Article: Wood Movement & Projects

It is essential to know and understand wood movement between the many different species of wood. In my opinion, the most critical part of woodworking. ;

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Wood absorbs water from the air during high humidity causing it to expand, during low humidity the water evaporates from it contracting it. After a project is finished this process is called movement in service. ;

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The greatest amount of movement occurs across the grain {width}. Thickness has a lot less movement and the movement lengthwise is inconsequential. ;

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I place each species of wood into one of four wood movement categories, they are minute, small, average and great. ;

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Woods should be glued {laminated} whether edge to edge, face to face or edge to face only to other woods within the movement category they are in. Then if the piece is moved to a different environment the woods will expand and contract at approximately the same amount insuring that there will be no unnecessary stress and tension on the glue joint or either piece of wood, which could cause damage {cracks} to the weaker wood. The exception to this would be if using very small pieces of wood {narrow and thin}. ;

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With cigar humidors for an example, they are normally lined with Spanish Cedar, which is in the minute wood movement category. When wood from one of the other movement categories is used for the box, the Spanish Cedar lining should be left floating or be slotted and stainless steel screws used to affix it. In this way the two woods can slide along one another resulting in no damage to the humidor when it expands and contracts. ;

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When laminating different wood species together, try to keep the grain patterns on all the pieces somewhat uniform and not a contrast such as gluing a tight closed grained piece of wood to a loose wide grained piece of wood. ;

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When several narrow pieces of wood are laminated edge to edge, look at the end grain and place them in an alternating grain pattern, hump up hump down. If you draw several circles inside one another, then draw a line down the center, the top half of the circle has the humps going up, and the bottom half going down. The end grain {growth ring pattern} will somewhat look like the circles cut in half. ;

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Make sure that your wood is between 6% and 12% moisture content. Wood that is kiln dried to between 6% - 12% is extremely stable. If the wood is above 12% do not use the wood for your project until it has reached acceptable moisture content. ;

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In this day and age owning a good moisture meter is essential to assure that the wood does not have an extreme change after your project has been assembled possibly ruining a future heirloom and costing you a lot of wasted time and money. The pinless type of moisture meters are best. From experience they are quite accurate. You do not have to worry about pin holes and sliding the meter on the face side of boards you can see the higher and lower moisture areas on the board, quite intriguing. ;

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You should make a project {all parts} from one length of lumber, larger projects from wood that comes from the same bundle and same business. Even within a species, pieces of wood will vary in specific gravity {from different areas of the country or world} causing them to expand and contract at different rates. ;

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Quartersawn lumber is more stable than Plain {Flat} Sawn or Rift Sawn lumber. To determine how a board is sawn, look at the end {butt} and at the growth rings. If the growth rings are 45 degrees - 90 degrees to the face it is Quartersawn, if the growth rings are 30 degrees - 60 degrees to the face it is Rift Sawn, if the growth rings are 45 degrees or less to the face it is Plain {Flat} Sawn. ;

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When gluing wood together {especially oily exotics} you should blow the piece off with compressed air {100 PSI} first {safety note: be sure to wear a high quality respirator mask.} This works especially well with opened pored woods to get the dust out of the pores. Then make a sort of tack cloth by applying cleaner to a lint free cloth. Wipe the surfaces to be glued for better glue adhesion. I have found Acetone to be excellent for this, as it leaves no residue, does not raise the grain of the wood, evaporates rapidly and does no harm to the wood. I also use it in the same way, aforementioned above, before applying the first finish coat for better coating adhesion. ;

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All glues have cons; it depends on what cons you want to live with. I have found polyurethane glues work exceptionally well especially on oily exotics. They are very flexible and expand and contract well with joints. ;

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Coating the wood with a finish will not stop the movement it just slows it down! The finish you choose to use, will determine how long it will take to reach EMC. For instance wood coated with marine spar varnish will take a lot longer than wood coated with tung oil to reach EMC. ;

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The below across the grain {width} wood movement measurements are the movement from 6% {EMC} equilibrium moisture content to 14% {EMC} equilibrium moisture content on a Plain {Flat} Sawn board 12" in width. If a board is 6" wide it will move one half the measurement and if you laminate boards together for a 24" width it would double. ;

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One thing to take in consideration is the largest percentage of climate controlled homes and offices will only vary 3% - 5% EMC during the 4 seasons of a year. ;

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