# Construction problem.



## Steve Maskery (1 May 2006)

Ladies and Gentlemain, as Frankie Howerd might have said, no, look, listen, here.

I've designed this 'ere chair:











The way I have drawn it, the back is only 12mm thick. I could beef up the lower part so that it is 20mm thick or so at floor level, but that still means 16mm max at seat level. My problem is obvious, how on earth do I attach the rails to the back? 

This is the joint that gets most strain, but I'm not at all convinced that a tenon that is only 12-16mm long would be sufficient. 

This is a bedroom chair, not a dining chair so the back won't be seen like a dining chair back would, so if I have to use brass caps it won't be the end of the world, but I'd prefer not to have to use them, I'd like to do some reasonably trad joinery here if possible.

These are the sort of things I mean from Woodfit:





I have seen chairs built with laminated backs like this before, but I've no idea what joinery the makers used. Anyone have any expertise in this area? It will be a lot of work to get wrong!

Cheers
Steve


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## OLD (1 May 2006)

How about fixing a block between the rails where they meet the back you then have a much larger surface area at the interface between back and combined rails so lots of ways to make the connection including your tenons as there could be other hidden fixings and large glue area


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## Steve Maskery (1 May 2006)

Hmm, good idea Old, hadn't thought of that. I could have two through tenons and another couple of blind tenons in between. Or indeed, all through tenons, if I reckon I could get them all identical.

I'll think on.

Thanks very much.

Steve


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## DaveL (1 May 2006)

Steve,

I think you need a set of nice metal brackets and a big slice of perspex. :twisted: :wink:


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## -David- (1 May 2006)

Steve

I am no cabinet or furnitur maker, just a DIY'er. So with that in mind my notion might be 'silly'.
I would not rely on the rails, you have a good hidden void between the rails, and sufficient depth to fix a wide batten to the back and then fix the seat to that. I am sure that would take care of the downward forces. As far as the twisting forces I would still use the rails. I would dowell from the back to the ends of the rails and the batten, making a feature of the dowell ends. 
Oh no!!
Just seen that OLD has got there before me.


David


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## Steve Maskery (1 May 2006)

DaveL":2y6dui7j said:


> Steve,
> 
> I think you need a set of nice metal brackets and a big slice of perspex. :twisted: :wink:



Absolutely! 

Yes, I thought the back was lovely, shame about the rest, really. I didn't like the way he'd mounted it into a cylinder at the bottom either, I reckon it would be very uncomfortable for the back, let alone the bum.

I've seen several fan-shaped chairs and alwways liked them. I found a nice one in a hotel room in Sweden about 20 years ago and always fancied making that. This is a hybrid of several really.

Well spotted!

Cheers
Steve

PS If anyone is wonderering what on earth we are on about, we saw a chair last weekend made from some nice wood and some horrible Perspex.


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## Chris Knight (1 May 2006)

Steve,

That is a tricky one. I reckon that it would might be strong enough with just the rails tenoned into mortices in the back but OLD has made a good suggestion to enhance the bearing/gluing area.

I think the problem of potential failure is likely to be compounded by the back flexing in the joint region which could effectively multiply the leverage stresses at the joint very considerably.

I suggest you make quick full scale mock-up and test it to destruction. You could use a flat piece of ply for the back to simplify the mock-up's construction.


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## jasonB (1 May 2006)

You could use some form of metal fixings like the ones you have shown. Then hide the heads by routing out a 1mm deep rectangular recess and glueing in some end grain veneer, I won't tell anyone they are not through tennons, you could even do some fancy diagonal wedging  

I once used this method on a table that had 50+ through tennons, eight of them took a lot of load so I used square metal bar epoxied in with a 3mm bit of endgrain to cover the hole and look like a tennon.

Jason


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## Steve Maskery (1 May 2006)

What if Iadopted an invisible mechanical approach? I could run 2 channels down the full length of each rail, bury 4 nuts in the back (before veneering, of course), and run 4 lengths of studding through the whole lot, tightening them up from the front of the cross-rail. If I made the seat so that it rolled down over the front rail, the tightening nuts wouldn't be seen.






?

Cheers
Steve


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## Bean (1 May 2006)

That would work Steve


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## cambournepete (1 May 2006)

What about using wedged tenons or wedged double tenons - would they be strong enough?

