In my last post I discussed orientation marks. In brief summary, orientation marks, like the cabinetmaker’s triangle, tell you where the parts go in the overall assembly. However, orientation marks are not the same thing as reference marks, which are used to identify a flat, straight, and square face, edge and corner. So today, I want to go over the reference marks.
If you come from the world of wood machining (like I did), you may be unaware of the usefulness of reference marks (like I was, before I started this whole hand tool journey 20 years ago). That’s because today’s powered jointers and planers can machine boards so precisely parallel in width and thickness (at least when they’re set up well), that we can get away with using either face or edge as a reference when machining joinery.
Reference marks hearken back to a time before the power jointer and planer did the heavy milling work in most woodworking shops. When all lumber had to be “four-squared” with hand planes, there was a process that was followed to do so, and the reference face and edge were an important result of this process. However, even if the process was followed very precisely, due to the nature of hand work, there could be minor inconsistencies in width and thickness in the board. These minor differences could wreak havoc on joinery without the use of reference surfaces. However, by using reference surfaces properly, these minor inconsistencies in width and thickness would not have any impact on the joinery.
When “milling” lumber by hand, boards are first crosscut to rough length and ripped to rough width. Then, one face of the board is planed flat, smooth, and without wind. With the face planed, one of the edges is then planed straight and square to that face. These two surfaces are then considered the reference face and edge, and are marked as such. The face is typically marked with a loop like mark, sometimes called a pigtail, with the origin of the mark at the arris between the face and edge. The edge is typically marked with a “V” shaped symbol, sometimes called an arrow or a carat, the apex of which would meet the face mark at the arris between the surfaces. These two marks are generally located approximately centered along the long edge.
The purpose of these reference marks is to aid in the layout of precise joinery. When a square or marking gauge is used to lay out the joinery on a part, it should only be referenced off of the reference face or the reference edge. As long as only these two reference surfaces are used for layout tasks, the joinery layout will be square, precise, and consistent between parts. If the non-reference surfaces are used, however, there is no guarantee.
Lets take a look at an example. The board in the photos above was purposely cut with a taper in both width and thickness to simulate what a hand “milled” board might look like, though the tapers have been exaggerated for demonstration purposes. Below, I drew two layout lines around the stock – let’s pretend that these are the shoulders of a tenon. Shoulder “A” was laid out by referencing the square only on the reference face (marked with a pigtail) or the reference edge (facing down in the photo). Notice how, regardless of the taper in width and thickness, the shoulder layout meets perfectly at the non-reference corner, ensuring that the tenon shoulder is perfectly square to the reference edge and face. Shoulder “B”, on the other hand, was drawn using a different edge/face to reference the square on, for each mark. As a result, the shoulder layout does not meet at the non-reference corner, indicating that something is not laid out properly, and that the joint will not be cut correctly if these layout lines are followed.
But even this doesn’t tell the entire story. Using reference surfaces is important, but it is also just as important that the correct reference surfaces be used. My rule of thumb is that the joinery surface should be the reference surface in most cases. I have found that there are very few exceptions to this rule. For example, below is a dry assembled headboard for a bed that I am currently working on. In this situation, my reference faces all face out, and my reference edges are all the edges where joinery is going to be cut. This example is very similar to how a table would be built – the inside edges of the legs, where the mortises are cut, are the reference surfaces on the legs, and the bottom of the apron is the reference edge (the top of the apron is planed flush with the tops of the legs after assembly and therefore would not be the reference edge).
Similar principles apply when making casework, boxes, or doors. The reference face of a case, or drawer, is the inside face, because that is what will ensure a square assembly (and will not be planed after assembly). The reference edge would be the bottom of the drawer, or the front of the case. In a door, the inside edges of the rails and stiles are the reference edges and the outside faces are the reference faces.
So if you are having some trouble getting your pieces to assemble nice and square, try paying closer attention to the reference face and edge when orienting the parts for your project, and when laying out your joinery. By referencing your layout tools off of fewer, more consistent surfaces, you stand a much better chance of your joinery being more precise, and your project coming together nice and square.