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Spline slots made easy

Author: Mark Salusbury
Photos: Mark Salusbury
Illustration: Len Churchill
Published: June 2024
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Splined bevel joints are strong and attractive. With this simple jig, they’re also very easy to accomplish.

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  • DIFFICULTY
    2/5
  • LENGTH/TIME
    2/5
  • COST
    2/5

Boxes or small cabinets are fine projects to make. Functional as well as decorative, they’re great accent pieces and excel­lent gifts. Bevelled corners, a popular option for corner joints, are very clean and crisp and offer the potential for con­tinuous grain and colour through a corner. This jig offers the opportunity to add strength and spice with inlaid splines in the same or contrasting wood as the box body. The key is to cut spline slots uniformly and attractively spaced. This simple jig makes that possible.

Made of four principal elements (nine pieces), the jig assembles in short order. It’s a strong, precise guide that’s a joy to use.

Glue the Runners On
Salusbury puts a few washers in the mitre gauge tracks to bring the top surface of each solid wood runner above the height of the table. He then adds glue to the runners, positions the base panel and adds some weight on top of it until the glue dries. Doing all of this with the base panel against the rip fence ensures the edges of the jig are parallel with the fence.

Glue the Runners On

Dadoes for Location
The base panel (centre) and the box cradle panels (left and right) have a dado in their faces to locate the triangular cradle supports. Notice Salusbury has marked the location of the kerf the blade will make on the base panel, as well as the centreline for positioning the first box cradle panel.

Dadoes for Location

Dry-Fit the Parts
Dry-fit the parts to ensure they fit well. Notice one long edge of each of the cradle panels is bevelled. This allows the edge to mate cleanly with the base panel.

Dry-Fit the Parts

A Helping Hand
Salusbury clamps a straight edge across the base panel, perfectly in line with the marked centreline. This gives the first box cradle panel something to butt up against during assembly. Just ensure the clamping pressure doesn’t deflect the base panel, as that will make a proper assembly impossible.

A Helping Hand

Right Angle
Once completed, the two upper faces of the box cradle panels should meet at 90°.

Right Angle

In Action
The jig runs in the mitre gauge slots as the user passes the jig and workpiece over the blade. Salusbury often uses a stop block clamped to his rip fence to limit travel of the jig. The T-square assists in making repetitive cuts.

In Action

Set your size

Size your jig by measuring the outer width of your table saw’s mitre guide slots, then add an extra inch or two to the width of the jig. My table’s slots span 11-1/8″ so I based my jig on a 12-1/4″ × 12-1/4″ panel, compact but capable of cradling boxes up to 10″ tall.

Cut your stock

Width and depth decided, it’s time to rip and crosscut the base panel and cradle panels from 3/8″ Baltic birch plywood.
Next, from straight-grained, quarter-cut hardwood, rip and accu­rately shape the two runners so they’re a press fit into the mitre slots and slightly thinner than the slots are deep.

Centre the base panel over the saw table’s mitre guide slots until it overlaps the slots equally on both sides. Bring the saw’s rip fence over flush with the right-hand side of the base panel and lock it to hold that position. Adjust the base panel so its rear edge is flush with the infeed edge of the saw table. Clamp a stop block to the rip fence, flush with the front edge of the base panel, to hold that setting.

Remove the base panel from the saw table and equally space four stacked pairs of washers along the bottom of each mitre slot to sup­port the runners as they are being glued beneath the base panel. Place the runners over the washers, making sure each runner sits just above the saw table’s surface and flush with its infeed edge, then apply a thin, uniform amount of glue onto each runner. Rest the base panel on top of the runners, making sure the runners are flush with the front and rear edges of the panel. Without shifting your setup, add weight to press the base panel down onto the run­ners; a couple buckets of sand or equivalent work well. Let the glue cure.

Rout a dado

Runners attached, it’s time to rout a shallow dado centred above each runner on the top of the base panel plus the underside of the box cradle panels. In my build, I set my router table fence 11/16″ away from my router bit (15/16″ on centre) and, using a 31/64″ bit (1/2″ Baltic birch ply is undersized), cut 1/16″ deep dadoes along the side edges of all three panels. This shallow recess will locate the cradle supports and assist with accurate alignment during assembly.

Speaking of alignment, on top of the base panel, locate and mark the exact left-to-right centreline, square to the sides of the base. Then, with the runners set in the saw’s mitre slots, locate and mark the future 1/8″ kerf slot. Avoid putting any metal fasteners near these lines as you complete the build.

