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Thread: Conference Table

  1. #16
    If you use the wood shrinkage calculator at woodweb with a 4% change in moisture content in plainsawn white oak you get a prediction of 1.8" movement in a 120" wide piece. The actual movement will no doubt be less as the grain orientation will not be purely tangential, but still a lot to plan for. If you have not yet glued up your parts you may want to turn them to a rift/quartered orientation as far as possible to reduce movement. There is a reason solid wood tables are typically built with planks running in the long direction.

    How are you measuring the mc of your wood? Pin meters are not reliable below 6%.

    It is always a good idea to know the projected in-service conditions for your work. I am guessing that in your location air conditioning is standard and the emc may well be 8% or so, but an unconditioned space may lead to much higher movement. You might want to let your wood acclimate to a higher relative humidity. What is the normal rh in your shop and how do you regulate it? If your lumber is stable at around 5-6% mc that would indicate an rh of 25% or so.

    Unless you encapsulate your top in epoxy it will gradually move to match any seasonal humidity swings.

    The one advantage of your design is that making the top in two pieces for easier transport will make only a minor difference in the overall look of the installed piece.
    Last edited by Kevin Jenness; 01-27-2022 at 8:40 AM.

  2. #17
    Thank you for your feedback Kevin.

    Quote Originally Posted by Kevin Jenness View Post
    If you use the wood shrinkage calculator at woodweb with a 4% change in moisture content in plainsawn white oak you get a prediction of 1.8" movement in a 120" wide piece. The actual movement will no doubt be less as the grain orientation will not be purely tangential, but still a lot to plan for. If you have not yet glued up your parts you may want to turn them to a rift/quartered orientation as far as possible to reduce movement. There is a reason solid wood tables are typically built with planks running in the long direction.
    Still, I've seen it done just like I'm doing it, with a steel frame underpin. I may segment the steel structure so that each steel section only need worry about the movement of the wood its underpinning.

    Quote Originally Posted by Kevin Jenness View Post
    How are you measuring the mc of your wood? Pin meters are not reliable below 6%.
    Surface moisture meter. None were over 6% so I assume its accurate that its at least under 6% which I tested variants of in the calculator.
    et140_beauty_r.jpg


    Quote Originally Posted by Kevin Jenness View Post
    Unless you encapsulate your top in epoxy it will gradually move to match any seasonal humidity swings.
    I was thinking about that, but it it would be hard to not have the pinhole in the ballon effect Mark mentioned.

    Quote Originally Posted by Kevin Jenness View Post
    The one advantage of your design is that making the top in two pieces for easier transport will make only a minor difference in the overall look of the installed piece.
    I haven't decided to do that yet, I'm going to see how stable it is when I get supports. Right now, its understandably unstable. If it continues to not be solid enough, cutting it in half may just be a necessity. Cutting in half wont impact the the look, but I need to cut it outside the floating tenons, so I'll have to see where mid-point is, and perhaps cut it slightly off center.

    Quote Originally Posted by Kevin Jenness View Post
    What is the normal rh in your shop and how do you regulate it? If your lumber is stable at around 5-6% mc that would indicate an rh of 25% or so.
    You are dead on there. I regular the humidity with a dehumidifier. As I'm in southern Texas, our main issue is high humidity. We are in a brief period of low humidity for winter, and its around 25%. I considered a humidifier but that seemed a bit ridiculous. Anything I put out will likely be similar humidity to my shop; lowered from the highs through HVAC, and low humidity in the winter.

    I'm going to continue work on this table this weekend, with my next step to add a temporary spine to the bottom to strengthen the rigidity of the joints while I work on it. I have been considering angle iron either surface mounted, or smaller angle iron in a V groove inset. The base itself has supports for 6', so I would really need to strengthen the vertical load for the 2' wings on each side.

