Hypotheitcal Real World wood movement question ...

[rjent]

OK, solid wood panel made of joined strips of wood (any species) 1.5 feet across and 1 inch thick 4 feet long on the grain. Allowing for a quarter of an inch per foot movement allowance, this puppy will expand and contract theoretically 3/8th of an inch. If you "mount" the panel in the center (so it can expand in both directions), you should only need to allow 3/16ths on each side to accommodate the movement. Is my thinking right?

I am just thinking out loud (and incredibly close) right now, but I remember a desk that had been made my a woodworker friend of my FIL (who was also a fantastic woodworker). That desk was "attached" to the front frame of the desk and he had dowel pins riding in grooves at the back of the desk that allowed the top, which was solid wood, to move at will. The desk was quite old even then and was holding up well. I always marveled at that and was my "introduction" to wood movement a long time ago.

So I was just pondering ....

[JPG]

OK, solid wood panel made of joined strips of wood (any species) 1.5 feet across and 1 inch thick 4 feet long on the grain. Allowing for a quarter of an inch per foot movement allowance, this puppy will expand and contract theoretically 3/8th of an inch. If you "mount" the panel in the center (so it can expand in both directions), you should only need to allow 3/16ths on each side to accommodate the movement. Is my thinking right?

I am just thinking out loud (and incredibly close) right now, but I remember a desk that had been made my a woodworker friend of my FIL (who was also a fantastic woodworker). That desk was "attached" to the front frame of the desk and he had dowel pins riding in grooves at the back of the desk that allowed the top, which was solid wood, to move at will. The desk was quite old even then and was holding up well. I always marveled at that and was my "introduction" to wood movement a long time ago.

So I was just pondering ....

I think so!

Another consideration is grain orientation of the strips. Tangential to the rings movement is greater than normal direction to the rings.

Most table tops have expansion slots and hardware to allow for the expansion. That may be easier than glueing the center.

[mountainbreeze]

Your thinking seems right to me. The thing that no one ever seems to talk about though is, for example, where in the expansion / contraction process is the panel at the time you are cutting it to size. I.E., is the panel currently at its narrowest and should therefore be cut knowing is will be 3/8" wider in the future?

[JPG]

Your thinking seems right to me. The thing that no one ever seems to talk about though is, for example, where in the expansion / contraction process is the panel at the time you are cutting it to size. I.E., is the panel currently at its narrowest and should therefore be cut knowing is will be 3/8" wider in the future?

I think in NM, it would be near as small as it will ever be.

[rjent]

I think in NM, it would be near as small as it will ever be.

Oh contrare immense one

We go from 5 percent humidity in the spring to sometimes 90 percent or more in the summer. The high humidity can last for a few weeks or at least a month or so then fluctuate between 40 to 80 percent through the late summer and fall. I have always estimated that we actually have a larger swing in humidity (and hence a larger movement of wood) that much of the country. Of course the temperature goes up so the relative concept changes the picture, but still....

That is one idea MB, what is the moisture level of the wood, that has always alluded me. How can you determine the moisture state of the wood when you build something. Is there an answer?

I thought that "setting" the panel in the middle wood (<-did you catch that ) allow the wood to move both directions and help stabilize the movement. This panel (hypothetically at this point) will be upright with vertical grain.

This is just a mental exercise for now .....

Dick

[JPG]

Oh contrare immense one

We go from 5 percent humidity in the spring to sometimes 90 percent or more in the summer. The high humidity can last for a few weeks or at least a month or so then fluctuate between 40 to 80 percent through the late summer and fall. I have always estimated that we actually have a larger swing in humidity (and hence a larger movement of wood) that much of the country. Of course the temperature goes up so the relative concept changes the picture, but still....

That is one idea MB, what is the moisture level of the wood, that has always alluded me. How can you determine the moisture state of the wood when you build something. Is there an answer?

I thought that "setting" the panel in the middle wood (<-did you catch that ) allow the wood to move both directions and help stabilize the movement. This panel (hypothetically at this point) will be upright with vertical grain.

This is just a mental exercise for now .....

Dick

Yes I caught that 'detail'. Thought it a good idea.

Since 'current' RH is 8%, I thought that a good assumption.

Get a 'moisture' meter. Yes they do exist.

[charlese]

Oh contrare immense one

How can you determine the moisture state of the wood when you build something. Is there an answer?

This is just a mental exercise for now .....

Dick

A moisture meter is used to obtain the moisture content of wood. With one you can determine MC when you first bring the wood home and at later times before you mill and install the wood.

If you wish you could also measure MC of wood in objects inside your home. You can measure the wood at various times throughout the year to satisfy your curiosity. This is good knowledge to have.

In my thinking, every serious woodworker should have a moisture meter in his shop. They are expensive, but so are our tools and the furniture we build/

If you are mathematically inclined you can use the figures found in the Forest Products Lab website http://www.fpl.fs.fed.us/documnts/fplgt ... ter_04.pdf You will find that wood movement is not as great as 1/4" per foot in actual practice.

I have worked through the formulas given in that chapter, and found it to be a good exercise.

An easier way to determining wood movement is to buy the latest issue of "Fine Woodworking " magazine. (August 2014 No. 241) They have a wonderful article about determining wood movement. They even have tables that allow one to avoid a lot of math.

[charlese]

Let's see what the "Fine Woodworking" mag article says about wood movement in New Mexico.

I will pick red oak as the wood - 8% tangential movement (flatsawn)

Average moisture content of interior wood in January is 6 to 7%

Average moisture content of interior wood in July is 5 to 6%

Change of Moisture Content (CMC) of 2%

Using the table - CMC of 2% crossed with Tangential movement of 8%
the table shows a factor of .00571

This is .006 (rounded up) wood movement per inch of flatsawn red oak.

Therefore a 12.5 inch flatsawn panel of red oak will be expected to swell and shrink .075 inches each year. O.K. we can increase this figure for safety. So we should allow at least 1/10 of an inch. (how about 1/8"?)

[JPG]

How about Truth or Consequences?

[rjent]

Let's see what the "Fine Woodworking" mag article says about wood movement in New Mexico.

I will pick red oak as the wood - 8% tangential movement (flatsawn)

Average moisture content of interior wood in January is 6 to 7%

Average moisture content of interior wood in July is 5 to 6%

Change of Moisture Content (CMC) of 2%

Using the table - CMC of 2% crossed with Tangential movement of 8%
the table shows a factor of .00571

This is .006 (rounded up) wood movement per inch of flatsawn red oak.

Therefore a 12.5 inch flatsawn panel of red oak will be expected to swell and shrink .075 inches each year. O.K. we can increase this figure for safety. So we should allow at least 1/10 of an inch. (how about 1/8"?)

Very interesting, so it is knot (<- ) as much as I thought. 1/8 inch is probably overkill, but assume an anomaly year, sounds good to me! Thanks!