Copper Marble Cannon

[cometboy]

Hello All,

So I have been moving along on a copper cannon and have run across another small mystery that I am hoping can be illuminated by the collective mind at SpudFiles.

The cannon is copper, 1 1/2 inch chamber, 1 1/4 inch tee and 3/4 inch barrel with a piston valve.

It has to do with the pilot valve. I had planned for the first tries at low pressure to use a Home Depot blowgun. But I was not able to use it as it did not trigger the piston. If I replace the blowgun with a 3/4" sprinkler valve, everything works well.

The piston is a piece of lexan tube, filled with hot melt glue and a rubber seal at one end. I originally had a groove cut in one edge to act as a fill for the chamber, but filled it with more glue after the blowgun pilot did not work.

The Lexan is a beautiful slide fit and moves very smoothly. the pilot volume is about 2 cubic inches. I am running tests at about 100psi and plan to use the cannon at 200-300 psi.

With all the wild looking plumbing on pilot designs that I saw on the other copper cannons, I didn't think that area of design would be too critical.

Obviously I was wrong.

Any suggestions?

Thanks.

[Gippeto]

If it activates with the sprinkler...differential area isn't the issue.

I have a 2" piston without o-rings that pilots with a modded blowgun...so it's not the piston size.

2cuin pilot volume sounds excessive...total piston travel should be no more than 1/2 the barrel diameter IMO.

Could be a combination of excessive pilot volume (for which Jack will surely chastise you ) and excessive flow past the piston.

Try reducing the pilot volume, and "tightening" the fit of the piston.

[cometboy]

Gippeto,

Thanks for the information. BTW, if this design does not work out, I'll be checking out a clone of yours.

I will try reducing the pilot volume.

The fit of the piston is pretty nice, though. Copper ID is 1.275 and the piston OD is 1.250". I can easily move it with a modest breath.

Tell me about modified blowgun, mine is cheap stock.

Another question about good sources for refrigerator compressors. I called a few recycle places in town and they will not even consider letting me have a discarded unit. Strange.

Cheers.

[jackssmirkingrevenge]

The Lexan is a beautiful slide fit and moves very smoothly.

Usually this is the problem, can you put a numerical value on "beautiful slide fit"?

2 cubic inches pilot volume is terrible, but I've seen much, much worse on this forum - I think the problem is that more air is leaking past the piston than you imagine.

If you can take some accurate measurements we cando the math

[Gippeto]

25 thou is too much. Should be closer to .004" difference.

Modded blowgun isn't hard. Take it apart, and look at the air passages...make them as big as you can...if you lucked out and have a brass oriface (some are plastic) smooth things out further with a dremel...just take off any sharp edges that are in the flow path.

This will increase flow fairly substantially.

Fridge compressors can be had on ebay...or check with an appliance repair place.

[jackssmirkingrevenge]

Ah, hadn't seen the actual numerical value had been posted. Yes, it's too loose.

Currently you have 0.05 square inches of area through which air can flow from the chamber to the pilot.

This means that your pilot exit diameter needs to be at least 1/4" for equivalent flow, difficult with a blowgun.

I would suggest tightly wrapping some duct tape or masking tape around the piston until it's a good fit, not jammed airtight but loose enough that you can move it with one finger.

[cometboy]

Gippeto, JSR,

Thanks again for the great information. I played with tape around the piston, no real payoff.

A 1/4" ball valve, though, works well. I ran a quick test at 100psi, which is about the limit of my compressor and the cannon pushed a 6 gram slug of delrin up to a very satisfying hit on a two by four. It left a half inch deep dent in the wood, so things are looking promising.

I will start looking at the appliance repair route for a compressor and hopefully get up to about 200-300 psi.

I will also look into getting a custom polypro piston turned for the unit. It might be problematic getting a .004" fit as I am pretty sure the copper bore of my unit is not quite round.

The cannon looks promising enough that I will start getting the support structure together. This will allow me to make some speed measurements.

Hopefully will get some high speed pictures in December.

I will try and publish a few here.

Thanks again for all the help. It must be a pain answering the same dumb questions over and over again.

In my area of high speed photography my favorite beginner question is, "What kind of camera are you using to get these pictures?"

[jackssmirkingrevenge]

I am pretty sure the copper bore of my unit is not quite round.

I've had that problem in the past with copper pipe, what I did withthis project was inlet a syringe tube as a piston support structure and fill the gap between that and the copper with epoxy.

Thanks again for all the help. It must be a pain answering the same dumb questions over and over again.

In my area of high speed photography my favorite beginner question is, "What kind of camera are you using to get these pictures?"

Not at all, we've had much dumber questions

I'm sure many people would want details of how you achieve such amazing shots if you're willing to share, at least some basic details of your setup, unless it's a trade secret of course.

[Gippeto]

Eh. Questions asked in an intelligent manner aren't so bad.

If you're going to have the piston made...why not have the tube in which it will reside made as well? Brass pipe bored and turned to suit comes to mind as workable and cost effective.

[cometboy]

Gippeto,

It's a good suggestion.

I am probably going to let things be as they are. the whole mess appears to be working pretty well.

As I mentioned at the outset, I am kind of fixated on the photography. The cannon is just a tool.

Normally I spend time trying to optimize things, in this case, 700fps is what I wanted and I already have it.

My next task is to try and minimize the decompression cloud. I noticed it with the crude PVC cannon and it kind of screwed up quite a few of my pictures. My target is only a foot away from the barrel (to keeep aim).

I may try and find a good high pressure N2 tank.

Unless you all have any other ideas.

Honestly, getting everyone's input has been fantastic. I would still be flailing about without it, probably for a few more weeks.

Cheers.

[jackssmirkingrevenge]

My next task is to try and minimize the decompression cloud. I noticed it with the crude PVC cannon and it kind of screwed up quite a few of my pictures. My target is only a foot away from the barrel (to keeep aim).

I may try and find a good high pressure N₂ tank

You'll always have some moisture in the tank. Some sort of suppressor is in order, on a basic level just drill some holes in the muzzle end of the barrel.

[jackssmirkingrevenge]

'tis the season to be jolly, tra la la la...

[cometboy]

jsr,

Have not forgotten about pictures. I have been sidelined with photographing another mode of destruction.

But I will be setting it up and when I do I will forward pictures.

So many things to destroy and so little time.

Cheers.

[jackssmirkingrevenge]

So many things to destroy and so little time.

Such is the curse of this hobby looking forward to what you come up with.

[cometboy]

JSR,

So after a day trying to get my old cannon set-up running I got some first results. Aim is bad, velocity low and timing is off.

I only had three apples. Apples are kind of my calibration.

120PSI velocity 650fps. Nylon slug, a cylinder @7 grams.

If you can give me instructions on how to post an image, I will do that.



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