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Piston help

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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Piston help

Unread postAuthor: covey12 » Thu Aug 27, 2009 5:11 pm

ok so i have tracked the problem down to my pistons having to much room between them and the casings, i have no access to a lathe so i need to make a non-oring piston, how much space between the piston and case should there be, i have only found breif comments like "very little space" are we talking so little that it barely slides?

and on a side not im working with 1" piston and 1/2" porting now so it will be cheaper if i screw up, whats the best piston you guys can think of, epoxy casted?
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Re: PISTON HELP AGAIN :(

Unread postAuthor: spudtyrrant » Thu Aug 27, 2009 5:28 pm

just use hot glue or epoxy, it will work fine for the piston size your making and its easy to use. by very little we mean less than 1mm, small enough that it slides with a little effort without lube and slides smoothly with lube
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Unread postAuthor: niglch » Thu Aug 27, 2009 5:33 pm

Just wrap your piston in some tape until it is snug in the porting. The more snug the better. If you are afraid that the piston will not move easily enough, just use some grease to lubricate the valve. By the way, what are you using as a pilot valve? You may also not be getting fast enough flow from it to actuate the piston. Blowguns tend to have very little flow and are difficult if you don't have a good seal around the piston. I use a 1/4" ball valve, and with a reasonably quick flick, my pneumatic fires almost instantaneously. I use a 1.5" end cap wraped in artists' tape in a 2" Tee porting.
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Last edited by niglch on Thu Aug 27, 2009 5:35 pm, edited 1 time in total.

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Unread postAuthor: FORE!!!! » Thu Aug 27, 2009 5:34 pm

You want the piston to be relativly tight,but not to tight,you should still be able to push the piston with one finger and move it without applying too much pressure and it should be able to hold its own weight in the pipe,if held vertically.

as for piston material ,epoxy cast is good its nice and light and relitivly easy to make, but without an adequate bumper you may chip/ding the piston when it hits back, but personaly id think a pvc one would be better but considering you dont have a lathe ,epoxy would be a good option..
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I've used various things like cutting oil, silicone lubricant, even butter

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This collection of containers on your bedside table must look very intimidating to any visiting young ladies

...but hey, lube shows you care;)
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Unread postAuthor: jor2daje » Thu Aug 27, 2009 5:38 pm

What material are you making this piston valve out of?
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Unread postAuthor: covey12 » Thu Aug 27, 2009 5:40 pm

thanks for the help, i have two options for the porting , considering that it is a 1" piston, i figured that a blowgun would work, if not i will used a 1" ball valve i have laying around

oh to the last comment i think i have decided to make it out of hot glue or a wooden dowel
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Unread postAuthor: boyntonstu » Tue Sep 01, 2009 7:53 am

According to Theairgunman, Bondo is easy to use, very light, and it works very well.


See Bondo in action here:

[youtube]http://www.youtube.com/watch?v=hBaNXMJHIV8[/youtube]

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Re: Piston help

Unread postAuthor: Technician1002 » Tue Sep 01, 2009 8:10 am

covey12 wrote:ok so i have tracked the problem down to my pistons having to much room between them and the casings, i have no access to a lathe so i need to make a non-oring piston, how much space between the piston and case should there be, i have only found breif comments like "very little space" are we talking so little that it barely slides?

and on a side not im working with 1" piston and 1/2" porting now so it will be cheaper if i screw up, whats the best piston you guys can think of, epoxy casted?


For space, without a ring, very absolute minimum but still free to move easily is best.
Image

If your casing surface is rough, it will pay to sand it smooth. Round and true is best. Piston should be a very close fit. I hope this photo helps. This piston casing was lapped to be very smooth and then the piston turnd down a shave at a time until it just fit. It is loose enough to barely move by gravity. If you have a drill press, this can be done on one in place of a lathe.

For material, something that has high impact resistance and changes little in size and shape with pressure and temperature is best. I like HDPE myself.
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Re: Piston help

Unread postAuthor: dewey-1 » Tue Sep 01, 2009 10:06 am

covey12 wrote:ok so i have tracked the problem down to my pistons having to much room between them and the casings, i have no access to a lathe so i need to make a non-oring piston, how much space between the piston and case should there be, i have only found breif comments like "very little space" are we talking so little that it barely slides?

and on a side not im working with 1" piston and 1/2" porting now so it will be cheaper if i screw up, whats the best piston you guys can think of, epoxy casted?


You may want to review and comprehend these drawings to help you understand the difference between to much gap and very little gap.

http://www.spudfiles.com/forums/viewtop ... tml#258397

It should give a little insight as to why some piston valves have problems.
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Unread postAuthor: geardog32 » Tue Sep 01, 2009 11:25 am

you don't need a lathe to make an o ring groove. i made an o ring groove in my piston using a miter box, saw, dremel, some files and alot of patients.
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Unread postAuthor: Hubb » Tue Sep 01, 2009 12:20 pm

geardog32 wrote:you don't need a lathe to make an o ring groove. i made an o ring groove in my piston using a miter box, saw, dremel, some files and alot of patients.


O-ring grooves can be cut into nearly anything with the correct amount of patients. I've done it using a hacksaw, a dremel, and a file. One can easily see the flaws that may develop in using these methods (namely, unevenly cut grooves). I do have to say that the two easiest ways I've done it would be to use a lathe type devise or a table saw.

The biggest problem with using a table saw is that the groove is usually cut too large (due to the blade, but this creates a "floating o-ring" which may not be bad).

The biggest problem with using a lathe is that most people do not have one. I don't have one but I have been using Tech's method of using a drill press. A cordless drill can also be clamped in a vice and used the same way. The lathe method allows better control over the groove.

As far as this piston goes, hot glue or bondo or epoxy (all of which have been previously named) will work quite well. Below are two links that may help you on your piston making adventure:

http://www.spudfiles.com/forums/making-epoxy-pistons-t8919.html
http://www.spudfiles.com/forums/epoxy-casting-how-to-t8916.html
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