# Parabola Routing Jig?



## totalrewind (Mar 6, 2012)

What I want to do is to be able to make a large wooden parabola for focusing sound.

I picture starting with a glue-up of rings with a router that cuts a path through them to refine the shape.

If you can read my MS Paint scribbling, something like this (cutaway side view):









Now, if I wanted a circle, that would be relatively simple. All I'd need is a central pivot. But my question is, can anybody suggest a mechanism to do a parabolic curve instead? (My gut tells me there ought to be some kind of linkage that could do this)


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## WalkerR (Feb 8, 2017)

Do you mean a parabola in one plane…or in all dimensions, like a satellite dish? You could do it with an adze and some patience.


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## totalrewind (Mar 6, 2012)

> Do you mean a parabola in one plane…or in all dimensions, like a satellite dish? You could do it with an adze and some patience.
> 
> - Walker


Like a dish. (Although even if I could figure it out in 2d, I could just put the thing on a lazy suzan and cut&turn, cut&turn, etc.)


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## DaveMills (Jan 17, 2020)

Here's a start… 




Getting to 3D will be interesting


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## tvrgeek (Nov 19, 2013)

Just get a Miro Snow Sled. As we learned in the early Microwave era, they were a really good parabola!

No help on industrustables either I see. Bust I can suggest is making a 2D template and sand to fit.


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## Dark_Lightning (Nov 20, 2009)

When I taught physics I made two 8 foot diameter parabolas for one demonstration. The structure was 1/2" plywood cut in the parabolic shape, and then had paper glued to them. That means that the sections of paper between the ribs were flat, not following a circular path. Still had plenty of gain for the purpose of the demonstration. How big do you want to make the reflector? I watched the video, but it was simple enough to lay out points and connect them with a pencil line and cut the ribs out. They were rectangular with 1/2 the parabola cut in each of them.


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## Kazooman (Jan 20, 2013)

Gonna make one of those microphones that can hear what the neighbors are saying? Or the opposite focussing a beam of high intensity sound to pulverize anything in its path? Either way I love the concept. I just have no clue how you would do this with a router sled as you suggested. Any router would have a good sized base to its frame. The bit would have to extend a huge distance to do the cutting while keeping the base from rubbing on the surface. Some sort of industrial overhead CNC machine with no need for a support base night work, but probably not a standard router.

I think the "many parabolic ribs with something in-between" like Steven suggested might be an easier approach. Figure out how to do the layout and cut them on the bandsaw.


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## WalkerR (Feb 8, 2017)

how large is large? Too big for a lathe?


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## waho6o9 (May 6, 2011)

Look Around 7:34






Best of luck


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## JackDuren (Oct 10, 2015)

This jig I found on Woodnet many years ago. You move the jig and not the wood. Change the radius you dont change the jig. I believe you can make this to a 12"- 24" radius….


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## totalrewind (Mar 6, 2012)

> Or the opposite focussing a beam of high intensity sound to pulverize anything in its path?


Close, though hopefully not pulverizing anything.  I was just thinking about doing one of those narrow-beam speakers like they sometimes have in museums, only for a lobby information desk. That way people standing in front of a "welcome" video would be able to hear it, and nothing else… meanwhile everybody else, vice-versa.

After looking into this a little more, now I'm thinking I may be making this too difficult. Why not do the same kind of router jig that people use to hollow out chair seats, only with a (yet-to-be-determined) parabola profile? Do you guys think that might work??


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## therealSteveN (Oct 29, 2016)

> Look Around 7:34
> 
> 
> 
> ...


I'm not sure which came first, looks like the same principal.

Seems if you based it on a Lazy Susan, you could swing 360 to make a dish. Arm stays still, and part rotates maybe? I also saw one years ago, where the swing went side to side from a pair of arms. Doing that you could drop the Lazy Susan.

https://www.woodomain.com/online-store/The-JKB-Universal-Routing-table-Ebook-p158404091


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## WalkerR (Feb 8, 2017)

I googled parabolic speakers and got some very expensive results, a few cheap "cell phone" amplifying devices that looked like little more then bowls. And a few DIY tutorials on how to make your own, using a bowl. You could try to find a wooden bowl that is more parabolic then spherical. Or make one on a lathe.

Honestly I think 3D printing is better suited for this. I have no idea what the cost is, but there are websites you can send a file and they will 3d print for you.

