Maximum Laminating Radius?

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8
Hello Everyone,
I am planning to build a large square parabolic sun reflector. After a lot of thinking and researching an economical way to accomplish this I've decided on using strip laminate bending to create the parabolic ribs of a grid to mount an array of square mirrors on. I have never done this type of thing before and my woodworking experience consists of building tree houses and skateboard ramps as a kid.

Based on what I understand, my plan so far is this is this: Lay up a bunch of long, thin wood strips and adhesive in a large parabola-shaped form, clamp them there, let them dry, and reapeat many many times.

I know I can handle building the form. My main concern is the wood springing back at all after I take it out of the form. In all the pictures I've seen of laminates this curve radii are much smaller that what I am going to attempt. ( My parabola will be around 8 feet wide and 1.5 feet deep) It needs to be as perfect a parabola as possible or the efficiency will go way down.

Do you guys have any tips on how to reduce or eleminate springback on such a shallow curve?

Thanks a lot.
 
As far as I understand it, there is no need to remove the profile ribs once made the parabola.

You are interested in the interior par not the exterior, aren't you? In this way you could have some sort of torsion box with a parabola on one of its sides.

Besides, you would get plenty of flat surfaces ( four sides + bottom) to fix whatever support or base needed.
 
Thats true, but the problem with that is it would take a lot of material and would be extremely heavy and hard to erect on a sun-tracking mount. I just want to make a whole buch of identical ribs that I can bolt together parrallel and perpendicular to eachother and have a nice lightweight frame. (wireframe-like)
 
Yeah that's a possibility but I don't know if it would be strong enough to hold sixty four square feet of glass mirrors. Especially in the wind. Ideally I would make the whole thing out of carbon fiber but that is way beyond my resources and ability.
 
how `bout an old satellite dish? bending wood to an exact radius isn`t usually something a novice jumps on (at least with success).....
if you`re dead set on trying holler back with a few more specifics besides depth/diameter and being able to support lotsa weight.
as a general rule metal is more cost effective than wood in stressed applications.
welcome to family wood workin`!
 
Do the ribs have to be an exact parabola curve or do they just all have to be exactly the same? Can you laminate up something long enough to cut all the ribs from (so they're at least all the same with or without springback) rather than try to get identical curves from glue-up to glue-up??
 
I figured metal would be more cost effective but wouldnt that me even more difficult to bend into a precise shape without expensive equipment? If you know a good way to get a curved metal shape I would love to hear it. Yeah an old c-band style dish would also work but I want something bigger than the common sizes. I also think it would look pretty cool to have mirrors on wood with a nice finish on it.

As for specifics of my design, basically all I need is to make sections of a parabola having the equation y=(x^2)/16 in cartesian coordinates on the interval (-4',4'). I think a 1"x1" cross section on the sections would be good.

All together I need 16 ribs.

thanks for the help by the way.
 
Do the ribs have to be an exact parabola curve or do they just all have to be exactly the same? Can you laminate up something long enough to cut all the ribs from (so they're at least all the same with or without springback) rather than try to get identical curves from glue-up to glue-up??


Yes it needs to be an exact parabola due to the fact that nothing but a parabola will focus all light from a distant source to one single point. They also all need to be exactly the same.

Is there anything I can do to totaly get rid of springback? So the layup will come out in exaclty the same shape as the form?
 
I know there are some formula's floating about the net for calculating the amount of spring back so you could compensate to get the final curve. Googling something like "bent lamination springback formula" would probably locate it with a bit of reading. But I've done some presoaking in hot water then clamped up (w/o glue) in the forms for a week, then glued, and gotten nowhere remotely close to the amount of springback that formula told me to expect. The thinner the plies, the better for minimizing SB too. But I think if you expect to get exact mathematical curves, wood aint the right material. Assuming you can even get it glued up exact, it's wood and even a lam is going to want to do what wood does - move some with changes in humidity. How were you planning on making the gluing form(s)? It seems like you'd almost have to have it cut by a CNC machine to get the kind of tolerances it sounds like you're after. If you got one wide lam done and calculated the SB and ripped the ribs from that, couldn't you recalc the curve equation and adjust the focal point to match your new curve? I think I'd be more concerned with trying to get the exact same SB results for doing each rib lam as a separate glue-up.
 
