What to Consider While Machining Wood?
It's a well-known fact that the properties of the input affect how you machine it. Hence, you may have noticed that you use different techniques while working with different metals. What many don't know is that the same principle also applies to wood.
“Working with wood is easy” is a classic misconception that may result in wastage and failed projects. The only significant advantage that comes with working with wood is that you can operate your CNC machine at a higher feed rate. Of course, the fringe benefits that come with CNC machining, such as automation, still apply here.
Here’s a look at the different wood properties to make it easier to handle and machine them:
Type of Wood
Naturally, one needs to start with the core properties of the material. Here are a few common categories of wood:
- Engineered Wood
- Medium-Density Fibreboard (MDF)
Let’s take a look at each of them separately.
Typically, hardwood comes from deciduous trees (broad-leaved angiosperms). These have a darker appearance. Due to its durability, hardwood finds use in construction, high-quality furniture, flooring, deck, and other such applications.
However, since hardwood plants take a long while to grow and mature, they are often more expensive than their softwood counterparts.
A few examples of hardwoods include ash, birch, beech, elm, cherry, walnut, mahogany, maple, and oak.
Softwood comes from coniferous trees (needle-leaved, gymnosperms). The wood bears a lighter appearance. When compared to hardwood, softwood is easier to cut but is equally easy to splinter.
However, its name can be misleading since, in some cases, softwood can be harder than hardwood, making machining them difficult. For instance, cedarwood had a Janka hardness of 900 against the 760 or 850 of birch and ash, respectively.
Softwood finds its most common use in the timber industry. Its other applications include furniture, paper making, doors, and windows. Some of the well-known softwood materials are cedar, cypress, fir, pine, spruce, redwood, and yew.
3. Engineered Wood
Engineered wood, aka composite wood, covers all types of wood particles, strands, and fibres held together with a binding agent. This wooden material then undergoes compaction to impart strength into it.
Naturally, these are easier to work with than solid wood. However, the adhesive can have an abrasive effect on your machine and its parts resulting in premature wear and tear. As a result, your machining capabilities depend on the binding material and content.
Medium-Density Fibreboard (MDF) and plywood are some common types of engineered woods.
By its very natural nature, wood is a composite material as it contains strong and flexible cellulose fibre bound together by the adhesive action of lignin and hemicellulose. As such, wood bearing larger cellulose fibres offer a coarser grain structure while fewer fibres translate to a finer grain structure.
Wood having fewer fibres or less strong fibres would be easier to work with and machine. Generally, hardwood has coarse grains, while softwood has a fine grain. However, this may not be universally true as the grain primarily depends on the fibre structure.
Interestingly, it is worth noting that the intensity and direction of warping also depend on the grain size and direction.
Moisture content, or MC, is one of the primary considerations while working with wood. If you have machined wood previously, you may have noticed that the construction tends to warp.
This warping is due to the variability of moisture in the air versus the moisture present within the wood. Changes in the moisture content can morph the workpiece’s shape and reduce its usability or aesthetic appeal.
Hence, one must consider the MC right from choosing the source material to the final resting place where the piece will come to use and take decisions accordingly.
Equilibrium Moisture Content
In continuation with the above factor, that is, moisture content, the equilibrium moisture content also plays a role in the wood performance while you work on it or at its resting place.
Equilibrium moisture content (EMC) is a condition where wood achieves a balance point in the MC with respect to its surroundings. The EMC may vary depending on the temperature and humidity of the surroundings and can even change from location (inside vs. outside) or geographical region!
The wood pieces could changes sizes by 1 to 4% in response to the change in EMC, which invariably affects the MC.
A knot is a region where the tree’s limb branches out (or connects with the trunk). This connecting point brings about a change in wood quality and properties as it breaks continuity and changes the wood fibre direction.
Typically, one may notice the stressed concentration of the material, making it tougher to machine the input at the spot. At the same time, it could cause cracking due to wood shrinkage (known as checking), which takes place after the knot dries. Thus, it dilutes its mechanical properties and makes it the product’s Achilles heel.
Keep the above wood properties in mind while working on your woodworking projects. With these considerations, you can operate your CNC machine at high efficiency to extract high-quality workpieces at all times!
About the Author: Peter Jacobs
Peter Jacobs is the Senior Director of Marketing at CNC Masters.
He is actively involved in manufacturing processes and regularly contributes his insights for various blogs in CNC machining, 3D printing, rapid tooling, injection molding, metal casting, and manufacturing in general.