add limited strength; biscuits can show through if the joint is later planed too thin or cut close to a surface.
Comparing the two
| Feature | Dowels | Biscuits |
|---|---|---|
| Tool needed | Drill and a jig | Biscuit joiner |
| Adjustment during glue-up | Little | Some, along the joint |
| Alignment accuracy | Depends on jig and drilling | Depends on fence and face reference |
| Added strength | Some, in all directions | Modest, mainly across the joint |
| Good for | Frames, edge joints, repairs | Panels, plywood and sheet goods |
Do you need either?
For ordinary edge joining, many woodworkers use neither. Careful jointing of edges and good clamping with cauls, which are straight scrap bars clamped across the panel to keep it flat, give flat results. Pay attention to wood grain direction when choosing which boards to glue together, and consider the effect of board warping on how flat they sit. If you do use dowels or biscuits, they act as insurance for alignment.
Safety
Wear safety glasses and hearing protection with power tools, and expect fine dust from a biscuit joiner. Clamp the work, unplug the tool before changing the blade or bit, and follow the manufacturer’s manual. Keep fingers away from the cutter. This is general information, not professional advice.