SpudFiles

I've recently put together a coax piston cannon with a 2" chamber, 3/4" barrel and 1" piston (all PVC sizes). My piston is only a millimeter or two larger in diameter than the seat but it fired fine using a 1/4" ball valve as a pilot. I tried piloting this cannon with a blowgun but it's not working. I would assume that with as small as this piston is, and as small as the difference in diameter of the piston and seat, it would fire with a blowgun. Below are some pictures to clear it up as well as my temporary testing setup with both pilots attached. Let me know what you all think I should do to make this thing be able to fire with a blowgun because I want to rig up an awesome trigger mechanism for it.

Piston in position


Piston fit


Temporary setup

IT looks like your piston is a little loose in it's housing. Try wrapping a couple layers of electrical tape around it, or make a new piston altogether that utilizes o-rings and a check valve.

The piston is very very tight. Not enough room for a layer of any kind of tape. I would put an o-ring on if I could find one that would fit.

To use a lower flowing pilot, as the others mentioned, you need less piston blow by. If the space is too small for tape, you can try a couple layers of paint.

I custom turned my piston to barely fit but still slide easily. Both the cylinder the piston moves inside was sanded smooth as well as the piston turned to fit exactly.

I will try to reduce blowby of my piston. What kind of paint would work best for adding thickness to my piston?

Try for a type of spray paint that is designed for bonding with plastics. A lot of people swear by Krylon Fusion. Personally I've never failed with Rustoleum Professional Enamel.

If your piston is lathe turned, you can paint it on the lathe. Just use very very light dustings from the spray can and before you know it, you'll have a perfect finish that you can polish further with micro mesh or something.

I wouldn't use spray paint anywhere near the machined surfaces in the lathe. Between the carriage and lathe bed, there are too many places the paint overspray can cause problems.

I forgot to mention to mask off the lathe *facepalm*

If I ever use any liquid on a turning, I always cover my lathe with plastic sheeting and towels.

Have you modified your blowgun for better flow? Also, I know it's too late now, but the elbow is just excess pilot volume and could have been replaced eith a coupling then bushing

I have a wooden dowel that is placed inside of the excess pilot area mentioned above. It has a ~1/4" hole drilled in it so it allows air to pass but reduces pilot volume. I think I am going to rework the piston, maybe even mAke a new one with better tolerances. I would go for a floating o/ring but I can't find the right size

Sorry to interject, but I don't understand how your cannon works if the piston fits inside the barrel. Could you explain that to me?

The piston is shown in the Pilot which is larger than the barrel. The reducer for the pilot is not shown yet in the second photo as well as the sealing face on the piston that would fit against the barrel.

The piston is 1 inch. The barrel is 3/4 inch as stated in the original post. The 1 inch piston won't fit down the 3/4 inch barrel.

I know the pilot looks like a barrel before it is cut to size for the pilot.

It does fit in the 1 inch pilot.

its not the barrel thats the chamber the barrel is in the center of it.

its like the 3 one

Yeah thanks to technician for clearing that up. Sorry for the confusion. I'd really like to know where you all get o-rings because it would be great to be able to add a floating o-ring. Then I might even be able to make a check valve ( would like info on ways to make these as well) and get this thing to fire from a blowgun pilot. I have a great trigger/handle system set up but it needs to be able to pilot from a blowgun. I will update this post with pictures of the trigger when I can and I'm looking forward to help on the o-rings and check valve.

I pick up o rings in the plumbing section of my local hardware store. The big box stores don't have bins of hardware. They carry a very limited selection all barcoded and such. Avoid them. Find a hardware store with bins of screws, bolts, nails, and o rings.

Another place to try is a Hydraulic supply house. A place that has repair parts for hydraulic systems and makes replacement hoses and such is another good source.

If in the US, the 1-1/16 inch ring fits my marshmallow cannon. You may need a slightly smaller size for the PVC instead of iron pipe. At about 20 cents US per small ring, I generally pick up the several near sizes and use one that fits best.

The size I have trouble finding is one to fit the 2 inch ABS pipe of my DWV cannon. They are either too small or too big.

You can order from here if you don't find a local supplier.
http://www.marcorubber.com/sizingchart.htm