the Carpentry Way: Can't See the Forest for the Trees (IV)                                                          

Can't See the Forest for the Trees (IV)

    
In the previous post in this series I looked at the outcomes of sawing a log through-and through in three different orientations:
  1. No adjustment for log taper (cuts A, B, and C)
  2. Ajust taper so pith of log is aligned to the cut (cuts A', B', and C')
  3. Adjust taper so bark side of log is aligned to cut (cuts A", B", and C")
I used the term 'cutting to the inside' in regards to cuts which followed the alignment of the pith, and 'cutting to the outside'  for cutting aligned to the outside surface of the log.

Cutting through and through to produce dimensional lumber involves a slabbing cut and then an edging cut. For methods (1) and (2) above, the edging cuts were made in alignment to the pith, while for method (3), in keeping with the logic of the log alignment in that case, the edging cuts were made parallel to the bark side of the planks.

An analysis of the three methods showed that whether taking slices from the log to produce boards which were flatsawn, riftsawn, or quartersawn, cuts made in alignment to the bark, sawn to the outside, generally resulted in boards with straighter grain. The difference was most pronounced with the flatsawn boards and least with the quartersawn boards.

A reader pointed out in one of the subsequent comments which I received, "I would think board C' would have the least grain runout, because you are taking it from the halfway down portion of the log. When you "saw to the inside" it seems you will have best grain alignment with a shim that sets the pith level to the mill bed.". I made a reply or two to that comment and then thought it was worth a more thorough explanation at the start of this post.

Here we have the log shimmed so that the pith is perfectly parallel to the deck of the mill, and therefore to the run of the cut:


The desired quartersawn board's cut lines are indicated by the lines marked on the log.

Now we'll remove the rest of the log and consider only the board slabbed out by those two cuts:


Now, if we execute the edging cuts so that they are aligned to the outside of the log, cutting to obtain a pair of wide quartersawn planks, we would obtain the following:


Tossing aside the trimmed bark side (mostly sapwood) and the tapered center boards with pith, and voila!, here are our two absolutely perfect straight-grained boards:


The result is perfect, however, this perfection is coming about largely by looking at the situation purely academically.

Getting to that desired board in the first place has its own set of difficulties. If it were the only board we cared about, and considered the rest of the log as waste, that would be one thing, however that's not a common scenario. The common scenario is to convert as much of the log as possible into usable timber.

If we went directly to the upper surface of that quartersawn board with our first cut, we would have a situation like this:


That can present a problem, as the massive offcut on top is difficult to maneuver, and it has to be removed before we make the next cut. In any case, in making the cut so far into the log there is a chance of the log pinching the blade, especially with longer logs, so extra steps to wedge the kerf of the log open may be necessary. Removing the massive offcut from the log deck would also incur the strong possibility that the setting of the lower portion of the log would be disturbed, requiring re-adjustments, and such a large offcut, if it were to be dropped or tipped off without care, poses a hazard to both sawyer and mill. Plus, when done dealing with our quartersawn board and the rest of the log below, the half-log must be reloaded onto the mill, which is hardly what one would think of when trying to be efficient.

More likely, if the sawing were to be executed through and though so as to obtain planks, the boards would be taken as so many layers to be sliced off, one by one, proceeding from the outside of the trunk until we reached our desired quartersawn board near the log center. However, given the fact that we have shimmed the log to the pith, all those other boards we slab off, to one extent or another, would suffer from having grain run out, as noted in the previous post. So, to produce one super high quality board, we produce an inferior grade for the rest of the material. This might be an acceptable trade off for some, but it cannot be said to be a great return on the volume of wood we have to work with overall, to obtain one excellent board and have the rest be also-rans.

