Mini-workbench: Part 4
Part 1 here, Part 2 here, Part 3 here
Marking Mortises in Miniature
Try saying that 3 times fast! Accurately marking mortises in miniature presents some challenges because the dimensions are so small. The Roubo workbench base relies upon 8 mortise and tenon joints to hold the legs and stretchers together. When building a full-size bench a standard marking gauge, or maybe a special mortising gauge is used to mark the mortises but such an approach doesn’t work all that well in miniature.
It is possible to use digital calipers or even a fine scale to mark out the mortises but I think I have a better way. My twist on the old adage “Measure twice, cut once” is that it is always better to measure once and mark as many times as necessary than it is to measure each time you mark.
This is because measurement in woodworking is more a matter of accuracy than it is about precision. Huh? Let’s look at the mortises. There are 2 of them on each of the four legs. You want all of them to be the same distance from the floor. In this case it’s 5″. And so I ask you, is it more important that they be 5.0000″ from the floor or that each of them be exactly the same distance from the floor? This is the difference between precision and accuracy.
And so, if you measure 8 mortise heights separately, you’ll have to make 8 measurements, each with the potential for tiny errors. But if you measure once and set all of the mortises this height, does it really matter if you measure 4.95, 5.00, or 5.05 when you do that measurement? I think not.
And thus, we will build a jig to set those dimensions. I started with 3 little pieces of wood one was a scrap that I picked up off the floor. Yeah, I know…I’m messy.
The larg piece is of no important dimension. The width of the skinny one is the distance from the front of the leg to the mortise (1 1/4″) while the length of the other piece is the distance from the floor to the mortise (5″).
The first task is to glue the two dimensioned pieces together. Cut the other piece into two pieces and create an L-shaped piece with it, like this:
Then glue these two sub-assemblies together, centering the dimensioned piece in the L of the other piece. You’ll see why “real soon” but don’t worry about being exactly centered. Just make sure there is some of the L on each side.
Ok, the jig is finished. Let’s do some mortise marking. I marked all of the legs with inventive codes like RF for right-front. I put that mark on the face that will point towards its end of the bench. In this way, I orient the tenon on top of the leg and by positioning the leg with its mark pointing outward, I know immediately which sides need a mortise. If you’ve never cut a mortise into the wrong side of a leg, you aren’t a woodworker. I think it’s a rite of passage.
You’ll find that one of the mortises will have its leg edge to the left while the other one will have the leg edge to the right. This is the reason for the jig being centered in that L as we can use it as a two-sided jig, like this:
In each location, use the jig to draw the bottom and one side of the mortise location.
It’s not hard to fill in the rest but what I did was cut a small piece of wood the size of the mortise. I stuffed that into the jig and drew the other side of the mortise like this:
And you know what? You can accomplish this in less time than its taken me to describe it (grin). Next time we’ll start cutting mortises.
Cheers — Larry
Full Chisel Blog
Logan Cabinet Shoppe
Renaissance Woodworker
Tom Fidgen's Made By Hand
Village Carpenter
Wood Whisperer
Fine Woodworking Magazine
Popular Woodworking Magazine
Wood’n Bits
Comments