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  • Dining table design question

    Hey all!

    I am thinking of building a dining table. I am still at the design phase. Keep in mind that this is going to be my first one. The room in which I want to but the table is large: 14' x 21'. I want to minimize cutting as much as I can.

    So I opted for a 48" x 96" veneered 3/4" plywood. Under which I'll glue and screw another regular select 3/4" plywood. That'll give me a table plate of 1.5" thick. Around the edge I'll glue and nail a trim. This table will not be cut in the middle to allow for an extension. It is made to be one piece.

    For the frame, I am going to make/buy tapered legs. My main concern is with the apron. I do not want to go with the straight one size piece of stock across the table. I'd like it to be arched (the cutting would be done by a pro with a template).

    Being 8' wide, there is going to be flex as you get closer to the center of the table side. I want this to be as minimal as possible... so I'd go the maple way as it has high density and little flex.

    Back to the middle of the arched apron and my first question. What would be the height of the apron at that middle point that would give me the maximum strength?

    Back at the table top now... 48" x 96"... quite a span here... If I were to stand up in the middle of the table... chances are that it would flex there too. My second question is how can I stiffen the plate? Running cross members on its width? If so at which interval?

    I've added a sketch of the side view of the table.

    Thanks in advance!
    Attached Files
    Last edited by HornyPotter; 09-16-2010, 04:54 PM.

  • #2
    Re: Dining table design question

    I don't understand your comments about the apron not being one piece.... it should be one piece. Unfortunately with an arch design, the beam (apron) is at is most flexible and also weakest at the center of the span where the bending moments are highest. You would want the beam to be thickest at the center... an optimized design for strength at minimum weight would make it maximum at the center and tapering to the end. An optimization for stiffness would alkso be tapered, but not a straight taper. Your question is, what dimension will make the apron the strongest.... the answer is, somewhat intuitively, the bigger you can make it, the stronger and stiffer it will be.

    Plywood is basically softwood veneers with hardwood face veneers. Since the veneers are arranged with the grain (and hence, the stiff direction) alternating in a perpendicular sense, plywood tends to be significantly more flexible than solid wood. Although at your 1.5 inch thickness (it will be a about 1/16 less since most 3/4 ply is really ~23/32) it should be pretty stiff. I make large tabletops exclusively out of solid stock. This has the advantage of making the top repairable... plywood is a often scrap if a mishap occurs that goes through the face veneer. Yes, solid wood is much more work and money. If you must use plywood, I wouldn't bother with the screws. They add nothing to the stiffness of the panels. The best adhesive to use is a laminating epoxy such as West Systems epoxy (sold at marine supply stores... pretty pricey but good stuff). Study the instructions carefully! If the epoxy is not in the cards, I would use regular yellow woodworker's glue like Titebond or Elmer's Carpenter's Glue. Aim for 100% coverage... it takes a lot of glue...and is difficult to achieve over full 4x8 sheets. Apply the glue, then stack sandbags or sacks of cement on the sheets for your clamping pressure.

    If you feel you need more stiffness on the top, running additional stringers in either direction will help. A grid of supporting members (that is, members running in both directions) will be best. Since you haven't specified a load, or how much deflection is acceptable, no one can answer this question. It's only a dining table... how much stuff do you plan to pile on to it? After getting the top laminated, you should be able to rig up a little test to determine if you need to actually add stiffness or not. Again, the more supporting members you add, the stiffer it will be. You might also visit some furniture stores and examine their tables... Some of the less expensive dining tables are plywood. You can see if there are clues to identify a ply table top.... for example, look at the edge where the leaf goes. Also, is the top veneer rotary cut? Planks don['t look that way so if it is, it's probably plywood.

    Maple is a decent choice but so is oak or cherry or most other hardwoods for that matter. Sweet birch is quite stiff. Density isn't a very good indicator of either strength or stiffness. Hickory is better than most hardwoods, as is Doug Fir or Loblolly pine. If you're interested you can find tables in books or on the internet that show the modulus of elasticity (also called flexural modulus of elasticity, or Young's modulus) for a variety of species. A higher number means it's a stiffer wood. But high modulus isn't the only consideration.... one must consider stability, appearance, ease of workability, how the material takes the inteded stain, etc.

