Monday, January 6, 2014

Can't See the forest for the Trees (V)

The previous post concluded by looking at how a log is typically sawn up: first into a squared-up cant, then sawn through and through to produce boards. Only a few of the boards produced by this method have truly straight grain, most will have some degree of grain run out. This method does produce a decently high yield of timber for relatively few cuts and log maneuvers however, and is thus cost efficient. That's why it is the most common method seen in cant cutting.

I came across a video for a new(er) and rather massive Woodmizer mill which illustrates the aforementioned cutting method perfectly. This mill, the WM1000, is designed specifically for sawing up large diameter logs:



This is  pretty grand piece of equipment and features some nice hydraulically-operated options which make handling a large log quite easy - beats hanging off a peavey any day. It also costs in the neighborhood of $55,000, so it probably won't be showing up in everyone's backyard anytime soon. A hardwood supplier in South eastern Massachusetts, Berkshire Products, recently acquired one of these mills, and that's where I first saw it.

There are other approaches to sawing a log up when the goal is to produce boards with very straight grain. With this, and some other methods, a greater amount of 'waste' may be produced, however it should be noted that 'waste' is often a value judgment, generally based on profitability as yield, or, at the very least, for the perspective of what humans can get out of it. Sawdust and wood offcuts may of course be converted into fuel, and even if left on the ground are food for many creatures. Wood is part of the natural carbon cycle and a tree left entirely alone by humans eventually dies and decomposes on the forest floor where it is of great benefit to many other organisms. Most cases, I don't tend to get overly-exercised by the idea of 'waste' when it comes to sawing logs or cants or boards up into smaller - I'd rather try and obtain the highest quality material I can as an operating principle, than to saw for volumetric conversion efficiency. That said, as far as 'waste' is concerned, a thin kerf blade is highly desirable when trying to squeeze every last bit of good material out of a cant of quality material. As a final caveat, it is certainly a shame when high quality material is sawn up into firewood, which has been known to happen from time to time, though IF it is a question of freezing to death or staying warm, that point admittedly becomes rather moot. Such was that fate of some fine pieces of classic Ming furniture during the cultural revolution in China.

Alright, before I wander too far off topic then, let's look at another way to saw a log. Of course, there are so many potential situations to consider and so many variables in terms of the logs themselves, which may not be especially cylindrical, or may have defects which need to be sawn around, that the cutting plans I suggest here should be taken a generalizations only. There's no substitute for experience, and anytime you open up a log there are uncertainties, things to be discovered which may or may not be good news. I can't possibly consider every variable, so please keep that in mind.

Here we have our log on the mill deck, and this time we have shimmed the log so that the outside of the trunk, the upper surface, is parallel to the cut:


Our first cut will be to slab off a chunk of sapwood from the upper surface, just as was done when preparing a cant with the log adjusted so that the cut followed the pith (shown in the previous post):


Again, though it is mostly sapwood, with a large log there's likely to be at least a usable board which could be cut out of that slab, but we'll set it aside all the same.

Now, for comparisons sake with the process shown in the previous post, let's say we still have the goal of obtaining a nice quartersawn plank out of the widest part of the log. Above that plank, we are going to cut a pair of larger slabs which will be re-sawn later into timbers:


Notice the straight grain visible on all the cut faces.

Removing those two slabs and setting them to one side, we arrive at the widest part of the log, and we look to obtain that nice piece of quartersawn material. We pull out the shim at the front of the log and swap in one which is about half the thickness, making the line of cut now parallel to the pith, as you'll see on the following sketch:


I've left the old thick shim to the side so you can more clearly see the comparison to the new shim.

We then slice off a piece to adjust the surface so that it is also parallel to the pith:



Notice that the waste piece is a large triangular slice. That is the form of waste piece that will be produced characteristically with this method. In this case, there might be enough meat in that slice to obtain some quartersawn board stock, if we were to re-saw the piece once again. At the very least, a shorter board could likely be obtained.

Next, we slice and obtain our quartersawn board of perfection from the log:


The same cut process can be repeated if desired with the other half of the log if so desired.

Returning to the two large slabs we off cut previously, we can now slice them. In this case, the choice is to obtain mostly square section timbers, mostly 7x7's, from those two slabs, though of course they could be re-cut for a variety of other shapes. Here's the result, including the lower quartersawn board:


Note that I have swung the camera around in the above sketch and we are now viewing the butt end of the log. Notice also the wedge-shaped waste pieces at the center of each slabbed piece of the log.

Next, the waste pieces are separated out and placed above the rest, so we can see what remains more clearly:


Most of the scrap pieces contain a high percentage of sapwood, and the center piece contains the pith, so they are not desirable, except to insects and fungi. There are two tapered boards which came from the middle section of the two large cants, and these may yield another slender quartersawn board or two. I think you'll agree that the quality of the boards obtained, lined up at the bottom portion of the sketch above, is very high.

In the previous post we looked at two methods of sawing:

  1. Sawing through and through
  2. Cant Sawing

I've been using the terms 'sawn to the outside' (parallel to the bark) and 'sawn to the inside' (parallel to the pith) in this series. Those are colloquial terms, in the same way that 'live edge' is a stand-in for 'waney'.  The technical industry term for sawing to the inside is "split-taper sawing", and the term for sawing to the outside is "full-taper sawing". The reasons for those two terms should be clear from the processes and set ups illustrated so far in the last few posts in this series.

