Thread: CHAIR DESIGN 101

I guess the amount of oil with in the Huon and closeness of grain is its attribute. While the Cedar is the opposite.

I doubt steaming will be happening today darn rain puts end to lots of things

Originally Posted by wheelinround

I doubt steaming will be happening today darn rain puts end to lots of things

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....neither rain nor hail nor snow will stop...... you get the idea!

I've had a bit of a breakthrough day ... more so than the Wallabies tonight but, as the commentators said for them as it is for me ... a win is a win!
i collected the cut ends of slats I've made in the last few weeks. These cut ends are as the slats came out of the steamer and there's clearly 2 different types.

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the red 'crusty' ends came from slats with a much higher success rate than the slats with the light coloured ends. I'm presuming moisture content/age is the difference. The red crusty slats were much more softened than the slats that came out of the steamer looking like they did when they went in!
I cut some sample pieces off planks of ARC and put them in the steamer hoping to find some of them would be 'crusties' ... but alas none.
I cut 8 slats from the 'crustiest' plank, steamed 4 and got 2 successes. However I changed my technique for the third and fourth and I think I might be onto something. As you can see from the video, my bending former is horizontal and I push the former halves together before engaging 3 clamps. Up until now, I have tightened the centre clamp and then nipped up the 2 outer clamps. The fracture, if it happens, is always in the centre bend and adjacent to the centre clamp and ONLY happens when I am tightening the centre clamp. On 3 and 4 today, I closed the former halves with the 2 outer clamps only and there is definitely some different mechanics at play here. When closing the centre clamp only and the 2 ends start to bend around the outer parts of the former, it transfers a competing stress on the centre bend that seems to result in a sharp bend and fracture. However, when closing with the 2 outer clamps the ends are trapped by the geometry of the former and this puts the workpiece into tension across the big bend in the centre.
So far, outer clamps only = 100% success. Slats from the same blank, steamed identically and clamped by the centre clamp = 50% overall but 0% today.
As noted earlier, I don't need to do any of this voodoo for any other timber I have tried BUT, having now decided to do it all with ARC, I will be practicing voodoo for about another 40 slats!
Another success today was trialling camphor laurel, it steamed and bent beautifully AND cleared the sinuses (sinii?) of every living creature nearby.
Anyway, home now, a win is a win....
fletty

Originally Posted by fletty

I'm going to the pub...
fletty

Seems lasts nights bender may have been good for the thought processes

Originally Posted by fletty

So far, outer clamps only = 100% success.........

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....now 7 out of 7, still 100%!

Originally Posted by fletty

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....now 7 out of 7, still 100%!

ah, the power of experimentation

Enjoying following this one fletty, especially since I have a stack of ARC in the garage ear marked for a chair. Not as daring as you though, i'm following some plans which call for bent lamination but as I don't have a bandsaw was going to try steam bending. Hopefully I can take something from all your experiments and come up with a satisfactory result without to much trouble.

I don't want anyone else going through the steam bending learning curve that has reduced me to the drivelling mess currently in front of you ... or was that the shiraz....?
So, I am going to write almost a data sheet on it and I have even titled it so that others might stumble on it when googling!
Don't worry though folks, I will soon return to my more adventurous prose and irreverence.
I am claiming little or no scientific nor botanic knowledge BUT I can now bend Australian Red Cedar (ARC) with reasonable certainty of getting the desired result.

Background
Timber is a material made of fibres or capillaries that are bound together by organic materials which are grouped together under the name of lignum. This lignum changes with age and external intervention such as forced or natural drying. Steam bending is a process whereby steam is used to soften the lignum and allow the timber to be bent (formed, rolled and twisted) which involves the capillaries moving in relationship to each other but being set in the new arrangement once the lignum re-sets. However, if the lignum is already permanently set via forced (kiln) drying or excessive age since the tree was felled, then the lignum can not be softened and hence the timber can not be bent without high probability of snapping or cracking. Green timber is by far the easiest to bend BUT this timber is then less stable and possibly even unsuitable for use on (say) furniture unless it is either air or kiln dried before use. Different timbers behave differently under this process and then the same timber behaves differently under different age or drying state. Our North American brothers and sisters swear by the bending prowess of white oak, the British swear by oak and yew and our Australian predecessors use spotted gum and related species very successfully for extreme bending such as wagon wheels.

Australian Red Cedar (toona australis, toona ciliata)
Australian Red Cedar (ARC) is a superb cabinet timber growing in Eastern Australia and north as far as India and was used and (particularly) exported from Australia in vast quantities during the 19th and early 20th centuries. It is now difficult to obtain in commercial quantities but we hobbyists can still source beautiful samples of the species for our use and enjoyment. It was used in some of Australia's earliest colonial furniture and has such a special place in our history that it has even inspired books. I particularly recommend you read 'Red Cedar in Australia' (various, ISBN9781876991197) and 'Rare and Curious' (Elizabeth Ellis ISBN9780522853797). After a number of searches, I have found that different sources advise that ARC has been declared both good and bad for steam bending! However, I have also noted that ARC has been extensively used in Australian manufactured railway carriages and these, by definition, need the internal lining to be bent to match the tunnel profile of these carriages

Mechanics of bending
If any material is bent, it is subject to stresses that come from the different forces that transition from the inside of the bend to the outside surface of the bend. In general. if the workpiece is unrestrained, the inner surface is compressed and the outer surface is tensioned. Both excess compression and excess tension can cause failure in the workpiece. If however the workpiece is restrained (for example reinforcing steel in concrete and steel strap plus end blocks with timber) these forces can be varied such that the workpiece always remains in compression (such as in reinforced concrete) or, for timber, both the tension on the outside and compression on the inside can be kept below failure levels. If the workpiece is thin (say <10mm) then most timbers do not exceed either their compression nor tension failure limits when bent BUT if the bend is extreme OR the thickness increases then either of both of these limits can be exceeded almost immediately if steam is not used or later if steam is used. The art is to get the right temperature, amount of steam and time, to bend the thickness you want bent around the radius you need without either a compression failure....

