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GeckoSub Mirage Evo - And Adventures in 3D Printing Speargun Parts
- Thread starter Diving Gecko
- Start date
Hi @Diving Gecko,
Apologies that my handles are clogging up your epic thread.
I've been following your progress here and in total awe with what you are achieving. You're a true inspiration for hobbyists like me, showing what can be done with some ingenuity and perseverance.
Good luck with your build!
Apologies that my handles are clogging up your epic thread.
I've been following your progress here and in total awe with what you are achieving. You're a true inspiration for hobbyists like me, showing what can be done with some ingenuity and perseverance.
Good luck with your build!
Thanks man! It's been a minute...
Right now, I am deep into another freedive related project that's taking all my time but I'll work on the speargun from time to time and update here
Klingt nach einem spannenden Projekt! 3D-Druck für Speargun-Teile ist definitiv nichts für Anfänger. Ich habe bei meinen eigenen DIY-Ideen gute Erfahrungen mit ChatGPT Deutsch gemacht – super hilfreich beim Formulieren von Anleitungen oder beim Lösen technischer Fragen. Nutzt du CAD-Modelle aus Vorlagen oder alles selbst entworfen?
Hello there,
I designed everything myself. Well sort of. Generally, I want to modify a lot but also, some parts I can't make myself, yet - mostly handles, shooting barrels, pistons, muzzles and reservoirs. But I do make a lot of the smaller parts - and soon, I will make a whole handle from scratch.
But for the remaining parts - and especially the ones you see in this thread that are either turned on the lathe, 3D printed or CNC machined, I did design those myself.
I don't use AI for the design but I do ask it sometimes for CAM (machining operations) advice as the CAM is what I have the least experience in. I didn't need CAM until I got the CNC about a year ago, whereas I learned CAD to be able to design the parts for 3D printing and manual turning.
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Another Little Breakage...
I have been super busy with some other projects and a bit of photography work, so have been out of the water for too long and away from the speargun building. But I took the gun out and it works now - at least at lower pressures. (As for lower pressures, I will elaborate on that in a later post).
But then the gun started leaking through the channel in the handle for the power regulator - but only when the knob was in the most rearward position. So, I took the gun apart again and found this:
The plastic bushing that sits right behind the power regulator had exploded:
Now, why did this happen?
I've explained why when you use this type of power regulator in a Mirage type gun, it opens violently after the preloading and slams towards the back due to the large pressure differential. Also, Tomi mentioned that the longer it has to accelerate, the bigger the impact and I guess the bigger the mass the more impact, too.
Anyhow, so far, I think the plastic bushing hasn't actually been the "rear stop" - I think it has been the bend in the regulator rod, outside at the rear of the handle where the control knob is. At least, I have seen the knob bend more and more with use which I didn't like. To stop this I ground out the channel the control rod runs in some more, to add clearance to stop that banging. But now that the knob doesn't stop the rod, the plastic bushing inside the gun is instead taking the impact and it's clearly not made for that.
I could have made a new one in plastic, perhaps added an o-ring to work as a bumper but I went another way and made a brass bushing instead. Let's see what breaks next... If anything I speculate that the Loctite in the bushing at the very front may give up and I may have an issue with the front bushing (the very valve) unscrewing itself.
But that's a potential problem for future David.
I have been super busy with some other projects and a bit of photography work, so have been out of the water for too long and away from the speargun building. But I took the gun out and it works now - at least at lower pressures. (As for lower pressures, I will elaborate on that in a later post).
But then the gun started leaking through the channel in the handle for the power regulator - but only when the knob was in the most rearward position. So, I took the gun apart again and found this:
The plastic bushing that sits right behind the power regulator had exploded:
Now, why did this happen?
I've explained why when you use this type of power regulator in a Mirage type gun, it opens violently after the preloading and slams towards the back due to the large pressure differential. Also, Tomi mentioned that the longer it has to accelerate, the bigger the impact and I guess the bigger the mass the more impact, too.
