Researching the options
Choosing a joinery method often comes down to what tools, skills and experience the shop possesses
There are approximately 30 traditional joints in Western woodworking and countless mechanical (plastic or metal) fasteners. Many of those offer the option of disassembly and reassembly.
But choosing the right joinery method begins with an understanding of wood movement. That includes issues with cross-grain lamination, expansion due to resin or water absorption, and changes in ambient humidity and moisture content. Harvested wood essentially moves across its grain rather than along it, while man-made sheet stock may move in all directions. Different species move at differing rates, and various cuts can affect movement. Rift and quartersawn boards move less than plain or flat sawn ones, so these cuts are favored for joinery in fine furniture. Joinery that restricts movement has a much higher possibility of failing over time, because one or more elements may change size and either shrink to create a loose joint or expand to cause cracks and fissures.
Dealing with stress
When considering which type of joint to use, a woodworker needs to consider how the connection will be tested in the real world. Stress on a joint is described as either compressive, shear, distortive (torsional), or tensioned. In the simplest terms, compression pushes parts together; shear pushes one part sideways or across the other and sometimes tries to snap it across the grain; torsion tries to rotate one or sometimes both parts; and tension tries to pull the joint apart. These stresses can be caused by adding a load (somebody sits on a chair); gravity over time; flexing and bending due to use (as in stair treads or boat masts); poor engineering and design (most often where parts are undersized); or activities that release captured pressure in the wood.
Every woodworker has heard of or experienced the latter, which is known as reaction wood. It’s the force that causes the two halves of a board to take off in different directions as they exit a rip cut on the table saw. There’s a lot of information about conifers, angiosperms, compression and tension that can help explain why this happens, but the overriding concern here is that wood which is misbehaving shouldn’t be used in joinery. It won’t grow out of its bad habits over time. Reaction is usually a result of the board being harvested from a trunk that hasn’t grown straight because of wind, location, drought or disease, or else it was milled from a branch that grew laterally (sideways). In that case, the growth rings are different at the top of the branch than the bottom, so when the wood is released from the gravitational orientation in which it has grown, those stresses attempt to equalize.
Beyond reaction during processing, boards that have essentially stabilized are often rejected for joinery because of crooking, warping, cupping, twisting or winding. These are all words used to describe boards that refuse to lie flat in various ways. They may be kiln dried and seem as though they are done moving, but they haven’t. These are not options for good joinery.
Wood strength is dramatically different along its grain than across the grain. Trying to split a wide plain sawn board across its grain is essentially impossible but splitting it along the grain is quite easy. It’s the main reason that we pre-drill for screws, and it’s a huge factor in choosing a joinery method.
Casework joints
Building boxes is mostly about attaching man-made panels to each other, so wood movement isn’t as critical an issue here as it is with solid wood parts. There are three primary options. The woodworker can choose butt or edge joints that are reinforced with biscuits, dowels, or pocket screws. A second method is to use glued profiles such as dadoes, grooves and rabbets, and apply clamping pressure until the adhesive cures. The more common solution in medium and large shops is to use a mechanical connector, some of which encourage the use of glue while others don’t. CNC processing facilitates the use of plastic and metal connectors, although almost all of them can also be manually prepped (the holes or recesses can be located with a tape measure and machined with drills and routers).
Choosing a mechanical connector requires some research. Some of the issues here are speed, strength, material thickness, cost per unit, and the ability to ship cabinets flat-packed and assemble or reassemble them on the jobsite.
Among the options are the Minifix connectors from Häfele America (hafele.com). One element of the connector, a small threaded housing, is inserted in the bottom or hidden face of a panel near its edge. The second half, an adjustable pin, extends from the face of another panel and the pin enters the edge of the first panel and keys into the base. The two parts are locked together by turning a screw.
There are numerous variations on the dowel, including the Twister DU 243 T from Hettich (shop.hettich.com). The receiver screws into a pre-drilled hole in one part using a coarse thread, and then the second part is attached by tightening a screw that draws together an expanding washer for a snug fit.
Lockdowel (lockdowel.com) offers plastic and metal ribbed dowels in a selection of sizes and configurations that provide a tool-free, glue-free, rapid assembly solution for joining panels and parts, and also for installing slides. At its core is a friction fit joint that can be machined on a 3-axis CNC or with a manual routing set-up.
