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

[AustinBinTX]

Definitely need a good barrel band to prevent the barrel from leveraging on the front of the receiver and breaking it off, IMO.... I can't imagine any plastic not reinforced with continuous carbon fibres (which you can't print) being anything like as stiff as aluminum, let alone the original, which was steel.... I have always hated the short barrel inset in QBs.... The barrel wobbles around something fierce unless additional setscrews are added from the sides at the back of the loading tray....

Don't forget that the forward force on the barrel and the rearwards force on the bolt both are equal to the pressure times the area of the front of the bolt.... For a .22 cal bolt, that is 0.038 sq.in., so at 850 psi it is 32 lbs. but at 2000 psi it is 80 lbs.... Increase it to a .25 cal (0.049 sq.in) and it goes up to about 100 lbs.... If you are holding the barrel in with a setscrew, consider the loads on the plastic at the threaded hole....

Bob

Bob,

I appreciate the insight.

I agree with everything you say, and I appreciate your concerns about the force on the bolt and the barrel set screw. I thought about the bolt, but I didn’t give any thought to the barrel set screw.

I do have to point out though, continuous carbon fiber reinforced plastic printing is now being done. It’s not exactly cost effective, but given some more time, I’d expect it to get into the price realm of us hobby guys.

https://markforged.com/materials/carbon-fiber/

I don’t believe that page specifies that it is continuous, but it is under their continuous composites section.

[AustinBinTX]

I suppose I should add that printing continuous fibers requires special slicing software as well. I’m sure they don’t let that go cheap either.

[Insanity]

I suppose I should add that printing continuous fibers requires special slicing software as well. I’m sure they don’t let that go cheap either.

I thought cura had several specialty filament profiles that include metal wood and fiber.

[AustinBinTX]

I suppose I should add that printing continuous fibers requires special slicing software as well. I’m sure they don’t let that go cheap either.

I thought cura had several specialty filament profiles that include metal wood and fiber.

I believe it does, but I don’t think it’s for continuous fiber. I can’t say for sure though. I haven’t used Cura ever since I switched to these Prusas and Slic3r PE.

I thought I had read that continuous fiber printing was still fairly new and that the slicing algorithms and software were still proprietary.

[Insanity]

Ya fiber fortified vs continuous probably is very different. I used repetier on my hacked davinci and using cura with my ender 3. So far I am quite impressed with the ender 3 it is hammering out parts at a resolution I thought I had to spend a lot more on to get.

[AustinBinTX]

Well, the multishot version was a failure.

All the dimensions are correct, but as expected, and as pointed out, the flex on the mag cutout was too much.

New design printing now. This one is using the full mag well that was suggested. It is also 28mm longer on the barrel end to increase engagement and allow extra barrel set screws. I’ve added one on top, and two on each side.

I went ahead and sliced this one with 100% infill instead of 50%. I expect it should do the trick. Print time is 6.5hrs, so I’ll check it out and post a photo in the morning.

[YEMX]

I wonder if this would be a good opportunity to try lost PLA and cast a metal version?  It would certainly cut down on machining time.  Not saying you, but maybe someone... 

I wish I had my forge made.

[AustinBinTX]

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.

[AustinBinTX]

I wonder if this would be a good opportunity to try lost PLA and cast a metal version?  It would certainly cut down on machining time.  Not saying you, but maybe someone... 

I wish I had my forge made.

Would definitely cut machining time.

I’d be willing to send one off if someone wanted to give it a go.

[YEMX]

I wonder if this would be a good opportunity to try lost PLA and cast a metal version?  It would certainly cut down on machining time.  Not saying you, but maybe someone... 

I wish I had my forge made.

Would definitely cut machining time.

I’d be willing to send one off if someone wanted to give it a go.

Ugh.  Now I really wish I had my forge built...   

[Rob M]

that looks pretty good Austin.. Yes some chamfering here and there.. Maybe your insignia on the side or something to break up the blockyness but it looks good.. Really your 98 % there.

[AustinBinTX]

that looks pretty good Austin.. Yes some chamfering here and there.. Maybe your insignia on the side or something to break up the blockyness but it looks good.. Really your 98 % there.