Anyhow, so far, I think the plastic bushing hasn't actually been the "rear stop" - I think it has been the bend in the regulator rod, outside at the rear of the handle where the control knob is. At least, I have seen the knob bend more and more with use which I didn't like. To stop this I ground out the channel the control rod runs in some more, to add clearance to stop that banging. But now that the knob doesn't stop the rod, the plastic bushing inside the gun is instead taking the impact and it's clearly not made for that.
I could have made a new one in plastic, perhaps added an o-ring to work as a bumper but I went another way and made a brass bushing instead. Let's see what breaks next... If anything I speculate that the Loctite in the bushing at the very front may give up and I may have an issue with the front bushing (the very valve) unscrewing itself.
But that's a potential problem for future David.
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Losing Weight - Chapter 1
When I made the aluminium nose cone I said I didn't care about it being heavy. In fact, I didn't care that the whole gun was heavy. I just needed it to work and use it as a base to prove out some other parts. But now that the bulkhead seems to work and I have dived with it a bit more, I am getting tired of it being this heavy.
So, I will make 2-3 weight saving improvements:
1). I will make a new, lighter nose cone
2). I will swap the stock Salvimar alu reservoir for a carbon fiber one
3). I may turn down the OD of the Pumping Barrel and possibly the Shooting Barrel, too
The last point depends on how heavy it is after 1 & 2 and whether I want to put a reel on this gun.
So, on to the new nose cone.
Here's the current one as a reminder:
I think it weighs about 75g.
Here's the new one:
It still feels really chunky but it's lighter even than the original Mirage:
And here's the new nose cone mocked up on a CF reservoir with a Vuoto muzzle - but I will be using my old, trusty UBL muzzle:
So, a few note worthy points to address (possibly, haha).
The nose cone is as short as possible on the outside. Normally, nose cones protrude forward of the reservoir a fair bit which allows for them to be more easily grabbed during assembly and disassembly and it also adds more room for placing sights and shooting line hooks.
But despite me liking those advantages and sometimes the looks - like on the Predathor nose cone - I always wanted to try this more "utalitarian" approach. And honestly, I think it looks cool.
I did add a line guide and the sights will just have to go on top of the reservoir instead.
But the main downside with this minimalistic nose cone is that it's really hard to grab for disassembly. This is why you see two small M3 threaded blind holes on the front face. The idea is that I can screw in two small eye bolts with a piece of dyneema between them to pull the nose cone out.
Also, since the new nose cone is in effect shorter (by 46mm), I can use a longer CF reservoir which also adds a bit extra buyoancy.
Some more nerdy pointers in no particular order:
When I made the aluminium nose cone I said I didn't care about it being heavy. In fact, I didn't care that the whole gun was heavy. I just needed it to work and use it as a base to prove out some other parts. But now that the bulkhead seems to work and I have dived with it a bit more, I am getting tired of it being this heavy.
So, I will make 2-3 weight saving improvements:
1). I will make a new, lighter nose cone
2). I will swap the stock Salvimar alu reservoir for a carbon fiber one
3). I may turn down the OD of the Pumping Barrel and possibly the Shooting Barrel, too
The last point depends on how heavy it is after 1 & 2 and whether I want to put a reel on this gun.
So, on to the new nose cone.
Here's the current one as a reminder:
I think it weighs about 75g.
Here's the new one:
It still feels really chunky but it's lighter even than the original Mirage:
And here's the new nose cone mocked up on a CF reservoir with a Vuoto muzzle - but I will be using my old, trusty UBL muzzle:
So, a few note worthy points to address (possibly, haha).
The nose cone is as short as possible on the outside. Normally, nose cones protrude forward of the reservoir a fair bit which allows for them to be more easily grabbed during assembly and disassembly and it also adds more room for placing sights and shooting line hooks.
But despite me liking those advantages and sometimes the looks - like on the Predathor nose cone - I always wanted to try this more "utalitarian" approach. And honestly, I think it looks cool.
I did add a line guide and the sights will just have to go on top of the reservoir instead.
But the main downside with this minimalistic nose cone is that it's really hard to grab for disassembly. This is why you see two small M3 threaded blind holes on the front face. The idea is that I can screw in two small eye bolts with a piece of dyneema between them to pull the nose cone out.
Also, since the new nose cone is in effect shorter (by 46mm), I can use a longer CF reservoir which also adds a bit extra buyoancy.
