Wednesday, March 18, 2015

Wood Basics


This post contains information on wood, the different species, its structure, different figures, and its use as a material for construction. It also provides information on wood veneer and plywood.

Wood Species

There are over 100,000 different species of wood. Some species are full size trees and others are from small woody shrubs. In common commercial use there are dozens of species harvested in the United States and many more from around the world.


Hardwood and Softwood

Trees are divided into two classes: Hardwoods and Softwoods. The hardwoods such as Oak, Poplar, and Maple have broad leaves. In general softwoods, such as Pine and Fir, originate from cone-bearing trees. Hardwoods come from trees that have their seeds contained in a seed-case.

Hardwood and softwood do not specifically refer to how hard the wood is. For example Balsa is a hardwood and yet is very soft.

The Structure of Wood

Wood can be thought of as a collection of drinking straws - where each straw is one of the fibers of the wood. These straws run along the axis of the tree, that is up the length of the tree or along its branches. These straws are what carry the sap of the tree which supplies nutrients to the tree between the leaves and the ground. The straws are the fibers of the wood and a view of them on edge is the what makes the grain of the wood.


The grain direction is along the fibers of the wood:

Grain Direction

As a way to refer to the faces of a milled board woodworkers often use the terms Face Grain,
Side Grain, and End Grain. These are shown below:

Figure

Figure refers to the appearance of wood on its face grain surface (what we think of as the face of the board). The figure can range from subtle to spectacular. Species which are sought after for the figure include, Maple (Curly, Birdseye), Lacewood, White Oak (Quartersawn) and many others.

Subtle Figure - Holly:

Highly Figured - Curly Maple:

Birdseye Maple: 

Quilted, Birdseye Maple:

Fiddle back:

Quartersawn Pine:

Quartersawn White Oak:

Quartersawn Zebrawood:

This image shows how Quartersawn boards are cut from a tree:


As you look at the end grain of the board you can see why the face grain runs in straight parallel lines - the growth rings meet the edge face of the board nearly perpendicular to it:

The diagrams below show the difference between plain sawn, rift-sawn and quarter sawn wood:


Image Sources: Hardwood Distributors Association.

Heartwood / Sapwood

Sapwood is the living, outermost portion of a branch, while heartwood is the no longer active, inner wood, which often makes up the majority of a branches' cross-section.

This sapwood is where water and dissolved nutrients (minerals) are moved between the roots and the leaves of the tree. In the outer growth rings of a tree (those closes to the bark - away from the center) sap flows through the "straws" of the wood.

Often the sapwood is a different color than the heartwood. In most species, the sapwood is lighter than the heartwood.

An exception to the sapwood being lighter is Sycamore. Its sapwood is darker than the heartwood.


How Wood is Measured and Sold

Lumber is often priced by a quantity known as a board foot. 

Board Foot

A board foot is a measure of volume. The proportions of the wood doesn't matter - only the space it occupies. A single board foot (1 BDFT or BF) is equivalent to a piece 12" x 12" x 1". That's 144 cubic inches of wood (12 x 12 x 1 = 144). A board that is 2' long, 3" wide, and 2" thick is also one board foot - that's the same volume  (24 x 3 x 2 = 144 cubic inches). Two board feet (2 BDFT) could be, for example, 12" x  6" x 4".

4/4, 5/4, 6/4, 8/4, 12/4, 16/4

Wood is normally sold using its thickness as a measure. You'll hear people discuss "four quarter", "eight quarter", etc. This refers to the thickness of the wood in the rough. For example 4/4 ("four quarter") is 4/4 of 1" thick, or 1". But that is not a usable dimension. The wood is in the rough, sawn right out of the tree, and not yet surfaced. That is, it has not been planed (run through a machine which flattens and smooths the face). Nor has it been jointed (run across a machine which flattens and straightens an edge). Once planing has been performed, the wood is thinner than 4/4. It is normal to assume that if you want a 1" finished size you'll need to buy 5/4. After planing it'll be a full 1" thick. 4/4 wood is usually about 3/4" thick after planing.

It is hard to come by wood thicker than 16/4. In fact for many species it is hard to even get 8/4 material. Here you can see a list of species available form one source in Michigan: Armstrong Millworks. Note how few species are available in 16/4 or even 12/4.

