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Tips for Managing FDM Dimensional Accuracy

Fused Deposition Modeling (FDM) is an additive manufacturing technology popular for prototyping and small batch production. It can quickly and affordably create a variety of useful parts saving its users time and costs. Unfortunately, no technology is perfect and understanding the limitations allows us to work around its shortcomings. Let’s look at a few ways we can manage FDM dimensional accuracy by adjusting layer height, seams, and chatter.

FDM Dimensional Accuracy Tips

One easy and impactful method to improve dimensional accuracy is tailoring the layer height resolution to fit the requirements of critical features.

FDM Layer Height

The biggest misconception associated with the FDM layer height is that a finer layer height will increase the accuracy of the print. This is not always the case. Selecting a finer layer height will, in fact, reduce the toolpath’s footprint, resulting in the ability to fit material in tighter sections along the x-y plane and can improve the overall surface finish of the part.

However, with respect to the z-axis, this may slightly offset critical geometries. Let’s dig into an example to explain this concept better.

FDM accuracy layer height

FDM layer height

We want to print the electronics enclosure in the image above with the trapezoid cutout as a critical feature. We know that our printer has a resolution capacity of .010 and .007 of an inch. It is also important to take the z-axis measurement of critical features, which happen to be 0.38 inches and 0.54 inches respectively.

Upon further inspection, you’ll notice that both these z-axis dimensions are easily divisible by the .010 of an inch layer height, but not easily divisible by the .007 of an inch layer height. This will result in the toolpath shifting the top of the cutout by .005 of an inch and making the base of the cutout higher by .001 of an inch. This example illustrates that finer resolution may in fact result in the critical feature deviating from its actual position.

The most important takeaway is that your design intent should take into consideration the layer height you want to print with for FDM dimensional accuracy.

FDM Seams

A seam in FDM printing is a linear blob up the profile of the part that illustrates the location where the tool path begins and ends. In most cases, the slicing software such as Insight or GrabCAD will do a decent job of hiding the seams by placing them in a sharp corner. This is not feasible with geometries such as holes or round slots.

If you are able to post-process the part, you can undersize the hole in CAD by about .015 of an inch in diameter and use a reamer to chase the hole. If the hole is to act as a clearance hole and doesn’t need to adhere to precisely fitting parts then you can add a corner to the internal diameter of a hole.

FDM seam corner for FDM Dimensional Accuracy tolerances

FDM seam corner for tolerances

GrabCAD Print and Insight can be useful in hiding seams as well.

In GrabCAD Print, begin by inserting your model into the build tray using the Add Models drop down. Please note for this to work correctly, you want to import a file type that is not a body file, like an .STL. Now that the model has imported, select the critical face. In this case, the critical face is the inside of the hole. Next select Model Settings > Face and make sure to check the “avoid seams” checkbox. This will help reduce the total size of the seam. However, it will not completely remove it if it risks the integrity of the part.

In Insight, upload the model into the software. Select Toolpaths > Setup > navigate to the toolpath’s parameter button. This will open the toolpath parameter menu. From there you may select the multiple contours from the fill style. Be sure to check the Link Contours box from the Contours section.

Mechanical Chatter

Corners and runs can impact both the geometric tolerances and surface finish of the part. When a FDM 3D printer makes a corner, the head ramps up or slows down. The movement of the extruding filament may be slightly out of sync, which can result in channel marks around the curve. To mitigate this issue, you should ensure that all of your rounds have a minimum radius of 1/10 of an inch. This provides sufficient time for the tool to ramp up and slow down as it prints.

Lastly, if you have a belt-driven system, it is important to ensure that the bolts and pulleys aren’t worn out or dirty, because that can also cause geometrical deviations in your prints.

FDM chatter marked reducing accuracy

FDM chatter marked reducing accuracy

I hope you’re able to apply some of these techniques to improve the overall dimensional accuracy of your FDM parts.