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Author: Subject: Cutting grooves on Al2O3 tube, carbide or diamond?
Heptylene
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[*] posted on 23-2-2019 at 16:14
Cutting grooves on Al2O3 tube, carbide or diamond?


For some experiments with phosphors, I plan to make a tube furnace from a fused alumina tube (O.D. 26 mm, I.D. 19 mm, 500 mm long) and some nichrome wire. I'd like to cut a helical groove on the outside of the alumina tube to hold the wire in place (think like the thread of a screw with the wire coiled around it inside the groove). I might add some refractory mortar on top of that if necessary (e.g. to prevent heating element oxidation).

I'm thinking I'll make a helical mark with a pen on the tube. Then slowly rotate the alumina tube and use a high speed tool (Dremel) to make the groove. Doens't have to be very deep, maybe 0.5-1 mm.

Now, alumina has a Mohs hardness of 9. So that narrows the choice of tool for the groove to tungsten carbide or diamond. I think a burr would be the best tool here.

Do you think tungsten carbide would work? It also has a Mohs hardness of 9, so think it won't last long. Would diamond last much longer to justify the investment?

EDIT:
Btw why do I want to use alumina? It has a very high thermal conductivity for a ceramic: 30 W/(m K).
Compare that to about 1 W/(m K) for glass. Copper is around 400. Lead and stainless steel are at around 30, the same as alumina.

Furthermore, alumina can withstand very high temperatures and has a low coefficient of expansion.

[Edited on 24-2-2019 by Heptylene]
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markx
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[*] posted on 23-2-2019 at 23:59


It is not a reasonable practice. You risk breaking the tube in the process and introduce fracture points into the alumina core that very likely shall contribute to cracking in the process of using the kiln later on.
I would suggest to leave the alumina tube as is and just wind the heating element straigth onto the core. You can secure it with some high temperature glass fibre material and refractory mortar, surrounded by a close matching case carved from soft fire brick. The heating wire is not going anywhere and you stand less of a chance to end up with a broken kiln core on first use.

If you still insist on trying to carve grooves into the core tube, then using abrasive diamond tools is the only practical option. They are widely available and cheap. Carbide tooling shall not be up to the task.
I would also suggest to build some kind of a fixture or jig so you can hold and rotate the workpice and guide the tool securely. Trying to perform such a operation in freehand mode is likely going to fail.




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andy1988
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[*] posted on 24-2-2019 at 01:01


See this note on the use of ceramic paper (I hadn't tried myself, but looks relevant).

I'm looking into laser assisted machining on a lathe myself (for ceramics).




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Heptylene
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[*] posted on 24-2-2019 at 07:06


markx: I hadn't thought about cracking the tube. You're right that could be a big problem. The thing is the wire will be in a rather tightly spaced coil (maybe 5 mm between wires) and upon heating the coil will expand and I run the risk of shorting adjacent loops of wire. Guess I'll use mortar as you suggested then.

andy1988: that ceramic paper could be useful. I planned on using silica sleeve such as this, but ceramic paper could be handy.
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Geocachmaster
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[*] posted on 24-2-2019 at 07:46


I agree with markx, just wrap the wire and use some mortar to hold it in place. I made a tube furnace like this using ~2 mm spacing between wraps and some Dap high heat mortar to attach it. It worked out just fine.



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Twospoons
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[*] posted on 24-2-2019 at 12:41


If you use Kanthal instead of nichrome you have less risk of shorting. On heating Kanthal builds a surface layer of aluminium oxide that protects the wire from further oxidation and also helps provide turn to turn insulation..



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