Cross bracing unstable wood

[Engineeruk]

Hi all,

First, a caveat - I’m very new to joinery so please excuse any unintended ignorance!

I’m edge joining 3, 200mm x 38mm boards to make a small (900mm long) tabletop.

The boards are plain sawn softwood and had some bowing in their cross section, though this has reduced after being dried.

I have limited tools, so was going to dowel and glue to join the boards. However I want to keep the thickness of the tabletop to a minimum so was going to omit wooden cross bracing such as skirts or aprons.

My question was with that constraint how best to prevent the boards from bowing?

I was considering using stainless steel bar underneath the tabletop to cross brace, as the thickness required in steel would be lower compared to wood, however, not sure how effective this would be. I was also considering using steel bar internally across the entire width (much like a very long dowel) but I feel this lacks elegance.

Would appreciate any advice from those who know much more than me!

 

[Jacob]

Hi all,

First, a caveat - I’m very new to joinery so please excuse any unintended ignorance!

I’m edge joining 3, 200mm x 38mm boards to make a small (900mm long) tabletop.

The boards are plain sawn softwood and had some bowing in their cross section, though this has reduced after being dried.

I have limited tools, so was going to dowel and glue to join the boards. However I want to keep the thickness of the tabletop to a minimum so was going to omit wooden cross bracing such as skirts or aprons.

My question was with that constraint how best to prevent the boards from bowing?

I was considering using stainless steel bar underneath the tabletop to cross brace, as the thickness required in steel would be lower compared to wood, however, not sure how effective this would be. I was also considering using steel bar internally across the entire width (much like a very long dowel) but I feel this lacks elegance.

Would appreciate any advice from those who know much more than me!

If they are going to bend there's little you can do to prevent it. Especially being so thick (38mm) which would generate a lot of force if it started to bow.
There are at least two possible solutions;
1 plane them thinner, say to 15mm or something. Then your structure will be more able to keep them flat
2 make sure they are dry to start with - plane them up square and straight, leave a few weeks and see if they still are. If not leave a few more weeks then do it again.

If you have sash cramps you will get a better join by plain butting them - as long as they fit together well and you cover both surfaces with glue. Dowels, dominoes etc are just for alignment and don't necessarily make a stronger joint.

PS If you constrain boards which are likely to bend they are likely to split instead. Might be better not to constrain them, let them bow and then plane them flat

 

[dzj]

Do you mean cupping instead of bowing?
A 90x20x3.8cm piece is more likely to cup than bow.
If it is bowing noticeably, best not to use it, because there's some kind of tension in it.

 

[Engineeruk]

Thanks both.

You’re right Dzj, I did mean cupping.

Does that change how I should work the boards?

 

[mbartlett99]

Not really - follow Jacobs advice. Plane them flat, let them sit, see if they move, rinse and repeat as necessary and only then plane the edges straight and square for gluing.

No need to go down to 15mm just get them flat and see if they keep moving. Sometimes you're lucky and its over in a day sometimes you'd swear the damn things are still growing.

How are you attaching the legs?

 

[Engineeruk]

Not really - follow Jacobs advice. Plane them flat, let them sit, see if they move, rinse and repeat as necessary and only then plane the edges straight and square for gluing.

No need to go down to 15mm just get them flat and see if they keep moving. Sometimes you're lucky and its over in a day sometimes you'd swear the damn things are still growing.

How are you attaching the legs?

Thanks - I’ll follow the advice here and won’t do any further cross bracing to avoid cupping

I’m going with steel hairpin legs so they’ll be directly screwed into the tabletop.

I was working on the assumption that because the wood is relatively thick that I wouldn’t need to do any further reinforcing, would you agree?

 

[Jacob]

.....
I was working on the assumption that because the wood is relatively thick that I wouldn’t need to do any further reinforcing, would you agree?

A small table at 38mm thick it won't need any bracing at all - you could just screw the hairpin legs straight to it, and then unscrew them later when you need to plane it all flat again!