December 29, 2010

Building a 6" oblate spheroid waveguide

Can't be that hard, can it?

It sounds simple enough. Draw up the profile, cut up some MDF donuts, glue together, turn it up on the lathe. Voila! Waveguide bliss. I was thinking about 3 - 4 hours and that I'd get to do other things that day. Well, it took about 3 days! 

It has an acrylic mounting plate because it's a bolt-on driver.

Here are the parts:

The tricky part is getting them all concentric. I spent about half an hour just talking with my construction advisor (aka Dad). That had me worried. I knew if it took half an hour just to talk about how to do it, my 4 hour plan wasn't looking good. 

Gluing - the easy part:

I had to leave a 1/4" hole through all the pieces, then line up with a metal rod. 


That had to be aligned to a metal flange that attaches to the lathe, which meant machining up another rod with an adaptor, which you can see above sticking out the top. 

Once glued, I then cut out the centre circles.

All this to avoid having to chisel out the whole thing very slowly on the wood lathe:

It's a fairly slow process where a chisel is held on to the metal guide. You have to keep stopping to test against the template (paper cut out). I have used this method in the past to create flared vent ends. 

Action shot: 

I realised that it would be quicker and easier in the metal lathe. The wood lathe actually isn't a lathe but a multi purpose shopsmith that works as a drill press, table saw and sander. The metal lathe is a proper lathe and more accurate but limited in the size it will turn.

Finished waveguide:

It was very time consuming to build and turn the first, but once it was done, the second was quick to turn up. I spent 3 days on the pair - probably about 16 hours. I could probably build them again in half a day.

The final result:

More about my prototype speaker >

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