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Problem with stock moving after machining

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markturner

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Hi, I seem to be suffering from a problem of stock that I machine to size in my workshop moving or distorting after I machine it? First noticed it on a piece of ripple sycamore that I machined to size, the blank was not distorted or twisted and I only took less than 3 or 4 mm off the surfaces to true it up and then ran it through the table saw to square it up. Within 2 days it has developed a noticeable wind of about 3mm and despite running it through the planer, it wont go.

Then today, I was preparing some timber to make my mothers Christmas present, a jewellery box, from Elm and pippy Oak. I had a nice plank of pippy oak, approx 20mm thick, a very slight cup in it, so planed it square. I ran it through the bandsaw to get 2 sheets of 10mm thickness for the top and bottom of the box, and as soon as I had completed the cut, both pieces sprang apart, and cupped, the faces are approx 5mm apart in the centre!!

What is causing this, and how can I stop it? Any way to make the cupped pieces flat? All timber stored and worked on in the same workshop in the same conditions.

Cheers, Mark
 

Steve Maskery

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Hi Mark
There are two issues here - moisture content and drying stresses.
Whilst maple is fairly stable, all burrs (and that includes pippy oak) and elm are notorious for their instability. It's because the grain direction is not uniform, it goes all over the place, which is why it's so attractive in the first place.
So these difficult timbers are often used as veneers on something more stable.
If you do want the solid, it would be better to resaw the board first, then flatten each board after it had been left to stabilise for a few days.
But the short answer is that you can't prevent wood from moving if it wants to!
S
 

woodbloke

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Steve Maskery":2rqbtbnj said:
Hi Mark
There are two issues here - moisture content and drying stresses.
Whilst maple is fairly stable, all burrs (and that includes pippy oak) and elm are notorious for their instability. It's because the grain direction is not uniform, it goes all over the place, which is why it's so attractive in the first place.
So these difficult timbers are often used as veneers on something more stable.
If you do want the solid, it would be better to resaw the board first, then flatten each board after it had been left to stabilise for a few days.
But the short answer is that you can't prevent wood from moving if it wants to!
S
Steve has it in a nutshell. These sorts of timber are notoriously unstable and if there are any stresses within the timber (elm 'specially) it'll move at the drop of a hat. I like elm, it's one of my favourite timbers, but I leave it for months to condition in the 'shop, then when it's rough sawn to approximate sizes, I leave it again and only then will I bring it down to a finished, usable size. If you have the patience to work with it, it's definitely great stuff - Rob
 

cmwatt

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I have experienced similar problems making a jewellery box out of elm. The elm was kiln dried, the problem is I was taking it between the house and the garage and the top warped about 5mm on one side of a piece approx 300x200mm. What I did was put it in a vice with some strips of softwood to keep it straight the entire length and then left if for 2 days like that in the garage. Made a big difference, it was now only about 1mm or so out, so I just too a plane to the top and flattened the rest and it has been fine until now. I've just left the box in the garage now rather than taking it between the house and there, and will then try and seal it as best I can.
 

jasonB

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markturner":1dlm3u2t said:
Within 2 days it has developed a noticeable wind of about 3mm and despite running it through the planer, it wont go.

Is that through a thicknesser or over a planer. If you put warped or tuisted wood through a thickness er it will come out just the same only thinner. You need to flatten one face by passing it OVER a planner then thickness it.

If a board is in wind you also need to pay attention to where you apply pressure as you run it over the planner otherwise it will twist as you move it along and not take material off the high spots.

What the others say about moisture and tension also apply

J
 

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