Hi Folks,
There is a term call “pre-stressing” or “pre-Tensioning” that I think was first coined by Mike Dunbar. It has to do with the length of a chair stretcher inserted between two legs. If you think about two legs splayed out, with a straight piece between them, the legs are normally drilled with a mortise, and the length of the stretcher is the lenght between the legs, plus the dept of each of the mortises. The tops of the legs normally are mortised into the seat at the splay angle as well.
At any rate, the tensioning technique adds 1/4 ” or so to the stretcher length. When the stretcher is inserted it actually spreads the legs beyond the normal distance and effectively jams the legs sideways into the seat mortises.
I think Mr. Dunbar came up with this theory with just anecdotal information, where as the chairmakers that I know such as Curtis Buchanan, Pete Galbert, John Alexander, and others vehemently disagree and say that there is no science behind it, and in fact is wrong, and contrary to good design.
I know that there are many who support this tensioning theory, and I dont want to start a flame, but rather I would like to hear from an engineer who could comment on the loads and stresses and come up with a scientific approach to proving or disproving the theory. Any comments?
Replies
I'm not an engineer, but I read an article that Mike Dunbar wrote about that subject not too long ago. IIRC, he said that it is a technique developed 250 years ago by Windsor chair makers to put the joints under compression rather than tension (which would occur when the stretcher is used without the additional length). He said that since the glued joints are essentially dowels which result in a lot of end grain in the joint, that the glue eventually fails and the joints become loose unless additional length of the stretchers applies compression to the joint. I also believe that he has been running a Windsor chair teaching facility in New Hampshire for many years. I'd be interested to hear for other knotheads on this.
Jim
HI Jim,
Dunbar does say that, but again I'm not sure there is any science behind it. Respectfully, because he says it doesnt make it so. John Alexander has examined thousands of old chairs and doesnt agree to my knowledge.
thanks for the input.
I understood Dunbar to be saying that as one sits on a chair that has splayed legs, the legs will want to flex to exaggerate or widen the splay angles. And over time, the gradual increase in splay will mean that the stretchers (which were sized to fit the new chair at its original splay and lack any wedges or other mechanical connections - e.g., the spoon bit shaped mortise and shrunken round tenon variety), will fall out. Other than glue, the stretchers will only stay in place as long as their tonons are long enough to bridge the gap.
All his 1/4 to 3/8ths of an inch really do is provide some insurance that if the splay increases some over time from weight stresses and the compression of the softwood weat mortises for the legs, the stretchers will still be plenty long to stay in place. I guess they also fight the tendency of the splay to increase (compression of the seat mortises) because pushing the legs apart has already put those leg/seat joints in compression.
Makes sense to me. What do all those who disagree do instead? I'm guessing that it is not trusting in glue. Probably they wedge those or use the greeen wood lateral shrinkage to their benefit with shaped mortise and tenons? Someone know what Curtis Buchanan does, for example?
Edited 11/20/2009 5:03 pm ET by Samson
Hey Samson
This is a great discussion to get folks fired up about Windsor Chairs and the "best construction" techniques to use when you build your baby.
I started making windsors about the time that Mike Dunbar's book came out. Mike has changed his tune once or twice ... since. Super dry tenons/green legs... yea.
Curtis Buchanan is a fantastic windsor builder as is Peter Galbert. Great. They are 2nd generation windsor builders from the Dave Sawyer school. Dave is something else. An MIT engineer gone pitchfork maker to windsors. He is one clever guy.
So... What's up? Mike has a theory on spreading the legs using a 1/4" idea. Poor.
Put the legs in the seat sockets and stretch them apart gently with your hands. How far apart are they? well if you take a 5/8" piece of pvc and cut it 8" long then insert a 1/2" dowel inside you have a telescope. Now with your holes bored in the legs, pull the dowel out against one of the sockets and the pvc against the other hole base and you know how long to make your stretcher. No math -- a good snug feel.
Curtis says NO. OK. Dry the tenon insert it in the moist leg socket and you have a tight fit. For a while my friend. The wood is going to relax at the joint.
This is the weak link in the windsor chain. One can argue "stretch it out" or "super dry the tenon and insert it in the green leg" in the end its like many have said: its an end grain joint and its going to fail. Why not think of the guy down the road doing a repair??
How did these old chairs make it? Repairs. Hide glue.
Mike is off the bullseye with his white glue friends. Its the wrong tool for the job. When that tenon comes out, you have a really tough problem to address. If it was hide glue, you could spread the legs a little, brush on some hide and squeeze it back together and you are good to go. With plastic glue, you have to get it apart and clean out that socket.
Good Luck.
dan
Edited 11/20/2009 6:11 pm ET by danmart
Good info. Thanks, Dan.
Funny you should mention the hide glue thing. I broke down and bought a pot today as I've got some veneer and chair projects on the agenda (hopefully soon).
