Quartersawn Douglas Fir Cupping!?!
A couple weeks ago, I glued up a 3’x6’x2-3/4″ benchtop using four pieces of quartersawn Douglas Fir. The wood is old growth and has been sitting in a garage for 5+ years. I recently had the 6″ thick boards resawn into 3″ boards. Most of the grain as seen from the end is less than 10 degrees off of perpendicular, though some at the edges is more like 30 degrees off, though only for two inches. After I glued it up, I flattened the top until it was flat flat. I roughly leveled the bottom, focusing on where the top would meet the base and not worrying as much about the rest. While I worked on the base, I stood it up on end against a wall in my basement shop.
Fast forward two weeks or so to today when I put the benchtop on the base. It rocks. At first, I thought that the base was crowned, but when I checked the top surface with a straight edge, there was atleast a 1/8″ gap of light under it at the center. And the opposite was true for the underside. Now, the fix is obvious. But why did my quartersawn top cup? Was my mistake waiting two weeks to install the benchtop?
Chris @ www.flairwoodwork.spaces.live.com
– Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. – Albert Schweitzer
Replies
chris,
How long did it sit after you resawed it? My guess is the the interior of the timber had different moisture level than the outside, and moved as it re-acclimated.
Ray
Chris,
I too think the wood needed some additional drying after resawing. It's tough to get the center of a 6" thick board to 6%-8% (or even 10%) moisture content.
I also think using 4 boards to make up a 36" wide top was asking for trouble. Look at the commercially available tops, they are 2-3 inches wide, probably for that reason.
Good luck, sound like you have some more flattening to do!
Lee
Lee, Ray, and Wilbur,
After resawing, I let the boards acclimatise for about two weeks, figuring that the 5+ years would have allowed the center to reach equillibrium. Also, I wasn't afraid of quartersawn stock cupping!!!
I think that stability might be part of the reason commercially available tops are made of very narrow strips. However, I think the real reason is availablity of wide stock. I'd imagine that in a factory, it would be a hastle to work with boards of varying widths, so they rip them all down to, say, 1-1/2" wide and glue them up.Chris @ http://www.flairwoodwork.spaces.live.com
- Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
chris,
The center of a 6" thick plank may be in equilibrium with the average yearly moisture content of the air where you are, and still be "off" from the specific moisture of the time of year when you opened the plank. Moisture changes will not be transferred immediately, or even quickly throughout 6" of lumber. So that, the planks may have been drier in the middle than their outsides, in a high humidity time of year, and will take on moisture, the converse would occur had you opened them in winter.
Given the one year per inch of thickness rule of thumb for air drying, after 5 yrs the planks may have just been getting dry in the center. Not trying to argue here, just trying to sow the seeds of confusion!
Ray
Ray,
Yes, you are right. I didn't factor in the current humidity in my shop to the humidity that it has been at for the past five years.Chris @ http://www.flairwoodwork.spaces.live.com
- Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
It might also be differential moisture from where it was placed. If your basement wall isn't insulated and sheathed, ie. a concrete wall, you would expect it to be somewhat moister than the air. The side leaning toward that more moist source might have gotten, or stayed wetter. The cup would be on the room side, with the wall side being convex, if my supposition were right.
Steve,
Good thinking, but not the case. The top is now concave. It was resting against an interior wall also, on a linoleum floor with about 12" of air behind it.Chris @ http://www.flairwoodwork.spaces.live.com
- Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
I agree with you. I experience slight room-side cupping all the time. I don't have concrete walls in my shop, but there's just more airflow on the room side of a board. I notice the same thing when I set a wide plank face down on a table for a couple days. It will cup toward the room side (up in this case). If I want it to go back I just flip it over for a couple days. Once you start to notice this you can use it to your advantage . . .I also agree with the others about more drying time after resawing.Brian
Chris ,
Resawing can cause MC variations and instability but you are not saying individual boards cupped and twisted rather the whole top has a cup across the grain .
