A key advantage of additive manufacturing/3D printing is that you can create an assembly of parts right out of the printer. Even something like meshed gears or other moving parts can be designed without thinking “How will this be assembled?” This freedom brings big savings in labor and opens up dramatically streamlined part designs.
Yet if you’ve held a 3D printed assembly with gears, you might wonder exactly how does the software “know” how to keep meshed gears separate? It’s actually quite simple, and this post will explain how it works.
First of all, separated parts will need to be designed in CAD as separate water-tight objects. Yet in the virtual CAD world, even separate parts can touch or even overlap—but sending those to be 3D printed means they’ll be fused together in the printed part. We need them separated.
The key is to design in a gap between the surfaces that’s at least a minimal distance value: what we’ll call the “minimum clearance value.” For meshed gears, for example, simply design in at least a minimum clearance (more on this below) between all nearby surfaces and your print will allow them to freely turn. That’s it! You don’t even need to make the differents parts as separate STL files (although you could); the space is all that’s needed.
The only question is “What’s the minimum clearance value?” Take a look at the two charts below and we will look at the components. Keep in mind that we are focusing on the Fortus FDM printers for this post. The same techniques apply to Objet printers using PolyJet technology, but the minimum clearance values are different.
Minimum Clearance Values for Fortus 3D Printers
Tolerance Values (X/Y) for Fortus 3D Printers
*Achievable accuracy for uPrint and Dimension printers are not stated. Assume minimum clearance of 0.020″ in X/Y for these printers, and double the layer height for Z.
**Accuracy is geometry dependent. Achievable accuracy specification derived from statistical data at 95% dimensional yield.
If you’re familiar with Stratasys Fortus FDM printers, you’ll recognize the various materials, layer thicknesses and tolerances in the charts above. These will all affect the minimum clearance, so let’s look at one scenario as an example. We’ll assume a part created using a Fortus 450mc, ABS, and the 0.005″ layer height.
You can see that for vertical (Z-axis) clearances we need at least double the layer height: 0.010″. On the horizontal plane (X/Y), features should have at least 0.012″ clearance. The X/Y tolerance for this printer is +/- 0.005″, so even a worst-case scenario (adjacent surfaces printing 0.010” closer than specified) will still allow for a physical separation of the components, because they will have a space of 0.002″ left from the specified 0.012″. Of course, you can always put more separation in there; these values are recommended for the tighest fit that will still have separation.
So for a particular assembly, once you know the printer, material, and layer thickness, you’ll have your minimum clearance values in Z and then X/Y. Because the values are different for vertical vs. horizontal features, it makes sense to design the clearances in your CAD software based on how the final build orientation will be in the printer.
In other words, decide how the part will be orientated ahead of time, and leave it unchanged from CAD through to the printer software. We advise to use the X/Y plane for part separation whenever possible as it is the most accurate plane.
Designers should also be considering support removal, especially when dealing with materials requiring breakaway support. Leaving ample room for breakaway support removal will shorten post-processing of the part.
The ability of Stratasys FDM printers to quickly create parts in real thermoplastics is a key advantage over subtractive machining approaches. Yet it’s this ability to have pre-assembled parts that allows for even more design flexibility. The resulting assembly can have fewer parts, eliminate the labor required for assembly and simplify the storage and maintenance of the simpler assembly.
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