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About wood hardness

Other factors to consider in choosing lumber

Wood shrinkage and its effects on furniture making

How do you create a continuous handrail longer than the original hardwood it was produced from?

First of all, hardwood lumber is virtually never available in lengths over sixteen feet, but our handrails are often in excess of 30 feet long without a visible splice. The way we accomplish this is by using a joint called a scarf joint. Handrails are always laminated to achieve the proper thickness and the thickness of each individual laminate is dependent on the whether the handrail is to end up as a straight handrail or a curved handrail. If the handrail is going to be straight, then each laminate will be 3/4" to 7/8" thick, but if the rail is to be bent to a specific radius, each laminate might be as thin as 1/8". Before the laminates are glued together to form the proper dimensions for a handrail, they are first end jointed to create the desired length by using one or more scarf joints. When the individual laminates are glued together to form the full thickness of the handrail, the scarf joints are carefully distributed throughout the length of the handrail so that no two joints end up in the same place in the rail. The reason for distributing the scarf joints is to make them as inconspicuous as possible and primarily to make the rail as strong as if there were no scarf joints in rail.

A scarf joint is created by cutting a long diagonal taper on the end of a board and matching the taper on another board. The two tapers are then mated to each other and clamped with glue to create one long board. The longer the angle on the taper, the stronger the joint but as the angle lengthens the usable board shortens. I've found that an ideal compromise is a true 20° angle. There are a number of ways to cut the angle but the method I prefer is to set up a jig on the table saw utilizing the slots for the miter gauge with the 20° angle locked in. I then cross cut the taper using a fine blade on the saw.

When clamping the new joint, care must be taken not to let the two tapers slip on each other as the clamps apply pressure to the joint. The clamps can be placed across the two boards at a slight angle to help retard the slippage factor. If the boards being scarfed are small enough, I've found that electrician's tape works well in place of bulky clamps. The tape can be stretched across the joint and wrapped back on itself with a couple of wraps since its adhesive doesn't stick very well on a wood surface. After the glue is dry, any excess glue on the surface can be scraped or sanded off and the board is ready to be laminated into a full thickness block.

It is important to make all of the scarf joints face the same direction, especially on a series of laminates that are being prepared for bending a handrail on a curved stair slope. As the boards are being laid up on the stair slope, the joint should take on an angle that is horizontal in attitude rather than vertical. The reason for this is that the joint will withstand the stress of bending better if it parallels the bend rather than running perpendicular to the bend. Once laminated, a scarfed handrail need not be treated any more carefully than any other handrail.

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Explain what wood hardness means?

All woods are not created equal and it is important when working with wood or when selecting a wood species for an architectural application or a piece of furniture that you understand some of the important characteristics of wood so that an informed choice can be made.

In this month's fact page I will attempt to explain the issue of hardness as a property of wood.

The most common way to categorize woods is to lump them into one of two general headings; hardwoods and softwoods. While most people feel this as much as they need to know when selecting wood for hardness, these categories are not at all accurate when concerned about the hardness or durability of a specific species of wood. The terms hardwood and softwood refer the botanical classification of a wood species and has nothing to do with its density.

Hardwood is a common term used to describe trees in the botanical division Angiospermae or angiosperms. Angiosperms are flowering plants with seeds born in a vessel, and with generally broad leaves. They more commonly lose their leaves in the fall but some species do remain evergreen. Examples of angiosperms are oaks, maples, cherry, and walnut.

Softwood is a term used to describe trees in the botanical division Gymnospermae or gymnosperms. Gymnosperms are not flowering plants and produce naked seeds, usually on the scale of a cone. Their leaves are usually needle-like or scale-like and usually, although not always, evergreen. Examples of gymnosperms are pines, firs, and cedars.

All woods which are sold commercially as lumber are given a rating for their density. While density is the best single figure to determine the hardness of a wood species, it is not without some glitches. Density refers to a ratio of how much a certain volume of wood weighs and is often given in the form of specific gravity, which gives it a comparative listing to the scientific standard of water. A wood with a specific gravity of .75, means that given the same volume of wood and water, the wood will weigh 25% less than the water. If the wood species has a specific gravity of 1.02, it will sink in water. Relative densities of various woods mean nothing until one compares those densities with species with known working properties.

Another way that density may be listed in the wood industry is using the ratios of kg/m3 or lb/ft3. Kg/m3 is very easy to convert to specific gravity since specific gravity of 1.0 equals 1,000kg/m3.

Here are the specific gravities of a few common woods.

Balsa .16, Western Yellow Pine .51, Poplar .45, Cherry .58, White Oak .76, Maple .72, Ipé 1.08.

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Are there other factors in choosing lumber?

