DIY Cylinder?

[Docv400]

I was looking at the cylinder of my ICS MP5 and two things crossed my mind straight away.

1) It's just a length of 1" diameter 20 SWG brass tube, about £6 for 12".(makes £10-15 or so for 3" seem just a tad too much, no?

2) The port is near the centre of the tube, so the piston's not actually doing anything useful until it's over halfway down it's stroke!

I'm off to my local tubing supplier later

Could just as easily use stainless or alloy.

[Aod]

thats quite interesting actually.

 

Hmmmm, B&Q for the win!!!

 

i wonder what kind of FPS gain one would see from placing the Cylinder port 25% down the Cylinder

 

front (nozzle end)

\/

................................../\ there

 

instead of here:

......................../\

[Docv400]

For what it'd cost it's gotta be worth a try. I'd have several small ports around the cylinder instead of just the one though. I've seen the four port ones, but why stop there? I can get hold of 'sticky-back' teflon sheet as well.

[Jimisin73]

More ports may be better, so that you don't create undue back pressure when the spring is compressing...you may still find that a higher torque motor is needed...can't wait to hear your results

[Docv400]

can't wait to hear your results

Gimme a couple of days...hopefully my shims will arrive tomorrow, if not then monday, and I'd really like to replace my pinion gear 'cos it's quite badly worn.

Hmmmm, B&Q for the win!!!

They don't stock anything that big in brass.

Try here...Metal supplier...

Do a search for 'Brass tube' and then it's the 1" 20SWG option (For my gun anyway)

I've also got some seamless stainless tubing somewhere, must dig that out too.

[bullzebub]

for optimal performance the cylinder volume should be equal to barrel volume. (not taken leaks ino account ofcourse)

 

if cylinder volume is bigger the piston will still be accelrating when the BB leave the barrel ... this is bad because opimal air speed is exactly when piston reaches the cylinder...

 

if you want to increase performance then you need something that has thinner walls (=bigger volume) and the port closer to the front of the mechbox...

 

or ... try to trim in the existing port so the piston hits the cylinder head exactly when the BB leave the barrel...

[Hissing_Sid]

It seems like some of you guys are just getting to grips with cylinders.

 

The reason that cylinders have ports is so that the piston can retract without sucking in air through the nozzle unneccesarily, thus reducing wear.

 

I gotta say that if it was up to me I'd just fit Type 0 (unvented) cylinders in every gun, just to standardise the production.

Having said that, airsoft companies rarely do anything unless there's a reason for it. I doubt they'd spend time machining ports in cylinders unless there was a good reason for it.

 

I just don't know what it it.

 

The cylinder volume actually needs to be a bit more than the barrel volume.

That's not just to compensate for air leaks either.

The BB should accelerate all the way along the barrel. Basically, you need a surfiet of volume to ensure there's always a pressure differential across the BB forcing it out of the end of the barrel.

 

In my experience, there's no such thing as "too much" air. I have an unvented cylinder in my P90 (from when I was messing with different barrels) and it makes no difference.

 

If anybody wants pictures, I'll see what I can do but, basically, it doesn't matter about the vent. Once the piston moves forward in the sealed portion of the chamber it will generate a pressure increase in the cylinder.

It will generate the same pressure increase in a vented or unvented cylinder. It'll just do it sooner in an unvented one (which is what you want if your BB has to travel along a longer barrel).

 

As long as the cylinder volume is enough to propel the BB out of the barrel then the vent is not an issue.

 

What I'm saying is that a vented and unvented cylinder in a P90 will yield the same power.

The only difference will be the amount of air vented behind the BB.

 

As for the idea of making your own cylinder; go for it.

I would say, however, that be aware that the inside of most seamed tubing is not perfect. It'll have pitting along the seam.

It might not be round either. It might have started off round but, after 6 months in a Jewsons warehouse, who knows what junk has been thrown on top of it?

[Docv400]

the inside of most seamed tubing is not perfect

I think most brass tube is 'drawn' and as such, doesn't have seams.