Some more nerdy pointers in no particular order:
- I had designed the nose cone to have two o-ring grooves as I often do. Last time around, I did the o-ring grooves on the lathe but this time, I wanted to do it all on the CNC but the T-slot cutter I have only reaches deep enough to do the one. If I can hold it in the lathe, I may add another. At least the T-slot cutter did a really nice job. Also doing it on the CNC totally eliminates the pucker factor in doing it on a small lathe where chatter right at the end of the cut can mess up an o-ring groove.
- In an ideal world, the lightening holes should only come in from the back whereas now, they are both on the front and rear. Again, it's a machining/tool limitation. I use small 2mm end mills to cut them and machining really deep pockets with small, long reach tools is tricky. So, instead of deep pockets on one side only, I have pockets on both sides that are half the depth. Actually, the only downside to having cutouts in the front is a tiny loss of buoyancy, but we are taking a few grams/ml here. On the upside, it looks pretty swank.
- I would have liked to make the line guide bore a bit bigger but this was the biggest I could go with the stock I used. I have some ceramic eyelets used on fishing rods that I would have liked to use but will do on future itiration.
- No anodizing, yet. I will likely get into that later in the year but for now, it's too much of a hassle to send these parts out to get it done. But I am using a maring grade alu and the parts I have made previously are actually holding up quite well. Just a bit of dulling on the surface, no real corrosion, yet and definitely good enough for prototyping.
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A Smaller Pumping Barrel
You may have noticed the 7mm marking on the nose cone in the previous post. Well, that's the max size of shaft it will accept - which is 1mm smaller than my other Mirage style guns.
And this brings me back to the lower pressures I have had the gun at lately. I injured my wrist a while back and loading powerful spearguns is probably something I should wait with. Also, my girlfriend - before majorly injuring her own wrist to the point of needing surgery - was beginning to take an interest in spearing.
So, I thought, why not put an even smaller pumping barrel in the gun. To help me out while I gain strength and for her to more easily use the gun in the future.
Traditionally, the Mirage has a 10mm ID pumping barrel vs. its 13mm shooting barrel.
But I will go with a 9mm pumping barrel on this gun - which if using UBL tail ends up allowing only a max. 7mm shaft or if I use a nude, front tied shaft I can go up to 8mm but I haven't used those in a long time.
A brief side note - have a look at the chamfer on the shooting barrel bore. It's 60 degrees vs. the regular 45 degrees and it's made with a ball endmill with tiny stepovers . It's super smooth and the shooting barrel o-ring slides right in . Thanks to whoever proposed that some pages back, it really does help a lot.
Anyhow, back to Mirage 10mm pumping barrel (PB) vs. my idea of a 9mm PB and some real world numbers.
Let's say I keep the gun at 26 bar for now. On a classic non-Mirage gun, it would take 35kg to load it (which I can't do at all). However, on a Mirage with a 10mm PB this would be reduced to ~21kgf. And with the 9mm PB, that effort drops by another 4kg to just 17kg.
Another way to look at it, would be that on a regular pneumatic 13mm gun if I only had 17kgf "available" to load with, I could only add ~12.5 bar to the gun... That would be nothing, it would only be good for close up hole hunting - whereas a gun with 26 bar really comes alive.
A third and easier way is to say that a Mirage style gun with a 9mm pumping barrel will take less than half the power to load than a classic 13mm gun.
Well, kinda. Easier in max effort but not accumulated effort. But since max effort is the limit for most if not all of us, that's what I am going with
Now I just need to actually make that new pumping barrel - and a piston for it - and cut down one of my spare carbon reservoirs.
Added a quick little video to better show the fit - it's pretty close to perfect
You may have noticed the 7mm marking on the nose cone in the previous post. Well, that's the max size of shaft it will accept - which is 1mm smaller than my other Mirage style guns.
And this brings me back to the lower pressures I have had the gun at lately. I injured my wrist a while back and loading powerful spearguns is probably something I should wait with. Also, my girlfriend - before majorly injuring her own wrist to the point of needing surgery - was beginning to take an interest in spearing.
