Aside from a dovetail joint, the box, or finger joint is one of the strongest joints to reinforce a corner. Before the era of corrugated cardboard containers, air filled packaging, and overnight shipping, wooden boxes and shredded wood were used to package fragile goods (think leg lamp in A Christmas Story). An inexpensive way to strengthen smaller boxes so they would survive the trip intact was adding finger joints to the vertical corners.
In this post, I’ll show you how to make a simple box joint jig for the modular crosscut sled.
It’s probably been a long time since you’ve seen a box joint. You’d think because it’s such a strong joint, it would be more prevalent in custom made woodworking. The intention of the box joint is more functional than decorative. If you’re going for the industrial look, then the box joint might fill the need. What you’ll find in most cases today is a dovetail joint. Not only is it stronger than a box joint, it’s decorative and conveys the craftsmanship of the maker.
Joint Strength
What makes a joint stronger than another? Surface area and mechanical connection. Many people will tell you grain direction makes a difference and end grain to end grain joints are much weaker than long grain to long grain. I don’t buy into that theory. Check out this video if you a firm believer in weak end grain joints.
Here’s a quick rundown of four common corner joints. Let’s compare a single corner of each joint assuming a 4″ tall box with 3/8″ thick walls.
Miter Joint
The miter joint has everything working against it. Because only two flat surfaces are in contact, it has the least amount of surface area. In the example box corner above, the surface area of the miter is about 2-1/8 square inches. The two mating surfaces are flat cuts, so there is no mechanical interlocking. To judge the mechanical effect of any joint, imagine if you assembled the box with no glue whatsoever. In the case of the miter, the box would fall like a house of cards as soon as you tried to move it.
Splined Miter Joint
A method to strengthen a plain miter joint is adding splines to the joint. The splines take advantage of increased surface area and a small amount of mechanical strength. The arrangement of the splines resist forces that open a corner while providing a moderate surface area increase. Three splines add more than 3/4 of a square inch of surface area, or about 40% more than the miter. You’ll find a slight mechanical grip, as tight fitting splines will allow you to lift the box if it were not glued. There would be very little resistance to racking from corner to corner though.
Box Joint
Like the spline joint, the glue strength of the box or finger joint relies on a large amount of surface area. The joint also adds a mechanical grip. A tight fitting box joint will resist some forces pushing the walls outward, but the strength in the vertical direction is equal whether glue is applied or not. Racking forces are resisted far better than the spline joint because of the increased number of mating surfaces and their added friction.
In our box example, assume the joint is comprised of 1/4″ wide fingers. The fifteen surfaces of the joint total about 5-1/2 square inches. That’s more than 250% the surface area of the plain miter joint. Couple this with the mechanical strength of the joint, and this joint is hard to beat.
Dovetail Joint
The dovetail joint is unique in it’s ability to withstand certain directions of force even without glue. Picture the most common application of a dovetail joint. Pull out a cabinet drawer on a higher quality cabinet and chances are good you’ll find a dovetail. The mechanical strength of this joint comes from the wedging action in the direction of pull in the drawer. The harder you pull the joint in the direction of drawer motion, the tighter it grips. In the side to side direction, it acts similarly to the box joint and has similar strength.
Because of the geometry of dovetails, the components are larger, so fewer pins and tails can fit in the joint. This will reduce the total surface area of the joint, but the mechanical strength will make up for the loss of surface area. In the example box above, assume the dovetails are spaced every 1/2″. The total surface area of the joint will be almost 3-1/4 square inches. That’s about 60% of the surface area of the box joint, but still 150% that of the miter joint.
Racking forces are also addressed with the dovetail. When a force is put on the corner, the wedging action of the joint surfaces will tighten and resist racking.
The biggest downside of the dovetail is the effort required to get them to fit properly. A small variation in the thickness of a material changes the fit of the joint. A box joint fits the same regardless of the material thickness and in a manufacturing environment, will be the most forgiving.
When boxes were mass produced, the box joint was the manufacturer’s choice. With advancements in tools and materials such as high tolerance plywood, and router jigs, dovetails are easier to execute. Today, they as much a mark of craftsmanship as they are a functional application.
Making the jig
On the plus side, this jig is simple to make. On the minus, it’s limited to a single finger width. This one is 1/4″. If I need a joint with 3/8″ fingers, I’ll have to make a new jig. Luckily, it can be built in a couple of hours.
There are only four wood pieces to cut. The key and the backplane are the two main elements. The base and back alignment pieces add repeatability when removing and reinstalling the jig from the sled.
Cutting the Backplane
Cut the backplane to size. The dimensions aren’t critical, and should accommodate the largest size box you’ll make. I made mine from a piece of scrap plywood about 12 inches wide by 6 inches high. Decide on the width of the finger you’d like and install the corresponding width dado stack.
Remove any installed sacrificial plate from the modular sled. The dado will quickly ruin a blade-specific plate you’ve made.
Find the center of the backplane, then measure and mark a line one and a half of the joint’s finger width over. Clamp the backplane to the rear fence of the sled with the line on the centerline of the blade. This isn’t a critical dimension either. You can eyeball it without issue. Being off center makes no difference to the performance of the jig, it just creates an equal amount of backplane on each side of the cut.
