Pre-cocked Gearbox Shenanigans

[The Chef]

Hola people.

 

Now, I don't know of what I am trying to acheive is indeed possible or not, hence me asking it here for all you technical wizards to encourage/put down (delete as applicable).

 

 

I am looking to make a DMR which will be predominantly run on Semi Auto. (SIG 556 for anyone that's interested).

 

I would like it to fire in a 'pre-cocked' state much the same as the PSG-1 or Army SA80, thus reducing that tell tale wind up noise that gives people the chance to dodge incoming rounds.

 

This is the major pre-requisite.

 

The second part is to have a 3 round burst instead of full auto.

 

 

That's the brief:

 

 

 

Now the questions.

 

Is this possible?

Am I going to need some form of MOSFET gizzardry (and if so, what am I looking for?) or is it going to be a Sector gear issue?

 

Sorry for such a noobish question. It's just I have zero knowledge when it comes to FET's and what they can do.

 

Cheers all.

Rob.

 

[ED-SKaR]

 

As i understand it, its a gear thing.

You will want a bevel gear with as many ARL cams as possible. the standard 2 is not enough, 4 is good but if you can find one with more then that'd be better.

You'll also want to move the cutof peg on the selector gear. Exactly how to do this i dont know, but you could easly cut it off and drill a new one in. You might be able to use the selector gear from an armyR85 as you mentioned.

After that, you will want very strong gears otherwise the teeth will shred off very quickly.

 

There are other ways to do it, maby better ways, but i dont understand them .

[Stealthbomber]

You need an L85 or PSG1 sector gear.

 

There's been a couple of threads related to this sort of thing recently but finding them might be a bit of a bugger since they were, IIRC, threads that sort of devolved into the subject from their original purpose.

 

Personally, I wouldn't bother.

 

I know I can't put you off by saying that so here's what I'd suggest:-

You will need a stonking battery (and an equally stonking motor) to drive your pre-cocked gun.

Buy the bits you need to build the gun, including an 11.1v LiPo and a Systema (or equivalent) motor.

Try to fire the gun.

If you still think you need the pre-cocked system you can then buy yourself a set of PSG1 gears.

 

I'm willing to bet, however, that you'll be happy enough with the trigger response from a properly built gun without the pre-cocked thing that you won't bother.

 

To get the burst function to work you're gonna need a whizzy fire-computer type thing.

Check out the mosfet thread for details of what's available.

 

FWIW, if you DO decide to go for this, I've got the sector gear I took out of my L85 somewhere around.

If I can find it you can have it.

 

The other thing to bear in mind is that, if the gun is controlled by an AB mosfet and it's firing semi or bursts you might be able to get rid of your ARL completely.

It's only neccesary to have if the gearbox can stop in any random position.

If the mosfet is controlling the gearbox so the gun always stops after a shot is fired you don't need the ARL.

 

*EDIT*

FWIW, it might help people offer more specific advice if you tell us what sort of gun you're building (AR15, G36, M14 etc) and what sort of power level it'll be shooting at.

[tome]

you'll still need the ARL. The mosfet only tells the motor when to stop, not the gearset WHERE to stop. So using a pre-cock gear without the ARL the motor will stop with the piston fully compressed. Then the spring will happily push the piston forward again (in most cases unless you have a mega motor like the M160 Charger or Systema Magnum)

 

I tried to get the precocking gear (G&G) to work in my ICS m4 but it would never stop before the piston was released.

[Stealthbomber]

you'll still need the ARL. The mosfet only tells the motor when to stop, not the gearset WHERE to stop. So using a pre-cock gear without the ARL the motor will stop with the piston fully compressed. Then the spring will happily push the piston forward again (in most cases unless you have a mega motor like the M160 Charger or Systema Magnum)

Sorry, yeah.

 

I meant that you can bin the ARL if the gun shoots in semi and burst WITHOUT the pre-cocked jobbie.

A braked mosfet will stop the motor AND gear train exactly where you want it to.

 

Having said that, with the Systema Magnum in my M4 the gears would never roll back anyway so the ARL was redundant.

