Make your projects strong

By Steve Maxwell

Photo by iStockphoto/Thinkstock

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Cut some wood, then put it back together again in new ways. This is woodworking described most simply, and the “put back together” part sometimes needs mechanical help.
Dowels, biscuits and floating tenons all create interlocking connections that boost the strength of simple wood joints beyond what glue alone can deliver. Most of the projects in this issue of Canadian Home Workshop include joints that rely on interlocking connections, and making these joints work begins with an understanding of why glue alone isn’t always enough.

Every brand of wood glue has more than enough strength to join the edges and faces of boards without mechanical help. In fact, tight-fitting butt joints involving edges and faces are stronger than the surrounding wood itself. But if one or both surfaces of a joint shows annular growth rings, the situation is entirely different. When growth rings are visible, the surface is called “end-grain”, and glue-only joints on end-grain are always too weak to count on. Dowels, biscuits and floating tenons all create interlocking edge-grain contact surfaces where only end-grain existed before. Of these, a dowel joint is one of the simplest and safest options for strengthening joints. All you need to create one is a drill and a simple jig.

Drill a hole on both sides of a wood joint, apply glue into these holes with a cotton swab, push a dowel into one hole and then slip the other drilled part over the protruding dowel. This is the “dowel joint” concept in a nutshell, but there are three reasons why the reality of making it work isn’t as simple as it sounds.

The two holes that house each dowel must be aligned perfectly or the wood won’t come together properly. The holes must also extend in precisely the same direction on each side of the joint line. Accuracy is key, and it’s why a drilling jig is always necessary for dowel joints.

The second potential problem is the dowel itself. Most dowel rods aren’t manufactured accurately enough for optimal joints. They’re often undersized and not perfectly round, making for a sloppy joint. You’ll get better results if you use factory-cut dowel pins. They’re already cut to popular lengths (1″, 11⁄2″ and 2″), they’re accurately sized and they include fluted edges on their sides to leave space for glue. These edges swell after assembly with water-based glues, making the joint even stronger.

Insufficient dowel-hole depth is a potential third problem, and avoiding trouble involves more than meets the eye. You’d think that each dowel hole needs to be only slightly deeper than half the dowels length, and a successful dry-fit of the joint without glue would seem to confirm this hunch. Trouble happens, however, when excess glue gathers in the bottom of the dowel holes during final assembly. If enough glue is present, it’ll stop the joint from coming together fully by hydraulic action. Avoid trouble by drilling dowel holes at least 1⁄8″ deeper than required on each side of the joint, and don’t apply more glue than necessary for complete dowel coverage.

Biscuits are factory-made ovals of hardwood that fit into slots cut by a handheld power tool. All models of biscuit joiners have features for facilitating the all-important job of aligning the biscuit slots that meet across a joint. However, slot depth can be one sneaky source of trouble, especially if your biscuit joiner is new and has never been adjusted.

Slot depth is crucial for biscuit joints, just as it is for dowel joints. If a biscuit is wider than the total depth of both slots, it’ll hold the joint apart, eliminating all hope for a tight assembly. To be safe, the depth of each slot should be about 1⁄32″ more than half the width of the biscuit.

The best way to test the adjustment of your biscuit joiner involves a pencil. Plunge a slot into scrap wood, push a biscuit all the way in, then use a pencil to mark the point at which the wood meets the biscuit face. Pull out the biscuit, keeping the markedside up, rotate it 180°, then push it in again. The pencil mark should disappear in the slot. Use the pencil again to mark another line where the wood meets the biscuit face. Pull the biscuit out and you should see about 1⁄16″ between the lines, representing the total excess slot depth.

With your biscuit joiner adjusted, there’s no need to check depth again—although it’s easy to cut biscuit slots slightly too shallow accidentally. Failure to advance the blade all the way while cutting or a small amount of debris on the tool can inhibit full assembly later. I always dry-fit biscuit joints before glue application. I’d rather discover a shallow slot before glue goes on than after.

Floating tenons are small, rectangular, shop-cut parts that interlock with grooves in neighbouring pieces of wood. They make especially good sense for joining stiles and rails around door panels, and that’s how they’re used in Ryan Shervill’s change table. Simply continue the panel grooves so they extend around onto the ends of the rails, insert floating tenons and you’re ready for assembly. The main issue with floating tenons is that their grain direction must run in the same direction as the grain of the rails. Anything else and your tenons will break under the slightest pressure.

Strong, accurate joints are an essential part of woodworking success. And like anything else in life, it’s not the least bit difficult when you know how.

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