During my work I came across several way to bend a rectangular waveguide. The copper is best softened before by a heat treatment. Then the choices are :
- fill it with sand and heat while bending
- fill it wit water, freeze it with LN2 and bend
- fill it with Wood metal melting at 60 Celsius and bend at room temperature
- fill with brass strips and bend at room temperature
Short of trying all, what are the better choices? Yes, I'm aware there are rectangular cast bends to solder in. They are limited the rectangular though.
We use the WR28 at 35GHz and the attenuation in a coax is dramatic. 5dB for a single meter are common. Even if the plumbing appears crude and tiring, we prefer waveguide if possible.
If you have no experience with any of these methods then you probably need to try some of them and compare the results (e.g. by way of swept return-loss measurements) because there is more to each method than you've mentioned. For example, using 'Cerrobend' you need to choose whether you're going to bend the waveguide: between rollers; by hand with one end in a vise; in a press with a form tool or caul; and so on. Each may yield different results and you may need to refine the technique. I've never heard of success using sand although I suppose it might work if the ends of the guide are suitably plugged. Ice doesn't sound likely to work - surely voids would appear as the guide was stretched during the bend?
However, if you intend to apply these techniques to circular or elliptical guide with the intention of preserving the cross section then you are asking a lot and you might be better off getting electroformed elbows made. If you do, keep in mind that electroformed copper can be gas-permeable - I made that mistake once!
Electrician's perspective: If it's a one-off (prototype or custom project) for a complex bend like an offset, or a J-bend...
I'd consider taking sheet copper, cutting the four sides with all the flat curves needed for each face, cut the flanges, bend in all the required squiggles and turns with a forming roller, and then braze or TIG weld it together.
You leave two opposing faces a bit wide so they overlap on the outside for easier assembly - make a fillet weld rather than acute on the outside corner - the weld penetration on an acute corner will mess up the profile of the inside corners of the waveguide.
Or combine both - TIG to tack it together and make sure it fits, then silver-braze so you don't spend all day on sealing it all up - brazing temperatures don't reach the melting point of the parent metal, so they won't make the TIG tack welds come apart.
And brazing metals can bridge gaps and fill grooves, almost like Bondo filler putty - just have to be delicate with the heat to stay in the plastic state.
Another possibility is to section the tubing and make a segmented bend, expand one side slightly with a flanging tool to make a step overlap for mechanical strength, and then braze the pieces together. If it fits and works as expected /then/ you can tool up molds to cast them, or figure out another way to mass produce.
On Fri, 14 Nov 2008 10:55:58 -0600, Rene Tschaggelar wrote (in message ):
in any effort to bend a tube, the metal on the outside curve wants to stretch, the metal on the inside of the curve wants to compress. Any of the various things to fill a tube are attempting to keep the tube from collapsing. If the filling material is sufficiently non-compressible, the metal on the outside of the curve must stretch. If it is not ductile enough, it will tear.
This is way out there, but could you make a dummy waveguide from solid plastic rod of the proper cross-section, and then plate a metal layer onto the plastic?
I think the Wood's metal or brass strip methods would work best. The problem with sand is that it relies on the fact that the geometric shape with maximum area relative to its perimeter is a circle. If you pack a circular tube with sand and bend it, the cross section can't deviate from a circle because, assuming the circumference remains constant, any change would reduce the section's area and the sand is incompressible. A rectangular section can distort into a more circular shape while allowing the sand to become loose. I imagine the ice would behave somewhat like the sand as it fractures.
The brass strips idea is clever, as long as you can pull the strips out after bending. It simulates the ID mandrels used for sharp bends in production.
Many folks have suggested flexible waveguide, which is probably the simplest method. I'll present two other alternatives.
You can design a mitered joint using only pieces of rectangular waveguide cut at the correct angles and soldered or brazed together. The design of both E-plane and H-plane corners of arbitrary angle is covered in Marcuviitz' "Waveguide Handbook". This was one of the volumes in the MIT Radiation Laboratory series published after WWII. The book was reprinted in
1986 by the IEE.
Finally you can mill out two halves with a CNC milling machine and bolt or braze the two pieces together. The seam does not have to be perfect if it is parallel to the E-field. Again see the "Waveguide Handbook" for a discussion of symmetrical slotted waveguide.
Both of these methods are easy with rectangular waveguide. If you use ridged waveguide, they are still possibilities, but you would need a good machinist!
One of my first jobs back in the 60's was for a place called "Waveguide Bending". And bending waveguide of all sorts of sizes was what they did.
The process they used was to force strips of spring steel into the waveguide and then bend them in hydraulic presses, then remove the strips to use again. Worked admirably for retaining internal size during and after bending.
Cut 4 pieces of flat stock and solder them together?
Ugly and a PITA, but you can avoid the crinkled / streteched sidewall and Wavy top and bottom.
My fuzzy memory about the little bit of rudimentary theory I used to know makes me think that the OUTSIDE dimensions and surface of the waveguide plays an important role. Or was that a dream?
Laser / plasma / Waterjet cut parts can be reasonably priced if you have a non- swamped shop near you.
Mark
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