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Difficult Loft for CNC Plasma cut


Alan Pike

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BellHorn (2).png

Sorry as you can see me be newbie.. I would like to create something along the lines of the pictured antique gramaphone horn but for use as a freestanding fireplace hood. I've tried to loft from two hexagons and am having trouble with creating the concave shape for the loft.

Also thinking ahead,, how can I flatten the concave shape in order to cut out with my cnc plasma cutter?? Or is this the right approach from the beginning??

Thanks

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It may make more sense to model one side, then Polar Array around the center. Post some general sizes - lets see what the forum members come up with.

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OK, once a side is created producing an array to look at the complete piece would be nice but not necessary.. So then to model just one surface would be accomplished...??? I've envisioned this hood as being based on lofting from a hexagon or maybe pentagon.. If it is to be cut from 18 guage plate i will need a structure around which to form and weld or braze the sections together.. So, if the loft is created from base hexagon with an 800mm side and the top of 200mm on a side about 700mm up the z axis. Two or three internal hexagons could be sized to create a Christmas tree structural form for fabrication..

As I mentioned, the flattening of the concave shape will be tricky (read I don't know how).. I've tried my Geomagic 2014 and it's 'convert to sheet metal' feature but it won't let me bend on the edges I need.. It strikes me that programatically, it would be difficult to take a concave 3d object and flatten it while accomodating dimensions that reflect the additional material that would have to be shown in a 2d rendition.. if that makes sense..

Somebody knew how back in the day.. As pictured above.. but maybe that's fabric stretched on a wire frame..

I'll try to upload a shot of the wire-frame model I've been working with..

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Where did the upload button go?? Doesn't seem to be these insert buttons..??

Put another way.. How do I attach the .plt file i've saved??

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Welcome to CADTutor Alan. :)

 

I must say, that this is definitely NOT your typical newbie 3D challenge.

I was able to model, a rough concept solid, based on your hexagon specs, of 8oomm along an edge at the bottom, and 200mm along an edge at the top with a height of 700mm.

I realize that I failed to create the sort of floral curve at the lower edge, but this could be cut with a Subtract command executed in TOP view.

 

It looks to me like the approximate dimensions you posted are in need of tweaking, and/or, I misunderstood what you meant.

The 3D Solid shown in the screenshots could be cut from a flat sheet and then rolled to the appropriate diameter.

I modeled this as 1.2mms in thickness, which was what I found online to be the metric equivalent for 18 gauge.

 

I only deal with 3D Solids in my work, and there are a wealth of wonderful Surface commands and functions about which I know zilch, there are people on the forum who are versed in working with Surfaces, and better at modeling than myself, and I will be curious to see what alternate approaches may be suggested.

definitely not your typical beginner modeling challenge.jpg

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Wow, Thanks for bending your thoughts toward this puzzle.. You are right, the floral curve at the bottom is unnecessary.

A couple of things: In the other dwf file attached, the drawing specs are quite different from 800x200x700 and are purposefully exaggerated for illustration. Your solution is based on a curve around a single point which would work, but.. Also, What is the x-ray view showing? I'll try to replicate and see what I can learn... Did my stab at explaining the evident need for the laid flat (to cut with cnc) profile or pattern would have to be considerably longer than a front view? I didn't realize how demanding the solution will be but am now really intrigued.. Maybe I'll have more luck with a sheet-metal cad..?? Thanks again. It's early morning here in Kenya and I'll not be able to get back for most of the day ahead..

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Wow, Thanks for bending your thoughts toward this puzzle.. You are right, the floral curve at the bottom is unnecessary.

A couple of things: In the other dwf file attached, the drawing specs are quite different from 800x200x700 and are purposefully exaggerated for illustration. Your solution is based on a curve around a single point which would work, but.. Also, What is the x-ray view showing? I'll try to replicate and see what I can learn... Did my stab at explaining the evident need for the laid flat (to cut with cnc) profile or pattern would have to be considerably longer than a front view? I didn't realize how demanding the solution will be but am now really intrigued.. Maybe I'll have more luck with a sheet-metal cad..?? Thanks again. It's early morning here in Kenya and I'll not be able to get back for most of the day ahead..

 

I haven't actually looked at the drawing you posted, I was just intrigued by the challenge of modeling it, so took a shot.

I didn't mean to imply that the curves at the bottom edges were not needed, just was focused on how to model the body.

The curves could be added, or, as I mentioned earlier could be laid out and subtracted from the 3D Model in the Top perspective.

 

Inventor may well be the software of choice for this, not sure, I don't use it, but I know that it incorporates some sheet metal type functions available.

I thought I was going to be busy today, but still just sitting here killing time, waiting to get busy.

I used the Sweep command instead of Loft, but I still have that drawing open, have just finished lunch, and I plan to play with it some more, barring the arrival of paid work.

If I manage to get it to where I think it is worth it, I will post some images.

The xray view was not really showing anything extra, I just like using it sometimes, though in this case, has no real meaning.

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Hello Alan,

Well, that will teach me to run out of work!

I redid it, added a bit of the scalloping at the perimeter, though it would look better with a deeper scallop, and have included the file, not because I think that you will want to use it, but so that it might help you to see what I did to model this without the need for me to describe every step of the process.

I'd love to see how JD, or one of the other knowledgable Inventor users would do this, with Inventor. Or for that matter, a better approach in Autocad, as there probably is one, as I stated earlier, it may involve modeling it as a Surface instead of a 3D Solid.

