How do I calculate board thickness/strength for a project.

[DomDru]

Hi all,
This may be a strange for for experienced makers, but How do I decide on a board thickness for a piece of furniture? Previous projects I made, where rather traditional, so I just checked what a similar piece would use and used same thickness boards. Now I'm designing a dining table with a central support. similar to a coffee table I have made last year.


First of all, how do I make sure the wood is strong enough and will not sag over time? I really want to keep the table looking as light as possible for the aesthetics, as well as the lumber cost. I think I have a good idea of what would work for this one, but I'm open for suggestion from someone who's more experienced.

Second problem is something I got wrong on my coffee table. My coffee table twists slightly. The central pillar is strong enough but can flex a little around the vertical axis. If pushed the top can twist around 5 degrees. That is not a problem for a small coffee table, but I really don't want this to happen on a large dining table. How do I redesign/reinforce the pillar so it's a lot stiffer and doesn't twist?

Thanks,
Dom

 

[Kicked Back]

Second problem is something I got wrong on my coffee table. My coffee table twists slightly. The central pillar is strong enough but can flex a little around the vertical axis. If pushed the top can twist around 5 degrees. That is not a problem for a small coffee table, but I really don't want this to happen on a large dining table. How do I redesign/reinforce the pillar so it's a lot stiffer and doesn't twist?

Thanks,
Dom

How are the two cross-sections attached to one another?

 

[DomDru]

Like a very tall cross lap joint. friction fit with a bit of glue.

 

[6x4]

On the second part of your question, that would seem to be the middle of the base assembly flexing under torsion from the top so yes, I’d agree that’s not desirable. Fixing could just involve going massive on your build, or the first thing I’d try would be triangular fillets in the inside corners of the top end of the cross, under the table. If you need more then try to design in big diagonals or better still triangles from 2 points under the table to an apex as far down the base support as you can, repeated on each arm of the cross. Hope that description makes at least some sense, good luck with the big build.

 

[blackteaonesugar]

Hard to give a definitive answer on the first question because there are so many potential variables involved.
If you're unsure of things on a given design, then that's where mock-ups and tests come in.

Your edge thickness, however meaty, doesn't have to be consistent through the whole table.

Visual-weight can be reduced by cutting undercuts at the table edge, producing a finer visual appearance while maintaining actual thickness.
I nearly always do this to a tabletop.
It's a great way of keeping integrity to the structure but reducing that bulk to the eye.

For the second question. That design if you think about it is one of leverage.
You're grasping the outside of quite a large radius and the force is only resisted by a very small vertical joint in the centre.
You're bound sure to be able to twist it with a mechanical advantage like that.
To reduce that you could put filler blocks at the top and bottom of each opposing 90 degree face, making it more like a solid block. (Edit - What @6x4 explained better than me above)

You'll still get a little twist and that's usually ok. It's inherent in that kind of design to a degree.
Most people don't go around trying to twist coffee tables though so I wouldn't worry too much

 

[Jacob]

Like a very tall cross lap joint. friction fit with a bit of glue.

Sounds very bendy!
By and large woodworkers don't "calculate" anything much. It's more play it by ear, copy what works, trial and error.

You have a pedestal with 4 brackets/legs.
Most obvious; they could be housed into a central post with a DT housing, just like a million tried and tested Georgian pedestal tables. The post could be turned.
Or a bit of a bodge - one pair should be constructed to go through together as one piece, the other two housed into its sides.
Or the four brackets could be staggered to form a box in the middle (instead of central post as above) the edge of each housed say 3" back from the face of the next one.
And so on.
Stability in the top is achieved in round tables by making the top material quite thin e.g. 20mm. This edge jointed then screwed (not glued) to a small apron around the circumference which can be shaped, moulded, left plain, veneered whatever.
The middle of the board then supported by a square H frame with the 4 corners of the H near the inside of the apron. The cross piece doubled and loosely M&T d into the sides of the H with space for movement. All screwed, no glue.
Sounds a bit complicated - have a look at some tables, don't try to reinvent the wheel!

 

[DomDru]

Thanks everyone for the answers!!!

@6x4 Fillets to make it less twisty sounds like a good idea. I was considering making it into a solid block, but because it spreads out a lot at the top and bottom I could not come up with a pleasing transition.

Hard to give a definitive answer on the first question because there are so many potential variables involved.
If you're unsure of things on a given design, then that's where mock-ups and tests come in.

I was digging around in some structural engineering textbooks and managed to find a technical term for what I'm asking. It's called "cantilever beam deflection". And yes, there are still many variables to consider, but at least it will give me a starting point. Found this handy Beam Deflection Calculator online for anyone looking for something more than just a guess.

Sounds a bit complicated - have a look at some tables, don't try to reinvent the wheel!

Reinventing things is the best part of this hobby. If I wanted something that already exist I would buy it. The item or the plans to make it.
But a frame sound like a solution I was looking for. And if I keep it away from the edge, apron may not be needed as the frame wouldn't be visible unless someone ducks under the table. It makes me feel a little stupid now, since my parents kitchen table is made like that Now just need to design one for an ellipse.

Thanks!

 

[Jacob]

....Reinventing things is the best part of this hobby.

But you haven't reinvented anything you've just designed it badly! There isn't much goes on in the way of reinvention anyway - it usually means just re-styling, decorative changes

........And if I keep it away from the edge, apron may not be needed as the frame wouldn't be visible unless someone ducks under the table.

The apron on a round table can act like a stiff hoop, helping to keep flat the material fixed to it.

It makes me feel a little stupid now, since my parents kitchen table is made like that.....

Always look at existing stuff first and copy wherever you can.

 

[thetyreman]

you need to have a plan first before making anything, draw it out fullscale, and then create a cutting list in that order

 

[Limey Lurker]

The on-line "Sagulator" might help you; if in this case it doesn't, it's a very useful thing to have in your Favourites bar.