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Flattening Cupped Bookmatch Tops


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Since it is possible to bend wood over drop tops using heat and water as a buffer, can cupped tops be brought back into line? Specifically, I have two bookmatched camphor burl tops which I would really like to save the match on so is this possible? I was wondering whether a good night in a water soak followed by heavy compression against a very flat surface (such as granite) for a good long while. Obviously if the wood wants to move, it will I guess.

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Yep, I've done it with ebony so surely the camphor can be made to behave. I wet the concave sides, placed them facing each other (so cup facing cup), set 'em on a bench and slowly (over a few hours) applied clamping pressure until I closed the gap. Then I transfered them to a flat shelf with some weight on top, and left 'em until I was ready to glue them onto a body (about a year later).

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if they're going to stay flat, be glued to a solid substrate, a bit of water (concave side needs to 'swell' to flatten) and optionally a bunch of heat (iron them flat while clamping, then sticker and stack to allow airflow all around while keeping things lined up, and you're good. If you have heat blankets, they're good for this, too...

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I had this problem with a piece of ash that I had bookmatched --the two halves curled pretty much right away (I let the plank get hot in the car, my mistake). I clamped them between two flat boards and left it that way for a few days. And they were almost perfectly flat again, enough to glue them down. I didn't use heat, it's possible I could have gotten them completely flat.

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Im wondering, because it was brought up for a maple top thread I was reading before, but if your going to glue the top to the body, wont that hold it in place?

I know when I glued my maple insert, I used 2 25lb weights and once the glue dried it was perfect.

how bad is the cupping?

Just saying, so you dont have to wait a year for a nice top, that would work out fine simply glued.. especially if its going to get carved.

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I had this problem with a piece of ash that I had bookmatched --the two halves curled pretty much right away (I let the plank get hot in the car, my mistake). I clamped them between two flat boards and left it that way for a few days. And they were almost perfectly flat again, enough to glue them down. I didn't use heat, it's possible I could have gotten them completely flat.

Mick, what you had going on there is the wood was stabalizing. In other words the core and outside of the pieces you cut did not have equal moisture. When you saw wood, many times the heat produced by cutting will dry the outer freshly cut surface quickly(this is very pronounced if the wood has a higher moisture content). The wood then has a big difference in moisture from face to face and this will cause it to move quite a bit. If you make it a practice to sticker and weight wood after resawing you give it a chance to stabalize(which is what you did, well you didn't allow for airflow but by trapping the surfaces it allowed for equalization in the wood and after you exposed it to air again it finished equalizing). It can be a very tricky thing to deal with if you have a case hardened board(one that has trapped moisture in the core). Don't count on this solution if you have an evenly dried piece of wood that has stabalized in a cupped position.

As far as a small amount of cupping on thinner pieces that have stabalized. The bit of moisture and sticker and weight method can work great. If you have a thicker board or a very cupped board you will not have a good fix. Again this is all for boards that have stabalized. Boards that are in the process of equalizing(you just recieved them or freshly cut) weight and sticker(keep that air flow equal) until they have settled down.

If you have heavy cupping or twisting heat(not the moisture) is the only way to permanently change the shape of the wood. It can be very tricky to get wood hot enough to re-shape it. Most woods soften between 250-300 degrees. Getting thick wood that hot(evenly) can be a real hard thing to do. Boat builders place wood in long pipes that they generate steam in(for several hours). Then pull the planks out and rapidly place them against the form(boat), and allow them to cool back down in place. Steam boxes or pipes are one method that can be used for thicker woods. For thinner woods heat blankets seem to be a sensable approch. If a heat blanket is not an option. Sealing the wood between sheets of metal with a bit of moisture applied(sealing the edges of the sheet metal to contain the steam) is another option, but this will require a sufficient heat source to achive high enough temp to get the steam working.

Hope that is helpful.

Peace,Rich

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I'll just add that I had the same thing happen to me with a 1/4" top. Only one half cupped but I managed to straighten it out by steaming it over a boiling kettle & then adding pressure while it dried. The wood actually started to straighten almost imediately without weighting it but the weights stopped it from bending back again.

