Practical 3D printing

07/09/2019 08:00

3D printing is a very paradoxical activity. Because of its nature and the fact that it is in its early days, you basically have to be a bit of a geek and an engineer to manage to do anything worth doing with an affordable 3D printer. This often leads to some sort of disaster as we try to exploit the full potential of the tools we are using, regardless of practicality of the result. This often results in overcomplicated models without much practical use. And for the avoidance of doubt, please feel free to include me in the lot !

Recently, I've had enough of this exercise and decided to print some things that were really meant to solve a simple and practical problem. It all started with a hanger for a headset I use for audio conferences, then I had enough of the falling apart handles of my wheelbarrow and decided to print a new set in Filaflex and eventually printed some pretty simple though very adapted hinge for the cat flap.

No big science here but some very satisfying projects that I decided to expose in this article starting from the simplest to the more complex one.

Headset hanger

This should really be the prototype of 3D printed model : nothing particularly smart but answering a practical need. I had enough of my headset using so much space on my desk and did not want to butcher the shelves of my library so I decided to print some sort of clip with a hook that should do the job.

The only real technicality of that model was to pick the right dimensions for the clip to be stable and grip to the shelf. And ideally you want this to print without the need to add supporting structures so it is cleaner and does not require too much of post processing.

I have some experience of this sort of clips having made myself some mounts for cameras in the past. The idea is that the U shaped clip will suffer from some effect of the flexibility of the material (PLA in this case) that needs to be taken into account. So what I've done is to design the U shaped clip exactly to the thickness of the shelf (you may take 0.1mm margin depending on the printing setup) and at the end of the bottom part, I've added a cylinder (in green on the picture) sticking out by 0.3mm. The purpose of this is double : make sure that the grip remains firm and make sure that the hook doesn't lean down in time.

For the rest, the design is not symmetrical, this is to avoid the need for supporting structure and the width of the U is 25mm which might be slightly on the safe side. The hook itself being 10mm. For the rest, do not forget to add fillets in the printing plane to smooth the movements of the printer's head and strengthen the structure. I used the medium quality presets in Cura for my bq Hephestos 2 and the result was as expected.

And there it is now, happily clipped to my library and doing its job for more than a year.

Wheelbarrow handles

This model is probably the most satisfying of my 3D models so far. The reason being that the result is nice, really useful and was good at the first attempt. The other satisfying bit is that it's one of the first models I printed in FilaFlex with exactly the right parameters for the purpose. One of the main parameter to master being the thickness of the walls which determines the flexibility of the model.

So there we go, the model is dead easy to design, there are only few things to determine before starting : one is the thickness of the walls, another one is the length of the handle and finally, the margin for the inside diameter so it fits without problem and stays in place without glue.

For my model I used :

Wall thickness 1.5mm except the base which is 2mm since it is not supported by anything and must be stronger.
Length of the handle 87mm. I have small hands and would recommand using slightly longer handles to be comfortable. 120mm might be a good option. But Filaflex being slow to print, I would not inflate that figure too much.
Inside diameter : 0.2 to 0.3mm more than the actual metallic structure.

I also added a small hole in the base wall just so that we don't have compression effects while fitting the handles and also to let the cavity breathe. This is an important one : the reason I have decided to print a set of handles for my wheelbarrow is not ease of use or comfort, it's mostly meant to avoid the water to get inside the metallic structure. When left under the rain, in the absence of handles, the structure collects some water which viciously remains inside the structure causing it to rust prematurely. That's why you need handles and that's also why I decided to leave some very small breathing hole.

Now that we have the major parameters right, we need to sort some details like adding a chamfer at the base and top, just to make it smoother and nicer to handle.

Note that the printing of Filaflex is long and it might be a good idea to print these one at the time.First it allows to test the model before printing the second instance, then it avoids the head moving from one handle to the other leaving some sort of printing scarce on the side of the handle. I found that Filaflex is nicely sticky to the printing bed when you get the offset right so I also decided to print without extra attachment structure. This is slightly risky for such a tall structure but the result is nicer at the base end.

Cat flap hinge

Ok, that one is definitely more complex than the two previous ones. Yet it's still not rocket science and the only subtlety there was to design the hinge in a way that would be easy to print and could minimize the number of elements to design.

Of course, there is a specificity in the hinge : it must allow at least +/-90° rotation around its resting position symmetrically. This is reducing considerably the options. I decided to have something as symmetrical as possible around a central rotation axis as shown in the picture below :

Now that of course is just one half of the hinge as you can guess from the open bits along the axis direction. That's when I tried to minimize the number of bits to design. I wanted to have the hinge clipping to the door in a U shape. Now this together with the presence of an axis and the mounting constraints was not going to give something easy to print. So I decided that each half of the hinge will be composed of two bits that should be identical. As a result, all I need to print is twice that set which will then assemble as shown below :

The green set on top is simply the same as the grey one at the bottom. Flipped 180° it fits in the holes left by its counterpart and covers the whole length of the axis (in red).

I have selected a 4mm axis to avoid premature wearing (again here better to use a smooth axis than a threaded one for the same reason) and took a piece of aluminium rod so it does not rust. Knowing that I would use low quality printing, I also decided to leave some margin and have the holes 5mm so there would not be too much post processing to do. The different bits mounting around the axis are separated by a 1mm gap so there is not friction while the flap still cannot drift too much from its original position.

Also note that the hinge is precisely designed to be mounted around a piece of door 10mm thick (which happens to be our garage's door) I included mounting holes which is important as you can't simply frill into a 3D model as the solid bits are normally mostly hollow.

So I decided to mount the hinge in replacement of the previous one which has been damaged by the cats rushing in and out in March (and sometimes deciding to go backwards while in the middle of the flap). I had to adjust by a couple of millimeters the height of the moving door but it soon was looking all good and operating without a sound.

Since the axis of the hinge is in the middle of the wall it clips onto, the axis is captive there and does not require any fastening. The price to pay for that is to be able to assemble the hinge and mount it to the wall after. This is guaranteed by the 2 bits design which can open before folding back around the mounting wall.

The flap has not been in service for very long but so far it's working fine. I am a bit worried at the stability in time with a complete season cycle and that's why I decided to go for a set of 6 bolts and nuts to maintain it properly clamped. I hope that this plus the axis taking part to the structural stability of the whole thing will be enough to help it surviving the weather and the intensive usage from the cats. Though for now I don't have any experience with PLA and very cold weather. We'll see what happens in winter.