Tuesday, December 31, 2013

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!

6 comments:

  1. Thanks for a good series of posts so far. I just checked all four of them.

    I bought an own old saw mill a couple of years ago, I like the idea of being able to take part in the entire process. I sometimes do the logging myself as well, but normally only if it is a single or a couple of large trees. The majority of my logs are delivered to me from a forest managing company.

    I normally adjust my log so that the cut aligns with the bark. My main saw mill is a circular saw type, so it doesn't take any shims to do that. You just align the log and saw away. here is a link to how I do it:

    http://mulesaw.blogspot.no/search/label/Saw%20mill

    I have an older mule saw that I am trying to get going. It is more like a horizontal band saw, except is a reciprocating blade. It is very slow, but I find it interesting because it is the first real mechanisation of saw mills.

    Regarding the quality aspect, I find it very satisfying that I don't have to meet any fixed efficiency goals. If one log delivers 50% usable timber, well then it is OK with me. I can use the off cuts for fire wood and the saw dust goes to the stable.

    Brgds
    Jonas

    ReplyDelete
    Replies
    1. Jonas,

      thanks for your comment and I enjoyed looking over your blog. As my posts in this series are somewhat generalized, I haven't addressed some of the differences that arise when using a circular saw type of mill instead of a bandmill, and I thank you for pointing that mater out in regards to the shimming.

      A mule saw seems pretty much like a powered version of a pit saw - it must be a fairly old machine, no? I've seen pictures of gang cutting version of such a saw in turn of the century equipment catalogs.

      ~C

      Delete
  2. happy new year sir, and thank you for your incessantly fascinating posts on woodworking from many, um, angles

    ReplyDelete
    Replies
    1. Colin,

      glad you are enjoying the series and thanks for the comment.

      ~C

      Delete
  3. Chris,happy new year. there are plenty times you would part the log in the center as a first cut. You may be quarter sawing, you may have defect on one side of the stick your trying to avoid, and if your cutting thru and thru , especially on a woodmiser, you might start that way so the piece will reference on the flat reference surface from that parting cut. Parting in the middle also let's you see what's going on in the interior that may tell you how to proceeed. Where as you cut straight down thru the top and you get what you get.
    Using the levelers ( instead of shims) you can then easily situate the log half on the mill for parallel to grain cuts.
    Other things come in to play like holding the log in place. In the field, a wood miser is very versatile but you will use many tricks to hold odd shaped pieces that the machine can not do on it's own. You will sometimes use shims to level it but only for short sticks less than 6', depending on the mill of course. Many of the local hardwoods here yield short log sections so shimming one end or both is common.
    Edging on a wood moser even wide boards is done as you pictured but some times you will use an auxilary board between the dogs and the log or board for greater support.
    The log is loaded with machinery that easily handles removing the off cut half log from the mill. After your first cut the roller arms will roll the log and the top half falls away back to the deck to be processed next.
    The wood miser can cut to 1" above the bed rails on the mill and the band does NOT get bound in the kerf ordinarily. The slabbing mills I have seen and used are slower in feed speed and the bar is 4" wide so you need to shim the kerf to keep from pinching the bar. Actually many times as you cut thru the you will see tension released in such a way that the kerf opens on it's own. That's some intenseforce when it lifts the whole half of the log 1/2" until it get to the end of the cut.
    It's interesting that different wood you might decide to cut off center if the pith is exceptionally large. Knowing ahead of time that it will spit in the center. Often mills leave the pithy junk on the edge of the quarter sawn boards. It is junk though most times.
    How you handle and break down the log will depend on if you do have an edger or not. Many of the small mills here edge on the woodmiser. Edgers are not so overly common.

    Good over all depiction of the process though. Must be many hours of work to draw all this stuff.
    If you could turn it into a video game you could corner a whole new market. Maybe Lumber Jack and Jill Sims? Could continue right thru delivery to the shop for virtual furniture making!
    pax
    Correy

    ReplyDelete
    Replies
    1. Correy,

      definitely some good points there, and I have amended the text in a few spots to insert a bit of qualifying language. There are so many kinds of mills and varying types of logs that might be sawn that I can only speak in generalities, and even at that these posts can get a bit lengthy.

      Many people have very basic woodmizer-type mills which are completely manual, with no hydraulic lifting forks, log rolling assistance, manual feed, etc., so I was taking that into account, and leaving discussion of those niceties out for the moment. Then there are the portable bandsaw mills, the chainsaw mills, etc. I happened to find a SketchUp drawing of a rough approximation of a woodmizer in the SU warehouse, so that's what I went with in these sketches.

      You're right that if one is intending to quartersaw, the log would ordinarily be split into halves and quarters first. I wanted to deal with the most common sawing practice of cutting through and through first however.

      It seems that quartersawing is pretty uncommonly done these days, commercially-speaking, except in a few species (like white oak), where that type of sawing also produces a more desirable figure as a side effect. It seems that mills which do quartersaw pretty much specialize in that way of sawing alone, and in just one species. A lot of guys I've run into with woodmizers seem to have had very little experience with quartersawing and have expressed to me fairly clearly that they see it all as a big hassle. It's too bad.

      And yes, the drawings do take a while, which is one reason for the gaps in time between posts. I certainly won't be the one pushing ahead with a 'sims' version anytime soon! It would be fun.

      Thanks for your comment and Happy New Year to you too.

      ~C

      Delete

All comments are moderated:

Anonymous comments will be deleted

Spam will be deleted

Comments containing links unrelated to blog content will be deleted