wood movement vs climate conditions

[mike calabrese]

I found this information on wood movement and posted it on another forum but thought I would duplicate it here.
Mainly it deals with base disc for segment turned bowls but the wood characteristics of movement can be applied with anything you might build.
DISCLOSURE>>>>
I WENT LOOKING FOR INFORMATION ON ACTUAL WOOD MOVEMENT VS CLIMATE CHANGE AND FOUND MR JONATHAN KATZ-MOSES WHO i CREDIT FOR ALL THE INFORMATION I SIMPLY RELAY & DISPLAY IT HERE FOR YOUR CONSIDERATION............... THANK YOU TO MR KATZ-MOSES WHO DID THE WORK.

I know all wood moves but after a few years of making saw dust I seem to have a sixth sense when wood movement is going to be an issue or not.
The key element is to know how much wood will mover over time ( change of seasons) and what to do to prevent it from ruining you day.
Ok here is where I disturb the dragon....
In a segmented bowl base floating or solid glued in there is a very simple way to calculate how much your base structure will expand or contract and thus give you an idea of how badly you need a floating base or if you are building a time bomb into you new creation with a fixed base.
The formula is real easy to work because our government ( US FOREST SERVICE) has done all the work for us and explained it all in a 509 page publication which we all can see here https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf.
If that is too much reading here is a you-tube video by Jonathan Katz-Moses https://youtu.be/EOr06I9JoAU that explains it all very clearly and details the formula method starting at 8:30 on the 15 minute video.
So what does this mean to you and me.
Lets say we are building a bowl and it is going to have a 3.5 inch diameter base disc or foot.
It would be good to know how much that disc will move over time so we could make an evaluation to build in a floating or fixed base and the potential risks associated there.
OK an example and the formula........
lets take SUGAR MAPLE which has one of the highest expansion coefficient for flat sawn lumber at .00353 (rift sawn would be about half that value).
And Olympia Washington which has the greatest change in equilibrium moisture content (EMC) from 19.9 in November and 13.3 in August
Now the formula WIDTH OF BOARD X COEFFICIENT OF EXPANSION X CHANGE IN EQUILIBRIUM MOISTURE CONTENT
SO A BASE DICS OF 3.5 INCH DIAMETER X .00353 X 6.6 ( 19.9 minus 13.3) = .0815 or slightly over 5/64ths of an inch
Your bowl will need to survive a base disc that will vary about 5/64the over seasonal changes. That I think would be floating base territory.

The same example in my neck of the woods SYRACUSE , NY would be 3.5 x .00353 x 2.9 = .0358 or slightly over 1/32nd.
I would take a chance here and skip the floating base especially that I like to make my bases out of several pieces with varying grain directions possibly further reducing the seasonal reaction to moisture.

I hope this will have you thinking about your next project and take some of the guess work of your design.
mike calabrese

[Dwayne Watt]

Great, quick summary of wood movement!
The only caveat is your project just might leave NY (in your case) and move to Portland, OR if it is sold or gifted or that favorite nephew/niece gets married and moves to Seattle/Portland area.

[mike calabrese]

Hi Dwayne
That is a definite possibility. In that case I would look into the Equilibrium moisture content (EMC) on my location and the destination and apply the values accordingly.
Not always possible to do if build first and ship later but there is always a way. At the end of it all a floating base with ample clearance might be the best choice.
I have only built 3 segment bowl and used a fixed base in each. Working on a floating base design at the moment.

I am thinking about building in some extra clearance but to keep the base from going off center I am going to use some thin packing foam to wrap around the base disc outer diameter. One or two turns around to act as an expansion joint. It would I guess make an excellent expansion space, joint, while comfortably capturing, holding, the disc. It would all be hidden in the recess of the bowl bottom ring and the keeper ring. I use this stuff to wrap my electronics for R/C airplane receiver and battery packs it is really quire easily compressible.
The picture here is a good example of the foam sheet I am thinking about.
Just an out of the box thought.
477838

[Edward Weber]

The problem with the JKM formula is this, The data used is for EMC, is for outdoor climate and relative humidity not Indoor. There is a huge difference.
Indoor changes in RH can be far wider and quicker than outdoor changes. This is fine if you're measuring for outdoor furniture but it does not always hold up for normal heated and air conditioned homes. Typically, interior climate controls (HVAC) dry the air out. So much so that many people need additional room humidifiers just to keep the RH at a comfortable level or the average 50%.
(ever hear of a piece under a vent cracking?)

