This article is quite long. If it had drawings and pictures for the sighted it would be much shorter. It's true. One picture really is worth a thousand words.
The jig has three fixed fences and one movable one. Initially the router is placed in the jig with the bit fully retracted or removed. The movable fence is adjusted so the router fits snuggly in the opening but moves smoothly. Two calibration blocks which are exactly the same width as the bit you plan to use are placed between the movable fence and the calibration stops. The stops are snugged up to the blocks and tightened down. The movable fence is loosened and moved back. The two calibration blocks are removed and replaced by two offcuts from the shelf material. The fence is pushed against the offcuts and tightened down. The jig is now adjusted.
Install a bit if not already done and adjust the depth of cut. Clamp the jig to the workpiece, run the router clockwise around the inside of the jig, and you are done.
There are two stopblocks for making stopped dados. These are optional but I recommend you make at least one. I elected to make two because of the way I make bookcases. I cut out one piece of material twice the width of one side plus a saw kerf. Then I cut the dados in this single piece and then rip it apart. This insures that I won't have the dados on one side offset by the width of a dado from those on the other side. If the design calls for stopped dados, both ends will have to be stopped. Hence two stopblocks.
I made the jig from 1/2 inch birch plywood from the home center. This is not the same as Baltic birch plywood but it is better than the common B-C plywood. Hardwood faced plywood has almost no voids and those that it does have are very small and only in one of the plies.
Note. My router table is not big enough to accommodate the stop blocks. It is a bench top table. I pulled a new piece of 1 by 4 poplar out of my stock and clamped it to the back of the table. Then I clamped shorter pieces at right angles at the proper distances from the router bit.
Install a feather board to the fence on the right side of the bit. Place the main board with the fence at the top on the table, under the feather board, and against the right hand stopblock. Make sure the workpiece is firmly against the fence and the right stopblock. Move the feather board down to firmly hold the piece against the table. Now you must raise the router by 1/8 of an inch while the router is running. This is easily done on my router because the raise and lower is a lead screw with 8 threads per inch. There is a knob on the screw. If your router doesn't work this way you will have to find another way. You might think of raising the router and then turning it on. I don't recommend this. If your stopblocks are more robust than mine you could raise the router 1/8 inch, hold the workpiece against the fence and right stopblock, lift up the left end of the workpiece, start the router, and lower the workpiece to start the cut. If you only have two hands you may find yourself running a little short of grasping appendages. Something I haven't tried but may work is to make a wedge that tapers down from say 1/4 inch to nothing. Raise the router, lift the workpiece and slip the wedge under the left end, turn on the router, hold the piece against the stopblock and fence and slowly pull out the wedge. If all else fails you may have to use a handheld router and kluge together a temporary jig to make this jig.
Let's assume the router is running and you have found a way to raise it to start the cut. Hold the workpiece firmly against the fence and move it to the left until you encounter the left hand stopblock. Turn off the router and don't move the piece until the router has coasted to a stop. Release the featherboard and move the workpiece back to the right side. Reinstall the featherboard. Turn on the router and raise it another 1/8 inch. Push it to the left and turn off the router. Repeat this until you have broken through the top of the fence on the workpiece. Do not get into the habit of placing your hands over the bit. It will break through on one of the cuts and you don't want to have any part of your body over the bit when that happens. It should take 8 passes to cut through two layers of 1/2 inch plywood. If you lose count the breakthrough could come as a surprise and I'll bet you don't like those kind of surprises any more than I do. The above procedure will cut a slot that begins one inch from each end of the fixed fence and is centered on its 2 inch dimension. Note. Because this slot is made with a 1/4 inch bit the actual end of the slot is 7/8 inch from the end of the fence. The 1 inch measurement is to the center of the router bit when the cut starts and ends.
The next step makes a counter sink slot for the head of the carriage bolt. Without moving the stopblocks or the router table fence remove the 1/4 inch bit and replace it with an 11/16 or 3/4 inch bit. Lower it to be flush with the table as before. Raise it 1/16 inch and make a pass as before. This is the clearance slot for the head of the carriage bolt. It only needs to be 3/16 inch deep.
