mike calabrese
04-19-2022, 3:39 PM
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
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