Taming the bark of an Avenger

[subscriber]

Hi Bob,

Thanks for spending time and resources to make these LDCs and test them.  Looking forwards to more results with pellets.  It would be useful to know the FPE, as more generally means louder.

The longer "Buck-Rail" style version has a multitude of closely spaced cones.  It would be great if you could also test the version with "plume" in the file name.   That has fewer chambers, where each successive cone is shallower and further apart.  So, each design is testing a different approach.

The "fatso" is large and should be more effective; especially when fired with a pellet.  As should the others.

[TorqueMaster]

My two cents on the 2-piece concept.  I had envisioned it having internal threads.  Same as "industry standard."  1/2"-20, is it?  However, if plastic is borderline up to carrying the weight with that threads size, go bigger internal then. Even the external thread idea is "ok", but I would want a cap to cover them if no external LDC is attached.  Strong, coarse, wide, deep threads, perhaps even custom threads should be used.  Appropriate for PLASTIC.  Whatever threads are chosen, an adapter could still be made to allow "industry standard" to be attached -- let the user decide if it feels too weak/droopy.  Quarter turn lugs seem a cool idea, but I'm not sold on ruggedness.  Turn too hard, plastic snaps.  With threads you can feel it snugging down and stop before there's damage.  I think that's all I've got for now.  Gotta finish my taxes ugggg.

[TorqueMaster]

Somehow I missed the "plume" version -- got it now -- I'll see if I can get it into the test mix.

Yes, I'll bump the reg, and test with pellets, and ... I'll do my best with FPE -- I don't think my barrel mount chrony will work with these cans in the way.  Wish I could test if/how they affect muzzle velocity, that would be interesting, too. Actually, I should be able to test that with the skinny 25mm one. 

Is it important to know the general FPE I'm shooting at, or the change in FPE (if any) due to the LDC?

[subscriber]

Thanks Bob,

Your thought on robust thread forms echo my own.   I have been thinking about my approach for a few hours, to figure the constraints for the starting point...


I don't expect any change in pellet FPE, with or without LDCs.  Not meaningful ones.  So, the request is to help quantify the size of the "job" an LDC is doing.   An already quiet .22 PCP that shoots at 10 FPE is not nearly as meaningful as one that shoots at 30 FPE.  I actually expect the fatso to shine when shooting at 40 FPE, while some of the slimmer designs may be just fine at lower power levels.

If you lack a working chronograph, projectile drop is a function of velocity.  With a known BC pellet, shooting at a target, zeroed at a known close distance, then shooting more of the same pellets at that tune, at 10 yard increments to 50 yards (or further, if possible) can be used to calculate the muzzle velocity.    Such drop testing should be done near level, or at least not at a steep angle.  Yes, the barrel has to point up to hit a target at eye height at 50 yards.  As long as the target to eye heights are nearly on the same level, that can be considered level enough.

The added piece of info is sightline center height above bore center.  Now, I don't have Chairgun or Sterlok, or whatever the best airgun ballistics program is for this, but I know who I would consult to convert drop data into muzzle velocity.  GTA has the brainpower to do it.

It may even be possible to sound record the shot from mid range to a 50 yard target, and use the average velocity to calculate muzzle velocity for a known pellet...


Some people might express doubts about your sound meter and test method.  The reason why I did not bring it up, is you are using what you have, and providing a relative measure, rather than an absolute one.   Most important to me, is that you are consistent in your method.

For what it is worth, your dB values seem low.  Especially from 2 feet off to the side.  If I was going to insist on a method and instrument, then I would need to provide you with that .  I just paid the taxman over 4k and am not in the mood to spend more money...

[subscriber]

It took me over 8 hours to model the interrupted thread mating LDC couplings shown below.  It is based on a 20 mm OD x 3 mm pitch thread.  I used "enough" diametral clearance between the male and female thread, so that the fist attempt to print these sample parts should have it real close to working.

There is a 60 degree included angle self-centering mating shoulder on the parts.  Without such a feature I fail to see what would center the parts - unless the thread flanks are able to achieve that without help.

I used three interrupted thread "lugs" because that makes each block of threads more robust, than four or more narrower ones.  Also, whatever stabilizing force can be generated to align the parts, three is a good number to avoid rocking.