So, I thought, why not put an even smaller pumping barrel in the gun. To help me out while I gain strength and for her to more easily use the gun in the future.
Traditionally, the Mirage has a 10mm ID pumping barrel vs. its 13mm shooting barrel.
But I will go with a 9mm pumping barrel on this gun - which if using UBL tail ends up allowing only a max. 7mm shaft or if I use a nude, front tied shaft I can go up to 8mm but I haven't used those in a long time.
A brief side note - have a look at the chamfer on the shooting barrel bore. It's 60 degrees vs. the regular 45 degrees and it's made with a ball endmill with tiny stepovers . It's super smooth and the shooting barrel o-ring slides right in . Thanks to whoever proposed that some pages back, it really does help a lot.
Anyhow, back to Mirage 10mm pumping barrel (PB) vs. my idea of a 9mm PB and some real world numbers.
Let's say I keep the gun at 26 bar for now. On a classic non-Mirage gun, it would take 35kg to load it (which I can't do at all). However, on a Mirage with a 10mm PB this would be reduced to ~21kgf. And with the 9mm PB, that effort drops by another 4kg to just 17kg.
Another way to look at it, would be that on a regular pneumatic 13mm gun if I only had 17kgf "available" to load with, I could only add ~12.5 bar to the gun... That would be nothing, it would only be good for close up hole hunting - whereas a gun with 26 bar really comes alive.
A third and easier way is to say that a Mirage style gun with a 9mm pumping barrel will take less than half the power to load than a classic 13mm gun.
Well, kinda. Easier in max effort but not accumulated effort. But since max effort is the limit for most if not all of us, that's what I am going with
Now I just need to actually make that new pumping barrel - and a piston for it - and cut down one of my spare carbon reservoirs.
Added a quick little video to better show the fit - it's pretty close to perfect
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I have used carbon fiber reservoirs for many years - so have Salvimar themselves on production guns. Let's see in another 5-10 years if any of them start failing.
That said, I have talked for a long time about trying to blow one up on purpose but haven't gotten around to it. Would be nice to have an idea of the burst strength. Maybe I will this year.
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Ooops...
I just realized I totally forgot to add a line hook...! I have the guide hole, but nothing to wrap the line around, haha. In earlier designs, I had a small threaded hole close to the bottom and the idea was have a line hook screw into that. Not sure where in the process I forgot about that and the M3 hole for extraction is too far up the side to do that in a sleek way now.
What a pity as I think I actually had just enough stock material to have a nicely sculpted one sit off to the side of the guide hole.
Well, I guess I have to do it like last time and like I did on the 3D printed nose cone and insert a piece of bent steel rod into some drilled holes. Alternatively, I machine one that glues into the lower lightening hole next to the guide hole.
I just realized I totally forgot to add a line hook...! I have the guide hole, but nothing to wrap the line around, haha. In earlier designs, I had a small threaded hole close to the bottom and the idea was have a line hook screw into that. Not sure where in the process I forgot about that and the M3 hole for extraction is too far up the side to do that in a sleek way now.
What a pity as I think I actually had just enough stock material to have a nicely sculpted one sit off to the side of the guide hole.
Well, I guess I have to do it like last time and like I did on the 3D printed nose cone and insert a piece of bent steel rod into some drilled holes. Alternatively, I machine one that glues into the lower lightening hole next to the guide hole.
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Ceramic Insert or Not?
I mentioned I had some ceramic fishing rod eyelets, but I don't think they are very good:
The idea would be to glue one into the line guide in the next itiration of this nose cone, but the bare alu - fileted in the CNC and polished with a rotary tool afterwards is surprisingly at least as smooth as those inserts. Maybe I got some duds.
So, I wont bother with the inserts for now. I will just polish the alu line guide bore. Of course, it will oxidize a bit and get a bit more rough but A). shooting some fish might have the line itself do some polishing and B). once I get into anodizing, that will help a lot. Anodized surfaces tend to have less friction.
Also, as a side note I mentioned that the bulkhead seems to be working perfectly now, at least at 26bar which is what I am keeping the gun at these days. How do I know? Well, on my second last outing, I took the last shot on low power and then three days later, I went out again - and the first loading effort felt exactly like it normally does after a low power shot which means, no internal leaks between the front and rear chamber.
