SpudFiles

Hi people! I'm a newcomer here (so I hope I'm not mistaken in the forum section ). Every since I've seen the Ukilliheal's paintball/battery shooting pneumatic machinegun ( I thought that one could make an airgun based on Metro 2033's Tikhar (or Tihar in some spellings; http://metrovideogame.wikia.com/wiki/Tihar). I was thinking about just letting you know when I was finished building it, but I thought that the technology would interest you in this community still.

In a recap, the Tikhar in the videogame is a sniper rifle class of weapon, so, naturally people will usually shoot it as semi-automatic. But it is capable to shoot fully automatically. Until I had watched Ukilliheals video, I thought that this was a gamey thing and wasn't possible with pneumatics, therefore I was quite surprised.

In the game, the Tikhar shoots 15mm ball bearings (therefore one would suppose steel spheres), in the other hand, my design will use the slightly more fragile glass marbles (because they are cheaper) which in spite of large tolerances (needless to say for a toy), are about 1 inch which is slightly larger. Maybe,once finished , I'll try shooting other things that fit the barrel and see what happens...

Ukilliheal's gun (maybe you already know) uses the already familiar sprinkler valve as a Q.E.V./pilot triggered valve, and a special bolt which makes it self loading. What enables it to be automatic is a safety valve (otherwise a valve that opens once certain pressure has build up) connected to the pilot of the sprinkler valve. Therefore, a reservoir (in his case, a compressor tank) will fill the chamber controlled by the sprinkler valve to a certain point where the safety valve opens releasing all the store pressure into the bolt, risen and repeat.

I find Ukilliheal's design quite inefficient because each time the safety valve opens, it will waste reservoir's air to the outside. So, I deviated somewhat from that. In the research I made, I've found out about the "HEAR" valve by GB Semi (which if I'm not mistaken, means "High Efficiency Auto Reseating"; http://www.spudfiles.com/pneumatic-cann ... c7899.html). He also uses the same self-loading mechanism as Ukilliheal and myself (although, we three are building it differently). Nonetheless, I disliked how complex looking and difficult in building his HEAR seemed to be. So I also modified that and in a quite heavy way. This HEAR valve acts as the sprinkler valve, safety valve and a regulator (I guess, must test that to be sure), all in one valve.

One of the tasks I've put up with myself is that I wanted to build this gun using pipings and fittings only. Finding pipes and plastic tubes in my town is not easy, plus, I bet that in the Metro 2033's universe, this is what people would use. I've finished making the bolt (or almost ) and a it's piston, which is drilled nylon rod, along with the HEAR's piston, which is an ABS tube, are the only plastic parts in the build. I'm considering also using a plastic tube for the barrel, but I'm having difficulty finding one in which the marbles fit. apart from that, the stock will be made out of MDF wood.

At the time of writting, I have all the essential parts made but I'm still struggling with the alignment in the bores of threaded unions in which the pistons slide (any tip? Should I tap a pipe instead of using off-the-shelf unions?). Also, I think that I'll be needing O-rings and gaskets, but first I need to buy an adapter for my tank compressor to try that out.

I will be posting pictures once I finish building. Until that, I hope my description will suffice. I welcome any suggestion or tip!

Discussed on this forum before http://www.spudfiles.com/pneumatic-cann ... 22741.html

Will you be trying to replicate the built-in pump too?



For a useful amount of shots, especially on full auto, you're going to need quite a decent reservoir at reasonably high pressures.

It happens that I've missed that thread! Still, it's refreshing to have a new one about actually building it.

It seems to there be a fuss for the trigger pressure, because everyone's thinking in terms of a blowgun. In my design the trigger is a mechanism similar to real weapons, but using a bicycle cable from the trigger position to the "finger" that blocks the HEAR valve (if you read about GB Semi's golf ball launcher, you'll understand). This way gives me plenty of freedom in choosing the positioning of the hand grip.

It is worth mentioning that my first prototype will vary significantly in aesthetics from the game's one. This because the Tikhar, firstly, is vague in how it works, second, whatever makes it work is quite exaggerated in effectiveness, then it seems to feature costume parts which would require me a lathe and even maybe a casting forge to make. Maybe a second version will be more like it if I have the money.
In that regard I'd like to make it bullpup and have magazine, reservoir, chamber and barrel top-and-under so it will look like a board . Because of this, I thought that there wouldn't be space for a pump, but after actually building it, there seems to be a large enough gap between the barrel and the chamber pipe for a shock pump. I'm just worried that it will output too little of a volume of air. The game's pump must be a shock pump by the power it outputs, but it seems to be a regular pump by the meter (supposing it is in metric units), which displays up to 12 Bar plus it will add pressure with each pumping, unusual for a shock pump with a container filled to rated pressure.

OMG! I hope this isn't too much of a TL;DR .

Question: Anyone has tips about fixing some unions which aren't centered equally (I.E. a tube won't flush from inside one to the next)?
Question: That fire extinguisher seems an interesting idea. do you know how much pressure it can hold and/or anything about modding it?

Dig_Gil wrote:It seems to there be a fuss for the trigger pressure, because everyone's thinking in terms of a blowgun. In my design the trigger is a mechanism similar to real weapons, but using a bicycle cable from the trigger position to the "finger" that blocks the HEAR valve (if you read about GB Semi's golf ball launcher, you'll understand). This way gives me plenty of freedom in choosing the positioning of the hand grip.

How do you plan on making that go full auto? It looks like it might "hang" without the trigger sear re-engaging.

there seems to be a large enough gap between the barrel and the chamber pipe for a shock pump. I'm just worried that it will output too little of a volume of air. The game's pump must be a shock pump by the power it outputs, but it seems to be a regular pump by the meter (supposing it is in metric units), which displays up to 12 Bar plus it will add pressure with each pumping, unusual for a shock pump with a container filled to rated pressure.

