Casting Experiment: Elliptical Horn Flare
split off from http://www.diyaudio.com/forums/multi...ign-build.html :cop:
My original idea was to cut this shape right into the front baffle, as I did here:
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.
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.
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 :rofl:
More of the design concept:
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:
That is the ellipse cut out from cardboard (Frosted Flakes cereal box side :up:) 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:
I then transfer that shape to cardboard.
I'll laugh too. :D
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:
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.
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:
More filling, sanding and several coats of polyurethane:
It is glassy smooth now and is getting several coats of wax as a release agent.
With the "plug" built, I built a form on it to make a negative of the shape:
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:
I'll let this set overnight and reveal my masterpiece tomorrow. :)
Good looking work on the horn casting! Look forward to seeing how that comes out :)
Very nice indeed John.
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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