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PVC thread retention question

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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PVC thread retention question

Unread postAuthor: ComradeBenedict » Wed Nov 10, 2010 7:02 am

Hey guys. I'm in the process of building a vortex cap BBMG with my friend. I was discussing our build with my dad, and he insists that threads drilled and tapped into the sides of the cap will not stay as soon as it's pressurized. Is there any pressure range at which 1/8NPT holes tapped in the sides of a PVC cap would strip(If they have something in them, of course)?
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Unread postAuthor: jrrdw » Wed Nov 10, 2010 7:18 am

All ways drill and tap through 2 layers of PVC, (if you didn't get that info all ready). As to your question, it's relative to how well/accurate you tap the threads and how much pressure you use. It has been mentioned before but is few and far between.
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Unread postAuthor: ComradeBenedict » Wed Nov 10, 2010 7:26 am

We'll probably use 150-200 PSI. I'm hoping that it'll be drilled with my dad's drillpress at work, so it should be plenty accurate.
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Unread postAuthor: jrrdw » Wed Nov 10, 2010 7:29 am

Just make sure ALL of you PVC pipe is pressure rated.
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Unread postAuthor: ComradeBenedict » Wed Nov 10, 2010 7:36 am

Yeah, I'm not in the mood for asploding PVC. Would adding a rupture disc for extra safety be worthwhile?
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Unread postAuthor: Crna Legija » Wed Nov 10, 2010 7:50 am

A pop off valve would be a better choices for safety. If you feed it 200psi you would be lucky to get 90 in the chamber, so i really doubt it would blow out, but of course if the barrel becomes jammed pressure will rise to what your feeding it quick.
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Unread postAuthor: Technician1002 » Wed Nov 10, 2010 10:59 am

The threads will hold fine. The hazard is starting a crack that will shatter the part under pressure.

1/4 inch pipe nipples are added to caps all the time for attaching trigger blowguns or ball valves to piston cannons.

Another common use for a tapped hole is for the addition of a pressure gauge or a meter pipe for an advanced combustion. These are not known for stripping out the threads simply from pressure.

A quick search shows this combustion with a gauge and meter pipe threaded into a coupling. This is Killjoy's FEAR.
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Unread postAuthor: ComradeBenedict » Wed Nov 10, 2010 7:39 pm

-_- wrote:A pop off valve would be a better choices for safety. If you feed it 200psi you would be lucky to get 90 in the chamber, so i really doubt it would blow out, but of course if the barrel becomes jammed pressure will rise to what your feeding it quick.


Right. That would be more fitted to the task, as I can't find any 200 PSI burst discs. Can't wait to get the ball rolling on this.
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