Why??!! Why why why why.....?

[Rallymantony]

Hello peoples of the woodworking world.

So I'm not completely new to wood but I am what you would class as a newbie, with that in mind I'm hoping I can get some guidance.

I'm trying to make a small table top, I've set my bench saw up as per instructed, blade square to fence and 90d to the table top, I've run the timber through the saw to give finished sides, glued, clamped and left to set.... Actually I've done this a few times and always the same result.... the timber bows! It's never 100% flat on surface no matter how many clamps I have on it, I clamp top and bottom, do each clamp up slowly bit by bit but always the same result...... Any help greatly appreciated..

 

[Valhalla]

Are you talking about bowing (along the length) or cupping (across the width)? If it's the latter then read on:

That's the nature of the beast....it's flat until it isn't. My guess is you're using plain sawn timber - big growth rings acrosss the ends (Quarter sawn is always better as it moves less than plain sawn timber, and if you're using oak you may get some nice medullary rays). Having said that, in quarter sawn timber the grain tends to be fairly straight and less interesting in terms of variation of grain and figure (and more expensive!!).

To deal with this it might be better to cut at least 1/4" oversize on the thickness and let it sit for a couple of weeks to let it 'do its thing'. You will probable find it will have cupped again....so plane it down some more and leave it some more. Eventually the cupping will stop as the timber dries out.

An alternative, if you are using wide boards (and the cupping is excessive) is to rip them down the middle. This will drastically reduce the amount of cup in each half. Plane each half as above and when you are satisfied that the shrinkage has stopped - plane them down so that they are reasonable flat to about an 1/8" or so over final size and glue them back together (you are not looking for a finished surface here - just flat or thereabouts). Once the glue has dried and you are satisfied that the shrinkage has stopped, plane them down to final size as and get it fixed to the frame as this will help reduce any further movement.

This is not a definitive process - there are other factors that come in to play such as grain orientation - grain running in opposing directions to help limit movement and then there's the look of the grain itself - this might dictate the positioning of the board - and so on................


Patience is the key here. - I learnt that the hard way

Cheers

 

[Jameshow]

Which way are you growth rings running.

If U U U U U

The. You need to alternate planks to become:-

n u n u n u n etc

Appologies if teaching you to suck eggs .

 

[Just4Fun]

@Rallymantony What is the story behind your username?

 

[Rallymantony]

Thanks guys, I really do appreciate you taking the time.
On this particular project I am using 20mm kiln dried chestnut tongue and groove, I sliced the tongue and groove off as the ends will be seen, the timber has been in my workshop for around a year and some of it has bowed so I've them to one side for the moment, the timber is actually for flooring so has the cupping grooves router on the rear, I milled this down with the thicknesses to remove. My problem comes when I glue and clamp, it always cups, I know its me as it happens every time... If I can work out how to upload a picture I will add one to show what I mean over the weekend..

 

[6x4]

There are some good answers above about letting the timber acclimatise after cutting and alternating growth rings.

Can I suggest also if you’ve thicknessed significant depth off one face, ‘cupping grooves router on the rear, I milled this down with the thicknesses to remove’ that would be a candidate for root cause as this will release stresses inside the wood on one side only and trigger movement. Do this if required but then go through the acclimatisation/re-plane cycle above, standard practice thicknessing is to take the same amount off both faces/alternate faces when removing any significant material. Good luck.

 

[Valhalla]

@Rallymantony What is the story behind your username?

I used to love reading the comic book stories of Thor and his hammer Mjolnir - I have a large carving mallet (made out of Lignum) that I call Mjolnir....
And it was a childhood dream that when the ship comes to carry me off that's where I'm going (unless they take a wrong turn...) although I don't know if I've got the credibility to get in .......not being a Norse warrior and all that

 

[Droogs]

I had Norse Pagan as my official religion on my army docs so that the army would have to give me a Viking funeral if I had headbutted a round by mistake.

 

[Spectric]

the timber bows! It's never 100% flat on surface no matter how many clamps I have on it,

Welcome to the world of wood, it is in my opinion one of the aspects of woodworking that is most frustrating and you need to understand, it still drives me mad. Start with the basics of wood, some good video's out there on the topic and if you start with shiete wood then you are going to struggle.

