Making horns.

[EddyCurrent]

I'm not sure you would get away with 240 grit and here's why. If you think about the cutting tool, probable a ball mill where the end is rounded, and imagine it cutting tiny furrows in the material, then the amount of stepover you give it will determine how far it moves across to cut the next furrow. So if the furrows are very close together it will take a very long time to cut and at some point there is a trade off between time to cut and desired finish. There's not a great choice of UK made machines, Exel was the one I was going for before deciding to make one.

[simonms]

Thanks Eddy, I am going to contact Exel tom. with similar info to what I have posted here to see which of their models they would recommend for machining these things and off course the price. I thought it might be asking a bit to much to go straight to 240, I don't mind a bit of elbow grease.

[Blackrat]

why does the horn have to be split to do it on a 3 axis ?

if you run a .3mm or .4mm stepover you will have very little sanding to get to smooth

whats the dimensions of the horn ? width and depth ?

[simonms]

I will be making several different sized horns the biggest at about 500mm wide 400mm high and 500mm deep (roughly). As I said I am a total novice and my comments on machining in two halves are simply taken on face value from this e-mail,

1. True 5 axis machining is complex and expensive, and should be avoided if possible
   
2. Cutter lengths are limited. Your item in one piece exceeds this. The cutter length for optimum machining is 50mm.
   
3. To accommodate 1. & 2. the machining solution is a hybrid 3 axis one. The first 48mm of material is positioned and bonded up. The surface is machined from 0 to 48mm in the height with a 12mm ball ended tool stepping over 1mm per pass, cutter orientated vertically. The next 48mm of material is bonded on, and machined from 48mm to 96mm in the height. Etc, etc, up to 216mm.
   
4. The inside surface of your horn could be machined using this technique. The outside will be the edge in plan projected down to the same plane as the small hole (ie not 25mm thick). Is this an option?
   
5. To cut the outer surface at 25mm thick the item need to be inverted. As it no longer has a flat bottom this way up it will require a male support jig, increasing the nominal cost by 50%. The item is now too deep for the 50mm cutter vertically, so has to be cut full 5 axis (expensive).
   
6. Another option would be to split the model into 2 halves. Each half could be machined front and back using 3. above, as there are flat joint surfaces for support when inverted. The outer surface would require some limited cutting with a 100mm long 12mm ball ended cutter, run at a slower feed rate to avoid it snapping off.
   

[Blackrat]

ok i see ... what sort of thickness would be acceptable for the horn ( the above sketch just shows a surface ie. 0 thickness)

[simonms]

Yes it does, just getting to grips with the drawing side, the walls would need to be 20-25mm thick, sorry forgot that info.

[Boyan Silyavski]

So the company that you asked gave you the correct answer.

If you want best finish,what you miss to understand here is what material and price. There are blocks or thick sheets of some compounds that could have perfect finish but they are not cheap. The wood will always need sanding from 60 grit if you use cheap pine. MDB will be ok, but then you need to treat it additionally.

So to resume what i believe is the correct answer:
1. IMO you need custom sturdy cnc with high Z axis/200mm or more/ and fitted with extra 3D printer head. 1kg of material is cheap 15eu or a bit more and you put it where you need it, no waste here and there. Then you machine it where and when necessary. Note should be taken that it will never be perfect perfect, you will need to sand and possibly flame polish for industrial look.
The bad thing is that it could never be a production, just a way to make cheap prototype. The 3d print i mean. And it will take you a lot of time to learn both.

2. for starters- cnc + what you are doing till now . Instead just implement 3d paths so the curves are smooth. Or do it 2 pieces. Of course if you go with 2 or more pieces split, you will need jigs about each step of the process.


One thing you should understand though. What you aim to do will always be done better if you make one piece, perfect it as much as possible and then cast it from whatever resin. If you can not, then hire somebody to teach you and make you the mold. That will be cheapest and fastest.

Of course that depends on what people would pay money for, if the idea is to be done artistically and finished by hand or they would care only for the custom acoustics.

[corkcnc]

On the other hand the stock is so light you are not going to have issues with torque so you could opt for a larger diameter cutter (for all convex work) which would keep the scallop height pretty low so if nothing else you could go with a 3 axis machine with a decent CAD/CAM package. You might still have to make it out of a number of blocks to optimize step over with different gradients but it might be 4 or 5 instead of the 3D jigsaw.
Best of luck with it
Noel.