Premium for Larger Sizes

Often wood is sold at a premium price for wider or longer boards. That is, even though the volume is greater for these boards, there is an additional charge per board foot for them. That's because it is harder to come by wider or longer boards. And therefore the sellers is justified in upping the price.


Gluing Wood

There is a lot of information about successfully gluing wood. In this post I'll provide a summary of some of the most important considerations.

Glue

There are many types of wood glue. For simplicity I'll focus on one manufacturer - Titebond. The company make a number of different varieties based on the conditions of the wood and the eventual use of the object you are gluing. For example they make a waterproof variety for outdoor use. You can use the Glue Product Selector on their main page for selection help. For typical indoor use, a good choice is Titebond I.

Applying Glue

Manufacturers recommend applying glue to both surfaces that are to be glued. This ensures that each gets "wet" with glue and there aren't any dry spots which don't get enough glue.

Timing is also very important in glue ups. The "open time" for a glue varies. For Titebond Original it is about 10-15 minutes. If you need a bit more open time you can use Titebond III, then you'll be in the 15-20 minute range. After that time the glue is already curing and is difficult to push joints together, or the glue will not develop it's full strength.

Temperature is also a consideration. Gluing in too hot or too cold a setting is a problem - particularly having the glue cure in cold conditions. You should strive to keep the temperature of the material and glue at 50 degrees or above. If the glue cures in colder conditions it will not develop its full strength.

Grain Direction in Glue Ups

It is imperative you respect grain direction when you assemble a project made from wood.
The important consideration is to refrain from a cross-grain glue up. When you glue wood together edge to edge the grain of the pieces should be running in the same direction.

Here's a clear example of what goes wrong when you glue wood without respect to the grain direction. This is one of the first pieces of furniture I made - a small Shaker bedside table. What a sloppy drawer fit! But it gets worse.

The table top has breadboard ends. These are meant to hide the end grain of the top when seen from the side. I naively simply glued them bread board edges on, which is a worst case situation of a cross grain glue-up. 

The wood expands or contracts in width whereas the edge pieces do not expand in width. Something has to give and the top cracks. 


This photo was taken in winter, when the heat is on, and thus the relative humidity is low, so the wood has contracted. But it is locked in place by the side pieces and thus cracks develop as it shrinks. In the summer these cracks close up and are nearly invisible.

The International Wood Collectors Society

I am a member of the International Wood Collectors Society. This is a group of people who have an interest in working with wood, its identification, and collecting samples.
I have about 220 species in my collection. The standard sample size our members collect is 6" x 3" x 1/2".

Wood Veneer

Wood veneer refers to thin slices of wood, most often thinner than 2 mm (1/16 inch). Veneer can be purchased as single sheets, sheets backed with a material added for strength, or as series of sheets in the same order they were cut from a log.

Veneer is created either by slicing away at the trunk of a tree or by slicing large rectangular blocks of wood known as "flitches". The appearance of the grain comes from slicing through the growth rings of a tree. The appearance greatly depends on the angle at which the wood was sliced. There are three main methods of veneer-making:
  • A lathe which rotates the log against a wide, sharp blade. The veneer is peeled off in one continuous roll. Rotary-cut veneer is mainly used for plywood, as the appearance is not as desirable as other methods because the veneer is cut concentric to the growth rings.
  • A machine which slides the board or piece of log against a sharp blade. The machine raises and lowers the wood repeatedly as the slices are made. This produces veneer that looks like it was sawn across the growth rings. This veneer is referred to as "cross cut". 
  • A lathe which operates on half-round sections of wood. The machines operates by turning and moving the wood in such a way as to expose the most interesting parts of the grain, creating a superior appearance. This veneer is referred to as "rift cut."
The video and web site below show examples of producing wood veneer and the wide variety of species and grains/figures available:

Plywood

Plywood is a composite material made up of thin layers or "plies" of wood veneer. The plies are glued together with the wood grain of adjacent layers rotated relative to one another. There are several important benefits to the cross-grain orientation of the layers:
  • It reduces expansion and contraction of the panels due to moisture changes in the atmosphere. This provides much improved dimensional stability over hardwood.
  • The strength of the panel is consistent across all directions. 
  • The wood is less likely to split when nailed or screwed near the edges of the parts. 
Below are some videos describing the process of making plywood, Medium Density Fiberboard (MDF) and Oriented Strand Board (OSB):