To Subscriber - Gamo Urban moderator insert similar to the Notos insert

[naptemp]

The insert's thin end is actually inserted into the barrel structure hole so I think it needs to be there.

The thin end does not fit completely into the barrel structure hole, there is still some space left.
So the between the barrel structure and insert there is space which acts as a first baffle.

[subscriber]

John,

Could you measure the location of the pin holes in the outer casing;
1) from the front end;
2) and the distance between pins?

This can be done by placing the pins in the holes (spanning the tube OD) with enough pin sticking out on one side to measure across the OD of the pins, the subtracting the pin diameter (or radius) to get the center of the hole.  If the pins are loose in the holes, find an undamaged drillbit shank that is as a good a fit in the hole (or gauge pin, if you have them).  If this measurement is not obvious I will create a sketch, but I think this s fairly standard method, so I start with a simple request.

I need to know the pin hole to should location as I plan to use that as the datum for everything else.

[subscriber]

What is the length of the outer casing, from front face to start of taper?

[naptemp]

What is the length of the outer casing, from front face to start of taper?

The outer casing length without the end cap is 133mm.
The outer casing length with the end cap is 137mm.

[naptemp]

John,

Could you measure the location of the pin holes in the outer casing;
1) from the front end;
2) and the distance between pins?

This can be done by placing the pins in the holes (spanning the tube OD) with enough pin sticking out on one side to measure across the OD of the pins, the subtracting the pin diameter (or radius) to get the center of the hole.  If the pins are loose in the holes, find an undamaged drillbit shank that is as a good a fit in the hole (or gauge pin, if you have them).  If this measurement is not obvious I will create a sketch, but I think this s fairly standard method, so I start with a simple request.

I need to know the pin hole to should location as I plan to use that as the datum for everything else.

I measured the location of the pin holes in the outer casing;
1) center of the pin to the front end - 6.1 mm
2) distance between center of pins - 28.6 mm

The pins are hollow inside and not completely circular.
They are about 2.2mm on the wider diameter and 2 mm on the narrower diameter.
I think that the holes are slightly narrower than the pins.

By the way, all measurements (previous ones too) have a tolerance of about 0.1-0.2 mm.

I am willing to print several versions until everything fits.
So that I do not print the entire insert every time, could you provide STL files only for the critical parts, like the area around the pins, etc
Then when all the critical parts fit I can print the entire insert.

[subscriber]

Thanks, John

Printing just the critical interface section to check and refine it is smart.  Will provide STL with that section.

[subscriber]

John,

I have mostly followed your spec wish list for the Gamo Urban LDC.  The STL for the full LDC and a test piece with critical dimension are attached.

I understood what you said about the "snout" on the stock insert.  From my perspective, that aspect is to flexible to align the rest of the insert to the barrel in a useful manner.  Instead, I have dispensed with the annular flow aspect of the stock design, making the insert body OD 32 mm in CAD.  Thus expecting it to print just under that.  The 1 mm section by 30 mm ID O-rings should protrude above the insert OD enough to take up any remaining slack and center the insert in the casing in a positive manner.  This O-ring is the intended part; or any one similar to it:  https://www.mcmaster.com/1295N435/

The space between the insert's first cone (external) and the casing near the barrel muzzle still form the first expansion chamber, so nothing is lost there.

The wall thickness around the retention pin holes at the insert OD was very thin; so I cut the part open into rounded, rather than square slots.  I used you pin location dimension to place the slots.  They should have clearance with the pins, except on the load bearing size, when the end of the stock casing meets the shoulder of the new insert.   Due to sag expected in printing the pins may have 0.2 mm interference when the insert is fully seated against its shoulder.   If so, this should be visible by looking through the pin holes, with the insert rotated to make the slots parallel to the intended pin location.   I few swipes with a needle file or drill should relieve that to make pin insertion possible.

I suggest you test fit the test piece to see if the 32 mm insert OD can go into the 32 mm casing ID.  If pushing the insert in while rotating the part back and forth does not make it fit; and you need to remove more material than you are willing, then I can adjust the OD in CAD by whatever measure is indicated by actually measuring the print OD.  You could also scale the part by a fraction of a percent to get a snug fit.  The O-rings should take up any slack, if there is any.

If there is interference elsewhere due to a misunderstanding on my part I will correct that.  If you prefer the extended section be small in than 50 mm in OD, I can adjust that.  It might look good matching the casing OD at 37 mm.

Note print direction, with external cone facing up.  Print 100 % infill and with supports off.  Use a brim for good platen adhesion.

If you are wondering about the proximity of the O-ring groove to the pin slots, note the load bearing surfaces, and the load path through the printed plastic part - last image below.

[subscriber]

The attached STL is for test piece version 2.  It has the pin slots 0.5 mm shallower, radially speaking.  Or without added clearance for measurement error; as the measured pin spacing is likely tighter than reality; if you measured outside to outside to get the pin spacing.

If you measured inside to inside for the pin spacing, the V1 is better.

In any event, V2 makes the part slightly stronger; and might help guide the pins better on assembly.  You will find out, if you print the parts...

[naptemp]

The attached STL is for test piece version 2.  It has the pin slots 0.5 mm shallower, radially speaking.  Or without added clearance for measurement error; as the measured pin spacing is likely tighter than reality; if you measured outside to outside to get the pin spacing.

If you measured inside to inside for the pin spacing, the V1 is better.

In any event, V2 makes the part slightly stronger; and might help guide the pins better on assembly.  You will find out, if you print the parts...

I have printed both test pieces, V1 and V2.

The pin holes align but need some improvements.
I could force the pins through but the holes are not overlapping well enough.

