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PVC Threads

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Unread postAuthor: zaphar » Sun Sep 18, 2005 1:09 am

I am building a potato cannon right now, but I am having trouble screwing on the back section of my cannon. Is there something I can do to the treads to make the back section screw on smoothly, when I screw the back in, I can't even make one full rotation without it jamming up. I've looked at the threads and they seem to be ok. Any suggestions?
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Unread postAuthor: saladtossser » Sun Sep 18, 2005 8:08 am

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Unread postAuthor: spudshot » Sun Sep 18, 2005 9:21 am

Teflon tape
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Unread postAuthor: boilingleadbath » Sun Sep 18, 2005 10:30 am

Note even one full rotation?!
Either your cross-threading it, or your definiton of a full rotation is messed up; the end will screw pretty much effortlessly most of the way, and then at the end it will get very hard.
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Unread postAuthor: zaphar » Sun Sep 18, 2005 9:54 pm

I'm pretty sure I didn't cross thread it because I've made sure to guide it on correctly and it still jams. I've looked at the burnt latke site and it didn't seem to help. I think something might be screwed up with the threading when I got the fittings because one part looks like it is offsetted too much or something. Would tightening the part all the way on several times work and eventally loosen it up a little? Would putting something on the threads to make them malleable work?
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Unread postAuthor: CS » Sun Sep 18, 2005 11:56 pm

Did you clean out the threads with a file?
zaphar wrote:Would tightening the part all the way on several times work and eventally loosen it up a little?

I thought you couldnt get one full revoultion? You could muscle it on there with a wrench or something, which can provide two outcomes, first you break it and strip the threads or second you break free what ever is stopping it from going on (should be able to see problem though).
zaphar wrote: I think something might be screwed up with the threading when I got the fittings because one part looks like it is offsetted too much or something.

What do you mean by offset?
zaphar wrote:Would putting something on the threads to make them malleable work?

Yes WD-40 will work if you wipe on both the male and female threads. I belive why this happens is because there is a vacum created behind the projectile and it "sucks" the cap on (feel free to publicly make fun of me if Im wrong on the last statement)
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Unread postAuthor: zaphar » Thu Sep 22, 2005 12:29 am

(1) I didn't have a file but I tried cutting out the little impurfections in the threads on both the male and female PVC parts, but it still seems to jam.

(2) The offset I speak of is what appears to be a imperfection in the molding process, the threads on one side don't match up exactly with the threads on the other. Basically what they filed out of the threads on the burnt latke site. I pretty much cut them away so the threads blend into each other yet it still jams.

(3) I haven't even fired the cannon yet but I understand what you mean about the vacuum thing.

I don't know if it makes a difference but the two parts are a 4" pipe to male threads and a 4" pipe to female threads. Here are some pics of the two parts. Note: I tried cutting down the threads a little because there were too many to begin with but the problem still seems to be there. I think it might be a problem with the female part.

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Unread postAuthor: boilingleadbath » Thu Sep 22, 2005 3:39 pm

My goodness... I think you mistook a perfectly good threaded conector for a "broken" one.

NPT is TAPERED! As you turn the threads, it will get tighter. Typicaly, you can turn it... a few thousand degrees before it gets tight, and then it gets tight VERY fast - you'll notice it getting a bit tougher, and within 500 degrees you'll be grunting for the next 45.

It is normal for the threaded fittings to leave threads exposed when tightened... but now, you'v cut off all your threads that would acctualy do anything. I feel your pain - those 4" fittings are expensive.
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Unread postAuthor: zaphar » Thu Sep 22, 2005 9:37 pm

wait, so it this the male threaded fitting no good anymore? It seems to screw on just as much as it did before. I just want to get rid of any exposed threads so the female back part is flush with the front male threaded part.
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Unread postAuthor: boilingleadbath » Fri Sep 23, 2005 2:16 pm

It's impossible for you to get rid of all (heck, any) exposed threads. I think a picture is in order...
<img src="http://img.photobucket.com/albums/v611/car2/NTPexplan.jpg" alt="Image hosted by Photobucket.com">

As you can see, all cutting the threads shorter accomplishes is making the threads shorter. It does not allow you to thread the fitting on farther, because the threads are tappered - kind of like a cork. (and the reason they are tappered is the same - to allow you to make a water-tight seal.

Also, even the best lubricants won't allow you to put the fitting on more than another turn.
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Unread postAuthor: zaphar » Sat Sep 24, 2005 1:49 am

oh, I understand what you mean about them being tappered, I mean actually cutting the threads off so there are only say about 3-4 threads on it, I cut about half of them off but there is still a gap but I'm not sure if it will stay on if I fire it.
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