Of course being woodworkers, we understand the fun and challenge of making it yourself. Even if the cost ends up being higher. I still say a solid piece of hardwood, an adze, and some time is all you need. Not that I've ever used an adze myself, but that Woodwright fellow makes it look easy!


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## Tony1212 (Aug 26, 2013)

Do you have a lathe and any experience turning bowls? This seems right up a turner's alley.

If not, I would go with the chair seat jig with parabolic rails.


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## Sylvain (Jul 23, 2011)

Maybe doing it in more phases:
1. doing a convex parabola (no need for a long bit)
2. making it abrasive (paint + sand?);
3. make a concave form approximately parabolic;
4. using the convex abrasive parabola to refine the convex one.

Now, 
- wood might not be the best sound reflector;
- it doesn't need to be perfect for this application (it is not an optical telescope) so an approximation with straight (conical in fact) segments is probably good enough.


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## gwilki (May 14, 2014)

If you have access to a CNC router, it would be and easy carve.

Otherwise, take a look at Izzy's Youtube site. He has another jig to cut parabolas/bowls on a table saw.


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## AlanWS (Aug 28, 2008)

There are plenty of ellipse cutting jigs around. A parabola can be considered an ellipse with the second focus an infinite distance away, so a sufficiently large ellipse is a decent approximation. But if you truly want to focus sound to a specific spot, you were using the parabola as an approximation to an ellipse anyway.

Making a template either by plotting it out and cutting to fit, or by CNC, which is pretty much the same idea mathematically, might be appropriate.

Or you could do what people who grind paraboloidal telescope mirrors do: first make a spherical mirror because it's easier to do with precision, then deepen the center a bit to correct it to a parabola. You can easily cut the spheres using a router on a pendulum, and do the corrections by shortening the pendulum and repositioning the center closer to the work. If you need a better approximation, you can use more than two spheres. In contrast to the situation when grinding a telescope mirror, you would need to smooth the transition a bit when you cut the spheres this way.


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## AlanWS (Aug 28, 2008)

Izzy Swan's "genius router jig" is for routing ellipsoids rather than paraboloids, but as I pointed out above, that would certainly work for your purposes, and might be better. But you'd need a pretty big jig.

Sylvain's procedure of refining a concave curve by sanding it with a convex curved surface is the approach used in grinding telescope mirrors. You can correct each with the other.

Susan Gardner's youtube videos on "Making a 15' radius dish" show how to make the spherical concave curve at a similar scale. She also uses this to make a matching convex curved sanding block. Correcting it by sanding the middle with a smaller radius curved sanding block, and the outer part with a flat block may do it.

The curvature at the center is the tightest, and the radius of the sphere (or sanding block) that fits there is twice the focal length of the parabola.

Yes, I am supposed to be working on something else and procrastinating. I hope this helps, or at least entertains somebody.


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## AndyJ1s (Jan 26, 2019)

Ditto on the ellipse vs parabola. Technically, a parabola is a two dimensional curve, and when that curve is rotated about the parabola's axis of symmetry, the result is a three dimensional surface called a paraboloid.

Similarly, an ellipse is a two dimensional curve. When the ellipse is rotated about its major (long) axis through its foci, the result is a prolate spheroid, which is what you want. Oblate speroids are the result of rotating the ellipse about its minor (short) axis.

The paraboloid will project sound from its focus out to infinity, along its axis.

A prolate spheroid will project sound from one focus to the other, but (in the case of a closed ellipsoid), once the sound converges at the other focus, the sound diverges past it, and is refocused at the first focus.

A sphere is just an ellipsoid with both foci at the same point.

Once I got to tour a spherical dome flight simulator for a fighter jet. The simulator was set up with the cockpit situated such that the pilot's head is at the center of the sphere, and imagery is projected all around the inside of the dome. Without the helmet on, anything I said was reflected back at me coherently from all directions. Thankfully, because my head was there, the reflected sound was mostly absorbed by my head, and did not keep reverberating. Very cool, but annoying!

There are lots of designs for jigs to cut an ellipse with a router (project plans and commercial products). Use one of these to cut the rails on which a router rides, and then pivot the rails around an axis through the two foci to create the "bowl".


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## tvrgeek (Nov 19, 2013)

The Miro sleds are very good parabolas. We made a lot of microwave antennas out of them. I poliched one and found the focus was less than half an inch in diameter.

You can buy a parabolic mic from about $60 to sky's the limit. We also used shotgun mics. If you are trying to transmit sound, then a phased array is more efficient.


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