Thread: Dinning table rock

The top in picture you've posted looks to be made up of 7 or 8 sticks edge glued together.
if the wood is at equilibrium moisture content, and the growth rings are alternated or arranged the right way, and the boards are thick enough, and the same number of finish coats is applied to both the top and bottom faces of the top, the warping due to humidity change -- what I think you mean by "rocking" -- will be minimal.

extra heavy steel sections are not really necessary, and without allowance for wood movement could lead to the table top self destructing.

As Ian has mention wood movement is going to be your concern. This is countered by having the screw holes in the steel legs elongated. If you are concern about the top sagging over time due to the length of the table then move the legs in a bit closer together. It also depends on how thick you will make the table top. If you add a steel support under the table then the steel will sag before the table top will. This is especially true with a flat steel bar as to a square steel bar both will sag over time.

My concern is the table moving side to side (long ways) over time due to there being no cross beam. But I think you are saying if the wood is thick, which it will be 7 sticks of 150x40 messmate then this wont happen. Good to know about alternating grain I wasn't aware of this.

I suspect you mean racking i.e. rocking along the length of the table. The legs look like they're only about 100mm wide and as the others have warned, you would have to slot the screws to allow for wood movement except for the centre screws. You might get away with it but I would weld two pieces of flat iron, say 300mm long, into the inside centre of each leg flush against the underside of the table. This would triangulate the racking forces and provide greater long term stability.

mick

Great, that sounds like a good solution and keeps it simple and cost effective (building for myself). So the cross beam doesn't need to join the legs to each other to stop the racking, that's good to know. What does slot the screws mean? I tried google/youtube but didn't get anywhere.
Sorry for my newbie questions. Really appreciate you guys taking the time to help out.

Originally Posted by Twist

..... What does slot the screws mean? .....

In this context they mean to make elongated holes in the steel parts.

A bit of Wood-technology 101:
Wood is a hygroscopic material, i.e., it absorbs water from the surrounding air, according to the relative humidity. As it takes up water at high RH, so it give it back when RH is low, and in doing so it expands & contracts accordingly. It will happen whatever you do, and the amount it expands or contracts depends on the species (some move more than others), and any coatings ("finishes") you apply (these will slow-down, but not prevent moisture ingress/egress). Because of the way wood is 'made', virtually all of the movement occurs across the grain direction, change in length is virtually zero, and can be safely accommodated by the elasticity of the material. The total amount of movement from season to season can be quite considerable, up to 10mm in extreme cases. You cannot prevent this, & the thicker the board, the more power it develops. If you try to "tie the thing down" with multiple screws & epoxy, as I've seen attempted on many occasions, the most common result is that the wood simply splits at the highest stress points.

The other aspect of wood movement is that the cells closest to the outside absorb/release more moisture than those closer to the heart, so they move more, which is why boards 'cup'. This is what Ian & Christos were referring to. There are two schools of thought on how to deal with that. One school says to alternate the boards, i.e., heart side down with one, then up with the next, & so on. That way, the cupping is "averaged out". The other school says to keep them all the same, and the fixings will control any cupping (provided you allow the wood to move sideways). My experience is that the former method works better with thick tops (30mm plus), and the latter with thinner tops (<25mm).

The conventional approach to dealing with table-top movement is to fix it to the under-structure in a way that allows it to slide back & forth a little, particularly at the outer edges. In your case that would mean putting elongated holes in the steel - around 5mm long, say, at the outsides, but progressively less towards the centre.
However, you are up against a conflict of wood structure & mechanics with your chosen design, because having the screws loose is going to increase the chance of wracking. While the design would work ok for a coffee table, perhaps, the longer legs on a dining table are going to put a lot of stress on the fixings when the table is dragged across a floor (as they inevitably are!). Leaving the screws a bit loose as you need to do is only going to increase the risk that the legs will work loose and allow wracking. A far better solution from a structural point of view would be to weld rails between the leg forms, so that the undercarriage is a self-supporting structure independant of the top. Then you can safely use slightly loose screws. Heavy-gauge 30mm square tube would do the job, I think, and should not be visible from any normal viewing point....

Cheers,

Originally Posted by Twist

But in my research I see a lot of simple tables that use steel legs with no cross beam basically a bigger version of the various coffee tables I have built. Any insight into when a cross beam is needed or is it a case of using think heavy steel with big tight screws to stop the rock.

Be aware that a lot of modern furniture is built to only last a few years. In old-school tables there was always some provision to minimise racking incorporated as part of the deisgn, whether it be an apron or some form of bracing.

With the trend to steel legs, they can get away without this in the short term because they can fasten the legs to the table with screws and the width of the steel acts as a form of bracing. (And to a degree because, in the past, this design was mainly used for coffee/occasional tables, and not something that would see the usage a dining table has to handle. )

However, as the others have mentioned, the outer screws should be slotted to allow for timber expansion. This means that only the centre screw can be fully tightened... and is thus the weak point. Stresses on the table will result in this screw stretching/working loose and, eventually, the table will rack. For sure, for sure.

Mick's (Glider) solution of adding steel tabs is the least that they should be doing for the tables to have some longevity and they wanted to keep the same appearance.

The best design, IMHO, for this type of table would be to make a square frame from square tube that affixes to the top of the legs, making them a stand alone assembly and then routering slots in the underside of the table top so the frame recesses in and is concealed from casual inspection.

Of course, that's more work and would eat into their profit margin...

Originally Posted by Skew ChiDAMN!!

Be aware that a lot of modern furniture is built to only last a few years. In old-school tables there was always some provision to minimise racking incorporated as part of the design, whether it be an apron or some form of bracing.

With the trend to steel legs, they can get away without this in the short term because they can fasten the legs to the table with screws and the width of the steel acts as a form of bracing. (And to a degree because, in the past, this design was mainly used for coffee/occasional tables, and not something that would see the usage a dining table has to handle. )

Agree that modern furniture is intended to last only a few years before becoming land fill or, if a woody is lucky, a kerbside find.

connecting the legs with a flat steel bar down the center of the table might mostly eliminate racking. At a guess the steel bar could be the same section as used for the legs.