    Comment


    • #3
      Re: Dining table design question

      If your concerned about stiff ness of the top,

      what I would do is to put a edge of ply or wood to make your 1 1/2 thick top, or even cut a solid piece of wood with a lip that would edge the plywood, I personally think that would be best,

      put your apron on, I am guess that will be about 2 to 2 1/2" in the center of the arch (at it thinest), I would put some ply wood strips on the under side inside the apron that run the length of the table to the end apron strips, glue and screw them, then put a 1/2" ply on the spacers, the that way the top would equivalent to a full 2" and you basically built a thin stressed skin panel, I think you find it adequately stiff, NO I do not suggest you dance on the table but it would most likely take it if it was done, and by using some strips you basically making the table a hollow box and some lighter, if one needs to move it some time,

      you may want to check on table cloths and see if you can find one that fits,


      I can assure you if one puts sliders in it I would not want to extend it fully and stand in the middle of it,

      our table is 42" by 98" approx, and table cloths are hard to come by, my wife made her own to fit it the way I remember, (our dinning room area is nearly the same as your room)

      and the 42, inches wide is enough to pass food back and forth across the table, if kids are the one food is being passed to 42 is plenty wide, I would re consider the 48" if it was me, (unless your big into center pieces and the like), you may want to take some saw horses and a sheet of plywood and make a temporary table for a few days or weeks and see what the working width is for you and your needs,

      ~~~~~~~~~~

      edit: I like solid wood as well for tables if ours was not solid it would be a real mess by now, it was built in 74, and has seen lots of kids, and there school and art projects and a few to many hot dishes sat on it, and needs to be refinished again,
      Last edited by BHD; 09-16-2010, 07:24 PM.
      Push sticks/blocks Save Fingers
      ~~~~~~~~~~~~~~~~~~~
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      • #4
        Re: Dining table design question

        Instead of laminating two 3/4" ply sheets (all the additional weight in the middle is working against you), have you considered lengthwise stringers under the table? A 4" center stringer will add much strength and will not interfere with anyone's knee clearance, or looks unless someone bends over low. For added strength, put in 2, about 6" apart. Taper it up on the ends where the deeper aprons have their greatest strength to eliminate a problem with the end seating.

        I agree with BHD on the table width. Personally, I would go with a solid wood top, about 1" + thick, but realize cost considerations quite often prevent the optimum.

        Just a thought

        Go
        Last edited by Gofor; 09-16-2010, 08:44 PM.
        Practicing at practical wood working

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        • #5
          Re: Dining table design question

          Hey Andy!

          OK... the 'one piece stock'. What I meant is that I do not want to use a rectangular piece of stock. This is not the style I want to give the table. I very well understand that the higher the apron, the stronger it'll be and that at the apex of the arch. The real question is at that point of the arch, how high should it be to be 'strong enough'. I intend NOT in dancing polka on the table... . This arched apron would be around 87" long and I am thinking of it being 2" high. Mind you, with a span of 87" (on a 96" table) it might also look silly and I might just make it bigger at 2 1/2" to 3" so that it looks pleasing to the eye.

          At first glance, I wanted to use fairly large 8' x 1' stock. Going for a 6/4 or 5/4, plane it, biscuit it, glue it. But the specialty lumber guy I asked if they could do it said that they only laminate to a max of 36"... and then he does not garanty the flatness of the resulting board because the wood would work overtime. He then explained to me that one guy bought a $6000.00 piece of african padauk (to make a dining table that he sold $32000.00) and after 1 month it caved in. ok he repaired the damage (turned it upside down, cut groves in it and poured epoxy and make it flat again). OK back to MY table top... I would hate to have a top that would crack or twist or become uneven. if the table were smaller, this would not be a problem. Also, the reason why I go plywood is because of simplicity, quickness, and yes cost. Solid hardwood vs plywood (for black walnut) is about 5 times cheaper.

          Running members across the table top, say at 16" oc would I think give it more rigidity. Putting cross members perpendicular to the members I think would not help as much since they would be screwed to the member and it would be that member supporting the load. But then again... it's a freaking dining table! Plates, forks, knives, a turkey and all that good stuff would not make it crumble! As you said: "the more supporting members you add, the stiffer it will be".

          Thanks for your answer and pointers! I'm making good note of them!

          Cheers!

          Comment


          • #6
            Re: Dining table design question

            Hey BHD!

            I did not think about sandwhiching a piece of ply between 2 pieces of stock...

            Years ago, I did build a 4'x8' table using an OSB 7/16", frame and legs made out of 2"x3" plus 2 members underneath the table. No one ever noticed what the table was made of... Damned... I even added a 4'x4' extension... we were lotsa people that one time! I was more concerned b the floor supporting that many people than the table!

            7/16" OSB was pretty stiff back then... the only thing is that there was a weird empty sound when you knocked on the table (much like a floating floor). This is why I am thinking of 2 times 3/4" ply... it would sound/feel more like real solid wood...

            If the veneered ply resist long enough... who knows I might just replace it in 20 some years from now for solid wood... mind you... i'll be 69 then!

            Cheers!

            Comment


            • #7
              Re: Dining table design question

              Hey Gofor!

              I did not think of it like that but heck yeah... this might be a cool way of doing it. I'm more concerned about flex on the length of the table than on the width...

              I'll run that my brain over the weekend... thanks for the tip!

              Cheers!

              Comment


              • #8
                Re: Dining table design question

                Originally posted by HornyPotter View Post
                Hey Andy!

                OK... the 'one piece stock'. What I meant is that I do not want to use a rectangular piece of stock. This is not the style I want to give the table. I very well understand that the higher the apron, the stronger it'll be and that at the apex of the arch. The real question is at that point of the arch, how high should it be to be 'strong enough'. I intend NOT in dancing polka on the table... . This arched apron would be around 87" long and I am thinking of it being 2" high. Mind you, with a span of 87" (on a 96" table) it might also look silly and I might just make it bigger at 2 1/2" to 3" so that it looks pleasing to the eye.
                I think you are on the right track. A lot depends on the look you are trying to get. An 87" apron that is 2" or even 2-1/2" high compared to a 1.5" thick table top, to my eye, will look too delicate... spindly, even. But only you know the look you are after.

                Originally posted by HornyPotter View Post
                At first glance, I wanted to use fairly large 8' x 1' stock. Going for a 6/4 or 5/4, plane it, biscuit it, glue it. But the specialty lumber guy I asked if they could do it said that they only laminate to a max of 36"... and then he does not garanty the flatness of the resulting board because the wood would work overtime. He then explained to me that one guy bought a $6000.00 piece of african padauk (to make a dining table that he sold $32000.00) and after 1 month it caved in. ok he repaired the damage (turned it upside down, cut groves in it and poured epoxy and make it flat again). OK back to MY table top... I would hate to have a top that would crack or twist or become uneven. if the table were smaller, this would not be a problem. Also, the reason why I go plywood is because of simplicity, quickness, and yes cost. Solid hardwood vs plywood (for black walnut) is about 5 times cheaper.
                I fully understand the motivation for plywood. Just be aware that there area tradeoffs. Solid tops are of course much more likely to cup warp or twist than plywood. On the other hand, many many table tops have been made over the past jillion years out of solid wood... success is never assured (life in general.. no guarantees, right?) but there are some things you can do to stack the deck in your favor.

                First, the selection of wood is important. Second, the best and most stable cuts would be quarter sawn or rift sawn, both of which are considerably more expensive than the common plain-sawn wood. Even more important is how long you allow the wood to season before using it. For a wide glue-up, you should sticker and stack the lumber for at least 6 months. This is true even if the wood is kiln-dried when you get it. There will be some fallout from this.... some boards will cup noticeably and others will bow. Those can be jointed and planed and used for something else... but not the table top. Once you have good wood, you will generally get a better good result if you limit each board to a max width of 4". If using plain sawn, alternate the cupping pattern on each board - concave up, concave down, repeat. THe narraow boards and the alternating end grain pattern will help a lot. Even if boards cup, becasue they are narrow the effect won't be as noticeable... and the next board, which cups the other way, will tend to cancel it out.