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Thanks for the support guys. These boards haven't been tested for moisture content yet, but that's a priority so I know what I'm working with. I accidentally left them standing against my cab and rushed off to work before popping them into the workshop. The sun hit the leading boards and, hey hey, cup goes the weasel.

Since these boards are about 10-12mm thick (7/16" > 1/2") I think I'll need to re-read the advice in this thread (just done a once-over relevance check as customary!) and get a plan of action into my head.

I actually want to thin one of the sets out by either re-thicknessing the non-bookmatched side or getting somebody to resaw and open out a new bookmatch on each piece. Given that in a perfect world I'd end up with two 4mm thick boards, how flat would they need to be to get them resawn nicely, and how many problems can I expect with the wood stabilising immediate after cut (I know this has been mentioned previously in the thread, will re-read) and during the re-equalisation process? If resawing 1/2" boards is impossible or has potential for the boards becoming write-offs, I'll ignore this path and get them thickness sanded. Either that or hold my horses and get them flat and stable first. Anyway. Motives clear mind open etc.

The other set will be going onto a carved top instrument of some description. No point in planning this too much lest the buildbug bite. Wood must settle Daniel-san.

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Thanks for the support guys. These boards haven't been tested for moisture content yet, but that's a priority so I know what I'm working with. I accidentally left them standing against my cab and rushed off to work before popping them into the workshop. The sun hit the leading boards and, hey hey, cup goes the weasel.

Since these boards are about 10-12mm thick (7/16" > 1/2") I think I'll need to re-read the advice in this thread (just done a once-over relevance check as customary!) and get a plan of action into my head.

I actually want to thin one of the sets out by either re-thicknessing the non-bookmatched side or getting somebody to resaw and open out a new bookmatch on each piece. Given that in a perfect world I'd end up with two 4mm thick boards, how flat would they need to be to get them resawn nicely, and how many problems can I expect with the wood stabilising immediate after cut (I know this has been mentioned previously in the thread, will re-read) and during the re-equalisation process? If resawing 1/2" boards is impossible or has potential for the boards becoming write-offs, I'll ignore this path and get them thickness sanded. Either that or hold my horses and get them flat and stable first. Anyway. Motives clear mind open etc.

The other set will be going onto a carved top instrument of some description. No point in planning this too much lest the buildbug bite. Wood must settle Daniel-san.

Sounds like you would do well to give the cupped site a very light surface spritz, and weight and sticker them. Let them aclimate and dry out well(at 1/2" range mostly dry wood will finish drying to a stable point pretty quickly). As far as re-sawing. The issue of heat causing cupping is a temperary thing as long as the wood had stabalized before you saw it(just weight and sticker for a day or two and it should relax just fine). When you are talking about re-sawing(given you have dry stable wood) it will be more up to how well the person who makes the cut and their equipment to get it done effectively. Given no odd mishaps and the wood is stable I slice soundboards @.17" or so one right after the next, and using Woodslicer blades the kerf loss is around 1/16". I would look close at the wood though if it is burled or spalted(sometimes that can be an issue if you get too thin).

P.S. To be safe it is best to weight and sticker any wood you recieve as soon as you open the packaging, and give it a week or so to aclimate.

Peace,Rich

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Thanks Rich - really straightforward info! I'd best search on *how* to sticker and weight wood pretty sharpish considering I received these boards on Monday...!!

BWAH!

Sticker- just place sticks 1/4 to 1/2" thick between the surfaces(lets air get to all surfaces). Weight- just put weight on the stacked wood. Also endgrain seal if you are storing wood for a longer period of time or if you are drying wetter wood.

Peace,Rich

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What Rich Said; space them evenly, exactly over each other on each layer, not too far from the ends, at least 3 for something sized like an electric top/body blank. Shellac or latex paint will do as a sealer if you don't want to go out and buy proper end seal. Most stuff you get should be sealed already, though. Also, if you don't have anything convenient to weight them down with, and you're using 1/2" stickers made of decent quality wood, or ply, you can use cable ties to tighten down each 'stack' of stickers. Basically using them like big bar clamps.

