Tuning FX Impact Mk2/M3 - slower first shot - no reg creep - quasi-scientific analysis

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Well, the first shot may have lower v0 because of a couple factors:

- reg creep - eliminated in my case. My regulated reg pressure is the same after sitting for many hours. I have a Sekhment digital gauge,

- valve poppet stiction - eliminated as I have a peek one.

The only remaining problem is a valve rod stiction. This is what Bob Sterne added:



It is also available on a GTA forum.

Conclusion:

- I will make a tight fit cone from peek. This will make the gap between the rod and the peek cone as small as possible,

- I will use 2.84x2.62FKM90 oring instead of FKM80,

The peek cone will also make the oring seal against the square face, not the conical one.

I don't have any other ideas without any plug modification.

How about that?
 
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I think the part need to be re-machined/re-engineered. I don’t see the end user glueing in parts as a solution. 

There are some pretty tiny grooving bars available in the Swiss machine world. 
if the rod is 3mm, what the major dia of the bore?

I also think FX needs to step up to the plate here. I’m pretty sure this issue drove me nuts on the mk2. I recall calling fx and telling them as it stands, my impact isn’t a hunting gun, as the first shot always misses. 


 
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I will use 2.84×2.62FKM90 oring instead of FKM80
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Please be aware FKM / Viton has poor abrasion resistance. It may or may not have a meaningful effect on longevity in this application. The stroke is short and it is a low duty cycle so I am inclined to think no problem except others have cited wear issues and it stands to reason that is why FX chose polyurethane in the first place. 
 
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That’s what happens when too many ideas and too many pages occur. I now remember seeing your diagram a few pages back. You just had to wait for us to catch up. I may have also dismissed it initially because cutting a 3mm internal oring is not fun. Will probably have to modify one of my micro internal grooving tools. At least it won’t be in a steel barrel. 
 
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The one thing you have to consider is air leaking around the peek guide. Do you cut an oring groove in the peek piece or put a dab of thread sealant on it before you press it in. It would be almost impossible to burnish it into the concave piece and expect it to seal like a poppet.
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The peek cone-like washer is there to serve two purposes:

1. Make the gap between the rod and the washer itself as small as possible,

2. Make the sealing face square for an oring.

There will be 2.84x2.62Dur90 oring sitting on the top of the washer.



So you just drop in the washer, top it with an oring and that's all.
 
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Seeker, if I’m understanding your reply fully, none of it addresses Vetmx’s point about air leaking around the outside diameter of the PEEK part. It needs an O-ring on the OD (as in my sketch) or some other feature/method to seal it.

[edit] Okay I see the sketch you added. The O-ring is sized to seal both the ID and OD. That way the pressure differential is responsible for keeping the O-ring seated against the PEEK part. That may work fine on the short term and perhaps long term as well but it may be apt to develop a leak in a cyclic action like this.


 
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Seeker, if I’m understanding your reply fully, none of it addresses Vetmx’s point about air leaking around the outside diameter of the PEEK part. It needs an O-ring on the OD (as in my sketch) or some other feature/method to seal it.

Vetmx, haha yeah I agree about grooving a 3mm bore. A primitive solution would be to cut a shallow counterbore on the face and bond a washer over it to create a groove to capture the O-ring. Tedious for sure but so is the alternative of grinding a tiny HSS grooving cutter.
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It covers Vetmx's point. The Dur90 oring will seal the outside diameter of the peek washer as well as the rod in relation to the washer.

It will be like that:



The washer needs to have smaller diameter (similar to oring's OD) and it will be conical. The picture is just to give you some idea.

It covers everything Bob's stated:



No need to modify the plug or make any oring grooves.
 
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Seeker, if I’m understanding your reply fully, none of it addresses Vetmx’s point about air leaking around the outside diameter of the PEEK part. It needs an O-ring on the OD (as in my sketch) or some other feature/method to seal it.

[edit] Okay I see the sketch you added. The O-ring is sized to seal both the ID and OD. That way the pressure differential is responsible for keeping the O-ring seated against the PEEK part. That may work fine on the short term and perhaps long term as well but it may be apt to develop a leak in a cyclic action like this.

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True. That's why I need to test it. I do not have any other idea without plug modification. If it works then a user just needs to drop in the washer and it should make him covered. There may be a need to loctite the washer so that it does not move back and forth during gun assembling.

Peek has very good abbrasion resistence properties. It is also used to make bushings. What is more - it should be rigid enough to withstand obout 60kg of force without flexing and jamming the rod.
 
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The ultimate goal of my proposal is to reduce/eliminate oring wedging caused by its extrusion. If the rod will be guided by the brass retention screw in mubhaur's solution and it will be a tight fit over the rod then it will also work and will be more elegant and reliable technically. However, who's going to machine such parts with offset threads? It is not that easy. The peek washer can be treated as a workaround.

P.S. Every Mk2 user may have such a problem assuming the plenum pressure high enough (130-140bars+)and v0 set on the knee but there is not that much of engagement from other forum members. We have a lot of smart people here but they seem to stay silent.


 
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I understand that my machinist can do it for me using his laith chuck with separately adjustable jaws.

The off set will no more look like off set once properly centered in adjustable laith chuck. 

To me it is easy and time tested arrangement. 

Bhaur
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Yeah…if he has a 3 jaw chuck where all three jaws tighten individually. U can offest the part.

I would just stick it to my collet chuck from the back. So the collet clamps on the smaller dia. Automatically centered 
 
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I think if I build one I’m going to make it approximately 12mm long with 2 outer o rings and one internal. Won’t even have worry about cutting that taper on the end of it. I will only lose a tiny bit of plenum volume but at 12mm, it will be stable in there. I’m just worried about seating the oring on a flat surface with the Seeker design. I think FX cut that concave area to purposely use the air pressure to pinch the oring onto the rod. But the side effects of the design are not so desirable. 
 
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It isn't a feasible for everyone to be able to cut a new oring groove into the outer section and I believe this will work the oring may not last as long as the way FX designed it but it should seal both inner on the rod and outer so air won't go around the peek it is a simple work around and I don't believe you will have to bond it to the rod the oring acts as the seal the peek is just keeping from getting to far into the pinch point 
 
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