Free

and plenty of others

or buy Peter Sefton Timber Selection Download

Also clamps, clamping could be another name for woodworking because with metal a few G clamps and you are away, with wood you need every shape and type of clamp going to try and control the work from doing it's own thing.

 

[Droogs]

Although he really doesn't like to blow his own trumpet Sgian Dubh of this parish has authored a book published by Chris Sxhwartz's Lost Art Press that is probably one of the best foundation reads for anyone who wants to understand working with wood and why it does what it does called "Cut & Dried". Think about getting it, you'll be glad you did.

 

[recipio]

There are a number of possible causes for this. Grain orientation is the most common as detailed above but are you sure the saw is cutting absolutely accurate 90 degree angles. ? A digital inclinometer is a good investment and much more accurate than eyeballing with a trysquare.
Some woods move when cut - I find Iroko the worst for this. Lastly you can correct a cupped board by running a sawkerf down the middle on the underside and then glue in a slip of the same wood. Clamp down on a flat surface until the glue is dry. Nor entirely invisible but it saves the piece from being scrapped.

 

[Doug71]

It's not really ideal using the edge from the saw as a finished edge but to check if it is the saw that is out you could cut one piece down face up and the next face down, this should cancel out any error of the blade not being 90 degrees.

 

[Valhalla]

I had Norse Pagan as my official religion on my army docs so that the army would have to give me a Viking funeral if I had headbutted a round by mistake.

I call your Norse Pagan religion and raise you a Jedi Knight in one general election

 

[RobTy]

I call your Norse Pagan religion and raise you a Jedi Knight in one general election

Trump that with Sith Lord in the census.

 

[Rallymantony]

@Rallymantony What is the story behind your username?

I'm a rally instructor and my names Tony........ In truth I've not competed for a few years now unfortunately

 

[Rallymantony]

There are a number of possible causes for this. Grain orientation is the most common as detailed above but are you sure the saw is cutting absolutely accurate 90 degree angles. ? A digital inclinometer is a good investment and much more accurate than eyeballing with a trysquare.
Some woods move when cut - I find Iroko the worst for this. Lastly you can correct a cupped board by running a sawkerf down the middle on the underside and then glue in a slip of the same wood. Clamp down on a flat surface until the glue is dry. Nor entirely invisible but it saves the piece from being scrapped.

Thank you for taking the time, I bought a digital inclinometer and it has helped and improved over using a square. I think my issues are from getting the clamping wrong somehow, going to invest in some some clamps as I only have 6 at the moment

 

[recipio]

Thank you for taking the time, I bought a digital inclinometer and it has helped and improved over using a square. I think my issues are from getting the clamping wrong somehow, going to invest in some some clamps as I only have 6 at the moment

Yes, using too much force with clamps will cause the wood to bow a little. All joints just need to be 'nipped up' until the glue squeezes out. Alternate the clamps top and bottom to even out the clamping forces.

 

[thetyreman]

it took 6 years and 3 attempts at flattening my workbench with a no7 plane before it's finally stopped moving.

 

[Rallymantony]

Yes, using too much force with clamps will cause the wood to bow a little. All joints just need to be 'nipped up' until the glue squeezes out. Alternate the clamps top and bottom to even out the clamping forces.

Thanks Recipio, I think that it is to do with to much clamp pressure, I do alternate them but as I said, I only have six clamps so a few more may help. I worry that if the clamps aren't tight as in as tight as I can get them then there will be gaps or the glue won't bond enough..... Trial and error I guess

 

[recipio]

Thanks Recipio, I think that it is to do with to much clamp pressure, I do alternate them but as I said, I only have six clamps so a few more may help. I worry that if the clamps aren't tight as in as tight as I can get them then there will be gaps or the glue won't bond enough..... Trial and error I guess

That's down to technique. You should have no hairline gaps before glueing. In fact you should lay the boards edge to edge and and have a virtually invisible joint line. It takes practice - most people advocate planing the edge but I get better results off the table saw. A good blade, a well set up saw and using the digital inclinometer are pretty much essential.
You can never have too many clamps but don't rush out and buy a bunch of cheap clamps - try one or two before committing the hard earned.