SpudFiles

Ragnarok wrote: I'm a little surprised at the mixing of metric and imperial units, but we'll move on.

that's how physics is taught here in the u.s. not the most efficient method I'll agree but its the way things are done

lozz08 wrote:Also, for my piston I was just gonna have a bit of pvc with an o-ring around it, like a plunger. Is that viable?

If you do it right, it should be, yes.

Actually, you might find these pages interesting. They're Nerf guns, but the same basic principles apply, and they might give you ideas:
http://www.boltsniper.com/BS-8/BS8.htm
http://www.boltsniper.com/BS-9/BS9.htm

His FAR is the reason I started doing this. My gun will be exactly the same, just a bb version.

650 fps is easily obtainable. Hell, if an electronic motor and gearbox can cock a 500 fpsish configuration, then there's no reason you couldn't quickly and easily cock a 600 fps one.

If you can avoid it, don't use an o-ring. For the (not telling) I'm building it uses a lightweight nylon piston with flared edges to seal with low friction. In the configuration I'm using, the O-ring would just end up destroyed before the piston even reached the end stroke of the first shot. (it will be moving at around 1100 + fps).

Good luck

how could I get my hands on one of these nice nylon thingies? Very creative. Some airguns use leather funnily enough.

My nylon piston is machined from nylon stock. Really only an option with a lathe or similar.

It will not create a perfect seal, but the pressure differential will be fairly low and minimal friction is required.

inonickname wrote:My nylon piston is machined from nylon stock. Really only an option with a lathe or similar.

It will not create a perfect seal, but the pressure differential will be fairly low and minimal friction is required.

If making a small diameter piston without rings, it is a good idea to lap the pieces together so the surfaces are very smooth with little friction. It is how I built the mouse musket. I used valve grinding compound and lapped the cylinder the piston rides in until it was perfectly round. With turned pistons to fit, the blow by is very low. It is low enough that if any grease is used, the piston requires an EQ port. Grease will seal it that tight.

Wow. Impressive stuff. I'm not 100% sure what you mean about the lapping though. Shouldn't a cy200linder of pvc be pretty perfectly round anyway?

If the piston is moving through that thing just by gravity... And is airtight... Wow. Nice job.

Anyone got a link o an explanation of the hammer valve? \Can't find one, search here doesn't work for me. Its strange, no matter what I type, it returns no results.

lozz08 wrote:Anyone got a link o an explanation of the hammer valve? \Can't find one, search here doesn't work for me. Its strange, no matter what I type, it returns no results.

Use the advanced search, not the quick search.

lozz08 wrote:Wow. Impressive stuff. I'm not 100% sure what you mean about the lapping though. Shouldn't a cy200linder of pvc be pretty perfectly round anyway?

If the piston is moving through that thing just by gravity... And is airtight... Wow. Nice job.

Lapping is the process of making two mating surfaces flat to each other by using a grinding compound in between and rubbing them together to remove imperfections.

In automotive use, the intake and exhaust valves are lapped against the seats so they mate perfectly. A valve grinding compound is used which is grease with the abrasive that sandpaper uses in it. After the valve is lapped, the grease and abrasive are washed away.

In the cannon photo above, it was lapped to a 1 inch wooden dowel that had a sheet of self stick sandpaper attached. With lots of rubbing, the inside of the pipe lost the galvanized iron pipe look and took on the finished machined look shown in the photo. It is smooth and seals well.

And yes, without air pressure in it, it can be opened with just a gentle shake. Pipe is round, but often far from a perfect inside surface is the norm. I polish off the high spots until they widen and reach the low spots. Then the pipe has a smooth surface with no low leakage gaps.

More info on valve lapping..
http://www.v6z24.com/howto/valvelapping

Polishing rocks with an abrasive is called lapidary.
http://en.wikipedia.org/wiki/Lapidary

I do have a short low grade video I posted on this.
[youtube][/youtube]

Thanks a lot for the info, it is especially important to me because this will allow me to make airsoft barrels from tubing with 6mm ID- I can lap it to about 6.1 or just under and make it perfectly smooth for the bb at the same time, so it will lose the least velocity it can going through the barrel.