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Skill Builder: Box and Finger Joints
Issue: December 2009
Posted Date: 10/16/2009
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by Sandor Nagyszalanczy
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Roll Over Images to Enlarge
Box joints on the table saw
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Photo 1
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| Photo 2 |
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Photo 3
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| Photo 4 |
One of the easiest ways to cut clean and tight-fitting box joints is with a table saw, using a dado blade and a sliding jig. You can build your own box joint jig by adapting your saw's miter gauge (Photo 1), or you can buy one ready-made jig, like the Rockler box joint jig (Photo 2). The jig consists of a tall vertical fence attached to a bar that slides in the saw's miter gauge slot. The forward-facing lower edge of the fence has an adjustable pin which is adjustable for size as well as for its position relative to the saw blade.
To prepare for cutting the joints, first install a dado stack in your saw, set to cut as wide as the width of the desired box joint's pins and notches: 1/4-in., 3/8-in., 3/4-in., etc. In this example, I'll cut 1/2-in. wide dadoes on 1/2-in. thick stock. This creates an attractive pattern of square pins; for stronger joints, cut pins that are narrower than the stock thickness: for example, 3/8-in. pins in 5/8-in. thick stock.
To set the jig for use, first set the size of the jig's protruding pin to the same width as the dado stack: 1/2-in. in this example (Photo 3). (If you're using a shop-made jig, install a wood pin of this width). Now, adjust the jig's fence so that the distance between the pin and near-edge of the dado blade equals the width of the pin (1/2-in. here - Photo 4).
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| Photo 6 |
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| Photo 7 |
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| Photo 8 |
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| Photo 9 |
Set the blade's depth to cut a notch that's a tiny bit deeper (1/32-in.) than the thickness of your stock. The pins will protrude very slightly above the surface of the assembled joint, making it easier to sand or plane the joint flush later (make sure and include this added length when calculating stock dimensions for your project).
To cut the first half of the box joint, butt the edge of a workpiece against the jig's pin and hold or clamp it firmly against the fence (Photo 5). Switch on the saw and slide the jig smoothly over the dado blade. Carefully lift the work out of the jig before sliding the jig back to the starting position. For all subsequent cuts, place the notch you just cut over the pin and cut a new notch (Photo 6). Repeat the process until you've cut notches all the way across your workpiece. It doesn't matter if the last finger isn't full-width or if the last cut leaves a notch instead of a pin - the other half of the joint will be cut to match.
To cut the joint's other half, space the stock a pin's-width away from the dado blade using a scrap piece cut just like the first pin you cut for the joint (Photo 7). Remove the scrap and cut the first notch (Photo 8) in the workpiece. Then, reposition this notch over the jig's pin, and cut all subsequent notches as before (Photo 9).
It's best to cut both halves of the joint in scrap stock first, and test the joint's fit before cutting your good workpieces. If the pins and notches' fit is too loose, shift the fence so that the jig pin is a tiny bit farther from the blade. Too tight? Move the pin a hair or so closer to the blade. Don't make the joint fit too tight - applying glue during assembly will swell the wood and make the fit even tighter!
If you want a plan for making an adjustable box joint jig, you can order one from Woodworker’s Journal here.
Box joints with a router
If you'd prefer to cut box joints with a router, can use either a template-style joinery jig with a portable router or a special sliding box joint jig or dedicated box joint bit with a router table.
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| Photo 10 |
If you already own a router dovetail jig, some can be adapted to also rout box joints. For example, there's a special box/finger joint template that works with Leigh Dovetail jigs (Photo 10). The aluminum template replaces the standard dovetail template and allows you to cut several different sized joints. A unique template shifting system built into the jig lets you cut both halves of the joint without having to manually realign the jig for matching joint parts.
After the desired size template is positioned and set up, as specified in your jig's manual, fit your router with a straight bit that matches the template you've chosen (in the photos, I'm using a 3/8-in. dia. bit with the 3/8-in. template fingers). For the cleanest cut, always use a spiral cut bit. Now, attach the special tapered bushing to your router's subbase, sized to match the bit.
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| Photo 11 |
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Photo 12
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Clamp a scrap backing board to the horizontal surface of the jig and clamp the workpiece vertically so that its top edge is bearing against the scrap. Rout out the notches and pins for one half of the box joint, moving the router counterclockwise around each template finger. To rout the matching half of the joint, slide the jig template to the left and lock it in place with the provided tapered pin (Photo 11). This shifts the template relative to the workpiece, so a notch is cut first instead of a pin.
Adjusting the joint fit's degree of tightness employs a clever feature of Leigh's variable guide bushing system: To make the joint fit looser, simply lower the bushing slightly into the subbase (Photo 12). Thanks to the bushing's slight taper, lowering it reduces the space between the bit and the template fingers. Thus, the bit cuts slightly larger notches that fit the pins more loosely. To make the joint's fit tighter (you guessed it), simply raise the bushing slightly.
Router table box joint jig
Another option for router-cut box joints is to use a special jig with your router table. The Rockler Box Joint Jig (available by clicking here) attaches atop any router table with a miter slot (Photo 13).
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| Photo 13 |
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| Photo 14 |
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| Photo 15 |
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The jig's sliding fence holds the workpiece vertically and guides it over the router bit. The jig works much the same way as a sliding table saw box joint jig (as described above), except that instead of having a pin attached to its sliding fence, the Rockler jig has a guide rail that's used to evenly space notches along the length of the joint. By switching rails and router bits, you can use the jig to cut 1/4-in., 3/8-in. or 1/2-in. box joints.