I definitely wouldn't suggest binning the ARL in any gun firing in auto or from a pre-cocked spring though.

[ED-SKaR]

The other thing to bear in mind is that, if the gun is controlled by an AB mosfet and it's firing semi or bursts you might be able to get rid of your ARL completely.

It's only neccesary to have if the gearbox can stop in any random position.

If the mosfet is controlling the gearbox so the gun always stops after a shot is fired you don't need the ARL.

 

*EDIT*

FWIW, it might help people offer more specific advice if you tell us what sort of gun you're building (AR15, G36, M14 etc) and what sort of power level it'll be shooting at.

 

 

If he is going to precock it then he does need the ARL or it will destroy itself! AND it wont precock, because it will unwind itself.

 

Its not like you to miss that he said he was using a sig 556

 

 

Other than that Stelthbomber is totally right, a well set up gun can give good trigger responce without the requirement of "shenanigans" like precocking.

[Stealthbomber]

If he is going to precock it then he does need the ARL or it will destroy itself! AND it wont precock, because it will unwind itself.

Yep, I addressed that above.

 

I guess it'd be unfair to turn this thread into a debate about ARLs so we should probably not dwell on it.

I was just putting it out there.

 

Its not like you to miss that he said he was using a sig 556

 

He must have edited his thread or summat.

 

Other than that Stelthbomber is totally right, a well set up gun can give good trigger responce without the requirement of "shenanigans" like precocking.

Indeedy.

 

I was considering sticking my G&G sector gear into my ICS M4.

It already has the spring decompressor so it should have been ideal.

As it turned out, after running the gun on an 11.1v LiPo, it just wasn't worth the effort.

 

Incidentally, on that note, I wonder if Tome was using an AB mosfet in his ICS M4 when he tried the G&G sector gear?

You NEED a braking circuit in the gun to stop the piston completing the cycle.

Both the PSG1 and the G&G L85 have a braking circuit built into the trigger contacts.

[tome]

Yep. Actually i left the precocking gear in there as i couldn't be bothered to take it out and shim another gear and it works much like any aeg at the mo hehe. Even with the AB MOSFET the gears would stop too late.

[The Chef]

Cheers Ed,

I was hoping you'd pipe up Stealth.

 

No worries on the gear, I've got millions of the buggers (helps having SA80's as clubby rentals).

 

Thought it sounded a bit 'complicated'.

 

 

I am looking to make a DMR which will be predominantly run on Semi Auto. (SIG 556 for anyone that's interested).

EDIT*** And no, I didn't edit the original, you must be off form or something today!

 

 

I'm hoping to use either a KA or JP Sig 556 as the starting point with a Magpul Stock (got the stock!) and extend the barrel a bit. Going to gut the gearbox and fit it with some decent stuff.

 

Looking to fire around the 380 mark,

 

Not sure what battery I can fit in it though. I don't know why, but I'm still a little hesistant to go down the Li-Po route. Might be me just being a big girl, but I'm still convinced they are going to blow up in a big nuclear stylie and raze my garage to the ground.

 

Ultimately, I just want it to shoot straight, but I'd like these little snacky additions to make it that little bit more gucci.

 

 

Surely, if it's actively braked without a reversal latch, the motor will be drawing current permanently in order to hold it in place (or am I being thick?). Edit** Seen you're edits/updates now. please ignore

 

Still, be pretty cool if I can get it to work.

 

**Edit**

 

Damm you guys type fast. all these responses whilst I was writing this one!

 

This is the look I am trying to acheive.

 

http://www.sigsauer.com/Products/ShowCatal...p;productid=218

[Stealthbomber]

Regarding the ARL (scuse me if I'm telling anybody how to suck eggs )....

 

When you shoot your gun in auto the gearbox can stop anywhere.

If it stops with the spring partially wound the gears start to roll back.

If you fire a burst and then release the trigger and then fire another burst while the gears are rolling back you'll probably strip the gears or at least blow the fuse.

 

To get an idea of this, stick your car in reverse, reverse back at around 5mph and then stick it in 1st and accelerate.... then get out and pick up the bits of your drive-shafts off the road.