In this post and the previous, I have gone about breaking down the log in a somewhat academic manner, and the patterns shown are by no means representative of the dizzying array of options which are out there. There are in fact some 8 different standard cutting patterns.  Through-and through is by far and away the most prevalent. Here are the eight patterns, for reference's sake:
  1. Live-sawing, split-taper
  2. Live-sawing, full-taper
  3. Cant; split-taper-split-taper
  4. Cant; full-taper-split-taper
  5. Cant; split-taper-full-taper-fixed fence
  6. Cant; full-taper-full-taper-fixed fence
  7. Cant; split-taper-full-taper-variable fence
  8. Cant; full-taper-full-taper-variable fence
'Live' sawing, as you may have gathered, is when you do not first slab down the log to make a cant. After the distinction between 'live' and 'cant' sawing, the next term, full-taper or split taper, describes the initial cut to produce a working face, and how the log is adjusted.

Those eight methods are illustrated and compared in the 1976 US Forest Products Laboratory Publication, Is there a 'Best' Sawing Method? which you can find online and download if you like. The methods illustrated are representative of a vertical sawing mill:


 The smaller circle we see is the skinny end of the log, closest to the mill head. The study generally concerned 'smaller' mill logs, those from 5" to 20" diameter. The study considered 'best' as being the method which produced the greatest yield. The conclusion was that method eight, sawing a cant and then breaking it down as 'full-taper-full-taper-variable fence' gave the highest recovery rate. The worst average recovery rates were accomplished using live sawing, split taper. Since this study was intended for the sawmill industry, where a consistent cutting method used over many logs is employed rather than making special individual considerations for each log to be cut. so, the recommendations from this article come less into play than it might with someone re-sawing their own material for personal use. Still, cant-based cutting, accommodating for full taper seems to make sense for both quality and quantity reasons.

Lastly, I'd like to share a picture from a Japanese book I have on log selection and sawing, showing the cant-based, full taper method employed there in many instances:


You can see three different possible destination for a log. At the top, the log is flatsawn for boards, as might be used for roof eaves, etc.. In the middle, we see that the log is to be divided into posts  (), and is considered akin to a 4-sided pyramid in form. To the the right of that we see the waste portions, here designated as useful for making chopsticks, etc.. Finally, at the bottom we see one potential treatment for using a bowed log as a beam, slabbed on two sides. There are other treatments besides, outside the scope of this posting.

And the method I illustrated in this post for breaking down a log into square section timbers, is quite similar to one termed hachi-men-biki (八面引き), or "eight-face drawing" in Japanese:


 Of course, how a log gets sawn up depends hugely on what sort of material you are looking to obtain. The above method shows a log sawn up for posts. One might equally saw up a log exclusively for thin boards, and in that case there are several choices for how it might be cut. Here are a few options in Japanese sawing:



And it is often the case that a log may be sawn up to yield both larger posts as well as thinner boards, and shapes in between.

There are many possibilities in terms of how a log might be cut, and I can't cover them all there. Just wanted to touch on the topic, and show the advantages to sawing to the outside is all. I hope I succeeded in getting that point across.

I'm thinking to do another post or so in this series, dealing with lumber selection relative to how it may move in service, which is a related kettle of fish. I hope you're keeping warm during the cold snap - the polar vortex - engulfing most of North America right now.

Thanks for coming by the Carpentry Way. Comments always welcome.

4 comments:

  1. Thanks Chris for this series, it addresses a dimension that I've not seen discussed before (taper along the axis) in woodworking books. Your CAD skills are quite impressively shown here to demonstrate your points. Excellent work!

    ReplyDelete
    Replies
    1. JMAW,

      I appreciate your comment and glad you found something of use in this series. I think CAD is a helpful tool in trying to explain the effects of various cuts on an idealized virtual log, so I'm glad to live at a time when such technology is cheaply available.

      ~C

      Delete
  2. This has been an interesting, but I am afraid, academic exercise. The issue(s) I find are: Very few logs are straight; and even splitting (hardwood term: riving) a Western Red Cedar of 10 or more feet length will create wandering and twisting planes of grain. And, this excludes the delight the trees show in torturous contours you see in end view for several inches under the bark. Then, the real killer is opening up the core to expose massive buried limb grain every couple feet.

    But it is fun to review the sawing methods.

    Bruce

    ReplyDelete
    Replies
    1. Bruce,

      thanks for your comment. True enough, this look at sawing has been an academic one, and I thought it was worthwhile to take such a look. There is theory and there is practice, and you need a bit of both to have a balanced perspective.

      Most of my bandsaw mill experience has been with softwoods, and generally the trunks are quite straight. Obviously, depending upon what sort of trees you saw, the conditions can vary quite a bit. There seemed to be little point in attempting to cover every potential circumstance faced when dealing with logs of various species. Surprises do await, as you note.

      I did want to show that the most common way of re-sawing does not yield much material with straight grain, and that is the case largely because live sawing through and through is treating the log almost like it was without grain, as if it might as well be plastic. And if little attention is paid to grain alignment when sawing perfectly straight logs, then it is unlikely to be any better when the form of the log wanders a bit away from straight.

      ~C

      Delete

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