  3. #18
    Quote Originally Posted by Kevin Jenness View Post
    Unless you encapsulate your top in epoxy it will gradually move to match any seasonal humidity swings.
    It would be interesting to see what would happen to a 10' perpendicular grain top fully encapsulated. I honestly still have zero faith in these large/wide encapsulated wood tops. I dont care how in-tact the encapsulation is there is simply no way the top doesnt either grow or shrink between encapsulation and its finial destination and for years to come. I would have a sneaking suspicion that a top like this, encapsulated, would be cracked through the epoxy at several locations in short order. Whether they would be blatantly visible defective cracks or just cracks who knows. There is simply no way a sheet of epoxy can accommodate even a half inch of tension or compression over 10'. Something would have to give.

    The answer is simply to know its going to move and accommodate it. Its definitely not impossible and at least the entire field is perpendicular so who's gonna know if the top is an inch longer or shorter as long as it doesnt wreak havoc on the finish which it wont with such small strips. Keeping it flat will be the key.

  4. #19
    Quote Originally Posted by Mark Bolton View Post
    It would be interesting to see what would happen to a 10' perpendicular grain top fully encapsulated. I honestly still have zero faith in these large/wide encapsulated wood tops. I dont care how in-tact the encapsulation is there is simply no way the top doesnt either grow or shrink between encapsulation and its finial destination and for years to come. I would have a sneaking suspicion that a top like this, encapsulated, would be cracked through the epoxy at several locations in short order. Whether they would be blatantly visible defective cracks or just cracks who knows. There is simply no way a sheet of epoxy can accommodate even a half inch of tension or compression over 10'. Something would have to give.


    The answer is simply to know its going to move and accommodate it. Its definitely not impossible and at least the entire field is perpendicular so who's gonna know if the top is an inch longer or shorter as long as it doesnt wreak havoc on the finish which it wont with such small strips. Keeping it flat will be the key.
    I basically agree with you that the slab will come to equilibrium with its environment, my point is that a sufficiently thick layer of epoxy will slow moisture transmission to a crawl and minimize seasonal moisture content swings. As far as the flexibility of epoxy over large areas goes, I don't see why it would be any more likely to fail over a 10 ft wide piece than a 10 inch one. I guess we will have to let Derek do the experiment and report back. As you say, keeping the piece flat while allowing for moisture related movement is the key.

    I did once glue up a similarly sized countertop of 1 1/2" square blocks at a customer's insistence. I think he had the material and didn't want to "waste" it. We got paid, he took it away and was never heard from again. As my friend once said, "We sure do get some lulus."

  5. #20
    Quote Originally Posted by Kevin Jenness View Post
    I basically agree with you that the slab will come to equilibrium with its environment, my point is that a sufficiently thick layer of epoxy will slow moisture transmission to a crawl and minimize seasonal moisture content swings. As far as the flexibility of epoxy over large areas goes, I don't see why it would be any more likely to fail over a 10 ft wide piece than a 10 inch one. I guess we will have to let Derek do the experiment and report back. As you say, keeping the piece flat while allowing for moisture related movement is the key.

    I did once glue up a similarly sized countertop of 1 1/2" square blocks at a customer's insistence. I think he had the material and didn't want to "waste" it. We got paid, he took it away and was never heard from again. As my friend once said, "We sure do get some lulus."
    Yep, agreed fully on the lag of expansion/contraction with rock solid encapsulation. Bonus as well is as you already mentioned in your reply HVAC etc... That said, I dont know many homes other than uber high end or museum level humidity control that ADD moisture in the dry times. Ive known tons of homes that have implemented humidification (steam, foam roll, etc.. ) but they all usually seem to fall into disrepair in short order, mineral buildup, and then are subsequently abandoned. So you have a structure that is doing a good job at pulling moisture out in the summer but is very poor at adding moisture in the winter. I personally believe this is why we all predominantly see shrinkage issues with finishes. Cabinet doors are a perfect example. Shrinkage cracks at panel/rail/stile are most always the norm. Spaceballs aside, you rarely have expansion issues though I guess those cracks would be a touch more hidden.