Raise the blade so it cuts a groove in the bottom of the backplane about 1/2″ tall. Remove the backplane from the sled. You’ll be using this as a reference to size the key.
Since I just got a new laser engraver, I thought I’d take advantage of it and add some graphics to the backplane.
Cutting the Key
Cut and mill a piece of hardwood that fits the width of the slot in the backplane. It should be snug but not tight, with no side to side movement. The length of the piece will be driven by the process you use for machining. Because I use a drum sander, I can get away with a short piece. If you use a thickness planer, you’ll need a longer piece to span the drive rollers plus a few inches. In any case, 5″ is about as short as you want to make it.
Cut a 1-3/8″ long piece and two spare pieces from the stock you milled using the crosscut sled. One piece will become the key and you’ll use the two extra pieces to align the jig later. Put a slight chamfer on the two top edges and front of the key. This helps later when cutting the joint.
Cutting the two alignment pieces
The length of these pieces aren’t critical, but the width is. These align the jig when placing it on the sled. The plate should fit in the sacrificial plate’s groove without binding nor side to side movement. The thickness of the bottom plate should be slightly less than the depth of the bottom sacrificial plate’s groove. The rear alignment plate will be cut from 3/4″ plywood and should be about as tall as the rear fence.
Place the rear plate in the fence’s groove and mark the location of the slots in the fence. Drill these through, then using a Forstner bit the width of the T-bolt head, cut a few shallow holes in the shape of a slot. These will retain the head of the T-Bolt without allowing it to spin. Because the head of the T-bolt will be sandwiched between this plate and the backplane, be sure the slots are deep enough so the T-bolts sit below the surface of the face.
Gluing up the jig
I glued the jig in two stages. First, I created an assembly from the two alignment plates. These are glued at a 90 degree angle taking care the edges are aligned. You can remove the sacrificial plate and use the sacrificial plate grooves in the crosscut sled as a fixture to glue these up if you like. Just be careful excess glue doesn’t squeeze out and adhere the assembly to the sled. I added a coat of paste wax to the sides of the plates and the sides and bottoms of the grooves to be sure. This also helps the finished jig slide in and out of the sled.
In parallel, glue the key into place in the backplane. After the key has had some time to dry, install the rip blade back on the table saw. Using the fence as a guide, trim the key and bottom surface of the backplane so the key is slightly shorter than the thinnest wall of the box you typically build. The final dimension of mine was about 1/4″ because I sometimes mill my box sides to 5/16″ thick.
Reinstall the dado blade in the table saw. Remove the clamps from the alignment plate assembly. Clamp the glued up assembly in place in the rear corner of the sacrificial plate’s groove using the T-track hold-downs. Raise the blade slightly higher than the top surface of the lower alignment plate and cut a dado through the bottom and back alignment plate assembly.
Remove the assembly and add spacers in the groove below and behind the alignment assembly. This will raise the assembly proud of the base and fence surfaces to ensure firm clamping of the backplane.
Slide one of extra key pieces into the dado you just cut in the alignment assembly. It should fit snugly. Drop the backplane into place and set the second extra piece of the key material between the key and the first piece. This will be the final jig spacing.
Using the extra key stock to align the backplane. When satisfied with the fit, apply glue to the back and bottom of the backplane and clamp it into place on the alignment plate assembly.
When the glue is dry, the jig is finished.
Using the jig
Using the jig is simple if you remember to turn the first board around when aligning the second board. Ask me how I know.
Install the proper width dado blade and raise it to the height equal to the thickness of the box’s panels. If the blade is too low, the fingers will not be long enough to reach the mating panel’s outer surface. This is a difficult situation to correct. If the blade is set too high, the fingers will protrude beyond the surface of the perpendicular panel. This is easier to correct with a plane and/or sander. The better scenario is to err on the side of the blade set too high.
Separate the side panels from the front and back panels. These will be cut as identical pairs.
Start by resting the face of the first panel against the backplane with the panel in a vertical orientation and the panel’s end against the base of the jig. Slide the edge of the panel against the side of the key. If the key is long enough to accommodate the thickness of a second board, you can do two at a time. Make your first cut, then move the board to place that cut over the key and make the next cut. Repeat until you reach the point when you would no longer make a cut. It’s possible you’ll end up with a very thin final finger. That’s fine.
Flip the top of that panel or set of panels away from you so the cut fingers are now facing up and the same edge you started with is resting against the side of the key. Make the next set of cuts exactly as you did the other end.
What you’ll end up with is a pair of end panels with identical patterns on each end.
Grab the end panel you just finished cutting and flip it left to right. It should be facing the opposite direction than when it was cut. Place the last notch on that side over the key. Butt the uncut front panel against the side of the end panel. Clamp or hold the front panel in place against the backplane, then remove the previously cut panel from the key. Without moving the front panel, make the first cut. Repeat for the rear panel if you’re going to cut them as a pair, or complete all the cuts on the front panel before moving to the rear panel. Again, cut the series of fingers across the panel.
Complete both ends of all four panels.
If you did this right, the fingers should interlock and the box will take shape. The joint should be tight, and you may need a rubber mallet to tap the joint home. If it’s too tight, you may need to add a thin spacer between two blades in your dado stack.