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or tension failure.....

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An example, bending ARC slats for chair splats
The job that inspired this thesis involved bending ARC around a (reasonably) sinusoid former. The splats are 8mm thick and 25mm wide. Rather than going through the whole experimental process before arriving at repeatable and predictable success, I will note the method that now works for me, using my equipment and my stash of air dried ARC
My steamer consists of an old laundry boiler connected to a double walled plywood box

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with the connection being via some irrigation pipe fittings

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and fitted with a domestic meat cooking thermometer

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which needs to indicates a temperature in the highest segment to achieve the desired bend.
The bending former is cut from a doubled sheet of MDF with the male and female portions separated by an 8mm wide gap

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The drying former is a simple frame to hold the bent workpieces in the desired shape after being bent in the bending former and until they have dried.

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The 2 lines drawn on the base of the drying former indicate (inner) the desired final shape and (outer) the actual shape of the bending former showing that the timber springs back about 50% when released from the bending former. This means that the bending former needs to have twice the amount of deviation as required for the final workpiece shape

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When dry, the workpieces are marked for length on the drying former....

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cut and sanded on the bobbin sander

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The process used on this example (as decided after EXTENSIVE experimentation)
The workpieces are soaked in water at least overnight. Until I inserted this step I was getting variable, unpredictable results and a failure rate of about 50%! I believe that the soaking is not softening the lignum but, as wood is a good thermal insulator, I believe that the soaked water allows the heat from the steam to get into the workpiece much more quickly and evenly.
The workpiece/s are steamed for at least an hour (forget "an hour per inch"!)
Quickly remove one workpiece from the steamer, put it in the bending former, and slowly but steadily close the 2 halves.
I found that a single clamp in the middle resulted in an unsuccessful failure rate. The geometry of the former seems to put an unsustainable bend in the centre of the workpiece when using one clamp BUT, by using 2 clamps simultaneously on the ends, the workpiece over the most extreme bend seems to be in tension rather than kinked.
The workpiece stays in the bending former for at least 10 minutes before being removed...
and placed in the drying former.

Conclusion
I am now getting a 100% success rate with consistent shape. Interestingly (and unexplainably), all of the workpieces since I inserted the soaking stage, demonstrate a much darker and more consistent colour. I am even considering putting the earlier pieces back through the process including the soaking stage to improve their colour and consistency.
ARC is NOT easy to bend! I have managed to drag a result out of the process only because I was committed to both the timber and the need to bend. If I had known the amount of work required I would have removed the need to bend and/or change the timber. As I experimented with ARC, I also sampled other Australian timbers, some of which bent beautifully and didn't even need sanding after bending!

fletty

One of the hidden benefits of woodwork is that on bl**dy cold nights you get to burn the evidence...

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... unfortunately i don't have any Brown Bros Liqueur Muscat left!

I have cut and radiused the remaining 24 slats tonight and put them into water ready to be bent later in the week but I have something else planned for Friday, Saturday AND Sunday .....,

fletty

Yep, very good write up.

We await eagerly for the next unfolding saga of the 101 Chair.

Fletty

thanks for the writing up upir observations

Originally Posted by fletty

but I have something else planned for Friday, Saturday AND Sunday .....,

I wonder what that might be

Yep, just goes to show what a little trial and error/perserverance will achieve.
I too partake of the sacrificial burning on these chilly nights.


Pete

I have been quietly working on the chairs and (finally?) enjoying the process. I have 'finished' the slat steaming process but unfortunately, due to breakages eating up the float, I have completed EXACTLY the right number which is dangerous! I'll cut another half dozen slats and steam them through the week to give me a few spares. By using the EP, I have determined that the slats can be inserted after the chair is assembled. I'll claim this as a design productivity feature but.........!
The plan now is to finish the chairs and then sort, select, insert and fix the slats using hide glue. This will allow for any future broken slat to be replaced easily.
I have started on the front leg to armrest joints which will be heavily sheer loaded even during normal use of the chairs. I am making 2 prototypes out of pine of the multiple finger joints so I can do a destructive test to compare glues.

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At SWWW, I spoke to our sticky marine brethren and so I will be trialling PURBOND and EPOXY glues. Having decided that I probably can't do a consistent test to the 2 samples by hitting them on the point with a sledge hammer, I'll probably use the bench vise or hydraulic jack?
While in Sydney last week, I visited the fabulous Museum of Sydney to see the Celestial City exhibition but also wandered into the Iconic Australian Houses exhibit. I was impressed by the content and quality of both the houses and the exhibition but particularly by the exquisite models. One house however, showed that I can no longer escape the steam bending of Australian(?) red cedar

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fletty

Yeah but it took him four years to get it right. You have eclipsed his efforts

Rockhampton.

Latitude 23° 26' 14.440" south of the Equator.

The Tropic of Capricorn divides the city.

Originally Posted by pjt

I too partake of the sacrificial burning on these chilly nights.

Oh, P-lease! It tried hard to snow here on three occasions last week.

Shall I get my granny to send you up some knitted booties for your cold little tootsies?