                The guy with the Paduak will probably have more problems. It is inadvisable to groove the bottom and apply epoxy! Although this may have been the only way to flatten it. The problem is, they have created an assymetrical structure. Wood DOES experience seasonal movement.... what they've done is increase the odds of trouble since not only is the bottom different structurally from the top, but also the rate of moisture permeation through the finish will be different between top to bottom. You want carefully selected, seasoned, straight true stock, assembled with some smarts, and finished EXACTLY the same way top and bottom. If I paid $32K for that table I would be ticked off. Wood is not very forgiving in this respect. You can't really make up for instability or poor methods.

                A final comment on the ply top.... remember that hardwood plywood is interior grade glue... not for moisure. The finish won't help much, and if the table is planned to be washes a lot this WILL affect the longevity. Having said that, I did a bathroom counter years ago out of hardwood ply and it lasted for years.... just gotta remember to not let standing water remain on it.


                Originally posted by HornyPotter View Post
                Running members across the table top, say at 16" oc would I think give it more rigidity. Putting cross members perpendicular to the members I think would not help as much since they would be screwed to the member and it would be that member supporting the load. But then again... it's a freaking dining table! Plates, forks, knives, a turkey and all that good stuff would not make it crumble! As you said: "the more supporting members you add, the stiffer it will be".
                I'm again confused reading your comments... forgive my density. So let's first establish unambiguous directions: The table will have a short direction, and a long direction. Okay?

                Now, it's tough to say for sure which direction to run the members that will give the best stiffening -- because you haven't settled on the sizing, and hence the stiffness, of your apron pieces (specifically the ones that run in the long direction). We also don't know how stiff your laminated plywood top is going to be... because we don't know the specific plywood you're going to use nor do we know how effective your lamination will turn out - it has a huge effect. Agree?

                If your design ends up so that the largest part of the deflection is due to the ply top sagging BETWEEN the two long-direction apron beams, then you want to to run your stiffeners across the table in the SHORT direction. If your top is deflecting for the most part because the the top, along with the long apron pieces, is flexing along long span... that's a different problem. In that case, beams running in the short direction are not going to address THAT flexibility. It is better in that case to run the reinforcing members in the long direction.

                Reality is that you WILL see deflection due to BOTH flexibilities, it becomes a question of "how much" each contributes. Members in both directions will always be best, but if you just attack one direction it iis likely to be fine. I like the massive look, and tend to make the aprons stiff..... in this case a few members in the short direction would be the approach.

                As it happens, I am currently designing an 84x43 inch dining table for myself... with a granite top I have left over. I had to buy an extra slab for my kitchen in order to make my kitchen counter, a large U-shape, with only one seam. So I ended up with a large piece of stone left over from the slab. The stone is a very unforgiving since it is extremely brittle. My design is twin pedestals... no aprons... with a reinforcing grid of welded steel tubing epoxied to the underside of the top. The tubing grid is powder-coated after welding for rust proofing, but you can't see it unless you look under the table. The stone and the steel are so heavy that the flexibnility of the floor comes into play! So I am having to completely calculate the design to provide the stiffness at minimum weight.

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                • #9
                  Re: Dining table design question

                  A 1x1 angle on the inside of the apron (screwed to the apron and the bottom side of the top) will do wonders in making it stiff. At least on the two long sides.

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                  • #10
                    Re: Dining table design question

                    Torsion box construction might give your the rigidity you seek. 1/2" veneer ply top and bottom with a ~2" deep (overall) torsion box would be very strong, and lighter than a solid 1.5" hardwood top. Aesthetically this will appear to the eye as too light but you can wrap it with an apron of greater depth using hardwood which you can shape to your desired profile as you will be free from structural issues since it is only decorative.

                    Move the legs in from the extreme ends of the table which will reduce the span, this will help too.
                    Last edited by Bob D.; 09-19-2010, 05:27 AM.
                    "When we build let us think we build forever. Let it not be for present delight nor for present use alone. Let it be such work that our descendants will thank us for, and let us think, as we lay stone upon stone, that a time is to come when these stones will be held sacred because our hands have touched them, and that men will say, as they look upon the labor and wrought substance of them, "See! This our fathers did for us."
                    John Ruskin (1819 - 1900)

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