F2000 craps out again... Angle Of Engagement?

[RostokMcSpoons]

Took the F2K along to the skirmish yesterday, had some fun and games trying to sort out the hop (at least it's hopping now)

Basically I think, thanks to not enough time on the range, so trying to do this on the field PLUS having my contacts in (so bad short distance vision) PLUS being stupid enough to fiddle with gloves on... I think I wound on so much hop I jammed the barrel.  Consequently the gun went 'bzzzzzzzzt' and that was that for its day

This is a project gun, and therefore this is just another part of the learning experience for me.  So I'm not too upset.

Disassembling last night, I was confronted with this:

(Arrrghh no focus.  Sorry.  Well anyway the pic (which I can't be arsed to re-take) shows a sprinkling of fine metal filings under the bevel gear

View attachment 85404

View attachment 85405

It turns out what really grinds my gears is the immovable object of the jammed gearbox meeting the unstoppable force of the high torque motor. 

And yeah, it looks like before, or perhaps after the jam, the sector gear was eating the piston... 

View attachment 85403

There's damage across the whole of the first tooth, and the top of the next 4 or 5 too.

Do you think the piston was damaged by a jam caused by the hop, or was gearbox going wrong anyway?

Am I going to have to learn the arcane ways of the Angle Of Engagement?  
Is this piston beyond saving already, or do I just need to file the first few teeth to make it easier for the sector gear to engage them?

As always, your thoughts would be much appreciated.

 

[Adolf Hamster]

metal filings under the bevel i'd be taking a close look at the shimming between the sector and bevel, and the engagement of the pinion. usually the latter is pretty tricky to do by eye (when the box is in its assembled state) but you can tell by ear when it's running if the box is straining.

one method is once you have the bevel shimmed as low as it can go to the left half (with all the shims in place on the top side too) is to take just the bevel and put it in the right half of the box and hold the motor in position to see if it's roughly at the right height. shimming the rest of the gears as low as possible gives the range of motion you've got to play with (eg if you've got 0.3mm under the bevel on the left side, you know you can't go lower than that) but you can go higher, which will help with the pinion engagement.

it can take a few test-tries, put the box together with just the motor and gears and run it to get a feel for the noise/strain it's under with no load (same idea as putting the box together with just the gears and spinning them by hand to check they spin free)

the piston is a bit tricker, one school of thought is that sort of damage is the sector gear naturally carving its own preferred engagement into the piston. when switching to the likes of steel rack pistons this can't be done, so you need to put some effort into making sure the aoe is smooth.

setting aoe isn't particularly difficult. you can get shims from ak2m4 that space the piston back from the piston head. the aim is to get the first pickup tooth to mesh smoothly with the sector gear (as a general rule i aim to have the face of the piston tooth and sector tooth meeting such that the full area is contacted). it should never require more than 1 tooth removed and in the majority of cases half a tooth is enough (i tend to buy the shs 14.5 tooth pistons for this reason). one of luke's more recent vids (i forget which one) has a good point about carving the back of the second tooth at an angle, to give clearance whilst also preserving as much of the second tooth as possible to catch the second tooth of the sector.

the way to check it is to have just the sector gear and piston/cylinder assembly in the gun (sans-spring), push the piston forward by hand and roll the sector around to pull it back, you'll feel if it's not picking it up smoothly. although bear in mind if there's any soft components in the cylinder (eg a mushroom piston head) then the piston might be slightly further forward when under spring tension.

where folk tend to go wrong with aoe is "correcting" guns that don't need corrected, or overcorrecting by shoving 1/4" of sorbothane in the cylinder regardless of how far back the piston actually needs to go.

 

[Skara]

I don't think it has anything to do with AoE.

Adolf already wrote a nice wall of text for you, I will just add that the damage on the piston could be caused by PME. Not because the gears are turning too fast for the spring to push the piston all the way forward, but most likely because of the hop jam.

What (most likely) happened is that the air trapped inside the cylinder couldn't be expelled forward into the nozzle so it pushed back on the piston, either stopping it or slowing it down enough for the sector gear to pick it up at a random spot.

Either way, I'm not a fan of 15t pistons, the 2nd tooth (first after the big, pick up one) is essentially useless and most aftermarket pistons already come with that tooth removed/lowered.

 

[RostokMcSpoons]

Thanks chaps, I'm probably not going to be able to test out all the shimming things today, Valentine's day being what it is

I've just looked at the motor and there's quite a bit of metal filings stuck to it.   No obvious wear on the bushing.  All the gears etc look ok.   

