Movable 3D-Printed Engine!

Hi I’m SunShine

And in this Instructables I’ll be showing how I designed a Movable Print in place engine model (Print in place meaning it does not require any assembly after printing).

While you might not be interested to know how to design a engine model, the tricks an techniques in this Instructable will be applicable to other projects and will enable you to create things that move and have more versatile functions way past the typical 3D printed bracket or figurine.

This is the 3rd Instructables I’m writing on the subject on designing mechanisms for 3D-Printing, and with these three you will have a lot of different techniques to face your upcoming projects!

The previous two Instructables can be found HERE and HERE, but this one can be read completely by itself. I already made a video on the subject showing of an in-line 3-cylinder engine and giving a rough explanation on how I made it, in this Instructables I’ll be designing a V twin engine. :

The STL Files of the engine models will be linked at the end.
(I included a picture of the components of an internal combustion engine with the names of the parts, I will be using these names during this Instructables.)

Supplies

Fusion 360

3D-Printer

Step 1: Finding the Manufacturing Challenges

Identifying the challenges of a potential mechanism is something that should come as early as possible in the design process. Knowing the limitations of your manufacturing process is key. In my case I’m planning on using an FDM 3D-printer, and the limitation of this type of printer is vastly different from say a DLP or SLS 3D printer.

In my case, one of the major challenges will be to print parts of the engine that will have to be able to move relative to each other (conrods + pistons), but manufacture these without them touching the build plate.

Other challenges of my project will be to find clearances that will allow parts to move and interact relative to each other without too much play but at the same time not sticking together during printing.

(The picture shows the parts of an internal combustion engine and their names)

Step 2: Solution Concepts and Design

A mayor component of this model is the Crankshaft. We were able to design it for it to be able to touch the build plate during the print operation. On the V-Twin the crankshaft only needs to touch the build plate in “one orientation”, but on the inline-3 we had to be smart about the crankshaft design to enable it to touch the build plate even though the crank pins are in 3 different orientations.

The crank “segments” were designed to be able to print in different orientations with flat surfaces at 0, 120, 180 and 240 degrees that are designed for build plate adhesion. This allowed for the same crank design on the V-twin as well as the inline 3 engine (see pictures)

Most people familiar with FDM 3D-printing will be able to tell you that you are not able to print into thin air, and if you want to, you will have to use some sort of supports. This is exactly what we are going to do, but unlike most designs, we will not have physical access to the supports to remove them post printing, so a different approach is needed.
My proposal for this issue was to design supports in a way that would allow us to break them away after the print is finished by simply spinning the crankshaft.

The supports for the pistons and conrods need to be designed as small as possible to make the break away as easy as possible. But making supports too thin can result in difficulties on printing onto them. So usually finding a happy medium is the optimal solution.

However, in our case, we have a very fortunate geometry around the "floating parts" (there are supporting walls near by), and in additionally we have a mostly symmetrical and light weight parts that need to be supported. So in our case we will try to produce the thinnest possible supports. Since we have parts in close proximity to the “floating” parts, just bridging across a single strain of filament seems to be a good option that we will test in the next step. (see picture for design, the support is highlighted in red)

Step 3: Testing

Now its time to put it to the test

As always, its more time efficient to test design features separately rather then everything at once and then trying to figure out what part of it failed.

For the crankshaft concept, we designed a single “segment” and printed it in the required orientations, as we expected, the crankshaft printed just fine since most overhang angles were below 60 degrees.

For the breakaway supports, we designed a custom test piece consisting of two blocks to bridge between and then the possibility to print on top of that single bridge.

In this test we were keeping an eye out for overly large movements by the part that prints on the single strain of support. It was observed that the symmetrical piston geometry made it easier to “balance” the part on top of the support strain. This effect is of course not directly available on the V-engine configuration (since there the connecting rods need to print at a relatively steep angle away from the support), but printing an engine in a V-Configuration is still possible due to there being two connecting rods on the same strain of filament balancing each other out a bit, and ultimately the connecting rods wrap around the crankshaft quite early in the printing process, witch ads an additional support point and allows the connecting rods to print successfully. (see pictures)

Step 4: ​Putting It All Together

Once you know that all the design features you want to use work by themselves, its time to connect the dots and put them together and make sure they work in harmony.

On the V twin and the Inline-3 this worked just as expected, the design features managed to work together without any interference, however when attempting to scale it up to a V8, a new problem arose, suddenly we were not trying to break the support of 3 connecting rods and pistons, but rather, 8! The forces involved with this meant that the original crankshaft wasn’t strong enough, and it could break very easily. In addition to this, the print was close to 8 hours long, an undesirably long print.

This shows that while designing everything in segments and combining everything in the end works a lot of the time, sometimes one needs to go back to the drawing board to solve unforeseen issues. In the case of the V8 I will try to adjust the crank-throw and try to make optimizations to the engine block.

There are other design features that were used but not described in detail in this Instructables, like clearances and elephant foot compensation, these features I have not changed since my Print-in-Place Spring Loaded Box instructables where they were described in detail.
But in short summary: I used 0.3mm clearances all around and 0.3-0.5mm chamfers to reduce the elephant footing effect from sticking together parts close to the build plate.

The singe strain of filament that was used as support was designed the same way as the strains of filament that made the compliant mechanism in my Instructables that showed how to use compliant mechanisms to create precise measuring tools.
In summary, for a 0.4mm nozzle at 0.2mm layer height, I simply extruded a 0.5x0.2mm rectangle.

Step 5: Summary

Congratulations, you now have an extra tool in your arsenal that you can use to design mechanisms that just a few years ago most people would have said is not 3D-printable on an FDM machine. And unlike my previous instructables you can now design movement and mechanisms that are not just in the build-plate plane!

The STL’s for the inline 3 and V-Twin engine can be found here:
i-3: https://www.thingiverse.com/thing:4575774

v-2:https://www.thingiverse.com/thing:4620846

And if you want to see the V-twin in action, you can see it in my newest video here (The video is not about the V-Twin, but you can see it in action here.)

You can reach me and find my work on the following sites:
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