How to make solid wood joinery with a CNC

Photo By Laguna

Joinery is the method that woodworkers use to connect two or more pieces of wood. In its simplest form, it involves merely gluing, nailing, or screwing two pieces of wood together. But it can get quite complicated when intricate joints are involved in the project.

Woodworking that requires both decorative and strong joinery is often found in cabinetry, furniture, flooring, doors, windows, and a host of other applications.

Some of the traditional joints in these projects include:
• Butt joint: two pieces of wood are connected by nails, screws, or dowels.
• Dado joint: two pieces of wood are connected by a groove in one piece that is the same width as the thickness of the inserted second piece.
• Rabbet: the rabbet is a wood joint that resembles the dado except the cut is along the edge of the board.
• Dovetail joint: two pieces of wood are connected by cutting a piece out of one and flaring the other, so they fit together. There are three types of dovetail joints – through, half-blind, and sliding.
• Half-lap joint: two boards are joined by removing half of each so they join flush.
• Miter joint: each end of the pieces of wood is cut on a 45-degree angle, and the pieces are joined together to form a right angle.
• Mortise and tenon joint: one piece of wood has a recess cut into it (mortise), and the other has a projection (tenon) cut on to it.
• Tongue and groove joint: used to join two boards along a long edge, each board will have a groove along one edge and a protrusion (tongue) that fits into it on the other edge.
• Box joint: a simpler alternative to the dovetail joints.

Of course, these are just a handful of the numerous types of joints that are possible using traditional woodworking tools and equipment such as chisels, table saws, bandsaws, and hand-held routers.

But how do woodworkers create those complex and interesting joints that make their work so special?

Photo: Laguna


CNC routers expand opportunities
Many interesting and even exotic joints can be cut relatively quickly on a 3-axis CNC. The equivalent joinery with traditional shop tools would be time-consuming and difficult.

The finger tenon joint is one of the simplest of the compatible tenons, with the joints exposed at the corners and plenty of surface area for glue.

Joining wood end-to-end or edge-to-edge gives it a more decorative effect and a resistance to pulling apart. With the lapped dovetail, the back edge of the joint would be a straight line. The precision required for this joint would not be possible without the accuracy of a quality-made 3-axis CNC router.

There are some considerations when you choose CNC for solid wood joinery. For example, sturdy fixturing will be needed since the bits will be exerting some force on the workpiece. There is also the issue of tear out with router bits, which you can prevent by using sharp tooling and adjusting your feed rate to get the proper chip load during cutting.

Also, you typically can’t cut square inside corners with a router bit, so you’ll have to design your joints with rounded corners. It’s essential to keep the router bit moving when you cut out corners. Allowing the tool to dwell in a corner creates excessive heat and tool wear.

In addition, make sure you have programmed clearance into your joints, or they won’t mate properly. For example, in a mortise and tenon joint, program the tenon to be 0.005 inch per side narrower than the cutout for the mortise. While this might sound like a fairly tight fit, the accuracy of the CNC router, combined with a sharp tool, should provide an excellent fit for your joints. If it's still too tight, increase the clearance to 0.010 inch per side.

Keep in mind that if you are adding a finish to any parts, the finish might add enough material to the joints to prevent them from fitting together. So, be sure to take any finishing operation into account when you are machining joints on a CNC router.

The thickness of the material you are joining is also a consideration. When you program the toolpath, you will do it for a particular wood thickness. If the thickness varies, your joints might not fit properly. You can compensate for these variations by using the "stock to leave" and "floor to leave" parameters in the CAM software.

The positioning of the joints in relation to the outer edges of the material is also important, and by using a CNC router and solid fixturing, the edges and ends of the material will line up perfectly when the two halves are joined together.

There are CNC machines on the market, including some specialty machines, capable of precisely and consistently routing any of the joints listed above. All the characteristics of excellent wooden joints – including appearance, strength, and flexibility – can be enhanced on a CNC machine with the addition of accuracy and repeatability.

Source: Laguna Tools. For information call 800-234-1976 or visit Read more blogs, how-to and project articles, or watch Tech Tips videos on the blogs and resources sections on Laguna’s website.


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