Another aspect worth mentioning is that of the edging. Larger mill operations will have dedicated edging saws, the boards slabbed off the log on the main mill and then transported, often by conveyor, to the edging saw. Some edging saws cut one edge at a time, however more efficiency is gained by the use of gang rip saws and the like to edge-cut several boards at once from each slab. In such a circumstance, it is less likely that the edging cuts on the slabs will be done in a fastidious manner, for each board produced. In fact, since the mantra of maximum conversion for economic reasons seems to be the main concern, you will often see lumber produced by such mills incorporating a good portion of the sapwood and the piece with the pith is kept and turned into yet another 2x4 or 2x6. A trip to your local lumber yard will confirm this.

If the mill operation is smaller, then the edging cuts are typically going to be done on the sawmill deck by the same saw, or they are not done at all and what is produced are waney boards.

If the edging is done on the mill deck, it is not done like this:


Obviously, a tall board like this would not be very stable during cutting, and it would be tedious to make it plumb and fixed firmly prior to cutting - some additional material would be needed to stiffen the board. It is not efficient to edge one board at a time generally. More typically, one would gang several boards together, and edge them as a group, and in such circumstances it is unlikely that the edging cuts on any one board in the gang are going to be in the optimal grain alignment for that board. Plus, slabbing the boards off, then reloading them on the mill to edge them is inefficient and most folks would prefer to handle the heavy green boards as little as possible if they had the choice to do so.

If the goal is to produce boards cut on on all four sides from a log, it is more sensible to turn the log into a cant, slabbing off the sapwood on four faces, and then re-saw the squared up cant into boards.

Here's a case where aligning the log so that the pith is parallel to the cut and the mill deck makes good sense - the boxed-heart timber:


In larger timbers, the slabbed offcuts may have enough volume to yield an additional board or two, but for the purposes of illustration, those steps have been ignored here.

In carpentry employing large timbers, the use of boxed heart material is fairly common. One issue with boxed heart timber is that that they are difficult to dry by conventional methods, and when they do dry, the outside dries ahead of the interior of the timber, which introduces considerable stresses. The way the timber resolves these stresses is to crack on the outside faces. If the timber has been processed into a component in a joined timber frame of some sort, and has squared housing and notches cut into the faces, then it is likely that the drying stresses will most readily propagate cracks from the corners of those housings and notches, as such 'geometric discontinuities' are stress risers

"Geometric discontinuities cause an object to experience a local increase in the intensity of a stress field. Examples of shapes that cause these concentrations are cracks, sharp corners, holes, and changes in the cross-sectional area of the object."

Cracks running through joinery can considerably weaken integrity of those structural connections, and the cracking and general distortion that results as the timber dries from the outside in may be considered a negative from an aesthetic perspective. If the presence of a heavy timber in an architectural space is considered as a show of integrity and strength, then the presence of cracks through such material, just like cracks through concrete, connotes very much the opposite I do believe.

An excellent solution to the problem of boxed heart timbers is to kerf the timber along one face, a saw cut taken down to the pith of the timber:


The kerf allows some air circulation to the interior of the timber, and as the timber dries and stresses intensify on the outside of the stick, the kerf acts as a relief mechanism. The integrity of the timber is little affected by the cut. As the timber dries, the kerf opens up into a wedge shape. This process can be assisted slightly by placing wedges along the kerf and driving them down as the kerf widens over time, but not pounded in so hard that they cause the timber to crack at the bottom of the kerf!

Once the kerfed timber has dried to the desired point, it will no longer be square in section, so it will need to be jointed and planed back to square and straight:


Then the timber can be placed in the building, the kerfed face oriented where possible away from sight, buried in a wall, or the like. If the timber is visible on all four faces, and boxed heart material is the only available option, then the wedge-like opening formed by kerfing and drying can be filled with a patch fairly seamlessly. Here's one such example:


 If the boxed heart cant is to be re-sawn further, it is simply laid flat upon the mill deck and sawn up as desired, keeping in mind that most of the boards produced will not have a perfectly straight run of grain:


Since the production of a boxed-heart cant in which all faces are aligned to the pith does not yield boards with especially straight grain, another method might perhaps be superior. We'll look at some other options next time.

Thanks for coming by the Carpentry Way, and all the best for 2014!