Thread: table top bowing, after getting straight

Sanding generates considerable heat, so much so that you can actually make the timber check at the ends when sanding end grain. At a guess, I would say the sanded side is now concave indicating the moisture content on the sanded side is now lower than the other un-sanded side? the fact that it is happening more in the middle third of your panel could be an indication that the boards you used were not properly seasoned before hand and the ends of the boards were dryer than that of the middle.

Also when storing flat panels, you either want to protect them from airflow by wrapping them in plastic (short term, I do it overnight to protect against the nightly humidity swings), or allow airflow to flow evenly around all sides of the panels by stickering them atleast 1" apart. If you stack them on the bench, one on top of the other. It is likely that the bottom two will be ok, but the top panel will either cup or crown depending on the conditions.

Originally Posted by Kuffy

Sanding generates considerable heat, so much so that you can actually make the timber check at the ends when sanding end grain. At a guess, I would say the sanded side is now concave indicating the moisture content on the sanded side is now lower than the other un-sanded side? the fact that it is happening more in the middle third of your panel could be an indication that the boards you used were not properly seasoned before hand and the ends of the boards were dryer than that of the middle.

Also when storing flat panels, you either want to protect them from airflow by wrapping them in plastic (short term, I do it overnight to protect against the nightly humidity swings), or allow airflow to flow evenly around all sides of the panels by stickering them atleast 1" apart. If you stack them on the bench, one on top of the other. It is likely that the bottom two will be ok, but the top panel will either cup or crown depending on the conditions.

interesting. I might put it through the thicknesser and see if stays flat. At this point I dont really care what happens to these laminated boards

My opinion is that Red Gum needs to be dead for many decades before it is stable.

Hooroo,

John

My guess would be that the timber is not properly seasoned. The bowing is most likely caused by a moisture imbalance between the two sides. It can happen when more material is removed from one side of the board in relation to the other. I learnt this (well, my client did) the hard way. He supplied me with the timber for a set of stairs, and asked me to dress as much as I could off one face as they were having problems with marks from the stickers. Sure, I machined all of the marks out, but created a moisture imbalance and all of the timber cupped. Not only was it the timber for the stairs, but also the entire floor in a very large house. The timber all came from one mill, and was treated the same way. i.e. most of the timber was machined extensively on one face in relation to the other.
Another cause could be the orientation of the growth rings. A piece of timber will shrink more around the growth rings that are closer to the outside of the tree than the rings closer to the centre. So, assuming that all of your boards are back sawn, if all of the growth rings are in the same orientation there will be a greater shrinkage on the side where the growth rings "round out". Hope that makes sense.
My money is on them being not properly seasoned though. I glued up a table top made from recycled hardwood roof battens about 12 months (probably longer) ago. It has sat in my shed since I glued it up, waiting to do exactly as you have described. Need to save my bickies for a domino, or find someone willing to let me borrow one. Anyway, it is as flat today as it was the day I glued it up. this timber was definitely well seasoned after spending countless years directly under a metal roof.

Was the sanded side sitting face up or down? On what kind of surface? Just flat on the bench? Assuming not quarter sawn, any changes day to day in moisture will effect the top and bottom sides of a panel unevenly in my experience and this can effect some timbers more abruptly and more obvious than others. I once glued up 300mm wide QLD walnut that was only 20mm thick and had bowing issues due to not stickering the panels. I stickered the panels, let them settle for a week prior to continuing and they were flat after that. Since then i've taken the precaution of stickering all work pieces between use rather than just stacking them on the bench or floor. I have a large supply of sticker material that I keep for this purpose. Either control the moisture/humidity in the environment or allow the wood to equalize evenly top and bottom would be my suggestion. If you want to attempt the waterfall edge, i'd cut the three panels and glue them up all on the same day to reduce the chances of further movement but wouldn't trust a waterfall miter joint with such timber over time.

thanks for the replies everybody.

Originally Posted by Wimmera Jack

My opinion is that Red Gum needs to be dead for many decades before it is stable.

Hooroo,

John

its not actually red gum. its mixed species of lighter colour'ed gum tree timber. basically what recycled timber suppliers in melbourne call 'messmate'. it wouldve been sawn and used green in the construction on houses.

Originally Posted by cjbfisher

My guess would be that the timber is not properly seasoned. The bowing is most likely caused by a moisture imbalance between the two sides. It can happen when more material is removed from one side of the board in relation to the other. I learnt this (well, my client did) the hard way. He supplied me with the timber for a set of stairs, and asked me to dress as much as I could off one face as they were having problems with marks from the stickers. Sure, I machined all of the marks out, but created a moisture imbalance and all of the timber cupped. Not only was it the timber for the stairs, but also the entire floor in a very large house. The timber all came from one mill, and was treated the same way. i.e. most of the timber was machined extensively on one face in relation to the other.
Another cause could be the orientation of the growth rings. A piece of timber will shrink more around the growth rings that are closer to the outside of the tree than the rings closer to the centre. So, assuming that all of your boards are back sawn, if all of the growth rings are in the same orientation there will be a greater shrinkage on the side where the growth rings "round out". Hope that makes sense.
My money is on them being not properly seasoned though. I glued up a table top made from recycled hardwood roof battens about 12 months (probably longer) ago. It has sat in my shed since I glued it up, waiting to do exactly as you have described. Need to save my bickies for a domino, or find someone willing to let me borrow one. Anyway, it is as flat today as it was the day I glued it up. this timber was definitely well seasoned after spending countless years directly under a metal roof.