Some of the text below will sound familiar. It is duplicated for those who did not cut the slot for the stop blocks. There are some differences so don't just skip over it.
Install a straight cut bit that is 1/4 inch in diameter and 1 inch long. Do not use a bit with a guide bearing. Lower the router until the bit is flush with the top of the table. Use an offcut to feel for the bit.
Install a feather board to the fence on the right side of the bit. Maybe in this case the left side would be better. Use which ever side is best. Place the main board with the fence on the left on the table, under the feather board, and against the right hand stopblock. Make sure the workpiece is firmly against the fence and the right stopblock. Move the feather board down to firmly hold the piece against the table. Now you must raise the router by 1/8 of an inch while the router is running. This is easily done on my router because the raise and lower is a lead screw with 8 threads per inch. There is a knob on the screw. If your router doesn't work this way you will have to find another way. You might think of raising the router and then turning it on. I don't recommend this. These stopblocks are more robust than the ones used to cut the long slot. Because of this you could raise the router 1/8 inch, hold the workpiece against the fence and right stopblock, lift up the left end of the workpiece, start the router, and lower the workpiece to start the cut. If you only have two hands you may find yourself running a little short of grasping appendages. Something I haven't tried but may work is to make a wedge that tapers down from say 1/4 inch to nothing. Raise the router, lift the left end of the workpiece, slip the wedge under it, turn on the router, hold the piece against the stopblock and fence and slowly pull out the wedge.
Let's assume the router is running and you have found a way to raise it to start the cut. Hold the workpiece firmly against the fence and move it to the left until you encounter the left hand stopblock. Turn off the router and don't move the piece until the router has coasted to a stop. Release the featherboard and move the workpiece back to the right side. Reinstall the featherboard. Turn on the router and raise it another 1/8 inch. Push it to the left and turn off the router. Repeat this until you have broken through the top of the fence on the workpiece. Do not get into the habit of placing your hands over the bit. It will break through on one of the cuts and you don't want to have any part of your body over the bit when that happens. It should take 8 passes to cut through two layers of 1/2 inch plywood. If you lose count the breakthrough could come as a surprise and I'll bet you don't like those kind of surprises any more than I do.
Without moving the stopblocks or the router table fence remove the 1/4 inch bit and replace it with an 11/16 or 3/4 inch bit. Lower it to be flush with the table as before. Raise it 1/16 inch and make a pass as before. This is the clearance slot for the head of the carriage bolt. It only needs to be 3/16 inch deep.
Check to be sure that the 34 and 1/2 inch piece of the movable fence fits between the end fences without binding. If it binds remove a small amount of wood until it moves smoothly. Apply glue to the short piece and stick it to the long piece. Check it against the jig to be sure that the long piece is centered on the short one. Clamp them together making sure that they do not slip out of position when the clamps are tightened. Often easier written than done. Set it aside for the glue to set.
After the glue has dried drill 1/4 inch holes one in each end of the long piece. These holes should be centered one inch from the ends and centered along the 2 inch dimension. Test with two 1/4 inch carriage bolts to see if the bolts will go through the slots and holes in the fence and the whole assembly will slide smoothly. If the bolts are binding because the holes are out of position determine which way the hole or holes must be moved to prevent binding. The fence on your router table should still be set to 1 inch. Install the 1/4 inch bit in lift it to full height. Place the hole over the router bit, turn on the router and move the workpiece in the proper direction to move the hole. Always arrange things so the workpiece is move to the left. Don't move it very far, it won't take much to correct the problem.
Drill two 1/4 inch holes in each calibration stop. Both are centered along the 2 inch dimension. One is 3/4 inches from one end and the other is 1 inch from the other end.
Drill two 1/4 inch holes in each stopblock. They are located on the crossbar of the T and are one inch from each end and centered along the 2 inch dimension.
Glue the 1 by 5 piece to the trunk of the T. After the glue sets test each one in the long slot at the top. If the thick part of the T binds against the top fence remove a little wood with a rasp or shoulder plain.
Install the calibration stop on the left end of the jig using two 1/4 inch carriage bolts, two fender washers, and two wing nuts. Install the movable fence. In the same way. After the squaring board is installed and glued you will not be able to install or remove the bolts on the left end of the jig.
