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Thread: What might've caused this? **Pics**

  1. #1

    What might've caused this? **Pics**

    I picked up a 'warranty job' this morning. This particular chair had some exceptionally beautiful wood in it, and chances of me having that much of the right size(s) at the same time again aren't very good. Thankfully, the customer understands. Anyhoo... on to the problem. Both the top and bottom rail cracked at the middle back splat (built probably 3-1/2 years ago, maybe. The splat itself is very, very much bowed out... maybe because it's under tension and working it's way out...maybe because it decided to bow after someone actually sat in it for a period of time. We've already decided the replacement will have four splats instead of three, because the middle one hit him in the wrong place. He says he used a pillow, which might've also contributed to the bowing... but I don't know. Doesn't matter why, because I'm going to replace it... but I'd like to hear theories. He says the rocker broke while he was sitting in it just after he called me a couple weeks ago. I don't know if the bow in the splats would push somebody back far enough to change the weight on the rocker or not... but it's a very clean break, and through all the laminations (no glue failure!). Comments and sympathy appreciated....










  2. #2
    Join Date
    Dec 2006
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    535
    Wow Kirk, sorry to see that! Repairs are no fun.

    As to causes, the broken rocker doesn't surprise me much, being that its mesquite. I once broke a 1" wide tenon on a large piece of mesquite casework just by pulling on the side to try and slide it a bit on a smooth concrete floor. Stuff can be a bit weak.

    As to the splat

    I'd discount the effects of use myself, if it were that easy to set a bend perminently, we'd also have figured out how to straighten them perminently. At a guess, I'd put it down to instability, but having never seen a splat change that much before....

    I've seen 8/4 mesquite table tops bend after several years, enough to shear the glue joints, but I always figured that the thickness aggravated the problem (that and the Sonoran wood is really twisty).

    Good luck with it, hope the repair goes well.

  3. #3
    Join Date
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    I don't see mention of type of wood in original message. Whatever it is, looks like it is simply weak wood. Perhaps not a type suitable for furniture or parts under stress.
    "Folks is funny critters."

    Think for yourselves and let others enjoy the privilege to do so, too. ~Voltaire

  4. #4
    Hmmm. Honey mesquite is very stable, and touted for it's strength. Now, as I mentioned, this is highly figured (might not show well in the pictures), and that might have been a factor. There's a lot going on in that wood, and the 3/8" mortise is centered on the 7/8" thick rail... so maybe 1/4" isn't enough to 'hold'. This is the first one that's broken to my knowledge... hopefully people would let me know.

    It's been awhile since I built one of these, and I can't remember whether I bend those slats a bit during lamination or not, and haven't bothered to get all the forms out yet to make sure. When assembled, it's not a 'significant' bend, but if I left them straight, that would certainly put stress on the rear of the rails when I put everything together. When I do bent laminations, I leave the pieces laying around for a week when I take them out of the clamps to make sure everything 'holds'.

    Next week after I take a sledgehammer to it I'll be able to see if that splat straightens out.

    Thanks for the responses so far... keep 'em coming. It may just be 'one of those things'...

  5. #5
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    When you look at the grain that broke out here you can see it actually follows the break line. That wild grain wood looks good, but it leaves weak spots in the piece. Not so good on a piece like a chair where there can be a lot of stress applied.

    Cheers

    Ian

  6. #6
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    Quote Originally Posted by Kirk Constable View Post
    Hmmm. Honey mesquite is very stable, and touted for it's strength.
    Yeah, I've heard those too, but after working with it 40 hours a week for two years, I realized there was more going on than a couple of wood bites (like sound bites? ). Its reputation for stability is based on the similar tangential and radial shrinkage figures, but completely ignores the fact that the grain often changes direction up to 90 degrees within 6" (Texan stuff may be straighter grained, but the Sonoran mesquites can be a nightmare). The times I've seen marked movement and joint failures in mesquite were all knarly stuff like this. As to strength, that's IMHO just a mis-interpretation of the wood's hardness and tendency to dull blades. I'd guess that the compressive strength is ok, but I've never seen much to impress me with its direct tensile strength or bending strength (I'm sure thats not engineering terminology, and the stuff might resist bending pretty good, but its terrible when it comes to bending to failure -- meaning the failure point is quite soon after it starts to deflect, unlike woods like hickory, ash, etc).

    You also mention possibly laminating the splat... I could maybe see this contributing to its instability, depending on how it was done. If all the lams are cut from the same piece, then I wouldn't guess it had too much to do with it... unless the glue was prone to creep? Looking at the first pic again, kinda looks like the top popped out of the mortise, maybe thats why its bent, and when you release it, the thing'll go back to its proper shape. I'd bet thats what will happen.

