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

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

    
In the previous post I delved into the topic of log scaling to a certain extent, and now want to move on to look at the sawing process. Just as there are various ways to scale a log and compute the volume and/or board footage, there are various ways to saw up a log, and pros and cons for each. Again, I don't speak as a professional sawyer, so my knowledge is limited in that respect. Still, nearly every woodworking project involves sawing material at some step, and having some grasp about how a log is cut translates into better understanding and planning when assessing a board to be cut.

I'm going to assume the log we will deal with is a tapered, truncated cone, or frustum. The  model tree is 48" at the base and reaches 100' high. This is the sort of taper one might see with a cedar tree. Other trees are less tapered, but using a stronger taper shows the results of the cuts more clearly, I think, so that's the direction from which I'll proceed.

I drew a tapered cross section, made it into a cone, and then chopped the bottom 10' off:


The resulting cut log tapers uniformly 2.4" each side, over 10' length, working out to about 1.15˚ per side:


I don't think there is anything especially unusual about this section of log, save for its large size and geometric perfection.

There are various saws with which we might slice up such a log, and we could make the cuts in a horizontal orientation or a vertical one. There are bandsaws that cut each of those directions, or angles in between for that matter. Sans power, there is pit sawing, where the cuts are made using a long handsaw, either by one person alone, or two working in tandem, top dog and under dog fashion, or two working separately:


Or one could sit by the side of the log and rip it horizontally as well.

So, for purposes here,  I'll simply assume the cutting will occur horizontally, the sort of thing that could happen on a Woodmizer or chainsaw-based portable mill, bearing in mind that the various ways of cutting we'll look at could equally be done on a machine which saws vertically.

Speaking of the various ways of cutting up a log, there are many, and most methods developed revolve around maximizing yield. we're going to considerably simplify matters though, and ignore yield altogether, focusing in on cutting the same log in three different ways, the aim being to cut three different boards. Those boards are, specifically, flatsawn, riftsawn, and quartersawn.

In cutting method one, we take the log section we have, and rest it on the deck of the mill 'as is'. This is an exotic softwood, heh-heh, having alternating green and white growth rings.

The three cuts we looks are are labeled on the end of the log, as A (flatsawn), B (riftsawn) and C (quartersawn):


In cutting method one, no compensation is made for the taper of the log. The log lays on the mill cross members 'as is'. The whole log, relative to its pith, is tilted down 1.15˚.

In the above view, the narrow end of the log is facing us, as this is the end at which cutting is always originated, proceeding down the tree trunk to the butt, for obvious reasons.

For board A, we would make a preliminary slabbing cut then follow it up with a second slabbing cut 2" lower, which would produce a waney board like this:


The 'cathedral' pattern on the face of the board is probably what most people think of when they think of 'wood grain'.

That 2' slab is then edged, the cuts run in alignment to the pith of the log, and a 2"x12" board is produced. I'm choosing this size of board for illustrative purposes - a wide board shows the grain patterns that result from sawing more obviously than does a narrow board.

We can execute a similar set of cuts for the other two boards,  B and C, slabbing and edging, until we have produced the desired three sticks, A, B, and C:


Let's set those 2x12s aside for the moment and start all over again from the beginning with the log. This time we will make some adjustment for the log taper, such that the very center of the tree, the pith, is aligned to our cut direction. The pith is therefore horizontal. We can call this 'sawing to the inside' of the log. Some sawmills have built-in devices to raise or lower one end of a log, and if we were cutting a long log we would need to look at supporting the log at intermediate points as well so as to maintain a straight section throughout.

Again, we see the three cuts we are after, which I will label A', B', and C':


Note the presence of a shimming board under the narrow end, roughly a 2"x4". To keep the pith of the log level with the sawmill deck, the log is tilted up 1.15˚.

We take the three cuts as before, slabbing and edging, the edging being done in alignment to the pith. That leaves three boards, A', B', and C':


We'll set those aside and start again.

The last cutting alignment we'll look at also has an adjustment for taper. This time though, instead of shimming the log so that the pith of the log is parallel to the cutline, we shim up the narrow end a bit more so that the top of of the log is parallel to the cut line, i.e., horizontal. Our three boards are now A", B", and C":


As you can see, the shim used is much thicker than when we cut in alignment to the pith, twice as thick to be precise, a 4"x4". Now the lower surface of the log is tilted up 2.3˚ relative to the deck of the mill.

Previously, we cut to the 'inside' - now we are going to cut to the 'outside' of the log.

The three boards are produced once again, labeled A", B", and C":


Now we have produced boards which are flatsawn (A, A', and A"), riftsawn (B, B', and B"), and quartersawn (C, C', and C"), we can compare them to see how the run of the grain compares. First, we look at the A set, flatsawn:


As you can see, the log sawn without regard to taper produces a flatsawn board with considerable grain run out, and numerous cathedrals on the tangential faces. Here's another view of the same three boards so you can see the run of the grain along the board edges, along with the pith-facing portion of the board:


Again, the log sawn without attending to taper produces the board with the most angling of the grain. The board sawn so along the pith line, A', exhibits less grain run out, while the board sawn to the outside of the trunk, A", exhibits virtually no grain run out. The alignment of the grain perfectly along the long axis of the stick makes the board sawn to the outside of the tree, A", the strongest of the three were they to be used as posts. If the above sticks were to be used as beams, again A", with the grain running straight down the stick, should bend the most evenly and predictably of the three I would think.

The other two methods of sawing, A' and A, produce some amount of grain slope on the faces of the sticks. While the end grain view of the three sticks is very similar, you can clearly see the degree to which the grain runs at a slope along the sticks by comparing the narrow edges of the boards.

The B set of sticks were all sawn in a rift grain orientation when viewing the end grain. Let's see how they compare:


Again, the sticks sawn with accommodation to taper, B' (to the 'inside') and B" (sawn parallel to the 'outside' of the log), show the least amount of grain slope, with B" showing the least amount of grain slope of the two.

Here's another view of the B set:


Finally we can compare the sticks which were quartersawn, C set:


Here, given that the grain runs 90˚ radially, the effect of sawing for taper (or not) is minimized, however it is still clear the sawing to the outside of the trunk, as in C", produces the least grain slope of the three cuts.

Another view:


We'll look more at 'sawing to the outside' in a subsequent post.  There are various reasons why we might want straight grained timber as a result of sawing practice - better strength, which is as critical in a dining table as it is in a building column, is one important reason, and unless sloping grain is desired as an aesthetic, conveys the line of a frame more clearly than does sloping grain, which, if extreme enough, 'fights' the lines of the piece.

Thanks for coming by the Carpentry Way.