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First Big Wall-art: it cupped...2/16/17
First, I'm a CNC guy. Having put all my energy into building the machine and writing the code, I swore I'd leave finishing to the customer.
Second, this is only my second attempt at a Big Piece (40x48x8/4 cherry, varying in thickness from 3/8 to 1-1/4--it was supposed to be at least 3/4 when done, but I made the newby mistake of not buying thick enough rough lumber to start).
However, I discovered that when I tried to go to bigger glued-up pieces of expensive hardwoods, ignorance wasn't the shield I'd counted on. The wood moved, quite a bit in my wood-heated basement workroom next to the double-door to the uncontrolled garage environment.
I was shocked! when the 8/4 panel cupped, and just as shocked that so great a curve disappeared with changing humidity. (Anything I learned about RH and EMC, I learned here in the past few weeks.) I don't have a decent MC meter, but I do have three humidity gauges which give me three different readings.
After it returned to normal, I quick-like-a-bunny CNC-carved my project and sloshed on copious amounts of Danish oil to all six sides--three times. And then, I sliced off the remaining 8/4 waste to release the carving--which had dwindled to only 3/8" thick in the thinnest places.
You probably guessed that it cupped.
After spending another day reading more here, I learned about warping from internal stress, and thought what I was seeing that because it happened the instant the thick matrix was removed. However I decided to test if it was moisture content by following a tip here to place damp towels on the curved up (driest) side, and sure enough, it began flattening immediately.
Following the Conservation of Misery Principle, as one problem recedes, another waxes: how do I/can I even stabilize it with a finish so it stays "relatively flat"?
Or is there another way to hold it flat so it can be used as a wall-hanging?
Failing that, I'm going to offer it as a piece to hang in the shower, or to display on the bottom of a swimming pool.
I feel like I woke up in another universe: wood always seemed so simple. But I bet mine is not the only heart it has bruised.
Open to any advice and admonition.
The use of very wide flat sawn stock does not help at all. Narrow quarter sawn stock is much more stable. How reliable of supplier did you use? Was it sold as kiln dried?
I used the go-to hardwood supplier, but I never asked whether it was KD. I know trim carpenters were coming in and out the entire time I was there, leading me to guess it was.
I had another supplier lined up, but drying was going to take another 30 days, and then I found this place with stock on-hand.
The truth: I wanted to run right through all of this without having to learn anything about it.
In fact, to make up my 40" width, I hoped to buy wide-enough 8/4 to have only two seams.
A fellow with experience insisted I rip them into smaller widths and make at least four, but he had to sit on me, and then babysit me to make sure I did the edge and face jointing and planing before I started gluing.
As I hinted, all this preparation removed more material than I expected, and I compounded the errors by not adjusting the project's parameters. Still, no matter that, the wood would have moved, because it moved while still in the nearly-2-inch plank stage.
I just checked again, and it's sitting nearly flat on the table. I have an idea to slip a dowel under its middle back and clamp the edges on either side, trying to give it a slight reverse bow.
But I wonder if anything can make it stay that way.
No such thing as working with wood and not "having to learn something". I've been doing it for over 40 years, still learning.
I've had things change shape during the finishing process where one side was covered and the other was waiting for it's finish.
One of the few ways you will stabilize that is to put a film finish on it that will slow the exchange of moisture. Wax is the best, the rest will let moisture in slowly and out the same.
I've straightened pcs out by placing them on a sawhorse and a heater below. When it's nearly there remove the heat and put the finish on and hope it's sealed good.
I am not going to add anything about the warping beyond that the side with the longest continuous grain is going to be the longest.
Beyond that. Is that a 3D topographical map you rendered onto cherry? Really cool. Really. I hope you get the bowing stress thing worked out (relieve the back?) so you can showcase the thing. It is fascinating.
I cut my teeth with one of the top carvers in the country. Lots massive carved doors and entries etc.....big stuff. Usually starting with 10/4 and milling ourselves. One thing I learned is take your time, like weeks. Rough size it, joint one side let it sit to adjust and do this with all remaining steps then finally glue it up for carving. One thing he did do which is time consuming for some doors to avoid bread boards on the back of doors we would pre drill the planks for all thread across the doors top and bottom to stop cupping.
If you could mount it on battens across the grain fastened tight in the middle and with sliding hold downs on the outside you might be able to get it to stay flat. Don't know if your situation would allow for that but I think the piece will continue to move as RH changes.
I wonder if, once flat you can carve out some of the back to relieve tension. Just a thought.