I was fairly happy with the shimming though - there was a tiny little bit of side-to-side movement available on each gear so it wasn't overtight.  My intention was to run it like that for the full day's play, then take it apart and check the shim job on the COL held out (bonus factoid: it's survived so far), and then tighten the gear shims if necessary.  Better to be too loose than too tight, I thought.

 

[Skara]

Better to be too loose than too tight

Yup.

 

[Adolf Hamster]

Thanks chaps, I'm probably not going to be able to test out all the shimming things today, Valentine's day being what it is

I've just looked at the motor and there's quite a bit of metal filings stuck to it.   No obvious wear on the bushing.  All the gears etc look ok.   

I was fairly happy with the shimming though - there was a tiny little bit of side-to-side movement available on each gear so it wasn't overtight.  My intention was to run it like that for the full day's play, then take it apart and check the shim job on the COL held out (bonus factoid: it's survived so far), and then tighten the gear shims if necessary.

the motor engagement is sort-of a separate issue to shiming (although it rolls into the shimming process)

think of it as doing the equivalent of adjusting the motor height (moving the pinion up/down) but for the bevel, there's a sweet spot where both will mesh nicely (the point where the motor sounds least labored and there's minimal gear noise).

your normal shimming will put a limit on this as you can only make the bevel go so low before it interferes with the idler, and so high before there's bad overlap with the idler/its interfering with the upper half of the case.

Better to be too loose than too tight, I thought.

absolutely, you don't need zero play in the gears, you just need to be sure that they cant move into a position where they might interfere with the box/each other, and that there's a good overlap (ie one gears teeth aren't halfway off the other).

hence when checking shimming, spinning the gears with the box at different angles to make sure that it's always spinning free.

 

[RostokMcSpoons]

I've just put the gearbox back together a couple of times today to check the shimming and I've just had some weirdness.   It occasionally decides to fire on its own
I've noticed that the nozzle is partly retracted, so it's partly pre-cocked.   Pushing back on the nozzle or bumping the gearbox occasionally gets it a fire a shot, or even a short burst, which is a tad disconcerting!
It doesn't always pre-cock.  And before this last re-assembly it was behaving itself (as far as I can tell, though I wasn't paying attention to the nozzle position)  so what could I have got wrong?

The last change involved moving a shim on the sector gear from the top to the bottom half, which seemed to help the trigger movement (it had seemed to stick slightly before), and also moving the front trigger contact as it was getting snagged on the tappet spring.  So trigger / tappet / nozzle were all feeling nice and smooth.

Before that I also smoothed off the main tooth on the piston, and filed down the second to a half height, but I'd test fired with that and hadn't seen the self-firing.
 

Has the Merf's active breaking gone doolally?  I don't think I can turn that off.  
Could the interaction of my shim on the COL's cam be interacting with the shim of the sector gear to make the sear action super-sensitive?

 

[Adolf Hamster]

sounds like there's some bare wiring making contact somewhere, possibly against the gearbox shell?

active brake will stop the motor based on when the mosfet isn't getting a connection, so whilst it should be reasonably predictable in normal semi-auto operation (as the col will trip at a predictable time), if it's in auto (which is the same as if it's getting a faulty contact signal) it'll just stop wherever it happens to be when the contact is broken.

 

[RostokMcSpoons]

Cheers, looks like that was a good shout.  One of the signal wires was in contact with the shell, the other one was close.   
I've bent the front one away from the shell, and it seems to be behaving itself again    hew:

 

[Adolf Hamster]

Cheers, looks like that was a good shout.  One of the signal wires was in contact with the shell, the other one was close.   
I've bent the front one away from the shell, and it seems to be behaving itself again    hew:

yeah, they can be tricksy hobbitses.

at least using the 2-wire setup you get random firing rather than a short (with associated bad smells)

 

[RostokMcSpoons]

Did some test firing on Friday that didn't go brilliantly well, with some failures to feed that I thought might be due to the weird positioning of the nozzle between shots, but I've just screwed the back plate in hard (ooooo-eeerrr Missus, yub-yub, wibble ker-snip) and fired a couple of hundred BB's through it down the garden.   Without any jams or problems.

View attachment 85854

Shot dispersion side-to-side was a bit more random than I'd like, but as this gun is now officially my Spray'N'Pray Nearly-Wanker-RoF Beast, then I think accuracy can be sorted at a future date (and after a future pay cheque).  I just hope I can actually have it run solidly for at least half a day before I do anything else to it!

