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Knots! How many is too many?

Started by nvr2l8, April 14, 2017, 08:21:44 AM

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nvr2l8

First off i'm greener then the trees i need to mill.

The stars are lining up for me. I need a sugar shack and timber frame is the only way! I have lots of hemlock and was about to pull the trigger on a woodland mill. Then a friend said "just come use my HD40" so trees are falling fast.

So I look at the butt log,  No shake, buck the the first log, second log, 3rd is started to get what I think is too many knots. Is there some general rule that the DIY'ers like me can use when sorting and choosing logs for Timbers?

ChugiakTinkerer

Welcome to the forum and good luck with the project.  Somebody with experience should be along shortly to help you out.  I'm as green as you, but I do know that depending on usage a knotty timber may be quite usable.  For a post in compression, knots and other imperfections will not be as deleterious compared to one acting as a load bearing beam.  Were I dropping logs now I'd plan on felling a few extra and perhaps try to avoid the ones that look excessively knotty.

The final timber cut from the log may look much different once you've trimmed off the rough exterior.  Also, you're going to need knee braces, purlins, etc.  You may be able to get good service out of an otherwise knotty timber by cutting it up for other purposes.  Just some thoughts from a fellow newbie.  Take some pictures of the HD40 in action, eh?
Woodland Mills HM130

DDW_OR

my advice is mill it all.
then dry the wood, the knotty boards may work for stickers or paneling
http://www.shadecustomsawing.com/drying/Drying_your_wood.htm

put the lumber into three groups
load bearing / frame
general lumber
scrap / firewood
"let the machines do the work"

Don P

Although you look at knots when grading what you are really looking for is slope of grain, or specifically short grain. The swirl of grain around a knot when cut through by the saw leaves grain that is not aligned with the axis of the timber. There is a short length of grain to the edge of the piece and it is easier to break. The knot itself is essentially the same as a hole in a timber, the slope of grain surrounding the knot is what to keep an eye on. The slope of grain in the timber is also the same concern. What you are looking for is a good tension strap along the bottom, tension, edge of a bending member. Knot restrictions on edges are tighter than in the center, neutral axis. The upper edge is in compression in a bending member and knots there are of less concern than along the bottom.

Generally timbers are sawn boxed heart. Knots grow from the heart and put on rings of growth just like the tree, so when you see one on a face you are seeing the wide end of a cone that extends into the timber to the heart. Stress is carried basically on the surfaces, particularly on the lower and upper faces and it diminishes toward the center neutral axis where tension and compression pass through that neutral zone of zero stress. So unfortunately where the knots and greatest slope of grain surrounding the knots are the largest is also on the surfaces where the stress is the highest. Be conservative, resist the urge to get stuck and have to use a poor quality timber. Step outside of your immediate need and view the timber objectively.

An old rule of thumb in grading is "quarter, third, half". Look at the timber, visualize the knots and the surrounding grain and decide whether the controlling defect compromises 1/4, 1/3 or 1/2 of that stressed grain. Essentially clear, straight grain is SS, 1/4=#1, 1/3=#2, 1/2=#3. Those rough rules tighten up around the edges and stressed faces. When 2 or more knots occur close together combine them in that assessment. If they appear on more than one face be conservative.

Think about how the stress relates to the strain in the member, for instance a simple beam that is uniformly loaded has the point of maximum bending moment at the center of the timber, max shear is at the ends. It would be better to have good grain in the center and at the ends. Avoid the temptation to "hide" defects by placing joints at defects, you are simply removing more fibers that are helping.

I do agree with using lower grade logs for posts, sheathing, etc

nvr2l8

Good explanation Don. You'd  have made a better stength and material instructor then the one I had in collage. He put me to sleep.

I've made a few bows so I understand the tension/compression relationship. The difference is in a bow you maximize difflection stress so you can't talerate any knots or twisted grain. I can visualize forces travaling through angle knots. This creates a turning moment that tares or shears sooner then a straight knot. Knots on the corners are bad. Correct?

You lost me at the 1/4 1/3 1/2 rule though. Are we talking 1/4 the area of a

I see trees as I write this with branches on one side. I'm starting to formulate a plan on how to mill it and where to it belongs in the framing.


Don P

I think you are thinking in the right direction, Check these out and holler back, heading down to do some wrenching, I'll check back;
http://www.spib.org/docs/timber_pg.pdf
http://www.spib.org/about-us/pocket-guides

hacknchop

I agree with advice given by Don P I would like to add a couple of other things about knots ,type of knot for instance sound or unsound in other words is it a live knot having the surrounding wood attached or dead knot where the tree has grown around the knot ,these are like a hole cut into a timber, when cut into 1 inch lumber will shrink and fall out .Knots and other defects add character to individual timbers making them either stand out or blend in but to many defects take away structual integrity whereas no knots or  no defects at all  makes for very unappealing bland looking structure so don't be to quick to reject and avoid being too easy going when it comes to building with timbers .I think the fact that you sought out advice on the matter is a step in the right direction ,you will do just fine. :) :)
Often wrong never indoubt

Heartwood

The visual grading rules and techniques for measuring defects in northeastern species are in the Northeast Lumber Manufacturers Association (NeLMA) grading book available as a free download on their website. It's similar to the SPIB guidelines that DonP posted. Once you grade the timber you can plug the design values for that grade into the relevant formulae to size the timber.

nvr2l8

Wow that's a lot of info. Question. When they talk of knot size is it diameter or area. Give the size of knots I've seen in standard lumber I'd say it is diameter. My quick glance through the NeLMA page and pdf didn't show. They also showed shack to be 1/6 the thickness. Can't figure out what that means either. Some shake is obvious to me sometimes not so much.

Don P

It is diameter but think deeper than what you see on the surface. Visualize a section slice through the piece, how much of the piece is compromised by the knot and short grain. There is a certain amount of putting on your X ray glasses involved in grading, think through the piece rather than just looking at the stick.

Generally, if you see shake, start cutting firewood from that end till you get out of it. I'm the same way, if I see a little by the time it's dry it is usually a good bit worse.

Heartwood

nvr:
Read the section of the NeLMA book on how to measure characteristics to find the answers. It requires some study, not just a quick glance. It's also a bit subjective, and experience helps.
The Guild sponsors an excellent grading course; I think the next one is in the fall in So. Carolina.

Don P

Please post details on that course when they become available. I'm working with 2 guys that were signed up last time and know of at least 2 more who would probably sign up.

A couple of pics might help
This looks bad;


Now look from the edge, the grain makes a pretty good strap around that defect, this is a legit #2


This through knot occupies about 1/3 of the section and there is a good strap of wood on the edges, another #2


This knot looks like a #1 if you just look at this face;


It is part of an eastern white pine whorl, no grade;

nvr2l8

South Carolina. To bad I'm in ontario so I'm gonna have to miss that one. I'll print off the NeLMA and do some studying. I have lots of hemlock and an evaporator that like shake ridden wood so I can be selective. Now I have to bug the Sawyers on how to turn a log in to Timbers that are the RIGHT size and not have a big pile waste slabs. I love this forum!

nvr2l8

Don. A picture really is worth a 1000 words.

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