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Thread: CHAIR DESIGN 101
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24th May 2014, 11:11 PM #16
dim sims, dumplings and hooch it is then !
Tomorrow, before the GREAT SPINNING WHEEL RESTORATION HANDOVER, I will be cutting strips of different timbers to see how they steam bend. I have been warned that Oz red cedar does NOT steam bend well?
i have a couple of options. If the red cedar doesn't steam bend well then I can use another timber and stain it to match OR use a completely contrasting timber such as huon pine. i should know by this time tomorrow!a rock is an obsolete tool ......... until you don’t have a hammer!
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25th May 2014, 08:09 PM #17
I've had one of those rare 100% result days ....... every sample broke!
20140525_190054.jpg
Admittedly I was trying something extreme, more than needed for my current project, but I wanted to know the limits of what I can do.
So, I cut identically dimensioned pieces of rosewood, straight grained Australian red cedar (ARC), wild grained ARC and huon pine. In spite of every reference saying "an hour of steaming at 90+degrees per inch of thickness" my 8mm thick samples were left in for an hour after reaching 90+ degrees. I endeavoured to bend them around a 200mm diameter former which is more extreme than I require for this job but should be within the scope of the process?
I THINK I had enough heat, steam and time but the results would suggest otherwise!
All of the failures were tension splits on the outside surface. I got to about 90deg of the required 180deg each time before fracturing.
A bit more research showed that many successful practitioners use a flexible steel strap on the outside AND blocks on each end of the strap holding the workpiece snug to length.
WARNING, SCIENCE CONTENT
When any workpiece is bent around a radius the inner surface goes into compression and the outer surface goes into tension. Timber, like most solids, has a tension limit which leads to failure if that limit is exceeded BUT compression has a plastic period, where the timber compresses, then a 'crinkling' phase where the material bulges sideways to allow more compression but then it stops, without failure.
SO, bending it around the former without a steel strap and limiting blocks gave me insufficient compression on the inside leading to excess tension on the outside and hence failure. I need to find a way to bend with a higher compression on the inside and less tension on the outside.
I have seen such a device on a Fine Woodworking video and will knock one up over the next few days.
For my chair project however, the bends are not as extreme as 200mm radius AND, more importantly, the require workpiece has both concave and convex bends so it is possible that the tension on the outside of a concave bend may balance the compression that it received from the adjacent convex bend and hence not exceed the tension limit .... maybe?
Next step,
- make up a steel strap with adjustable limit blocks and retry the extreme radius
- make a male and female former for the chair slat, drop a steamed workpiece in .... and clamp it within an inch of its life!
I've got an unfinished glass of Krinklewood 2011 Shiraz somewhere............
flettya rock is an obsolete tool ......... until you don’t have a hammer!
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25th May 2014, 08:36 PM #18
Bending any timber 180° around a 200mm former is a big ask Fletty. I have a list of MoE and MoR values for Australian woods somewhere. Acquired for bow building but they will give you an idea of which timbers are more suited to steam bending.
Timber frame bracing strap is often used as the outside support strap. I think you can get it in a number of widths.Those were the droids I was looking for.
https://autoblastgates.com.au
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26th May 2014, 01:45 AM #19
Hi Fletty, don't know whether you have seen this one but shows what is possible with steam bending by Micheal Fortune
https://www.youtube.com/watch?v=610d...layer_embedded
Pete
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26th May 2014, 06:21 PM #20
beggar me! that's just plain humiliating
a 1" radius .....!
and, as for his timber stash.........!
AND, she's prettier than me..........!
flettya rock is an obsolete tool ......... until you don’t have a hammer!
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27th May 2014, 01:45 AM #21
The process and jigs all speak to me of someone who's been doing this a while, all of the bending was done with a steel strap on the outside, of note was what seemed to be a substantial bit of engineering at the start end of the bending arm, the clamping to the arm and the lever length to assist with the bending, also Micheal looked like he was bending the timber back and forth at one point, maybe softening/loosening the fibres to get it to conform
I say it would still be an effort to get the timber round, the bench was wobbling there a bit and there was a foot on the bench to push against.