 

Point is that if you're running a gun that shoots in semi-only and is controlled by an AB mosfet you can definitely remove the ARL.

All that happens is that when the trigger contacts are released the mosfet shorts out the motor terminals which makes it hard for the motor to spin.

 

Very few mosfets are truly "active" braking, meaning that they pump a momentary reverse current though the motor.

The term "active braking" actually comes from the world of r/c cars where you could make your car stop quicker by programming the transmitter to send a short burst of reverse to the car when you pull the stick to the "stop" position.

 

Point being that a proper "active" braking circuit will only pump a momentary burst of reverse voltage through the motor and a normal mosfet will work by shorting the motor terminals.

In either case, there's no continual supply of leccy drawn.

 

The whole thing with the ARL is slightly dodgier with the burst function.

Again, it depends on how the fire-computer works.

It doesn't help that the people who sell these things might not always tell you the whole truth.

 

The simplest way to set up a "burst" function is with a timer circuit.

Basically, the timer will work ALL the time but when you shoot in semi the trigger will cut off the leccy before the timer runs out.

When you shoot in auto the timer will allow leccy to flow for, say, 0.5 seconds and then stop it.

In both cases when you release the trigger the timer resets.

 

The trouble with this sort of timer is that it isn't exact.

0.5 seconds might be enough to fire 3 shots, or 4, or 5. Etc.

It also means that the gun is, effectively, firing in auto which means it stops firing at a random position in the cycle.

In this case you cannot remove the ARL.

Note that it also screws up the pre-cocked system because your next shot in semi will only serve to pre-cock the system and be a dud.

I keep banging on about this. The biggest flaw in a select-fire gun that uses the pre-cocked system is that when you change from auto to semi your first shot will almost always be a dud.

Given that a shot in semi is likely to be an aimed shot this is a bad thing.

 

Anyway, some people who make fire computers now claim that their burst function is NOT simply a timer although they don't actually explain how it does work.

If they use a sensor of some kind then the computer WILL be able to control when the motor stops accurately and, thus, control the gears which means you can, in theory, remove the ARL as long as the gun isn't pre-cocking.

 

The main thing to take away is that a pre-cocking system in a select-fire gun will never shoot reliably in semi on the first shot.

It can't. The design simply doesn't make it possible.

The first shot in semi will only ever wind the piston to the point where it pre-cocks and then stops.

 

That's the main reason to ditch the pre-cocking idea.

Or ditch the idea of shooting in auto or bursts.

[tome]

when the AWS MOSFETs with burst function finally come out, they will use a sensor on the sector gear to determine its position. I wonder if you used this in conjuction with a pre-cock gear whether it would give the behaviour you're looking for

 

http://www.awsairsoft.com/manual

 

[The Chef]

hmmmmm.....

 

sounds like I should stop being a perfectionist and just run a high power/speed set-up and keep the 3 round burst part firmly in the control of the Mk1 Finger.

 

I think it's going to prove as difficult to implement as getting the bloomin' gun is!

 

Thanks guys, I'll look at other options. Was a nice idea though.

[RSM]

For what its worth here's my foray into precocking on building a DMR.

 

Stock ratio CA gears with G&G sector gear with first three teeth to engage piston removed.

Extreme Fire SW-AB-Long mosfet (an active breaking yet relatively 'dumb' mosfet compared to ones out there now)

G&P M160 motor

Guarder M160 spring.

 

With an 8.4v 4300mah battery the gun had no problems running in precocked mode at all, it was only when I tried to run an M170 in it that I needed to use a 10.8v 2200mah pack and found that to use the ARL release I was infact starting to bend the extension arm on it.

 

On adding a newer mosfet computer I found that as it needed 5 shots to program it on power up, as the first of these 5 shots isn't a complete cycle I would assume it threw the timing off so I would end up with a failure to fire every couple of rounds as it sounded like the gears were trying to turn over but couldn't despite the fact the only thing that had changed in the set up was the mosfet.

 

Switching to some triple torque up helicals I had lying around things went back to normal.

