Thursday, April 25, 2019

Log Rivening

This is a bit of the out of the box type of work that i often engage in.  It is all around the economics of wood manufacture.  It is inspired by a YouTube video in which medieval log revening was replicated by a team using their methods.  What reallystruck me is that the yield of rafters, properly hewn as well is eight rafters compared to a possible four rafters using our methods.

This problem of modern productivity has been an open question for me for decades.  It turns out that any log passing through our cutting system does several things;

1]   Half the material ends up in the chipper
2]   Real strength is variable, as is warping and twisting.  All adds additional post manufacturing losses.  For this reason boards are generally used only as cladding or for short length reinforcement.  Even robust two by fours are uneven in strength and must be watched.
3]   The heartwood or outer shell is completely wasted in all but the largest logs while we fully utilize the weakest wood in the core.  What heartwood that is preserved is also ill positioned in the plank for providing more strength and general integrity.

As a purist, this always bothered me.  You take a remarkable piece of natural engineering and you cut it into a lot less lacking even reliability unless you over - engineer your structures.

Now a saw log can easily be spun to remove the bark and even shaved clean with little wood loss.  We do it all the time in preparation for veneer manufacturing.

That same log can be advanced back and forth over rollers and easily re-positioned in  terms of angle.  What we do instead of actually cutting is to use  roller blades to split about an inch deep the full length of the log.  In fact we can make this automatically  and do it back and forth while rotating the log a measured angle.  Chewing out a deep gouge may make sense as well to remove any overage in widths while providing ample room for release.

You then do it over and over until you have reached the desired depth..  At that popint you actually split the log down the center. Then you use the roller blade to split off the individual planks from the center core which my well be only six inches across.

What you are producing is a set of uniform planks with a tapered cross section.  You may actually only use this technique on the outer ten inches of the log in practice.  The remaining split core can be additionally split or even sawn depending on need.  My point is that all this preserves all the heartwood the full length of the produced plank.

Those planks can be easily planed on one surface at least to leave the rest rough depending on intended use.  Again planing and sawing will produce other useful material if you want a fully squared plank.  You will still be using all the heartwood as your feedstock and wastage can be nicely minimized.  Better yet, you will be producing a maximum strength plank consistently from each such plank.

This is a huge improvement in productivity from a saw log.    Starting with a three foot diameter saw log and splitting forst to the eight inch depth mark we reduce the diameter by 16 inches leaving a twenty inch log able to produce further boards and planks.  Been pith wood, i would simply use it to produce boards and that should reach a high level of productivity with little wastage on the edges.  The heart wood shell should produce almost thirty planks three inches on the outer edge and around two inches on the inside edge. This means at least thirty  two X eight planks of the highest quality.  Wastage should stay around twenty five percent.

I should also add that once we have our log prepped and mounted in the processing bed, it also becomes practical to actually use circular saws as well to cut boards from the heart wood directly.  all this creates a high yield of heartwood based lumber.

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