Stub tenon and groove joinery has long been one of my go-to methods for building frame-and-panel doors. It's straightforward, strong, and works especially well when the grooves in the rails and stiles also hold a plywood panel. But I've found a way to simplify the process even further: Instead of cutting integral stub tenons on the ends of the rails, I use separate plywood splines.
The idea is similar to loose-tenon or biscuit joinery. Matching grooves are cut in the mating parts, and a separate spline connects them. In this case, the same groove that accepts the plywood spline also captures the panel, so you can form most of the joinery with a single router table setup.
There's another advantage. Without integral tenons to account for, you can cut the rails and stiles to their finished lengths before routing the joints. That simplifies the layout and allows you to concentrate on accurate crosscuts rather than calculating additional material for the tenons.
With a 1/4" slot-cutting bit, a few plywood splines, and a straightforward glue-up sequence, you can build accurate frame-and-panel doors with clean, square joints. Here's how I approach the process.
How Plywood Spline Joinery Works
A spline joint uses a separate strip of material to connect two workpieces. Instead of forming a tenon on one part and a matching groove in the other, you cut grooves in both pieces and insert a spline between them.
For a frame-and-panel cabinet door, I cut continuous grooves along the inside edges of the rails and stiles. I also rout matching slots in the ends of the rails. Short plywood splines fit into the rail-end slots and extend into the grooves in the stiles, connecting the frame.
The plywood panel fits into the same continuous grooves. This eliminates the need to make integral stub tenons and keeps the machining process simple.
I like to use MDF-core plywood for this method because some products measure very close to 1/4" thick. That makes it possible to cut the grooves with a 1/4" slot cutter in a single pass. However, plywood thickness varies, so check your actual material before selecting the cutter. The splines and panel should fit snugly without needing to be forced into the grooves.
Because the panel is plywood, it can be glued into the frame to contribute to the assembly's strength. If you substitute a solid-wood panel, leave it free to expand and contract rather than gluing it into the grooves.
Step 1: Lay Out the Rails and Stiles
One of the benefits of spline joinery is that the router table setup replaces much of the detailed joinery layout. Once the rails and stiles are cut to size, you mainly need a reliable way to keep track of their orientation.
I use a simple triangle-marking system. Arrange the frame members in their final positions, with the show faces oriented correctly. Bring the two rails together and draw a triangle across them. Repeat the process for the stiles.
The marks provide a quick visual reference throughout the process. The point of the triangle indicates the top of the assembly, while the edge crossed by the triangle identifies the inside edge of the frame.
When I'm standing at the router table, the marks also help me keep the correct face up. It's a simple habit that prevents routing a groove on the wrong edge or end of a workpiece.
Step 2: Set Up the Router Table and Slot Cutter
For this joinery method, I install a 1/4" slot-cutting bit in the router table. The bit forms the grooves along the inside edges of the frame and the matching slots in the rail ends.
A slot cutter works especially well because its horizontal cutting edge produces a consistent groove at a set height above the router table.
Set the Slot Cutter Height
I use a setup block to establish the bit height. For the original project, a 1/4" setup block provides the reference needed to position the groove in the workpieces.
Since the groove is cut in a single pass, you don't need to make multiple cuts to center it. The important thing is to use the same reference face for every workpiece so all the grooves align during assembly.
Make a test cut in scrap material of the same thickness as your project parts. Check that the groove is properly positioned and that the plywood fits before routing the actual frame members.
Position the Router Table Fence
Next, position the router table fence to control the depth of the groove. In this setup, the fence is aligned with the bearing on the slot-cutting bit, allowing the cutter to reach its maximum depth of approximately 1/2".
A steel rule makes it easier to check the alignment between the fence faces and the bearing. With the fence properly positioned, the workpiece has support as it enters and exits the cut.
Lock the fence in place and verify the setup with a test piece before proceeding.
Step 3: Rout the Grooves in the Frame Parts
With the router table set up, cutting the grooves is straightforward.
Start by routing along the inside edges of the stiles and rails. Use push pads to maintain consistent downward pressure and keep the workpiece firmly against the fence. Feed the stock steadily past the cutter, following the router bit manufacturer's recommended direction and cutting limits.
The ends of the rails also need grooves to accept the plywood splines. Because these cuts are made across relatively narrow workpieces, I use a backer block to support each rail and keep it square to the cutter.
The backer block also helps prevent tearout as the slot cutter exits the wood.
Keep the orientation marks visible and double-check each part before routing. The slots in the rail ends need to align with the continuous grooves in the stiles for the splines to fit correctly.
