A Practical Guide to FDM 3D Printing Tolerances for Engineers
Fused Deposition Modeling, or FDM, is a powerful tool for producing functional prototypes, jigs, fixtures, and end use parts with complex geometries. However, designing for FDM requires a different mindset than designing for traditional CNC machining. A primary consideration is dimensional tolerance. While many services quote a general accuracy of plus or minus 0.2mm, understanding what this means in practice is critical for any engineer looking to get functional parts back the first time.
This is not a process where you can specify a tolerance block on a drawing and expect the machine to hold it. The tolerance is an inherent result of the process variables. Understanding those variables is the key to designing parts that meet your functional requirements.
Sources of Dimensional Variation
The stated tolerance of any FDM part is an umbrella term for several sources of potential variation. Unlike subtractive manufacturing where a tool cuts away material to a precise coordinate, FDM builds objects layer by layer from molten thermoplastic. This introduces thermal and mechanical variables that a designer must account for.
First and foremost is thermal contraction. As the extruded plastic cools from its extrusion temperature to ambient temperature, it shrinks. This is the primary reason why features like holes tend to print undersized and external dimensions may measure slightly smaller than the nominal CAD model. The material pulls inward on itself as it solidifies.
Second, the physical characteristics of the extrusion process itself contribute to variance. The diameter of the nozzle, the precise width of the extruded line of plastic, and the layer height all play a role. A part’s geometry is ultimately an approximation built from these discrete lines and layers. A curved wall, for example, is not perfectly smooth but a series of stacked, semi cylindrical strands.
Finally, mechanical factors of the machine itself are a source of variation. The rigidity of the motion system, the tension of the belts, and any backlash in the mechanical components can affect the machine’s ability to place the nozzle precisely where it is commanded to go. Maintaining a large scale print farm requires constant calibration and preventative maintenance to minimize these effects.
What a Plus or Minus 0.2mm Tolerance Means
Let’s translate this into practical, real world terms. When you design a part and send it to print, a general tolerance of ±0.2mm means a 50mm cube will likely measure between 49.8mm and 50.2mm. However, this is not the whole story.
Internal features, especially vertical holes, are most affected by thermal shrinkage. A 10mm diameter hole in your CAD model will almost certainly print smaller, often measuring around 9.7mm to 9.8mm. The material pulls inward as it cools around the perimeter, reducing the effective diameter. For holes that require precise fits, it is often necessary to design them undersized and perform a secondary drilling or reaming operation to achieve the final dimension.
On the other hand, a 10mm boss or pin will print closer to its nominal size, though it may still be subject to the general ±0.2mm tolerance. This creates a challenge for designing assemblies. If you design a 10mm peg to fit in a 10mm hole, it will not fit. The hole will be too small, and tolerance stacking works against you.
A good rule of thumb for a clearance or slip fit is to design in at least 0.4mm to 0.5mm of total clearance. For a 10mm peg, designing the corresponding hole at 10.4mm or 10.5mm is a safe starting point. For parts that need to be press fit, testing is essential, but starting with a 0.1mm to 0.2mm interference might be a reasonable first iteration.
Accuracy in the Z axis (the build height) is typically better than in the XY plane, as it is controlled directly by the high precision lead screws of the motion system. Layer heights are discrete and precisely controlled, leading to less variation in overall part height.
Designing for Success
Instead of fighting the nature of the FDM process, you can design your parts to work with it. We help engineers in the Houston TX area do this every day.
- **Prioritize Dimensions:** Identify the most critical features and dimensions of your part. If the fit of a specific hole is critical, plan for a post processing step. If the overall size is less important, you can design to nominal.
- **Be Generous with Clearances:** When designing assemblies, always add clearance. It is far easier to add material back into a CAD model and print a tighter fitting version than it is to sand or machine a part that is too tight.
- **Consider Part Orientation:** The orientation of the part on the build plate can affect its accuracy. Features in the XY plane are generally more dimensionally accurate than features built up with vertical layers (aside from overall Z height). We review part orientation as a standard part of our manufacturability analysis for all our Houston TX customers.
Understanding how these factors interact is key to leveraging FDM for functional engineering parts. By accounting for thermal shrinkage and process variables, you can design parts that assemble correctly and perform as intended, right off the print bed.
Ready to print your next part? Fixed price. 7 business day turnaround. Free manufacturability review. Visit www.splinearc.com or email Hello@splinearc.com.