Kitchen table design

[BSevier]

I have been asked to work on a kitchen table project that this person has designed. I have some concerns about the stability of the table and am asking for anyone who may have some knowledge on these things.

In the attached photo, he shows just three legs to a round column that goes up to a 50" round top. One of my concerns is having just three legs, the other is that he wants the column to be just 2" in diameter.

I was thinking of using a bent lamination for each leg that would then join at the column. With just three legs - each projecting 18", will this be stable? With the column being just 2" in diameter, will it provide enough support for the table top?

I would hate to get through the project and find out it will be a failure, but I also cannot calculate if it is feasible. I have searched google for images of other tables like this, but haven't found any. I would think that if this was feasible, someone would have done it.

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Any ideas?

Thank you,

Bryan

 

[JohnnyR]

Just from looking at it I can't see this being stable. With only three legs, the 18" from center becomes about 14-15" for the fulcrum point, and all the weight is up high.

 

[jazzflute]

More questions (of course...):

What size is the table top?
What material and thickness? (top and legs/column)
How is the column to be attached to the top?
How much usage will the table get, and how severe? (i.e. a showpiece in the corner vs. a kid trying to sit on it...)

K

 

[Dave Richards]

Based on those proportions, I don't think you can get enough stiffness in a 2" dia. column made of wood. Steel, maybe, but i'd bet the thing would still be wobbly. If the client is dead set on that design, I'd consider turning down the job.

 

[BSevier]

The entire table is to be made from Cherry - the top will be 3/4" by 50". To attach the column to the top, I was thinking of having a tenon at the top of the column feed into a connection that will then screw to the bottom of the table.

This will be the kitchen table, so usage will be heavy.

 

[jazzflute]

The entire table is to be made from Cherry - the top will be 3/4" by 50". To attach the column to the top, I was thinking of having a tenon at the top of the column feed into a connection that will then screw to the bottom of the table.

This will be the kitchen table, so usage will be heavy.

I know my reputation for serious posts precedes me, but I am actually serious with this suggestion:

In order to not fail, I think that the center post needs to be iron or steel, and the table needs to be fastened to the floor.

The max viable tenon size for a 2" shaft is slight less than 1" square, and the leverage of a 25" lever leaning on that single joint is sure to bring about either tenon or mortise failure. Basically you're talking about using a tenon the size of two 10mm Festool Dominos to attach the table. Not so good.

K

 

[Endless Pursuit]

It looks pretty on paper but if the intent is to use it for more than a resting place for a potted plant placed dead center, I'd pass. If the column was 4" OD and the feet extended 2 ft from the centerline, maybe.

Best thing to do is draw to 1/2 scale and see what it looks like.

 

[BSevier]

Thanks for the feedback.

Basically, you are confirming my concerns. I just dont have the engineering degree to say this will fail.

My thoughts were to have a minimum 4" (preferably 5") column with 4 legs.

The customer wants the legs to appear blended - not "attached" to the column. That was the reason I was going to use a bent lam for each leg and join them to a central column that then goes into a round connector ~18" in diameter that then screws to the bottom of the table. It would then be a bunch of hand work with a spokeshave to get the legs to the final round/oval shape.

However, I am open for ideas.

 

[Dave Richards]

I just did a quickie sketch to try to get the proportions. As drawn the top is 50" dia. and the column is 2" dia. I guessed at the height above the floor where the legs meet the column but the extend out 18 in. from the center line. I did a fast center of gravity calculation. You can see it winds up just under the top.


It's not going to take much to tip this table over if the column or the attachment at the top don't fail first.

 

[BSevier]

Dave,

Thats awesome - thanks.

Can I ask if you did that drawing with the free version of SketchUp?

If so, I guess I need to spend more time with the program. I use it for most of my drawings, but had trouble with some of the angles needed with this table.

I also didn't realize it could do CG.

 

[Dave Richards]

Bryan, I use the pro version but there isn't anything I did that couldn't be done with the free version. The CG calculation was done with a plugin.

 

[BSevier]

Thanks, I will check out the plugin for it and hit youtube for some training.

So I realize the CG is way to high for this design, and I know you want the CG as low as possible to make it stable.

But, for a 'typical' table where would you expect the CG? I figure if I can get it close to a 'typical' table, it will be feasible.

 

[smallboat]

I think the issue is the height of the CG coupled with the narrow base.
Look at Dave's second drawing which represents the view on an axis parallel to where any two legs contact the floor.
It wouldn't take much to push that high CG out past the feet.

The leverage of the wide table top just makes it worse.

In a typical table the CG is well within the footprint and the overhang is much less extreme.

 

[Dave Richards]

I would expect the CG would be relatively high on any table but the legs on most tables would probably cover a larger footprint. Pedestals of this sort made of metal would tend to be weighted to bring the CG down.

Edit to add: smallboat said that better than I did.

Be careful with the YouTube tutorials. Some are good; some aren't.

 

[SteveHall]

It's a simple lever. By my calculations, the length of longest overhang (of the eccentric base shape) is about half that from the fulcrum to the opposite side:



Ignoring the weight of the base since it is so close to the fulcrum, a force only twice the weight of the top will tip it over. (Including the weight of the base might buy about half the bases's weight more resistance to tipping.)

I'd guess this table tips from less weight than a three year old hanging on its edge. There's a reason sturdy kitchen tables have legs within an inch or two of their edges!

 

[Bill Clemmons]

But, for a 'typical' table where would you expect the CG? I figure if I can get it close to a 'typical' table, it will be feasible.

For a single pedestal table like this, I would want the center of gravity no higher than the intersection of the legs w/ the pedestal. Even then, I would still be uncomfortable w/ it.

This is similar to tables made by the Shakers. They used a wider pedestal, usually wider at the bottom than the top. The legs covered a wider area, relative to the top. And the top was seldom more than 24" in diameter.

 

[Dave Richards]

Excellent, Steve!

And considering the intent is to make the legs from laminated cherry, the weight of the base will be rather insignificant.

 

[BSevier]

found the plugin - played around with it - pretty easy to use.

i am still struggling to attach the round legs to the round column and have them descend at an angle. back to the tutorials...

 

[Dave Richards]

Maybe this will give you an idea or two.



Draw a line representing the centerline of the column and one leg. Include an arc for the curve at the transition. Place a 2" dia. circle on the top.
Run Follow Me.
Select and scale the bottom end of the extrusion to make the leg taper. The taper will start and the lower end of the bend. That may not really be where you want it to start but it'll probably work for what you need. There are other methods to do this if you really need to show more detail.
Select the entire extrusion and copy/rotate it to make the other two legs.

 

[SubGuy]

You can add more legs. That increases the fulcrum point out closer to the leg extension distance. 4 or 5 would make a big difference. But I would suggest something more like this: I made a table that was I think around 54" and my original base was this:

It was 19". NO STABILITY AT ALL. then went with this:

If I remember right the span is 38 or 40" and it is thoroughly stable.