I have been attempting to design a table out of solid wood. I realize that nearly everyone will question me as to why I don’t just use veneer over a stable base. Let’s avoid that topic. As everyone should know, and I have been learning, solid wood is continually expanding and contracting.
With that in mind, I have been trying to calculate approximately how much movement will occur tangentially across a solid top approx 32″ wide made out of 7/8″ black walnut. Based on the formula I came across, I estimate the tangential movement with 14% air humidity to be 5/8″. However, my main question, is this accurate, especially after the piece is finished? I intend on at least using waterlox on both sides and then a top coat, to be determined. I realize a finish does not completely prevent a piece from exposure to moisture, but does it still effectively move as much as though it were not finished?
My appreciation to anyone that can enlighten me!
Z
Replies
There are various woodworking calculators you could try at http://www.woodworkersweb.com/woodcalc.htm
The one you want is the "Shrinkulator". I don't know how accurate it is, so a grain of salt should be applied ;-).
-robert
Zombee,
In practice you will probably be closer to 3/8".
The finish slows down the movement, but will not prevent it.
There are many ways to fit the top to it's base to allow free movement, I have my own trademark, but you can also purchase hardware etc.
Hi Willie,
Any chance of learning what your trademark / technique is for attaching tops to bases? :)Z
Z,
I see you live in Arizona. I've just moved to Prescott. I don't yet know how much seasonal variation will occur in this location. But the project you describe will easily move 1/2" to 5/8" in the midwest and east.
Rich
Edited 9/9/2005 12:36 pm ET by Rich14
What you're needing to know zombeerose is how much a 32" wide top will move in typical atmospheric relative humidity (RH) conditions. Given that typical seasonal RH varies from summer to winter in your area it should be simple to work out what you need to allow for movement.
Here in the UK I expect interior RH to generally hover between about a minimum of 30% and up to 60%, or maybe a bit more. This equates to the wood possibly drying out to as low as about 6% moisture content (MC) up to about 12% MC. Knowing this range of MC allows me to make calculations about how much the timber will move as long as I know how much the wood moves for each 1% change in its MC. Tangential movement is always greater than radial movement so you can make calculations for each type of cut, and you can average them out to work out typical expansion and contraction values.
American black walnut is considered to exhibit "small" combined movement values-- William A Lincoln, World Woods in Colour. Combined values refers to an average value for tangential and radial shrinkage and small means <3% movement in the RH range 60% to 90%.
There's enough information there for you to extrapolate the allowance required in your conditions where you expect the furniture to be situated.
However, as has been pointed out by others there are factors such as polish to consider, where film type polishes slow down the movement of water vapour in and out of the wood.
Not mentioned so far has been the fact that your bit of walnut could end up anywhere in the world-- you just don't know. My furniture made in the UK twenty years ago sits in houses in Texas and other southern US states, and furniture I made when I lived in Texas has been on a container travelling over the Atlantic back to the UK where I again live. And who knows, but one or two of my sticks of furniture could end up in a desert environment such as Arizona or Saudi Arabia, or perhaps in the damp and dreich Falkland Islands.
The point is that I often find it's counterproductive to get too excited about working out exact ranges of movement under a specific set of limited RH conditions. It's easiest to make a generous allowance to cope with all conditions and just move right along and make accordingly.
Therefore, in conclusion, if you allow 6 mm of movement for every 300 mm width (1/4" per 12" width) you just won't go far wrong. This means the wood might shrink or expand by 1/8" per foot width, with some slight adjustment for the season in which the piece of furniture was built, your workshop conditions, and what the wood MC was when you started working it. That value applies to English oak, a wood that moves a lot.
It's too much allowance for most timbers, but it's a pretty safe allowance for everything. If you reckon that your walnut top of 32" width might sometimes be about 31-11/16" wide at one extreme and at the other extreme perhaps 32-5/16" wide you won't go far wrong.
You'll notice that range of movement is 5/8", the same figure you got from an involved formula, whereas my method of making an allowance came from a long established and simple 1/4" per foot 'rule of thumb' that's served cabinetmakers well for decades. Slainte.
Richard Jones Furniture
Richard, you have snuffed out a potential storm with a concise and informative answer there. All these tables, rules etc are good for reference purposes, but in practice one should not get frantic -as you say it is counter-productive, and people should merely make generous allowance to cope with a reasonable range of conditions, given the type of timber they are using.
I have basically repeated what you have said because I believe strongly that this is what should happen in practice.Philip Marcou
Hi Richard,
Thank you for your insight and new perspective. I tend to be a massive over-thinker sometimes, which is probably due to my desire for perfection. As I can only imagine where my furniture will end up during its lifetime, using your advice is my best bet. I will keep it in mind and make a general allowance for movement.On a side note, I thought I should mention that your website does not format/display properly if using Firefox (from Mozilla). As Firefox is becoming considering more popular lately, you might consider adjusting your site so it will function properly.Thanks again!Z
zombeerose, you're not the first to say that about viewing my website on Firefox. I don't have the expertise to make the adjustments myself and rely upon someone else to do my website for me.