In the last information page I discussed the difference between hardwood and softwood and their relationship to density. Now I would like to discuss other physical characteristics of wood which are important to the selection process when choosing a wood species for a selected application.. Commercially sold lumber species all have a series of tests performed on them to rate them for a variety of characteristics.

One of the characteristics our company is most interested in is a woods bending strength. This rating tells us how thick we should mill individual laminates for a curved handrail and what precautions are necessary in initiating a bending procedure. Basically this rating tells how far a given thickness piece of wood of a specific species will bend before it breaks. A related rating is stiffness which tells how much pressure must be applied to the same thickness wood before it bends or breaks. This is the important rating that determines what kind of load can be placed on a beam or joist or any structural horizontal timber. This is also important to determining the size of a long column which may be subject to bowing under load.

Crushing strength or hardness is the property that tells how easily the grain can be collapsed by heavy or concentrated pressures. This is important in determining a wood's fitness as a flooring or as a column which may be subject to large amounts of pressure. Shock resistance tells how well a given species withstands momentary blows or changes in pressure. Some woods, while capable of bending without fracturing, if done slowly, will break when forced into the same bend instantly. Good examples of where this characteristic is important is in wooden handles for hammers or the interior laminates of a bow or downhill skis.

Movement tells how much a given species of wood will shrink or swell with changes in humidity. All wood expands when it increases its moisture content and contracts when its moisture content is reduced. As humidity changes throughout the year, so does the moisture content of wood. You may have noticed that a particular door will rub against its jamb in the rainy season but not when its dry and sunny. It is important for exterior doors to be manufactured of wood that has minimal movement with changes in moisture. Another application where this characteristic is important is hardwood flooring. People all too often look at hardness alone when determining hardwood flooring material but not all woods with good hardness have small movement. A prime example is maple which is hard but moves a lot with changes in moisture. This causes unsightly cracks between planks when its dry and possible floor buckling when the humidity goes up.

Preservation is the characteristic which tells how well a species of wood resists attack by fungus or insects. This characteristic is important in determining a wood's durability in an exterior or damp environment such as siding, decking or fencing.

There are several working properties which are important to manufacturers.. The blunting affect on cutting tools tells how often one must resharpen milling equipment. Gluability is a characteristic which becomes important when trying to glue a species of wood to itself. Many woods have a natural wax or oil which tends to repel certain types of glue, particularly water based glues. A way to minimize this affect is to wash the surface to be glued with lacquer thinner just before gluing. Also using a non water based glue such as epoxy will help the gluing process. Millability tells how well wood handles the stresses of high speed cutting knives. Two major factors are the straightness of the grain and the intergrain strength. Woods that have poor bending characteristics tend to mill poorly because of the same weakness in the natural adhesion between cells. Nailing and screwing capacity is also a characteristic requiring good intergrain strength. Woods with poor gluing and screwing properties tend to split easily when nailed or screwed without pre-drilling.

If you are searching for the best species for an application, your lumber supplier should have information on the above characteristics as assist you in your decision.

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Question: We are a furniture manufacturer in Mainland China. We use wood from South East Asia and sometimes export our furniture to Europe and North America. The wood furniture we export usually shrinks and Cracks. Do you have solutions for us to solve this problem. I know I can have the wood kiln dried but that is very difficult to accomplish. Besides this method, would your company suggest other solutions for us?

Mainland China

Answer: Your question regarding problems with your wood product shrinking and cracking when exported to locations with less humidity is one that has plagued woodworkers the world over. All wood shrinks and swells with changes in moisture content. Some species of wood change less than others and it would be important to find woods which have minimal movement to use for production and shipment to a less humid climate. Another precaution is to test the moisture content of the wood being used in production. In a humid, tropical environment the wood should stabilize at a residual moisture content of 12% or less. If the moisture content is higher, the wood should be air dried until the moisture content reaches stability. Expect that in a temperate climate such as Europe or North America that the residual moisture in wood will stabilize at 8% or less. Once you have determined that you are using the best wood species possible and you know that you are using the driest wood possible in your climate, the rest of the solution will have to do with the furniture design and wood selection. It is important to allow the wood to shrink naturally without being restricted from the movement you know it will make when its moisture content is reduced. For instance, let panels float freely inside of dadoes rather than gluing the panel in its dado. Seal all the end grain so that the wood won't dry and shrink more quickly on the ends. That will reduce cracking or checking due to non uniform shrinkage. Wood does not shrink evenly on each of its axes. It shrinks the least in length so avoid gluing wood together in opposite grain directions. Wood also shrinks less when cut in a quarter sawn manner, exposing a vertical grain surface rather than a flat grain broad surface. By using quarter sawn lumber, you may reduce your shrinkage factor by 60%. If you need more help designing furniture for shrinkage, let me know. I would be glad to examine your furniture designs to look for ways to alleviate your problems.

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