Anyway, that's what my lathe's for.

 

 

 

Having said that, airsoft companies rarely do anything unless there's a reason for it. I doubt they'd spend time machining ports in cylinders unless there was a good reason for it.

Maybe it's a cheap and effective way of limiting the fps and keeping the rest of the gubbins standard, and making loads of dosh selling 'upgrade' cylinders , or am I being too cynical?

 

 

It will generate the same pressure increase in a vented or unvented cylinder. It'll just do it sooner in an unvented one (which is what you want if your BB has to travel along a longer barrel).

Not quite. If you compress air in a sealed cylinder the pressure rise is in proportion to the travel of the piston. If you only start compressing the air about half way through the stroke, you'll get about half the pressure. But this is only strictly true if there is a constant resistance to the air escaping, which in the case of an Airsoft gun doesn't happen. The BB will start to move when the air pressure is enough to overcome the resistance of the ................. (insert correct name for the rubber tubey bit that the BB feeds into and stops it moving too soon i.e. before a decent amount of pressure has built up behind the BB) and no, I don't have a clue what that pressure is. Would depend on wear/lubrication/BB size and quality, etc. If you could ensure that the BB only started to move when the piston was almost at the end of it's stroke, wouldn't you get the highest achievable (for that particular spring/piston/cylinder combination) fps?

I'm thinking, have a really tight fit for the BB so it only 'pops' when a pin that is attached to, or is part of, the piston head and extends through the cylinder head and nozzle, strikes it near the end of the stroke, thus the pressure is highest when the BB is released. Obviously, it would have to be far enough before the end of the piston travel otherwise the piston would be starting back before the BB had left the barrel.

 

Is this feasible d'you think?

[Hissing_Sid]

If you compress air in a sealed cylinder the pressure rise is in proportion to the travel of the piston. If you only start compressing the air about half way through the stroke, you'll get about half the pressure. But this is only strictly true if there is a constant resistance to the air escaping, which in the case of an Airsoft gun doesn't happen. The BB will start to move when the air pressure is enough to overcome the resistance of the [hop-up]

<{POST_SNAPBACK}>

You've corrected your own error there.

 

To be pedantic, the case of a sealed vessel, the smalle volume still "wins". If you have a cylinder 10cm long then you have to move the piston 5cm to build up 1 BarG inside the cylinder.

 

If you have a cylinder with a vent half way along then, obviously, you build up no pressure at all until the piston has passed the vent.

However, once it's passed the vent you have, effectively, a 5cm long cylinder. The piston then only has to travel 2.5cm to build up 1 BarG.

 

TBH, having said that, I wonder if this IS the main reason for a vent?

you are, effectively, creating a smaller sylinder which increases pressure faster.

In theory this means that a BB might accelerate to a given velocity faster. Which is what you'd want in a short barrel.

 

In reality I suspect that what happens is that as soon as the piston creates a couple of PSI pressure the BB will begin to move.

If this is the case then both BBs will accelerate in a linear manner as long as the piston continues along it's stroke.

Basically, this concludes, in the case of the vented cylinder, with the BB exiting the muzzle and then the piston expelling a couple of CC of air behind it.

In the case of the unvented cylinder the BB will pop out and there will be maybe 10cc of air expelled behind it.

 

It's not the volume of air which dictates power. It's the speed at which the piston moves. That's why we fit stronger springs to increase power.

[Sale]

I would think that as the piston gets to accelerate without moving the BB until the piston head O-ring passes the vent, it accelerates BB's faster.

 

Another theory I've heard is that "too much air" might cause erratic muzzle velocity compared to a gun with suitable vent hole in the cylinder.

 

As a rule of thumb I like to lean towards the idea that TM put the hole in there for a reason. Thus, I get cylinders that match the barrel I'm using.

 

-Sale

[Kristoffer]

http://www.systema-engineering.com/English...eunchiku-1.html

[Hissing_Sid]

Another theory I've heard is that "too much air" might cause erratic muzzle velocity compared to a gun with suitable vent hole in the cylinder.