Along the underside of each of the cradle panels, crosscut a 45° bevel along one long edge.

Shape your triangles

Measure the distance from the base panel’s centreline to its lead­ing and trailing edge, along the bottom of the dadoes. That will be the length needed for the two short sides of the four triangular cradle supports; 5-7/8″ in my case, but I cut them 5-1/2″ so I could add complimentary hardwood edging to my cradle supports after assembly.

Build time

Begin assembly by clamping a straight edge exactly flush with the centreline drawn earlier across the base panel and then, without distorting it, clamp the base panel to a benchtop, providing a firm reference point to build from.

Next, hold one of the cradle panels, bevel edge down, flush with the base panel and snug to the straight edge, and then slide the base edge of one cradle support, long edge up, within the dado in the base panel until it seats fully in the dado under the cradle panel. Repeat this dry-fit in the other dado under the cradle panel. Once you’re satisfied that the four parts mate perfectly and create a 45° angle, disassemble, apply a thin veneer of glue to the bevel edge of the cradle panel and the base and hypotenuse edges of the cradle support and reassemble the parts firmly by hand. Make sure the bevelled edge of the cradle panel is flush with the base panel and the clamped reference edge. A triangular layout square placed beneath the cradle panel should indicate a perfect edge-to-edge 45°. Let this sub-assembly cure undisturbed.

An hour later

Remove the clamps and straight edge, re-clamp the base panel to the benchtop and repeat the process with the other cra­dle panel and supports, this time using the bevelled edge of the previously assembled cradle as the reference edge so the two cradle panels butted together form a per­fect 90°. While the glue cures with this assembly, you can make the T-square.

“T” time

A simple lap assembly, the T-square pro­vides a square edge to be clamped onto either of the cradle panels for repeatable spline slots left or right of the saw’s kerf. I suggest cutting the two parts long, squar­ing them up, clamping them dry, drilling for countersunk #6 × 3/4″ screws through the thinner arm part, then applying glue and screwing the assembly squarely. Once cured, cut the parts to final length.

Add reinforcements

Install countersunk #6 × 3/4″ screws; one centred up near each end of a runner (avoid overtightening) and two or three down through each of the cradle pan­els into the supports below. If you opt to add trim to the vertical edge of the cradle supports, as I did, now’s the time. Lastly, tune the runners for a smooth, stable glide.

Blade choice

Raise the blade you’ll use for slotting about 1″. A 24-tooth ripping blade with a 1/8″ kerf, ground to a square, flat-top profile is my suggestion. Advance the jig so the saw blade passes just through the box cradle and no further. I clamp a stop block to my rip fence to limit forward travel. Withdraw the jig, and with the saw turned off, mark lines squarely up from both sides of the kerf inside each cradle face for future reference.

Lastly, a coat or two of finish seals the build.

Put it to use

In use, choose where you want to install slots/splines on a box. Clamp the T-square to set that position. Adjust the height of the blade to give you the spline depth wanted. This might be best done on a piece of scrap wood. It’s now set up for cutting the splines in your project.

Holding the box firmly within the cra­dle, snug to the square, advance the jig forward until the cut is made, creating a slot. I aim for about 3/4 of the depth of the box corner, but another approach is to adjust the depth of cut to ensure the ends of the splines are in line with the thickness of each side. For example, if the project has 1/2″ thick sides, the exposed splines will run 1/2″ along each side. Lift and remove the box, slide the jig back to clear the blade, rotate and reinstall the box flush with the T-square to rip the next spline slot at the same elevation in another corner. Repeat for each of the four corners.

Reposition and repeat for all the slots you’d like to decorate and reinforce your box or cabinet.

Spline material can be contrasting or matching. Machine it to thickness so it fits snugly, but not overly snug, as adding glue will only make the fit slightly tighter. Once glue is applied to both the slot and spline, tap it into place and let it dry before flush cutting the overhanging spline and sanding or hand planing it flush with the outer surfaces of the box or cabinet.

Materials List

mitre slot jig materials list

Illustration

mitre slot jig illo


MARK SALUSBURY - [email protected]

Whether it is joinery or turnery, Mark has enjoyed designing and making furniture, decorative and functional items and home remodeling ... anything to do with woodworking, for over 35 years.

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