3D printed a new QB78 breech. Multi shot coming soon

[Insanity]

Id like to get a small mill and lathe like the bench top ones for what little I want to do it would be perfect. I have the luck too if its going too smooth I stop because Murphey comes by during the final stretch lol.

[Rob M]

having machined a few breeches myself  , theyre a PITA>> too time consuming , too many operations.. too many setups.. Theres no margin in selling machined breeches unless you happen to have a 4 axis cnc and all the g-code already panned out..
The 3d breech , if done correctly and posssibly sleeved could be a huge loophole in these otherwise large obstacles. Especailly for lower power guns , once we start dealing in hpa this assumption goes out the window,.. I myself had thought extensively about mass producing breeches at one time , even with a 3 axis cnc in the shop, one mill and one lathe it made no sense .. This was in part due to fixturing and offsets.. The ideal way to do it would either be 4th axis on a rigid machine or multiple machines ( i.e having 2-3 mills set up for dedicated operations , where the breech only stops at each mill one and there is no tear down of the indexing set ups and offsets on the mill table )
EVEN then there is the challenge of longer drilling  , sounds simple i know.. The problem with smaller diameter drills is they tend to wonder based on the diameter / depth of the cut.. So when dealing with 7/16 or even 15 mm holes that are long , this presents its own challenge irregardless of set up or reaming.. Yes , boring would be accurate but now we have a tool stickout issue.. The issues go on.. Can it be done ?? yes.. Guys a few yrs ago were cranking out manually machined breeches that were awesome and a good price.. But i suspect they had the machining and mutiple operational setups well planned or a ton of spare time (-'

[Rob M]

now conversely ( i apologize if im  rambing ) with 3 d printing there are a seperate set of challenges.. Conveniently theyre all Up front.. In other words, 99.9 % of the labor intensive stuff is getting to  point where the g-code and slicing is perfected.. ( this often includes orientation of the part based on surface finish or mechanical toughness ) .. Once you have the perfect file and its relaible , youre kinda good to go.. Its intellectual property as opposed to sweating on a hand crank..This sounds like a nice segway but oftentimes overcoming the limitations of the printer or the materials make this an uphill battle.. Ive been through designs where after 10 iterations it was simply never going to work.  One idea I shared with a friend recently was insert voids.. Basically when you have a longer part where there may be bridging stress on the part, id draw in 2 long voids 1/8 th diameter each.. when the print was done , install 2 steel rods in the voids ( interference fit + jb weld or whatever )  to give the part massive spine strength without any visual inperfections to the part.. I dont have a great example of this but for the breech idea this may offset any flex tremendously

[AustinBinTX]

I’m hoping the hunchback has eliminated most of the flex, but it’s not ideal.

Since it’s printed vertically, there isn’t near as much strength in the span as there would be printing it horizontally. No real good way to go about it if I keep the hunch other than making it a tongue and groove fit. Print the hunch as a second piece, but print it horizontally so it has more strength. It would have to either be filament welded or CA glued, or both.

I’m also not even sure how how the scope is actually going to sit, or have to sit to clear the hunch at this point. It could all be a wash other than the practice in CAD.

Dowels for stiffening are a good idea. 1/8” sounds a little small and flexy. There’s a decent amount of material left in the breech on either side of the centerline to go bigger I think. I was thinking at least 3/16 to 1/4” round.

I’m not sure I’m going to actually worry about stiffening anything up anymore if it works as is. I do plan on chamfering all the edges and streamlining some of the blockiness if possible.

[Rob M]

I’m hoping the hunchback has eliminated most of the flex, but it’s not ideal.

Since it’s printed vertically, there isn’t near as much strength in the span as there would be printing it horizontally. No real good way to go about it if I keep the hunch other than making it a tongue and groove fit. Print the hunch as a second piece, but print it horizontally so it has more strength. It would have to either be filament welded or CA glued, or both.

I’m also not even sure how how the scope is actually going to sit, or have to sit to clear the hunch at this point. It could all be a wash other than the practice in CAD.

Dowels for stiffening are a good idea. 1/8” sounds a little small and flexy. There’s a decent amount of material left in the breech on either side of the centerline to go bigger I think. I was thinking at least 3/16 to 1/4” round.

I’m not sure I’m going to actually worry about stiffening anything up anymore if it works as is. I do plan on chamfering all the edges and streamlining some of the blockiness if possible.

yea 1/8 was just an example.. 1/4 would be super stiff and possible eliminate the overhead hunch solving a few issues.

[mackeral5]

Woke up this morning to a finished hunchback of Notre Dame print. Looks like the very bottom had some cooked filament from a roll change yesterday, but otherwise, it turned out as expected. No chamfering on the drawing yet as it’s a pain to go back and edit any dimensions without having to delete all the previous chamfering anyways.

I had to put my own supports in the mag well instead of auto generating. It looks rough, but it’s really not bad. I see where I forgot to add two supports as well. It should all clean up with a small file anyways.

This one is much stronger, and much uglier. It will work, but jeez does it hurt to look at.

I made it so the barrel will need to have the loading sled turned off, and then and extra 2mm. Instead of doing an internal o-ring, I’ll just do one on the end of the barrel similar to the Diana Chaser.

I apologize for the bad photos. I was in a rush to get out the door and on my way to work.

Very nice design, I like it.  I would like it even more in aluminum and on one of my guns.

Great job!

[QObsession]

How has the breech printing gone? Any improvements? I am very interested?

[AustinBinTX]

How has the breech printing gone? Any improvements? I am very interested?

I ended up dropping the idea of printing them.

No matter which filament I could print with, the strength and safety was never there. I printed with all of the toughest most exotic filaments aside from PEEK, and they all eventually showed stress failures.

The .step files have been proven to be good, but it’s still cheaper to order one from Umarex than it is to buy the drop and mill it.