 

So, if you want just single shot, then precocking with a simple AB mosfet should be fine, however if you want to use a computer for 3rnd burst then I would stick to conventional gears. I did find however that on average precocking was about 1fps more consistent in line with a theory I had that if all that happens on firing is the piston being released then more consistent results should be achieved.

[banjo]

I'd best give some details of my stab at pre-cocking then as well....

 

The resason for doing it..... like everyone else, I'm used to using a bolty, so want the same kind of consistency and, to much time on my hands and an over active imagination

 

G&G M14 & AB MOSFET is fitted amongst other toys and trinkets, power is 350 fps

 

The gear set I chose to use is a Systema Super torque set, as I have a PSG gear set and thats what they use only with the obvious differences.

 

I'll kid you not I had 1 hell of a time trying to re-locate the sector gear nub, but after much trial and error, not to mention head/ball scratching, I came out with a position about 120 degrees farther forward of the orginal position, I re-manufactured it out of, believe it or not, layered super glue, filed to shape.

 

It took me the best part of a day to sort out, and I'll be honest, even though I felt a great deal of satisfaction to get it working, I was a little underwelmed with the results and reliability of it, as every now and then I have a trigger lock up that is only cureable by releasing the spring and then re-cocking it, this is obviously a PITA when the target is in the cross hairs and you get bugger all feed back from your weapon

 

I guess it just needs some fine tuning, but after about 10 hours fudging with it my impitus was not quite what it should of been

[RSM]

Just noticed something that I should add on, I didn't make any other mods to the G&G L85 sector gear other than to remove the first three teeth that would engage the piston, tappet nubs/semi cut off lever cams were left as they were.

[ED-SKaR]

Not sure what battery I can fit in it though. I don't know why, but I'm still a little hesistant to go down the Li-Po route. Might be me just being a big girl, but I'm still convinced they are going to blow up in a big nuclear stylie and raze my garage to the ground.

 

 

You dont need that. If i was to go DMR (and i will with my L86 when i can be bothered to set it up) the first thing is to set up the gearbox how you want it, big spring, torqey gears and whatever. but then for the battery put in a custom 9.6v nimh. I recon i can get a 4-5 Ah batery in the front of my L86 but thats another story.

 

Huge largetype or preferably custom 9.6 will give you a good trigger responce even if you are pushing 400fps.

[skag187]

or you could do it electronically, http://www.youtube.com/watch?v=kCG_PVhvwrI

as I understand available soon

[ED-SKaR]

Or you could try throwing in a torque motor and a lipo?

 

I have an m4 pulling just over 400fps: It has stock ICS gears, a mosfet, and a turbo motor. Surprisingly it feels like a regular AEG pulling 320ish, doing about 18-20rps. On an 11.1v it felt like it was doing at least 25rps. That's off a stock tube 11.1v 15c li-po. A 30c or 40c lipo would be even snappier.

 

 

he specifically asked about not using lipos, they are unnessesery

[Docv400]

...Point is that if you're running a gun that shoots in semi-only and is controlled by an AB mosfet you can definitely remove the ARL.

All that happens is that when the trigger contacts are released the mosfet shorts out the motor terminals which makes it hard for the motor to spin.

 

Very few mosfets are truly "active" braking, meaning that they pump a momentary reverse current though the motor.

 

Point being that a proper "active" braking circuit will only pump a momentary burst of reverse voltage through the motor and a normal mosfet will work by shorting the motor terminals.

In either case, there's no continual supply of leccy drawn...

 

You're basically correct, but it needs clarifying a bit...

 

A 'proper' Active Braking (AB) MOSFET set-up has a 'P'-channel FET in addition to the usual 'N'-channel item (in a simple switching set-up), it puts a momentary reverse curent to the motor to stop it spinning when the trigger is released (or the semi cut-off is activated).

The shorting method, or Passive Braking (PB), is done by a secondary set of contacts in the trigger block (as in the ARMY L85 and others), a MOSFET isn't neccessary for a PB set-up, and you couldn't achieve it with just a (single) MOSFET.