You could glue a block to the back and then joint into that - not sure if that suits your design though - which I rather like btw.


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## engineer one (1 May 2006)

my little engineering brain reminds me of one inevitable thing
Steve, people ALWAYS lean back on chairs, and by that i mean
go onto the back two legs and lean further back. in your case, 
the very shape of the back, and its thickness will add to the
bending moment.

Firstly as Old has said, i am sure that a cross bar under the seat
is vital, but i also wonder whether you should consider an
angled bar from near the floor up to the front of the seat. 
This would stop the seat bending at the front, which i think it would
with the front legs not further supported. I know it takes away
some of the initial beauty, but in these circumstances, i always think that
referring back to the mackintosh style chairs with high backs
is a useful lesson.

Your mechanical supports have some value, but i wonder whether the bending moment will not cause the ply to shatter because it of the 
localised stress when tightened.

The design is great, but why do conventional chairs have lower
stretchers????? :-k :-k 

I know you have more experience than me in actual construction, 
but i think the bending moment is greater than you think it may be.

good luck
paul :wink:


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## Scrit (1 May 2006)

Steve

Have you considered bonding-in something like some BigHead fasteners in the back and using these with your threaded stay rods? These fasteners were actually designed to be embedded into fibreglass or plastics on boats, etc and would probably work quite well in your situation as the point load on the joints would be spread onto the full area of the plate (and they make some fairly large plates). If you then end the threaded rods in 40mm diameter holes on the inside of the rails it would be possible to use one end of a kitchen worktop connector (in fact just the long hex nut and half-moon bushing) to provide a completely hidden joining system with a lot of strength

Scrit


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## Steve Maskery (1 May 2006)

Wow, Scrit that is EXCELLENT! I didn't know such things existed!

There is just the thing in the photo, but I can't find it in their catalogue. Methinks a little phone call is in order. 

Thanks very much indeed.

Cheers
Steve


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## Colin C (1 May 2006)

Hi Steve
I think you want tot look at an antique hall chair and see how they are fixed.
What about fixing a rail between the back rails ( dovetailing it to both rails and you can still have through tenons )
you could also put a surporting peice at the back to help strengthen the joint and also make a feature of it.
If that is clear :?  
What was wrong with that perspex chair :^o 8-[ 
You are right it did look unfinished


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## engineer one (2 May 2006)

steve,
i am still concerned about the back breaking around the seat fixing, and no matter what you do with the fixing, it will still be very weak there.

so i have another thought to make it easier, i think.

can you not also bend the back a little in the horizontal plane, so that it is part of a cylinder in both planes. in that way you will truly strengthen the whole back, and you could use 12 mm all the way. 

as for the tenons, how about through ones, and tusks???

i do not think the fixing is the problem, i am convinced it is the 
bending moment.

paul :wink:


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## OLD (2 May 2006)

I think that engineer one may have a point about stress areas however we all tend to over engineer things, but the answers may come from research into furniture of the 1960 where bent ply and minimal lines were common i believe, although i don't have any information to hand.


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## engineer one (2 May 2006)

i agree that woodworkers tend to "over-engineer",
however ,in this litigious society,
we have to be careful because the client NEVER tells the
truth, by that i mean they will tell you they will use the
item in the way designed, but they never do.

i only caution that you look carefully at the relevant areas,
and old is right to suggest looking at the plywood furniture of
the 50's and 60's

paul :wink:


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## Nick W (2 May 2006)

I think I'd join a solid piece of timber between the rear of the two rails, probably tenoned into them. Shape that piece carefully and then glue and screw through that into the back. It should give quite a large area of long-grain to long-grain join. And yes, taper the back so that it is 16mm at the join area. Maybe even have the back an even 20mm up to the join area and taper it from there up?


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## Steve Maskery (2 May 2006)

Thank you all for your excellent suggestions.

I'd like to keep it as clean as possible, so I thnk tusk tenons are probably not what I want - but thanks for the idea.

Looking at the furniture of the 50s and 60s is a good one, I'll have to do some research.

I've just ordered the catalogue from BigHead. I like this idea a lot. The biggest problem with it will be mounting it so that it is in line with the rails, rather than perpendicular to the face. But maybe some of the ones on a strap will be bendable enough to be moulded to the shape of the back.

