Casting Experiment: Elliptical Horn Flare - diyAudio
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Old 30th January 2010, 02:44 PM   #1
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Default Casting Experiment: Elliptical Horn Flare

split off from Large 3-ways design / build



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Originally Posted by MJL21193 View Post


Moving to a compression driver is something I've been meaning to try and this is a good opportunity.
The horn flare will not be a problem to construct - I won't need a CNC (there were and are people that can fabricate accurately without them) and I'll try to go through the process with a few pics to show what I'm doing. I'm a bit pressed for time right now with a few different things happening but I may find time on the weekend to get started on the flare..
I don't want to cross the HF driver too low - 2-2.5k is where I want to be.
I'll take a side track into the wild world of horn flare design and construction.
The concept:

118.PNG

Top view:
119.PNG

Side view:
120.PNG

My original idea was to cut this shape right into the front baffle, as I did here:
Picture 801.jpg

That is labour intensive and pretty much one-off, my hard work goes into the baffle and that's it. I wanted to play with mold building and casting so where to find a better place to start than with a horn flare
Basic idea is a elliptical flare, conical in shape with 90 degrees horizontal dispersion, 40 degrees vertical.
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Old 30th January 2010, 02:56 PM   #2
Magura is offline Magura  Denmark
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Make a positive mold, throw it in an oven at like 250C, with a piece of 3mm PET on top. Wait a few minutes, take it out of the oven, trim.

Presto! and you got a real good looking horn flare.


Magura


EDIT: I think it's about time, to direct your attention towards your stats.....now we are 13 whom will laugh when it won't work
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Old 30th January 2010, 03:06 PM   #3
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Quote:
Originally Posted by PKNJ View Post
I have to say I'm impressed with the thought put into the cabinet design. And you even thought out the construction in detail as well. I convinced it will work the way you think it will. Most of the naysayers seem to have fallen away. I voted "It'll Work" early.
Thanks

More of the design concept:

121.PNG

A section view through the centre axis vertical and horizontal showing the details of the throat and mouth.
To construct a mold for this shape is tricky (obviously) but not outside the realm of possibility. Suggested was a CNC machine to cut the profile but I don't have access to one of those. I'm relatively old school in any case and can usually come up with a way of doing things differently.so...

It starts with cardboard. From cardboard I lay out and cut an ellipse of the mouth. This will be where the sides of the flare meets the front baffle, without a roundover. I then make a cone, ordinary straight round cone from paper. Trial and error gets me to the exact dimensions after a few tries and I wind up with this:

NPX_102.JPG

That is the ellipse cut out from cardboard (Frosted Flakes cereal box side ) forcing the cone into the correct shape. The throat opening of the cone is 1".
Using a careful, steady hand I trace around the cone where it meets the cardboard, cut on the line on the paper to get this:

NPX_128.JPG

I then transfer that shape to cardboard.

Last edited by MJL21193; 30th January 2010 at 03:13 PM.
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Old 30th January 2010, 03:11 PM   #4
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Quote:
Originally Posted by Magura View Post
Make a positive mold, throw it in an oven at like 250C, with a piece of 3mm PET on top. Wait a few minutes, take it out of the oven, trim.
That's not a bad idea! I'll try that I think.

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Originally Posted by Magura View Post
EDIT: I think it's about time, to direct your attention towards your stats.....now we are 13 whom will laugh when it won't work

I'll laugh too.
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Old 30th January 2010, 03:27 PM   #5
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next step is to make the form. I use a scrap of OSB and screw a large dowel to the centre, making sure the it is perpendicular. The dowel I used (old broom handle) was a bit small so I had to add a couple layers of cardboard and masking tape to build it out to 1".
The cardboard elliptical cone is then slid in place with the dowel through the throat hole:

NPX_112.JPG

I line it up and trace around it where it meets the OSB. I use silicone on this line to glue the cone in place.
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Old 30th January 2010, 03:40 PM   #6
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After the silicone cured, I sprayed on some polyurethane to seal the cardboard then I drilled a few hole in the back of the OSB to access the cavity. I used spray foam to fill the cavity and let that cure overnight.

Next came some filling and sanding:

NPX_115.JPG

More filling, sanding and several coats of polyurethane:

NPX_123.JPG

It is glassy smooth now and is getting several coats of wax as a release agent.
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Old 30th January 2010, 06:34 PM   #7
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With the "plug" built, I built a form on it to make a negative of the shape:

NPX_129.JPG

While the plug is very near perfect, it can be improved upon and this gives me the opportunity. Making a negative from plaster of Paris will serve two functions: First, it will allow me to create a smoother and more perfectly shaped positive for future castings since I can correct any of the defects in the negative before casting a new positive.
Second, once the new positive is created, the negative can be cut in half and modified to form the outer cast for backside of the horn flare.

It took 5 litres of plaster to fill:

NPX_133.JPG

I'll let this set overnight and reveal my masterpiece tomorrow.
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Old 30th January 2010, 06:41 PM   #8
Dr_EM is offline Dr_EM  United Kingdom
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Good looking work on the horn casting! Look forward to seeing how that comes out
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Old 30th January 2010, 06:55 PM   #9
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Very nice indeed John.
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Old 30th January 2010, 07:12 PM   #10
SunRa is offline SunRa  Romania
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This is a great tutorial Keep it coming!

By the way, what did you use for making those rendered images / drawings of the waveguide?
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