  6. #21
    I believe I'm going to build a steel undercarriage for the table out of 3/4" box steel inset about 5" from the edge. I'd like to weld on steel 90 degree bracketed for the top with slides of at least 1" to accommodate the discussed travel. Does anyone have a recommendation on a steel bracket (no thicker than 3/4") that will accommodate this design? I could not find anything with that level of travel built in.
    Last edited by derek labian; 01-29-2022 at 2:10 PM.

  7. #22
    Seems a tremendous amount of over engineering but have at it.

  8. #23
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    If you are fab'ing this yourself, why do you need to buy a bracket? Am I understanding correctly you are fab'ing the base up yourself? Make a long hole (or weld up pieces such that you get effect) and then screw the table down..

    Maybe I'm missing something / over simplifying?

  9. #24
    Quote Originally Posted by andrew whicker View Post
    If you are fab'ing this yourself, why do you need to buy a bracket? Am I understanding correctly you are fab'ing the base up yourself? Make a long hole (or weld up pieces such that you get effect) and then screw the table down..

    Maybe I'm missing something / over simplifying?
    I was just looking for a bracket with an existing slot that I could just weld onto the frame. Just one less thing to do.

  10. #25
    Quote Originally Posted by Mark Bolton View Post
    Seems a tremendous amount of over engineering but have at it.
    I'm seeing that the table is too unstable without a steel under-frame. The 2' on either side will not carry any weight without the frame, and making a full frame allows the top and the base to be safely separated for transport. I'm not sure how I could simplify further, its just a rectangle made of box steel.

  11. #26
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    One way to do it is to use two pieces of angle iron where the back of the L is kept spaced out from the other by a certain distance, creating an endless slot..

    Example:
    I made this way back when so don't make fun (not a fan of my old stuff), but I could have used 1/4" (or whatever thickness you wanted for your bolt thread diameter) flat steel as shims between the angle irons that are butted up to each other and created a slot of nearly any length without any machining / plasma cutting / etc. Considering you need a big slot length (2" or so), this might be easier than finding bracket and since you could make the steel underside function as both structural and your bracket at the same time, I think it would be more efficient than a bracket. And you could way over plan on wood expansion / make the slot much longer than necessary.

    Install metal female thread into the table and use a long bolt to fit between the angle iron gap you created with your flat steel shims. The washer under the bolt head sliding along the angle iron's edges.

    Or play with the idea with different metal forms (sq tubing, etc), but I think the idea itself will make life easier.


    bistro table 6a.jpg
    Last edited by andrew whicker; 02-02-2022 at 12:25 PM.

  12. #27
    Quote Originally Posted by andrew whicker View Post
    One way to do it is to use two pieces of angle iron where the back of the L is kept spaced out from the other by a certain distance, creating an endless slot..

    Example:
    I made this way back when so don't make fun (not a fan of my old stuff), but I could have used 1/4" (or whatever thickness you wanted for your bolt thread diameter) flat steel as shims between the angle irons that are butted up to each other and created a slot of nearly any length without any machining / plasma cutting / etc. Considering you need a big slot length (2" or so), this might be easier than finding bracket and since you could make the steel underside function as both structural and your bracket at the same time, I think it would be more efficient than a bracket. And you could way over plan on wood expansion / make the slot much longer than necessary.

    Install metal female thread into the table and use a long bolt to fit between the angle iron gap you created with your flat steel shims. The washer under the bolt head sliding along the angle iron's edges.

    Or play with the idea with different metal forms (sq tubing, etc), but I think the idea itself will make life easier.


    bistro table 6a.jpg
    Hi Andrew,

    I like that idea. I originally wanted to use angle iron but it wasn't rigid enough. Even if I stick with the box steel, I could attach an angle iron runner to it in the way you suggested just to anchor the top with the floating washer. Thanks.