I agree making the back cupped would increase its stiffness. Good idea. But it does make the joinery even more difficult. Hmm, tricky one.

Anyway lots to think about, thank you all very much indeed.

Cheers
Steve

PS Neil, you've let me down. I had a bet with myself that you'd suggest Miller Dowels! :wink:


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## engineer one (2 May 2006)

steve,
like everybody i was trying to suggest a quick way to build
up strength, not unfortunately make the joinery easier.

i do however wonder whether there might be some logic in
moulding the cross piece that has been suggested onto the former
when you shape the back, i assume that you may well use some
kind of vacuum press to retain the shape. if you put retaining nuts into
the cross rail at the back before you glued it onto the back, maybe that would work. i.e. captive nuts hidden at the back of the cross piece.
it could be quite a thick section, and then the front legs could be studded into it.

another bit of stirring for the mix :evil: :evil: :evil: 

good luck

paul :wink:


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## Steve Maskery (2 May 2006)

Hi Paul,

What you suggest would be a good solution, I think. I cross piece could look quite attractive, not as "in yer face" as a tusk tenon would be. But if I can have it invisible, then that would be even better.

I'll let you all know what happens when I see this BigHead stuff. Orientation is going to be the problem, which, I admit, would be easier with a cross-piece as you suggest, which could be easily shaped to fit the shape of the back. I might even make a mock-up of both.

Cheers
Steve


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## engineer one (2 May 2006)

steve i was actually suggesting the cross piece goes inside, not outside,
not least because i think it would be better to make the rear of the seat
have a widthways support, this would give it more strength in shear, and that might well strengthen the back.

the other reason for suggesting cylinderisation of the back is that i wonder how you are going to get it stop flopping around if you only use 12mm ply,
it looks quite long, and i don't actually see how you are supporting it at the top. :? 

glad to help you think outside the box 

paul :wink:


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## Newbie_Neil (2 May 2006)

Steve Maskery":jlewpphk said:


> Neil, you've let me down. I had a bet with myself that you'd suggest Miller Dowels! :wink:



ROTFL

Cheers
Neil


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## Steve Maskery (2 May 2006)

Neil - 

Paul
Oh, I see. But I don't see how that helps to fix the back to the rest of the structure (whether it's the rails or a crossrail), you still have the problem of fixing something which is only 12 or 16mm thick to the rest of the chair. Doesn't that still rely just on the strength of the laminations? Or am I missing something?

Cheers
Steve


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## engineer one (2 May 2006)

well steve, not least you get a BIG gluing area on the inside, 
which could for instance be fixed with contrasting dowels from the outside,
to make a feature. or "miller dowels" :lol: :lol: 

if you think about the older plywood chairs that you saw in 
offices etc, they often used quite small screws to hold the ply to the
metal work, and they often use a captive nut to hold things in.

paul :wink:


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## Bean (2 May 2006)

Steve BigHead Fasteners are expensive to buy, they do send out excellent sample bags though, try to bag one of those first

Bean


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## engineer one (2 May 2006)

ok steve, more thoughts

from an engineering point of view, i believe that the bottom of the back is as unsupported as the top will be, so i feel that you need a stretcher to the front legs, so i pondered, and thought , well that's what i call it. :lol: 

how about a stretcher from the lower back curving at a kind of tangent
to the back, toward the front. 

as for my idea about the under seat rear cross piece, surely a two pack 
epoxy used for yacht building would give you strength and location??

good luck
paul :wink:


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## Steve Maskery (2 May 2006)

A stretcher would solve all the problems, of course, but, ah, it would change the design too. It would be clutter it up.

Actually I'd not thought about the strngth of the front leg joint.

I want to have my cake and eat it.



