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Making a big tabletop, would this be acceptable?

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Forum topic by Jimothy posted 07-24-2021 01:56 AM 563 views 0 times favorited 8 replies Add to Favorites Watch
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Jimothy

78 posts in 2153 days


07-24-2021 01:56 AM

Hey everyone, I am making a tabletop out of solid massaranduba, about 75”x35”. This is meant to be a sort of “modern” and “basic” table, and the client already metal legs that screw directly to the top that they want me to put on, so I was thinking of doing this:

Gluing up the tabletop/panels, and underneath I would screw on 3 perpendicular pieces across the width: near both ends, and in the middle. I wouldn’t put the screws in super tight, nor would I put more than maybe 1-2 screws max per board. The idea behind this is to allow for some movement of the top while also preventing excessive warping.

If I didn’t do any of this and just screwed the metal legs into the tabletop without the supports i described underneath, would the table warp ? Such a big table I just want to make sure it doesn’t warp at all.

What do y’all think?


8 replies so far

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Lazyman

7801 posts in 2600 days


#1 posted 07-24-2021 03:04 AM

You’ve got to use elongated holes in any batten that go across the grain so that it has room to shrink and expand with seasonal moisture swings. Whether the wood will warp if you simply attach the legs directly is hard to predict. Amoung other things, how thick is it, current moisture content wood, the grain orientation of the individual boards you will glue up, the current conditions in your shop and the conditions of that space where it will live all play a roll

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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CaptainKlutz

4859 posts in 2707 days


#2 posted 07-26-2021 03:06 AM

+1 LazyMan questions/comments.

Wood moves when humidity changes. Make sure your fasteners have slots to allow for movement.
Use the Shrinkulator calculator to figure how much it will move.

massaranduba? That is a new wood for me…
Called Bulletwood too: http://www.hobbithouseinc.com/personal/woodpics/bulletwood.htm

Looks fantastic. Cousin of Chico Zapote, which I have used. Super hard and oily species. Make sure you use proper technique for gluing oily wood.

Please post pictures of table top when done.

Best Luck!

-- If it wasn't for bad luck, I wouldn't have no luck at all, Doom, despair, agony on me… - Albert King - Born Under a Bad Sign released 1967

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Tony1212

580 posts in 2947 days


#3 posted 07-26-2021 12:55 PM

Wood movement is fairly easy to deal with. Some enlongated holes and not-very-tight screws should do it.

But at 75 inches long, I think about sag. My concern would be that it is perfectly flat along the 35” but U shaped along the 75 inches between the legs.

You don’t mention how thick the wood is, where you plan on attaching the legs, nor if you plan on using an apron. There are some online sag calculators that will tell you how a plank of wood will sag over a distance and weight load based on the species and thickness of the wood.

What kind of legs would you be using? If you’re using 4 individual legs bolted on in the corners, no need to worry about wood movement. But if the legs are welded together, then you have to figure out how to deal with movement.

Any chance that you can attach the legs to the battens?

-- Tony, SW Chicago Suburbs

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Jimothy

78 posts in 2153 days


#4 posted 07-26-2021 10:58 PM

The Legs are not going to be a problem at all in terms of movement. Thanks for the reply guys, but I’m not sure what you guys mean by elongated holes for the screws?

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Jimothy

78 posts in 2153 days


#5 posted 07-27-2021 01:37 AM

I described how I wanted to put 3 perpendicular boards across the back of the tabletop in replacement of an apron.


Wood movement is fairly easy to deal with. Some enlongated holes and not-very-tight screws should do it.

But at 75 inches long, I think about sag. My concern would be that it is perfectly flat along the 35” but U shaped along the 75 inches between the legs.

You don t mention how thick the wood is, where you plan on attaching the legs, nor if you plan on using an apron. There are some online sag calculators that will tell you how a plank of wood will sag over a distance and weight load based on the species and thickness of the wood.

What kind of legs would you be using? If you re using 4 individual legs bolted on in the corners, no need to worry about wood movement. But if the legs are welded together, then you have to figure out how to deal with movement.

Any chance that you can attach the legs to the battens?

- Tony1212


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Tony1212

580 posts in 2947 days


#6 posted 07-27-2021 01:30 PM



I described how I wanted to put 3 perpendicular boards across the back of the tabletop in replacement of an apron.

- Jimothy

Hmmm, maybe I’m envisioning this incorrectly. In my mind’s eye, I see you having 4 or 5 boards that are 75” long glued up to make up the 35” width of the table. Then there will be 3 boards shorter than 35” perpendicular to the long boards (the battens). Is this correct?

If so, those long boards will sag in the middle if not properly supported by either an apron or the thickness of the boards themselves. This photo is an exaggeration, but it conveys the idea.

Looking at bulletwood in The Sagulator, it says that even 3/4” thick boards spanning 75” will not sag very much with 30lbs of evenly distributed weight (estimated weight of tableware and food). That’s some pretty impressive wood.

Also, “elongated holes” means drilling the holes in the battens bigger than the screw diameter. You probably don’t need much. If the screws are 3/16” diameter, you might use a 1/4” drill bit. You also may need a washer on the screw.

-- Tony, SW Chicago Suburbs

View Lazyman's profile

Lazyman

7801 posts in 2600 days


#7 posted 07-27-2021 02:05 PM

Tony, I suspect that you left the default button for attached at the ends in the sagulator set. If you select floating, the sag on 3/4” board becomes excessive (0.24”), though technically the span will less than 75”, assuming the legs are inset some from the ends. IMO, 3/4” is just too thin for this span without aprons or some other support along the length. 1” is marginal so I would want a minimum of 1.5” thickness.

BTW, If I am doing the math correctly, at 67 lbs/cuft, a 1” thick slab will weigh over 1200 lbs.

EDIT to add: with that kind of weight, you will need very good way to attach the legs without aprons that won’t rack.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Robert's profile

Robert

4714 posts in 2694 days


#8 posted 07-27-2021 02:16 PM

75” long table with legs inset 16-18” = app 40” of unsupported top. Shouldn’t be an issue even for a 1” thick top.

A lot depends on thickness, type of wood and use.

That said, looking at Tony’s pic, if this was for a lawyers office that was going to have volumes of books stacked on it, maybe not…..............

-- Everything is a prototype thats why its one of a kind!!

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