Weird.
A different dowel broke on my Roorkee today. It was a different stretcher, but it was in exactly the same place as the last one that broke.
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| Owwwwie! |
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| Different stretcher, but it was in the same mortise as last time. |
This got me to thinking that perhaps it isn't the stretcher, but the mortise that is not square. Every time I sit in it a bit rough, an immense amount of stress is placed on that part of the stretcher.
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| Broom handles to the rescue! |
I am starting to think that these stretchers do not need to be too thick, as the weak part is not in the center of the stretcher, but clearly on the end. I can't imagine the kind of stress needed to crack a tenon off cleanly across the grain.
Perhaps pear is a bit brittle, but still...
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| The cheapest of the cheap. |
These Brazilian mango wood broom handles cost a whopping 1.20 Euro at the local grocery store, and one broom handle gave up two stretchers.
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| Not the nicest looking, but the chair sits. At least until the next tenon breaks. |
At this point, I have lost a bit of confidence in this chair. My plan for a permanent fix is to replace all of the wooden parts with a different species, with a bit more care in making straight mortise and tenons.
The good news is I have a couple of small projects coming up that will likely be made with some beautiful pear wood!
Great fix. My bet is the stretchers from the broomhandles will last you forever, just to show you they can ;) I think you made a great fix and still have a chair to sit in. Thanks for all the great posts. You are inspiring with your will to keep woodworking in small places with limited tools.
ReplyDeleteTake Care
Chris
Thanks, Chris! I bet you are right.
DeleteRight now this chair is my main lounging chair, so I need to be able to sit in it until I get a permanent fix.
And I appreciate the compliment. You are far too kind.
Cheers!
"mango de madera" means wooden handle. That looks like pin or other soft wood to me. Mango wood, at least in ecuador, is red and with a nice tight grain in a typical tropical wood look. I'm really enjoying seeing you struggle with dry Spain, you have some nerve to work on that floor. Lovely chair btw.
ReplyDeleteHi Sebastian, thanks for the clarification. My Spanish is terrible. Hopefully not for long.
DeleteWorking on the floor really isn't so bad, as long as your joints can handle it. It is a very solid surface!
Thanks for the compliment, cheers!
Brian,
ReplyDeleteit sucks it broke again on you but it's nice that I have company. I did a bonehead layout mistake in the shop today. Took a break and read your post.
I hate boneheaded layout mistakes. Hopefully you caught it before wasting too much wood.
DeleteJust speculating, but what if you make the mortise a little wider so that the tenon can go in deeper. That way you have greater thickness where the stretcher meets the leg.
ReplyDeleteHi Roy,
DeleteIt's possible that could work. The danger being the wider the mortice, the weaker the actual leg. No quick fix for that. I'm certain another species more apropriate for chairs will do the trick.
First, thanks to Sebastian: that translation is one of the funniest things I've seen in a while.
ReplyDeleteAbout the actual break though, here's something to think about (I have no practical experience here, so it could be completely meaningless): according to CS in TADB, the right wood for the lower parts of staked furniture should be "a stout species that is easily rived" which includes oak and ash. Easily rived is so that the grain is very straight. The stretchers aren't equivalent to the staked legs, but perhaps some crossover. I haven't dug through all of his writings on Roorkees to see the specific recommendations. The wood database for pear (http://www.wood-database.com/pear/) has several numbers for wood strength. The most relevant looks like "modulus of rupture". The value is about 12K lbs/sq.in. Various types of oak and ash run from 14K to 15K. Not a huge difference, but maybe enough?
(Of course, the real reason I suspect for the breaks is that there was no mention of a towel during the build).
Jeff
Haha! Hi Jeff.
DeleteIndeed, there was no towel during that build. I'm sure that's the problem.
The grain was sawed straight, the wood should be strong and thick enough, there must be a defect in construction. Being this is the first one I made, using a bit and brace, I didn't know how to dial the angle in perfectly. I'll correct that shortly.
I can't imagine how much stress it would take to break any hard wood straight across, but I did it twice. I imagine a lot. I think next, we'll try elm.
Btw, that was page 33 in TADB (The Anarchist's Design Book for those who don't follow Mr. Schwarz). He also mentions maple, which (assuming hard maple) has an MOR of almost 16K.
DeleteDang! Hate to see that. It does seem that something is twisting to find alignment and thus creating a good bit of torque. I've not built one of these so this is just a guess. Are your straps tight enough? Loose straps could let the legs splay too much and create enough leverage to result in the type of break you are getting. Being on a slick tile floor could exacerbate the problem. Again, just spit balling.
ReplyDeleteHi Greg,
DeleteI'm not too broken up over it. In fact, I like to see complete failures like this. I guess I'm weird that way. I look at it as a good learning opportunity. I made a couple of small mistakes that wound up being critical. Hopefully, I can avoid doing that again.
You could be right about exacerbating the problem. The arm leather did stretch a bit and was a little floppy. However, the chair itself is designed to be a little loose, and tightens itself up as you sit in it. This makes it ideal for outdoor use, as it self-levels on uneven surfaces.
If nothing else, I've learned where the single biggest stress point on this chair is. Another way is I know the weakest part of the wood.
Cheers!
Hi Brian.
ReplyDeleteI am sorry, but I don't think that the wood or the tenons/mortises are the problem.
I see one common demeanour in this problem....(you)
You just moved to Spain so it is understandable, the tapas available are delicious. and so is the red wine.
Brgds
Jonas (Who is still just shy of 0.1 tons)
Haha! You might be on to something. Tons of fun!
DeleteThe slick tile floor might be the problem. As the left-right stretchers are below the front-back stretchers, when you sit in the chair this will tend to bring the front and back feet nearer together. This would apply a bending torque on the front-back stretchers.
ReplyDeleteSylvain
Hi Sylvain, you might have a point there. I could put some rubber stickers on the feet to mitigate that.
DeleteThanks!