Wood Strength Is About More Than Janka Scale
With fine woodworking you soon discover wood strength can be a relative thing. To give an example there are four different ways of measuring the strength of wood.
Compressive strength speaks to how much weight wood will take parallel to the wood grain. Depending on species it is obvious this will vary.
Among the same species the trees age, the climate it grew in, and also the type of cut which made the board, can also cause variation.
Bending strength pertains to how much stress the wood can withstand perpendicular to its grain.
Stiffness will tell you how much deflection the wood will have under load.
Then there is the Janka Scale which relates to the surface of the wood and how resistant it is to denting that surface.
There are relational aspects concerning strength and wood density as well. Wood density is measured by volume of weight from species to species.
While this is relational and strength can at times be relative, these standards are what have defined the wood industry.
There Are Limitations In Calculating Wood Strength
Not all wood artists are completely up to speed on the science behind all this. Most do not need to be.
Yet there are times when this lack of knowledge can make or break their work.
Strength will always be relational to the grain direction and how the load is applied to it. This is critical to be mindful of when making cuts for your designs and the function they intend to serve.
How you cut the wood and apply that piece to the design could be in error. This kind of error can reduce the load capacity merely because of not accounting for grain direction before the cut.
Knowing the why behind grain structure and how longitudinal cells form wood grain will save some headaches. A perfect example of this is the process of making hinges in wood art boxes.
In furniture making this is also true. An error here can cause the ankle of a table leg to crack or break under load.
An area where many craftsmen and artists can be caught unaware is in wood bending. It is important to understand the limitations of what the wood will allow.
An interesting note is composite woods made of plastic and wood dust surpass the strength of natural wood. While this is not included within standard rating scales, it is an interesting wonder of chemistry and natural materials.
Wood hardeners work on this basic principle to aid in protecting and enhancing wood strength.
Wood Grain Anatomy Impacts Wood Strength
Any woodworker must understand the anatomy of a tree and the wood grain they are working with. As a tree grows the grain is not the same from live edge to live edge.
There is a lot which takes place through the years of growth and it can be seen in a board. At the center of a tree is what is called the pith.
The pith can be softer than the surrounding heartwood of the tree. The next portion of the tree moving out from the center is heartwood.
The cells within heartwood are dead. It is the hardest part of the wood within the tree and provides structural support allowing the tree to stand.
Moving outside of this heartwood is sapwood. The sapwood carries life supporting water and minerals from roots to leaf.
The sapwood is typically the softest portion of the wood. Outside of the sapwood is the cambium layer. This layer is living and growing.
It is this layer that year after year produces the growth rings we see in a tree. Outside of this cambium layer is the tree bark.
This basic understanding allows for a novice to know there are stronger portions to lumber than others. When not using dimensional lumber from the store you must select the needed portions of wood to use for your projects.
There are times when the design is beautiful for these layer transitions. Within wood art one simply must know how to not compromise the needed strength for the sake of beauty.
Knotted wood for example can be beautiful when used for wood art. Knowing how to best use it can offer beauty and maintain strength within the piece.
Cellular Structure And Liginin Gives Wood Its Strength
If one has been woodworking for any amount of time you have heard the terms rip cut and crosscut. These cuts are describing how the blade cuts across the lumber in line to the wood grain.
A rip cut goes with the grain exposing what can be generally called the long grain. A crosscut is perpendicular to the grain and it exposes the end grain.
These terms are directly related to the growth of the tree and the cellular structure which formed the wood. Earlier I mentioned the term longitudinal cells.
While the process is more complex than I will state here, know that it is this cellular structure that ultimately forms the wood grain that you see. So how does this play into woodworking and strength for our use?
Wood is in of itself a natural polymer. The longitudinal cells combined together form cellulose fibers that in turn are held together by liginin.
This liginin is a binder which hold the fibers together. It is the direction of these fibers when bound together that give wood its strength.
Naturally if wood is cut and used in a project with a load placed in the direction of, or parallel to, its grain it will be stronger.
Likewise if wood is cut and a project is built with a weight load applied perpendicular to its grain, the wood is much weaker.
This goes back to the opening concept of how wood strength can be a relative thing. It is not merely a species to species or a like kind tree to another like kind.
The very application in how it is used can determine how strong the wood will be.
Right and Wrong Applications For Maximizing Strength
I will give an example of how what you have read can be used in the wrong way.
I own a table from a very nice store. In its time it was quite expensive. Yet this table leg and its wood grain runs parallel to the floor.
What this means is that as weight rests on the table, gravity pulls the weight down perpendicular to the legs grain. This is a wrong application on a very nice table.
How long will my table last? Again things are relative. That table leg is quite thick, and due to this alone it has survived three generations in my family.
It was handed down from my grandmother to my father, and then ultimately to me. For this reason I do not expect it to give out anytime soon.
Nevertheless if the design was any different, as in a thinner leg, it would not have survived.
