Understanding Wood

© Joseph Jourdain – Josephus Harp Shop – 2019

 

Wood is a very complex material. When used as an engineering material, proper understanding of its characteristics are essential. Here are reference materials that will help you to come up with a solution to a specific issue. There is so much to know. It takes years to truly have an intuitive knowledge of different woods for critical application such as the harp.

Wood Handbook : This book is a must. Free download at: https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf .

Chapters 2, 3  and 4 should be read. This is your most important book and it is free. Thanks to USA department of Agriculture.

Have a look at the table of contents HERE

Understanding Wood : by Bruce Hoadley. It  is a great book to buy if you are serious about understanding all aspects of wood. Technical books will make more sense afterward. It is especially written for hobbits and woodworkers. Even if you are already an experience woodworker I guaranty you will learn something from it. For instance in the "Carrying capacity and stiffness of beams" section he explains the two formulas used to indicate strength and stiffness of beams. Mathematics is as much about relationships than numbers and relationships are intuitively easier to understand with words than numbers. He has a full page with graphics and descriptive axioms about juggling the parameters of the formulas. You can read them below. I have marked in red what is not obvious at first.

- If you double the length, you will cut the load-bearing capacity in half
- If you select a lumber grade that is twice as strong, the beam will carry twice as much
- If you make the beam twice as wide, it will carry twice as much
- If you double the depth, it will carry 4 times as much
- If you double the load on a beam, it will deflect twice as much
- If you select a grade with double the Modulus Of Elasticity (MOE), the deflection will only be half as much
- If you use two beams side by side the deflection will only be half as much
- If you double the span of the beam, it will deflect 8 times as much
- If you double the depth, you will reduce the deflection to 1/8

 

 

The Machinery’s Handbook 26th Edition. Free download at: http://theminimachineshop.com/Technical/Machinerys%20Handbook.pdf

This book is more a shop reference book and is a must for proper structural designing of any thing you want to make, especially a harp.

 

 

Here is a material chart that compare different material in terms of  modulus of elasticity and density. This tells you why we use wood instead of modern material for our best instruments. Also that wood has both solid and foam properties.  It is a true solid for properties that are parallel to grain and not so for properties perpendicular to grain.

 

 

Here is a humidity chart correlating temperature and moisture content in wood. This is a must for your shop and for drying soundboard. I got it from Forintek Canada Corp sometime in the 1990's. It is the most useful chart to have and this is what I use. You need a good hydrometer and thermometer to determine the moisture content of your climatized wood storage. Then you know at what temperature you should set your kiln to dry it to a specific wood moisture content. The left axis is the relative humidity in the air and the right axis is the moisture content in the wood at sea level.

 

 

The functions below are theoretical mechanical properties of wood based on density alone. There are significant variations in wood characteristics for a specific wood variety. Unless you do your own testing it is difficult to estimate how far your sample is from the standard values. This is also a good method when you do not know for sure what wood you have or there is no data for it. This method is based on the theory that strength of wood is a function of the lignin content in wood which varies according to the cell types and structure of each wood species.  (table from the Wood Handbook)

 

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