HI,
I am in the early stages of design for the looong awaited dining table and chairs. The first design idea is a stretcher, joined for most of its length, that slits into two curved pieces to attach to the legs (sort of a curved Y at both ends). I have seen plenty of photos of this approach but not a made piece. I think that I can scale the table legs and this stretcher onto the front legs an strecher of the chairs to make a dining set with a well integrated design. The trick will be to finish with a 90* bend to form the tennons into the table legs.
Ah yes, the other bit is that I have most of an oak tree available that has been slabbed and air dried for 3 years; but have no experience in working with oak. (expect questions re finish etc)
My current assumption is that I will cold laminate the stretchers. Does anyone know the formula for calculating springback. I have seen one that involved the number and thickness of the plies as well as the length and degree of curve. Second, is the timber likely to be suitable for the bend. not absolutely sure of the species, there are apparently about 25 separate species of oak that were planted as street trees in Canberra, but it could be Pin Oak (Quercus palustris).
Replies
http://www.woodweb.com/knowledge_base/Calculating_Springback.html
Rick, I have not ever done my own test of that formula, and have seen it brought up over there a few times. It is nice to have such a simple formula, however it seems just a little too simple. The fact that he only considers the height of the arc, without taking the length of the arc into the equation leads me to wonder how far he took the formula to the test.If you go back and look at the illustration on your link. When I look at the bending form, the gray arc with the X marking the height. Imagine if you will that the length of what is shown were to multiplied say 5 times, and also divided 5 times without changing the height or thickness of plies. Would the y spring-back stay the same for both of those samples?Patto, There is another thing to consider in your design. Regardless of whether you are steaming or laminating your bend. You can not bend all of the way to the end of you part. Eight times the thickness of the parts are needed on the ends to provide the leverage needed to get the bend made. If this is your first attempt at this sort of thing, and if your sense of logic is steering you toward using thicker / fewer plys, because it will look better, as I did when I was learning to bend, I would encourage you to let go of that notion.If the wood is the true pin oak that you have listed, I think you should be OK. However in my part of the world, willow oak is often mistakenly referred to as pin oak. If that is the case for your wood, I would have some questions about that one, I think it is a little too brash for this purpose.Finally, the most important thing to consider is to choose straight grain wood.
It's not most scientific calculation. It's a generalization by someone on FWW. It's certainly not etched in stone so the results are going to vary somewhat. The formula does work and thinner plies and a harder glue decreases springback. Also pre steam bending will help. Air dried stock is best. Another way is to use a heated pipe to prebend the strips. A technique more common to guitar makers. There are oval shaped forms with heating elements in them or the simple way is to mount a pipe in a vice and position a propane torch in it from behind. Guitar makers can pretty much prebend the sides before the finish clamp up to get the final bend.
Not knowing the geometry of your required curve I offer the following:
1. Keep your individual lamination pieces under 1/8" thick (less for tight curves). If you can, plane both sides of the individual pieces for maximum surface adhesion.
2. Spring back on cold press laminations is quite minimal (read: throw away any formulas you may have)when compared to steam bent.
3. If at all possible use a two-part bending form, that is, sandwich the lamination between the form pieces. This means with deadly accuracy draw your male or female form, draw your lamination then your corresponding form piece. A CAD program simplifies this step greatly.
4. If you are using clamps to do the forming use as many as you can possibly fit on the forms.
5. I use yellow wood glue but many people use urethane glues as there is more working time. If using wood glue careful planning is needed to ensure you can glue and press the whole shebang within 15 minutes. Having a buddy or two for help should be considered.
Good luck!!
If you want to eliminate the issue of spring-back, first steam bend each individual laminate and after they have cooled, lay-up the laminates. While it requires two steps, the final lay-up is greatly simplified. Which ever approach you use, I would recommend that do at least one dry run so you know what how much pot time you need for your glue or epoxy.
Winwood1 addressed most of the points I would have offered, so I'm just going to elaborate on his:
Most of this comes from stuff I learned doing this project
http://forums.taunton.com/tp-knots/messages?msg=20211.4
1. Keep your individual lamination pieces under 1/8" thick (less for tight curves).
I think if your radius were big enough, you could go thicker, but in general I agree with Winwood. Getting the thickness consistent from ply to ply is probably more key than actually getting them at a specific target thickness. When you're done thicknessing, dry clamp your stack together tightly and measure your total thickness for item 3 below.
2. Spring back on cold press laminations is quite minimal
I would guess that the max springback I got on that project was maybe 1/64" - small enough that I could correct for it by just cutting the tenon a hair off center on the end.