Originally Posted by hurcorh

Was the sanded side sitting face up or down? On what kind of surface? Just flat on the bench? Assuming not quarter sawn, any changes day to day in moisture will effect the top and bottom sides of a panel unevenly in my experience and this can effect some timbers more abruptly and more obvious than others. I once glued up 300mm wide QLD walnut that was only 20mm thick and had bowing issues due to not stickering the panels. I stickered the panels, let them settle for a week prior to continuing and they were flat after that. Since then i've taken the precaution of stickering all work pieces between use rather than just stacking them on the bench or floor. I have a large supply of sticker material that I keep for this purpose. Either control the moisture/humidity in the environment or allow the wood to equalize evenly top and bottom would be my suggestion. If you want to attempt the waterfall edge, i'd cut the three panels and glue them up all on the same day to reduce the chances of further movement but wouldn't trust a waterfall miter joint with such timber over time.

in terms of seasoned vs unseasoned - it is recycled timber pulled out of old houses. I can only assume its seasoned as it would be many decades old. It was stored dried by the timber merchant and I've had it in my garage for a couple of months dried. I never checked it with a moisture meter though so it is plausible.

In terms of overnight storage - yep i just had it stacked. I cant remember which side was up or down.

Does the moisture/humidity theory answer the question why bowed pretty quick the next day? (after I re-sanded it flat with drum sander, flipped it over to sand other side and noticed that the flat side had bowed again)

Regarding bowing, was the grain direction across the panel alternated or anything like that? My only other thought aside from moisture content changes is that by thicknessing off of one side more than the other it cause internal stresses in the wood to react more on one side but i'm not so sure. I know none of this is definitive but there are a few variables it could possibly be.

I have had a similar issue with some Meranti that I glued together.

I had the Meranti stacked in a dry shed for 7(?) years and it was furniture grade when bought for a renovation.

One of the boards warped and bowed. No matter what I did, including puting the glued boards through a thicknesser, strapping it down, inverting it etc it still exhibits the warpage.

Sometimes it is just best to start again....

Originally Posted by hurcorh

Regarding bowing, was the grain direction across the panel alternated or anything like that?

did you alternate the grain rings -- one board arching up, the next arching down, ect. across the panel?
If you didn't this is the most likely reason for the panel bowing.

Originally Posted by hurcorh

Regarding bowing, was the grain direction across the panel alternated or anything like that? My only other thought aside from moisture content changes is that by thicknessing off of one side more than the other it cause internal stresses in the wood to react more on one side but i'm not so sure. I know none of this is definitive but there are a few variables it could possibly be.

i cant remember if they were alternated - its something ill check and get back to you on.

Originally Posted by cava

I have had a similar issue with some Meranti that I glued together.

I had the Meranti stacked in a dry shed for 7(?) years and it was furniture grade when bought for a renovation.

One of the boards warped and bowed. No matter what I did, including puting the glued boards through a thicknesser, strapping it down, inverting it etc it still exhibits the warpage.

Sometimes it is just best to start again....

haha damn. yeah ive given up on this and have started again already . so much wasted timber, resin and time!

rip the panel along the glue lines, turn every 2nd board over and re-glue -- should give you a usable panel

It’s good practise to rip and thickness the boards to just a tiny bit oversize, then leave in your workshop for two weeks to acclimatise. Any stresses or seasoning issues will be become evident - before you proceed to build the table.

You have all probably seen the random finger jointed wide boards in the green sheds, the top one in particular is cupped badly, the next about 1/2 as much, its not tuntill you get down to about the 4th or 5th one that they are ok, all for the lack of stickering to allow equal ventilation to all the boards

Hi
In my joinery business we used to manufacture 100's of bench tops/tabletops for a retailer every year. Timber being timber, with all the care taken, sometimes warp, that just a fact of life. The trouble is what do you do when this happen? As a manufacturer we cannot just throw them away or use them as firewood otherwise I would have been out of business in the first month!
Before processing any timber make sure it is seasoned, where I live around 12%, depending on the timber, is ok. Machine the timber straight and square. Do not glue up if the timber is warped hoping to straighten it while clamping as you are only adding tension into the top. If you do, there is every possibility the top could warp. If you have a wide thicknesser, machine alternative sides (this help in getting the top dead flat) then sand. I disagree with an earlier post that sanding produces to much heat and can warp a top, never seen that in 45 years, if the timber is too hot after sanding either your feed speed is to slow or your belt is blunt.
Remember if you have to wind up your clamps so hard to get the top together, there is something wrong and you are only putting tension into the top, this naturally can cause the top to warp.
So after all the care in the world you top is warped what do you do? Well this is what we did and is pretty standard in the industry.
Firstly if is warped across its width, just cut down the center, re machine the edge square and re glue. If it along its length you do not have many options I am sorry, there is no way to fix it other than cut along the glue lines and start again.