Boxguy’s Spline Slot Cutting Jig
This hard working (and dusty) jig is used on almost every box I make, and is quick and simple to build. It has a wide plywood board for a base, a long “trough” supported by 45 degree triangles cut from a 2X6 and a handle so you can pull it back to you. (I just made the handle out of a forking branch.) The long trough lets me use this for boxes that are large or small. The wide base lets me run the jig along the saw fence. Sliding the fence over so it is snug along the jig removes all the play in the slide and makes the jig extremely accurate.
If you spread the two sides of your “trough” apart a little, your box will touch both sides, but rests on the bottom. Touching the box to the bottom board is important because it lets you set the height of your saw cut accurately using the base of the jig as a starting point for your cut. I can now set the slot depth using those long square brass spacer bars to raise my blade to the proper height by just sliding the jig forward and setting the bars on top of the jig base. (Hint: when you are making the jig. Affix the back side of the trough so it aligns with the back end of the jig, then just set a box into the trough to align and set up the front side so the box touches the jig base and then fasten the front trough board in place.) There are two important things to notice in the above picture. First, I have drawn a pencil line on the jig that aligns with and is as wide as the dado blade. Second, there is a strip of 2 inch masking tape running across the back side of the trough. More about these next…
You can see the tape and the line on the outside top of the trough here. (The mark on the trough takes little looking, sorry.) Though I have posed this with a finished box, you can get the idea.
First, I mark where I want each spline cut to be made on the box blank itself with a pencil. (In this case I would have marked three 1/4 inch wide lines for the three splines.) Second, I align these penciled-in marks for the splines with the pencil marks on the trough that indicate where the blade will cut. Third, using the box itself as a ruler, I draw a line along the top of the box and across the masking tape. This pencil line across the tape will now become my indicator for locating the box on the jig for each cut.
I draw one line across the tape for each row of splines (in this case three marks) and put a number next to each line. (If you don’t number the lines, I find it is too easy to lose track of where you are in the sequence and accidentally cut the same slot twice.)
To cut the slots I hold the box blank firmly in place with both hands, and use my body to shove the jig forward and over the dado blade. Do not bring the box blank backwards through the blade. Instead, after cutting the slot, lift or tilt the box blank up and above the blade then pull the jig back into place for the next cut. Making more than one pass through the dado blade will widen the slot slightly and you will not get a nice, tight fit on your finished spline. I cut all three slots in one corner then roll the box to the next corner and cut three more slots.
I like to use the back of the trough as a foot so I can stand this large jig out of the way on the floor next to the saw. That is why I cut the hole-handle in the base board.
This shot shows the back of the jig. Notice the maple stop at the top so you don’t push the jig too far forward with your body as you make the pass over the blade. It also shows the runner that fits in the saw’s miter slot.
It took me far longer to tell how to cut slots than it takes me to actually cut the slots in my boxes. With practice, blade height set up, locating the slots on the box blank, marking the lines on the masking tape, and actually cutting the slots with the dado blade only takes 5 to 10 minutes.
If you have questions or comments, just ask. Thanks for reading.
-- Big Al in IN