Once all the grooves are complete, dry-fit the parts to confirm their alignment.
Step 4: Cut the Plywood Splines
The splines are short strips of plywood that connect the rails to the stiles. Their thickness should match the grooves, while their length should be slightly less than the combined depth of the two mating slots.
For example, if each mating groove is 1/2" deep, the spline should be just under 1" long. This leaves a small amount of clearance so the ends of the spline don't bottom out and prevent the rail shoulders from closing tightly against the stiles.
The spline width should also allow it to fit within the rail-end slot without extending into the area needed for the panel.
Cut four matching splines for the frame, then test-fit each one before applying glue.
Step 5: Assemble the Frame-and-Panel Door
I like to divide the glue-up into three stages. This keeps the number of loose parts manageable and makes it easier to align the frame before tightening the clamps.
Glue the Splines into the Rails
Start by applying glue to the slots in the ends of the rails. Insert the plywood splines, making sure they're fully seated and properly aligned.
Check that the splines don't extend into the panel groove. Otherwise, they may interfere with the panel when you assemble the frame.
Allow the glue to set for approximately 20–30 minutes, or until the splines are stable enough to handle according to the adhesive manufacturer's instructions.
At this point, the splines effectively act like integral tenons, making the remaining assembly easier.
Attach the Rails to the Plywood Panel
Next, apply a thin bead of glue in the grooves along the inside edges of the rails. Fit the rails over the corresponding edges of the plywood panel.
The edges of the splines and panel should align, or the panel can be slightly inset. Make sure the panel seats fully in the grooves and that the rails are correctly oriented.
Using a controlled amount of glue will help minimize squeeze-out around the inside corners of the frame.
Install the Stiles and Clamp the Door
The final stage is attaching the two stiles.
Brush glue onto the exposed splines and apply glue to the corresponding grooves in the stiles. Then bring the stiles into position, fitting them over the panel edges and the projecting splines.
Once the frame is assembled, apply clamps across the door to draw the joints together.
As you tighten the clamps, check that the faces of the rails and stiles are flush and that the frame remains square. Use only enough clamping pressure to close the joints without distorting the assembly.
After the glue has cured, trim any exposed spline material flush with the frame.
Customize the Door with Decorative Molding
The basic spline-joined assembly creates a clean, square-edged frame-and-panel door. That simple appearance works well for many furniture styles, but it also provides a starting point for adding decorative details.
Because the frame is assembled before the molding is applied, you can choose a profile that suits the project without changing the underlying joinery.
Two options are a substantial bolection molding or a smaller applied bead.
Option 1: Make a Custom Bolection Molding
Bolection molding creates a pronounced transition between the frame and panel. The profile overlaps the inside edge of the rails and stiles and extends onto the panel, adding depth and visual interest to the door.
I make the molding from an extra-wide blank, which provides a safer and more stable way to rout the profile before cutting the narrow molding strips free.
Start by routing a cove-and-bead profile along the edges of the blank. An ogee profile is another option if you want a different appearance.
Next, use a point-cutting roundover bit to form the upper bead and fillet.
A straight bit then forms a groove or step that matches the offset between the frame and the recessed panel. This allows the finished molding to bridge the transition cleanly.
Finally, cut the molding free from the blank at the table saw. Miter the pieces to fit around the inside of the frame.
The result is a decorative profile that adds dimension to the door while leaving the spline joinery unchanged.
Option 2: Add a Simple Bead Molding
For a more understated detail, I like to use a small applied bead molding. It fits into the junction between the frame and panel, providing a subtle shadow line without overwhelming the door.
Start by routing a bead profile along the edge of an oversized blank. Then rip narrow strips from the blank and miter them to fit around the inside of the frame.
Both molding options demonstrate how a simple, square-edged frame can be customized to suit different furniture styles. Just be sure to attach applied molding in a way that's appropriate for the panel material. A solid-wood panel still needs room for seasonal movement.
A Simple Alternative to Traditional Stub Tenons
Plywood spline joinery offers an efficient way to build frame-and-panel cabinet doors without cutting integral tenons on the ends of the rails.
By using a slot-cutting bit to form matching grooves, you can prepare the rails and stiles to their finished dimensions, cut the joinery with a consistent router table setup, and assemble the frame with separate plywood splines.
The result is a strong, accurate door that's easy to customize with decorative molding. And once you've used the technique on one project, you'll find plenty of other opportunities to put it to work.
For other ways to make frame-and-panel joints, explore our related Woodsmith guides to cutting stub tenon and groove joints on the table saw and making stub tenons with a router table.