 

Possibly by opening such a CAD file, the 3D Solid, as shown in the next to the last step of the progression, could be imported to Inventor, and treated with the sheet metal functionality, not sure.

It may not HELP you, but it might just teach you a few things about one of the many different ways this might be modeled in Autocad. :)

gramaphone horn from Hades.jpg

gramaphone.dwg

Edited by Dadgad
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Back to my school tech drawing days a pencil and a sheet of paper, if you can make the curved edges a pline by exploding etc Dadgad dwg ? Then divide into small steps this will approximate the curve length, drawing a series of lines horizontally (in 3d) at these steps joining both sides, these are the lengths in a plan view for a flat pattern join the edges and use spline to smooth. Its approximate but will be close if you use small steps. Hope that makes sense.

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Good points BIGAL. :beer:

 

The length of the polyline which I drew, as an approximation of how it might look, is 962 mms long.

But the shape I drew based on the numbers Alan provided in his earlier post, looks appreciably different than the photograph, and quite different from the .dwf image, which after figuring out what it was, I opened. That one is about twice as tall as it is wide.

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Rather than lofting between the hexagon cross sections I would loft between two adjacent seams of the horn pedals. The shape show below was created by lofting between two seams and then using thicken. The scallop bottom was created by performing a Boolean Intersect with a solid the had a rounded bottom.

 

To create the flat shape development for the pedal I would measure the width at various lengths along the curve and use those values to create the flat shape.

horn1.JPG

 

Note, I eyeballed the measurements for this model.

~Lee

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I did have a go at a solid and exploding etc then used divide, the points do turn up on the curved edge so you could make an approximate flat version. As it will be welded a bit of tolerance should be ok. ScJ nice answer but how much. As it is a 1 off a bit of time spent would not go to waste. I would make a cardboard version as a test.

 

 

Come to think about it you don't need a solid at all just an edge, use 3drotate to stand up, copy and rotate in the Z plane so you have 2 edges join the horizontals move to flat plan z=0 and add a nice arc. The curved/flat length would be all the mid points of the lines joined up.

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I took another shot at modeling it, because I wanted the scallops to be more like in the picture. :beer:

 

I used a spline to generate the curve of the plate this time, instead of an arc, so a better approach, but still wondering what the real dimensions are meant to be.

This is still loosely based on the dimensions Alan gave in his earlier post.

gramaphone horn from Hades.jpg

gramaphone.dwg

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MMmm looks like I just lost a fairly long reply because didn't post before noticing the link in scj's reply and clicked myself into seemingly outer space..

Jochen, I just had a look at your site.. So that is where the, not a hatch not a loft fill comes from.. It looks like you are somewhat understating the task when you mentioned that Bigal's suggestion, while correct, would best be done automatically..

So it begs the question.. How do I do dat?? So to speak..

It looks like I have posed a question and CAD issue far more complex than I had imagined. Apparently you specialize in unfolding complex 3d objects. I'd really like to learn more about you. Is this a service that you offer?? Is it an add-on to autocad?? If i am correct about this and what I need to do is unique (or at least in the vocational metal fabrication world), I am very keen to learn more.. Let me send this before i muddle on somewhere else..

Also, thanks Bigal i do indent to follow your contribution too. And IRM too.. Thanks!!

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Hi Alan,

I'm not offering service. I'm coming from Mathematics and don't know much about material properties (k-factors etc.)

So I can only offer single geometrical sample service.

Thank you for your interest.

Regards

Jochen

PS.: You've got PM.

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I had some time this morning so I thought I would see what I could do with the unfolding of the shape.

 

Here's my process.

 

1. I started with a spline on the XZ plane. I then created a series of horizontal lines at random intervals placing the lines closer together where the curvature was greatest.

f01.JPG

 

2. I next rotated a copy of the spline by 60° about the z axis and each of the horizontal line by 120° about a vertical axis with a base at the beginning of the line. The green line is also at 60° and is used in the next step to trim the red lines.

f02.JPG

 

3. A 3dpoly (cyan) is constructed from the midpoints of the red lines.

f03.JPG

 

4. A UCS is next defined by the yellow line (a line at 30°) and a point on the Z axis. This is the plane perpendicular to the unfolding (flattening) of the shape.

f04.JPG

 

5. Successive use of the 2D Rotate command is then used to unroll the geometry. The Reference feature of Rotate is used to specify the angle and the base point is the next vertex of the exploded 3DPoly. Here you can see the intermediate shape after one rotation.

f05.JPG

 

6. And here after 4 rotations.

f06.JPG

 

7. Here's the final flatten shape (green lines).

f07.JPG

 

This process does not take into consideration the shrinking or stretching of the shape due to bend allowance but that could be included by stretching the shape.

 

This process could be automated and may be already available but if you are only doing it once you might consider this approach.

 

~Lee

 

Note, the Rotate Reference command is not the most intuitive command but with a little practice it becomes easier to use.

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Nice approach Lee. :beer:

Isn't it amazing how tedious some things are, even difficult things, in Autocad?

 

I should think JD might rise to take the bait, and show us how easy this would be with a modern modeling program? :huh:

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Hmmm food for thougt back to my single egde rotated in 3d, if you make a UCS that represents the X axis from two of the edges you can work out to the centerline of the panel this is a fixed X value, a new 3d pt and also a corresponding horizontal line length. A bit more clunking and you can produce a flat 1/2 pattern, you would add a 1/2 arc to bottom edge but this arc is not critical, the cardboard test would reveal the shape required.

 

I worked it out get a gramphone and a bit of cardboard and trace it :shock:

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