Manufacture of Molybdenum kathode

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Finn Hammer
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Manufacture of Molybdenum kathode

Post by Finn Hammer »

All,

This thread intends to detail the manufacture of a cylindrical kathode, produced from readily available molybdenum.

The first kathodes I used were made from stainless steel, but when the power level exeded 1kW, the kathode saw katastrophic failure: it melted.
This put me on a search for materials with a higher melting point, and properties that allow machining with ordinary tooling.
This lead me to the so called refractory metals, and my choise fell on Molybdenum, due to the relatively good machineability of this metal. It machines quite similar to cast iron, and has a similar appearance.
I buy it at Metall-Maier in Germany:
https://www.sintermetallshop.de/
Trying to make this design in tungsten would require the availability of either advanced grinding machines or EDM machinery, something I don't have immediate access to. I have a lathe, and if you want to go this way, you are going to need one.
Some of the shapes would seem to require a CNC mill to produce. I don't have one of those either, but before CNC was invented there were Toolmakers making these things by hand, and it so happens that I am one of those pre-CNC toolmakers, now retired, so the lack of machinery is not a stopping factor.

A new part starts with scriblings on a napkin, or some other paper, I will spare you that, instead start with the presentation as seen in the 3-d modeling package, where the rest of the fusor was also modeled. The tricky part of the design is the junction between the kathode body and the stalk, where I want a thicker section to facilitate machanical integrity and heat transfer.

KathodeSW.JPG

I start by chucking the 17mmØ rod in the 4 jaw chuck, excentrically, with 2mm of runout, so that I can turn an external diametre of 15mm on to it. I also drill the hole to facilitate the parting off operation.
After this, I chuck on the 15mm cylindrical part in the 3jaw scroll chuck, and turn the other end of the cathode, and finish the hole. Chamfers are made as I go along here.
What I have now is something looking like this:

20211001_122138.jpg

Most of the excentric ring has to be removed, and this is where the real "handmade in Denmark" starts: With a cross-cut 6 inch flat file, I start to work the excentricity down to the cylindrical surface, to get to this point:

20211001_122123.jpg

and in another view:

20211001_122152.jpg

You can see the process in this video, where I file the very last part off, and it becomes apparent that this is where you will get your fingers dirty. Proceed with caution!

https://youtu.be/PgjtQqo9fZw


I repeat this operation on the other side to get to this stage:

20211001_125617.jpg

The pyramidality of the stub has to be smoothed out, and this is done with a half round needle file as shown in this video:


https://youtu.be/BuoSskr96UU


The final product looks like this, where the rough surfaces has been worked over with 300 grit emery cloth.


20211001_153359.jpg

20211001_153405.jpg

It also becomes apparent, that cutting a thread on this material is not straight forward.

20211001_153414.jpg

I hope this can serve as inspiration. Filing stuff is an aquired skill, and from the videos you can deduct some of the clues to a good end product. These are: try to work the file slowly. Check the produced surface regularly, chuck the part as far down in the vice as possible, to avoid squeal, and finally: Practice, practice, practice. That's all, really.

This Kathode has already survived operation in excess of 2kW (75kV, 30mA) without breaking a sweat.

Cheers, Finn Hammer
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Richard Hull
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Re: Manufacture of Molybdenum kathode

Post by Richard Hull »

You are better man than I Gunga Din! I gave up on the cylinder cathode in 2019 with my cross system. As a matter of fact I am thinking about adding another hoop to my Tungsten wire grid. 8 beams and maybe a belly band Equatorially. More beams, still. I was fortunate to leap into the old "Orphan's pile" at Midwest Tungsten back in the 1990s. They had a huge selection of Molybdenum and Tungsten wire and deposition boat sheeting materials. These were very usable cutoffs, sheeting trim, etc. They were just giving the stuff away, virtually. It is gone now.

I am amazed at your patience in such matters. A bit too tedious for me. As I use the spherical chamber the wire grids are best. I have melted a tantalum grid back in 2000. But I have used Tungsten since then, I have not burned out a wire grid since 2000. Of course I keep all fusor work here under 1.3 KW.

A beautiful piece of scraping away metal from a small billet to make such a precision item. Well done, indeed.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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