3. If at all possible use a two-part bending form, that is, sandwich the lamination between the form pieces. This means with deadly accuracy draw your male or female form, draw your lamination then your corresponding form piece.
Yup - you need to account for the exact thickness of your lam stack between the male and female halves. I faced my MDF forms with formica (you can also just use clear packing tape) to stop any glue squeeze-out from sticking to the forms. I goofed up one of the forms and didn't account for the formica thickness and had to scrap the first laminate and redo the form to correct it.
4. If you are using clamps to do the forming use as many as you can possibly fit on the forms.
It will help if you have opposing flat areas on the backs of the forms for straight-across clamping. It will also help if you have some runners screwed to the bottom of the forms (cover the glue exposed area with clear packing tape) so that the male/female interlock and cant slip side-to-side as you're pulling them together.
5. I didn't use yellow glue, but urea glue from Nelson Paint for more open time. But I'd still suggest having a buddy to help you spread glue, get it into the forms, and close up the clamps.
Like some others have mentioned, pre-bending is a big plus too. I used a low-tech method of soaking the lam stack in hot water for about 30 minutes - drying everything off - then clamping up in the forms without glue for about a week to let everything dry off. When I pulled them out before doing the glue up, a lot of the curve was already in the pieces. I think it made the glue up go a lot smoother. The glue will make things slippery until it starts setting up. Wrestling the stack into the forms and clamping up is a bit of a workout even with the pieces already curved much less bending from flat. Pre-bending with steam or a hot pipe is probably even better.
Waddaya mean it wont fit through the door?
Use the correct glue and springback will be zero. Use a glue like UniBond 800 which has a rigid glue line and no calculations will be required. Use a white or yellow PVA glue which is flexible and your guess will be as good as anybody elses.
Steve
I have to agree with previous posters--do not use PVA glue. It creeps under load, allowing springback. Thin laminations glued up with a urea formaldihyde glue like Unibond 800 will give a lamination that will fit the form several days later. The dried squeeze-out of this glue is really sharp and will do nasty cuts if you're not careful.Jim Eddy
[URL=http://www.jameseddywoodworks.com]James Eddy Woodworks[/URL]
If you laminate the bent Y you will get almost no springback
Once the glue dries it will be solid as all get out.
Wicked Decent Woodworks
(oldest woodworking shop in NH)
Rochester NH
" If the women dont find you handsome, they should at least find you handy........yessa!"
Just me so...
Laminated curves.. OAK is a BUMMER! ASH do what you want and stays there!
Thanks everyone, I have done a little marginally successful steam bending before but these pieces are intended to be bigger. so I think laminations are probably the go. The idea for the legs and stretcher came from a photo of a reproduction piece in FWW last year (Louisiana purchase table); I just didn't need anything that big. 2.4m would be nice and will just fit. 2.2m and cosy seating might have to be the result. I'm still trying to sort out sizes.
The driver for oak is opportunity. My favorite wood supplier has a complete trunk that was slabbed 3 years ago and has been air dried in a shed since (its down the back so is going to be a B* to get out). This is more than long enough to do a 2.4m table with 200*40mm planks, all with matched grain. I am going to have to laminate the legs, but square section pieces that were taken from either side of the pith are more than sufficient for decent chair components such as turned front legs and solid curved side rails.
This thread has given me enough to tackle the bending.
My last challenge is to design matching chairs. Current thought is to scale the turned table legs and stretcher. back legs probably with a curve in one dimension only, but splayed to create a wider top compared with the seat level. Might need a bit of a taper to emphasise this.
So what do I do for a crest rail etc that complements the table? All I know at this stage is that it will have an upholstered seat and timber back. I would like two carvers by adding arms to the base design.
By the way, just so you know, if you are going to laminate these stertchers you better have a boat load of clamps.Wicked Decent Woodworks
(oldest woodworking shop in NH)
Rochester NH
" If the women dont find you handsome, they should at least find you handy........yessa!"
And you can never have too many boats :-)
you better have a boat load of clamps.OR 'wrap' the sticks in that packin' tape with them 'strings' in it..
Bend it around your form and away you go!
Calculating springback
The traditional calculation for anticipating springback is:
Y = X/N^2
where
Y = the springback (i.e. decrease in arc height)
X = arc height of mold
N = number of layers in the lamination
As many woodworkers have discovered, this doesn't tell you how to get to a desired final arc height, only to what degree your lamination will fall short. That is, the desired arc height (let's call it D) equals the arc height of the mold (or glue form) minus the springback.
or
D = X - Y
but we already know Y = X/N^2, so
D = X - X/N^2 or
D = X(1-1/N^2)
Now just solve for X, and you will know what to make the arc height of glue form to achieve desired arc height D
X = D/(1-1/N^2)
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