TBH, this would have been my first guess but,as I say, I left an unvented cylinder in my P90 and have noticed no difference. It's no more powerful and no less accurate.

Also, I've recently been using an upper gearbox out of an ICS M16 in my M4. Obviously, I couldn't compare the power since it's a different spring in my M4 upper to in the M16 upper but the accuracy was still perfect.

 

If there is any effect, maybe it really only kicks-in at the limits of performance?

Maybe if you fitted an unvented cylinder in an MP5K you'd see something odd happening?

 

As a rule of thumb I like to lean towards the idea that TM put the hole in there for a reason. Thus, I get cylinders that match the barrel I'm using.

<{POST_SNAPBACK}>

Very true.

 

What's more, though I hate to say it, Airsoft companies rarely build things in such a way as to make them user-friendly, higher-quality or more accurate.

The only reasons they seem to build things in a given way is to make manufacturing easier or to stop the gun breaking.

 

Being the cynical git that I am, if excess air caused inaccuracy I'd imagine they'd leave them unvented and leave adventurous amateurs to put the vents in to increase accuracy.

I'm more tempted to think the vents are there to stop something breaking.

[snorkelman]

could be that the optimal config (from a reduced wear/breakage pov) is forward vent ..but as barrel length increases vent has to go further back untill they have to accept the wear hit of no vent at all?

 

otherwise as you say they may as well go unvented on them all and have have excess air shot out of shorter barrels

 

stevie

[Docv400]

If you have a cylinder with a vent half way along then, obviously, you build up no pressure at all until the piston has passed the vent.However, once it's passed the vent you have, effectively, a 5cm long cylinder. The piston then only has to travel 2.5cm to build up 1 BarG.

That would only be true if the resistance to the pressure build-up stayed constant (i.e. the BB didn't move at all) until the pressure you quoted was reached (and where did that 1 BarG figure come from?) Otherwise a cylinder twice as long will give approximately twice the pressure if the resitance is held until full travel is acheived.(taking into account leakage, spring rate, friction, etc)

But this all depends on when the BB starts to move doesn't it. The hop-up has two functions 1) To hold the BB until the pressure builds to a predetermined level, whatever that level may be,(those of you who have used a blow-pipe of any description will know it's far more effective to build pressure in your mouth and then discharge it as quickly as possible with a sort of "Tuh" action than just to blow hard) and 2) to put backspin on the BB. Am I right, or am I missing something fundamental here?

[snorkelman]

you build a gearbox to propel a bb down XX length barrel based on a piston compressing air in a cylinder YY length

 

you build a new weapon with a barrel X length that only requires a cylinder Y length

 

simplest option is to retain the same gearbox piston etc for both but shorten effectve length of the cylinder to Y - by cutting a vent in the YY cylinder so it doesnt act as a pressure source till half way along its travel?

 

1 gearbox suits both and your parts bin count remains the same (bar need to vent some of your cylinders)

 

an unvented cylinder would work in both full length barrel its designed for and the short barrel (excess air is wasted) a vented cylinder on a full length barrel might not

 

or am i totally missing the point and tieing myself in knots? (wouldnt be the first time lol)

 

stevie

[Docv400]

or am i totally missing the point and tieing myself in knots? (wouldnt be the first time lol)

No idea mate. What I do know is that it's one thing having a complicated idea in your head, and another thing entirely to try and type it all down in an understandable and coherent way

[Sale]

an unvented cylinder would work in both full length barrel its designed for and the short barrel (excess air is wasted) a vented cylinder on a full length barrel might not

 

or am i totally missing the point and tieing myself in knots? (wouldnt be the first time lol)

We all have observed this by now. The question is why do they make the extra vent hole in the cylinder for a short barrel, if a full cylinder would work as well?

 

It might be accuracy, ROF or simply how long the gun lasts. Theoretically a full cylinder with a short barrel stresses the mechanism more than with a vented cylinder, but I'd assume the difference isn't that big. A manufacturer wouldn't make the vent holes without a reason, and a marginal increase in accuracy might not be enough to push them to do that. We are clearly missing something here.

