long range ammo MiniBoy Mark I

[saefroch]

Epoxy is a lot easier to remove than you'd think, so don't worry if you happen to spill any where it's not wanted.

I advise testing the reinforcement somehow (fire something of comparable weight, perhaps with lubricated wadding), behind a significant amount of shielding.

[MrCrowley]

Epoxy is a lot easier to remove than you'd think, so don't worry if you happen to spill any where it's not wanted.

The problem is that with the area where I will do the fibreglassing, a car will be driving on that area within hours. Rather not wait for the epoxy to set on the cement before removing it. Unless you know of a good way to remove non-cured epoxy?

I'm starting to think firing 100g frozen lemons out of the hybrid was never a good idea.

I'm fairly confident wrapping the first meter of the barrel in a 3mm thick layer of fibreglass will be enough but I'm worried about the ABS threaded fitting. I'll have to find some way to reinforce that as well I imagine. Even then, the threads could still crack inside the galvanised adapter it threads in to.

[ramses]

Unless you know of a good way to remove non-cured epoxy?

Acetone/torulene,MEK, maybe isopropyl alcohol.

I agree that the stress will likely concentrate in the threads. Although, they will be supported by whatever they thread into.

Theoretically, there should be very little axial force on the barrel. If one or more threads is left to atmosphere or unsupported, that will be the failure point (from hoop stress)

The only axial force should be from projectile friction and drag from the working fluid. I'd worry about whatever is directly opposite the barrel along that axis.

[saefroch]

Unless you know of a good way to remove non-cured epoxy?

The only thing that I can suggest is a gas torch, but why not just do it in a different place? Like somewhere with a smooth floor (easier to remove epoxy)...

[MrCrowley]

P_yield = YS * (d_o² - d_i²) / sqrt((d_i² + d_o²)² + (d_i² - d_o²)² + (d_i² + d_o²) * (d_i² - d_o²))

Where d_o is the outside diameter of the pipe, d_i is the inside diameter of the pipe, YS is the yield strength of the material, and P_yield is the yield pressure of the vessel.

I tried both DYI's and btrettel's equation for the aluminium tube and for DYI's calculation I got 3000PSI and for btrettel's I got 352PSI

I used 6000PSI for yield strength of the material as I have no idea what type of alloy it is. Just to clarify, the '*' symbol used in the equation is a multiplication sign?


@ramses,

Hmm, that is a worry. As the ABS threads are threaded in to a galv. iron fitting, some of the threads are exposed and I'm worried that if I try and thread them all the way in they may crack.

I'd worry about whatever is directly opposite the barrel along that axis.

If I understand correctly, you are talking about what the barrel threads in to (an adapter in the tee) and maybe even the piston itself?

Perhaps this is LeMaudit's elaborate plan to kill of a NZer...what did we ever do to you Canadians?

Like somewhere with a smooth floor (easier to remove epoxy)...

Well I would do it on the smooth floor in the garage/carport, hence the problem about cars running over epoxy. I can't do it in the garden as I have a dog and my dad is a bit irrational and probably would think it'd kill off the entire flora and fauna within a 5 mile radius.

[DYI]

If you got 352psi using btrettel's equation, you screwed up somewhere - his will always return a higher yield pressure. It's rather long to be doing by hand all the time. I'd suggest a quick spreadsheet to make sure it gets calculated properly.

The * symbol is multiplication here, and 6000 psi is a very low yield stress for any sort of structural aluminum alloy - that number is closer to the yield stress of ABS.

You could always add another galvanised fitting which goes inside the fiberglass wrap and attaches to the ABS barrel (probably by casting inside a larger pipe like I did for the ME20H-41 barrel). Dead space would be increased somewhat, but it'd eliminate the concerns over barrel rupture for this particular shot.

[MrCrowley]

Ah, I must've screwed up the equations somewhere as i'm getting different figures now. That'll teach me not to use a computer calculator.

What do you think is a reasonable yield strength for aluminium of an unknown alloy? Wikipedia lists 10,000PSI for 6063-T4 I believe. Your and btrettel's equations are giving me 285PSI and 596PSI respectively for 10,000PSI...unless I buggered those equations again.

