Lams Cupping?

[mmattockx]

As I work on my first lam bow I am learning in large chunks. One thing I have found is thin lams cupping after grinding. They are tip wedges that are 0.096" thick, 10" long and taper to zero thickness over 4" on one end. They are straight grained red oak, bias cut.

The board I cut them from had a bunch of internal stresses and both sides of the rip cut along the length bowed noticeably (away from the center of the board if it matters). The thin piece for the wedges also cupped across the width, towards the outer surface of the board. In grinding them to thickness all the cupping was removed and most of the long bowing came out at well. They sat in my unheated shop for a couple days and were cupped again (maybe 0.020" across the 2" width) when I pulled them out to grind the taper on the one end.

Is this a common problem with lams? The thicker lams (~0.150" thick) have not shown this so far and I would be happy to keep it that way.


Thanks,
Mark

[Shredd]

The first thing I thought of was to check the moisture content...  Should be under 12 %...

[mmattockx]

The first thing I thought of was to check the moisture content...  Should be under 12 %...

I have no way to do that, but it was a board from a big box store that would have been kiln dried and I live in a semi-arid area so it won't have picked up much since the kiln.


Mark

[Flem]

I'm with Rich, cupping is almost always moisture related and with such a thin profile piece, it won't take much. You can get digital moisture meters for under $30. these days. Otherwise you will just be speculating as to the cause of your cupping. I live in an arid zone also, but I always check moisture with an old Lee Valley analog meter. If anything goes wrong, at least I will know its not moisture.

[Longcruise]

Also, kiln dried wood could be the culprit.  The outside drys first and shrinks collapsing and damaging the inside cell structure.

[mmattockx]

I'm with Rich, cupping is almost always moisture related and with such a thin profile piece, it won't take much. You can get digital moisture meters for under $30. these days. Otherwise you will just be speculating as to the cause of your cupping.

Ok, so you guys think it was probably wet and then dried after I cut and sanded the piece. Could this go the other way and be a dry board gaining MC from a more humid environment? It is dry here but the outside RH is often in the 70-80% range when it is cold because the amount of water the cold air will hold is so low. The board came from a heated store and then sat for maybe a week in my unheated shop before I cut it.

Are there any other causes of cupping? I figured it was due to the internal stresses in the board and was gone after I ground the lam flat the first time. I was surprised to see that some returned after it sat for a bit.

If the cupping is not bad and will flatten out on the form during glue up is there any reason not to use the lam?


Thanks,
Mark

[Shredd]

  I would think that by the time you cut a piece of wood that small all the internal stresses would be gone...   Why don't you try this...  Mark the edge of your board with two 45* pencil marks to keep track how the pieces where oriented when cut...  Cut 2 pieces about an 1/8" thick...  Now lay the pieces on a dry surface...  Put one upside down from the other piece...  Come back the next day and see what happens...  if they are both cupped upwards it is probably a moisture problem...  If one is cupped upwards and the other downwards it is internal stresses...  If nothing happens run them through the sander on both sides...  The frictional heat may cause them to warp...  lay them both on the table over night again...  One up and one down...

[Flem]

I was thinking that wood is going to try to reach equilibrium and the smaller you make that piece, the fresh surface is going to be drier than ambient and absorb moisture and like Rich is saying the exposed surface is going to absorb more creating the cupping effect. Once again its all guess work without numbers.

[Shredd]

Absorb moisture if the air is wet and dry out if the air is dry...   If the moisture in the wood is high and the air is dry the wood will cup upwards when laid on a dry surface...

[Mad Max]

when I make my thin lams/wedges/overlays what ever, I stack them up and lay a board on top of them on a flat surface  to keep them flat.
I always make that stuff last when it tapers to 0

[mmattockx]

  I would think that by the time you cut a piece of wood that small all the internal stresses would be gone...

That has been my experience, especially after grinding both sides. That usually lets everything relax and straighten out.

   Why don't you try this...  Mark the edge of your board with two 45* pencil marks to keep track how the pieces where oriented when cut...  Cut 2 pieces about an 1/8" thick...  Now lay the pieces on a dry surface...  Put one upside down from the other piece...  Come back the next day and see what happens...  if they are both cupped upwards it is probably a moisture problem...  If one is cupped upwards and the other downwards it is internal stresses... 

It is too late for that on these, but I am going to keep it in mind on future projects. I brought them into the house from the shop to get the lower RH air and laid them cupped side down on my desk overnight. They straightened out a bit, so it may have been a combination of internal stresses and a bit of moisture. They are certainly close enough to use now, what is left will easily flatten out in the glue up.

when I make my thin lams/wedges/overlays what ever, I stack them up and lay a board on top of them on a flat surface  to keep them flat.
I always make that stuff last when it tapers to 0

Good advice. This is all happening slowly as I finish the form pieces and work through the various things that come up when learning a new technique. I expect the next bow will go a lot faster with a finished form and some experience on grinding lams.

There is some excellent info here, thanks to all for the replies.


Mark

[williwaw]

Did the cupping settle down?  After reading  https://www.esf.edu/wus/documents/EMCofWoodFPL268.pdf, I was reminded that equilibrium moisture content varies with temperature in addition to humidity.

[Flem]

Did the cupping settle down?  After reading  https://www.esf.edu/wus/documents/EMCofWoodFPL268.pdf, I was reminded that equilibrium moisture content varies with temperature in addition to humidity.

Excellent info that everyone should be aware of. Although it might be a moot point in this instance.