Make Perfect Half Laps, Grooves and Dadoes Without Measuring

Versions of this little jig have been around as long as table saws. It lets a woodworker make absolutely tight grooves without having to spend a long time measuring and setting up fences or stops, and it eliminates trial and error fitting. It’s easy to build and once you make one, you’ll use it over and over again. It’s perfect for sizing grooves and dadoes in casework, especially when building with ‘nominal’ thickness plywood, and it excels at setting up the saw for half lap joints (A).

The walnut base on our jig here at The Woodshop is about 7” long, and this jig will measure for kerfs from zero to about 1-1/2” wide. The two poplar sliding blocks have a small tab on their bottoms that slides in the T-slot in the top of the walnut piece (B), and this is easy to make on the table saw. Simply set the blade height for a little less than half the thickness of the wood and run the flat edge against the fence. Make two passes, one top and one bottom. Use toilet bolts to slide in the T-slot and wing nuts to lock them. The wing nuts can’t extend past the width of the poplar blocks.

To set up the jig, lock the block on the right in the picture so it extends past the walnut by EXACTLY the thickness of your saw blade (C). The line is there to visually make tiny adjustments, in case the lap joint or dado is a hair sloppy or too tight. Slide the other block so the gap between the two poplar blocks is exactly the same as the width of your part. That’s it.

To use the jig, set the stop on the miter gauge (just clamp on a block if you don’t have one) so that when the jig is up against the stop and the work piece is up against the jig (which is lying on its side, D), one edge of the cut is lined up with the blade. The work is touching the poplar block. Make the cut. Now stand the jig upright, slide the work to the left so that it’s touching the walnut block (E) and make the second defining cut. Now just nibble away the waste between the two cuts and check to see how the joint fits. You can use this jig to set up grooves and dadoes in plywood in exactly the same sequence.

If you’d prefer not to make the jig, we found small and large quite sophisticated versions of it on the Bridge City Tool website for $49 and $79 respectively.