Sunday ToolFoolery - A Pair of Japanese Marking Gauges (Kebiki)

After completing a large shoji screen last year, I vowed to make myself a couple of Japanese marking gauges that were robust enough to lay out large tenons.  A quick trip through the Japan Woodworker catalogue gave me some inspiration about the design, as well as a source for the razor-sharp marking knives that are the business end of this contraption.

First of all, I'm no tool fetishist.  I believe that Wharton Esherick said it best when he explained that he would "use any tool that'll do the job.  If I have to use my teeth, I'll use my teeth."  That being said, I do like the look and feel of a nice tool and I've never regretted it when I spent a few extra bucks on a high-quality item.  But tool-making for me is a busman's holiday, and I take a fast-and-loose approach.

Marking gauges (kebiki) are deceptively simple.  An arm with a marker on the end slides through a fence.  The arm is locked in the fence (thus locking in the distance of your knife mark from the edge) by means of a wedge.  I chose quarter-sawn white oak for most of the gauge because it is tough, and walnut for the wedge as it is softer and that little amount of give should help the wedge to stay in place.  I also think that walnut and oak is a really nice combination.

I also made it a bit chunky, as I wanted to be able to whack the pieces on the bench in order to loosen the wedge, adjust the bar, and lock the wedge with three easy taps.  Given the tough and stringy nature of this piece of oak, I opted to use a router to cut the two overlapping slots that hold the arm and the wedge - 1/4" and 3/8" respectively.

This also assured that the arm and wedges would protrude from the fence at a 90 degree angle. (The hand-tool gods had their revenge for not morticing by hand -- they took a big chip out of my 3/8" bit.)  Now use a chisel to make a flat shoulder where the larger arm slot meets the smaller wedge slot.

The Wedge

The wedge slips into the smaller slot.  It is important that the wedge is angled at less than about 12 degrees in order to get the right purchase.  It set this angle on my bevel gauge and cut the wedge accordingly and cut it with a coping saw.



After a bit of spokeshaving and sanding I was pleased with the result.  I then moved on to adjusting the angle of the slot in the fence.  Using the same setting in the bevel gauge, I cut a guide block to insure that the angled surface of the wedge would ride against a matching angle in the arm slot ensuring maximum contact. Now, regardless of where the wedge is placed in the slot, it will be perpendicular to the fence.

The Fence

Since I ganged up the two gauges on one board, now all that is left to do is cut out the design.  I didn't push the boat out too far with this as I went with the traditional (I think?) Kebiki outline.  Once rough shaped, I popped it on the bench for a little spokeshave action.

The Arm

The fit of the arm, the profiling of the rounded edge, and the placement of the knife in the arm are the most important parts of this project.  I first brought the thickness of the arm down (by hand) to 3/8".  I want there to be some friction as it moves, and as I've yet to round over the edge, I checked it by sliding one corner into place.  The edge rounding was done with a block plane -- a #3 round plane, a router bit with a 3/16" radius, or a scratch/bead plane could be used.  When the arm could inch along the fence, I considered it done.

To install the knife, I started by drilling a 1/4" hole, one inch from the end of the arm.  This allows you to cut a slot on only the edges the hole to fit the knife into place.  Here is where you will use geometry and physics to your advantage.

If you angle the knife by say 2 or 3 degrees (facing the arm as you will use it, this means the knife slot will angle toward your left shoulder -- for righties) the cutting action of the knife will pull the fence tight up against your work.  I cut this slot with a naked coping saw blade.  Proceed slowly, with the knife in hand.  You will only need to cut the slots a bit to guide the blade.  Insert the blade and give it a couple of taps so that it protrudes by about 1/16".  This friction fit will hold the blade in place, but allow you to remove it for sharpening.

A couple of test cuts show that it holds its line, even with the grain. The lines are deep enough to aid in making those little angled chisel cuts that allow you to get precision lines when sawing tenons by hand, and great shoulder lines on dovetails.  And having a couple of them lying around will save time resetting gauges.

If you have any questions on the X's and O's of this -- or have a better way to do things -- just make a comment.  Thanks.