After answering the question, if the threads work easily and smoothly, I would like to know if the timing of the thread to shoulder needs adjusting.  Ideally when the threads are tight, the three sets of lugs will be at 100% overlap.  I can adjust that by moving the shoulder closer or further away from the threads.  I also need to verify if there is any interference or contact at any other than the cone shoulders.


EDIT:  I have updated the part model for the female thread to look like the last simplified image.   The central hole diameter near the cone is now the same as the ID running inside the rest of the part.  Less chance of interference that way.

Will hand off the STL files for these two parts so motivated people can print them...

[Outer_Rondacker]

Perhaps you guys are correct. Two piece mods are a bad idea. I have other issues to deal with at the moment on this avenger. I have to attempt to print a AR style stock for the rifle. The cheap plastic junk that comes on it is wobbling all over the place and I feel like the next time I pick it up its going to bust in half. Not bad for being a few weeks old. It is becoming almost impossible to stay on target.  I found something on thingivers but states its not finished.

Someone else who has better sound meters and chronographs can test in there back yard for now.

[subscriber]

Matt,

The two piece design is not a bad idea.  It has challenges.  I just put in an all-nighter towards making that work.

Anyway, the above interrupted thread test piece STL parts can be downloaded from this link for 7 days:  https://we.tl/t-d81e0kNTHl

I hope "someone" prints these test parts, after insisting a two piece design was the way forwards.  In exchange, describe what the stock or stock adaptor parts need to look like (with key dimensions), and I will create STL files for that.

[Mr.P]

I don’t have an avenger but am still enjoying this thread.  The detail on those models is outrageous. 

[TorqueMaster]

The tri-thread lug looks great.  My first question, though, was...why?  I see that it looks like it can slip on 95% of the way, and then a 1/3 rotation should secure it, so that is cool.  What happens if it is installed in one of the other 2 "wrong" orientations?  Is this stronger than having full threads that need rotated 10 times?  This ideally does end up in a -- how do they say -- registered position -- so you know up is up, but I don't think an LDC is affected by that. 

After typing that, it looks harsh -- not meant to be, just an honest first impression.  At first glance it looks like "normal" threads with three sixths removed.  I now suspect if I look at it closer, the "lugs" are symmetrical -- so any of the 3 installation positions will allow it to rotate 1/3...err 1/6 turn and lock down.  So ok, if that's the case, it is a robust plastic "lug" and I am starting to get it.  I'll add it to the to be printed queue.

You mentioned not knowing if just threads would allow correct alignment -- if designed concentrically -- and both parts have mating flats -- like a nut on a bolt -- seems very aligned to me.  That said, the taper you used looks even better to keep it all co-axial.

Re:FPE -- I have a chronograph, it attaches to the barrel.  It will fit the shroud.  It will not fit a big LDC.  I can test FPE without an LDC, then remove it and go about testing the sound levels. 

Some quick research says some smartphones have automatic level adjustment, which may corrupt any level meter app from getting consistent results.  I'll have to watch background levels and see what's going on.  I have a standalone analog sound meter -- between the two I should be able to get some consistent results, at least level CHANGE, if not actual "correct" levels. 

[subscriber]

Bob,

You are astute.  Why an interrupted thread?  Because a quick install/remove was requested.  Is it really that hard to turn something 10 times (or 5 with a coarse thread). compared to 1/3 turn?  Well, it is faster. 

While the individual threads are more robust than a finer thread to stand frequent removal / installation, the combined installed strength is not stronger than a fully circular thread, because half of the full thread is missing.  While my design works like a bayonet fitting, the fact that the threaded lugs are backed up by closed rings, without long open cantilever sections, makes the interrupted thread stronger than a simple bayonet fitting.

The downside, as you suspect, is that one needs to have registration marks, like on a reflex camera lens mount, or the timing with the shoulder will be off.  This could be two dots of nail varnish or paint in appropriate places.   So, just match the two features, insert and rotate 1/3 turn.  The features can be discretely molded into the design if you want to do this in the dark, although shooting in the dark could be tricky. 

If the thread was a true multi-start, then it would not matter how you install it.  But, a long coarse strong multi-start thread has a very steep helix angle, compared to regular thread.  This means it does not have the same friction holding for a given tightening torque.

One may be able to get away without the conical shoulder for alignment, but there is no downside to having it.   For one thing, the cones work to increase rotational holding friction, at a given tightening torque.  Research conical clutches and brakes.


