Saturday, March 14, 2009

Timbering, Part III -- Drawing Layout Lines

Now that the weather is considerably more temperate, I decided to return to my (nearly perennial) workbench slab-top project. In the dead of winter, I began constructing a hewing/bucking cradle from a simple design that was churning in my head. The cradle consists of two stationary elements braced together, two moving elements, and two pairs of opposing wedges, or chocks, fixed to the elements.

Sure, I could have notched some logs and created something simpler that would've worked. But I wanted a cradle that was very stable and also usable for logs of any size. With my design, once the log is on the fixed portion of the cradle, a few good taps from a sledgehammer drives the movable elements in, completely locking the log between the chocks. The photo below shows the maple trunk securely held in the cradle:

I did some fine tuning of the cradle the past few evenings. Basically, I shortened the chocks a bit to provide a little more clearance on both sides of the log. I also deeply counter-sank the lag screws fastening the chocks and brace to the elements, just in case a misguided axe were to come down on top of one of the screw heads. Having the heads deeply recessed in the wood minimizes the chances of damaging the cutting edge of the axe bit.

This morning, I started scribing a layout for the slab. Nominally, I want the slab to be uniformly 6" thick. But I am allowing an extra 2" on the top of the slab to absorb any inadvertently deep scoring strikes, as well as to accommodate the infinite amount of planing I am ultimately going to have to do to achieve a fair surface on the bench top. Starting at the butt-end of the log, I used a level to draw two horizontal, parallel lines, 8" apart, with the pith of the log just about in the center of the nominal 6" slab. You can see those lines in the photo above.

I then drew a pair of lines in the same manner on the two stems at the top end of the log. This is pictured in the photo below:

Of course, the stem on the left is too small to accommodate the lower line. But so be it. It will be very interesting to see how this crotch turns out in the final table. Let it be rough and funky; the slab top is the only place where I demand a precise surface.

The next step was to scribe lines on each side of the log, intersecting with the upper horizontal lines at each end. This was going to be tricky; were the log straight, a simple chalk line would've sufficed. But this log is extremely irregular, and highly concave on one side. So I tried fastening a long, aluminum angle on the extremely concave side, with each end fastened just slightly below the heights of the upper lines at each end of the log. The angle is sufficiently rigid, of course, so as not to sag, and I fastened it at each end of the log with wood screws:

Then, I took my log scribe, which is basically a compass with adjustable levels built into it, and, making sure to keep the scribe in a level plane, ran it along the length of the angle, with its attached pencil drawing a scoring line length-wise along the side of the log. This seemed work reasonably well, though it was tricky keeping the scribe level, and I had to stop every so often to either extend, retract, or adjust the angle of, the pencil to keep it in contact with the log surface:

Next, I fastened the angle to the other side of the log, and similarly scribed a line:

To make the lines stand out better (and hopefully last a little longer), I finally took a large, black crayon, and traced it on top of the lines on both sides, as well as at the ends:

The butt-end of the log is about 20" in diameter. My guess is that I will not end up removing very much material from either side of the slab; rather, adze them to eliminate any high spots and get them reasonably planer. I may attempt to make the less concave of the two sides completely straight, but I haven't quite decided yet.

However, what I am contemplating for the extremely concave side (the side shown in the photo above) is shaping yet another, smaller slab, with a straight side and the other side crescent-shaped and convex, so as to key into the concave side of the first slab. In this case, the bench top will be generally rectangular, though composed of two interlocking "leaves" (so to speak). It will be approximately 24"-26" in width, except at the upper end, where the stems of the crotch measure about 31" across.

So now the first maple log is all set for scoring, and then hewing. All I need to do is get my axes honed, roll my sleeves up, and get to it...which is much easier said than done! This last photo is of my broad axe (hewing) and felling axe (scoring), both waiting to get tuned up (maybe on Sunday):


West Coast John Poole said...

Do you have any photos (or sketches) of the support assembly without the log in it. I didn't completely understand its design based on the first few photos, as I read more I think I see how it works. Others not as well versed with wood-working may have the same problem I did envisioning how the two parallel assemblies worked.

Nice to have thick lumber (10" or 12"?) to build the support system, the posts appear to be one piece and not laminated.

John Poole said...

Hi John,

There was an earlier posting here that I wrote when I first began constructing it, but as I look at the photos there, I don't think they provide any better explanation of how it works, either.

I will post a few additional photos that do.

Regarding the four main support elements, each consists of two 6x6 pressure-treated posts, cut to 4' lengths and bolted together. The cross-brace is a 4x6 PT post, also cut to about 4 feet. This is not all that obvious from the photographs, though.

- John

Shellmo said...

I like seeing how this develops! I wish you could give my hubby some lessons!