  7. #7
    Not taking the time to read all the other excellant explainations ... My conclusion is that the wood (due to either the curvature of the piece and the poor selection of the orientation of the grain to the curved lines) the wood was cut so that weight & pressure was subject to the short grain structure of these pieces. They were doomed to failure at the beginning.

    The SLAT is/was not deep enough into the recess/rebate/mortice to allow for enough surface area to contain the pressure excerted. If that is the Center SLAT then it truely was uncomfortable and recieved excessive pressure holding weight off of the other two because it would be directly in line with the person's Spine. All the pressure would be directed between the SLAT and the Spine, the other two shared none of the weight.

    Being a Figured wood you were cursed with wandering grain and resulting short grain pieces. It would have been more wise to use a straight grained segment for the rockers or an acceptable substitute wood stained to match.

    Orient the replacement rocker so that the grain is longer at that (obviously) stress point. Add additional SLATs so you can offset and better distribute the weight between all SLATs and Make sure there is NOT one in the center so the spine carries the load.

    At least that is my observation. And opinion...
    Last edited by Bill Simpson; 05-13-2007 at 09:29 PM.

  8. #8
    Thanks for the subtlety.

    I used to call them SLATS... then I saw a complelling argument somewhere for the use of SPLAT... so that's what I've been calling them. I never bothered to look it up until now. So here you go, from the American Heritage Dictionary:

    splat 1 (splāt) Pronunciation Key
    n. A slat of wood, as one in the middle of a chair back.

    I don't know if mentioned before that the rockers were laminations.

    In 10 years (give or take), I've just never had problems like this from mesquite, so it's been a bit of a letdown.

    Thanks for all the replies.

  9. #9
    Quote Originally Posted by Kirk Constable View Post
    Thanks for the subtlety.

    I used to call them SLATS... then I saw a complelling argument somewhere for the use of SPLAT... so that's what I've been calling them. I never bothered to look it up until now. So here you go, from the American Heritage Dictionary:

    splat 1 (splāt) Pronunciation Key
    n. A slat of wood, as one in the middle of a chair back.

    I don't know if mentioned before that the rockers were laminations.

    In 10 years (give or take), I've just never had problems like this from mesquite, so it's been a bit of a letdown.

    Thanks for all the replies.
    I don't know where you got that defining remark but around here a Splat is the sound you hear behind a cow that has completed her "dinner"

    Rockers were laminated and still failed that way???
    seems odd.... Perhaps if you were laminate in a fashion so that the swirly short grain criss-crossed and would make a "Plywood" effect.

    As for the Slat or Splat in the center, I looked at a couple of chairs around and saw that the ones with an odd number of Splat/slats were more curved than the ones with even number of slat/splats.... My little old knucklehead started thinking and figured the curved backs took more of the pressure off the spine and onto the shoulders. Suspect those old codgers figured that out (but didn't tell anybody)

    I admire the finish on the piece, say it had been a couple of years(?) and still looks that good.... I think I would have planted that Splat/slat a little deeper into the rail (Top & bottom) and it would have geven more area to allow for the bowing which drew the tenon out of the mortice slot. Which eventually caused the side to blow out. Did just the center bow? Was it a skinny old dude and only rested on the center Splat/slat.


    And , Perhaps sense you refered to it as a SPLAT, it figured that that was what it was suppose to do.... "Splat!!!" (like a Cow Pie)

  10. #10
    Join Date
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    In all honesty, I have to wonder if the problem is due to use rather than construction. Though I would have used another wood for the rocker, it is surprising to see a laminated piece break like that, and as before, never seen a splat fail that way. Real question will be answered when you get it ripped apart, whether the splat is really bent or if it flexed enough to cause mortise failure and then stayed wedged in that position. The more I look at the first pic, the more I think its the latter, but then why didn't the rocker flex too? Shop I worked for did about 75% mesquite work, and failures occurred, but weren't common -- never seen a splat fail, even in mesquite -- spindles broke easily, but fortunately we didn't do many of those. Mortise's would sometimes blow out like in your pics, but usually when cutting, not in use. Occaisional chair leg with angled grain would break. Just part of the fun, and part of the reason the shop charged more for mesquite pieces.

    As to splat/slat, I picked that up from reading antique books and some articles on making chippendale chairs from old FW mags, can't say as I've seen 'em called anything else, though slat does make sense in an A & C design.

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