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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.

table

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  • 2
    Base of table is too narrow. 100 kg is EXTREMELY heavy, and this table is very much top-heavy.
    – Nelson
    Oct 3 at 8:16
  • 4
    "As strongly as I could with an impact driver" doesn't inspire confidence. What kind of wood is the table top? What size screws did you use? Did they have hex bolt heads? Phillips? Square drive? Was it just the 8 screws that I see?
    – popham
    Oct 3 at 9:32
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    Have you talked it over with the "nice company" that supplied it?
    – popham
    Oct 3 at 9:37
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    Can you please clarify if you designed the table by combining the top from Company A and the legs from Company B, or did you purchase the whole thing from "Nice Company"? If you bought it as a kit from "Nice Company", then you need to go back to them to ask them about the wobbliness, because no, it shouldn't wobble. If you designed it yourself, you're going to need some diagonal bracing as noted in a couple of the answers.
    – FreeMan
    Oct 3 at 16:22
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    I watch a guy on YouTube who makes heavy wooden slab tables like this, generally with a similar leg arrangement. He never uses wood screws straight in for the legs; he glues in metal inserts, and uses thick bolts, a bunch of them in two rows for each leg. Definitely more than four per leg. Oct 4 at 19:34

3 Answers 3

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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.

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  • Lateral diagonal reinforcement is sufficient, not necessary.
    – popham
    Oct 4 at 0:43
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    @isherwood, I was complaining about the use of the word "needed." Diagonal bracing will totally wreck this table's aesthetics. It will solve the problem (as could a sheet of plywood), making it sufficient to solve the problem. Fortunately there exist non-bracing solutions (that won't wreck the table's aesthetics), making bracing sufficient, but not necessary.
    – popham
    Oct 5 at 15:48
  • Fair, but I disagree. 1/16" cable between the cross points and a loop under the table center could actually enhance the aesthetic.
    – isherwood
    Oct 5 at 15:50
  • @isherwood, wire rope guys going up to the center of the table? Am I reading that right? This would require a horizontal compression strut to equilibrate the tension. I suppose you could anchor the table to the floor and use that as your compression strut. Any diagonal brace, I think, would need compressive strength to avoid this compression strut, but that is my subjective aesthetic judgement.
    – popham
    Oct 5 at 15:57
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    @isherwood, I see tension rods in the wild frequently, where they've been stretched and no longer do anything (without the horizontal deflection that stretched them). That table's wobble would be back in no time, requiring constant intervention to re-tension the wire rope.
    – popham
    Oct 5 at 16:03
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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.

enter image description here

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  • 1
    Buy 'hairpin' table legs at $50 a pop. google.com/shopping/product/…
    – Mazura
    Oct 3 at 23:34
  • Diagonal bracing is sufficient, not necessary.
    – popham
    Oct 4 at 0:44
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    Plenty of big, heavy-topped tables use straight vertical legs with no bracing, just screwed to the underside of the top. Bracing is far from an absolute requirement as you imply.
    – Chris H
    Oct 4 at 11:02
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    @ chrisH, The answer was in reference to the legs the OP has on the table. Sure there are other legs that may be more suited to the slab. However that was not the question.
    – RMDman
    Oct 4 at 11:24
  • Bracing the trestles independently, from the centres of the Xs, should be sufficient and give more clearance under the table. IMO. Oct 4 at 18:04
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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:

enter image description here

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.

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    Thank you @popham for your suggestions. A few questions to make sure I fully understand your suggestion: What do you mean by "ear plates": is it the small metal plates on the feet, that are horizontal and flush below the table? Also, by fastener, do you mean simply a screw? Finally, by horizontal steel tube, do you mean the top of the feet? Thanks!
    – DevShark
    Oct 4 at 18:52
  • @DevShark, I can confirm your ear plate definition. There are 4 ear plate locations, with 2 ear plates per location. Each ear plate appears to have one and only one fastener, where I've been assuming that these are the only fasteners attaching the legs to the table surface.
    – popham
    Oct 4 at 19:10
  • @DevShark, I can again confirm your horizontal steel tube definition. I couldn't tell from the photo if this was a plate or the same material used for the X's.
    – popham
    Oct 4 at 19:11
  • @DevShark, by "fastener" I intended screws if that's what has already been installed. If you haven't punched through the top, then why tempt fate? Use more of what certainly works. Personally, I would use threaded inserts and mating socket cap screws.
    – popham
    Oct 4 at 19:11

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