Keep the thoughts coming

S


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## engineer one (3 May 2006)

"my candle burns with such a light, since it burns at both ends, but ah my friends and ah my foes will it see the night"

aas you said to me recently on another subject steve, we all strive for 
the impossible, but must take what we can get. the essence of your chair is great, BUT, can you sit on it on a regular basis????? :lol: :? 

as i said at the beginning the customer, or their kid, or someone will
always lean back on the b thing, so we must look to avoid the sawing 
motion that would tend to reduce the strength of the joint, and 
make it an expensive piece of firewood.

whilst all my suggestions have been wood inspired, the design does lend
itself to a more fantasy approach, very "round table", so then i thought
what about using metal??

i do think that you need to restrain the front legs, as well as hold up the seat, so then i thought about wires between the various areas, you could make these cross toward the bottom, and they would be quite thin, so 
not so visible.

but overall the real concern has to be the thickness of the ply you want to use for the back. i do not believe that 12/16 or 20mm will be strong enough to stand the bending moment, let alone the other stresses, but happy to be proved wrong.

now i have to get back to making my book cases more attractive, and relatively quick to construct.

best wishes
paul :wink:


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## Steve Maskery (3 May 2006)

I woke up with another thought ths morning.

If I make the back curved round, to increase its rigidity, I could do that by adding more layers in the centre. It would be possible to keep the edges the same thickness (so it still looks delicate) but be considerably beefier in the centre.

This would, however give me a 3D piece of wood, so how on earth do I veneer that? I have enough trouble with my seams on flat stuff!

I think I'm going to take up wood turning.

Cheers
Steve


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## CHJ (3 May 2006)

Steve Maskery":xlsv3pnx said:


> ...snip...This would, however give me a 3D piece of wood, so how on earth do I veneer that? I have enough trouble with my seams on flat stuff!
> I think I'm going to take up wood turning.
> Cheers
> Steve



You will find veneering on the finished product even more of a challenge :lol:


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## Chris Knight (3 May 2006)

Steve,
I have said it before and I'll say it again.. I recommend "Veneering A Foundation Course" by Mike Burton ( http://tinyurl.com/z8hm9 ) In it he shows how he uses his iron on method to veneer a bombé chest with 3d curves. Never done it myself but he makes it look perfectly practical.


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## CHJ (3 May 2006)

If I remember correctly, a lot of the 50's & 60's furniture relied on the flexing of the ply to relieve the stress loads at the actual joint interfaces.

Just for the hell of it try bonding two lengths of 12mm ply at 90deg. with a 24mm ply (two strips) corner fillet (use ordinary PVA) and see if you can break the joint after 48 hours, I think you will find it stronger than envisioned.

As for fixing the seat side rails to the back, some form of hidden steel angles across the back member (bonded/laminated in) and running onto/into the inside of the seat rails (barrel nuts?) would take a load off any glue joint in that area. 

An internal horizontal member dovetailed into the side pieces set back far enough to accommodate a similar piece bonded to the back would hold the side rails at fixed distance and provide fixing to the back rail.

Rambling I know, but just my musings trying to think outside the box a bit.


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## Nick W (3 May 2006)

Surely embedding bits of steel into relatively flexible ply will have the effect of concentrating stress at the interface between the two, making matters worse. Of course the same will happen using a solid lump of timber, but to a lesser extent. Seems to me it's time for a prototype (or six) starting with simple wood to ply joints with a decent glue, and working up if necessary.


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## engineer one (3 May 2006)

nick. to an extent, you are correct about concentrating the loads, 
but that is one of the reasons that one for instance in other circumstances
you put larger washers on any fixing.

but one of the reasons for building a prototype (sorry Steve!)
is to check the load paths, and the point stresses.

in general if someone is using the chair properly, then captive nuts supporting the seat and holding the front to the back would be
used properly since the load would be spread downward properly,
it is only when people use the chair in an "inappropriate way :lol: " that you may have problems, and that is what we (i ) are trying to avoid.

as for the veneering i thought you might have a vacuum bag steve to do both jobs at once, i.e. form the shape, and also fix veneer at the same time, or is this a case of finding how to use a new technique :twisted: 

i think if you study our old friend mackintosh, you will see a number
of fine looking chairs which evolved from the first sketch to the finished
product, and maybe ended up being jam mixed with butter not just jam.

you realise this will unlike the chair keep on running.

by the by are you costing the forum discussions into your price??? :lol: :lol: 

good luck
paul :wink:


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## Steve Maskery (3 May 2006)

Paul

The chair is for our own use, so price is immaterial. It wont get a lot of sitting on either, I'll just use it in a morning to help me put my socks on. But that's not the pont, of course 

Re Mackintosh. I love his stuff, but his was a classic case of beautiful aesthetics and very poor construction methods. I've seen some of the originals in Glasgow and they are not all a pretty sight (well the chairs are, but the joinery isn't).

