Construction problem.

[engineer one]

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:

 

[Steve Maskery]

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

 

[engineer one]

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:

 

[Newbie_Neil]

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

ROTFL

Cheers
Neil

 

[Steve Maskery]

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

 

[engineer one]

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:

 

[Bean]

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

Bean

 

[engineer one]

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:

 

[Steve Maskery]

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

 

[engineer one]

"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:

 

[Steve Maskery]

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

 

[CHJ]

...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:

 

[Chris Knight]

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.

 

[CHJ]

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.

 

[Nick W]

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.

 

[engineer one]

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:

 

[Steve Maskery]

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

 

[engineer one]

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:

 

[Scrit]

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

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

 

[engineer one]

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: