.22 LDC bore clearance study print files

[subscriber]

Glad to see someone else generating and exploring ideas, Bruce

I am not going to predict the performance, even though prediction is a hobby of mine.  My only comment is about the ability of FDM printing to create small holes:  Small holes tend to close in a lot, percentage area wise.

[WobblyHand]

Dang, woke up to a plate of red spaghetti  .  Printed 153mm worth out of 166mm.  Got two threads at the top, and the LDC fell over.

I don't know.  Used to have an over adhesion problem, now under adhesion.  I'd guess this is so heavy and tall that the bed slinger eventually knocks it over.

Might redesign the chambers to have less ribs and more air.  Had 15 degree sectors with 20 degree spacing.  Probably will double the sector width and make the ribs slightly wider.

Is there a way to add external support?  Add a brim and supports from there?

I'd attach this file but it's way too large for this forum, zipped it's 18MB. 

[subscriber]

Bummer

Should have a brim.  Some of the other printer guys might give better advice.

[WobblyHand]

Bummer

Should have a brim.  Some of the other printer guys might give better advice.

I figured at 40mm diameter it didn't need one, but I was wrong.  This thing needs external supports to prevent rocking/vibrating.  Wouldn't be surprised if the top was waving around while printing.  Guess that's why they have other kinds of printers.

If I kick another one off, it will have a brim for sure.  Going to increase the sector size, though.  Too much filament usage.  The reject print weighs 103 grams.  I have to reduce print time, 9 hours is just too long, especially for a prototype.

[WobblyHand]

As another thought, someone mentioned TPU I can't imagine being able to complete something too complicated with it.  At least when I tried, I got a LOT of stringing.  Can't imagine it would resist gravity that well either.  But, it got me to think.  What if one changed the filament properties?  Say we selected a denser material?  That may make the printed object act either more ideally, or less.

The only filament that I know of with significantly higher density is one with tungsten powder in it.  Anyways, just some rambling on my part.

Really should get back to my CAD program and figure out where my FEA stuff is.  I'd love to be able to sim one of these.

[WhatUPSbox?]

Brim. You could also model in some supports.
Also check your bed leveling. If the first layer gap has opened up and the first layer isn't getting squished down, that will affect adhesion.

[WobblyHand]

Brim. You could also model in some supports.
Also check your bed leveling. If the first layer gap has opened up and the first layer isn't getting squished down, that will affect adhesion.

Good advice.  I will check the bed again.

[TorqueMaster]

Dang, woke up to a plate of red spaghetti  .  Printed 153mm worth out of 166mm.  Got two threads at the top, and the LDC fell over.

I don't know.  Used to have an over adhesion problem, now under adhesion.  I'd guess this is so heavy and tall that the bed slinger eventually knocks it over.

Might redesign the chambers to have less ribs and more air.  Had 15 degree sectors with 20 degree spacing.  Probably will double the sector width and make the ribs slightly wider.

Is there a way to add external support?  Add a brim and supports from there?

I'd attach this file but it's way too large for this forum, zipped it's 18MB.

Definitely a brim.  If you use supports, choose "bed only" so they are not internal and inaccessible.  If it's cylindrical, I doubt external supports will be attached automatically, you'd need to design and attach them.  Shouldn't really be necessary though.

I use PETG on a PEI textured bed and it sticks like the dickens, no worries until I get over roughly 5:1 height to width.   How fast are you printing?  That would obviously affect the forces the print sees.  I print pretty slowly ~30mm/s on average.  Also if you are overextruding a bit, sometimes a glob on top will get smacked by the printhead and knock the print over. 

[WobblyHand]

Dang, woke up to a plate of red spaghetti  .  Printed 153mm worth out of 166mm.  Got two threads at the top, and the LDC fell over.

I don't know.  Used to have an over adhesion problem, now under adhesion.  I'd guess this is so heavy and tall that the bed slinger eventually knocks it over.

Might redesign the chambers to have less ribs and more air.  Had 15 degree sectors with 20 degree spacing.  Probably will double the sector width and make the ribs slightly wider.

Is there a way to add external support?  Add a brim and supports from there?

I'd attach this file but it's way too large for this forum, zipped it's 18MB.

Definitely a brim.  If you use supports, choose "bed only" so they are not internal and inaccessible.  If it's cylindrical, I doubt external supports will be attached automatically, you'd need to design and attach them.  Shouldn't really be necessary though.

I use PETG on a PEI textured bed and it sticks like the dickens, no worries until I get over roughly 5:1 height to width.   How fast are you printing?  That would obviously affect the forces the print sees.  I print pretty slowly ~30mm/s on average.  Also if you are overextruding a bit, sometimes a glob on top will get smacked by the printhead and knock the print over.