I think a prototype is a good idea, and as for spreading the pressure, that's what is so attractive about the BigHeads.

Re Vac. I do have and us a vacuum bag, but the point is that the seam will not be a straight line. I might have to take Chris's advice and do it the old-fashioned way with dead animals.
And if the author makes it look dead easy, I bet it isn't!

Cheers
Steve


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## engineer one (3 May 2006)

Steve, i agree about the construction skill of mackintosh,
but lets be honest some Hepplewhite is not exactly made as
we believe that it should be today, particularly behind the scenes.

even though i worked for vauxhall, i was still taught that form follows
function, and that if it looks right, it is.

from my perspective, the chair looks nice, but not "right", which is why 
we are having the discussion i guess.

but since you are putting your socks on whilst sitting on it,
you are infact going to cause some of the problems about the 
bending moment so that is what the prototype will show,maybe

anyway thanks for letting me and us into the process of thinking things
through

paul :wink:


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## Scrit (3 May 2006)

Steve Maskery":1nxj2gmi said:


> I think a prototype is a good idea, and as for spreading the pressure, that's what is so attractive about the BigHeads.
> 
> Re Vac. I do have and us a vacuum bag, but the point is that the seam will not be a straight line. I might have to take Chris's advice and do it the old-fashioned way with dead animals.
> And if the author makes it look dead easy, I bet it isn't!
> ...


I first came across Bigheads in the late 1960s - my brother used to use them to repair his Berkley T60 and later a Lotus Elite (the original pre-Elan mkI variety). These are both fibreglass monocoque structures with no chassis, so all the suspension loads, etc are fed into the structure through Bighead anchor points. I doubt that your chair will equal the loads involved and if the rails are deep enough you should be able to avoid the turning moment where the rails meet the back quite well.

As to 12mm plywood not being that strong., well, I've laid-up a few batches of curved tub shells for 1930s style chairs in the past, and whilst a 1/2in (12mm) shell can flex it is remarkably strong. If you're really concerned I'd go for poplar veneers with a face veneer top and bottom - poplar is one of the strongest lightest woods for lamination which is why it was so popular in early aircraft construction. Putting a curve in the back would certainly increase the rigidity of the structure and I can recall seeing some chairs at an exhibition in the 1970s where the laminae were glued using epoxy (West System?) and extra strength derived from using a couple of layers of either fibreglass or carbon fibre in the middle of the structure...... If you want to get rid of the flexing conventially, then going to 20 or 24mm thickness would certainly get the extra rigidity but possibly at some cost in terms of the aesthetics of the piece. I'm unconvinced that having a tapered form (wing-cross section with thinner edges - a way to "lighten" the look of a thicker back) would work as well as your original design. In any case - who'd lean back on a Mackintosh chair and expect it to survive? They're pieces of art rather than functional pieces of furniture. :lol: 

Scrit


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## engineer one (3 May 2006)

scrit i think you are right, but my concern about the 12mm is the
height at the back, whereas you talked about a tub which by 
definition suggests more equal sizes all round.

having been involved in early 1970's formula one car testing i know
what kind of loads are put through the smallest of areas, and 
how well that can be spread, but often they used mallite
which had a balsa wood centre so who knows/

the mosquito certainly was fixed and flew "fast" with very early glues
so maybe i am being over -sensitive.

but in my limited defence at the beginning i thought steve might be
making something to sell so was concerned about the H&S problem too.

as for rocking back on a mackintosh chair, who would have thought you could fall down a staircase in a museum and break not one but three japanese vases and call that an accident???

unfortunately someone will always do the unexpected, even if it is a visitor.

but i do feel that we are all learning more.
paul :wink:


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## Neil (3 May 2006)

Hi Steve,

This chair which the guy on Cutting Edge Woodworker made looks to be constructionally similar to your (very nice, btw) design. He just used M6 threaded inserts set into the back, so the big heads should be absolutely fine. He also did a double taper so that the widest part of the back was at the joint. Anyway, you might glean something useful from the pdf.

