3D Drawing/Printing exercise in design

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Jul 8, 2009
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I got a Wild hair, Brain fart, idea the other day and decided to run with it for a while to see where it led.

I was watching a video on YouTube, tutorial about how to build a wooden rubber band gun, and I started thinking I could 3D something like that. I started looking to see if anybody had already posted the STL files for something like that. I took a look around on Thingiverse but couldn't find anything that really jumped out at me.

I wanted something fairly small and kinda cool looking, kind of like 007's gun, and I'd REALLY like to make it clip fed. The slide will be the rubber band clip. I want an actual size sorta' replica of a Walther's PPK, BUT not be a true replica. I'm NOT a rivet counter, and I'm trying to make this thing easier to 3D print - not make a museum quality replica..

Hmmm....Bond's Walther PPK. Can't get much cooler than that and it's small enough that it shouldn't take forever to print the parts. I love watching the 3D printer build a part, but I HATE waiting for it to get done. I've got a Freebie g-code print simulator that gives me a pretty good idea of how long it will take to print a part. For complex parts that will take HOURS to print, I'll start the printer before I go to bed and hope that by morning the printer hasn't turned out several hours of junk.

Here's what my little design exercise looks like so far. The first attachment is a screen shot of the 3D PDF file for my sorta PPK, the second attachment IS the 3D PDF. You'll need to download it and then open the file with Adobe or whatever your software of choice is.

This is mainly just a solid model so far. I have carved some hunks out it where I know it' will be required. The trigger assembly so far consists of the trigger and the "hammer", but nothing between them.


View attachment PPK.bmp

View attachment PPK RBG.pdf
I came across plans/files on thingiverse a while back for a single action rubber-band revolver (think Colt Peacemaker) and I've printed 2 of them - one at full sized and one at 70% for the kids to use. They work well and are loads of fun. I'm interested to see how this one turns out, though for some reason the pdf is blank for me.
That does sound like a fun project. When I saw the rendering of the Walther, I just had to search for a picture of the real one as what was in my mind from the Bond movies didn't match.

You Learn Something New Everyday: It looks like the graphic artists and prop guys from the Bond franchise never saw the real thing.

My current 3D project is a [ame="https://www.youtube.com/watch?v=C5hEarotKx0"]WWII Tiger tank[/ame]. I'll use a more conventional RC control since I have them as well as gear motors and servos. I plan to use an Open Panzer control board. I felt it needed a suspension so I redesigned the base section with Fusion 360 to be 3D printed with torsion bar suspension. Most of the tank is now being printed as I write in PLA, but I want to try printing the track and wheels with PET. We'll see how it turns out.

Lohring Miller
What materials will you use for the print? Looks good. Is there a grip texture going on the handle? :)

I currently just print in PLA, I don't mind the smell of it while it's printing. That's an important consideration since my printer is in my living room.

I ran a test print of the slide yesterday, to use up the last of the roll of white PLA and to see if there were going to be any problem areas in the print. I found a couple and I modified the model to include some built in sacrificial supports in those areas. I use the Prusa version of Slicer and don't like using the support generator that's built into it, why support everything if you've only got a couple of spots that need help? I try to design with 3D printing in mind - I try to avoid overhangs that would require support. I think I get a stronger better looking finished product that way.

I'm thinking about checkering the grips, but I'm not thinking too hard about it yet for two reasons. First, those little pyramids would be a bear to model. Second, my printer is just a relatively cheap Prusa I3 clone, I'm not sure how well it would handle printing the checkering. If/when I get this designed and printed to a point where I'm happy with it, I might try printing checkered grips just to see if it would be as much of a disaster as I think it would be.

Yea PLA is certainly the most living-room friendly material! Made from corn and smells pretty sweet like popcorn almost (to me anyway!) Good pick on that
I remember my first printer was a DLP resin printer, B9 Creator. I also had it in the living room next to the TV, resin stench and noisy mechanism - it was a miracle my girlfriend tolerated this eccentricity..

