Interlocking dado shelving

[scott vroom]

I recently built a wall mounted shelving using interlocking dado joinery. I don't have a picture of the finished product yet but will post one later after the finish has dried. What I do have are a couple of pics of the miniature prototype I first built to confirm the design....those are attached below.

The problem I encountered with the interlocking dados are small gaps, clearly visible in the assembled proto. I was able to tighten up the dado cuts on the final piece, but was unable to eliminate the gaps completely without making the joints so tight that I had to dead hammer them together, which caused some visible scrape marks on the boards. The initial project was paint grade so the presence of wood filler was not a problem. I'm now preparing to build several similar design stain grade shelves and am looking for advice on possible alternative methods of joinery that would completely eliminate the gaps, or suggestions on how to modify the interlocking dado method to reduce or eliminate the gaps (without having to hammer the pieces together).

Here are 2 pics showing the joinery and the gaps....please keep in mind this was a miniature proto-build and not the finished product:

[Michael Dunn]

How did you cut the dadoes to make the half laps? If the cut is square and cut to the same width as the thickness of the wood you should have no gaps. Table saw with a dado stack would be work unless the wood is wider than the dado stack can cut. Or you could cut them on the band saw using the fence to make a straight cut. I made a similar project on a much larger scale. The large 10' boards I had to use a jigsaw and cleaned up with a router.

[Dave Richards]

A common solution for hiding the gaps is to cut a shallow dado on each side of one board and cut the notch narrower on the other piece to accommodate the dadoes.

[scott vroom]

Michael, the shelving is 4-3/4" deep therefore I was able to make the full depth cut with my 8" dado stack in one pass on the TS (the first project was poplar so a single pass was easy; next phase will be QSWO so I expect multiple passes will be required to reach max depth).

Clearly the key is precision. On the finished full sized shelving just completed, I cut the dados into fully milled boards that were just barely thicker than the dado width. This allowed my to "sneak up" on the fit by running the dadoed boards through planer until a tight fit is achieved. This proved to be a challenge as both interlocking boards had to be thinned simultaneously. Way too time consuming for a low revenue project, and still not yielding a completely gap-free joint. More practice?

[scott vroom]

Dave, thanks. I viewed the pic you PM'd me.....you're basically creating shoulders similar to a mortise/tenon joint, on one side of the joint. Still, the dados must match the board thickness to completely eliminate the gap. I'll play around with it. Would be good if you could post that pic on this thread for others to see (I didn't want to post it without permission). I believe your idea would diminish the gap potential.

[Ken Fitzgerald]

Scott......use a sled or miter gauge and stop blocks. Make the dados in two passes. Use a stop block to position the "leading edge" of the dado. Use the actual material to establish the thickness of the dado and the 2nd position of the stop block (trailing edge of the dado). Using the actual material as a gauge diminishes the chance for errors and improves repeatability.

[scott vroom]

Scott......use a sled or miter gauge and stop blocks. Make the dados in two passes. Use a stop block to position the "leading edge" of the dado. Use the actual material to establish the thickness of the dado and the 2nd position of the stop block (trailing edge of the dado). Using the actual material as a gauge diminishes the chance for errors and improves repeatability.


Ken, thanks. If I understand correctly, you're suggesting using, say, a 5/8" dado for my 3/4 slots. Make the first dado pass, then make a final pass using an actual milled shelf to set the second pass stop. Very clever and repeatable. I don't have a good sled at the present so I'll use my miter gauge and attach the first and second pass stops to my TS fence....should work. I'll give it a try tomorrow and let you know how it works out.

[Richard Coers]

What dado head are you using? If a stack unit, add paper shims to get the exact fit. Most paper is .003" thick. Also, a thickness sander lets you sneak up on a precision fit better than a thickness planer. Besides, the fit will change after you sand it anyway. Nobody said good woodworking is easy. Quality work takes a lot more than throwing boards through a machine and nailing it together like they show on TV. Take you time.

[Dennis McDonaugh]

I make the dado exactly the same size as the shelf then take a pass with a Stanley 98 or 99 to make a tight fit.

[johnny means]

I use a similar technique to build knockdown horses. I cut the slots with the board flat, using the rip fence as a stop. Set up the dado cutter and fence to cut the outside edge of the edge of the slot, but narrower than your stock. Cut all four, flipping the boards end for end, so that each cut is identical. Adjust the fence away from the cutter and repeat, widening the slot a hairs breadth at a time until you get a fit you're satisfied with. With a little patience, you should be able to get a fit right enough to not need glue