[Stealthbomber]

You're basically correct, but it needs clarifying a bit...

 

A 'proper' Active Braking (AB) MOSFET set-up has a 'P'-channel FET in addition to the usual 'N'-channel item (in a simple switching set-up), it puts a momentary reverse curent to the motor to stop it spinning when the trigger is released (or the semi cut-off is activated).

 

Nope.

 

Mosfets are, as we should know, simply switches. An N-channel mosfet is a normally-open switch and a P-channel mosfet is a normally-closed switch.

When you apply power to the gate pin the mosfet changes state.

In a simple two-transistor mosfet all that's happening is the P-channel mosfet goes open-circuit when power is applied, thus allowing the motor to spin.

 

This is a detail I find irritating when it comes to the description of mosfet units for sale.

People apply the term "active braking" when they either don't understand what it means, don't know what the item they're building actually does or are knowingy telling fibs.

 

To create a truly ACTIVE braking system you'd either need a circuit which includes a capacitor and resistor (and possibly even a third transistor) or a fancy computerised system.

 

 

*EDIT*

I realise that part of what you're saying is that a single-transistor mosfet won't have a braking circuit. Fair comment that the term "mosfet" should be clarified.

When we're talking about the braking effect of mosfets I assume people realise we're talking about 2-transistor devices that, together, are described as a "mosfet" in airsoft vernacular.

[skag187]

In the 1st post the OP said he wanted Pre-cocking in semi for DMR use but also wanted 3 round burst, as far as know, the only way to have both is with some sort of fire-control computer his newest version the Lion will feature pre-cocking as one of it features, shown in this video

I mean yeah. its $100.00 (I am guessing its not out yet) but it does it all, its small, its open source (so anyone can write for it) and as easy to install as a mosfet

[jond36]

or you could do it electronically, http://www.youtube.com/watch?v=kCG_PVhvwrI

as I understand available soon

 

Wow! This could give a great advantage.

[skag187]

Wow! This could give a great advantage.

 

I know I am definitely getting one ASAP

[The Chef]

Thanks guys.

 

Skag, I'll take a look at that in a bit.......

 

 

Darthwite is correct.

I am not averse to using a LiPo, just, shall we say, apprehensive. I've heard some real horror stories and with that combined with my innate talent to have things go bang around me, I just wonder whether I am tempting fate that little bit.

 

If I can get an ultra snappy trigger response, then indeed the pre-cocking might not be necessary, especially if it's going to be prohibitively ridiculous to implement. I am trying to reduce that delay before the shot is fired, preferably as close to zero as possible.

 

The three round burst again, is purely a 'would like' and not a definite. If its not possible, then obviously I can drop it with out tears. but it would be nice.

[Stealthbomber]

FWIW, the simple burst function based on a timer will probably be fine in use.

They don't require any additional work at all. It's just like fitting a mosfet.

 

As I tried to say before (but possibly failed) it's just a timer that starts to count down from half a second every time you pull the trigger and resets when you release the trigger again.

When you shoot in semi the cycle is complete and the cut-off lever stops the gun shooting before half a second is up and you release the trigger, thus resetting the timer.

In auto the gun shoots for half a second and then stops. You release the trigger and the timer resets for another go.

 

If I sounded negative about this earlier it was in conjunction with the pre-cocked doodah cos the timer-burst would leave your gears in some random position where it's entirely likely that your next shot in semi would be a dud.

 

In a regular gun the burst gizmo's are well worth a look.

 

I am in the process of trying to standardise all my kit so part of that plan is to fit an RA-Tech burst trigger in my M4 GBB and then fit a burst gizmo in my ICS M4 so it operates the same way.

 

 

Main issue with LiPos is where they fit into the gun IMO.

If you're trying to shove a LiPo into a buffer tube you're really asking for trouble if it's tight.

If the battery is going into a space that's a nice fit it won't get damaged and will probably last forever.

 

Personally, I have an old .50 cal ammo ox and I store all my LiPos in it and also drop the charger in it when I have to charge them up.

Least that way if one does catch fire it won't melt a hoel in the carpet or table.