Have you tried acetone vapor polishing for a smooth and glossy finish? Can try it at a basic level with a biscuit barrel and kitchen roll. (would mean a switch to ABS though)

Looking forward to seeing the print results!

I haven't tried any vapor polishing since I've only been printing in PLA. I've got a drafty old house and from what I've read PLA is a lot more tolerant of temperature changes than ABS is.

I think that there is a solvent that would be usable for vapor polishing PLA, but I'm also pretty sure that it is nasty stuff.

I got some more design work done on the RBG. I THINK I have a workable trigger assembly designed, but the proof is in the pudding as they say so I'll have to get it printed out and see if it works. I also think I have a slide lock that just might work but I have been holding off on printing anything else until I'm relatively sure that what I've designed will actually work.

The first attachment was SUPPOSED to be just the trigger assembly, from trigger to rubber band wheel. I had everything but those objects turned off in the drawing, but when I saved it in a 3D PDF format I got the whole freakin' thing. So the first attachment is the 3D PDF of the entire rubber band gun. Apparently my 3D software thinks it knows what I actually want to show better than I do.

The second attachment is just the trigger with its' built in springs. I need to get the trigger printed out ASAP so I can find out how hard/soft those springs are. That way if I don't like the way the trigger pull feels, I can play with the spring design to get what I want - at least that's the theory.

At this point I'm not sure whether or not I should design a separate frame to act as a test-bed for the trigger assembly and/or rubber band magazine/slide, or just use what I've already got designed as the prototype.


View attachment RBG.pdf

View attachment RBG Trigger.pdf
I have discovered that I need to re-design the trigger assembly, or at least the sear. I was planning on using a portion of a ball point pen spring to return the sear to it's normal non-firing position. I had estimated that spring diameter to be about 3mm and designed the sear so that the sear spring would sit in shallow pockets to keep it in position. My sear and all the rest of the trigger assembly is about 4.5mm thick. I measured a few springs today and discovered that about the smallest I could find was about 3/16", or close to 5mm. This obviously will NOT fit in a 3mm hole in a 4.5mm thick part.

I can see 2 possible solutions:

Option 1 - I print a small pin instead of a pocket to keep the sear spring in position on the sear. I allow about 0.3mm clearance between printed parts, that gives me a free fit without too much slop. With the clearance I already allow I SHOULD be OK as far as part thickness and sidewall clearance is concerned. I may have to make the pocket in the slide that allows the sear to move a little deeper. If I do have to modify the pocket then I'll allow a little more clearance in the slide around the sear at the same time, but that would mean that I'm still only modifying 2 parts.

Option 2 - I make the entire trigger assembly thicker, this assembly consists of several parts that would need to be modified. Doing this would also affect the main frame because the business end of the trigger sits in a big honkin' pocket in the main frame, part of the sear projects into this pocket. The sides of that pocket in the main frame are already as thin as I care to make them, so I'd have to make the main frame thicker. This in turn affects both the left and right side frames and/or the left and right grips. I also need to alter the interior pockets in the slide that parts of the trigger assembly project into. If I don't want to alter the exterior of the slide too, then I've got to compensate tor the extra thickness somewhere. I'd probably take it out of the grips since I've got a quit a bit going on in the left and right side frames. Parts of the clip and the slide lock mechanism are already going through there.

Nope, Option 2 is NOT a viable option, I'm going with Option 1. I can already foresee a printing problem with the overhang caused by the pin so I'll design a sacrificial support piece for it.

I printed the slide and part of the frame. There were a few trouble spots on the slide, too much overhang and not enough radius on that part of the slide and part of it didn't print well - it's not too noticeable, bit I know it's there. Also I'm not happy with the back part of the slide. Right under where the actual hammer would be there is a noticeable gap that I need to fix on the next print. At 0.2mm layers, 3 perimeters, and 40% in-fill rate the slide takes almost 11 hours to print - so I don't want to print too many versions of it that don't work.

