3D Drawing/Printing exercise in design

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You'd need steadier hands than I've got to do that.

My original intention with the Colt was to unilaterally arm my nieces, nephews, great-nieces and great-nephews this Christmas, we're gong to be getting together when my brother comes up from Mississippi in a couple of weeks. When it didn't look like I'd get the bugs worked out in time I went to Plan-B and printed out some puzzle boxes instead. The plan now is to take the prototype along and I'll take orders for the rubber band guns. That way "mom and dad" have a say in whether or not the kids are armed.

I expect that there'll be a little "field-testing" by the kids and maybe we can get some video of that.

I once printed out a puzzle box for my kids. Basically a round tube with a maze on it that slides inside a hexagonal tube with a locating lug. The idea was you have to solve the maze blind to get the tubes to separate and get to the rewards inside. I chucked some cash in there and let my kids loose, not really expecting they'd solve it. 10 minutes later they had it apart and pocketed my cash! Turns out the locating pin wore down very quickly as they worked it and it eroded enough that it wouldn't engage again. One of these days I'll have a look at fitting an improved locating pin without showing its location on the outside.
The puzzle box you described sounds a lot like the ones that I printed. Especially since the first one that I printed did about the same thing as you described. However, I beefed up both the number of perimeters and the infill percentage and I haven't had that problem since.
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.


Do you have any files available for the final gun?

I can dig up the STL files. The CAD files probably won't help you much, I use Designspark 3D as my CAD. It uses Rsdoc as its' file format and I'm not sure if I can save them in any other useful format. I'll check on saving the CAD to a different file format when I get home tonight.

I could probably post the STL files here, or PM me and I'll send you the files. These files will be for Colt 1908 wannabee. If I remember correctly it takes about 7-8 hours to print the parts, I usually print at 60mm/sec so YMMV. It takes a couple of hours to clean and assemble the parts after printing, and then do a little gun-smithing to get it to work reliably. The fasteners I used were M3 button-head cap screws nuts. I used a piece of 4mm OD brass tubing for the rubber band wheel axle. It uses #10 rubber bands for both the ammo and the springs.

Looks like DesignSpark only creates STEP/IGES files if you buy the paid Exchange add-on module, which goes for the low price of $558!

But DesignSpark Mechanical is free, so I could open the files in DesignSpark if I wanted to.

The STL files would be nice.

I'll send you a PM.



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