You need to keep weight or clamp the glue ups to something flat , by waiting you gave it time to move a tad , chances are how ever you fasten the top will pull it flat .
luck to you dusty
Chris: All of the responses have thoughts that are certainly possible, but here's one that hasn't been mentioned yet:
If you used "ordinary" wood glue (i.e., PVA-based) to glue the top up, you introduced quite a bit of moisture into the center of a very thick top. Given that you've stated that the boards are not quite quartersawn, you'd expect the wood to move differentially between the top and bottom. That is, the perfectly square but rift-sawn pieces will move with increasing moisture into a trapezoid.
Admittedly, the trapezoid would be hard to measure, but it only takes 15-20 thousandths of differential expansion to cup a 36" wide top by the amount you state.
Further - if your shop is not precisely humidity controlled, you can expect this to happen again from time to time, requiring you to re-flatten the top.
Given that it's pretty much certain that a moisture content change from some source (the glue, the air in the shop, a moist basement wall) caused the cupping, I would suggest that you use that to your advantage. Re-flattening the top with planes or a wide belt sander is not worth the effort, because it won't stay that way. Instead, lay moist (not wet) towels on the cupped side of the top, and check it every 30 minutes or so. You will find that it will flatten out in short order, and then will start to cup the other way.
The trick is to catch it when it's very close to flat, then bolt it/mortise it to the base. Wait a few days for everything to settle down, then handplane it flat.
Mr. Keller?,
I did use PVA glue.
I was hoping that someone would suggest an alternative to resurfacing. I'll give it a shot and if it doesn't work, back to Plan A.Chris @ http://www.flairwoodwork.spaces.live.com
- Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
One point I forgot to make - the "damp towel method" was something I picked up from a book or magazine ages ago, and I've used it fairly frequently when I've glued up and surfaced a panel and then set it aside for a couple of months. It seems, even with very tight humidity control in my shop, that the panel always needs tweaking to get it flat enough to use as a case side or a table top.
I haven't had to use the adjunct to the damp towels, but as I recall the magazine tip suggested drying out the other side (the bowed side) with heat lamps or a hair dryer. I've also seen a similar tip published that suggests laying the panel out on wet grass, with the sun providing the heat to dry out the other side.
I've never had occasion to require the heating part of the equation, but considering how thick your top is, it might help speed things up a bit.
And - you're right about stability of narrow strips versus wider, quartersawn boards. Assuming that the wider boards are indeed quartersawn with the end grain nearly vertical, narrow quartersawn strips glued together will be no more stable than the wider boards of the same species. The trouble, of course, is finding 24" wide quartersawn boards - it happens occassionally when one of these tremendous white oaks falls in the neighborhood during one of the frequent hurricanes we get in NC, but it's hard to get to it with a chainsaw lumber mill before the homeowner cuts it up into firewood...
I have also heard of it before, sometimes used in conjunction with an iron to create steam for the same effect. I thought about laying it out on the grass, but was afraid of too much movement too quickly, as well as the end-grain checking.
I spent about three hours last night monitoring the benchtop and noticed no changes at all. So I decided to try to wipe the entire top of the benchtop down with a damp towel and cover it with plastic so that the moisture would migrate into the wood cells instead of evaporate. I just checked it and it is super flat (probably within 1/64"). Thanks for the idea - it payed off big time.Chris @ http://www.flairwoodwork.spaces.live.com
- Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
Good show, I'm glad the idea worked. It's saved my butt on more than one occasion!
I told you you should have sent some of that wood to me. :@)
Seriously, it seems that not waiting long enough after resawing would lead to the cupping.
Quick question: why flatten the top before it is actually on the base? I would think that would be the last thing you would do for a workbench.
Edited 7/21/2008 2:24 pm ET by wilburpan
A couple reasons I didn't flatten the top after attachment:1. I was excited to see what it looked like2. The base wasn't yet built3. I wanted to flatten the bottom as well (actually, I had so much fun flattening the top, I decided to do the bottom as well. Also I needed a couple flat spots so that it would rest on the legs without rocking.)Chris @ http://www.flairwoodwork.spaces.live.com
- Success is not the key to happines. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
If you had the top standing on the concrete floor it may have taken up moisture thru the endgrain. 1/8" isn't terrible ,but I imagine you will have to reflatten then add your supports.
mike
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