Measuring pellet velocity with the LDC off will provide the info required.

I think that observations such as, "all you hear is the hammer dropping" are rather encouraging, regardless of the sound measurement device.  Especially if without the LDC your Avenger is uncomfortably loud.  That said, a peak hold sound meter that is able to read above the values you expect would be useful. 

If you want to compare results with others on the forum, then placing the mic level with and 1 meter to the side of the muzzle would be useful.  That is with the muzzle 1.5 m off the floor.  Or you can use your own distances, as long as it is stated and repeated for comparable measurements.   We already have non-standard measurements, and I have no problem using that data to compare different devices, tested the same way.

I bought this meter a couple of years ago, because it was what Pyramyd Air used at the time:  www.amazon.com/gp/product/B000EWY67W  .   There are other similar looking meters for less money out there.  I think MrBulk's thread about testing 20 silencers (a sticky on this forum) had a few options listed in it.

Many sound recorders have auto-levelling.  I thing smartphone apps to measure sound levels have to look at the raw data, or everything would measure the same...  That said, how good are they at differentiating short sharp sounds?  No idea...
 

[WhatUPSbox?]

If you have an iPhone, the CDC has an app (free) that they developed.
https://www.cdc.gov/niosh/topics/noise/app.html

[subscriber]

Thanks, Stan

From your link, the app can be downloaded here: 
https://apps.apple.com/us/app/niosh-slm/id1096545820

[TorqueMaster]

I did some testing at ~1750psi reg and maxed out the hammer spring.  It got the .22 14.3gr CPHPs zipping along around 963fps for 29.45fpe.

I did not realize how air hungry it is at this level, so unfortunately part/much of my test were off reg, and I don't know when it fell off.  I'm not convinced my phone will suffice for such a short burst of sound anyway, I need to test that first.  Multiple Android apps measure room ambient volume similarly -- at 10-20db which seems way too low -- and some can be "calibrated" by adding a linear offset it appears.   My analog meter is NOT good for short bursts.  I may look into one of the newer low-end electronic models.  I can try borrowing an ipad and use with that CDC NIOSH app, too...

What I did learn is that the plastic threads that mate to the shroud-- I printed them "inclusive" in Cura to maximize their OD -- are not holding.  Could be partly because I printed at 0.28mm layers, but it is still a slightly snug fit all the way in, so I'm not convinced printing with thinner layers would help.  So...both the "air stripper only" and the slim buckrail were launched 20 or so feet.  No damage.  I started electrical taping them to the shroud after that and no further escapees.  I may just need to print a hair wider yet, or try PLA, since its a bit stronger than PETG.  The stock cap seems to be a harder type plastic, not metal, so plastic should be capable...

I also printed the tri-screw-lug.  Seems almost perfect on first run, awesome!  Also printed at 0.28 layers, so the tiny bit rotation is off could be due to that lack of precision.  It seems plenty strong, so as-is or fully-threaded would be fine.  Photos attached...

[subscriber]

Thanks Bob.   Your feedback is motivating.  I try hard to incorporate feedback into future designs.

If one were to actually hand tighten the two parts of the interrupted thread, would they line up closer to 100%, or overtravel further?  If I interpret correctly, there is slight overtravel?

Even as shown, that is very close to full overlap.  It also hints at why I did not use 12 threaded lugs, where there is an angular mismatch (loss of contact) at each mating surface.  Three lug sets has the lowest total joint strength loss for a given angular error.

I was wondering when the LDC would be blown out, considering the very small thread mass on the Thingyverse thread design.  Buck-Rail Avenger LDCs seem to stand up to use, but their website states the threads are "chased", suggesting he prints oversize, then cuts away the excess with a die. 

I was contemplating conjuring M24 x 1 mm dies, as this thread size is not standard.  Else modifying a nut to cut plastic is rather simple.  Then again, importing such dies from China would probably be the cheapest; even if there are shipping delays.  I know that user Soupyyy has an M24 x 1 die that he uses to chase Avenger LDC threads.  That would be the most logical first step - to ask where he got his.

[WhatUPSbox?]

