Material Dilemma

[Moonbogg]

Hey, what about this idea. I could make aluminum end caps and have the OD of them thread right into the chamber tube, like huge pipe plugs. I would use straight threads and seal with an o-ring. With a single thread that huge, it should hold the pressure easily and I could totally eliminate any tie rod or steel weight altogether. I would just have to figure out how much thread to have engaged and how to torque those bad boys correctly.

[SpudBlaster15]

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[Moonbogg]

Is 7075-T6 aluminum out of your price range?

No, I looked at that as well and its not much more expensive for this design. It is strong, but it is very susceptible to stress corrosion cracking. It will look shiny and new, then suddenly crack and fail on you without warning. With such a bolt preload stressing the flanges constantly, I would never consider using this material despite its strength. Also, there are some issues people have mentioned about anodizing seriously reducing fatigue resistance of this alloy, and I plan on anodizing.

EDIT: When I first started spudding, I never expected that designing a cannon could be so involving with so many things to take into consideration. Then again, these issues never presented themselves until I moved up into the higher pressures. The risk factor has motivated me to think this thing through long and hard.

[inonickname]

You people suggesting titanium obviously have no idea of:

a) The price of raw titanium stock
b) The extremely poor work ability and hazards of working with titanium

[Gippeto]

Hey, what about this idea. I could make aluminum end caps and have the OD of them thread right into the chamber tube, like huge pipe plugs. I would use straight threads and seal with an o-ring. With a single thread that huge, it should hold the pressure easily and I could totally eliminate any tie rod or steel weight altogether. I would just have to figure out how much thread to have engaged and how to torque those bad boys correctly.

It's a direction I'm leaning in as well. Cleans up the appearance considerably.

Inside thread major diameter should not use up more than 60% of wall thickness. Put the o-ring AHEAD of the threads, such that pressure is contained to the full wall thickness of the tube. I know many manufacturers of pcp rifles expose the threads to pressure...but I've read of an AA going kablooie from an overfill and a Zoraki pistol cracking the valve chamber (in the threaded section) from over pumping. Can't for the life of me figure WHY they would do it that way...when a slight alteration increases the safety factor so significantly.

The design I have been using, with countersunk shcs has the advantage of being strong and EASY to calculate, but...you see the heads of the screws. (Countersinking puts the head of the screw in shear rather than the threads...more meat = more strength....and much reduced bearing stress on the tube wall. )

This layout also reduces the requirement for wall thickness. ie. You don't need the extra meat in the tube to cut a shoulder and threads.

Torquing shouldn't be a big issue...trying to turn that plug under pressure will test your strength to be certain. Snug enough for the o-ring to seal is all you'll really need.

Engagement?? One diameter should suffice, but this is best calculated to be certain.

[velocity3x]

You people suggesting titanium obviously have no idea of:

a) The price of raw titanium stock

Titanium: 0.5" x 12" x 12" $547.00 from Online metals. Pretty pricy....I'd say!

[Moonbogg]

You people suggesting titanium obviously have no idea of:

a) The price of raw titanium stock

Titanium: 0.5" x 12" x 12" $547.00 from Online metals. Pretty pricy....I'd say!

Yeah, its ridiculous. I'd rather have the cannon made of lead.

@Gippeto

Thanks for the feedback on the threaded plugs idea. I will take a look at it and if I go that way the o-rings will seal infront of the threads.