Looks like the steel strap is a big help
This is a pic of some bending I did a while back, I used a lever block with the chain around the outside
single piece.jpg
Nothing like a 1" Radius this is about 1300mmR
Pete
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27th May 2014, 09:53 AM #22
Have 1x1/8x6ft steel flat here as well as chains and puller gear if needed Fletty.
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31st May 2014, 06:58 PM #23
more WIP .... less WHIP!
After 4 days of work work I snuck back to the shed. Unfortunately the recently completed steamer had also decided to have a break and the boiler element had quit! Would you believe the local sparky supplier had a replacement element for a 1950's laundry boiler IN STOCK! I lost a few hours but was steaming again by lunchtime.
I cut the profile of the back splats into some 4mm ply, fixed it to a piece of MDF and used an 8mm diameter bearing bit which then created the right male and female shaped formers to form 8mm thick slats of ARC between them
20140530_144318[1].jpg
I then used the 2 half formers, some more MDF and a copy bit to double the thickness
20140530_155519_1.jpg
this wouldn't have the capacity to go into production of the 64 needed slats but would allow me to at least work out HOW to make 1!
I covered the machined edge with clear packing tape and lay them flat on the workbench
20140531_140447_1.jpg
In the meantime I had put 2 pieces of 8mm thick anonymous hardwood (anonymousii hardwoodii) in the steamer (90++deg, 30+min) and then quickly put the 2 of them into the former and clamped it up
20140531_143441.jpg
All research had mentioned SPRINGBACK which is the amount that the workpiece would retreat from the shape of the former when released. I allowed A BIT (= 37 poofteenths) of exaggeration of the required shape but my actual 'retreat' from the shape of the former became a full blown rout! The 2 shaped pieces can be seen laying beside the former to which they have only a passing resemblance!
20140531_153137[1].jpg
Well 'obviously' hardwoodii anonymousii must be a VERY POOR timber for steam bending so I cut a real blank, 25 x 8mm straight grained Australian red cedar, as will be needed on the chair, steamed for 40min, clamped in the former, left to cool and laid it beside the hardwoodii anonymousii ........ hmmmm .... I think I have greatly underestimated springback!
20140531_170344.jpg
I am now wondering whether to trash the current former (or glue it together and recut it), make it a more extreme shape OR live with the wimpy shaped splat?
I already know the answer .... new more extreme formers tomorrow!
fletty
PS, while Grandad was bending bits of timber, my grandson had used a sandwich maker to bend himself double and hide INSIDE a kitchen trolley
IMG_7484.jpg
I received this pic with a message advising to "cancel the search party, boy found" .... he gets it from his mother!
flettya rock is an obsolete tool ......... until you don’t have a hammer!
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31st May 2014, 10:34 PM #24
See this is the good side of living out west you'd never find a part like that on the north side. Hope it wasn't to exy I mean tradesman can charge like Dr's when they feel like it.
I reckon 90C in 30mins is pretty good means the Chicken soup & steamed vegies will be ready quicker along with the dumplings.
Just sent you further reading material might be of help.
Is that No1 GS snuggled under one of Grandad's works of art?
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31st May 2014, 10:44 PM #25
This may be of help. Some times its best to leave the timber out in the sun to pre-warm it but dampen it or wet it down first.
PLASTICIZING THE STOCK
The purpose of all plasticizing treatments is to soften wood
sufficiently to enable it to take the compressive deformation necessary
to make the curve. Hot wood is more plastic than cold wood,
and wet wood is more plastic than dry wood. A combination of heat
and moisture is therefore most effective in softening wood. Treatments
with hot water or steam are used commonly to prepare wood
for bending. Some chemicals soften wood. However, research has
not produced a satisfactory explanation of the phenomenon of
plasticization, and softening methods are still based largely upon
trial-and-error experience.
Softening With Steam or Hot Water
Despite considerable experimentation with various chemical treatments,
plasticization with steam or hot water remains the most
practical and satisfactory method of softening wood for bending
purposes. Water alone softens wood somewhat, as evidenced by the
fact that green wood bends more readily than dry wood. Likewise,
heated wood is more readily bent than cold wood. Together, heat
and moisture can produce a degree of plasticity roughly 10 times
that of dry wood at normal temperatures.
