The Craftmans Shop > Backyard Ballistics
Gifts for relatives
ddmckee54:
We have a winner, I went with an escapement mechanism for the sear. Printed out the internal parts last night. I had to do a little tweaking, files and razor saws are great for removing the offending extra bits of plastic. I attached a picture of the test gun sitting on a CD jewel case.
I had two problems, first the sear wouldn't move far enough to release the hammer/wheel. I must have modified that part of the sear and not realized that I had changed it. Second, once I got the wheel to release, the trigger bar wasn't rotating the sear enough to release the wheel. I put a couple of windows in one of the frame halves so that I could see what was happening. I had a ramp that pushed the trigger bar down to ensure that the sear was released to return to it's normal position. The ramp was causing the trigger bar to release the sear too soon. I removed the offending ramp and I now have a functioning 6 shot semi-automatic rubber band gun.
I'm using #16 rubber bands, they are too large but they were all I could get at the office supply store. I've ordered some #12's and some #10's to see which will work best.
Last night I copied the internal parts of the test-bed gun into the 3D model of the Colt 1908, so I now should be able to print out the actual RBG's.
Don
Alphawolf45:
My first "gun" was a single shot rubber band firing contraption that looked nothing as nice at this one.
I hope to see you develop a mag fed repeater.. That would be awesome. Per your own words- I want to get this figured out, because what started this whole mess was me deciding - a long time ago - to build a clip fed semi-automatic rubber band gun. I'm close to getting a reliable firing mechanism, now I've got to get the clip part worked out.
I scratch build actual rifles and pistols for serious competition but the interest that I have begun as a child with rubber band pistols and later it was spring fired small shafts with suction cup tips.. Then a BB gun and then pellet rifle and finally actual smokeless powder firearms.. You may be starting a junior or two onto a lifelong appreciation of firearms. Good for you!
ddmckee54:
I printed out the real version of the RBG, damned thing don’t work. When I checked into what was going on I discovered that I hadn’t modified the sear properly. There are two tabs on the sear that catch the hammer/wheel. One releases the wheel and the other holds it in a half released position, about 30° rotation, until the trigger is fully pulled. At that point the sear will slide past the trigger bar and snap back to its’ normal position.
On my test version I cut and filed this catch to make it work. I modified the sear’s print file before I reprinted everything, but I didn’t give it enough clearance – it still wouldn’t release the hammer. I then scribed where I had cut/filed the test sear onto the newly printed version, intending to measure how much more it needed to be modified. Turns out there were no flat surfaces that I could get an accurate measurement from so I had to guestimate them. I wasn’t right, but it’s real hard to guess at a 0.5-0.75mm distance, especially for an Imperial guy. I really had cut it off twice and it was still too short.
Now if I fully load the RBG the damned thing shotguns, releasing all of the bands at once. However with just 2 bands loaded, it dry fires just like it’s supposed to - so I’m real close. Hopefully all I have to do is modify the sear one more time and this thing will be ready to print multiple copies.
Don
ddmckee54:
I did a little "gunsmithing" yesterday, I now have a working prototype 6 shot, semi-automatic, Colt 1908 rubber band gun.
The attached picture is a family shot of the working prototype and "most" of the reject pieces. Counting the versions that are in the gun, I made 2 versions of the trigger bar, 4 versions of the trigger, and 6 versions of the sear. Plus 2 versions of both the left and the right half of the frame which aren't shown in the attached picture. This was all to fix 2 problems, the shot-gunning problem that you already know about, and the problem of the trigger jamming.
The first modified version of the sear fixed the shot-gunning problem. From then on it was trying to determine why the trigger would jam, usually in the fully pulled position. It took me a long time to determine that the rubber band used hold the trigger bar up was getting pinched between the trigger bar and the sides of the frame. The trigger bar runs through a cavity in the frame, there's some clearance, but not too much since the cavity is used to keep the trigger bar and sear in approximately the same plane. I increased the depth of the cavity at the trigger end to allow room for the rubber band, while keeping the original depth at the sear end so that the sear and trigger bar stay lined up.
This mostly worked, the trigger no longer jammed in the fully pulled position, but it didn't ALWAYS return to it's normal position. This I eventually tracked down to the trigger catching in the surface of the frame.
The way that I print the frame halves is with the outside surface up, and the flat inner surface on the print bed. This gives me the best surface finish on the outside where it is most exposed. Unfortunately this also means that the first printed layer over all of the required cavities will be a "bridge". I don't care what 3D printer you've got, the bottom side of a bridge layer will be a corduroy surface at best. I finally figured out that the square leading edge of trigger was sometimes catching on this corduroy surface. I radiused the leading edges of the trigger where they contact the sidewalls, and problem solved.
Don
ddmckee54:
OK, time to fess up.
When I started this project for the relatives, my original intention was not to arm the great nephews and great niece for a parental insurrection - that was just a perk. The real reason was more self-centered. Sitting way back on the back burner is a project that's more or less for my own funsies. I'm designing/printing/building a 1:1 mostly scale replica of a Walther's PPK. I wanted it to resemble a PPK, be semi-automatic, have the same capacity of "ammo", and be clip-fed.
I took the guts of my PPK as the first iteration of the Colt 1908. Since that design didn't work in the Colt, I know it won't work in the PPK. Now that I've got a working design in the Colt, I need to start transitioning that design back into the PPK. I wound up moving bits and pieces around a lot in the Colt, so I won't know if that transition is actually possible until I actually fire up the CAD software.
I'm currently using the slide of the PPK as the "clip". I want to try and keep this feature since it seems like the simplest way to pre-load the rubber bands. Loading the rubber bands is a fairly slow process and I can appreciate why the early gun designers wanted to have some form of rapid re-loading system.
I'm going to play around with the 3D CAD a little and see how difficult getting the working Colt hammer/trigger group design into the PPK is actually going to be - while still keeping the slide as the "clip".
Don
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