 

[Adolf Hamster]

Good to hear she's shooting again.

Is the side-side dispersion more than the vertical dispersion or just in general wider than preferred?

 

[RostokMcSpoons]

I'll see if I can record some video or put some rounds on paper tomorrow... But at 15m some of the shots were missing a target that's about 9" wide.  Quite a few were smacking into the middle, but it's feeling less accurate than the DE with its shorter barrel. 

It was of course a windy day, maybe it was gusting and I just didn't notice

In unrelated news, unexpected gnu in bagging area, more fun and frolics to come!  (Apologies in advance) 

 

[Adolf Hamster]

I'll see if I can record some video or put some rounds on paper tomorrow... But at 15m some of the shots were missing a target that's about 9" wide.  Quite a few were smacking into the middle, but it's feeling less accurate than the DE with its shorter barrel. 

It was of course a windy day, maybe it was gusting and I just didn't notice

wind would definately do it, if that weren't a factor i'd be thinking that kind of dispersion there's maybe some grit in the barrel (assuming there's no supressor shenanigans that the rounds might be clipping)

In unrelated news, unexpected gnu in bagging area, more fun and frolics to come!  (Apologies in advance) 

and so it begins

 

[RostokMcSpoons]

Bah humbug.

Well I never got around to saying that I took the gun along to the skirmish a couple of weeks back, and it stopped firing on the range after only a few shots.

Having just taken it apart, I could see the trigger was jammed - so it wouldn't go back far enough to actually make contact for single or full auto.

I meddled with the spring and the lever from the left side of the gearbox, and the trigger released.  So I tried a test fire.  
Ughhh.   First I made the mistake of using my DE mag which is the one the F2000 really doesn't like, so I think I had some double feeding.  Then I tried another, and things went downhill fast.
It spat out a few shots, not very accurately, and then I got the death scream / rattle.

Yup, it's eaten another piston.
 

View attachment 86794

Ho hum.  Any recommendations for a replacement piston? 

Is it worth trying a 14 tooth one? 

And should I avoid metal teeth in case it tries to do the same again?

Anyway, in other news... spotted off the starboard bow... 
 

View attachment 86795

Yes, another one


 

 

[Adolf Hamster]

normally i tend to go for the standard (not swiss cheesed, you can save more weight by not running a thrust bearing on the piston end) rocket/shs pistons from ak2m4. either 14.5t or a 15t and file down a tooth as appropriate.

steel rack can push the problem elsewhere, but they can still fail, especially if the rack isn't glued in proper.

 

[RostokMcSpoons]

In the absence of actually being able to go skirmish, I've been making holes in my waste paper basket and throwing BB's around my garden.   The good news is that the F2K is now running apparently quite well with the Nuprol 14 tooth piston I sourced from Action Hobbies on a whim, along with the mushroom piston and cylinder head combo I bought from @ak2m4 a while back.

I've also re-shimmed and so was rather surprised to see the rate of fire at a positively wanker-ish 26rps, but I think that's being 'improved' by the fact the air seal with this piston head and cylinder head combo is pretty poor.  When I put my thumb over the nozzle there's resistance but its quite easy to push the piston all the way in.  So it'll make the piston be able to cycle back and forth very nicely, but the muzzle velocity is all over the place, varying from 200-255fps (0.28g BBs, hop applied, spring guide not fully extended)
 

So I need a new piston head / cylinder head combo?  Or is there some bedroom-tech shenanigans I can get up to?

 

[Skara]

Where is the air escaping from?

Does the nozzle have an o-ring?

Also, mushroom (or silent, in marketing language) piston heads aren't made to stand up to high RPS abuse, or at least that's what I heard back then.

 

[Adolf Hamster]

is the piston head the polymer body variant?

because i had one of those (funnily enough in me g&g f2000...) and i found they'd got texturing on the plastic that was right round the whole piston including the o-ring track meaning awful sealing.

 

[RostokMcSpoons]

It's not the nozzle the air is escaping from, I did check that.

Here's the kit I bought: 
https://www.ak2m4.co.uk/xt-v2-v3-cylinder-piston-head-aluminium-silent?search=xt67

View attachment 89304

Obviously it's all tucked away inside the gun so I can't inspect it right now.
Should I try swapping the o-rings over?

I presume the cylinders all have the same internal dimensions?  Certainly the old piston + head made a good seal with the new cylinder + head.  (But then the piston got eaten)
Here it is for reference anyway...
https://www.ak2m4.co.uk/index.php?route=product/search&search=xt77