Dan,
I have been trying to put my NASA experience to good use in updating the design of Windsor chairs. To remove the wear in the joints while keeping the lightness, I am thinking of using titanium. Now if I could just come up with a way to make milk paint stick to titanium. Let me know if you have any ideas.
MelMeasure your output in smiles per board foot.
"Now if I could just come up with a way to make milk paint stick to titanium."Use hide glue as a primer. I seriously think this would work.Chris @ http://www.flairwoodworks.com and http://www.flairwoodwork.spaces.live.com)
- Success is not the key to happiness. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer
Samson,
Thanks for your response.
I cant speak for all the others, but I have worked with Curtis Buchanan (and Pete Galbert) who say the pre stressing is BS. Among other things, he says that adding the extra length, makes the stretcher instead a "spreader". And if spreading would be a good thing, why not remove the stretcher all together?
I have worked with Curtis (and Pete Galbert) and I try and answer the questions of what they do.First, he (Curtis) does not rely on glue. His design uses super dry tenons, going into mortises at EMC. (Equlibrium moisture content). When the tenon dry, the joint gets much tighter. He does use hide glue that he mixes himself to insure freshness and to allow repair later.
Again thanks for the discussion.Secondly he uses very tight joinery in the seat mortises. Curtis maintains that tensioning the stretcher actually weakens the seat mortise joint. Third he uses a 6 degree angle on his seat mortise where Dunbar uses 11 I think. While the higher degree tenon would give more downward strength I suppose to hold up the seat, the smaller angle gives a much tighter joint. This is true for the tuning pegs for violins etc. where a very small angle gives a very tight joint to hold strings in tune.
I have heard a quote from Dave Sawyer when he was looking at a Dunbar reamer, who said in reserved New England manner. "Dunbar reamer, wrong angle".Curtis does wedge the through tenons in the seat.All the other chairmakers I have referenced could of course elaborate better than I, because of their experience and they have extensively researched and discussed it.
More great info! I'm loving this thread. THANKS!
Does anyone try wedged through tenons on the stretchers (and medial)? Seems like that would be strongest of all, especially if one were to have a reverse taper on the outside (create an hourglass shaped mortise). Dunno, but just thinking out loud.
Edited 11/21/2009 10:53 am ET by Samson
Samson,
I dont know about through tenons, but I do know that some chairmakers carve a v groove around the tenon app. 1/2" from the end, which when inserted, the green wood from the mortise fills in and actually makes a locking joint. A lot of the small mountain chairmakers have done this for generations without any glue at all.
A fox tenon might also be an option if you don't want to mess with green and super dried wood and don't want to go through.
I believe Mr. Dunbar tailors his techniques so that he is able to teach a chair making class a week. He needs to be able to take a rank amature and send them home with a chair at the end of the week.
His classes have changed from being concerned with the transfer of knowledge to that of the "Happy Customer" going home with a finished piece.
He is an excellent businessman and very good at marketing his product. I am not sure of his academic endeavors into period chair making. Folks like Jennie(was John) Alexander, Dave Sawyer and Peter Follansbee have more thoroughly examined the period construction techniques of green woodworkers.
Anyone aspiring to be a furniture maker should take at least 1 class from Mike Dunbar. He has a thriving business and is not too shy to talk about it.
F.
Floss,
Thanks for responding. Again I have not met Dunbar, but some of the things he has written about just seem plain wrong. He may be a good marketer, but I am not sure thats good business if missinformation is given or the product isnt as good as it could be. I have met many Dunbar students that are much happier going after going to one of the other chairmakers. I would like to hear from anyone else who has met both and hear their opinions. I have to disagree about taking at least one class with M. Dunbar. For myself, I think I could get much more from Buchanan, Galbert, Sawyer and the bunch.
Commonly done in for concrete construction. That's been a few years, let's see, concrete has outstanding compressive strength and poor tensile strength, so pre-compressing concrete prevents concrete constructions to ever have withstand tensile loads.
No idea about mechanical properties of wood. At first thought the proposition to pre-compress wood appears to be of rather questionable merit form the engineering perspective with but with strong marketing potential. The implementation you describe sounds to have been conceived in a rather intuitive fashion. Without proper stress/strain analysis I bet can do more harm than good when pre-stressing members of complex structures.
Concerning advantages of pre-tensioning with respect to the durability of glue joints: I support the opinion that good joinery and proper furniture construction does not require any glue for structural purposes, so pre-tensioning to somehow strengthening glue joints is a mute point for me.
Chris Scholz
Dallas/Fort Worth, TX
Galoot-Tools
Chris,
This is the kind of info I was looking for, though you lost me with the compression/tensile things. I guess thats why I am not a engineer.
thanks,
I have heard of pre stressing concrete....I didn't realize that it was pre compressing. I have not worked with pre stressed concrete, so I don't know if that is what is done or not.