If you go down the iPad route for measuring sound, there is an interesting evaluation of using an external mic. The calibrated mic they use is relatively inexpensive (<$25).
https://asa.scitation.org/doi/full/10.1121/1.4964639
https://www.amazon.com/s?k=B00ADR2B84

[TorqueMaster]

Thanks Bob.   Your feedback is motivating.  I try hard to incorporate feedback into future designs.

If one were to actually hand tighten the two parts of the interrupted thread, would they line up closer to 100%, or overtravel further?  If I interpret correctly, there is slight overtravel?

Even as shown, that is very close to full overlap.  It also hints at why I did not use 12 threaded lugs, where there is an angular mismatch (loss of contact) at each mating surface.  Three lug sets has the lowest total joint strength loss for a given angular error.

I was wondering when the LDC would be blown out, considering the very small thread mass on the Thingyverse thread design.  Buck-Rail Avenger LDCs seem to stand up to use, but their website states the threads are "chased", suggesting he prints oversize, then cuts away the excess with a die. 

I was contemplating conjuring M24 x 1 mm dies, as this thread size is not standard.  Else modifying a nut to cut plastic is rather simple.  Then again, importing such dies from China would probably be the cheapest; even if there are shipping delays.  I know that user Soupyyy has an M24 x 1 die that he uses to chase Avenger LDC threads.  That would be the most logical first step - to ask where he got his.

Happy to help...until I find a new rabbit hole to dive into.  They are as hand tight as I can get them given the geometry.  It is slight undertravel.  So slight that it is fine now, and may be even better if there's more to grip, and as the parts wear.  And printed at a finer layer height may move it either way.

Quick fleabay search shows a "US Stock" die for $16ish.  Nuts are even more.  I can buy a lot of electrical tape for $16   I would have no major to issue with drilling the shroud to secure the inner LDC with small screws if necessary.

@WhatUPSbox? Thanks - a mic is cheaper than a new sound meter.  Hopefully I can make do with what I have on hand...

[Outer_Rondacker]

If you have an iPhone, the CDC has an app (free) that they developed.
https://www.cdc.gov/niosh/topics/noise/app.html

Thats what I was using for sound.

Bob's quote. "I did some testing at ~1750psi reg and maxed out the hammer spring."       Not telling you what to do Bob but your wasting air. Back off the hammer spring a bunch. I get 100 shots with the reg set at 1750 on a fill.

[subscriber]

Folks,

I am looking for interested volunteers to print and test the two-part detachable LDC shown below.  The STL files can be downloaded from this link for 7 days: 
https://we.tl/t-osYckrIoxV


The design uses interrupted threads to attach the external LDC to the in-shroud 2-stage air stripper section. 

To fit the female thread inside the shroud, I had to scale that aspect down to 85% the size of the one Bob printed two days ago.  So, now the threads are a nominal 16 mm OD with a 2.55 mm pitch.   

The most significant aspect is that the front and rear sections of the thread are now integral to functional in-shroud and external LDC sections.

The thread pair still has a boatload of clearance to avoid the need for "shaving off a few tenths" before the parts will mate.





EDIT:  I check STL files for integrity to see if they will open with Cura.  The image below indicates the orientation on the print platen that is required.    I don't know if there is a reason to avoid printing them side by side.  Perhaps they are stronger or straighter if printed one at a time.

[TorqueMaster]

If you have an iPhone, the CDC has an app (free) that they developed.
https://www.cdc.gov/niosh/topics/noise/app.html

Thats what I was using for sound.

Bob's quote. "I did some testing at ~1750psi reg and maxed out the hammer spring."       Not telling you what to do Bob but your wasting air. Back off the hammer spring a bunch. I get 100 shots with the reg set at 1750 on a fill.

Yes I am.  Was trying to keep testing somewhat uniform and no hammer spring adjustment was given.  So I tried zero, three, and max, and kept getting velocity increases (but the last was only maybe +5 fps) so I left it there.  I figured that a typical "more is better" consumer would probably do the same.  I further figured that more meaningful test results might be had if more air is blasting through the LDC.  If I were shooting at this reg power level, I would shoot for somewhere in the 915-935 fps range, not the max 963ish.  I didn't record the numbers, but I think even zero turns was faster than that. It's really up to subscriber... crank it up like a joe six-pack or dial it back like an enthusiast would?

For consistency of testing, we should be the same...

[Outer_Rondacker]

Yup it should.