I printed the sear and the rubber band wheel/hammer too, all the moving parts in the firing mechanism only take about an hour to print. I'm pretty sure that the sear and wheel will work as advertised, but I'm also pretty sure that my trigger, in its' current configuration, will not work. The trigger needs to slide back pushing the sear to release it. In order to get the semi-automatic operation I had planned on the trigger sliding past the sear, that part's easey-peasey. The hard part is getting the trigger to slide back past the sear when the sear is in its' locked position as the trigger returns to its' normal non-firing position. This can obviously be machined out of metal, but I want to 3D print these parts and I want it to be a simple mechanism - simple is good, complex is bad. I've got a PDF copy of Ingenious Mechanisms someplace, maybe it'll give me some ideas.

I've also come to the conclusion that 3D printed springs are pretty much useless, at least in this size. If they are printed thin enough to flex properly they don't exert enough pressure, and if you print them thick enough to exert the proper pressure they will not flex enough and they break - at least in PLA that's what happens. I've got a couple of 305mm lengths of pre-wound 4mm OD spring material ordered in 0.4mm wire diameter. So no more scrounging to find the right sized ball point pen springs, I can just cut off the length of spring I want. (Ebay, like Google, is your friend.)

No pictures, I haven't figured out a way to take a picture of a black object and have it show a reasonable amount of detail, never claimed to be a photographer. If it helps any to visualize this stuff looks like, I've been told that I'd get in a lot of trouble if I tried carrying it into an airport. I've got a cousin that works Homeland Security at LAX, but I still don't think I'd try walking through there with this thing assembled. I may be crazy, but I'm not that kind of crazy.

Since many people are of the opinion that "If you don't gots no pictures it didn't never happen." I've got a couple of pictures for you.

I've heard a rumor that you are supposed to be able to copy and paste images, let's give it a shot. The first is an assembled overview of what I've got designed and printed so far.
It's printed in black PLA, 40% infill, and 0.2mm layer height, the slide itself took almost 11 hours. The blue painter's tape is holding in the pin the keeps the trigger in the correct position. This pin will eventually be sandwiched between the left and right side frames, only the left side frame has been printed to this point.
Slide removed.JPG
This shows the slide, "actually the magazine/clip", removed. Yes it is "loaded", but at this point it cannot fire. I've found that I was a little too loose with my tolerances. I've got enough play between the slide, the hammer, and the 2 spacers that I get "jams". There's enough of a gap that the "ammo" gets jammed in the gap. - can't have that.
Bottom of slide.JPG
This shows the bottom of the slide and the space in the slide that the barrel slides through. This locks the front of the slide in position on the front of the gun. The side frames have wedges on them that lock the slide down on the rest of the "gun". The slide will be locked in position by the "clip". The clip will fit into the recess at the bottom of the handle and 2 locking tabs will extend through slots between the side frames and the hand grips - thus locking the slide in position - or at least that's the plan at this point.

Like I said earlier the current trigger design with plastic printed springs is trash. I've been working an a different version and I'm waiting for my spring stock to get off the slow boat from China so that I can test it. I think I can use part of my original sear design, but it will need to be modified and re-printed.

I have been working on a design for "checkering" the hand grips, but the more I think about it the more I'm leaning towards scrapping that design. It looks good on the screen, but I don't think it will print well - if at all.

Well the slow boat from China made landfall yesterday. I now have two 305mm lengths of 4mm OD spring made from 0.4mm diameter stainless steel wire in my grubby little hands. I guess the yard can wait for a while, I've got more important things to do.

I have re-designed the trigger assembly around my spring stock, actually more of a design tweak that a re-design. The down side is there were enough little changes that I will have to re-print every part I have already printed. The re-design includes spring pockets to ensure the springs stay in the correct location, and modifications to the side frames to eliminate a gap at the rear of the slide. Leaving the gap wouldn't hurt anything, but at the right angle you can see daylight between the slide and the frame at one spot - can't have that.

