Better 3D Printing: a Gaelic Box With All in One Twist Latch Using Minimal Support

The goal of this Instructable is to help you make better 3d prints in-spite of specific limitations of your printer and general limitations of fused deposition filament printing.

To get the most from our printing we need to:

1) Know Our Software. Learn it, especially your modeling software, its difficult but your great idea is only imagination until you can model it in software. I’m depending on you to do this yourself.

2) Plan Ahead. To plan you need to know what your printer can and can’t do. Test it out, make measurements, experiment with your materials, print speed and temperature. I’m depending on you to also do this yourself and I include test object you can start using.

3) Design Smart. We are going to use our knowledge of fused deposition printing to make designs that push the limitations of 3d printing. I’ll show you some of my ideas but I’m sure that you can find even better ideas on the internet and come up with them on your own.

The object we are going to print is a Gaelic style box. It has some decorations on it, a double hinged 2 flap lid and a unique (as far as I can tell) rotating joint latch I designed that is what they call all- in- one or “impossible” since it is all printed as one piece yet can’t be removed from the box without destroying it.

Here are pictures of an un-cleaned box printed in woodfill on the left and the cleaned box printed in PLA on the right with the lid closed.

Here are the links to the STL files for the 2 lid pieces:

http://www.thingiverse.com/download:1410969

and the main box body:

http://www.thingiverse.com/download:1410970

And here is a video showing the features of the box in action!, illustrating the turning latch and how the double hinged lid functions.

https://youtu.be/WpKsjL0TdP8

The box stl file is too complex to be properly sliced using slic3r. I used Cura, it worked well. I can’t vouch for other slicing software.

To best understand this Instructable, you can, and should print the Gaelic box yourself. The design was done using Blender. (TIP: If you are using Blender the units should be set to meters but set your “scale” to 1000 right before you make your stl file and tell your slicer the units are mm. This prevents truncation and rounding errors.) Again, the box stl file is too complex to be properly sliced using slic3r.

Materials for the project?

This is all about your printing so its your printer and your filament.

software – its your software! I use Blender and Cura. It is up to you to learn your modeling and slicing software. Knowing your software is key to implementing smart designs.

Step 1: KNOW YOUR PRINTER

The current printer at our Makelab, that I used for this project, is warped and the support arms are wobbly and worn out. We will be building a new printer this summer, but you can still get good prints out of the old printer printing with thick (350 um) layers and slow speeds.

What can your printer do? What are its best settings? You don’t know until your test it. Here is a little test object I made. Try it out on your printer, repeat the prints over and over using different layer thickness, filament temperature, print speed and an extruder cooling fan if you have one. The results will also vary with each type of filament you use.

Here is the link to the STL file:

http://www.thingiverse.com/download:1410951

So lets be brutally honest, my test prints look terrible. That is why we do them, to find the limits of our Printers. What my tests taught me is that through holes (used for the box’s lid hinge) are really tough to print. The top sags on every printer and the hole fills in with junk if it is under 1mm diameter. On our printer, the height of 2 mm holes sagged to 1.4 mm. I also found that I can’t print bridging, even 2mm of it, since we don’t have an extruder head cooling fan. To get useable through holes, I must print at 60 mm/sec or slower. A friend repeated the test print on his personal, tuned, higher resolution printer (shown in Black PLA) and even though spatial resolution of that printer is repeatedly measurable within 10-20 um, through hole height on our test sagged to 1.8mm on 2mm holes (100 um layer height). When designing hinges, take the sag into account and make the holes big enough to accommodate the hinge pin.

Step 2: OVERHANG

While any printer can print walls that are straight up and down, it gets tougher, but much more interesting, when they aren’t vertical. This I because part of each filament tracing is now unsupported, hanging in mid-air.

The portion of the filament that is printed in air, has nothing to push against as it is extruded and sags down, and rounds up. In this diagram I show that at 45 degree overhang, the filament shape is still enough to support the next layer on top of it. As the angle gets closer and closer to horizontal, the support gets worse and worse until the wall collapses.

The good news is that pretty much every printer can reproducibly print a strong wall with a 45 degree overhang. Remember: design using 45 degree overhangs where needed! So what can you do with that limitation? You can make nice raised and impressed (pushed in) decorations!

This raised ridge makes a pretty and durable raised design. Unfortunately impressed (or depressed) designs do not 3d print well, especially on top surfaces. To compensate, I made impressed decorations of waves for the top lid of my box by printing them on the underside of the printed lid, where they print more faithfully. When done, I flipped the lid over, cleaned the grooves and they look nice. The key is to keep the walls of the groove at 45 degrees. Since the groove is upside down, it is essentially made of 2 overhangs meeting in the middle.

Step 3: THE ALL-IN-ONE ROTATING JOINT LATCH

In addition to decorative designs, you can use the principle of keeping overhang at 45 degrees to make a really cool “impossible” rotating joint. What makes it so cool is that it is printed all in one piece. It is not assembled and you can’t take it out without destroying the box. The joint is self aligning and turns smoothly since all sides of contact are the same. Obviously its not impossible since I did it but many folks are puzzled by how it is done.