I attached a photo.
The part in yellow is where the plastic can be seen through the hole.
So a small part there will have to be removed.
I think you can achieve that by moving that groove a small amount (about 0.5mm) towards the rifle barrel.

Need to move the groove on both sides of the insert.

[subscriber]

John,

I moved the pin grooves 0.5 mm as you requested.  Test piece and muffler Version 3 STLs are attached.

It is obvious from the non-sectioned image that the pin grooves are not annular rings; while the section image could depict a continuous groove all the way round.  A continuous groove might make assembly easier because there is nothing to align, but it weakens the part, compared to two discreet grooves.  This is a general comment for anyone wondering about it.

[naptemp]

John,

I moved the pin grooves 0.5 mm as you requested.  Test piece and muffler Version 3 STLs are attached.

It is obvious from the non-sectioned image that the pin grooves are not annular rings; while the section image could depict a continuous groove all the way round.  A continuous groove might make assembly easier because there is nothing to align, but it weakens the part, compared to two discreet grooves.  This is a general comment for anyone wondering about it.

I am fine with having the pin grooves on 2 sides (not continuous, all around), the original cap has the grooves on 2 sides too.

I printed test part V3.
It is better than V1 and V2 in terms of clearance of the pin holes, but there is still a small obstruction of the pin hole.

I think you should make the pin groove wider by another 0.5 mm, towards the barrel.

Just moving the groove 0.5mm (towards the barrel) may not be enough since there may be a variance between various rifles.

You could make the wall thicker, just in the area with the pin and o'ring grooves, if necessary to make it stronger.

[subscriber]

I am starting wonder if the shoulder is meeting the front face of the stock casing - as in the printed part not going in as far as intended.  Else, the continuing interference seems odd. 

Anyway, try test piece V4. STL attached.

[naptemp]

I am starting wonder if the shoulder is meeting the front face of the stock casing - as in the printed part not going in as far as intended.  Else, the continuing interference seems odd. 

Anyway, try test piece V4. STL attached.

I printed test piece V4.
The pins go through with little resistance.
But, now the test part has a small play.
It can turn left and right a few degrees even with pins inserted in the holes.

The shoulder contacts the stock casing front as it should, so that is not an issue.

[naptemp]

Subscriber, I just realized that in a previous post I said to move the groove towards the barrel.
I meant to say to move the grooves down towards the barrel, not make the grooves deeper.

If you made the pin grooves deeper that would explain why there is that play in test part V4.

[subscriber]

John,

I just made the pin grooves shallower, without changing anything else.  There is a V5 and a V6.   V5 has its groove depth reduced by 0.3 mm per side over V4.  V6 reduces the groove depth an additional 0.2 mm per side over V5.

I think the reason for this struggle is that the upper edge of the groove has a pure overhang, and is distorting in printing.  If I was to make that a 45 degree shoulder it would print closer to the CAD dimensions, even though on the face of it, a flat wall seems to make a  better shoulder.  For one thing, pin to pin spacing would matter a lot more if the shoulders were at an angle.  Hopefully V5 or V6 nails it.

[naptemp]

John,

I just made the pin grooves shallower, without changing anything else.  There is a V5 and a V6.   V5 has its groove depth reduced by 0.3 mm per side over V4.  V6 reduces the groove depth an additional 0.2 mm per side over V5.

I think the reason for this struggle is that the upper edge of the groove has a pure overhang, and is distorting in printing.  If I was to make that a 45 degree shoulder it would print closer to the CAD dimensions, even though on the face of it, a flat wall seems to make a  better shoulder.  For one thing, pin to pin spacing would matter a lot more if the shoulders were at an angle.  Hopefully V5 or V6 nails it.

Peter,

I printed and tried both test parts V5 and V6.

V6 is perfect, pin holes are well aligned and pins go in smoothly with just a little resistance.
Also, there is no more play in the part.
Also, the shoulder aligns well with the front of the casing.

Let's continue with V6.

I was thinking that you could provide several versions of the insert.
One of the versions should just replace the original baffle and end cap, and have nothing beyond the barrel.
I can try that version first (since it's less to print) and flush out any problems.

I have many o'rings and I am sure I can find some to fit.

All my prints are done in PETG, although that should not matter much.

[subscriber]

John,

I will send the STLs for a "flush" version and extended version, based on the pin groove geometry of test piece V6.  Should be done around 11 PM your time because I need to eat something first.

[subscriber]

Short and Extended LDCs in same zip file - attached

Short version shown below:

[naptemp]

I printed the short insert.
The insert will not go all the way in, because its diameter is a little too wide for the casing, in the area closer to the muzzle.
It goes into the casing OK, but as you keep pushing there is more and more resistance.

I measured again the original baffle.
Its outer diameter is about 29mm along its entire length.
There are some ridges towards the front and back, which make the OD slightly larger in places.
The original baffle goes easily in the casing, so I assume we can make the insert slightly larger.

The 3D printed short insert OD is about 31.5-31.8 mm.

Can you make the insert outer diameter about 29.5mm?
You could keep the insert front with the grooves the same and decrease the insert OD past the grooves, using a small all taper.

Also, can you make the the o'ring grooves about 0.5mm wider.

Please send just the short insert, since it is less printing.
I will print it overnight since the short insert takes about 4 hours to print.

[subscriber]

Why not reduce just the rear end of the insert diameter (near the barrel muzzle); and keep the front part that you have already established to fit well?

How deep does the new insert go in before it jams?  Or more importantly, how much is hanging out from being fully seated?

The stock insert has a deliberate air space around the OD, with those "gills" on the sides.  The stock insert is not the model for what we are doing, as the whole gill aspect is discarded.