Radial Horn Plan and Secion Combine to Model - diyAudio
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Old 14th July 2015, 03:17 AM   #1
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Default Radial Horn Plan and Section Combine to Model

These are drawings from a horn made years ago by a French builder and posted on his site for DIYers to use. When I look at them though I don't understand how they can be used to create a 3D model. If I print them both to scale and overlay them at the throat, the mouth end of the center line of the Section view does not stay in contact with the arc of the Plan view mouth as you rotate it out to the side corner. It seems to me there is another radius for the Plan view arc that isn't given. So my question is, can the horn shown actually be correctly made using only the two drawings ?

Thanks
Attached Images
File Type: jpg Artec Plan.jpg (80.2 KB, 200 views)
File Type: jpg Artec Section.jpg (81.8 KB, 193 views)
File Type: jpg AR4320-1.jpg (36.3 KB, 195 views)
File Type: jpg AR4320.jpg (37.4 KB, 194 views)

Last edited by Hearinspace; 14th July 2015 at 05:00 PM.
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Old 14th July 2015, 09:37 AM   #2
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If that's a drawing of the horn contour then no, that's not enough....which is how it looks. You would need a "flattened" drawing of the curved walls to make it.
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Old 14th July 2015, 12:12 PM   #3
xrk971 is offline xrk971  United States
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The top and bottom are curved compound surfaces. Flattening won't work - you need to use the profile as a guide to cut a surface of revolution in the azimuthal direction. The how sides are flats so that is not problem. For a curved tractix though like an Edgar horn - the flattening works because it is curved only in 1 direction. For that case I would model it in 3d and then "flatten" it using sheet metal tools in CAD.

To make this one will require making a jig and router or rasp on wood or soft material like plaster or plastic. Or a large 3d printer can make it.
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Old 14th July 2015, 04:40 PM   #4
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I'm not sure I understand your answers in the way you intend them so . . .
My question is that even if I make templates the problem is that while the two drawings line up at their center axis, as the section view is rotated out to the mouth corner the two arcs diverge, leaving the section view too short. Looking at the photo of the actual horn, it's difficult to tell if the knee of the curve is changing as it progresses out to the side wall or if it remains constant.
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File Type: jpg DSCF0620.jpg (356.5 KB, 146 views)
File Type: jpg DSCF0619.jpg (404.0 KB, 33 views)
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Old 14th July 2015, 10:24 PM   #5
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Hearinspace, that's because what you have printed is the contour at the zero axis. The mouth of the horn is a radius, so your profile will be different where it meets the side wall, unless they've designed it to be constant and it appears they have not. I've attached a very poor drawing of what I was talking about wrt flattening.

xrk, at least with SolidWorks it looks like you're right. I can only make straight lines and radii into sheet metal features to be flattened.
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File Type: png neo 8 profile and flat wall copy.png (55.9 KB, 44 views)
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Old 15th July 2015, 12:19 AM   #6
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Thanks ! That's what I'm asking.

Presumably, the drawings were originally posted in order that people could make their own, yet it's not clear to me how they can. If using CAD I thought the order would be to spin the profile around its center axis to get a trumpet-like horn. That would give the throat. Then the question is whether to pivot it at the throat as I did in post #4 or whether it moves in some other way - perhaps laterally following the plan view arc at the mouth, I don't know.
I see a knee in the profile happening roughly mid-way between the 55mm and 82mm marks. If that point stays a constant distance from the throat center then how is the part of the profile below that determined to keep the end of the curve on the mouth arc? (This is cumbersome speech - I hope you follow my thinking. I'm new to horn terminology.)
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Old 15th July 2015, 12:45 AM   #7
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From what I know about radial horns, the cross section is constant at any azimuthal angle - that is sweeping it 360 deg will produce two discs of the correct profile then cut the discs to get the side profile as a straight projection up and down but following the horizontal expansion profile. Really, a flat would probably work well enough. Most of the action of a radial horn is the profile defined by the top and bottom. So in summary, a surface of revolution defined by the centerline axis should work. It even looks that way to me in the photo.
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Old 15th July 2015, 01:34 AM   #8
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Well, that's originally what I thought and in a way what I was hoping you would confirm. But it still bugs me that when done that way the horizontal arc in the drawing does not correspond. If the designer followed his own design then the two have to work together somehow.
I wondered if it he could have taken the profile and moved it laterally - the profile tip following the horizontal arc with the profile plane still parallel to the center of the horizontal expansion (zero axis) like in the sequence below. (Open the first one then use arrow key to toggle through the sequence.)
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File Type: jpg DSCF0623.jpg (459.1 KB, 36 views)
File Type: jpg DSCF0624.jpg (436.2 KB, 17 views)
File Type: jpg DSCF0625.jpg (471.3 KB, 19 views)
File Type: jpg DSCF0626.jpg (313.7 KB, 17 views)
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Old 15th July 2015, 03:51 AM   #9
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By horizontal arc you mean the line defining the side walls? They are simply a vertical projection through the two discs that form the top and bottom and slice the discs with that profile. I guess I don't see the inconsistency. Make a 3d cad model and then do vertical projection of that side wall profile. It should work.
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Old 15th July 2015, 05:29 AM   #10
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No, I mean the arc across the mouth in the "Plan" view of the first pic. in post #1 and the one lying flat on the table in the pics in post #8.

I was a house builder for a time so I started off using that jargon, calling the diagram that is standing in the post #8 pics the Section view - what I believe you call the vertical profile.

If we line up the entry of the throat of both diagrams (right hand end) as in the first pic above and pin them together with a common axis and then drag the leading (left hand) edge of the vertical profile toward us , it will not stay in contact with the arc lying flat on the table. It can only line up with the that arc if the right hand end of the vertical profile is released from the mouth. In other words, the vertical profile is too short to reach the mouth edge at the sides of the horn. (Ahah! That's how to say it!) ) So there is a problem in my understanding of the radius common to both vertical and horizontal profiles.
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