    Derek

  13. #28
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    Cool, have fun. : )

    I think angle iron gets a bad rap because of it's tendency to twist, but if you design out that twist (like butting two pieces together or building a frame, for example) it's a super efficient way to build in my opinion. I love tubing, but there is definitely a simplicity in using angle iron (less to cut, easier to cut, low cost per foot / pound, drill thru only one surface, etc).

    Of course, the mill scale can be super frustrating on hot rolled and angle iron doesn't necessarily look very good.

  14. #29
    I fabricated a steel frame with 1/16" box steel. It took me a while to get all the welding supplies and learn how to weld. Sadly, the table still flexes because there's too much mass and not enough anchors to the frame. I'm going to add 1" steel strips to the box steel along with 1" angle iron on the edge. That should add the needed rigidity. I can use the angle iron as a slide for expansion instead of drilling through it. I realized I need a lot more than the 10 screws / 10 brackets I fabricated to hold it to the table. This will allow me to add 20 or 30 screws to the angle iron edge and the wood to expand up and down it. I found wood screws with a 3/8" head, so I think that should be sufficient for movement.

  15. #30
    Quote Originally Posted by derek labian View Post
    I fabricated a steel frame with 1/16" box steel. It took me a while to get all the welding supplies and learn how to weld. Sadly, the table still flexes because there's too much mass and not enough anchors to the frame. I'm going to add 1" steel strips to the box steel along with 1" angle iron on the edge. That should add the needed rigidity. I can use the angle iron as a slide for expansion instead of drilling through it. I realized I need a lot more than the 10 screws / 10 brackets I fabricated to hold it to the table. This will allow me to add 20 or 30 screws to the angle iron edge and the wood to expand up and down it. I found wood screws with a 3/8" head, so I think that should be sufficient for movement.
    Caveat: I haven't read this all; just a quick skim thru ....
    I'd be glad to suggest a bigger screw, shorter/longer slot, a simple rib, but the issues and limitations you describe seem a bit too complex for a fix via this post in this thread.

    This is also not the place to try to teach mechanical/structural design, but to support your chosen profession I would humbly suggest you pursue more knowledge in this area - either via the university of hard knocks (experiment, if time allows), or something a bit more formal. Even the 'tube can be some help: search for 'Mechanics of Materials', 'Strength of Materials', and possibly 'Statics'. I see occasional courseware even from MIT. I have watched 2-3 on the first and they make for interesting refreshers (I no longer practice in this arena). These videos won't make you an engineer, but you'll at least be more familiar with the basics of how to maximize strength with minimal materials - skipping over some of the errors of the trial-and-error method. You could identify the load's direction and how best to orient material to resist the resulting deflection.

    Quick example:
    A given angle iron size is significantly stiffer/stronger when a leg is parallel to the load
    ('L' <---load)
    vs non-parallel
    ('>' or '^' <--- load)
    Either CAN work, but one is better, stronger, lighter, cheaper, efficient - - pick your own term. And for your welding prowess, all common structural steel shapes have their properties widely published in look-up tables. The concepts apply to both wooden and steel structures.

    Architects design beautiful unique buildings, then shamelessly get a Structural Engineer to make sure it won't collapse. For you, maybe an alternative is to contract with a Mechanical Engineer to assist with material selection and orientation of a proper structure - using YOUR aesthetic design - for your deliverable. How many pro woodworkers have posted examples of their work, and 'admitted' they subcontracted the finishing - - or wide-belt sanding, or CNC work, etc? A 10' commercial/retail table is not the ideal place to make a structural mistake. ...Know when to ask for help.

    And best wishes for a happy (repeat) customer on this one!
    (Apologies if this sounds 'scolding' (NOT my intent), but hard to control the tone here.)
    Last edited by Malcolm McLeod; 02-16-2022 at 11:17 AM.

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