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Best homemade alternative to SV ?

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 4:59 am

Trial and error is what you're stuck with if you don't do the math ;)

The easiest solution I can suggest is progressively wrapping the piston with tape (duct tape might be a bit too thick, maybe try masking tape or ordinary clear tape) until it's tight enough that it seals but you can still push it with one finger, it's an arbitrary rule of thumb but it usually works.

One other thing, did you mod the tee with your improvised bore-thingy, and if so, are you sure it's a perfect circle?
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Unread postAuthor: Crna Legija » Fri Nov 19, 2010 7:36 am

if you cant find a something that fits, your best bet is to cast. Hot glue or epoxy are best but you can use car body filler but it wont last very long.
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Unread postAuthor: Technician1002 » Fri Nov 19, 2010 12:40 pm

Should have built like the lower part of the illustration.


The slightly smaller valve seat is a good idea. The bad idea is to neck the piston down in size. This can cause poor alignment and reduces strength. Leaving the piston full diameter the full length is better. It supports the seal better and can't get turned sideways out of align. An o ring if you can add one works wonders. This piston is a sticky in the How-To Database.
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Unread postAuthor: Selador » Fri Nov 19, 2010 2:15 pm

Well, I got it to work, after a fashion.

I wrapped clear packaging tape around the piston, until it is very tight in the bore.

So tight, in fact, that if the bore didn't have a thin film of silicone grease in it, it wouldn't move at all.

I was under the obviously wrong impression that in order for this particular kind of piston to work, it needed to at least slowly fall out of the bore, if turned on end.

This bugger is STUCK. Turn the valve on end, and you still have to force the piston out with a stick, from the barrel end. Not just a push. You have to -force- it.

I wouldn't have thought it would work at all, like this. All I can think of, is that JSR was correct in his last thought. I caused an irregularity in the piston bore.

Consider me embarrassed, but learning.

:)

~~~~~~~

Tech, did you use a punch to cut those circular pieces out of the rubber sheet ?
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 2:51 pm

Oxbreath wrote:Consider me embarrassed, but learning


No embarrasment in learning.

As to the force requred to move the piston, given your piston is 3/4" and the barrel is 1/2", then the area of exposed piston (where the air can act on to open it) is 0.245 square inches, at 100 psi that's almost 25 lbs of force trying to open it on vacating the pilot so friction is relative.

I would propose you consider the casting route, if efficiency is your ultimate goal and you're unwilling to just buy a QEV :)
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Unread postAuthor: Technician1002 » Fri Nov 19, 2010 3:29 pm

With piston friction, a low friction piston opens much faster and is less likely to stick open or closed. It is one of the reasons I like o rings in larger pistons and lapped seals in smaller pistons.

I caused an irregularity in the piston bore.


Have you considered sanding it true? Sandpaper on a dowell works great to remove the high spots until the high spots are the same as the low spots.

This cylinder bore was sanded to make it true for a close fit. The piston can move by gravity.
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Unread postAuthor: Selador » Fri Nov 19, 2010 3:48 pm

jackssmirkingrevenge wrote:
Oxbreath wrote:Consider me embarrassed, but learning

No embarrasment in learning.

I would propose you consider the casting route, if efficiency is your ultimate goal and you're unwilling to just buy a QEV :)


Exactly so.

And you can be noncy about learning. Or you can ask, and accept the hard questions/statements. The ones that get right to the core of the matter.

I appreciate that I have gotten the direct questions and advice that I have gotten here. Also appreciate that no one has mistaken my own direct approach, for any sort of aggression/insult.

~~~

If I make another 1/2" piston valve, I will either go with the casting route, or the washers and o-rings that I have seen some others here, do.

As for buying a qev, I will, as soon as I can afford one. I'll want to build a small metal spudgun, at some point. I figure the qev will allow me to keep it as compact as possible.

~~~

Tech, Good idea. I may come back to this one in the future, and try to true it up.

I had to laugh, though. Because sandpaper and a dowel rod is how I got the bore into the untrue state it is in now. Albeit, applied differently than you meant.

Lapped seals ?
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Unread postAuthor: Technician1002 » Fri Nov 19, 2010 4:09 pm

Lapped simply mean ground together. The piston was too big to fit originally and with some fine grit in grease the original pistons were shoved in tight and rotated, moved in and out until they just barely fit, then the surfaces were cleaned. Car valves are ground to shape, then with a valve grinding compound, the valve and seat are ground against each other to make a perfect metal to metal seal.
This video does not show the addition of the valve grinding compound.
http://www.youtube.com/watch?v=fhXsH12Rg6s

I used marker like they mention in this video to see that is is even.
http://www.youtube.com/watch?v=96a44QxyCRk

On one of my old broken pistons you can see the worn off sharpie pen marks.
Image

Here are two more marks on the piston. The piston is a PVC pipe cap. The face is smaller than further back which is why the marks remain at the nose, but are sanded off further back. It was slightly out of round. Forcing it into the hole while sanding, made it round and sanded the wide part down so it fit very well. This made a very tight pound it in the hole fit into a nice lose fit with tight tolerances.
Image

The lapping sanded the hole round in the process too. The initial sanding was done with sandpaper on a dowel. It was finished with the lapping.

It is quite dirty after a year of use. Before the dirt was added, the sharpie marks showed the piston wear as it became the right size to fit well.

Not many spudders go to this level of detail to get tight tolerances.

1 inch PVC pipe cap inside a 1.25 inch pipe.
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