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Blow forward bolt, presssure and springs

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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Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Sun Jan 11, 2015 12:32 pm

I have turned and drilled an aluminum housing for my blow forward bolt design, I tried acetal bolts, 1st one accidentally got 850psi up its behind ( lack of thinking) this caused an earsplitting bang and a 2 piece bolt! I made a second and am having trouble with it sticking forward as I am using alot softer spring (sorry no way of measuring the spring but first one you could probably half its length squeezing hard as I could between thumb and 1st finger) so I got some 1mm wire springs which are easily squashed by hand.

I am to use in the range of 300-500psi what kind of spring rates are normally used, I realise alot depends on piston area but I need to find a balance of returning piston quickly but also at the same time absorb the impact as the piston is halted from its forward motion.

I am now making a brass piston as I feel this will hold up better and also allow a smoother action.

Sorry if its all abit vague but asking for any advise anyone has, I am listening...

Details - breach/cylinder main bore of 19mm (allows taping for 1/2" bsp threads for air/barrel fittings.
Piston just under 19mm diameter with 13mm hole in base to sit over air supply tube.
Ammo 9.5mm steel ball bearings.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Fri Jan 16, 2015 3:47 pm

This is my main projectt and am determined for it to work, it will be powered by 2 lithium rechargeables, probably 18650 which will be going though a voltage booster circuit to get 12/24volts dependant on solenoid used, part of this will then be pulsed by another circuit switching a relay which will then feed power the solenoid for the blowforward action, I will use 3 large capacitors to ensure the solenoid will not lack any power for instantaneous current.

It all sounds abit of a jigsaw but hopefully it will work I have a 13ci paintball bottle with buttstock with options for various psi, I will hopefully mount this all in aluminum channel/box and aiming will be via a green laser dot sight I have.

I will update progress here, hopefully my brass bolt will run smoothly! Ammo is 9.5(2)/ 3/8" bearings. These are planed to be fed from a horizontal magazine using a 'zippy' key ring round a small pulley to pull a sledge to keep the ammo supplied. The capacity will only be small maybe 15 rounds.

Photo of breach/bolt and then when assembled (to follow as need resizing for forum)


Redrilling barrel test from 9.4mm to 9.6mm so ammo fits.




PS magazine zippy and blow forward system all borrowed ideas, but hopefully completed gun wiil be slightly original!
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Ianbuckwell
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Sun Feb 01, 2015 8:48 am

Had a problem with the bolt jamming forward, after a bit of thought it was found the bolt was going a bit too far past the air entry tube and not reseating, so I reprofiled the inside of the back corner to give a better angle, tested and it was fine, tried with some ammo and I am pleased with power. Just need development of the magazine and then some form of stock/bodywork.

Any suggestions on magazine, it doesn't need to be huge as the 'stock' is a 13ci paintball tank as aiming for around 20-30shots if lucky. I have ordered a couple of paintball magazine springs hoping I may be able to do something with these as I really don't want a magazine sticking upwards.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: jackssmirkingrevenge » Tue Feb 03, 2015 10:34 pm

Interesting, is this for a semi or a full auto? You can reduce the pressure on the bolt by having a smaller area for the air to act on, can you show us a diagram of what the internals look like?
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Wed Feb 04, 2015 1:35 pm

Sorry don't have a diagram it is basically what is shown in photo, hopefully the blow forward force will be reduced now I have about a millimetre until it angles inwards to help reseating on the supply tube, it is also a less than ideal fit in the aluminum tube as it was the only piece of brass I had, I will use a magnet to secure the ball bearing or a (can't remember name) basically a bolt with spring and ball bearing in it to hold in place until it gets the main blast of air.

My aim is full auto if I can get a reliable magazine designed, the air will be pulsed by a Chinese eBay circuit that allows adjust of pulse duration and frequency. This will then control a solenoid valve to send pulses of air (well that's the plan!! :) )
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Last edited by Ianbuckwell on Wed Feb 04, 2015 1:43 pm, edited 1 time in total.

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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Wed Feb 04, 2015 1:36 pm

If not already mentioned I am basically copying Bill Gilmour YouTube video with slight variation.
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Re: Blow forward bolt, presssure and springsfs card

Unread postAuthor: Ianbuckwell » Thu Feb 05, 2015 1:40 am

Jackssmirkingrevenge - I have no experience of any CAD programs and if I found a computer and used MS Paint it would look like a 3yr old.had done it, but if photos of any parts help just say what you want, least I can do as after all it was you and a few others who helped me out when making my first gun and without that I would probably have called it a day, so thankyou, your help to myself and others is really appreciated.
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Re: Blow forward bolt, presssure and springsfs card

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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Solar » Mon Feb 09, 2015 10:28 am

Just a few notes on solenoid driven blow forward operation. They may not relate at all but I am going to bring them up to hopefully save you some trouble. The launcher in my avatar is a variation of a blow forward system as the blow forward operation(especially at 40mm) produced a downward kick over 200psi from the weight of the bolt in a previous design. This version uses a 3/4" bore cylinder to operate the bolt and then a seperate solenoid valve to launch the projectile once the bolt is closed. The timing is controlled by a small computer and timed instead of using a hall effect or limit switch to activate the main valve. Some of the issues with this design regards the need to regulate the triggering solenoids for the bolt and main valve. Since ballast pressure is higher than the triggering pressures for the solenoids the timing can be skewed once the regulators settle. With stated pressures up to 500psi I am wondering what solenoid valves you plan on using to trigger this. Regarding spring force, etc. is this a true blow forward that seals the oitput until e bolt is closed? If so ypu can also utilize an air spring with an adjustable bleeder hole to change the air spring force.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Tue Feb 10, 2015 4:57 pm

