3D Drawing/Printing exercise in design

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Keep the updates coming Don. I'm finding all this very interesting as well.

I stumbled across an interesting video Tuesday night, but I couldn't find the link today to save my soul. The video was the first part of rebuilding a Creality Ender to rival a Prusa Mk3 using a Duet board and other parts. That in itself was interesting, but the really interesting part was he redesigned the filament cooling fan shroud and how he tested it. He used a shallow pan of water on the print bed and brought the nozzle down low enough to see the air flow pattern in the surface of the water.

Well I said I was going to do some experimenting, so last night I did. My cooling fan shroud used to look like this, with the axial fan at the back of the printer.

I switched to this shroud when I got the blower to use instead of the axial fan. Again, the blower is on the back side of the printer just like the axial fan was.

As I said earlier I found that I had plenty of air movement, too much of that air was directed at the extruder hot block. In fact so much air was hitting the block that the heater couldn't maintain extruder temperature with the fan speed set at 60%. I had to lower the fan speed to about 10-15% in order to maintain extruder temperature.

When I first tested the shroud with the pan of water, I found that I also had plenty of air hitting the surface of the water. Unfortunately where the air streams hit the water was about 6-8mm away from the nozzle on all sides. Out came the aluminum foil and the masking tape and I started blocking off parts of the shroud openings. Bottom line, I've blocked off the entire opening on the back side of the hot block and I've blocked off all but about the from 8-10mm of the openings on both sides of the shroud. This gave me two indentations in the water about 6-8mm long on both sides of the nozzle, about 1-2mm away from it. Sorry, didn't think that I should have gotten some pictures of that until I was typing this - but I promise it did actually happen. (The checks in the mail too - just in case you're wondering.) I did find out that with the discharge blocked off as it is now I can maintain extruder temperature, even at 40-50% fan speed.

Anyway, this is what the next iteration of the filament cooling shroud will look like. I left the walls on both sides of the openings plenty thick and solid, about 1.5-2mm. Just in case I need to get out the file and do some more "adjusting" to the air flow later on.

We'll see if this works.



  • upload_2018-8-30_12-46-35.png
    52.6 KB · Views: 397
I changed the fan shroud over the weekend and repeated the water test to check the airflow with then new shroud.

I'm halfway there, the air from the right-hand discharge is hitting right where I want it to hit - at the tip of the nozzle. The left-hand side is still hitting about 6-8mm away from the tip. I can maintain extruder temperature with the fan running at 60% constantly and I can bump the fan speed up to 100% temporarily for bridging, just like you're supposed to do.

Definitely making progress, along with a bunch of junk prototype parts - but that's another topic.

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.
I use foxit reader which has a beta program for showing 3d pdf files, I can rotate it but only see a solid black image.

I use Designspark for my 3D work, the price was right - free - so I'm not too sure about the "quality" of the PDF file that it generates. I'm able to "see" the file on several machines using the freebie Adobe runtime package.

I've been threatening to devote the time to learning Fusion but just haven't done it yet.

Been thinking about the checkering on the grips of the PPK and how I could 3D model/print that. While I was printing a part for another project I had a epifany, epipheny, brain fart. I got to wondering what a part that was printed with cubic infill would look like if you printed NO top layers. I whipped up a quick test part, 40mm X 40mm X 5mm with a 3.5mm dia. thru hole.

The attached file is is what I got. The best description I can think of is that it's kind of an inverted checkering. It's a crappy picture, and this was the BEST of the bunch. I covered part of it with a green highlighter to try and get a little better definition - didn't help THAT much.

I printed this with a 0.4mm extrusion width, 0.2mm layer thickness, 6 bottom layers, NO top layers, with 10 perimeters. This SHOULD give me about a 4mm border around the part, never did measure it to verify that. I don't think I could use this trick on any part that didn't have a flat top, at least not with my current Slic3r. I think that you can print multiple parts simultaneously with Simplify3D and use different slicing settings for each part. If you can print a solid part with infill, NO perimeters and NO top - this might allow you to sculpt the top of the infill.



  • 100_0569.JPG
    70.2 KB · Views: 516
OK, it's about time I got back to this project.

Scratch the idea about printing a part with no perimeters and then sculpting the top of the infill. When the infill is printed the printer doesn't print one line across the part then just move to the next line to be printed. Nope, it prints a line across the part, then prints around the inside of the perimeter until it gets to where the next line across the part will be, then it prints the next line across the part. If I tried to sculpt the top of a part with no perimeters I'd still get lines where I didn't want them. The only way I can see to do that right now would be to model the infill myself and I'm not that crazy dedicated.