My cheap digital calliper says the diameter of the metal disk on the sabot (the only part of the sabot that can't be modified easily) is 47.4mm which means it could fit inside this aluminium tubing. If we decide the aluminium may be strong enough (I'd be willing to fibreglass a portion of it) and we can figure out a cheap way of adding threads, I'd much prefer it over the ABS barrel.

[jackssmirkingrevenge]

Apart from gloves, a well ventilated room and a newspaper covered floor, any other precautions I should take?

Plastic covered floor, don't use anything porous unless you have very thick layers of it.

I don't see how the sleeve and epoxy filled gap is more expensive though.

Heck, binding with some hefty rope then pouring epoxy over it would probably be much stronger than fibreglass.

Leather cannon style

[SpudBlaster15]

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[jackssmirkingrevenge]

No, no, no. You have to cover the entire room in plastic to ensure you don't leave any trace launcher building evidence behind.

Nice one Dexter

[MrCrowley]

You didn't give my 50 IQ points back.

I knew that loan was a bad idea

I don't see how the sleeve and epoxy filled gap is more expensive though.

Well if I could find the right pipe for $30, it'd be fine but I don't imagine I could find some pipe that would sleeve over with a small gap relatively easily and cheaply.

I'm increasingly more worried about the ABS threaded adapter now, perhaps it is better just to get an aluminium or similar barrel.


Edit: The aluminium in question is grade 5005.
This shows the yield strength for sheet 5005 aluminium ranges from 19-28KSI.

This doesn't provide any yield strength figures but has some information about its heat treatment etc.

[LeMaudit]

Perhaps this is LeMaudit's elaborate plan to kill of a NZer...what did we ever do to you Canadians?

I can't remember... you must have done something
And if it wasn't you then it was your brother. Or someone close to you.

[Goats spudz]

What do you think is a reasonable yield strength for aluminium of an unknown alloy? Wikipedia lists 10,000PSI for 6063-T4 I believe. Your and btrettel's equations are giving me 285PSI and 596PSI respectively for 10,000PSI...unless I buggered those equations again.

Not knowing the alloy or the level of artificial age hardening is a problem, as the strength of aluminum alloys varies substantially across the board. For example, 6061 is easily the most commonly used grade for structural aluminum. In the quenched state (6061-0) you're looking at a yield strength of ~8KPSI. However, when fully precipitation hardened (T6), this alloy has a yield strength of ~40KPSI.

You really need to know precisely which material you're dealing with before attempting any kind of failure analysis.

Plastic covered floor, don't use anything porous unless you have very thick layers of it.

No, no, no. You have to cover the entire room in plastic to ensure you don't leave any trace launcher building evidence behind.

...what did we ever do to you Canadians?

You didn't give my 50 IQ points back.

Is that A kill room?

[ramses]

@ramses,

Hmm, that is a worry. As the ABS threads are threaded in to a galv. iron fitting, some of the threads are exposed and I'm worried that if I try and thread them all the way in they may crack.

I'd worry about whatever is directly opposite the barrel along that axis.

If I understand correctly, you are talking about what the barrel threads in to (an adapter in the tee) and maybe even the piston itself?

You understand correctly. The reaction force will be applied to whatever you see when you look down the barrel. This spot must transfer whatever force is needed to acccelerate the rest of the cannon to, well, the rest of the cannon.

Since the barrel doesn't have any ends attached to it, it theoretically experiences no axial stress. It will see a force equal to whatever the force of friction is on the projectile, and whatever force is necessary to accelerate it at the same rate as the rest of the launcher from recoil.

It will experience hoop stress from the pressure, which along with bending forces from off-axis distribution of the recoil force on the launcher as a whole. This is where that thread worries me. I guess you could epoxy a short bit of threaded galvanized thread directly into the barrel. The steel would take care of the stress around the crack, and the epoxy joint should only see shear stress, which should be negligible for the reasons above (except perhaps recoil forces. Use a barrel brace to mitigate this)

[Moonbogg]

Why is there a problem where shooting this sabot round is concerned? Is it too heavy or something? I don't see how it would cause a problem when you've shot the cannon before. What size is your barrel and what threads are attaching it?