Friday, September 18, 2009

Hewing a Roubo Workbench Slab

Some of you may recall that earlier in the spring, I began working in earnest (well, sort of) on a green-wood carpentry project to construct a Roubo-style joiner's bench from several large sections of a very old silver maple on my property that I needed to take down the previous year. This had become something of a background project that I'd regularly put aside for stretches at a time to honor more pressing commitments, and then return to whenever I found a spare hour or two.

Jacques Andre Roubo's bench, from Plate 11 of L'Art du Menuisier (ca. 1769)

This past week, I declared success in removing bulk wood from the large maple log that eventually will yield a slab for the bench top. A hefty cant (although I actually rather enjoy calling it a proto-slab) had finally emerged from the log:

So, on a brisk, but very sunny, early-fall afternoon, I began hewing (or, shaping) the cant:

For any one unfamiliar with the techniques and tools being described, the earlier process of bulk wood removal (scoring) was accomplished using a traditional felling axe to essentially slice away sections of wood within the boundaries of scoring lines drawn on the log. Scoring lines basically define the intended shape of the timber.

Hewing, on the other hand, also involves wood removal, but is more focused on defining and squaring-up the actual sides of the timber. This is accomplished primarily with a broad axe, which, compared to the felling axe, has a wider cutting edge, a much shorter handle, and is considerably heavy for its relatively small size. A broad axe is used, in general, to shape wood.

The photo below shows a face of the cant up-close. The face is oriented vertically, using an adjustable cradle that I had built specifically for this purpose. You can easily differentiate the hewed area on the left from the scored area on the right:

One strikes the broad axe about thirty degrees to the direction of the wood grain (not necessarily as shown in the photo above), using a series of short, well-controlled strokes. The cutting edge must be kept extremely sharp to avoid tearing the wood, as well as to minimize the number of strokes required to do the job. Since the axe head is quite heavy, you don't need to force the stroke that much; rather, just initiate the stroke and allow the weight of the head to do most of the work.

You do need, however, to use quite a bit of muscular effort to carefully control the placement and angle of the blade. So it can be rather tiring work. During the initial hewing, we're mainly concerned with clearing out the roughly scored wood and leaving just a reasonably clean surface. On subsequent iterations, we'll attempt to smooth and flatten the surface as much as possible, periodically checking it with a level or plumb-bob.

Of course, the ultimate goal is to shape the cant into a relatively flat and squared slab of wood suitable for fashioning into a precise bench top. This means that one face (the top of the slab) must be as planar as possible. The other face (the bottom) need not be that perfect, but it should still be reasonably clear. And at least one of the two lateral edges must be completely planar and at a right angle to the top so as to accommodate stock being edge-planed. So these are the requirements for true-ness that the hewing process ultimately needs to support (not to mention the countless hours of planing that will ultimately follow hewing).

In an effort to make this work as precise as possible, I cut a reference edge along one of the scoring lines at one end of the cant, and pared it down to a smooth, straight surface:

The reference edge provides a good, solid place to position a level or plumb-bob to ensure that the cant is always in the same orientation every time I return to hewing it. It's simply easier to hold the level against a squared piece of wood than attempt to align it with a drawn scoring line. Periodically, while hewing the faces, I'll check this reference edge to make sure its completely vertical, just in case the cant shifted slightly while being worked. I'll also check the area of the face I'm currently working with a plumb-bob to ensure that it's heading in the direction of relative planarity, with a parallel orientation to the reference edge.

If I happen to flip the cant over onto one of its faces, I'll likewise make sure that the same reference edge is completely horizontal as the cant lays in its cradle, adjusting or shimming the cant as necessary. I might then attempt, for example, to hew either of the edges into a vertical planar surface, or perhaps adze the exposed face to remove more material using a different group of muscles. As hewing progresses, I would expect to occasionally establish other reference edges at various locations on the cant, as well.

The photo below shows the reference edge. I used a small carpenter's hatchet to finish the edge to a straight line:

This next photo shows the hewed surface of the stem where the reference edge was cut. When you take great care to use the broad axe precisely, you can get a surface that is nearly as smooth as what could be accomplished with a hand plane. Although the photo might not do an ideal job revealing it, this is the case right in center area of the upper stem, where a rough oval of spalt can be seen. Note also the beautiful bands of colors (including the silver-blue) running through the surface of the wood. This will eventually make for a beautiful bench top when finally finished:

Yet another view of the cant is shown below. The face of the cant shown here (and in the preceding photo, as well) is destined to be the bench top, and the bottom edge (the edge the cant is currently resting on in its cradle) will be the front edge of the bench. This particular face was selected as the bench top mainly because the bottom edge exhibits far less sweep (or end-to-end curvature) than the top edge, and will be that much easier to straighten:

And since this edge will face the woodworker, it must be shaped so as to form a vertically planar clamping surface with the bench legs on that side. I also intend to fashion the smaller branching stem into a crochet for holding the far end of stock secured to the bench-front for edge planing. That should make for a very functional and also aesthetically pleasing feature of the bench.

The bifurcated area of the cant, where the two stems of the living tree had split-off from the main trunk into a Y-shape, was perhaps my single biggest motivation for using this particular log for the slab. In addition to providing a base for an integral crochet, I am anticipating that its additional mass will add to the overall heft and stability of the bench. And from a purely aesthetic perspective, the wood in such areas of trees often yields myriad unusual coloration and grain patterns, lending itself quite well to the creation of a unique piece of furniture.

So, it is for all of these reasons that I am constructing a bench top from a whole section of a silver maple, rather than assembling one from pre-fabricated slabs or milled lumber. Not to mention the fact that the tree had been in my yard for many years; hence, using it in this manner also provides quite a bit of meaning and relevance.


beth♥ said...

And... cue lumberjack music. =)

juliana inman said...

Wow - this looks like fun! You are wearing your steel-toed boots, es verdad?

Colleen said...

And I always wondered what you did to take our your frustrations! Why do I hear the Monty Pyton Lumberjack song repeating over and over in my head. :D

Colleen said...

And just for completeness


John Poole said...

Beth & Colleen ~ And you two both thought I was rugged, huh?

Cousin Juliana ~ Definitely steel toed boots! My axe is sharp enough to easily cut through boot leather if I lost my grip. ;-)

AlexandraFunFit said...

Next time wear tighter jeans to complete the picture of complete Hotness! You're quite the Stud Muffin.

John Poole said...

Aw shucks! Thanks! But you know, I always wanted to be a lumberjack... :-)