https://www.woodcraft.com/blog_entries/woodsense-wood-movement

The Formula in the Woodcraft article uses a FSP of 30% as a base and commonly known Radial and Tangential shrinkage values. There is no need to use 20 year old U,S, Forestry location data. While 30% FSP is not perfect, is it a good close estimate for most species (there are always exceptions)

Just another way to solve for the same

[mike calabrese]

Hi Edward
Thanks for your reply
Yes I agree with your points but at the vary least it serves as a bench mark and a provocateur for awareness of the condition. Personally I am not a big fan of floating bases as for me it limits the design lines in most cases presenting a flat bottom in an otherwise nicely flowing form. Additionally after all the careful work to get the best possible joint presentation in the main bowl the probability of seeing a gap in the bowl bottom to me is a distraction. On the other hand so is a broken bowl.
mike calabrese

[Edward Weber]

Wood is like a muscle. A thick piece has more potential to move than a thin piece.
Your disc of 3.5" diameter, if it's 3/4" thick it has more potential to break the glue bond and move than the same diameter disc of 1/4" will want to move. If you make the base properly, there is no need to worry about using floating bottoms in all but the most extreme cases. The adhesives we use today combined with properly assembly will keep a base secure and in tact for years without failure

[Richard Coers]

Wood is like a muscle. A thick piece has more potential to move than a thin piece.
Your disc of 3.5" diameter, if it's 3/4" thick it has more potential to break the glue bond and move than the same diameter disc of 1/4" will want to move. If you make the base properly, there is no need to worry about using floating bottoms in all but the most extreme cases. The adhesives we use today combined with properly assembly will keep a base secure and in tact for years without failure
I don't agree with your potential to move comment. Thickness has nothing to do with the amount of movement, that is determined where the board was taken out of the log. That refers to the tangential vs. radial movement. A very thin piece will have more flexibility, but it moves just as much as a 2" thick piece. Also suggesting that modern adhesives will prevent failure ignores the possibility of a weak point in the wood. The joint may not fail, but wood failure is always a possibility. I've seen lots of antique furniture with a sound glue joint, but the wood has cracked.

[Reed Gray]

Furniture repair places in the SW states have more work than they know what to do with because of the very dry climate. Generally I think of indoors as being more stable than outdoors because of temperature controls.

robo hippy

[Edward Weber]

It can move just as much BUT a thinner piece can be constrained from moving by adhesives, while a thicker piece can overcome the glue bond and cause joint failure.
I did not intend to say modern adhesive prevents wood failure. I intended to say joint failure due to movement. Yes wood can still crack.

Plywood is an example, it's made from thin woods so that the adhesive can restrain it's movement. if it were made from thicker sheets, the potential for the glue joint failure is higher

We are also talking about the OP's concern about using floating bottoms. This is a specific usage using worst case scenario values for wood movement. A 3.5" diameter disc has the potential to move X but only at the center point, the movement is less as you move away from the widest point. Just because wood can move X or has the potential to, doesn't mean it will.

Wood shrinking to the point of structural failure or cracking due to shrinkage is another story

[Edward Weber]

Furniture repair places in the SW states have more work than they know what to do with because of the very dry climate. Generally I think of indoors as being more stable than outdoors because of temperature controls.

robo hippy

Indoor climate can be stable but when you factor RH & Temperature it's not so straight forward.
If you ever get an electro-static shock in the winter, your house is too dry, low RH but the temp is the same 72 degrees you set it at. Some areas have high humidity in the summer but no need to turn on the AC, so now you have High RH (sticky) and the temp is the same 72 degrees.
Outside temp and RH move more in conjunction, dry and cool, hot and warm. Indoors, it's whatever you want. Every time you open the door, you exchange what ever was outside for what ever was inside. Every time the heat of AC comes on, you're drying the air or lowering the RH but maintain the temperature.

I have no idea why or if SW furniture repair is busy but most furniture is made in North Carolina, so a trip to the SW is an abrupt change and can cause failures.
It's not the environment wood is in that causes problems, it's drastic changes that usually cause the most grief.