In general, hardwoods are more readily softened than softwoods,
and certain hardwoods more so than others. The degree of softening
is one index of bending quality.
It is rarely, if ever, necessary to soften wood to its maximum
degree of plasticity for bending purposes. Indeed, excessively softened
wood may fail sooner than wood that is not so soft; presumably,
softening weakens wood. Evidence of the effects of overplasticization
is found in the results of Forest Products Laboratory tests on
the steaming of white oak (table I).
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1st June 2014, 10:22 AM #26Skwair2rownd
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A most interesting thread Fletty!!
The compressability of timber is an interesting topic.
I was talking to the son of an Italian cabinet maker in Leeton many years ago
and he had been to Italy to sus out some new machinery. He brought a couple
of samples of what I thought were carved feet for chair legs back with him. They
turned out to have been made by steaming a chunk of timber and compressing
it in a a mold under tremendous pressure.
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1st June 2014, 11:14 PM #27
20140601_203522.jpg
Sydney's rainy day shortened my morning run and gave me time in the shed. Although family duties saw me out and about for a happy afternoon, by tonight I had a successful result. YES, I have the first usable one of the required 64 steam bent splats!
The theory might be beyond me but experimentation is a valuable teacher.
The grubby pic above is a tracing of all of the data point shapes and was taken with the template on the (severely MDF dusted) floor. (sorry fence furniture, I'll clean up tomorrow!)
A = the datum and represents the unbent workpiece
D = the profile that I need and is the shape of my first former
B = the shape of the slat bent in D
E = the new 'more extreme' former that I made by guess work this morning AND
C = the shape of the slat bent in former E and is very bl**dy close to D which is the shape I need
Using some rather dodgy maths....
the first slat was bent in a former that has 105mm displacement but spring back reduced that to 50mm on the dry workpiece (= 48%)
the second slat was bent in a former that has 175mm of displacement and spring back reduced that to 90mm on the dry workpiece (= 51%)
SO, I'm prepared to say that for my setup, my way of working and air dried timber I need a former providing twice the displacement I actually need on the workpiece and yes, I wish my new former had been 210mm not 175!
I'm happy with the result and can now get on with the other 63.
For those who are enjoying the mysteries of steam bending ....I have another!
When I put todays new blank into the steamer, I also put in the workpiece shown as B above. I was going to steam it again and put it back into former B ... I'm not sure why but it seemed a productive thing to do ?
I put it into the steamer on its side i.e. looking like a slow S from above. Well, after working on the piece that ended up giving me a good result, I removed B from the steamer and ... IT HAD PERFECTLY STRAIGHTENED OUT! Being on its side, gravity couldn't have done it so it appears that the timber has retained some sort of memory. By the way, after putting it back into former D it gave me an exact repeat performance and returned to shape B!
Anyway, I now know how to make the slats I need and will thicken the former E so that I can at least get 2 correctly steam bent slats per session .... at least 32 days
flettya rock is an obsolete tool ......... until you don’t have a hammer!
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2nd June 2014, 07:53 AM #28
Fletty,
This is interesting reading !
Do you think any of the first pieces you bend will return to straight (or return towards straight) while you bend the others ?
I wonder if there is a way to store them bent for the month or two or three while you complete them all ?
Nice work !!Glenn Visca
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2nd June 2014, 11:55 AM #29
Hi Glenn,
It's strange you should say that! Last night I dreamed that I had done all 64, one per day, and put them into a dark cupboard. When I got them out on the 65th day .... THEY WERE ALL STRAIGHT!
I'm going to leave the first one 'unrestrained' and measure the variation over time, if any. However, for all the subsequent ones, I will at least restrian them in length and hope that will work?
flettya rock is an obsolete tool ......... until you don’t have a hammer!
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2nd June 2014, 12:32 PM #30
Tee hee ...
Sent from my HTC Sensation Z710a using TapatalkGlenn Visca
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