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Machining a QDV

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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Machining a QDV

Unread postAuthor: FighterAce » Sat Jun 11, 2011 8:09 am

I'm soon getting a mini lathe and my machinist is kind enough to teach me how to use it. I designed a QDV which encompasses just about every lathe operation I will be using on it... turing to outside/inside diameter, inside/outside threading, facing, parting, internal/external grooves...


The QDV is made up of 7 parts... the inner part (yellow) will be brass. It houses the piston, has 25mm threads on one end to attach the breach and O rings to seal against the aluminum shell (white). Two aluminum plugs (green) hold the inner part in place. Ports will be 3 equaly spaced ⌀6 holes. Maybe I should file them to rectangles... I'm not sure. The piston (red) will be ⌀10 polyethylene with 2 O ring grooves for 6x2mm O ring. Connected to the piston via M3 thread is a stainless steel rod (gray). Almost all of the walls are 5mm thick so I should get about 1000psi working pressure with an adequate safety margin. Bumper (white) will be a sandwich of 2 pieces of polyamide and a spring. It is not to the right length in the model.

Modeled everything in cross section except the piston and the metal rod. Program is google sketchup and no it doesn't have hatch lines included... I've been looking for a plugin to add them but had no success so far.

Image
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I'm just not sure about 2 things... Should I round the shoulder where the 20mm piston housing goes up to 40mm (toward the O rings)? And is it possibile (and how) to machine those 3 inner O ring grooves in front of the piston. Those O rings are 14x2mm (14mm is the ID) for the 14x2 (10mm ID) copper barrel.

Theres no doubt in my mind someone will argue with the way this thing comes together and that this can be built in more than one way. I have no objection to hearing it since I do tend to overcomplicate things in my designs :D

btw. I keep thinking I saw the exact same thing here on spudfiles with threads and O rings working together but I'm not sure. Has anyone else used this design?
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Unread postAuthor: Labtecpower » Sat Jun 11, 2011 10:39 am

From what I know about lathe tools, machining the inner 3 grooves will be pretty hard, especially since there is little room in your QDV.

I suggest that you connect the piston to the rod in a similar way as I and Technician1002 did. It will give faster opening times, and a safer gun, because there are no sharp rods launched backwards.
This makes it a little more complicated, but you seem to like that :D

This QDV should give some very nice results when finished :)
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Unread postAuthor: Gippeto » Sat Jun 11, 2011 10:45 am

Striker actuated qdv? :)

For the most part, I think the design shows good thought and sound practices. You shouldn't need more than one o-ring to seal each piece though...if the first one seals...what's the second one doing??

Applying a radius to the inside shoulder of the piston housing will enable it to better handle stress, easy enough to do, but not really needed in this case.

The o-ring grooves can be made with a boring bar, or form tool...your machinist will be familiar with form tools...ask him about it. Again...one is enough.

Good work. :)
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Unread postAuthor: FighterAce » Sat Jun 11, 2011 11:15 am

Labtecpower wrote:I suggest that you connect the piston to the rod in a similar way as I and Technician1002 did. It will give faster opening times, and a safer gun, because there are no sharp rods launched backwards.


Totally forgot about that... Thanks!

Labtecpower wrote:This makes it a little more complicated, but you seem to like that :D


Actually it makes it simpler because I can use a smaller diameter rod and not worry about piston wall thickness :D

Gippeto wrote:You shouldn't need more than one o-ring to seal each piece though...if the first one seals...what's the second one doing??


The second one is keeping leaks at bay when the first gets shredded by the threads :D I hope it wont come to that... but still its nice to know you have some redundancy.
On the barrel sealing O rings there will be a very sloppy fit. I've had just 4 O rings in my latest breech system and it leaked. Now I have 7 O rings for 100% seal while keeping the action smooth... ie. its not a put-it-in-a-vice-to-get-it-out fit

Gippeto wrote:Applying a radius to the inside shoulder of the piston housing will enable it to better handle stress, easy enough to do, but not really needed in this case.


I thought its necesarry because of the air pressure...
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