 

-Sale

[Hissing_Sid]

A manufacturer wouldn't make the vent holes without a reason, and a marginal increase in accuracy might not be enough to push them to do that. We are clearly missing something here.

<{POST_SNAPBACK}>

Isn't it funny when, after 10 years of production, we all kinda go "Well, I dunno" at the same time?

[Docv400]

It's a conspiracy I tell you...

 

Had a look in my scrap metal stash and found some nice seamless stainless tube. I can get the brass locally but it's 5m minimum order and £40 (for 5m mind, but I'll never use all that. It'd make 50+ cylinders)

I've got enough stainless to make up about 6 or so cylinders, so I'll be experimenting soon...

Might have to turn up a few different alloy piston heads too 'cos the stainless is a few thou' bigger/smaller (There's two different guages)

p.s. My shims came in the post today...

Shim topic

[Pendra]

Ok here is my theory about the holes:

As we know the compressed air pushes the BB out of the barrel. The higher the compression, the higher the FPS. However, air starts to flow through the nozzle right after the piston starts to move. As the piston goes closer to the cilinder head, the compression increases, the speed of the airflow increases and the BB is accelerated further. I guess you have the best compression when the piston is really close to the head.

However if you use a hole less cylinder with a short barre, the BB will leave the barrel before the piston achieves the maximum compression. So the BB will not be accelerated to its highest possible speed. This extra air will just go through the barrel and that is all.

With the hole, the piston will go closer to the end of the cylinder without moving the BB. So it will achieve the the highest compression faster, while the BB is still in the barrel.

As I can't imagine a AS cylinder blowing up because of overpressure, I guess there is no other reason for the hole.

So I think it is about achiving the best air compression/acceleration while the BB is in the barrel.

And yes, I think someone should really try an M16 barrel with an MP5K cylinder and the way around.

[Docv400]

And so it comes back to an earlier assumption of mine...

I assumed that the hop-up was one unit that did two jobs, 1) the back-spin bit we all understand, 2) holding the BB securely to a) stop it just rolling out if the firing sequence ended with a BB 'chambered' and also to physically stop it moving until a decent pressure had built to give a good fps. But having had a delve into my hop-up yesterday, I'm inclined to think it doesn't do the last bit. Mine stops the BB going past under it's own weight, but it only take a light puff (from me) or even a tap of the upright barrel on the bench to knock it through (that's through the initial entrance 'lip' not the hop up protrusion).

Is that normal or is it worn?

[MDK_Marshal]

Sounds worn to me. But then, i have to experience with decent hop rubbers, realy

 

I should have done this for my physics practical, it'd have much mroe science, and be much mroe fun than spring constants and the like

 

Incedentaly, does anyone have a reliable way of measuring the FPS of a gun WITHOUT a chrono? And I'll scream if anyone says "Coke Can!"

[Hissing_Sid]

It's normal.

[Docv400]

So do you think it would be preferable to have a bit more resistance at the entrance for the previously mentioned reason?

[Hissing_Sid]

TBH, I'd be worried that the hop-up would release the BB in a manner so as to put side-spin on it as well as back-spin.

 

 

Tell you what would be interesting would be to shoot a gun with a vented cylinder AND an unvented cylinder and see what the difference in noise is using waveform software.

 

It might show something like the muzzle crack and the piston hitting the cylinder head etc.

 

I might have a go at that in a little while. I've got a ICS M4 with a vented upper and an unvented one off an M16. They produce shots with 1J energy so it might be interesting to see if there are any obvious differences in the firing noise.

 

[edit]

Just finished recording my M4 shooting with vented and unvented cylinders.

 

The results aren't even worth posting. Do distinguishable difference in sound and no difference in power.

 

Go figure.

 

FWIW, I had the mic by the muzzle. I might try it again with the mic taped to the receiver but there's nothing obvious to see.