Oh, and the gun finally shot two fish. Just two small suicidal jacks but this place is one of the hardest places to catch fish that I have ever speared at. And... I am terribly out of shape. I guess you can't stay in front of a machine and laptop for months and not have it impact your diving, haha.
I mentioned I had some ceramic fishing rod eyelets, but I don't think they are very good:
The idea would be to glue one into the line guide in the next itiration of this nose cone, but the bare alu - fileted in the CNC and polished with a rotary tool afterwards is surprisingly at least as smooth as those inserts. Maybe I got some duds.
So, I wont bother with the inserts for now. I will just polish the alu line guide bore. Of course, it will oxidize a bit and get a bit more rough but A). shooting some fish might have the line itself do some polishing and B). once I get into anodizing, that will help a lot. Anodized surfaces tend to have less friction.
Also, as a side note I mentioned that the bulkhead seems to be working perfectly now, at least at 26bar which is what I am keeping the gun at these days. How do I know? Well, on my second last outing, I took the last shot on low power and then three days later, I went out again - and the first loading effort felt exactly like it normally does after a low power shot which means, no internal leaks between the front and rear chamber.
Oh, and the gun finally shot two fish. Just two small suicidal jacks but this place is one of the hardest places to catch fish that I have ever speared at. And... I am terribly out of shape. I guess you can't stay in front of a machine and laptop for months and not have it impact your diving, haha.
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Losing Weight - Chapter 2 - Cutting Down Carbon Fiber Tubes
The aluminium reservoirs are strong but a tad heavy for short Mirages with the extra internal hardware that goes with them. The stock Salvi alu reservoir that I've used so far is 280g, the carbon one will be about 162g so a 118g weight saving which is quite a lot.
I've cut down a lot of these, normally with a hacksaw blade and then hand sanding square. You can print a jig so you keep the sanding perpendicular and I guess I could CNC mill one, too now. But I tried something else this time.
First I made a fixture out of a block of Polypropylene (PP) plastic. It's like the poor man's Delrin/POM. Not as stiff but stiff enough for this use. The machining pics are blurry since I still don't have much light on the machine and because it moves much faster in plastic than in alu:
In the end, it looked like this with some tapped M10 holes:
I had intended to make a similar top shell to get really good, even clamping pressure but lut let me back track a bit first.
I had cut down the reservoir a mm or two oversized with a small cutoff wheel on my little electical grinder. So, the CNC was just to make sure everything was properly square and I realized there really wouldn't be much tool pressure during the machining so I decided to try without a top shell but some clamps instead:
The above pic is actually the remaining part of the reservoir which I was using to test the setup. I think the only challenge here was to not over tighten the clamps. I then took a cut with a 12mm endmill but no pics of that. And then I finished with a sanding wheel, this time on the real reservoir:
I was worried the sanding wheel would perhaps be compressed unevenly and give a wavy cut but with small stepdowns and just "skim cutting" it worked well. The edge is even and perpendicular as far as I can tell.
Now, honestly, I think the faster and perhaps even better solution would be to do the rough cut with a hack saw or cut off wheel and mount a sleeve/jig to block sand against. When I used the end mill it ripped the inner fibers a bit. Not the end of the world as I have to epoxy coat the inside anyways. It's kinda the wrong cutter for the job but all I had - but supposedly burrs are better. But I wouldn't have had this issue with a cut off wheel + hand sanding.
Anyways, I wanted to try this and it was fun to machine some plastic for a change.
Also, after I cut the tube down to size, I was left with a shorter 250mm piece which I cut again, so now I have two 125mm long pieces. I think I may finally get around to blowing one of them up. I will need to make some parts for this first and then borrow a scuba tank but I have a work trip coming up so it will have to wait a while longer.
The aluminium reservoirs are strong but a tad heavy for short Mirages with the extra internal hardware that goes with them. The stock Salvi alu reservoir that I've used so far is 280g, the carbon one will be about 162g so a 118g weight saving which is quite a lot.
I've cut down a lot of these, normally with a hacksaw blade and then hand sanding square. You can print a jig so you keep the sanding perpendicular and I guess I could CNC mill one, too now. But I tried something else this time.