The orange dowel system from FastenLink (fastenlink.com) is a two-part, very strongjoinery system that includes the plastic dowel (called a link) and a ramped pocket that receives it. Standard CNC routing and horizontal boring machines are all that are required to turn the links into a simple and fast manufacturing solution.
Several manufacturers offer pocket screws in bulk, and a few make complete joinery solutions based on this technology. Those include Castle USA (castleusa.com), Kreg Tools (kregtool.com), and Armor Tool (armor-tool.com).
Cabineo 12 from Lamello (csaw.com) is a one-piece cabinet joinery option that can be used with any CNC because only surface machining is required. The Cabineo is installed in one board, either in the workshop or on the jobsite. Then the connector is screwed into a 5mm hole in the second workpiece.
OVVO invisible connectors (ovvotech.com) are installed during the manufacturing process so the end-user or the installer can click furniture together without tools, glue or screws.
Festool (festoolusa.com) makes a wide range of panel and cabinet connectors that take advantage of the capabilities of the company’s portable Domino mortising tool. Among them are corner and center panel connectors in a variety of configurations.
Cabinotch (cabinotch.us) has developed an innovative joinery system for assembling face-framed and full-access cabinet boxes. It involves a patented interlocking joint, and the manufacturer delivers custom-sized, RTA cabinets that include the face frame, sides, backs, bottoms, tops and shelves.
Striplox (striplox.com) is a range of strong and versatile mechanical joining systems that allow simple, safe and efficient assembly, without complex methods, processes or multiple parts.
Hoffman (hoffmann-usa.com) makes a clever plastic floating dovetail system that includes both the connectors themselves and a range of tools and machines for installing them.
Starting with connecting systems for furniture, Knapp Connectors(knappconnectors.com) has expanded to create wood connectors for case goods, stair makers, timber construction, and more. All the wood connectors are hidden, self-tightening and can be snapped together for fast assembly.
Titus (titusplus.com) makes a full line of cabinet and furniture connectors that are homeowner friendly.
And for joinery on the CNC that requires non-ferrous fasteners that won’t harm bits during contact, Lignoloc (beck-fastening.com) is the world's first shootable wooden nail, and Raptor (raptornails.com) makes plastic composite staples, nails, and specialty fasteners that can be cut and sanded without damaging router bits, saw blades and sanding belts, and also provide complete corrosion resistance.
Furniture joints
These are most often connections made between solid wood parts, rather than man-madesheet stock. The most common are repetitive drawer joints such as the box/comb/finger, or various iterations of the dovetail including blind, half-blind, through and sliding. Box joints are easier and faster to mill and assemble than dovetails, and while they’re not as strong, they are more than adequate. Both joints can be milled with jigs for table saws and router tables, or with specialized machines such as the PantoRouter(pantorouter.com) or Shaper Origin (shapertools.com).
Traditional assembly of parts such as aprons, legs and frames calls for single rather than repetitive joinery including bridles, castles, and tenons. A bridle is essentially a mortise with one open wall, and a castle is a more complex joint where three parts meet, such as two parts of an apron at the top of a leg.
Tenons are the most versatile joints as they can assume many forms. The floating tenon has two mortises, and some mortised tenons are secured with pegs or wedges. The woodworker can add visual impact by using a through tenon that has an exposed end which lies proud of the surface.
Mitered joinery is an option where thinner structural or decorative parts meet at a corner, and miters can be keyed, splined, locked, coped, or butted. Solid wood panels can be joined with butt edge joints that are often reinforced with biscuits or dowels (which help alignment and add shear strength), or with locking rabbets, or tongue and groove joinery. And long, thin pieces can be joined with scarfs or half laps, to add both visual appeal and strength.
Choosing a joinery method often comes down to what tools, skills and experience the shop possesses. A lot depends on how frequently jobs call for specific joints, and whether that volume of work can justify an investment in automation. There is great satisfaction in using hand tools to execute a perfect dovetail, but if that’s more than an occasional activity then a jig, machine or CNC may be the best joinery option. Esoteric experiences are good for our souls, but they’re not always the best solution when it comes to paying the bills.
Originally published in the May 2024 issue of Woodshop News
