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Walnut cupping after glue up.5/4/15
I cut and planed 3 perfectly straight pieces of 3/4" plain cut walnut. I wanted to glue them together to make a small panel.
I clamped & planed the edges in pairs to get a quality glue joint, I carefully glued and clamped them together using clamping cawls to match and correct any irregularities along the length as the glue dried and reduce any sanding needed.
The day after, I went to make some cuts and noticed that the panel had cupped.
Why did this happen?
Is there any way to correct it so I can cut a straight miter along the length of the cup.?
How do I stop this from happening again in future?
Thanks in advance for any help.
The panel cupped because one side lost moisture faster than the other. I would guess the panel was laying flat on a work bench and cupped up. It might work to simply turn the panel cupped face down and let it sit for a day, or so, and see if it goes back. In the future, store panels on edge or cover them with cardboard or scrap plywood. Also let wood sit in shop (on edge) for a couple days before milling.
did you invert each plank so that the grain is opposite to the next one ?
there's a good video here https://www.youtube.com/watch?v=Jk7EHOQOZIQ
Inverting rings is a myth.
I agree that alternating rings on flat sawed panel glue up is a myth. To me, it is much more important to match grain pattern on a panel with clear finish.
Alternating rings is not a myth.
It doesnt stop individual pieces from cupping, it just makes one panel cup the opposite direction so you dont have a large cup in the same direction.
I will add my vote to the myth.
If we have a piece of wood that is at a uniform MC and is flat to begin with, as the case here it seems, and then a day later it is warped, we know that the moisture content has changed...100% certain. Further the moisture change is more on one side than the other.
However, if one side gains or loses 1% MC than the other side, this MC difference and the difference in shrinkage from side to side will cause cup. The amount of cup due to MC differences from face to face is much greater than the cup due to differences in shrinkage due to bark vs. heart sides. Note that the shrinkage difference bark to heart will be larger for thicker lumber, but even then it is small.
Appreciate that we often get cup when drying lumber, but in drying we have large moisture losses, so a wee bit of difference bark to heart times a 25% MC change will be 25 times more than with a 1% MC difference. So, even if there is a cupping issue in drying, with the small MC changes seen in service, the cupping in service will be very slight.
In the wintertime when plants are heated and therefore humidities are often below 30% RH or 6% EMC, it is common to see the top panel in a stack which is maybe 8% MC, being cupped upward. The wood is a bit too wet, it dries only on the exposed top surface and therefore the top surface shrinks, giving us cup. It does not make any difference on ring orientation, they always cup upward...every top panel. Obviously, better incoming MC control, so that there is no MC changes, will eliminate the problem. Otherwise, make sure that top and bottom have the same MC at all times.
"At all times" means during construction and after finishing...we urge people to finish top and bottom identically for this reason, especially with a large panel when even a little warp can be objectionable.
Hope this lengthy discussion helps provide understanding. I had the same question last month and so you will see this discussion in FDMC magazine.
Did you joint the face on the jointer before running them through the planer to assure a flat surface?
Alternating the growth rings simply gives you a sine wave table top when it moves.
If you keep all of the growth rings consistent you know which direction it will cup and you can mechanically fix it in place allowing for movement. It is much easier to match the grain pattern if they are all in the same direction.
In theory if you design your table top so that it is concave and pull down the outer edges with slotted screws the ends do not have to be fixed. The table will stay flat.
I try to book match panels not only because they look nice, but also because they move consistently across the face.
If you were going to panel an entire room with cherry, it wood be foolish to alternate the growth rings. We keep all of the sap to one side, it is a defect. You can match the grain of cherry perfectly if you stick with the middle of the tree.
I think Hoadley's book has some decent pictures of this.
Plane those walnut, and leave it for weeks to dry, with lots of ventilation. Stack them if you can to maintain the shape. Then only glue them. Some dry wood expands with heat, some wood bend if you are not put properly..
I am sure your glued wood will not work after a few weeks if you are not using poly glue. Don't use water based glue for wet wood.
Wood does not expand a practical amount when it is heated. In fact, what does happen is that the heat lowers the humidity and this causes shrinkage, plus it might soften the glue.
You do not need to alternate the strips. The amount of cup is so very small that this orientation is not important.