The wood grain in this instance should be running in the direction of the force that will be exerted on it. In other words the wood grain should be running the length of the leg from end to end.
Visually when installed on the table the grain would appear a bit diagonal to the floor rather than its present parallel appearance.
The grain in relationship with its load should be placed from point of contact with the table on one end, to the other point of contact which rests on the floor.
Wood Strength In Application To Wood Art
Wood burning has become popular in recent years. This is a really cool fad that too few understand.
The burning process itself will compromise wood strength. Rather, wood charring is a better method that should be considered.
In smaller works of wood art this principle becomes critical with some applications such as wood joining. Yet there are other applications people attempt to only become frustrated not knowing why they fail.
I gave an example earlier about wood hinge making. If one were to attempt to drill the hole for a pin within a cross grain cut hinge barrel, it would never survive.
In this application the drilled hole for the hinge must be in the direction of the wood grain. It is the only way to maintain strength.
If the hinge barrel were thick enough perhaps it could survive a cross grain drilled hole. Yet here its strength would be compromised.
Constant friction from wood hinges opening and closing would add stress. An accidental drop could shatter the barrel.
It is important to know when one must follow basic principles for wood strength.
Other works like spindles for chairs must follow this same rule. Due to how small a spindle can be care must be taken with the species of wood. Often wood such as maple or hickory is used in these applications.
If strength is of absolute necessity do not look to the wood hardness alone. Many mistake the hardness to mean strength.
A better indicator for measuring relative strength is by its density. This measurement is called its specific gravity and is measured by volume.
Know that this is still an indicator not an absolute. There are exceptions to the general rule.
The hardness of a wood is a component of strength but not the value of that strength. To better know a more absolute value one would have to factor in its compressive strength, and stiffness, as well as how flexible the lumber is.
Despite all of this talk about strength we know homes in North America are built on pine 2x4s. It is a softwood and it is cheap by comparison.
No one seems too worried about their house collapsing. In this relationship structural codes are made in accordance with the wood strength to ensure the wood will carry the load of a home.
Wood Strength Can Depend On How Wood Is Milled
We need to explore how lumber is milled for it plays directly into application and strength. More importantly it is what happens to the wood after it is milled.
For wood artists our lumber does not come in dimensional pieces. Our lumber is most often cut from live edge, through the center of the tree, and out to the opposite live edge. This renders a wide plank.
Live sawing, or what can be called gang sawing, is not what is traditionally done for lumber that you see at Lowes or Home Depot.
Lumber you see in those places are cut from logs to obtain the maximum number of pieces for maximum strength per log.
In wood art the lumber we use most often has a completely different set of terms from store lumber. Our lumber is measured in what is called board feet.
A board foot consist of 144 cubic inches rather than a measurement of 1 linear foot.
In art the term often used for the wood is called rough sawn lumber. Even this is a very general term. It can further be broken down by defining what type of lumber this is.
Fractions are used to give its thickness such as 4/4 lumber is 1 inch thick. 5/4 = 1.25 in thick, 6/4 = 1.5 in thick and so on.
The greatest advantage to this type of milled lumber is the ability to get thicker slabs that reach from live edge to live edge. The beauty it renders is superior to other cuts. Yet there are drawbacks and issues that can arise from live sawing.
Boards will almost always have cracks or splits that form at the ends of the boards running along with the grain. Additionally due to the cell structure of the grain, a live cut allows the board to cup, twist more easily or bow.
People can get picky with a 2×4. After working with the volume of rough sawn wood that I have, I do not even care about a crooked 2×4 anymore. Rough sawn can get pretty bad at times.
Imperfections In Rough Sawn Lumber Impact Wood Strength
To relate this issue to the wood strength we must look at how the lumber responds. This cracking, twisting, and bowing can affect its application especially for larger projects.
I have seen boards which were surfaced to later continue cupping or twisting. If a piece is to carry a load one would not want to apply weight in a way to further complicate the issue.
There are ways to help identify boards for certain applications but much of this is hands on experience. The end grain will tell you a lot in conjunction with what condition the board is in.
Going back to the cellular structure you cannot alter how tight the liginin has bonded these fibers. Surfacing can at times help. Minor issues can be worked back with wood clamps and bending the wood.
Yet this is not always the case.
Live sawn wood is beautiful but it is temperamental. It has its own personality. At times it will decide what you can or cannot do with it.
Selecting the right species for the project is where it starts. You want strength but also relative ease of use and then function in the work.
Sometimes resourcing the lumber becomes its own art. Finding beautiful pieces and having it meet the applications needs can be challenging.
For those who are new to working with this type of wood it can be frustrating at first. It will take time to understand some of the principles behind the process.
To get the most wood strength for your project or artwork, one must take care to see the signs and follow the general rules of woodworking.
The best way to learn is to dive in. Make some mistakes and grow from there. Take your time and explore what can be done along the way.
It will not take long for you to become a specialist at certain wood species and its application.