How to cut a large wooden parabola - Woodworking Talk - Woodworkers Forum
 
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post #1 of 8 Old 06-06-2013, 08:10 PM Thread Starter
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Talking How to cut a large wooden parabola

Hi all! This is our first post here!

We are building a solar hot water heater using a sheet of galvanized steel covered in Mylar and shaped as a parabola with 5 Birch ribs to concentrate the sun on a black painted copper tube.

We are having a challenge shaping these Birch ribs. They are about 48" x 12". Our little skill saw just isn't getting the precision we need - well to be honest, maybe our not-so-steady hand has something to do with it...

We have been looking into some sort of CNC setup, or maybe pre-cutting the pieces and shipping them to some place in the U.S. to be finished, or ???

Our questions are - is there another way to shape these ribs?

Would a 2 axis CNC do the job?

Our woodworking skills are pretty limited so we get to learn as we go.

Any help would be appreciated. Have a great afternoon! :)

"The earth is one country and mankind its citizens."
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"La tierra es un sólo país y la humanidad sus ciudadanos."
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post #2 of 8 Old 06-06-2013, 08:20 PM
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First Welcome aboard, first check an make sure you have a good blabe in the saw if not go get one from the box store an practice on a piece of scrape. That should work for ya if not some others will pop in that have CNCs
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post #3 of 8 Old 06-06-2013, 10:48 PM
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Lay out the parabola on some 1/4" hardboard. Cut with the skill saw close to the line. Use rasps, files, and sandpaper to get it perfect.

Use the hardboard piece as a pattern and mark out your ribs. Cut the ribs close to the line, then clamp the pattern back on each rib and use a router with a guide bearing to trim the rib to the pattern.

Keep the pattern to use in the future.

For just a little more, you can do it yourself.
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post #4 of 8 Old 06-06-2013, 11:24 PM Thread Starter
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Quote:
Originally Posted by ed_h View Post
Lay out the parabola on some 1/4" hardboard. Cut with the skill saw close to the line. Use rasps, files, and sandpaper to get it perfect.

Use the hardboard piece as a pattern and mark out your ribs. Cut the ribs close to the line, then clamp the pattern back on each rib and use a router with a guide bearing to trim the rib to the pattern.

Keep the pattern to use in the future.
UUUU, i like this one. Fairly simple. i don't know what a "guide bearing" is but i suppose the guys at Lowe's will. i'll only need to invest in a router.

Thanks, ed_h! Have a great evening! :)

P.S. Is there a way to do this with a 2 axis CNC setup?

"The earth is one country and mankind its citizens."
Bahá'u'lláh
"La tierra es un sólo país y la humanidad sus ciudadanos."
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post #5 of 8 Old 06-06-2013, 11:53 PM
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My first thought was to cut them with a band saw if you can get the use of one. CNC sounds like over-kill.

Ed's idea to use a pattern with a router to get things precise is a great idea, too. Here's a good video that shows how it works:

Bill
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post #6 of 8 Old 06-07-2013, 03:12 AM Thread Starter
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dodgeboy77, thanks a lot! That looks really easy. Now all we need is a router and to make our template exact enough. But at least it looks like something we'll be able to do ourselves.

Again, thanks a million and have a wonderful evening! :)

"The earth is one country and mankind its citizens."
Bahá'u'lláh
"La tierra es un sólo país y la humanidad sus ciudadanos."
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post #7 of 8 Old 06-07-2013, 08:16 AM
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I have a Small CNC, and access to a large one at work. Your ribs would be a trivial challenge for any CNC router. A minimum of a 3-axis CNC would be able to cut your 2D project. You have X and Y coordinates to plot the outline, but you need the Z axis (up and down) to step down into the wood. Several ribs could be cut in one session from the same sheet of birch plywood. Every rib would be cut perfect and the same (assuming they were drawn perfect), or each one could be different if they needed unique holes or slots or pockets. You would need to provide the pattern(s) as a digital file, and most CNC shops can use DWG, PDF, DXF, AI, and a few other vector (line) formats. Depending on where you live there may be a local shop or two that could CNC cut the parts for you.
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post #8 of 8 Old 06-07-2013, 11:23 AM Thread Starter
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Dear 4DThinker - wow! This place is full of great help and information! Uf!

i would think that if we could at least get our template made through the CNC process - it seems to just be more exact as you mentioned - maybe that would be the way to go. At this point we've got a PDF and some sort of Linux file.

Mexicali is on the U.S. - Mexican border right across from Calexico, California, 92231. However Calexico is pretty small so we'd probably have to send/take it to San Diego, which wouldn't be a problem - it's only about 1.5 hours west of us.

The info about the difference bet. 2 and 3 axis was very helpful - thanks again and have a great morning! :)

"The earth is one country and mankind its citizens."
Bahá'u'lláh
"La tierra es un sólo país y la humanidad sus ciudadanos."
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