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becoming a Pneumatic gunsmith

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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becoming a Pneumatic gunsmith

Unread postAuthor: Marmaladekamikaze » Wed May 30, 2007 4:40 pm

Hello,
I've a couple of questions that are bugging me and was just hoping some of you could help me out.

(1) Is there any rule of thumb when making pneumatics concerning bends, I've had a bad feeling about having a system with two 90 degree bends in pneumatics..like by how much does a bend reduce performance?

(2) what is the effect of ''dead space'' as in the space between the projectile and the valve in a common pneumatic spudder, does it increase or decrease speed?

http://tinyurl.com/27eabm
(3) I was also wondering why the GGDT calculation for the above golf ball gun is showing signs of sucking ass, basically I would like a muzzle velocity of around 150m/s and hopefully the assembly will be about 5 foot long. and also
could someone give me values for the flow coefficient, open time and dwell time for a generic 1 1/2'' modded sprinkler valve please?

(3) I always thought a good C:B ratio for pneumatics is 3:1 but The GGDT program seems to think a 7:1 is optimum for me, and also I always thought that a long barrel was good for accuracy etc. but according to the program my barrel should be shorter than my chamber?
http://tinyurl.com/2hjsfh

(4) I was looking at good hand and foot pumps to fill this future assembly, does anyone have any experience with using both, and if so which is better?
and what valve system is best to use schrader or presta?, I'd like the least amount of gas to leak out when disconnecting.
I was checking this one out
http://www.bikepro.com/products/pumps/silcfloor.html


I did read the ''read this first'' thread but since I had a few questions not exactly related to GGDT, I thought it best to make a thread rather than exchange mails with the creator of the aforementioned thread. Thanks for the offer though!
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Unread postAuthor: lukemc » Wed May 30, 2007 5:07 pm

well i know that bends and dead space decrease performance but by how much its hard to tell.

also your useing a half inch sprinkler for a golf ball gun? i think tat might be your problem try going to a 1" sprinkler
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Unread postAuthor: pyromanic13 » Wed May 30, 2007 5:42 pm

I'm not going to read that until you change the color back to normal... In fact I literally can't

edit: not font, color. sorry
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Last edited by pyromanic13 on Wed May 30, 2007 6:12 pm, edited 1 time in total.
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Unread postAuthor: Fnord » Wed May 30, 2007 5:55 pm

Huh? I dont see any weird fonts.
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Re: becoming a Pneumatic gunsmith

Unread postAuthor: joannaardway » Thu May 31, 2007 2:33 pm

1 - The rule of thumb is to keep bends to a minimum, after you have used the number required to create a practical launcher shape (many people use 45 degree bends to bring the chamber and barrel closer together in an under/over configuration).
They have a small effect on performance, but little enough that you can use as many as is necessary.

2 - Dead space is a "grey area". Too much can lose performance, but contrary to popular belief - a small amount of it can actually aid performance, as it provides an air volume to keep pressure up when the projectile has attained higher velocities, because the flow is less limited than that through the valve.
Keep it down, but don't go to the trouble of making it miniscule.

3 - Your GGDT sucks because your dwell time is wrong. 999 times out of a thousand, you want to set it to an arbitarily high number, like 1000, or 10000, because most valves don't reseal during firing (which is what dwell time represents).

A sprinkler will have an open time of about 10 to 15 ms unmodified, 2 to 5 modified, and a dwell time as explained above.

I recommend going into the options and setting flow coefficent to "Efficency". Most common sprinklers have an efficency of about 20 to 25%. The program will automatically calculate flow co-efficent from the seat diameter (of 1.5" in your case), and efficency, saving you the trouble).

3b - I recommend ignoring the GGDT optimisers - they may list the most powerful/efficent setup, but they typically design an impractical launcher.

A huge C:B ratio produces an inaccurate, loud, inaccurate and inefficent launcher. A small one produces an underpowered one.
I personally use C:B ratios of around 1:1 to 2:1. I consider anything above that to be too inefficent - especially if to be filled by a manual pump. Other's might not.

Here's the general method - choose a barrel length. A long one obviously creates a bigger cannon, but also provides more power and accuracy. But don't expect much from it accuracywise- spudguns are like lightning - they never hit the same spot twice - porting the barrel can help though, but it also limits power (but can actually improve range, due to improved round stability)
Don't make it too big you can't use it though.
Then just make it so that your chamber is about twice the volume of your barrel.
Easy.

4 - What are you making your chamber from? You will be limited on your pressure by the valve and chamber strength - those particular pumps go rather beyond the recommended pressure limits of the pipe.

You could still use them, but don't use them to their maximum if you do.

Neither presta or schrader is "better". They'll both leak a tiny bit, but that can't be helped. Quite a few people use a check valve immediately after the valve (but before the chamber) to help to minimise this.
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Unread postAuthor: Hotwired » Thu May 31, 2007 4:56 pm

I didn't trust either a presta or schrader valve to not leak on its own. I tested both and there was always an air leak when removing the pump head.

I eventually just gutted a presta valve and stuck a check valve in after it so no air would escape when disconnecting.
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Unread postAuthor: Marmaladekamikaze » Thu May 31, 2007 9:24 pm

pyromanic13 wrote:I'm not going to read that until you change the color back to normal... In fact I literally can't

edit: not font, color. sorry


I don't understand? what would I have to do to the colour for you to be able read it?

thanks Joan for the great feedback! I've subsequently tweaked the GGDT like you said.

I know what you mean about the pumps, they do have the capability to burst a lot of sch PVC pipe.
for the chamber I was thinking of using SCH 40 3'' O.D

I was thinking of making the barrel about 1 m long and the chamber .5 m

just out of interest ''hotwired'' could you tell me what make of check valve you used... and also how I may connect the presta to the check valve correctly?

and thanks again for clearing up the confusion Joan!
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Unread postAuthor: Counterstriker » Sun Jun 03, 2007 7:54 am

I also have a question about PSI ..ok..If I have a 2 foot long 2 inch diameter air resevoir that says it could hold 280 psi and a ball valve that can only hold 150 how high can i go?
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Unread postAuthor: Hotwired » Sun Jun 03, 2007 9:06 am

I used one of these ones but the 1/4" one I recieved was totally hexagonal.

I connected the presta valve by locking it into a 8mm compression - 1/4" male bsp fitting.

Cut back the rubber around the presta valve so it can just fit in the nut of the compression fitting.

Find a suitable size washer to put inside the nut to help keep the valve stem in - I had to use my dremel to grind it down a little.

Put valve stem of presta through the washer and the compression nut and screw it down onto the 8mm compression fitting - do not include the olive.

That was then screwed into the 1/4" check valve.
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Unread postAuthor: spudthug » Sun Jun 03, 2007 9:06 am

just out of interest ''hotwired'' could you tell me what make of check valve you used... and also how I may connect the presta to the check valve correctly?


u dont need a check valve for a sprinkler gun.. it will only leak about 1/2-3 psi out and thats no big deal...hotwired used one because it could have pilotted his qev accidently because qevs are sensitive...sprinklers are pilotted from the bottom though...

I also have a question about PSI ..ok..If I have a 2 foot long 2 inch diameter air resevoir that says it could hold 280 psi and a ball valve that can only hold 150 how high can i go?


1 off topic...

2. obviously the lowest pressure rated part on the gun.. 150 psi..
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