Replace broken chair leg with plywood?

[Jim Allen]

The leg is curved requiring a piece 23" long by 2 3/4" wide and 1 1/2" thick. It's painted so I can't see a downside to using 2 pieces of 3/4" plywood laminated together.

Am I missing something, or would it be ok?

[Jamie Buxton]

A pic would help.

An issue with painting plywood is that the edge grains telegraph through the paint. Of course, if the chair is built so that nobody sees the plywood edge, you're good.

[Richard Coers]

You'll have to edge band the plywood. But I guess if you are going the cheap and easy route on the leg, you could skim coat the edge grain with bondo and sand it smooth before priming and painting.

[Jim Allen]

The chair and the broken leg.



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[scott lipscomb]

Id go buy a 2x4 or a 2x6 (whichever works), before I would use ply-my two cents.

[Carroll Courtney]

I think the repair will work for maybe yr or so then the layers will start splitting. Popular from HD say a 1x6 may work better only cost few dollars more but last longer. Chairs I believe don't get baby any and kinda take a beating.
Here' another ideal if using plywood, maybe take some of that super glue and kinda put it on the edges of the plywood which will make it harder,just thinking outside box. I think pen turners do that.

[Brian Tymchak]

Regardless of what you use for materials, I recommend putting only 1 threaded insert into the new leg. It's clear that 2 inserts left too little material to be strong enough to handle the load. With a lap joint only 1 fastener is needed as the shoulders should keep the leg from racking. Unless the joint is too loose I guess.

[Jim Dwight]

I would not use plywood for this because you are effectively making the leg half as thick. The wood that goes horizontal cannot support the loads the chair sees, only the vertical grain wood supports the load. I'm a bit nervous about chairs since hearing a chair I built has a broken leg. It is made of cherry and lasted about 15 years. My son in law was sitting in it and he isn't small but it still shouldn't have broken. I haven't seen a picture yet but my guess is it's a front leg which is about 1.25 by 1.6 inches and straight. I may not have used a piece with vertical grain allowing a failure where the grain ran across the leg at an angle. Just guessing. It's a Woodsmith plan chair and I have four in oak that are around 25 years old and fine.

With this experience I am more interested in being careful with grain orientation in chair legs. If you could find a piece where the grain follows the curve of the leg it would be the strongest.

[Jim Allen]

Good point Brian, I like that idea, it's obvious that is the weakest point.

Thanks everyone for your comments. Scott, with your suggestion, it dawned on me that I keep a cash of 2x4s in the rafters in my garage, never gave um a thought until your suggestion.��

[Jim Becker]

I would not use plywood for this because you are effectively making the leg half as thick. T

I didn't see that at all...I assumed he would laminate more than one piece of quality multi-ply (BB or similar) to create his replacement leg.

[Jim Dwight]

Jim, while the actual thickness may be the same, I am arguing that the portion of the leg that adds strength is cut to roughly half because of the non favorable grain orientation. The loading on a leg is predominantly vertical so grain that is horizontal doesn't help. It would tend to split easily with slight off axis loading. The plys with vertical grain would resist that but that is only about half the thickness.

[Ole Anderson]

Anything less than a good hardwood with proper grain orientation is going to result in less strength than what broke once. Don't do it.

[Andy D Jones]

Jim, while the actual thickness may be the same, I am arguing that the portion of the leg that adds strength is cut to roughly half because of the non favorable grain orientation. The loading on a leg is predominantly vertical so grain that is horizontal doesn't help. It would tend to split easily with slight off axis loading. The plys with vertical grain would resist that but that is only about half the thickness.

I'm not sure I agree with this strength analysis...

Though the cross-grain layers do not provide significant tensile strength, they do provide substantial compressive strength, while also providing spacing between the long-grain layers, which amplifies the stiffness of the long-grain layers (like a torsion box structure).

I agree that the strength is not equivalent to that of a full thickness of long-grain hardwood, but it is significantly more than that of a half thickness of hardwood.

I would also be concerned with the bottom of the foot splitting and delaminating the plywood. At a minimum, a generous round-over at the foot would be preferable, over a square or even chamfered edge.

Overall though, I agree that plywood is not the best solution for this problem.

-- Andy - Arlington TX

[Jim Becker]

Yes, to be clear, I wasn't suggesting that the plywood was the best choice for this; only that it would still be strong if properly done, keeping in mind the design for this particular stool.

[lowell holmes]

I would make sample legs and when it was perfected, I would make the real leg.

My SWMBO keeps me lined out. :rolleyes:

[Jim Allen]

I did a sample with a 3/4" piece of plywood - kinda proof of concept, worked well so I made the leg out of a Douglas Fir 2x4, look ok have no clue how strong it is. The inserts were very beefy, threads were metric less than 1/4" dia. I used 1/4" inserts,required much smaller hole than the beefy ones.

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