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Unread postAuthor: wyz2285 » Tue Sep 25, 2012 12:01 pm

21% of chamber volume. When I was talking about damage the chamber I meant corrosion.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: shardbearer » Fri Nov 16, 2012 10:26 am

Are PT threads BSPT or NPT?
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Unread postAuthor: Lockednloaded » Tue Nov 27, 2012 11:32 pm

Looking to build a large bore hybrid, and I've got a safety concern:

If I'm using a 5x mix with a 14" x 2" chamber, would a 5 foot 1.5" sdr-26 barrel survive if I sleeved the first 2 feet with 2" sch 80 PVC?
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Unread postAuthor: MrCrowley » Wed Nov 28, 2012 12:35 am

It will definitely burn IMO. I don't think the sdr barrel will last long because it will deform, don't think it will shrapnel however. Considering I used ABS pipe at 10x mixes, without signs of burning whatsoever, I can highly recommend that for 5x mixes. Keep in mind that the wall thickness of my ABS barrel was 6mm and the pipe was rated for pressure (roughly 220PSI I believe).

If you still go ahead with it, I'd definitely sleeve the entire barrel. I don't use my ABS barrel any more and probably will never use it again, it's just not worth the risk and dampens the experience when you're worried about the barrel.

I managed to find aluminium pipe that was perfect for golf balls, might be a bit tricky finding ABS or steel pipe for golf balls as I never found any.

TBH, I get bored of shooting golf balls from the hybrid. I can't even see them if I'm aiming at something only a few hundred meters away, they just shoot off and curve in any odd direction. If you want to go for distance whilst also trying to hit something in the distance, I'd try making some cheap PVC rockets.



____


I have a question of my own. Whilst wiring up my new ignition circuit, I tested the stun gun circuit sporadically throughout the assembly and it worked perfectly. I didn't even wire the stungun up to any other components, I just did tests with a 12v battery to make sure it still sparked. When I put it in a project box however, it stopped sparking all together. It's done this in the past but I've usually got it working again pretty quick. I also noticed that the 12v battery is now really hot and down to ~7.5-8V but using a new 9V battery doesn't get the stungun working either. Any ideas what's going on here?
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Unread postAuthor: Lockednloaded » Wed Nov 28, 2012 12:42 am

Mr.C, good to know, I think I'll go with sch10 aluminum pipe. IIRC D_Hall used it for golf balls on a high pressure pneumatic. As far as ammo is concerned, I don't really need to hit anything at a considerable distance. I've got a friend who's got a huge ranch with a golf course and lake on it, so we'll mainly just lob them off into the distance or destroy some things at close range.
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Unread postAuthor: PaperNinja » Wed Nov 28, 2012 7:12 pm

About how much psi is there per mix number with hybrids? I want to know how far I will...and won't be able to go.


Also, will the connection between the spark plug and the end be hazardous pressure wise? Or not, is a threading a threading?
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Unread postAuthor: MrCrowley » Wed Nov 28, 2012 7:15 pm

PaperNinja wrote:About how much psi is there per mix number with hybrids? I want to know how far I will...and won't be able to go.

For each mix, the chamber will have ~0.6PSI of propane added to it or ~0.7PSI of MAPP gas if I recall correctly. For each additional 'mix' above 1x, you add roughly 14.5PSI of air on top of that. So the pre-ignition pressure for a 10x propane mix is in the region of 140PSI. This will produce roughly 1000PSI when ignited.
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Unread postAuthor: PaperNinja » Wed Nov 28, 2012 7:18 pm

So you multiply by about 10 to get the ignition pressure? Thanks for the quick reply btw

On an unrelated note, does anyone know the name of non-lead good .177 pellets?

Edit: nvm those weren't so hard to find... still wondering about the spark plug thing tho
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Unread postAuthor: MrCrowley » Wed Nov 28, 2012 7:20 pm

PaperNinja wrote:So you multiply by about 10 to get the ignition pressure? Thanks for the quick reply btw
Roughly. I don't think our hybrids will produce as much pressure as HGDT says simply because our mixing isn't always spot on and other factors like that but 800-1000PSI can be expected from a 10x mix propane/MAPP hybrid I would say.
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Unread postAuthor: PaperNinja » Wed Nov 28, 2012 7:37 pm

Thank you



More worries... will the high pressures of 20-30 X "shear" or flatten threading and have the pipes burst apart? Or does that not happen?
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Unread postAuthor: saefroch » Wed Nov 28, 2012 8:26 pm

At sufficiently high pressures, a failure will occur. Do some quick reading via wikipedia on shear stress/shear modulus and tensile stress/Young's modulus. There's also compressive stress/bulk modulus, but at the level most of us are at we don't bother considering it.

If you're wondering if your specific build will fail, you need to know the material and wall thickness. Threads are usually not an issue, they're designed so the pipe walls will fail first.

If you're wondering about the failure mode of metals versus plastics, I can tell you that metals are far favorable because they tend to just split open and deform a little along a weak section, instead of bursting into shrapnel like most plastics.
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Unread postAuthor: PaperNinja » Wed Nov 28, 2012 8:30 pm

I plan on using schedule XXH steel pipe. It's good to know that metal doesn't blow apart, though I don't plan on it's rupturing anyway :?

I hope you're right about pipe breaking before threads, because my pipe isn't breaking... but I'll read on wikipedia.

Also, how will I know if my spark plug threads are as deep/wide and stuff as my pipe threads? Just by the size, assuming thicker schedules change ID, not OD?
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Unread postAuthor: saefroch » Wed Nov 28, 2012 8:54 pm

I have no idea what XXH is. You should really be good with sch 80, but again I'd use an online calculator or have somebody here spit out some numbers for you.

Few spark plugs have NPT threads. You'll have to look on ebay if you want to find any. I found some 1/2" NPT spark plugs there a while back.
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Unread postAuthor: PaperNinja » Wed Nov 28, 2012 9:38 pm

What threads do spark plugs tend to have? I'm sure (hope) there's an adapter on mcmaster.

and XXH is the next level past 80, rated to 6000 psi. Theoretically it should be good up to 40X, but I wouldn't use it past 30X just because I'm paranoid.
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Unread postAuthor: saefroch » Wed Nov 28, 2012 10:41 pm

Manufacturer ratings are usually extremely pessimistic. Do you have any idea how much sch XXH pipe weighs?

I think spark plugs typically have machine threads, which are in no way compatible with pipe threads. I was unable to find an adapter anywhere.
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