The first step before cutting a joint is to fit the jig's sliding fence with a sacrificial face, cut from a scrap of 1/2 MDF or plywood that's notched to fit over the rail. The distance between the bit and the rail has to be the same as the diameter of the bit; in this example, that's 1/4-in. (Photo 14). Make the setting by adjusting the jig's side-to-side position in the miter slot: the jig has two expanding strips that lock into the slot. Now, adjust the height of the bit so that it's just a hair higher than the thickness of your workpiece (in the example shown, the workpiece is 1/2-in. thick, so the bit's cutting height above the jig is 1/2-in. plus 1/64-in.) (Photo 15). Now butt one edge of the workpiece against the rail and hold it firmly against the fence as you slide it through the spinning bit. Reposition the stock to the left so that the notch you just cut fits over the guide rail, and repeat the process of routing notches along the length of the joint. To cut the mating workpiece, offset the first cut by the width of a pin, then continue routing as before. You can adjust joint tightness by shifting the jig relative to the bit: less space between the bit and guide rail produces a looser fit; more space yields a tighter fit (Photo 16).
Box joint router bit
An interesting alternative to standard router methods is to use a special box joint router bit (Photo 17). These bits consist of a series of 5/32-in. wide slot cutters separated by spacers and stacked on an arbor with a 1/2-in. shaft. The bit forms a series of evenly spaced notches and pins that create half of a box joint in a single pass. The catch is that the bit will only cut stock that's 1 9/16-in. wide or less. This is great for quickly routing corner joinery for shallow trays and small drawers, say, for a jewelry chest or a machinist's style tool chest with lots of skinny drawers.
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| Photo 17 |
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| Photo 18 |
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However, with careful setup and routing, you can box join stock that's twice that wide by flipping the workpiece over and taking two passes on each joint end.
To use a box joint bit, chuck it up in a heavy-duty router (this is a big bit which takes some power to run) and mount it in a router table. Set the bit's depth of cut so that the bottom of the lowest kerf cutter is flush with the router table (Photo 17). This will form a full notch at the bottom edge of the stock. Next, set the table's fence to yield the desired pin length (Photo 18), which should be slightly greater than the thickness of your workpieces. Even though many bits come with pilot bearings that ride on the bit's shaft to control the cutting depth, I prefer to use the router table's fence to limit depth of cut. Once the bit is set up, cut your workpieces to a width that yields a full 5/32-in. wide pin at the top of the cut. This will save time and trouble: Rout one half of the joint as usual, then flip the stock over (top to bottom) for routing the second half of each joint. You don't need to reset bit height for the matching halves of each joint.
To guide the narrow workpiece through the cut, use a miter gauge or other sled jig you've made for your router table (Photo 19). Fasten a scrap strip to the face of the miter gauge/sled to act as a backing piece to support the work and prevent tearout. As usual, rout a test corner and check the joint fit. If it's a bit too loose, you'll need to increase the size of the pins slightly: Remove all the cutters and spacers from the box joint bit's shaft, and add thin shims between each pair of cutter/spacers. If the joint fit is too tight, reduce the size of the pins by removing all components from the bit shaft and then substituting slightly thinner spacers between the cutters. Make sure and replace the backing piece on the miter gauge/sled once the joint's final fit is perfect.
Finger joints
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| Photo 23 |
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| Photo 24 |
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Another kind of joint that's useful for joining solid wood parts with great strength are finger joints. These resemble box joints, but the individual interlocking members are tapered along their length, instead of being square, like box joints. Finger joints are most commonly used to join parts end to end, but they could also be used when gluing parts edge to edge, as the joint's extensive surface area provides a lot more wood-to-wood contact for the glue.
About the only practical way to cut finger joints is using a special router bit designed for the purpose (Photo 20). A finger joint bit cuts all the fingers and spaces between them in a single pass. Like the box joint bit, a finger joint bit is large enough that it must only be used with the router mounted in a router table.
To set up a finger joint bit, adjust its height so that it will cut a full finger at the bottom edge of the stock. Set the router table's fence so that the full profile of the bit is cutting into the stock. Just like with the box joint bit, use a miter gauge (or sled jig) to guide the workpiece past the cutter (Photo 21). For the mating piece, simply flip the work over and rout without resetting the bit's height or depth of cut. The parts should mate and fit together snuggly (Photo 22). If the finger's fit is loose, decrease the depth of cut by moving the fence very slightly closer to the miter slot. For a tighter fit, move the fence slightly in the opposite direction.
You can also use finger joints to add strength to miter connections, say, for a carcass or furniture frame (Photo 23). After mitering the ends of the work, you rout them with the finger joint bit as usual, only using the miter gauge as a guide with its head set to 45 degrees. There's a small caveat: The ends of the members will overlap at the finger joint (Photo 24), hence you must allow for extra length in one member and then trim the overhanging fingers at the miter's tip. Additionally, the joint will show at the inside heel of the miter (Photo 25), but only on the surface of one side of the frame; there's still one "good" side of the frame where the joint won't show.
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Sandor Nagyszalanczy is a furniture designer/craftsman.
writer/photographer and regular contributor to Woodworker's Journal.
His
books are available at: http://sandorsworkshop.com/Books.html
©2009 Woodworker's Journal
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