First I made a fixture out of a block of Polypropylene (PP) plastic. It's like the poor man's Delrin/POM. Not as stiff but stiff enough for this use. The machining pics are blurry since I still don't have much light on the machine and because it moves much faster in plastic than in alu:
In the end, it looked like this with some tapped M10 holes:
I had intended to make a similar top shell to get really good, even clamping pressure but lut let me back track a bit first.
I had cut down the reservoir a mm or two oversized with a small cutoff wheel on my little electical grinder. So, the CNC was just to make sure everything was properly square and I realized there really wouldn't be much tool pressure during the machining so I decided to try without a top shell but some clamps instead:
The above pic is actually the remaining part of the reservoir which I was using to test the setup. I think the only challenge here was to not over tighten the clamps. I then took a cut with a 12mm endmill but no pics of that. And then I finished with a sanding wheel, this time on the real reservoir:
I was worried the sanding wheel would perhaps be compressed unevenly and give a wavy cut but with small stepdowns and just "skim cutting" it worked well. The edge is even and perpendicular as far as I can tell.
Now, honestly, I think the faster and perhaps even better solution would be to do the rough cut with a hack saw or cut off wheel and mount a sleeve/jig to block sand against. When I used the end mill it ripped the inner fibers a bit. Not the end of the world as I have to epoxy coat the inside anyways. It's kinda the wrong cutter for the job but all I had - but supposedly burrs are better. But I wouldn't have had this issue with a cut off wheel + hand sanding.
Anyways, I wanted to try this and it was fun to machine some plastic for a change.
Also, after I cut the tube down to size, I was left with a shorter 250mm piece which I cut again, so now I have two 125mm long pieces. I think I may finally get around to blowing one of them up
. I will need to make some parts for this first and then borrow a scuba tank but I have a work trip coming up so it will have to wait a while longer.
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New Pumping Barrel (w. "Zahar Valve")
As mentioned, I wanted to try a pumping barrel with a smaller bore than the classic 10mm of the Mirage. Both since my wrist is not super strong these days but also very much because I want my GF to have a chance at loading this gun
In terms of weight saving, the original Mirages came with 10x14mm (ID x OD) pumping barrels (PB) but for my latest DIY Mirages, I had already downsized the OD to 13mm to save a bit of weight. The tube I am using for the new PB sticks to my new standard of 1.5mm wall thickness so it is 9x12mm.
I also wanted to try the o-ring relief valve that @Zahar proposed some pages back. While it's pretty much the simplest to make, the challenge in my particular case is that I only have 1mm of clearance between the pumping barrel and the reservoir. Same between the pumping barrel and shooting barrel. So, I can't use very thick o-rings. There's only room for rings with a 1.78-2mm cross section - which means not as much surface of rubber to seal with. I think a 2.5-3mm thick ring would make things easier:
Anyhow, I still wanted to see if I could make it work. So, first, I made a little test piece out of the 9x12mm alu tube:
The grooves you see are mostly practice grooves. And the two red pieces above the tube are PB pistons and the two below are end caps I need for a leak test.
No pics of the vent holes at the bottom of the groove, though. I made them on the CNC but just in manual mode so was too busy holding the part by hand in a V-block and then jogging the drill down with the hand wheel so, no pics of that and I forgot to snap one later.
I used 0.9mm drill bits intended for circuit boards. Cheap, which is good since they are so brittle and snap easily.
I drilled four holes in the last groove I made and then added the o-ring. I also put some tissue inside the tube before closing it with the end caps.
Tissue since...:
Just a tad over 4 bar of pressure as my compressor doesn't go much higher and I couldn't be bothered to hand pump (I normally take this water filter tank up to 7 bar when testing dive watches after battery swaps).
And the o-ring valve didn't leak - at least not at 4 bar of pressure. The tissue was dry apart from a bit of dirty oil, but no water:
I then made the same valve on the longer tube meant for the real pumping barrel (the o-ring is just pulled out of the groove on this pic to show the vent holes, there isn't two grooves):
But I was sloppy when I held the tube in the CNC and it shifted a bit so not all the holes are on center -and I also broke two drill bits because of this.