Because the stopblock slot which is cut along the top of the jig has a little give to it, install a 1/4 inch carriage bolt, fender washer, and wing nut in the center of the slot to stabilize it. As you tighten the nut the slot will tend to spread. Place a clamp across the jig, up and down, and tighten it lightly. Adjust the tension of the clamp so the slot does not expand when the wing nut is tightened. Set a try square to the width of the fixed fence at the ends and make sure it is the same width at the center. After the wing nut is tightened without deforming the jig, remove the clamp.
Turn the jig over and place the 5 by 13 inch squaring board on the left end with the square 13 inch side on the right. Using a framing square, square it up with what is now the bottom edge, it was the top before you turned it over. Clamp the squaring board to the jig leaving room to put in screws at the upper left, lower left, and lower right corners of the squaring board.
No, I haven't left out a step. The alignment is very critical here and a small error will render the jig useless. Surfaces with glue on them are very slippery. Make sure the right edge of the squaring board is exactly at right angles to the bottom edge of the jig. Release the clamps and retighten them as many times as necessary to get it right.
Select screws that will go through the squaring board, the main board of the jig, and one of the fixed fences without breaking through on the other side. Being careful not to hit one of the slots drill pilot holes in the upper left, lower left, and lower right, corners of the squaring board. Drive screws into the holes.
Remove the clamps and the screws. Spread glue on the surface of the squaring board which was against the jig. Don't get it too close to the locations of the slots. Place the squaring board back on the jig and reinstall the three screws. Clamp the upper right corner. Wipe off any glue squeeze out and hope there is no squeeze out in the slots. This completes the assembly of the jig.
Here is how I did it. Start with a piece of hardwood stock that is 8 inches longer than the distance from the front of your saw table to the front of the blade. Set the rip fence to cut a strip that is somewhat wider than the diameter of the router bit. Use a feather board to hold the stock against the fence and a push stick to push it through the saw.
Now set the rip fence down to reduce the width of the piece to a scoshe over the size of the router bit. Using the end of the long stick as a handle employ a feather board to hold it against the rip fence and push the end of the stick into the blade for a distance of approximately 4 inches. Turn off the saw before attempting to remove the stick.
After the saw stops spinning pull the stick out and check the end. If the blade removed enough material to make a kerf, use a small hand saw to cut away the thin piece. Try the piece in the dado you made above. If you've been living right it will fit snuggly. If you have done a few things wrong it will almost fit but will be a little too big. If you have been really bad it will fit very loosely and you will have to start over.
If it almost fits move the fence just a little and push the end of the stick into the saw again. Be sure to orient it correctly. Test it again. Repeat until it fits.
Use a hand saw to cut off two 2 inch lengths of the stick. These are the calibration blocks for this bit.
Note: Selecting a bit. You are not necessarily locked into one particular size of bit. I had originally thought that I would use a 1/2 inch bit. While the jig was under construction I changed my mind and have now decided to use a 3/8 inch bit.
You can change bits at any time except for one issue. The cutout that the bit will make in the next step will be very useful in aligning the jig to place the dado where you want it. The edge of the slot closest to the fixed fence will be on one edge of the dado. If you make the initial cut with a large bit, say 11/16, and later put in a 1/2 inch bit the edge of the dado that will be cut will not be at the edge of the slot. If you cut the slot with a given size of bit and use the same bit to cut dados the top edge of the dado will always line up with the top edge of the slot.
Note: You can always go up to a larger bit but if you go back to a smaller bit you will no longer have this easy way to position the jig in relation to the wanted dado.
Additional note: Any time you change the bit you must make new calibration blocks to match the size of the bit.
The router. I am assuming that a plunge router will be used. It is the only safe way to make stopped dados.
Note: Definition of a safe place. A place where you will never be able to find it the next time you need it.
Don't make it that safe.
As long as you continue to use the same router bit you won't have to move the calibration stops.
Note: You might not have it positioned as described above but upper left refers to the way the jig has been oriented for all descriptions.
Move it along the fixed fence until it reaches the right end, move it down to touch the movable fence, and move it back to the left. Move it up to cut away that last little piece.
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