That's exactly what happened.  Glob above the z height got smacked.  I saw it happen.  And for the third time.  What gets me is this hasn't happened before.  Don't recall seeing globs.  The globs were pretty small, only 0.5mm or less.  But that's enough to make full contact with the nozzle.

[TorqueMaster]

That's exactly what happened.  Glob above the z height got smacked.  I saw it happen.  And for the third time.  What gets me is this hasn't happened before.  Don't recall seeing globs.  The globs were pretty small, only 0.5mm or less.  But that's enough to make full contact with the nozzle.

I have a purely anecdotal, but similar situation -- I had a print fail at the same place in the print (near the end of the print, of course) at least twice.  Maybe it even got me a third time.  Printhead either stuck to, or ran into the print and knocked it loose.  This was an item I'd printed many times before, so the failures were thought to be a fluke, until they weren't.  These failures were all using the same gcode file.  I made a trivial change to the slicer parameters, enough to change the print path a little bit, resliced, and the resulting gcode printed as well as always on the first try.  I never figured out what caused the "bad spot" --  I just chalked it up to a slicer glitch.  Maybe something about that particular path created a blob big enough to cause an issue, I dunno. 

[WobblyHand]

Bob, @TorqueMaster,
What might such a change look like?  Say changing from aligned seams to random?

These printers seem to require a lot of odd tweaking.  My mill and lathe generally don't.  Is this because of the materials, the way they are built or some technology limitations?  I've printed several LDCs and none have run into this issue.  They were the same height, but not as wide.  They would have been easier to knock over as the base was narrower.

Used the same slicer.  Hmm, I recall some updates to the profile were done.  After that, I started to get blobs.  Or it's coincidence.  Makes me suspicious.  I may see if I can redownload the slicer and decline any further changes.  Only problem is this effect doesn't seem to be obvious until the layer count is high, which makes for a frustratingly long test cycle.  Maybe I can make a simpler object that shows the problem in less time.  Say 10mm OD and 45mm high.

[miksatx]

Printed Chimera 1&2. Chimera 1 clipped 2 pellets it’s out for now. Reprinting it on my other printer.
Chimera2
Gun Vet Compact shooting 18ftlb 770fps gun w/o moderator 3 shots
91.5 92.7 92.6 dB
 W moderator
88.3 88.3 89.5 dB

[miksatx]

Printed Chimera 1&2. Chimera 1 clipped 2 pellets it’s out for now. Reprinting it on my other printer.
Chimera2
Gun Vet Compact shooting 18ftlb 770fps gun w/o moderator 3 shots
91.5 92.7 92.6 dB
 W moderator
88.3 88.3 89.5 dB

The New Chimera1 print same problem clipping at the muzzle.

[subscriber]

Thanks, Mike

I wonder if the long skinny tubes has the airflow trying to flip the pellet end over end?  I am vary of using such long tubular sections in anything but an experimental LDC.

[WobblyHand]

Thanks, Mike

I wonder if the long skinny tubes has the airflow trying to flip the pellet end over end?  I am vary of using such long tubular sections in anything but an experimental LDC.

Why would it do that?  Why doesn't it do that in other designs?  The pellet should have sufficient gyroscopic stability to travel in very close to a straight line, especially near the muzzle.

But I do agree that this sort of design is experimental.  My last working LDC with a long small ID tube inside isn't showing any signs of clipping.  Just looked at the exit for signs of clipping and I don't see any.

My printer and I seem to be having a spat.  Can't get a good print out of it.  Have numerous issues to sort through.  Going to be out of action with respect to printing new ideas for a bit. 

But I did get a microphone.  So I'll try to get a test set up working.

[subscriber]

A waisted pellet's skirt helps it to fly nose first due to air flowing front to back in normal flight.  When the pellet is in a narrow tube that does not constrain the pellet (compared to what happens while the pellet is in the barrel), the dominant air flow is back to front, due to the high pressure air still behind the pellet.  That airflow acting on the pellet skirt tries to flip the pellet end over end. 

Yes, the spinning pellet is gyroscopically stiff on its axis, so it does not actually flip, but it can get bumped and then oscillate in yaw until the pellet is out of the tube.   If the extent of the yaw is large enough, the pellet could contact the tube and then bounce to an even greater yaw angle. 

What Mike reports could start in the first narrow tube section, then cause a crash as the pellet enters the second narrow section.   

The degree of spin stabilization depends on the barrel twist and the pellet velocity.   How much the air from behind might affect the pellet depends on the airgun tune; as in pressure and volume of that air; with or without hammer bounce adding extra air.

