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Build a dedicated setup that creates perfect joints every time.
Imagine the path perfect box joints whenever you want, without having to waste time setting up your milling machine. This notion may seem halfway through, because box joints can be difficult to fit. But it isn’t! This jig lets you go directly from milling parts to cutting seams – it’s perfect for carpenters like me, who like to make multiple boxes.
A box joint consists of pins and sockets which all have the same width. To create a box-like joint, a series of slots (the plinths) are milled which are spaced so that the remaining wood between them forms the pins. Properly sizing the pins so they fit snugly into the sockets is the fussy part.
In order for the joints of the box to come together, the pins and sockets must be offset, where one side has the pins, the other has the sockets. Sometimes called “finger joints” because the pins look like fingers in a folded pair of hands, box joints are popular because they are strong, attractive, and easy to make as long as you have a reliable jig.
Think of the jig shown here as a miniature milling table dedicated to making a specific joint. You can build it in a day, using hardwood plywood or MDF, a couple of chunks of solid wood, and a small router. Dial the perfect size once (it’s easy, I’ll show you) and the mask is ready to be used again and again.
Make the jig
Start by cutting a nut pair into a blank that contains the jig’s plywood base and sled (Photo 1 and Fig.A). After cutting the blank in the two parts, notice that the nuts are on the top face of the base and on the bottom face of the slide. Flip the sled over and cut a nut for the fence 2-1 / 4 “from the rear edge. This nut is perpendicular to the nut for the runners.
Make the runners and fence out of solid wood and plan them to fit the nut. The skids must allow the sled to slide smoothly on the base, without binding or wobbling. If the skates are too loose, the mask will not be accurate. If they are too tight, the mask simply won’t work. The fence should fit more tightly, almost needing to be forced into place. This pressed fit is necessary to allow for fine tuning of the jig. Cut the fence so that its length matches the width of the sled so that later on, when tuning, you will be able to feel small changes in position between these two parts.
Install the router base and guides on the template base (Photo 2). Mark a center line for the router and then drill a hole on this line that is large enough for the router clamp to pass through, 4 “from one end of the base. Remove the router base plate, center it over the hole that you just drilled, then carefully transfer the location of its mounting holes onto the base of the mask. Use these marks to drill holes for the mounting screws, countersinking them on the top face of the base. You will need 1 “long flat head screws. to mount the router base. I decided to use larger screws as well (5/32 “), so I plugged the mounting holes in the base of the router to fit them.
Fix the guides with the screws after having drilled the countersunk holes through the underside of the slide.
Dedicate the sleigh
Attach the base of the mask to the corner of the workbench. Mount the sled to the base and make sure it slides properly. Screw the stop block to the base, positioned to stop the slide when the cutter is fully seated in the enclosure.
Remove the sled and install some in the router. An upcut spiral tip works best because it minimizes tearing. The width of the pins and sockets you want to make determines the size of the tip you will use. The choice of the bit also dedicates the slide to the realization of a specific box joint: the slide shown here has been made with a diameter of 3/8 “. Bit, therefore it is dedicated to the realization of boxed joints with pins and sockets of 3 / 8 “wide. If you commonly make box joints in a variety of sizes, simply create additional shoe assemblies dedicated to the appropriate router tips.
Make a slot in the slide (Photo 3). Lower the tip below the surface of the sled. Place the sled flush with the rear end of the base and lock the two parts together. Start the router and gradually raise the bit to make a hole through the sled. Turn off the router, lower the tip to 1/8 exposure and remove the clamps. Now beat the slot. Start the router, push the sled forward to the stop block, and pull it back flush with the back of the base. Raise the bite another 1/8 ″ and go again. Repeat this process until you have gone through the sled. Finish the job by lifting the tip above the surface of the slide the same size as its diameter (3/8 “in this case) to complete the slot in the guide.
Install the key
A solid maple key indexes the workpiece for routing each socket (Photo 4). This key is installed in a second square slot which is routed into the fence. The space between these slots corresponds to their width. On the mask shown here, the slots measure 3/8 “wide, so they are spaced 3/8” from each other.
Start by milling a long, square key blank to match the width of the slot in the sled. The resistance you feel when you insert the key into this slot will be the same resistance you feel every time you index a piece on the mask, so a snug but not forced fit is ideal.
Mark the location of the second slot on the fence. Then reposition the guide on the sled, carefully aligning the mark with the opposite side of the slot in the sled. Remove the mask stop block. Then run a square slot for the key through the fence. Complete this slot in several steps, raising the bit incrementally. Then install the key and reinstall the stop lock.
Perform a test joint
Raise the tip slightly higher than the thickness of the stock you intend to use (Photo 5). Cut the parts that we intend to join oversize in width. It’s best to wait until all pins and sockets have been routed before cutting these parts to their final width, as it allows you to align any uneven edges before making the final cuts.
Route the first socket by holding the test piece firmly against the key (Photo 6). Use the socket you just routed to index this piece for next socket routing (Photo 7).
Once you have routed all the sockets, flip this piece over to the opposite face and use it to mill the first socket into the second test piece (Photo 8). Then route the remaining sockets (Photo 9). Assemble the joint to test the fit (Photo 10).
Fine-tune the joint
If the joint is too tight, tap the right end of the guide to reduce the gap between the key and the milled slot (Photo 11). This reduces the width of each pin. If the measurement is too loose, tap the left end lightly to make each pin wider.
Since the fence and sled are the same size, you can use your fingers to accurately measure these small adjustments. Run additional test joints to evaluate your adjustments. When the joint fits snugly, install screws to lock the fence in place.
Fit the sled
You can use the same slide assembly to cut box joints in different thickness media, but you will quickly discover a problem: when milling the joints in a thin material after milling the joints in a thicker material, the top of each socket tends to tear the back side. This tear occurs because the routed slot in the fence is too high to fully support the shorter sockets required by the thinnest support. The solution is to install an auxiliary fence with a shorter slot that will fully support these shorter sockets (Photo 12).
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