Cheers,
Neil


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## Steve Maskery (3 May 2006)

Hi Neil,
Thank you, again 

It has occurred to me that the rails underneath do ot need to be maple. They could be ply, and indeed, could be solid, right across. That way I could get 6 nuts in quite easily, they could be in line with the face of the back (no need to insert them at an angle to match the rails) and the strength of the rails won't be compromised, because it is the same in all directions.

Ooh I can feel a brainwave coming on.

Cheers
Steve


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## cowtown_eric (4 May 2006)

Eric from the colonies here...

how about this idea. 

your two curvitational underseat rails that butt onto the curved back lamination, they are, I agreee where the majority of stress is gonna occur, so I'd draw upon the concept of the "clenched nail" or the wedged tenon, and suggest that you place a block between the two rails, perhaps substantial, as in 8.4 material, and temporarily attach the seat assembly to the back. Then, judiciously cut 3/8 x3/8 moritices through the back into the seat rails, perhaps add a few in a curvilinear orientation into the 8/4 block. 

then, taper the top and bottom sides of the hole with a 3.8 chisel, perhaps to dovetail angles, ..yer getting set up for wedged tenons here.

You glue the block between the rails to the back, not clearly strong enuf, cause it would just lift off the top ply under stress, but every little bit helps.

You could always use dowels on the ends of this block to attach it to the side rails if you wanted, or just pocket screws. 

anyway, you mill up some 3/8x3/8 stock and glue it into the side rails, trim it off just a tad longer than yer 12mm back thickness, and cut a horizontal slot for the wedge. 

you put the back in place over the tenons you've created, clamp it up, slather some glue on some wedges and drive em home, creating captured dovetails. Not easily removed or fixed, unfortuanately.

Any stress would concievably be carried throughout the entire thickness of the back, as opposed to just off the front frace. 

If the 8/4 block was dowelled into the side rails, and the same wedged tenons were used along it's length, you could likely even triple or quadruple the strenght of the jointery.

OTOH, you could just use wood screws and plugs and metal dowels to achieve the same strength

This is sheer conjecture on my part, it would be a simple task to do a mock up and test it to assess the strength. 

I betcha that Rennie Mackintosh fella did the same with his chairs. That high back spells substantial leverage to me. 

Eric
in Calgary
we is still just using saddles, eh!


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## Newbie_Neil (4 May 2006)

Hi Eric

Welcome to the forum.

Cheers
Neil


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## Steve Maskery (4 May 2006)

Hi Eric, Thanks for your thoughts.

Well, all those who are still rivetted by all this, what do you think of this? I've moved the rails a bit further apart and filled in with a stack of birch ply. I can then get 4 bolts in perpendicular to the surface of the back:







and






At M8 (5/16" if you are over the Pond), I think a horse could kiddle on it.

BTW Had some bumf from BigHead today, inc a few samples. I think they would be great in the back as I wouldn't have to remove as much material (although then I would be relying on the weld to hold).

Cheers
Steve


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## engineer one (4 May 2006)

interesting solution steve.

i quite liked the chair on the pdf, but note that he was using 13 plies
wonder how thick it came out at???

still worry about the flexing.

great drawings though
paul


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## OLD (5 May 2006)

I think you are in danger of forgetting just how strong modern glue is your bolted solution would work with out glue .
If i had the skill to design and build this chair i would as recommended build a 'tester' glue and screws (original design + block) laminate a back following scrits recommendations and abuse it, not compromising the design is worth the experiment and if the back needs to be stiffer then finding a material to incorporate will be a interesting exercise.


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## Scrit (5 May 2006)

engineer one":39bftwf5 said:


> I quite liked the chair on the pdf, but note that he was using 13 plies - wonder how thick it came out at???


Assuming standard laminating veneers at 1.0mm +/- 0.3mm and 0.2 to 0.3mm glue line, probably around 18 to 20mm thick. 



Steve Maskery":39bftwf5 said:


> BTW Had some bumf from BigHead today, inc a few samples. I think they would be great in the back as I wouldn't have to remove as much material (although then I would be relying on the weld to hold).


Bighead will actually give you some leading factors for their stuff if you ask. I'd say that the wood to Bighead bond would probably fail before the weld, though, and I'm assuming an epoxy bond

Scrit


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