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I know I'm going to get a reprimanded by some here...

But I ended up just clamping the living hell out of my top and when I held my breath, undoing the clamps, it came out great...

My top was slightly cupped upward, meaning the center of the top was flush with the mahogany back...but the sides showed a small amount of space. Less than 1/8" each side.

I glued up the pieces, used alot of extra clamps and went to town. It's been almost 3 weeks since I loosened the clamps and things look ok.

-F

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you took a big chance there...just clamping stuff willy-nilly like that you run a tremendous risk of your top cracking...you really should have straightened it first.it's like punching a cow...sometimes the cow just looks at you..sometimes the cow tramples you...and then as you are picking yourself up off the ground and limping off you think"i can't believe i was stupid enough to punch a cow"(yes...that story is based in reality...never punch a cow) :D

you are not out of the woods yet either...it could still crack...though every day that it stays in one piece lowers the chances of that

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Its hard to imagine you're actually an adult rhoads56. Bum slapped in other thread? By who...

You're someone quite special, you really are. Any form of confrontation on these boards and you're in there. Even when theres nothing going on you'll start all the same.

And whats this about advertising? Whats that under your name? "Mods confirm" Jeez, you really do sound like the kid at school with no mates except teacher... Tbh, i'm really suprised you'd want to advertise your business when you're attitude on these boards stinks.

Edited by JonnyC
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Its hard to imagine you're actually an adult rhoads56. Bum slapped in other thread? By who... you?

You're someone quite special, you really are. Any form of confrontation on these boards and you're in there. Even when theres nothing going on you'll start all the same.

And whats this about advertising? Whats that under your name? "Mods confirm" Jeez, you really do sound like the kid at school with no mates except teacher... Tbh, i'm really suprised you'd want to advertise your business when you're attitude on these boards stinks.

Why you getting all worked up, it was a simple question :D

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you took a big chance there...just clamping stuff willy-nilly like that you run a tremendous risk of your top cracking...you really should have straightened it first.it's like punching a cow...sometimes the cow just looks at you..sometimes the cow tramples you...and then as you are picking yourself up off the ground and limping off you think"i can't believe i was stupid enough to punch a cow"(yes...that story is based in reality...never punch a cow) :D

you are not out of the woods yet either...it could still crack...though every day that it stays in one piece lowers the chances of that

Yup, I realize that....at the time, I *knew* was taking a chance...but something in me said "#$% it" and I went for it. This build is progressing so slowly anyway, that if the top cracks I wont have been too far into it...granted I will have lost what time I put into it...and the wood....but so far so good.

I definitely wont do this again...but this one time, I decided to take the chance.

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So yeah, anyway - saving my cupped boards....

I like the addition of cows to this one. I approve. You two bumslappers get a room will ya? :D

Given that the boards are fairly small and in small quantities, I think I'll go with your advice Rich and Mattia. The boards will be put in the workshop on a flat surface with stickers underneath (one at each end and one in the centre) and layer them up before weighting with a couple of concrete blocks. A bit of dampening "in" the cupped curve sounds like a plan.

I'll advise on the boards progress. I'm not furthering the Mockingbird build until I get the fretboard anyway :-D

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Hi P,

I wouldn't dare try forcing cupped wood to bend, remember that bloodwood drop top that cracked? It was from clamping it and it split.

What I now do is plane the wood it back flat.

Imagine the wood is cupped in a C shape (ie plainsawn or flastsawn wood that is arched up in the middle). I start by running the piece with the raised arch center through and planing that side flat. Then I flip it over and run it through to plane down the 2 raised edges. So it goes from C to |

You lose material, but if the other option is a permanently warped piece of wood or a cracked piece of wood, then it's better to have a thinner top that's flat.

Now for Riftsawn wood that's got a twist in it, a wide Jointer would be best. Otherwise, you can always use a palm sander or hand plane to get it as flat as possible.

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