I think I would approach its construction a bit differently. Rather than trying to construct a rigid parabolic dish, I would go for a rigid, lightweight frame structure (with 16 straight ribs) on which you could mount the flat mirrors at the geometrically correct locations, heights and angles. It would be either a 4 or 8 sided outer frame of plywood, with the ribs running radially to the center. The bottom of the frame and the ribs could be stiffened with plywood flanges. The ribs would be straight (equal width) or triangular rather than parabolic.

The mirrors would be attached using risers that are each cut to the geometrically correct length and angle to form the parabolic form. With a regular spacing, working out the length and angle isn't very difficult.

I haven't built anything quite like this, or run any numbers on it, but I would guess that 1/2" ply (marine for the outdoor environment) would be stiff enough. You may find a need to tie the ribs together at the center and mid-spans with stringers.

Dick
 
I have some experience grinding telescope mirrors to a parabolic shape. My first reaction is there's no way you're going to build a structure out of wood, especially with small flat mirrors, and get a true parabolic shape.

What you're trying to do, I believe, is to focus light from a far distant source (the sun) to a single point for heating some object. As I understand your setup, you will move the entire array to track the sun.

You can do this without a parabolic arrangement. I'd have to think about it more, but I think you could mount the mirrors on a flat surface and focus them on the object to be heated. You would then move that whole flat surface so as to keep the light shining on the heated object. The heated area (the area of focus) would be about the size of your individual mirrors, if you do a great job of focusing all of them to a single location.

One way to control the movement of the array would be to monitor the temperature of the heated object and search with the motors to find the highest temperature, which would be your best position. If the object to be heated is reasonably large, you would not have to search continuously because it would take time for the sun to "move" enough to move the focus off the heated object. So maybe you could search every minute or so. Remember that it takes time for the temperature to change so you have to take that into account when searching for the best alighment.

If you decide to try this approach, one problem will be aligning each of the mirrors. You can use the sun for this alignment by aligning one mirror as the master mirror (of course, align the whole structure reasonably well before you start). To align the next mirror, align the whole stucture so that the master mirror is shining on the focus, and align a second mirror. Then cover the second mirror and repeat for the third mirror. Keep doing this until you get all the mirrors adjusted. You'll need help and it'll be a pain but it can be done.

Good luck! Let us know what you decide.

Mike
 
You are both right about it being easier to mount mirrors at angles on a flat surface rather than build a parabolic surface. I built on like that a few years ago that worked great. It had 115 six inch square mirrors on a flat piece of plywood. I used it to make steam. It had a few problems though: It was heavy for how small it was, I couldnt fit as many mirrors as I would like on it, it was ugly, and it was boring. This new one that I want to build would solve all those problems.

Maybe I could bend square tubing over a wooden form and then heat it to relax the stresses?
 
I agree with Tod, and on top of that I thing that having to hold so many mirrors calls for metal structure, besides having to withstand wind and exposure makes it more difficult.

Why not reduce weight by using mylar mirrors? They may not be as efficient as glass ones but definitely lighter and safer to manipulate.

Astronaut blankets, as we call them here are a good source and cheap enough:dunno:
 
Brandon,
First and foremost. welcome to the boards!

Yes it needs to be an exact parabola due to the fact that nothing but a parabola will focus all light from a distant source to one single point. They also all need to be exactly the same.

Is there anything I can do to totaly get rid of springback? So the layup will come out in exaclty the same shape as the form?

One thing about working with wood that everyone has to learn and deal with is that Wood moves. Moisture content changes the wood constantly. There is no magic solution, unfortunately. Plywood might help in that it is relatively more stable than just regular boards due to the alternating grain of the layers, buy given the complex shape you're working with I'm not sure it would help you.

Sounds like aluminum or a steel frame may be the way to go on this one.
 
Thanks everyone for steering me in the right direction. You have convinced me that wood is probably not the best material for a project like this due to its unpredictable and changing nature. I've made my fair share of solar reflectors and concentrators and I want this one to be the best. I want it to produce at least 3 kilowatts of heating power so it needs to be precise. Looks like a metal structure is my best option. Anybody know any tricks for acurately puting shallow bends in square aluminum tubing? Probably not the right forum!
 
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