I have mocked up the trigger mechanism using the 4mm spring stock that I got a couple of months ago and have discovered that I need to modify the pins I'm using to keep the springs in place. That or glue one end of the spring in place. I need something to keep the springs in the proper place while the Rubber Band Gun is assembled. Once it's assembled they are trapped in position and won't go anywhere. I'm re-designing the pins now and will try printing a prototype tonight.

Haven't updated this in a while. I have been working on smaller simpler version RBG that I had hoped to be able to arm the great-nephews/nieces with for Christmas - probably not gonna happen.

I planned to use the simpler RBG's to field test the trigger mechanism. I printed a prototype, my version of a Colt 1908, and it failed miserably - at least that's my opinion. It dry cycles just fine, but when I load it with rubber bands, it does not behave consistently. Sometimes it would cycle through 4-5 shots properly, sometimes it wouldn't fire at all, and sometimes it would fire all the rounds at once. Totally unacceptable behavior and not something that I'd ever try passing off onto a kid, even if they are over 30. Since I copied all of the internals of the PPK directly into the Colt, I know that the PPK won't work either.

I'm re-designing the internals, both to get a more consistent release of the rubber band, and to get rid of those fiddly little springs. After losing the 4th or 5th spring, either on assembly or dis-assembly, I realized that those things have got to go. I don't know of too many boys that won't try tearing something apart if they get the chance, and losing a custom made spring turns this into a paperweight. I'm trying to replace the itty-bitty springs with rubber bands, gotta love it when you can use the ammo to replace a broken spring.

I printed a new test frame last night, and I'll print the new internals tonight. I'll let you know if we have a winner, or just another dud.

Well, it's not exactly a winner, but it's not exactly a dud either. I did print out the re-designed internal parts last night, they appear to work correctly. In my haste to get this thing working, I didn't pay too much attention to the appearance of the beast, so it looks a bit like Frankenstein's monster with bits added on or clipped off here and there.

I've got a good feeling about the release mechanism, it seems to be working properly. I got rid of the custom fiddly itty-bitty springs, instead I'm using a rubber band. You gotta love it when you can use the ammo to make replacement parts. I was originally making the wheel/hammer and the two spacers as three separate parts. I found out last night that loading the rubber bands onto the wheel would separate these parts allowing the rubber bands to jammed in the gap. The hammer/wheel/spacers are now printed as one part.

Right before I quit for the night I found that the rubber bands are getting jammed in the gap between the wheel and the frame at the shooty end of the RBG. What's happening now is that when the trigger is pulled, the sear releases the wheel and the wheel/hammer starts to move, but the wheel gets jammed by the rubber band pinched in the gap. I'm going to open up this gap to see if this corrects the problem, then I think I'll have a working semi-auto RBG - only time will tell.

If/when I get it working tonight I'll take some pictures of this monstrosity in all its' glory? I'll let you know tomorrow if we've got a working RBG, or just another plastic paper-weight.

There's still no joy in Mudville, and I don't have pictures but I've got the next best thing a bitmap and a 3D PDF.

The bitmap below shows the current working bits of the RBG.

This shows the RH side of the test frame that I have printed, the Hammer/wheel, the Sear, the Trigger and the link, labeled Trigger2, between the trigger and the Sear that causes the Sear to release the Hammer/wheel. For some reason, that I have yet to figure out, I cannot change the layer/color of the trigger, the link or the sear - they're always blue. (Maybe they're sad because they don't work?)

The problem that I'm having right now is that as a single shot RBG this works just fine. When I load multiple "rounds" onto the hammer/wheel, it goes into MetalStorm mode discharging everything at once. While impressive, it's not exactly the result I was after. I've even tried loading just one rubber band, then rotating the wheel as if multiple rounds were loaded. This should cause the RBG to dry-fire until the loaded band comes around. Nope, wheel just rotates.

Sitting here looking at it I may have had an idea, it was either that or a brain-fart - too early to tell. I'm starting to think that the problem is where I have the rubber band that I am using as the sear spring attached to the sear. It is currently wrapped around the sear, so I don't think the rubber band has much leverage to move the sear back into position to stop the hammer/wheel. I know it's possible to make this work, I've watched videos of a guy building a semi-auto RBG out of plywood that uses a similar set-up - principles are the same anyway.

I'm going to try adding a lever at the bottom of the sear that will project toward the rear of the gun. I should be able to double or triple the amount of leverage that the sear spring can apply on the sear. Of course I'll have to modify the existing cavity for the sear, but that's what mills were invented for wasn't it? If it works, then I'll modify the print files, print the parts again, and see if it still works.

Attached is the 3D PDF, it doesn't have the rubber bands modeled, or the additional text.



