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How do I shore up table legs?

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Forum topic by Frankell55 posted 02-27-2019 05:51 PM 348 views 0 times favorited 7 replies Add to Favorites Watch
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Frankell55

5 posts in 824 days


02-27-2019 05:51 PM

Making a copy of Osodiy’s plywood herringbone coffee table.

All glued up and cut into shapes.

Started sanding and discovered the legs flex laterally. Afraid any non vertical load on the table will cause failure.

First thought is to put into compression with threaded rod but that requires drilling very straight for 18”.

How do I fix these legs?

Thanks


7 replies so far

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HokieKen

11986 posts in 1742 days


#1 posted 02-27-2019 06:28 PM

Maybe you can saw them in 1/2 with a band saw then route in a slot on each side then epoxy a metal rod into the routed recess and glue the halves back together?

-- Kenny, SW VA, Go Hokies!!!

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Woodknack

13008 posts in 2984 days


#2 posted 02-27-2019 06:50 PM

IIRC the original was built in Baltic birch, the expensive stuff. To be honest, I suspect the original isn’t very strong either.

-- Rick M, http://thewoodknack.blogspot.com/

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clin

1076 posts in 1600 days


#3 posted 02-27-2019 08:34 PM

I agree with Rick, the high quality Birch plywood would be much better. And as Rick said, even it probably isn’t all that strong, though it may be quite a bit stiffer. But plywood lamination glue joints aren’t all that strong when used like that. It’s pretty much like building with a solid piece of wood with all the grain running the absolutely worst (weakest) direction. I could easily imagine someone bumping the table and a leg snapping.

At this point I like the idea of drilling and inserting something. But understand the concern of drilling over that length.

I’m not sure compressing it with threaded rod would be the best option. While I think this would stiffen and strengthen it at first. The wood is going to relax (give) under the tension and I think the rod would loosen over time and not provide nearly as much strength as it did originally. Just humidity changes would likely cause the rod to go to high tension (when humid), crush the wood a bit, then when it dries the it would be that much looser.

Even a 0.25% change in the length due to humidity would result in >1/16” of change over the 18” length. A metal rod isn’t going to give that much, so the wood will. And again, this is because of cross grain. While plywood is very stable across it’s length and width, it’s still just stacked layers of wood and will expand and contract in thickness more then in length and width.

Having said all that, I’d consider drilling it out and gluing in as large a hardwood dowel as possible. Something that by itself would be strong enough for a leg. Or even gluing in a metal tube. In this case you aren’t relying on preloading the wood by compressing it. You’re just putting something inside that by itself is strong enough to prevent the leg from bending enough to snap the plywood. The plywood becomes more a decorative veneer than a structural member.

An entirely different way to strengthen it would be to cover it in fiber glass. This would still give you a completely clear, though thicker, finish. You’d also have to glass the top just so the finishes matched and aged the same. I’m not really recommending that approach, but the tensile strength of the fiber glass would greatly reinforce the legs.

Yet another option would be to embrace a change in the look. You could cut slots in the legs to insert hardwood splines. Maybe do the same along the edge of the table top to tie the deign together.

-- Clin

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CaptainKlutz

2215 posts in 2098 days


#4 posted 02-27-2019 09:30 PM

Stacked plywood is not that strong laterally. Be surprised if those support any side loading.

If it were mine: would probably saw the legs down middle and then laminate a single 3/8-1/2” thick piece of BB plywood in middle. Would add strength along the leg axis, plus it would fit the design motif with end grain on 2 sides, and end grain/side grain in other 2 sides.

Problem is you still have ply(s) of plywood as the only thing preventing the internal plywood from bending on side load of leg. Probably still need to laminate 2 layers of 4oz fiberglass cloth with epoxy to outside of legs the help reduce the side movement. 4oz cloth will turn transparent inside a nice clear laminating resin, and let the grain show. Most folks won’t even notice the fiberglass on legs unless they look at feathered edges. Epoxy top coat mimics a oil based poly finish pretty well. Meaning, you would not have to use epoxy on table top if poly was used.

Best Luck.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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Aj2

2649 posts in 2401 days


#5 posted 02-28-2019 12:18 AM

If you have Kids I recommend something like this.
.

I’m just kidding sorta :)

-- Aj

View bilyo's profile

bilyo

959 posts in 1706 days


#6 posted 02-28-2019 03:51 AM

My solution would be similar in that I would split what you have down the middle. But, rather than sandwiching in a metal rod or more plywood, I would sandwich in a complete leg of maple or other hardwood. As mentioned above, the plywood you have would then be just a veneer on either side. Granted, this would change the look some, but would still have a pleasing look and would be strong enough for strength and stability.

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Frankell55

5 posts in 824 days


#7 posted 02-28-2019 10:52 AM

Thanks for all the great options. I will keep the group advised.

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