I've heard from web savvy types that it's a common problem. Some say the compatibility issues between Explorer and Firefox are a fairly small issues, and others say the issues are difficult to fix. I don't know, but perhaps at some point I'll have a look at it-- or more likely I'll pay my web guy to see what can be done, ha, ha. Slainte.Richard Jones Furniture
Yes Richard it is a common problem. I also have that formatting problem. What is not common is the supply of "web savvy" types who can get it right a little more often.I can't summon the interest to get to grips with this web siting thing myself, and I still haven't found a tame one-maybe a nice lady type would do better...Philip Marcou
You'd think Firefox would go to the effort to make their browser capable of showing web pages in the same format as the most used browser in the world, i.e., the Microsoft offering But it always seems to be that the makers of alternative browsers can't manage that trick, ha, ha. Slainte.Richard Jones Furniture
On the contrary Richard, Firefox actually follows and complies with web standards. That is the single biggest downside of Internet Explorer. Yes, it is undeniably the most common browser thanks to the financial backing and monopolistic status that Microsoft has. However, regardless of the status that Microsoft possesses, it does not give them the intelligence to follow what the general community has defined as a standard.Lastly, the small alternative browsers, just like your little "mom&pop" shops, are usually more innovative and aware of what the public actually wants as compared to the giants that only care about their bottom line. In fact, if it wasn't for Firefox and all of its new features, Microsoft would not even be releasing another version of IE.Not trying to argue but rather attempting to clarify a misconception.Z
Well hopefully the new Microsoft version will be compatible with the Firefox layout, but I don't know as I really make no attempt to keep up with all these things myself.
As I said before, I am aware of the issues, but so far I lack the will, energy or inclination to 'repair' my website to suit other browsers. Perhaps one day I'll get around to fixing it? Slainte.Richard Jones Furniture
Min EMC |-----------------------------------------------------| Max EMC (where it will live)
^ EMC at time of build
You need to figure out these 3 numbers. Mininum EMC inside, Maximum EMC inside, EMC when attaching the table top.
For instance, if you build it in Arizona and move it to Houston, there will be mostly expansion after construction is finished.
The EMC inside a house in Arizona in the winter time can be quite low (4% - 6%). Of course that depends on whether the house uses a humidifier or not. But, assume that it does not.
There are web sites that will show the EMC extremes for lumber in all the states.
If the table top has a skirt, then just use pieces of 1/2" channel about 2" long that can slide against each other.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)
PlaneWood
Using the "Shrinkulator" at http://www.woodbin.com will give you an accurate answer. Input the species, range of relative humidity, starting width and the output will be the expansion/contraction seasonally. It is based on the work done by the US Forest Service, Wood Products Lab and published in the Wood Handbook.
Finishing has little affect on the wood movement. However, it will slow down the changes but will not change the magnitude of the movement.
"I realize that nearly everyone will question me as to why I don't just use veneer over a stable base."
Actually I applaud the use of solid wood table tops and the fewer boards the better. I'm constantly looking for trophy wood to make my table tops. As long as you work it correctly and design for the movement it should be fine. I have a 2 board Koa dining table that's 42" wide and I just finished (2) 1 board walnut side tables 21" dia. To my eye nothing looks as good as continuous grain.
John O'Connell - JKO Handcrafted Woodworking
The more things change ...
We trained hard, but it seemed that every time we were beginning to form up into teams, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing; and a wonderful method it can be for creating the illusion of progress while producing confusion, inefficiency, and demoralization.
Petronious Arbiter, 210 BC
I'm happy to know that I'm not alone in my determination, or as some might claim - stubbornness, to use solid wood.Thx for the info. Z
zombeerose,
Welcome to the world of solid wood.. Don't fall for the myth that it must be glued up or it isn't safe! plenty of solid wood stuff has been made over the centuries and some of it hundreds of years old still is great.
Want more assurance? your house is made of wood and if you believe everything the "experts" say about it , it should be move by several feet!
Ok wood will swell and all you need to remember is that if all the grain is going the same way then everything willl move the same amount.. you only need to worry about cross grain movement and there are lots of ways to deal with that...
Frenchy,
Your comments remind me of a shocking experience I had this past week. My wife and I had lunch at the Longfellow's Wayside Inn in Sudbury, MA. They have a few antiques displayed, one of the nicest is a plum pudding mahogany high boy with beautifully fitting drawers and a magnificant finish. I pulled open one of the drawers and saw the worse fitting dovetails I've ever seen....the pins didn't come close to matching tails. I also noticed the drawer sides had shurnk in thickness and the pins were quite proud of the tails. Lastly, I noticed the dovetails had been cut in reverse, that is, the half blind dovetails on the front panel began and ended with the tails instead of the pins. I wish I could tell you what wood was used for the drawers....so our pieces won't look like that 400 years from now.....lol
This forum post is now archived. Commenting has been disabled