  • DUMMY.pdf
    119.6 KB · Views: 383
Last edited:
A little difficult to tell from the pic alone, but to me it looks like the trigger rod would be acting on the sear in such a way that it would be next to impossible for the sear to return to position without a lot of spring force. Basically it looks like the trigger would be overriding the ramp on the sear.
I think you beed an escapement type of.mechanism. When the sear releases the rubber band wheel it should simultaneously insert a stop that limits tje wheel rotation to a fixed angle.

When the trigger is pulled to the stop what you are calling the trigger rod slides past the sear, allowing the sear to swing back to the stop position. I'm starting to think I've got too much trigger travel between when the sear allows the hammer to release, and when the trigger rod releases the sear. I REALLY need top look at the geometry and timing of those actions.

I've thought about an escapement, but I must have a block of some type when I think about them. Every time I come up with an escapement design, it releases the rubber band on trigger release, not on trigger pull - and that just feels wrong. I must be doing something wrong.

Yeah, you need to have the escapement set so that the rubber band comes off when the escapement is in place. I checked out a commercially produced multi-shot rubber band gun, and it used an escapement mechanism, coupled with a rubber-band lifter connected to the trigger. I may take a look at your system and see if I can figure out how to design such a mechanism for your gun.

My post yesterday was a bit brief as I was dealing with a horrible toothache that started while I was typing. Again, it's difficult to tell from just the drawing, but to me it looks like the part of trigger 2 that acts on the sear to fire a 'round' must then release the sear to let it revolve exactly as the round is fired. If there is any delay in the trigger 2 holding the sear after the round is fired the hammer will freewheel from the tension of the next loaded rounds and the thing will go full auto. It looks like trigger 2 can/does pivot downwards as the sear resets, but to my eye the sear cannot put the downward force on trigger 2 until the catch that activates the sear has released. I think this timing is wrong and that's giving you the issue.

I don't know the terminology for it, but how about a sear that has a sort or 'U' shape? So as the sear is rotated to fire a round, the back portion of the 'U' becomes a stop for the hammer wheel so it can only rotate enough to release one band and holds the hammer wheel at roughly 3/4 cock. Then as the sear rotates back to position the back part of the U releases the wheel and it is caught by the front of the sear at full cock, ready to fire again. I could draw it easier than I can explain it, but hopefully you get my meaning.

I was trying to get the sear to release the hammer with a single action, I have since given that up. I HAD the internals re-designed by late Christmas Eve when I decided to make one final change. I should have left good enough alone, that last change caused the software to lock-up and I lost all my work. I've been working with computers for decades, and I STILL haven't learned that I need to save - OFTEN!!!

Between having to re-create my work and needing to do some emergency printer repairs, I didn't get the printer running again until late on the 25th. With all the changes, I have to print everything over, there isn't one part of this RBG that didn't get modified, even the hammer. I went back to a multi-part hammer because it just prints better that way, but I did modify it to make the assembly easier.

I printed the frame halves yesterday and I'm going to print the internal parts tonight when I get home, according to the slicing software they should take about an hour to print. Then we'll find out if the latest idea is a winner, or if I just got another paperweight.

During the day the printer is busy cranking out gift/puzzle boxes for Christmas presents. It's printing the last one now, takes about 8 hours to print.

You could either use, as others have said, an escapement mechanism so that the wheel is trapped into only "one band" rotations, or, you need a mechanism to definitively move the trigger2 bar off the sear, so that only the hammer/wheel is supporting the sear and preventing it from rolling back.

Weak leverage pulling the sear back into position might contribute, but, your larger problem is that once the trigger2 bar pushes the sear far enough that it disengages the wheel, it disengages the wheel - and it holds the sear disengaged. It's not until the trigger moves far enough to let the sear begin to be supported by the hammer/wheel that there's any possibility of the sear falling back into the hammer notches. If you manage to get that to work, it's only going to be by fiddly luck, and as tolerances and friction change, it'll almost certainly stop working and need readjustment.

The escapement mechanism is foolproof. If you're determined to use just a plain single-disengagement sear, then you need to _force_ the trigger to disengage as soon as the hammer/wheel starts turning. You could make the trigger2 bar "sprung" in an upwards direction, and add a link such that rotation of the hammer pushes the trigger-bar down, disengaging it from the sear and allowing the sear to come to rest on the hammer. This is still fiddly but would allow the sear to ride the hammer and stop it at single-band rotational increments as you have envisioned the mechanism.
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 ands 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.



  • Test gun.jpg
    Test gun.jpg
    72.1 KB · Views: 446
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.

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.



  • Working prototype.jpg
    Working prototype.jpg
    75.1 KB · Views: 427

Latest posts