In my opinion (which is probably wrong, but here it is) if the stretcher is longer than needed, it would create increased stresses in the joint at the seat. It would amplify when a load is placed on the seat (someone sits in the chair). Granted, different wood has different compressive and tensile strengths but over time, I would imagine that is where the failure would occur. Granted, I have not done any type of analysis on this, this is just my feable meanderings from thinking about the question posed.
Hope I haven't muddied the water to much with that.
I'm pretty sure Mike Dunbar did not invent this prestressing technique though there is not a lot of information about how windsor chairs were made 300 years ago. Dunbar had to more or less rediscover the methods when he started making chairs.
By prestressing the stretchers they keep the legs spread apart, rather than being used to hold them together. I think that's the right way and I use this method myself. I don't see how the legs will become more spread apart as time goes by unless the legs actually bend (unlikely) or the mortises get larger (possible). The stretcher joints usually fail because eventually all joints will fail. The leg to seat joint could also fail but is less likely since everytime you sit in the chair you are making the joint tighter. If the mortise is tapered it will be especially difficult to come loose.
That's my experience from both making windsors and repairing old ones.
Hi Ben,
Thanks for responding. What I dont get is why would you want the legs spread apart. I seems like they should be to the exact distance and then stay there.
Stevo
If the chair seat is a hardwood then you are right. In fact, there are windsor chairs that do not have stretchers, they are called Welsh Stick chairs or Cardigan chairs. But many or most windsors have seats made of pine or poplar because it's easy to carve. Stretchers would then be necessary because softwood has too much give.
HI Ben,
I know about the cardigans,and in fact just stopped to set Don Weber in Kentucky in september. Don does them both with and without stretchers usually in Elm seats. My question is if the stretcher is in fact a spreader, why would it ever be needed? If stretching is good, I guess it distorts the joint in the seat, but thats about it. I'm not sure that it would hold up to science is my point. I did ask an engineer friend of mine who didnt know offhand, but did say maybe an experiment would be in order. I'm not even sure what to test. Certainly strength downward, but not sure about the rest.
thanks
Stevo.
Its about triangulation. Racking, twisting and stability.
dan
Well, for an elm seat you don't need stretchers. A softwood seat would evetually let go of the legs; by pushing the legs apart a little, via a long stretcher, the legs remain attached to the seat.
I suppose if the chair were made of metal or plastic stretchers might not be necessary but wood moves with changes in relative humidity so eventually the joints loosen up.
I have read that chairmaking in the 17th century was very competitive and it was tough to make a dollar (some things never change). If they could have sold chairs without stretchers to save a few pennies or dollars they would have. Maybe we don't see too many stretcherless 17th century chairs today because they fell apart and were consigned to the wood stove?
The way I see it, a chair with stretchers that are a little long will last longer (before needing to be reglued) than a chair that has stretchers of the "correct" length.
After taking several classes with Mike Dunbar I took a ladderback chair class with Brian Boggs. I felt that Brian's emphasis on tight, dry to super dry joinery, correct grain orientation and the use of hide glue made much more sense and can be applied to Windsor chair construction as well.
I have also worked with Curtis Buchanan and Pete Galbert. Brian, Curtis and Pete all agree on the basic elements of chair joinery:
Tight joinery (within 1 or 2 thousandths) so that the whole joint has wood-to-wood contact
Superdry tenon going into a dry mortise so that the tenon will expand within the mortise as it reaches equilibrium moisture content.
Correct grain orientation -- basically wood shrinks very little over it's length so when you place a tenon from a rung or stretcher into a leg you want to do it in a way that takes advantage of the natural movement of the wood. In this case the wood of the tenon moves less in the radial plane (from the center of the tree out to the bark) than it does in the tangential plane (parallel to the growth rings) -- so you want to place the radial plane so that it parallel to the length of the leg -- in this way the movement of the tenon complements the movement of the leg. If you looked at the end of the tenon the growth rings would be parallel to the floor.
Hide glue. Brian, Pete and Curtis all use hide glue, although they vary a bit in it's application. Brian and Curtis size the joints first -- this is a very thin coat of hide glue applied to both the mortise and the tenon that is left to dry completely. Once the sizing is dry you apply a second coat then assemble the joint. The theory is, is that when you assemble a tight joint you shear off a good bit of the glue because there is simply no room for it. With hide glue, the second coat of glue will activate and bond to the sizing coat (which has already bonded to the wood) yielding a very strong glue joint. Brian says that you can even get a good end grain glue joint if this is done correctly. Pete is not convinced of the value of sizing and as far as I know doesn't do it. One other thing -- Curtis does not size the leg to seat joint since he says that it can split the seat.
Here's a link to an hour-long video of Brian Boggs at last year's Woodworking in America Conference in Berea, KY. It's a wide ranging discussion about chair design and construction and specifically talks about many of the technical aspects of chair joinery that I've mentioned above:
http://blip.tv/file/1501931
It will be an hour well spent.
Lefky
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