How can I make this X-legged table more stable?

Question

I bought a heavy (100kg) slab of wood and feet from a nice company.

There is a problem though: when I push the table on the small side, the table oscillates for several seconds. The amplitude of the oscillation is small (maybe 5 mm), but I find it strange. No other table that I have does that.

My other tables have much more structural components though, whereas this table only has 2 feet screwed independently to the table.

I've screwed in the screws as strongly as I could, with an impact driver.

Does anyone know:

1. Is this oscillation normal?
2. What is the cause of it? The fact that the feet are independent? The X shape of the feet? Something else?
3. What can I do about it?

Additional details from OPs comment, below:

• the wood is oak.
• Screws are only the 8 screws, Philipps head.
• I have talked about it with the nice company, they only suggested to drive the screws fully.
• I did not design the table in any way, I bought all from the company.
• I think the inward slant is only due to the picture. In practice, they are perpendicular.

Answer 1

Lateral diagonal reinforcement is needed. At the moment the feet are only protected against lateral tilting by the flimsly plate at the top. Try to immagine the resulting forces (lever law) when pushing the table in the lateral direction.

An possible solution would be to weld an strong steel plate on top of the feet, assuming the feet have enough wall strength.

Should that look unpleasant, one could hide the plate in an milled cavity.

Answer 2

You need to add diagonal bracing or struts as shown in the pic.

Without a dissertation about the laws of physics that apply, you simply need to use the strength of the triangle.

Even 1 inch square tubing can be used. You will probably have to find a welding shop to fabricate the struts. Then paint them yourself.

You could also try to find more stout table legs that are suited better for the weight of the top.

Thus is the result of trying to make your own table.

Answer 3

Nontrivial gaps are probably opening between the steel and the tabletop when you push horizontally. Insufficient stiffness of the steel ear plates is probably the root cause. The torsional stiffness of the horizontal steel tube flat to the table's underside could also be insufficient. A strategy to fix the problem without welding or adding braces is to use the tabletop's stiffness to harden things up.

The ear plates are deforming when your horizontal load shows up as a moment acting at the ear plate connections. The deformed shape looks something like the following, where I've exaggerated the scale of vertical movements:

The moment, M, activates a withdrawal force, W, in the screw and activates bearing pressure, C, between the wood and the steel ear plate. My strategy for stiffening this up is to install additional fasteners on top of that lump to restrain it against opening. The designer probably put his single fastener at its current spacing because as the distance between W and C increases, the magnitude of the screw's withdrawal force decreases. Minimizing the withdrawal force minimizes the risk of the screw tearing out of the wood, so that was certainly what the designer had in mind. My narrower fastener spacing would increase that risk if I proposed a single screw, but by using two fasteners at my narrower spacing, the tear-out risk will actually go down.

I would locate 2 fasteners per ear at about 1/2" away from the face of the horizontal steel tube. If you're worried about over-tightening the screws out of fear of tearing the wood, don't be. You can abandon bad holes and drill additional holes (or install threaded inserts) as a work-around.

You may think that drilling holes in steel is outside your skillset, but it's not. Hardware store drill bits quite commonly are designed for drilling steel. Just look for "metal" on the packaging. Be sure to wear safety glasses to minimize the risk from metal chips.

If the table is still too flexible, then the horizontal tube is flexing between the steel ear plate connections and the vertical components. The space between the vertical components and the ear plates looks so short in your photo that I'm skeptical that this is contributing significantly to the table's stiffness problem. Again, tiny gaps may be opening, but this time they're between the ear plate location and the vertical component location. Same strategy, different position: Drill holes through the horizontal steel tube and then install additional fasteners to restrain these gaps against opening. I would install a total of 16 fasteners, installing four 2X2 grid patterns at 3/4" in from the outside faces of the horizontal steel tube and evenly spaced between the vertical components and the steel ear plates.