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post #1 of 13 Old 08-03-2013, 08:00 PM Thread Starter
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Help, why is this happening

My husband makes furniture from cypress pine. he is self taught and does so well with little woodworking knowledge. we have had two tables out of many that have pulled apart on the table top were the tongue and grove is! My husband clamps glues and screws everything. He uses 19mm cypress pine tongue and groove flooring and its been kiln dried so we can't understand why it's shrinking slightly!?!? Can anyone give us some advice?
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post #2 of 13 Old 08-03-2013, 08:22 PM
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A picture would help a lot to see what you are up against. Just because the wood has the tongue and groove doesn't necessarily mean the joint will be strong. The wood would have to be very straight and the tongue and groove would have to be well matched before you can successfully glue it up. Then you mention screws. A wooden panel isn't like a piece of steel. It will expand and contract a lot and if you secure the outer part of the width of the panel with glue and screws and the wood shrinks it will bust in the middle to relieve the stress.
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post #3 of 13 Old 08-03-2013, 08:33 PM
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All wood expands and contracts whether is was kiln dried or not. As the relative humidity changes, the wood will absorb or emit moisture. This means that joints that are fixed across the long grain of the wood must be constructed in a manner that will allow the boards to freely move. If the movement is restricted, the board may warp, split or the glue joint may fail. Cross grain joints must be constructed with movement in mind.

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post #4 of 13 Old 08-03-2013, 10:37 PM
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Originally Posted by Keenwoodworker View Post
My husband makes furniture from cypress pine. he is self taught and does so well with little woodworking knowledge. we have had two tables out of many that have pulled apart on the table top were the tongue and grove is! My husband clamps glues and screws everything. He uses 19mm cypress pine tongue and groove flooring and its been kiln dried so we can't understand why it's shrinking slightly!?!? Can anyone give us some advice?
Wood will expand and contract depending on its moisture content. If its drying it will shrink. This happens across the grain, not in its length. The thicker and wider the wood is the more of a propensity to expand and contract (swell and shrink). Finishing the wood on all its surfaces may slow down the process, but likely will not stop the possibility of it happening.

As in many materials, wood is also subject to heat and cold, which ambient conditions like relative humidity can bear on its stability.






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post #5 of 13 Old 08-04-2013, 01:41 AM Thread Starter
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Thank you all for your advise I have attached a photo of the boards pulling apart! so the support peices under the top should not be glued to allow for movement? Is this what you mean? My husband carefully chooses the boards for the top making sure that they are straight! Is tends to be overally cautious with making his products strong I think it could be to his detriment :(
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post #6 of 13 Old 08-04-2013, 06:03 AM
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Thank you all for your advise I have attached a photo of the boards pulling apart! so the support peices under the top should not be glued to allow for movement? Is this what you mean? (
Exactly!
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post #7 of 13 Old 08-04-2013, 07:33 AM
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Thank you all for your advise I have attached a photo of the boards pulling apart! so the support peices under the top should not be glued to allow for movement? Is this what you mean?
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Exactly!
+1. I agree.

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My husband carefully chooses the boards for the top making sure that they are straight! Is tends to be overally cautious with making his products strong I think it could be to his detriment :(
How did he assure that the boards were straight. Did he joint the edges to get them straight, and square, or some other way, like just sighting the edge, or laying them to what was thought of as a straight edge or surface?

Properly jointed edges have no gap when placed together. As for gluing them up, jointed edges would require very little clamping pressure to insure a good joint.

But back to the separation problem. Once you have the boards glued up, any cross bracing under them should not be glued. What you can do is pick one board near the middle and you can secure that board with a screw. Then, towards the sides of the top you can make slotted holes in the support piece to insert screws with a washer head, or a pan head, just snug, so that if any one of the boards decides to expand, it can do that from the center one (that's screwed) out to the edges, without being restricted. The rest of the boards will move (or be pushed) in the same direction. Likewise if one or more decides to contract...the action/reaction can happen.






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post #8 of 13 Old 08-04-2013, 09:22 AM
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It's harder to get a good mating surface if the boards aren't perfectly flat, which would increase the chances of the glue up failing. Boards are best flattened and straightened using a jointer in conjunction with a planer to provide a flat reference face and a perfect 90° adjacent edge.

It could also be from using old glue. It's best when fresh.
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post #9 of 13 Old 08-04-2013, 11:02 AM
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It's not possible to select straight boards for glue up. The board have to be machined straight on a jointer just prior to glue up. You have to machine them and lay them out on a table and dry fit them together to the point where you couldn't slide a sheet of paper between. Then the tongue and groove could be put on the boards if you wished to use it. Personally I would rather use biscuits or a spline joint rather than the tongue and groove.

The support pieces under the top should never be glued but fastened with a minimal of screws. You could use more screws but you would need to elongate the holes and use pan head screws with washers and not tighten the screws completely. That way it would allow the wood to shrink. It helps to have kiln dried lumber but the wood is going to swell up in humid weather and shrink in dry weather so it's constantly on the move and the older it gets it will shrink more and more to where a table top may be 1/4" or more smaller in width in 20-30 years.
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post #10 of 13 Old 08-04-2013, 05:20 PM
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I'm well aware of what cypress is and of what pine is. Is there a such thing as "cypress pine"? If so, that's news to me. Or are you saying that he's making tables using both cypress AND pine?

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post #11 of 13 Old 08-04-2013, 05:53 PM
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The picture shown appears to be cypress - is it possibly AU Cypress?

The guys above have hit the nail on the head - wood moves and there is no way to prevent that so joinery / construction has to take this into account. The means of doing that is pretty broad though. In no case would the apron or battens be glued to the top but there are a few different ways to attach it.

Slotted holes for screws is a common one.

Another is a cleat or Z-clip.

A sliding dovetail is an exhaustive approach (with hand tools).

I particularly like how mid- late 17th through 18th century French and subsequently Acadians and Creoles went about this with nails. The top is composed of relatively narrow tongue and groove boards nailed down to the apron and battens but not glued together. This allows the expansion and contraction (excessive with a lack of climate control) to take place in the T&G joints. That was a direct evolution from similar means using pins rather than nails.

Last edited by firemedic; 08-04-2013 at 05:56 PM.
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post #12 of 13 Old 08-04-2013, 05:58 PM
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I never heard of cypress-pine either but it's for real. http://en.wikipedia.org/wiki/Callitris
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post #13 of 13 Old 08-05-2013, 05:20 AM Thread Starter
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We are in Australia and yes there is such thing as cypress pine it's a semi hardwood we buy our timber from the mill tongue and grooved they use for flooring. We will try not glueing the boards underneath to allow for movement!! The boards are perfectly straight when placed together. Thanks for your help :)
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