I think I will make a fixture for it tomorrow and re-do it. Also, I put six 0.9mm in this tube - up from four in the test piece.
I did a few quick test pumping strokes and I can't really feel much, if any, throttling so I think six holes is enough and so is one single groove.
Thanks so much for the idea @Zahar. I hope I can make this work since it really is the simplest way to do this . Also, it's easier to get good o-rings than it is to get good tubing needed for the sleeve type of valve. But first tests are promising, fingers crossed that it will keep working.
Side note: the surface finish of these tubes isn't great but don't worry, the inside is really smooth. I haven't had a single of them leak and I have used them for a long while by now.
As mentioned, I wanted to try a pumping barrel with a smaller bore than the classic 10mm of the Mirage. Both since my wrist is not super strong these days but also very much because I want my GF to have a chance at loading this gun
In terms of weight saving, the original Mirages came with 10x14mm (ID x OD) pumping barrels (PB) but for my latest DIY Mirages, I had already downsized the OD to 13mm to save a bit of weight. The tube I am using for the new PB sticks to my new standard of 1.5mm wall thickness so it is 9x12mm.
I also wanted to try the o-ring relief valve that @Zahar proposed some pages back. While it's pretty much the simplest to make, the challenge in my particular case is that I only have 1mm of clearance between the pumping barrel and the reservoir. Same between the pumping barrel and shooting barrel. So, I can't use very thick o-rings. There's only room for rings with a 1.78-2mm cross section - which means not as much surface of rubber to seal with. I think a 2.5-3mm thick ring would make things easier:
Anyhow, I still wanted to see if I could make it work. So, first, I made a little test piece out of the 9x12mm alu tube:
The grooves you see are mostly practice grooves. And the two red pieces above the tube are PB pistons and the two below are end caps I need for a leak test.
No pics of the vent holes at the bottom of the groove, though. I made them on the CNC but just in manual mode so was too busy holding the part by hand in a V-block and then jogging the drill down with the hand wheel so, no pics of that and I forgot to snap one later.
I used 0.9mm drill bits intended for circuit boards. Cheap, which is good since they are so brittle and snap easily.
I drilled four holes in the last groove I made and then added the o-ring. I also put some tissue inside the tube before closing it with the end caps.
Tissue since...:
Just a tad over 4 bar of pressure as my compressor doesn't go much higher and I couldn't be bothered to hand pump (I normally take this water filter tank up to 7 bar when testing dive watches after battery swaps).
And the o-ring valve didn't leak - at least not at 4 bar of pressure. The tissue was dry apart from a bit of dirty oil, but no water:
I then made the same valve on the longer tube meant for the real pumping barrel (the o-ring is just pulled out of the groove on this pic to show the vent holes, there isn't two grooves):
But I was sloppy when I held the tube in the CNC and it shifted a bit so not all the holes are on center -and I also broke two drill bits because of this.
I think I will make a fixture for it tomorrow and re-do it. Also, I put six 0.9mm in this tube - up from four in the test piece.
I did a few quick test pumping strokes and I can't really feel much, if any, throttling so I think six holes is enough and so is one single groove.
Thanks so much for the idea @Zahar. I hope I can make this work since it really is the simplest way to do this . Also, it's easier to get good o-rings than it is to get good tubing needed for the sleeve type of valve. But first tests are promising, fingers crossed that it will keep working.
Side note: the surface finish of these tubes isn't great but don't worry, the inside is really smooth. I haven't had a single of them leak and I have used them for a long while by now.
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I do not recommend using green O-rings. They have residual deformation and will let air through when changing position!
Most sources list FKM (green) as having more resistance to "Compression Set" than black Buna/nitrile/NBR rings. But I think NBR is more flexible. So, not really sure what the conclusion is for our use. I assumed lower compression set means less deformation so longer life? But maybe for an o-ring that will be expanded and contract with every pump stroke something else is at play?
But here's Parker's o-ring Handbook showing less compression set (lower is better) for FKM than for NBR:
No data for 20-30C though and even Parker's handbook warns against getting too hung up over compression set as other parameters can be more important.
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