Thread: wood bending questions

did I post wrong ? hs the technique changed? I expected lots of replies by now

Zed , I'm not an experienced woodbender , and probably someone will correct me in a minute - but I think that for such a gentle curve you could use much thicker strips than 2.5 mm - the stairbuilders bend with 6 or 7 mm thick oak , and I've done S-forms with 3 mm Ash for some quite tight radius - like a car wheel.The lamination was 18 mm , too (6 strips)

radius ~ 1 m means circumference is about 6.3m so your curve is about 1/6 of the circumference

there is a formula for spring back, something like
spring back = (the vertical distance between the chord and the top of an arc) / (the number of plies squared)

so the more plies the less the spring back

in your situation, the spring back with 3 plies would be about 3x that with 5 plies, and 4x that with 6 plies

sorry, but at this time of night it would be more accurate if you measured the vertical distance between the chord and the top of you arc rather than I try to calculatre it -- maybe it's because I should be in bed, but I can't make sense of your dimensions

but the above should get you started

Originally Posted by Zed

Hi,

I haven't posted much for a while as I've been "Away" and not doing that much ww. I was hoping that some of the experienced benders may be able to answer my questions.

a couple of questions from the benders if I may please.

1) how thin do you need to make the individual tas oak / vic ash strips to do a bent wood lamination to a thickness of about 10mm where the bend radius is approx a metre ? (Ie sum is:

L = 1100mm (overall centre of timber length from tip to tip as a straight line not an arc)
L- = 1090mm (outside edge of L = the "stretched" side)
L+ = 1110mm (inside edge of L = the "compressed" side)

(just a gentle arc really)... I'm thinking 2.5mm or so should be ok.

2) how much springback will I get after the form is dry?

3) should I wet the strips a bit to facilitate glue absorption ?

4) will I be able to push the springback back to the size I want when i attach it to the carcase?

5) anone know of any online resources that specifies characteristics of aust species ?

ta

Zed

I can't claim to be a bender (maybe just bent) but if I was gonna do something like this..
1... cut varying thicknesses and just see how it feels, even take it until u hear it creaking then u know how varying thicknesses will behave.
2... it will most likely spring back some, my guess would be u'll get less springback with thinner laminations and more with thicker.
3.... no I wouldn't think that it will be necessary to wet the wood, I'd be using a 2 part epoxy or similliar not that pva won't hold but for a better long term result an expoxy will most likely be better and may even be stiffer, less springback.
4... yep I'd reckon so.
5... not really.

hope that helps, might start some discussion...


Pete

Yep, Ian's onto it with that calc. divide by number of plies squared means an increase of just one ply has a big effect on less springback

Pete

Welcome back Zeddy, good to see you still have a penchant for woodworking.

Have you thought about using bendy ply and then laminating a veneer to the exposed faces?

Or possibly you could try kerfing the piece you want bent.

Have you considered steam bending?

Geoff

Originally Posted by Geoff Dean

Have you considered steam bending?

Geoff

Or Kerf bending?

Originally Posted by munruben

Or Kerf bending?

That would be the suggestion I had just above the steam bending.

Good Morning Zed

10mm Tas oak will steam bend to that radius very easily without laminating. Ian's formula for springback is a good start, but every piece will spring a little differently - wood's like that.

What are you making? How critical is precise bend retention?

Cheers

Graeme

making a cabinet 1100 high with slightly bent sides. I think I got what I need to know considering the very slight bends Im applying.thanks for your input.

Originally Posted by Zed

making a cabinet 1100 high with slightly bent sides. I think I got what I need to know considering the very slight bends Im applying.thanks for your input.

Hi Zed

attached is a sketch of what you've described.
(personally I think I'd use an eliptical curve rather than a circular one, bur that's bye the bye)

given the dimensions, spring back would be about 16mm with 3 plies, 9mm with 4 and 5mm with 5.

HOWEVER, it would be better to treat the springback as academic and build the carcas to match the curve that comes off the former -- rather than trying to get the curves to conform to the carcas.

why thank you Ian. I do believe I will use 5 x 2mm and use an epoxy glue that is nice and hard - it will resist the spring back. but then I dont really expect much spring back with such a gentle curve.

the wife wants a symetrical curve so thats what it will be. My preference would have been opposing ellipses (ie one upside down to the other. maybe I can still talk her into it

are you Ian Green aka AWR jap woodworking author?

Hi Zed
leaving aside the benefits of the very long open time of epoxy, my preference for that lamination would be PVA -- and if I had the gear I'd use a vacume bag given the wide area of a cabinet side.

I don't think rigidity of the epoxy glue is a particular benefit.

also I'd definitely do the curves first and build the cabinet to the shape that came off the former -- rendering any springback irelevant


I wish I were that Ian, my wife has a love of things Japanese

Originally Posted by Zed

why thank you Ian. I do believe I will use 5 x 2mm and use an epoxy glue that is nice and hard - it will resist the spring back. but then I dont really expect much spring back with such a gentle curve.

G'day Zed, 2mm lams are very thin I think you could possibly go 3-4mm even with Tas Oak (KD I'm guessing) with almost negligible spring back IMO.
Have a read of this thread in boat building.
I've done dry celery top without steaming for a 500mm dia curve 4mm lams and the spring back was about 1.5 mm over about 800mm.

Cut twice as many lams as you need though for the second go.

Just re read, are you making a cabinet side of say 300 deep I thought we are talking legs sorry. For a side (deep) I think there may be other issues and what ian said, I don't know.