If there is a little axial misalignment between barrel bore and plastic tube for any reason, that could help kick off the initial yaw that is then exaggerated by the back to front air flow.

A long skinny tube with a definitive but narrow clearance between tube ID and pellet, that allows air to flow rapidly forwards between bore and pellet over a long distance, is the very opposite of an air stripper.  If we accept why air strippers are useful to minimize air blast from causing pellet wobble, then anything that exaggerates the problem air strippers are intended to alleviate, is "bad".   In extreme cases, bad enough to cause clipping.

If my explanation does not satisfy; the fact that it happened is evidence that it is possible .  It may be on the edge of happening with other skinny tube designs, but depends on the wobble frequency of a given pellet type whether a crash results or not.   

[TorqueMaster]

Bob, @TorqueMaster,
What might such a change look like?  Say changing from aligned seams to random?

These printers seem to require a lot of odd tweaking.  My mill and lathe generally don't.  Is this because of the materials, the way they are built or some technology limitations?  I've printed several LDCs and none have run into this issue.  They were the same height, but not as wide.  They would have been easier to knock over as the base was narrower.

Used the same slicer.  Hmm, I recall some updates to the profile were done.  After that, I started to get blobs.  Or it's coincidence.  Makes me suspicious.  I may see if I can redownload the slicer and decline any further changes.  Only problem is this effect doesn't seem to be obvious until the layer count is high, which makes for a frustratingly long test cycle.  Maybe I can make a simpler object that shows the problem in less time.  Say 10mm OD and 45mm high.

Changing from aligned seam to random would surely change up the print paths.  Back in the day, before cura 5.x, I could change the linewidth slightly, say from 0.45mm to 0.46mm and that would stir things up a little bit.  That might still work, it sort of depends how much of you print is walls, and how much is "infill."

Slicing software is still in it's infancy, so unforseen things could happen with combined settings, of which there are quite a few to keep track of.  Every update fixes things, but could muck up others.  For a free product, Cura does pretty darn well at keeping things working while improving.  I generally stick with a version that's doing well, and try upgrades when a cool new feature is worth trying.  On Linux it lets me install multiple versions, hopefully windows does too.  I can always switch which version I'm using.   

I'm not a machinist, but I think it's safe to say there's a lot more going on in software to determine the printhead "toolpath" than your other machines.

Some updates in the past have caused blobbing.  Moisture will also cause it, PETG absorbs it over time, a good roll could "go bad" sitting on the machine for a few days, depends on your RH.   A dehydrator would dry it out.

Also, are you sure you are in a good state of tune?  Extruder calibrated, percent flow adjusted to not over or underextrude (by very much) ?  Failures like globbing at the top could be overextrusion dragging along on the nozzle, building up more, the taller it gets, until it's a big enough blob to impede the nozzle.

Someone also mentioned getting a good first layer, with good "stick."   That should help in safely smoothing any small blobs that block the printhead, but not if they are larger. 

Good luck, keep trying!

[subscriber]

Wave cancellation:
https://physics.aps.org/articles/v16/196#:~:text=Cavities%20at%20the%20sides%20of,new%20techniques%20for%20protecting%20coastlines.&text=Goodbye%20wave.

[WobblyHand]

Wave cancellation:
https://physics.aps.org/articles/v16/196#:~:text=Cavities%20at%20the%20sides%20of,new%20techniques%20for%20protecting%20coastlines.&text=Goodbye%20wave.

These techniques work fairly well for narrow bands of wave lengths.  It's harder to do for wider bandwidths. 

It's one of the reasons I wanted to search for my old fashioned book on waveguide filters, as there are some wide band filters in there, and a methodology to design them, that is not brute force numerical simulation.

[WobblyHand]

A long skinny tube with a definitive but narrow clearance between tube ID and pellet, that allows air to flow rapidly forwards between bore and pellet over a long distance, is the very opposite of an air stripper.  If we accept why air strippers are useful to minimize air blast from causing pellet wobble, then anything that exaggerates the problem air strippers are intended to alleviate, is "bad".   In extreme cases, bad enough to cause clipping.

If this does happen, then it would seem easy enough to add small holes into the narrow tube to act as a stripper.  So we get our cake and eat it too?  The small holes would empty into the surrounding cavity?  Worth a try?

Honestly don't know if what you have said is valid, (it's outside of my experience) but it would seem one could test it out.
It's unclear to me what happens as the bore decreases.  Air is a strange fluid.  There can be turbulence effects, viscosity, and other things, which may or may not affect behavior, which strongly depend on the dimensions (or relative dimensions) of the structure under consideration.  That's a fancy way of saying, "it's complicated".  Or maybe it isn't.  Complicated may mean that the problem is not fully understood.