The design is based on a central shaft, that is made of 2, 90 degree cones pointed at each other that intersect at their tips to form an hour-glass shape. The shaft then sits in a hole that is the same shape, but with 500um clearance on each side. The beveled shape of the hole through the wall is similar to drilling part way through the walls from each side. To keep the shaft strong, it tapers to a minimum diameter of 5mm. Since the shaft gets wider in each direction you can’t push it in or pull it out. Because the shaft, and walls of the hole it sits in, are all made up of 90 degree angles they are quite strong and in this orientation all the slopes are at 45 degrees from vertical. In this model the latch is horizontal but a vertical orientation works well too.

You make ask about the overhang at the top of the hole. Luckily, slicers make the gcode to print the periphery of each layer as a continuous pass. This means that as the layers get closer and closer to the top of the hole, the layer is being printed as a loop which tends to bulge a bit more and makes a bit fatter but more self supporting layer. So yes, the overhang approaches horizontal right at the middle of the hole BUT it is support by 45 degree walls connected to it. Unfortunately, you do lose a bit of clearance since this bulge is into the space between the rotating joint and the hole. I experimented with this clearance and 500um clearance is fine for PLA but the clearance is a bit tight for PLA-woodfill.

I have not found this rotating hinge joint in other designs. I think it is original but I certainly could be wrong. I hope you try designing with this joint, it takes some practice with your software to design it correctly. The complexity of the section through this joint is tough for slicers to handle. I found Cura does a good job.

Cool as this intersecting cone, rotating joint may be, you won’t be able to incorporate it unless you can design some pretty intricate support material.

Step 4: SUPPORT IT YOURSELF

In my experience, any surface flatter than 45 degrees, prints better using support. The problem is that no automatic slicer program can generate support adequate for complex designs. We want the shaft of our joint to rotate. A slicer program, if it could handle it, would generate support along the cutout to the shaft and from the top of the shaft to the top of the cutout. The shaft is not going to be able to rotate after that! Instead, we need to design our own support. We need to support the shaft from its ends, from the inside and outside of the box walls. Since the shaft is made up of 45 degree surfaces, the part of the shaft within the hole doesn’t need support! Here is a cross section through the shaft, the orange arrows show the support material that I built into the design.

The requirements of support material are that it must hold up the parts while printing, not interfere with the function of the supported parts and be easily removable, if desired, after printing. It is also nice if it is as small as possible. Just like any other 3d printed part, no support material surface should be flatter than 45 degrees. You can see how this was done clearly looking at the support on the inside of the box. It makes a loop from the end of the shaft, attaching back to the inner wall of the box. I taper each part of the support to just 500um. That provides plenty of strength and is easy to remove with simple tools.

Generating the support is easier using true 3d cg modeling software, such as Blender or Maya, but it can be done in CAD programs as well. It is a slow process and takes some time. You must be meticulous.

Step 5: HINGES

The first time I printed the Gaelic Box, I made the walls and hinges 4mm wide and the hinges fell apart with use. 3D printed material is very weak to forces along the direction it is extruded. Put a hinge pin through the hole, lift up and the layers delaminate far too easily. I re-designed the hinges to 8mm thickness and they are much more sturdy. For the hinge pin I use a large paper clip. It is interesting that this box is designed with 500 um clearance between all surfaces. This works well for the rotating joint but was not quite enough for the hinges, so they require a bit of sanding before they all fit together but when it all fits it folds perfectly.

Step 6: CLEANUP

tools- its your choice

I like small exacto or cheap razor blade knife

Large metal file – take off the wooden handle and use the handle too

smaller square and rounded fine files

emory cloth for sanding (the Blue cloth/sanding trips from the box)

small screw driver for prying out support material

tiny electronics pliers

Paper clips and a cigarette lighter - helped clean out the holes for the hinges

you won’t see me use a dremel much as it can destroy your print before you can finish a sneeze!

I like to start with the knife and pare off the dangly bits. You can also carve down a straight corner if the blob-like rounding needs to be removed.

The square end of a file is strong and perfect for drawing along the impressed groove decorations. They need to be cleaned up since the first print layer tends to be extra wide and partly covers the edge of the groove.

The flat blade of a large file is great for filing down the square faces such as the inside of the hinge joints. I also like to wrap the course emory cloth around the file blade to do rough sanding yet keeping the edges flat and corners true.

To remove the support material I often twist a small flat head screw driver to pop the support up, pull it off with pliers and cut with the knife then sand off what I can.

Clean out the hinge holes by bending the tip of a paperclip at right angles. With this design it should work in pretty easily. If its stuck, and a through hole is slogged, heat up the paperclip with the cigarette lighter and working it into the tight hinge hole, softening the plastic around the hole temporarily. This is also likely to fuse the layers more tightly and may make the hinge stronger.

Take your time on cleanup, don't break your new creation!

Step 7: Wrap-up

I hope you’ve enjoyed discussing ways to improve the design of your 3d printed models. Try incorporating these points:

1) Test your printer until you know your printer.

2) Take the time to really learn your modeling software.

3) Think about what you know, and learn through trial and error, which design features you can print well.

4) Design models intelligently. For example, look what we can make even while keeping overhangs at 45 degrees or steeper.

5) Design your own support material when it will help.

6) Plan the cleanup of your print in a way that doesn’t damage it.

you can find out more about the open source 3D modelling software Blender at

http://www.blender.org/

The design of this box and its component features are copywrite under creative commons, non-commercial use. It can be copied, modified and shared as long as the use is non-commercial.