I had thought over the weekend. Since this is wall art you could possibly integrate 2-3 clean looking stainless steel cables on the back that would act as a tensioner to take the cup out and something to hang the piece by. You'd have to be careful when tensioning it you wouldn't want to crack it.
One more idea. You could put in the sun cup up for awhile and see if it'll warp back straight. If it does batten real quick.
Here are a few things:
The wood was probably dried poorly. The thicker the wood, the more likely it is not dried correctly. Sounds like you went to a hobby shop. Professional lumber vendors will dry thick wood correctly and it can stand being cut into and not warp. As rich dais, the center was wet and as it dried, it shrunk, cupping your work.
It is not to be assumed that all wood will warp. Things can and should be done to not cause it yourself - stable comfortable temps and humidity, deep cuts made slowly, and plan to hold it flat, since once it is out of your hands, it may cup.
Frame and panel was developed to help control large panels of solid wood. It has worked pretty well for over 500 years. It accommodates wood movement.
The TWO RULES are : All wood moves, and all woodworkers disagree as to how much, why, when etc. But it is a science and as such, is predictable.
Wood will move and nothing you or I do will stop it. So-called primitive people cut stone by drilling a line of shallow holes in the stone and placing wedges of hardwood that were desert dried into the holes, then wetting them. The wedges swelled with the water, and exerted pressure along the line, eventually cracking the stone. Pyramids could then be built, with water and wood.
We have all been humbled by wood. You have a nice piece of work, call it a learning experience and enjoy it from that added perspective.
What day is this? The 24th? Gak. I apologize, but you'd be proud of me: I was readin.
Based on the recommendations all over WoodWeb, I ordered Bruce Hoadley's _Understanding Wood_ from the library and it arrived today. However, I continued hunting information on the web, which UW now corroborates.
My Biggest Mistake was letting the air in the work area get too hot (the Forest Products Lab Wood Handbook FPLGTR113 recommends storing wood in cooler temps and under a plastic covering after manufacturing).
My second biggest was believing that the Watco Danish oil sealed the wood by virtue of being absorbed. I understood that the product was a mixture of varnish and oil and I assumed that with each new coat, the varnish was built up, leading to a sealing coat.
First @Bart, I had that very idea: I made a "spine" out of a 3/4" dowel with a flat side for the center back and stretched cables side-to-side across it. I found stainless steel "mirror hangers" in the hardware store, about an inch wide and in a squared-off J profile, and bought a very scary circular saw for the dremel tool to mill slots in the edges. That bit was dicey because the edge is only 1/4" in places.
I put the hangers in the slots, and attaching a turnbuckle to one side with an S-hook, and the stretched 18-ga galv. wire between the turnbuckle and an S-hook in the hanger on the other side. I was very surprised that there was still enough travel in the buckle to tension it up, even with my clumsy construction.
The wires pluck like a banjo. I've got 8 of them space roughly 4-1/2 inches apart.
I don't know if steel wire is going to be strong enough for cherry... :^) I'll rig some other way to hang it.
I had let the piece rest on the machine's table for several days, but not, as you guessed, raised to allow air movement across the bottom. There had been some cupping even in the pre-cut 8/4 stage.
As soon as I unclamped it, I was ready with "spar varnish". 2 coats. "If it's good for ships," I assumed, "the only thing more robust would be what they put on submarines." Wrong again. Firstly, FPLGTR113 indicates that a minimum of THREE coats (and 6 or more for real ocean applications) are required to get good finish, but secondly, it recommends 2-part polyurethanes above clear varnish.
Even after unclamping, a cup of perhaps an inch remained. However, I was able to take that out with my "wire braces."
@David R Sochar
I got it at a hardwood supply warehouse. I only assumed it was kiln-dried. (Unfortunately, I haven't been back to the place to show the piece and pick the owner's brain.) Just as you say, learning experience. For all the heartache, I wouldn't trade the experience.
I actually thought, "Hey, I have the model in the computer: I could cut its negative out of the back and have a model that was a uniformly thick shell." (But then I laid down until that went away.)
Thank you for the encouragement. Today, I confess to investigating "wood alternatives".
@rich c again:
A carpenter I showed it to suggested treating the entire piece as the (large) panel in a ficticious door and routing stiles and rails to make a stout floating frame for it.
Thank you all and each. I have some residual questions but will try to remember them for a separate post.
Sorry to leave you hanging. Here's a picture of the status. (I expect to hear "pings" in the middle of some night when the wires start letting go. :^) )
Thanks again all.