Cutting accurate fitting dados can be a time consuming and frustrating task. For example, the actual thickness of 3/4" plywood can be as much as 3/64" less than the nominal, or stated, thickness. This means multiple setups and test cuts if done on a table saw or buying multiple, specialized plywood sized router bits. However, using this easy to build router jig makes the job fast and easy while providing great results. If you are fortunate enough to own a router, this is the way to go since this jig will cut dados of up to a 24" long cabinet size.
Step 1: See the Results for Yourself, Then Download the Plans
These photos provide proof of the accuracy of this jig. If you like what you see you can download a free set of plans by googling: adjustable dado jig - woodsmith shop. An online PDF file will provide easy to follow instructions.
So, why this Instructable? Simply because a few of the steps present potential difficulties that can be avoided if you know what to expect ahead of time. I've outlined a few of the problems I encountered in the following steps along with an explanation of how I solved them. If you decide to build this jig you may find them helpful.
Step 2: Basic Assembly - Issue #1
The instructions in the Woodsmith online plan call for building the body of the jig as a single piece (as seen in the first photo) and then ripping it into equal halves. However, I found a problem with this approach.
A good glue up requires lots of clamps and the instructions don't mention this.
Since hardboard is slippery, gluing the layers together with good adhesion over the entire surface area can be difficult and you'll need a lot of clamping pressure at as many points as possible to obtain the desired result.
Even though I have a fair number of clamps I didn't feel I had enough to span the assembly as called for in the plan (photo 1 looks better than the actual result), so I made a second jig by building each half as a separate piece. Photos 2 thru 6 show my glue up and use of clamps. I was able to clamp each half of the jig in one step by positioning the pieces as seen in photo 6.
The only problem this created was squaring the jig's guide strip (for the router) and cutting a channel for the sawdust. I solved this by taking some two sided carpet tape, attaching it to a piece of scrap (photo 7) and then sticking the scrap piece to the jig (photo 8). Then when I flipped the piece over to run it through the table saw it stayed perfectly level while making the cut.
Step 3: Installing the T-nuts - Issue #2
Two T-nuts, one located at each end on one half of the jig accept a threaded adjustment knob to facilitate proper sizing of a dado.
Installing the T-nuts isn't easy. Here's the thing, it's called hardboard for a reason - it's hard! Since the T-nuts have to be recessed in the hardboard, use a spade bit to get the hole started, going just deep enough into the surface of the hardboard to match the thickness of the nut head. Then finish drilling the hole with a regular drill bit.
Now, here is the hard part. To set the T-nut you have to hammer it into the hardboard. I did this by taking a 2" hex bolt and screwed it into the T-nut (first put a regular nut on the hex bolt to use as a stop, preventing damage to the threads of the T-nut). Next, I took a hammer and repeatedly hit the head of the hex bolt until it was properly set.
NOTE: Don't be surprised by how much effort this takes and the possibility of seeing some cracking or separation in the hardboard. I had to glue a few cracks that were the result of my excessive pounding but I don't have an answer on how to avoid this.
Step 4: Getting the Cleats Square to the Jig - Issue #3
THE TWO HALVES OF THE JIG ARE HELD TOGETHER BY HARDWOOD CLEATS LOCATED AT EACH END OF THE JIG. THE CLEATS ARE FIXED (SCREWED) TO ONE HALF OF THE JIG WHILE THE OTHER HALF OF THE JIG OPENS AND CLOSES VIA A SLOT CUT IN THE CLEATS. THE SIZE OF THE JIG OPENING IS LOCKED IN PLACE BY AN ADJUSTMENT KNOB THAT CONNECTS THE CLEATS TO THE T-NUTS ON THE MOVABLE HALF OF THE JIG.
To properly position the jig a cleat is pressed flat against the edge of the work piece receiving the dado. For this reason, if the cleats aren't installed perfectlysquare to the jig the dado will wander off line, so take extra care on this step. (My first jig somehow ended up out of square.This forced me to redo the entire project since I glued the cleats to the jig per the instructions).
SQUARING THINGS UP: I took a T-square (any decent framing square will do ) and marked 90 degree lines on my work surface by which to position my cleats. After careful alignment I clamped everything in place and screwed the fixed half of the jig and the cleats together.
On my second jig I opted NOT to glue the cleats in place as instructed in the plan. I wanted to cut a few test dados before committing to a final glue up. When I was satisfied with the results I could then disassemble the jig, add glue and reassemble. At this point in time I'm not sure if I will add glue later or not. I feel confident the assembly is strong enough without it and like knowing I can take the whole thing apart if I ever need to.
Step 5: Your First Perfect Dado
Once the jig is completed you can make dados that perfectly match the thickness of your wood. The Woodsmith plans include all the information you need to create several types of dados and grooves with the jig so I won't touch on that here. However, as seen in these photos the process is really simple.
Photo #1 - Sizing the Dado - Put the piece of wood for which a dado is needed in the jig. Then close the jig around the board and tighten the knobs to lock in the dado size.
Photo #2 - Securely clamp the jig to the board needing the dado. Make sure both are fully secured to your work surface.
Photos #3, 4 & 5 - The results after cutting the dado.
ONE THING TO REMEMBER:The jig works with only one size bit. Since most of the dados you rout will be approximately 3/4" wide, build your jig for use with a 1/2" straight bit. As explained in the Woodsmith plan, by following each router guide (in a two pass process) you will end up with a perfectly sized dado every time.
For more great woodworking tips and free plans check out Woodsmithshop.com. They have been a great help and inspiration to me over the last few years and are part of the reason I enjoy sharing Instructables with all my online friends and followers.