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pneumatic questions

A place to ask general spud cannon related questions.
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pneumatic questions

Unread postAuthor: regre7 » Sat Jan 14, 2006 3:43 pm

Hello again. I've been gone for a while, but I'm back.

I'm about to start putting together the pneumatic cannon I was talking about earlier here, but I have several questions. I've never used a tap, but I think I need to tap the holes for my pressure gauge and schraeder valve. I ordered both of them off of SGTC. How do I know what size tap to get for these? The website recommends drilling an 11/32" hole for the pressure gauge I believe. I've also read that by simply screwing the gauge in you can tap the hole. Is that method reliable? It seems like if you mess up a little you're out of luck.

In short.... how do I mount a pressure gauge and schraeder valve?
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Unread postAuthor: saladtossser » Sat Jan 14, 2006 6:43 pm

1/8" 27 npt
1/4" 18 npt
http://www.engineersedge.com/tap_drill_chart.htm
that should point you in the right direction

and yes, tapping is very reliable, but you can always buy a crap load of reducers and threaded bushings.
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Unread postAuthor: jrrdw » Sat Jan 14, 2006 7:11 pm

hi, i'm a lawnmower machanic, so i retap plenty of threaded holes, sometimes new parts require you to tap a predrilled hole EX: craftsman blade spindal houseing ie: mandral, it's been my expearince if you have a part with standerd size threads, you will need a metric sized tap, most presure gauges have pipe thread, not fine, not machine but pipe thread, its in a class of it's own, different size diameters, sae or metric, you should have parts in hand when makeing these discesions, it would be a better way to do it.
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Unread postAuthor: nicholai » Sun Jan 15, 2006 12:26 pm

Since its not metal that you are putting threads into, yes you can just drill a hole and screw the threads into the unthreaded hole. I find that this method works fine with the exception that if you dont screw it in straight your threads will be crooked and it wont look "pretty". The threads on the pressure gauge (1/8" pipe thread?) are pretty small so if you do screw it up you can just re-drill and tap to a larger size and add a threaded insert (threaded on the outside and inside so you can use the same thread pattern that you previously screwed up)

I personally self-tap with the threaded piece, its quicker and im damn good at getting it straight. I think it makes a better seal because it forces the metal threads to cut a thread pattern into the plastic. When its been previously tapped and you attempt to screw it in you run the risk of screwing it in crooked and cross-threading the hole.
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Unread postAuthor: regre7 » Sun Jan 15, 2006 1:27 pm

Okay, I think I'll be able to get it reasonably straight. Do most gauges and valves come with a nut to secure it from the inside, or do you just rely on the threading?
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Unread postAuthor: drac » Sun Jan 15, 2006 3:51 pm

Just the threads.
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Unread postAuthor: regre7 » Sun Jan 15, 2006 5:34 pm

Just because I'm paranoid....

The theads will hold the pressure gauge and schraeder valve in place when SCH 40 pipe is pressurized to 100 PSI?
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Unread postAuthor: regre7 » Sun Jan 15, 2006 6:25 pm

Another question.

I'm using the over-under form for this cannon, and I'd like to know if putting the valve before or after the 90 degree elbows (or perhaps between, although that would be awkward) would be better?
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Unread postAuthor: nicholai » Sun Jan 15, 2006 6:29 pm

Absolutly, if you are that paranoid, fill the tank with water and then pressurize it. Go to home-cheapo or lowes and ask the guy in the plumbing section for a couple tablets of leak detector. they should be free. Its a small red pill that you put in your toilet to test for leaks. It makes the water a bright red color so its very easy to see.

Fill your chamber with water, drop in the leak detector pill and pressurize. at 100 psi a pinhole will piss out a stream over 5 feet high (unless you have a real big leak)and will be quite visible. Pressurizing with water is much safer too.
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Unread postAuthor: regre7 » Sun Jan 15, 2006 6:34 pm

Was that meant to be sarcastic or did I just read it incorrectly?
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Unread postAuthor: nicholai » Sun Jan 15, 2006 10:14 pm

no, but if you stripped out the hole and tried to use a gauge in it anyway it would probably blow off, or the cannon could just explode, sending shards of PVC shrapnel everywhere (unlikely). i test all my air tanks/systems for pneumatic guns using the colored water method.
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Unread postAuthor: regre7 » Sun Jan 15, 2006 10:27 pm

What do you mean by stripped out? Messed up the threading?

While we're on this subject, should I wrap the gauge threads in teflon tape?
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Unread postAuthor: nicholai » Mon Jan 16, 2006 5:55 am

yes, when threads are stripped out they dont hold pressure. This happens from overtightening and general wear from screwing and unscrewing. Id wrap that sucker in teflon tape before you screw it in and if you're still paranoid, use some plumbers putty type epoxy around the base of the threads
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Unread postAuthor: regre7 » Mon Jan 16, 2006 10:50 am

Will any type of epoxy work, or does it need to be specifically for pipe?
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Unread postAuthor: nicholai » Mon Jan 16, 2006 5:44 pm

well pretty much any will work but a putty type of epoxy will be much more easy to work with since it wont dribble down and make a mess, the other types are typically more of a liquid and have to be mixed with a popsicle stick or something like that. Besides plumbers putty is cheap, bonds very well to PVC and gets hard as a rock.
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