Some shock pumps will go up to 40 bar, but as you say they have a tiny stroke volume and it will take you forever to get up to pressure, as well as giving you arms like tractors.

Question: Anyone has tips about fixing some unions which aren't centered equally (I.E. a tube won't flush from inside one to the next)?

Epoxy casting.

You can center things pretty well:

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Either that or buy yourself a nice little lathe

You can make pistons on a drill press, Tech has an excellent tutorial here.

Question: That fire extinguisher seems an interesting idea. do you know how much pressure it can hold and/or anything about modding it?

A CO2 fire extinguisher can take at least 70 bar, how you hook it up depends on the actual extinguisher.

I'm back from finally assembling the valves. Wanted to have that done before replying here. That and also, I made a sketch to better explain how I've implemented the concept of GB semi's HEAR Valve. I hope these drawings aren't too large:

This up here is a simplified (for clear understanding) version of my HEAR valve. Along with the mechanism that engages and disengages the Sear (or "finger" as I mentioned before). That sear detail drawing is colour coded in respect to what collides with what. I'm thinking about using acrylic plates to make that mechanism. The green back has a threading so that it attaches to the valve in a convenient way. The black thing with an arrow is the bicycle cable which attaches to the trigger at the grip. As you may have guessed, we can use leverage in these mechanisms and therefore, pressure against the trigger finger is a non-issue.
The mechanism is designed to allow semi-automatic fire by being sort of a ratchet where the pawl forces the sear (a one tooth gear) to rotate and then it slips out of the tooth's range, therefore re-engaging the sear. The axis of rotation of the lever which carries the pawl can be adjusted so that the pawl never lets off the sear, and so enabling auto-fire. The purple lines are rubberbands or springs.

This up here is how it cycles each shot, and so answering JSR's question on how to make it full auto. I admit that the O-ring (orange dots) positioning wasn't well thought out. I was hoping that it wouldn't need much of O-rings, but after testing, this valve failed.

The valve, as-built, has the ends tapered because they kept getting stuck or blocked due misalignments in the fittings. The piston is a tube with both ends corked with nylon and glue. Holes were drilled all around the tube and not just the side facing the chamber and reservoir as the pictures suggest (the picture is a bit naive with how the air flows around the tube). I think that the holes weren't positioned correctly in my piston because when the piston is fully inside the valve, it gets stuck there. Otherwise, the piston shoots out the valve how it's supposed. Actually, I think that this valve might work with the holes right in the middle, but I digress.

Apart from this failure with the HEAR valve, the Bolt works quite well! even without O-rings. It worked successfully at about 7bar (~100PSI). It leaks air like crazy, but it could shoot a marble through a 60centimetre barrel across the room (maybe 2.5metres?). I'd like to compare with a rifled barrel, but I haven't the necessary tools for that. Unfortunately, the fitting's alignment still plagues the bolt too: while screwing it has a sweet spot (or rotation angle) where it won't get stuck.

Epoxy casting.

You can center things pretty well:
(...)
You can make pistons on a drill press, Tech has an excellent tutorial here.

I fancy the epoxy piston tutorial, but I feel more comfortable with Technician's nylon rod method. Next step, I'll redo the HEAR's piston this way.

Fascinating, yet one thing appears odd - if the piston is of constant diameter, whither the pressure imbalance that would encourage it to move once the sear is released?

I re-uploaded your images to spudfiles for clarity:

Dig_Gil wrote:I fancy the epoxy piston tutorial, but I feel more comfortable with Technician's nylon rod method. Next step, I'll redo the HEAR's piston this way.

A sound decision, epoxy casting can be effective but getting it right is often the path to insanity

I'll try to make it clear...

You have to bear in mind that the side of the valve with the sear is open to the outside air. The opposite end is an exhaust to the Bolt (hopefully also at atmospheric pressure).

Most of the time, the piston will tend to move out of the valve. But, at stage 2 of the cycle diagram, it will be stopped by the sear and stick while the chamber pressure is higher than the atmospheric pressure. At this point air can't flow to the chamber from reservoir and the chamber is evacuating, so the piston acts as a membrane or something that follows the side with least pressure. When Chamber pressure is equal to atmospheric pressure, and because of the sear, the piston will shoot back into the valve, causing the exhaust port be closed and the path from the reservoir to the chamber to be opened.
At least that's the theory.

I wonder what Bryce "clide" Bell would think of this...

This is what I meant (click on pic to embiggen):

I don't know... In the test I made, it indeed moved as expected (partially; although it didn't return into the valve), but then it was an improperly built piston so I can't comment on the concept itself. I suppose that (given that it works) when blocked, a piston will behave as if there was infinite pressure on the blocked end. Actually is just an hunch and this explanation doesn't sound right to me. That's why we test things out!

Edit: On second thought, I'd say that maybe the narrowing caused by the exhaust port could make this difference in pressure momentarily...? Maybe I'll end up to have to throw in some springs .

Dig_Gil wrote:On second thought, I'd say that maybe the narrowing caused by the exhaust port could make this difference in pressure momentarily...? Maybe I'll end up to have to throw in some springs

This looks like it could have helped, because physics says your design shouldn't work and had you made it with o-rings I'm fairly certain it would have remained resolutely static.

Another point worth mentioning is the fact that the exhaust port will be connected to the Bolt which is a piston where a spring will make it unavoidable that the pressure in it will be slightly higher than that of the outside.

Dig_Gil wrote:Another point worth mentioning is the fact that the exhaust port will be connected to the Bolt which is a piston where a spring will make it unavoidable that the pressure in it will be slightly higher than that of the outside.

Hmmm... diagram?