Last week I was running low on cash so I went to the money store to trade my signature for some more folding money. They always have an off-duty cop sitting in the lobby so I figured “Why not” and went out the car and got the little black bag containing the parts for my gun. I handed the bag to the cop and asked him how much trouble the bag's contents could get me into. Which for some strange reason, made him open the bag REALLY slow. He took the parts out and assembled the gun. He told me that legally speaking I was fine, because this wasn’t really a weapon. However professionally speaking, it was just too damned realistic making it hard to determine that it wasn’t an actual weapon. But he said it was REALLY cool – which I already knew.

After I double check the design and re-print the parts I'll take some more pictures so that you all know it actually did happen.

Printed out some parts over the weekend, actually most of them. The only thing I didn't re-print was the slide, that's about a 9.5 to 10 hour print and I ran out of weekend.

The spring pocket idea turned out to be a bust. Putting a 4+mm pocket into a 4mm part doesn't leave a whole lot of wiggle room. My printer does a fairly good job, but sometimes the edges aren't exactly what you'd call crisp - especially in an overhang, and crisp edges are really what you need with parts this size. I don't mind a small amount of fettling to get the parts to fit properly, but the present design involves more fettling than I want to do. I think that next I'm going to try to print a small pin that the spring can locate over as opposed to a pocket that the spring has to fit into. I'm also not happy with the fit of the clip to the clip detent in the frame, it's a little too snug. I can fix that at the same time I fix the trigger spring pocket in the main frame.

Fortunately these changes will only affect the trigger group, the slide, and the main frame. All toll, that's about 12-13 hours of print time.

Well I finally got back to this project, that darned work thing kept getting in the way of the really important stuff. I'm going to have to do some experimenting, the temperature settings that I was using in May and early June resulted in semi-melted blobs of goo when I tried printing last night. Who'd of thunk that a 15 - 20 degree rise in ambient temperature would have made that much difference.


Thanks, for a while I was wondering if there was anybody out there. I was pretty sure that I was just talking to myself - again, and that gets old after a while. I mean it just seems a little - one-sided?

This isn't the first time I've noticed a lack of heat dissipation on small-ish parts. I've found that when printing a large part each layer had enough time to cool before the next layer was printed. The way I solved this issue in the past was printing multiple small parts, of about the same height, at the same time. That way each part had some time to cool before the its' next layer was printed. This time I was trying to print the entire trigger group which is 6 pieces: the two parts of the actual trigger, the sear, the hammer, and the two spacers that center the hammer in the slide. When I finally decided to cancel the print, the top of all of the parts looked like molten puddles of goo.

I guess it's time to re-design the filament cooling fan/shroud - again. I've been using a 40mm x 10mm axial fan and I've been debating whether I should switch to a blower for filament cooling. Last night I was watching a couple of videos where they were testing various fan/shroud combinations, printing the same part on the same printer. They came to these conclusions:
An axial fan with no shroud is better than nothing.
An axial fan with a shroud is better than just the fan.
A blower with the same shroud will be better than the axial fan and shroud.
This is because blowers handle the higher pressures that the shrouds cause better than axial fans do.

There are a couple of 50mm by 15mm blowers on their way to me, courtesy of our friends at Flea-Bay. By the time they get here next week I'll have the shroud re-designed and hopefully printed. It should be a big enough part that layer cooling won't be that much of an issue.

Well, I got the new radial blowers in the mail yesterday. I also got the filament cooling fan shroud redesigned and reprinted over the weekend. So last night I changed from the old fan/shroud combo to the new fan/shroud combo - HOO-BOY howdy what a difference. I need to do some more work on the design of the discharge from the shroud, it's blowing onto the extruder hot block too much. I had my fan minimum speed set at 60% which was never a problem before. But when I tried a test print last night and the fan came on, my extruder temperature dropped over 30 degrees before I figured out what was going on and turned the fan speed down to a point where the extruder heater could cope. I plan to experiment a little bit with some masking tape and aluminum foil until I get the discharge shape worked out, then I'll print a new shroud.

This should also help my overall print quality, especially on overhangs and when bridging which were always problem areas before.

Keep the updates comming please. I bought my first 3 d printer (Zortrex 200 ) a month ago, So i am following with interest.

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