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Old 14th May 2009, 04:10 PM   #1
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Question CAD modelling of waveguides for CNC milling?

Anybody have any experience modelling waveguides and getting them CNC milled into wood? I'm thinking multiple sheets of plywood stuck together for the depth.

I've seen the oblate spheroidal equation, how would one go about putting this through a CAD program? I've got sketchup....


How about a non-axisymmetric profile? Like the 18sound XT1086 but bigger to match bigger woofer...




Ta
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Old 14th May 2009, 05:18 PM   #2
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Yes, I'm doing exactly that. I'm actually waiting to put my motorcycle engine back together before starting to cut a 320hz LeCleac'h horn. Sawdust in the crankcase = no good.

The details of drawing a curve like that in a Cad system vary with the system, but typically you'd draw enough points to describe the curve and then fit a spline through them. In my very limited experience with it, Sketchup is not what you want - it is for sketching, not for accurate technical models. That said, if it can export IGES or STL files, you might be able to make something useable with it but I would definitely check that first. Personally I use Pro/E or Autocad. Other packages to look at would be Solidworks, Unigraphics, Catia, etc. Solidworks is probably your best bet. The others are expensive. I haven't used the latest version of AutoCAD, but in the older ones (just a few years), you can't draw something like an elliptical horn as a solid model.

Another thing to watch out for is what CAM software your CNC shop has. If they don't have something moderately complex, they won't be able to cut out 3D shapes like this - well, they won't be able to import a 3D model and from that program the cutting paths for the shapes. A lot of shops with operators who are more experienced with the machining side and less with the computer and cad side don't have the necessary software in my experience, as this software is also not cheap.
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Old 14th May 2009, 06:40 PM   #3
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All my test wave guides are CNCed. The problem with wood is that it is not smooth enough for the throat to perform as designed.

As for the curve in CAD, it's possible to compute the points and import them using a program such as "points import".
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Old 15th May 2009, 12:04 PM   #4
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Cool, I'll check out solidworks...

Quote:
not smooth enough for the throat to perform as designed.
Is this true of all woods? How bad is it? What about when painted/sealed? (I obviously have no experience here ) I notice people building Gedde's kits painting the waveguides, but I'm not sure if that's all the way into the throat.

Are there any other materials suitable for this sort of thing? I'm only building 2... These aren't for prototypes, and they'll get used outdoors sometimes.



Does anyone have equations for an elliptical waveguide?


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Old 15th May 2009, 12:37 PM   #5
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I have not had too much experience with varous wood, but. It seem that if wood has tought fibers, then it would be difficult to mill. The spinning direction and angle at which the cutter crosses the fibers causes various degrees of roughness. Other alternative might be arificial wood.
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Old 15th May 2009, 01:28 PM   #6
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I've had no problems with plywood or MDF. Using the appropriate bits, speeds and feed rates should work on real wood as well. I'd always expect to have to sand at least a little though.
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Old 15th May 2009, 01:31 PM   #7
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Quote:
Does anyone have equations for an elliptical waveguide?
What about Geddes Data?

I think I saw a spreadsheet somewhere but I can not find it now.

btw, Im very interested in this stuff also.
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Old 15th May 2009, 09:51 PM   #8
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I've seen Gedde's Bi-Spheroidal waveguide patent, but it didn't seem to have any easy step by step guide
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Old 15th May 2009, 09:56 PM   #9
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Look in the thread here called 'Geddes on waveguides'. I believe the formula is given and there are several revisions of an excel sheet posted.
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Old 16th May 2009, 02:30 AM   #10
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Was that for the oblate spheroidal profile or an elliptical profile?

Is it just a matter of computing two different oblate spheroidal profiles (1 for horizontal, 1 for vertical), then interpolating between them somehow to produce an ellipitical?


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