I have one solenoid that is working ok at 350psi, if not I have another Chinese copy of the Burkert 5404 which is rated at 25 or 30bar, my aim is to build an automatic gun, I would like high power but that is not my ultimate goal. I had considered bleeding a bit of air bit now have the spring working. I found this solenoid, its low flow is what concerns me, I will try what I have and see how they go if necessary I will lower my pressure goal.

Solar - sorry you lost me in the last paragraph, I am just a newbie, not sure what system it is realy the bolt just blows fowward until the barrel ( I aim to machine barrel/bolt so the bolt actually penetrates the barrel to help sealing). Do you have any threads on your gun build? I am just learning from here and youtube.

Thanks for the help.

Any advise on magazine design, should I pressurize the magazine tube from the end, just seal it or drill a hole in end cap to allow any leakage to escape? Which is caused by my machining. I have a suitable size tube now (aluminum) and will use paintball magazine spring with end piece to keep balls at the correct end for constant supply, they will then be held by magnet and possibly a detent to ensure they are not released until higher pressure as the barrel is currently only around 18" (may replace and extend if required). If the gun works in principle I intend to plan and rebuild with tighter tolerances rather than design/make as I go along.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Solar » Wed Mar 11, 2015 3:44 am

You should be able to have the side of the bolt cover the magazine tube during the shot to prevent air getting in there. you will need a hole in the base of the magazine tube anyways to let the follower expel air under it so you don't get a 'jammed' magazine as you load it. If you don't put e hole in the bottom you are basically making a tire pump. Flow is a problem with the higher lressure solenoid valves. take a look at www.clippard.com at their 3-way valves and pilot actuators in conjunction with a lower pressure rated solenoid to run the pilot.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Tue Mar 17, 2015 7:19 pm

Thanks for the pointers and tips, much appreciated. This project will probably need remaking as it has been a learning experience in using a lathe which I haven't done before, hopefully this one will at least work, therefore encouraging me to improve it. Main problem at the moment is due to the rather loose fit of my brass bolt in the aluminum tube, as it returns it sometimes jams on the air feed tube, I have hopefully overcome this now as it seems it was more to do with the threading in the end of the tube causing it to pull out of alignment when tightened.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: jrrdw » Wed Mar 18, 2015 11:39 am

Ianbuckwell wrote: Main problem at the moment is due to the rather loose fit of my brass bolt in the aluminum tube, as it returns it sometimes jams on the air feed tube, I have hopefully overcome this now as it seems it was more to do with the threading in the end of the tube causing it to pull out of alignment when tightened.


If you have enough thread lenght you can face the mating surface and that should stop the tightening from causing the misalignment.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Wed Mar 18, 2015 9:11 pm

Caan I just check I have understood you correctly as there is plenty of thread and if not I can lengthen them, you are saying rather than using the O ring to compensate I can slightly angle the end of the aluminum outer tube to match the angle of the brass fitting that is the air inlet to the cylinder, which would then stop one side from touch first which is currently causing the inlet supply to go out of parralel with the aluminum tube?


Thanks for the suggestion, I hadn't thought about it actually fixing the problem, I had only considered quick fix ways to get around it, but this would be much better as it would allow the joint to be tightened up properly.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: jrrdw » Thu Mar 19, 2015 6:01 am

Everything I said is assuming that the threaded hole is centered and straight on the X axis, correct? If not the bolt will always be mis-aligned. If the threaded hole is true and correct and the 2 flat mating surfaces (where the o-ring is placed) are true and correct the end result will be a true and correct assembly.

Quick test: Assemble the parts outside of the breech lay them of a flat surface (workbench / table with the hex head hanging off the edge) and roll the assembly. If the end of the bolt wobbles 1 of 2 things are wrong or both. 1. The hole is not drilled straight and or 2, the bolt is not true turned to center.

Excessive clearance between the bolt and breech can also cause jamming.
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Re: Blow forward bolt, presssure and springs

Unread postAuthor: Ianbuckwell » Thu Mar 19, 2015 7:00 pm

Thankyou for replying, I will check the alignment, I know the air entry tube is pretty straight as when put in the lathe holding on the brass fitting it did not really move, piston/bolt is kind of loose in outer aluminum housing but not drastically, although the bolt is true as this was machined, I have a feeling that the threads I put in the aluminum are probably the major cause as these were done by hand rather than on a lathe so are probably not aligned properly.

I think it is going to be a case of using this as a learning experience and if I can get it to work satisfactoraly I will remake the parts, but this time paying closer attention to tolerances.

Thanks for the help, this is my first 'spud gun' to have parts made by myself and my first time using a lathe to attempt to make something accurately and I got caught out as I thought the bolt was jamming due to the bases diameter so turned it down a bit, but then to find that it was not the cause so it ended up smaller than I needed it! I also need to measure more and also believe my calipers.
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