I had thought about doing something on the side, but I’m printing these standing up with a .6 nozzle. It’s pretty hard to get any real detail on the sides when they’re printing vertically. I’d try printing horizontally, but I’m afraid it would mush the bolt and barrel bores out of round too much. On a side note, I should have taken a picture of the breech on the print surface. I’ve got 210mm of build height on my Z axis, the breech is 200mm. On the final layer with a 2mm Z hop, there isn't much left. Was definitely some picker factor the first time I watched it.

I’ve got an enclosure I can put one of the Prusas in. I may give CF reinforced polycarbonate another shot. Last time I tried it without an enclosure and I had terrible warping issues. It’s gotta print hot, and stay fairly hot to prevent it.

So far this has been a fun experiment. I think there’s more potential for FDM in the airgun world. Hopefully some of the industry technologies will drop in price so is hobby guys can afford them and prove it.

[Rob M]

yes.. breeches are incredibly hard to make.. The amount of machining involved justifies that.. 3d printed breeches could reduce the cost by a large factor , and possibly the overhead also. If your printing vertically, a weaver rail on the top surface instead of a 11mm would really break up the lines.. and thingiverse has  some rails that print perfectly  vertically.

[Rob M]

" justifies the price is what i meant to say.. ( edit is gone for some reason

[AustinBinTX]

yes.. breeches are incredibly hard to make.. The amount of machining involved justifies that.. 3d printed breeches could reduce the cost by a large factor , and possibly the overhead also. If your printing vertically, a weaver rail on the top surface instead of a 11mm would really break up the lines.. and thingiverse has  some rails that print perfectly  vertically.

When I machined them on the mill I did picatinny rails, but that’s a lot of overhangs for printing. It’s only a few thousands under 1/8”, but it’s a whole lot of chances for the printer to mess up.

I noticed the edit feature was fine too. Why is that?

[AustinBinTX]

yes.. breeches are incredibly hard to make.. The amount of machining involved justifies that.. 3d printed breeches could reduce the cost by a large factor , and possibly the overhead also. If your printing vertically, a weaver rail on the top surface instead of a 11mm would really break up the lines.. and thingiverse has  some rails that print perfectly  vertically.

When I machined them on the mill I did picatinny rails, but that’s a lot of overhangs for printing. It’s only a few thousands under 1/8”, but it’s a whole lot of chances for the printer to mess up.

I noticed the edit feature was fine too. Why is that?

I assumed the edit feature was just missing because of the new phone and Safari. All of the other random stuff in my posts is because I can’t seem to get iPhones autocorrect to stop fixing things that don’t need to be fixed.

[Rob M]

yea.. maybe just get it fully operational and later on worry about the fine details.. another way to approach it is to print the 2 sections of rail seeperately horizontally and attach em after the fact.. Still the same cosmetic affect but more work sadly.

[Insanity]

I suspect  that when you machine one out of metal you could take some of quasimodo hump off.

[AustinBinTX]

I suspect  that when you machine one out of metal you could take some of quasimodo hump off.

I actually have never had the hump on them when I mill them. They end up looking basically like the Gauntlets breech, give or take a few millimeters here or there.

I’ve used 7075 the two times I’ve done it. The extra rigidity probably wouldn’t offset the extra cost of a larger drop of 7075 and the extra time to machine it.

I’ve been fighting the urge to just make a new one from aluminum on one of my nights off, but I’ve got my mill up for sale and I’m too lazy to keep recleaning it so it’s inspection ready. I got caught up in bigger is better, and now a huge chunk of my little shop is taken up by a mill I only use once every 3 months or so.

[AustinBinTX]

I suspect  that when you machine one out of metal you could take some of quasimodo hump off.

I actually have never had the hump on them when I mill them. They end up looking basically like the Gauntlets breech, give or take a few millimeters here or there.

I’ve used 7075 the two times I’ve done it. The extra rigidity probably wouldn’t offset the extra cost of a larger drop of 7075 and the extra time to machine it.

I’ve been fighting the urge to just make a new one from aluminum on one of my nights off, but I’ve got my mill up for sale and I’m too lazy to keep recleaning it so it’s inspection ready. I got caught up in bigger is better, and now a huge chunk of my little shop is taken up by a mill I only use once every 3 months or so.

You also have to double the cost and time for me to machine one. It never fails that I’ll mess something up on one of the last revolutions and trash the whole breech the first go round. It’s usually a powerfeed going in the wrong direction, or an incorrect tool diameter punched into the DRO.