As I mentioned in part one of the Modular Crosscut Sled series, I built the modular sled to make small boxes. One way to make a box stronger is adding splines to the beveled corners, so this jig is essential for me. The jig is easy to make and uses minimal materials.
You’ll notice I used two different types of plywood. That wasn’t a design choice, it was driven by the scrap materials I had on hand.
Cutting the triangular supports
As you can see in the initial design rendering, I planned to drill holes in the triangular supports so I could vacuum any sawdust trapped between the back and the box holder. I found it easier to cut the corner of the support at the junction of the base and back of the jig out of the outside rear triangles. Tilting the jig and shaking the sawdust out will remove most of the sawdust. Any other debris can be vacuumed out through the holes drilled in the back of the jig.
Cut the triangle supports on the sled using the bevel jig. Calling it a jig might be a bit optimistic. It’s just a triangle with a hole to clamp it in place against the fence on the base of the sled. You can also use a miter saw to cut the bevel.
Making the base
Cutting the base is straightforward. There are two pieces of plywood comprising the base. The bottom and the back. Both will clamp to the sled using the T-Tracks.
Drill the holes for the hold-downs to match the T-Track in the sled base and rear fence. Slot those holes to the edge of the plywood with the band saw or by hand. When adding slots for T-Track bolts, always cut them parallel to the T-Track so the T-bolts can slide in after the jig is set in place. In this case, the base slots are cut parallel to the saw blade and the fence slots are cut perpendicular to the saw blade.
I also drilled a couple large holes in the back plate to vacuum the sawdust from behind the box holder. I drilled them between the locations of the triangular supports. When the jig is assembled, this area is nearly closed off,. Debris will accumulate, and you’ll need a way to clear it.
Round over the edges and corners. Finally, glue the back to the base making sure the two parts are perpendicular. Add the center triangle brace to add rigidity to the assembly. When you place the center triangle, offset it to one side of the saw blade or the other. You don’t want to cut into the support with the blade.
Cutting the box cradle plates
An important consideration when making the spline jig is the depth of cut of your spline. If the box is held too high off the base of the sled, the blade will have to raise up higher. Not a problem for the jig, but you’ll be cutting a deep slot into your sled’s front and rear fence. To minimize the blade height, the corner of the box should rest as close to the base of the sled as possible. To accomplish this, the box holding cradle plates are beveled and that bevel is glued to the jig’s base. In this configuration, the corner of the box will essentially be resting on the jig’s base.
Cut the box cradle plates to size with the bevel on the long edge. Next, dado the groove for the T-Track using the table saw. Unless your dado blade is a perfect fit for the T-Track, remove one chipper and cut a groove slightly narrower than the track. Move your fence a little at a time to sneak up on the width of the slot so the track fits tightly in the groove.
To finish the plates up, round over the upper corners, then cut the T-Tracks to length and fasten them into the plates.
Gluing up the rest of the jig
With the base and back already glued up and dry, glue the outer two triangles in place against both the base and the back. Glue the rear box cradle plate into place onto the three triangles. Use glue on the bevel as well to adhere it to the base.
Clamp a 90 degree cabinet clamping square on each end of the cradle plate. Add glue to the bevel of the second cradle plate. Place it on the base and clamp it to the squares. Add glue to the other three triangles and slide them into place under the cradle. If the they are cut accurately, the the triangular supports should match up and the glue will secure them in place without clamping. Because this is a cumbersome clamping operation, secure the triangles with brad nails or screws if you prefer.
Making the fences
Cut the pieces for the fences and drill the holes for the T-Bolts. Round over the exposed corners and edges. Glue them up making sure the bases are perpendicular to the wall of the fence. These are quick and easy to glue up. Use brad nails or screws if desired.
Aligning the jig
Cut a piece of wood to fit the width of the sacrificial plate’s slot in the sled. It should be snug enough to slide in and out of the slot without side to side play. This will be your alignment reference when placing the jig into the sled. It should be as long as the jig is deep in the direction of cut. Finally, cut the bridge for the front end of the jig. This will hold the front of the jig together after the first cut is made.
You’ll notice in the initial design rendering, the sacrificial plate extended beyond the base. I built the jig this way and realized the extension served no practical purpose. As well, installing the jig to the sled was more difficult with the extension. I later glued a bridge directly to the front of the base, then cut off the extension after the glue dried.
Add enough layers of masking tape to the bottom of the alignment plate to raise it slightly above the base of the sled. Add a layer of waxed paper or packing tape to the sled base slot to avoid any glue squeeze out from adhering the jig to the sled. I doubt you want this jig to become a permanent part of the modular sled.
Add a layer of glue to the alignment plate and lower the jig onto the sled. Be sure to align the slots with the T-Track on the base as you lower it. Clamp the jig using the T-bolts and let the glue dry.
Install the fences and the jig is complete.
Using the Jig:
When building a box, it’s best to add the splines and finish sand the entire exterior before cutting the lid from the box. This will give a more consistent result and result in a perfectly fitting lid.
Install the blade in your saw you’ll use to cut the splines. A flat top blade like a Freud heavy duty FTG rip blade will give the best result. FTG stands for Flat Top Grind.
For the strongest joint, set the blade depth to cut through as much of the corner of the box as possible without breaking through the inner corner. Do this by setting the box in the cradle next to the blade and sighting down the cradle to see the blade against the corner of the box.
Set the fence to the desired distance from the blade. Set the box into the cradle with the top against the fence. Keeping pressure on the box, hold it against the cradle and the fence, then cut the first slot. Be Sure Your Fingers Are Nowhere Near The Saw Blade. Rotate the box to its next corner and repeat on all four corners.
If the splines are symmetrical from the top to bottom of the box, flip the box over so the bottom is against the fence and cut the other four splines using the same setup.
Move the fence to its next position and repeat the process until all the slots are cut in the box.
In most cases, you’ll only need to use one fence, but if you have a very narrow (shallow height) box, you may want to use the second fence to trap the box in the jig. This will keep your hands away from the blade.
Cut your spline material to a thickness that slides into the slot without excess resistance. Too tight and it will push out the glue when you insert it. Too loose and you’ll see the glue line.
Cut the splines to a length where they protrude fully from both sides of the slot, then glue them in place making sure they’re fully seated in the bottom of the slot.
Once dry, cut them with a flush cut saw. Sand everything smooth and you’re done.
Not only does the spline joint add strength, it adds a pleasing design element to your work.
