The most common mistake in a 3D printing order: a machined-part design gets sent 1:1 to the printer. The result is disappointing – not because the print was bad, but because the part wasn’t designed for the process. Understanding the logic of FDM printing means designing correctly from the start.
Layer Orientation: The Single Most Important Decision
In FDM printing, material is built up layer by layer. The bond between two layers is always the weakest point in the part – comparable to wood grain. Tensile forces along the layers are well tolerated. Tensile forces across the layer direction (Z-axis) can lead to delamination.
What this means for design:
Plan the layer orientation before exporting. Ask yourself: in which direction do the main loads act? That direction should lie in the XY plane, not in Z.
Example: a lever loaded in bending should be printed standing up, with the bending axis in XY. Printed lying down, it will crack on the bending side due to layer separation.
Let us know at inquiry time which load direction is critical – we’ll orient the part accordingly in the slicer.
Wall Thickness: Not Too Thin, Not Too Solid
Minimum wall thickness: 1.2 mm. Below that, the printer can’t extrude a complete line. Recommendation for load-bearing walls: at least 2.0–2.4 mm (= 4–6 extrusion lines at 0.4 mm).
Uniform wall thickness is better than strongly varying thickness. Large thickness jumps create stress during cooling and can lead to warping.
Thin, unsupported ribs under 1.5 mm wide and over 20 mm tall tend to vibrate and produce poor surfaces. Stiffen them with small gussets or increase wall thickness.
Solid material vs. infill: Fully solid parts are rarely necessary and unnecessarily increase print time. 40–60% infill with a lattice or honeycomb structure is enough for most applications. For extreme loads, we recommend locally reinforced areas instead of 100% solid infill.
Avoiding Overhangs and Support Structure
Support structure costs time, material and rework – and leaves marks on the surface. With targeted design, it can often be avoided entirely.
Rule of thumb: the system prints overhangs up to 45° from vertical without support. Up to 60° is often still possible, depending on material and print parameters.
Practical tips:
- Horizontal holes in the part? Design them as a teardrop or diamond shape instead of a circle – the roof of the hole bridges itself.
- Long horizontal edges? A small 45° chamfer on the underside often eliminates the need for support.
- Internal cavities? Plan an opening on the underside so support can be removed – or omit it entirely if the cavity isn’t functionally necessary.
Tolerances and Fits
Realistic tolerances for calibrated FDM systems:
| Dimension | Achievable accuracy |
|---|---|
| XY plane | ±0.15–0.20 mm |
| Z-axis (height) | ±0.10–0.20 mm |
| Bore diameter | ±0.10–0.15 mm |
| Outer dimensions | ±0.15–0.20 mm |
Fits: FDM printers tend to print outer dimensions slightly oversized and inner diameters slightly undersized. For play-free fits (e.g., shafts, axles) we recommend:
- Choosing the shaft diameter in CAD 0.2–0.3 mm smaller than nominal, or
- Reaming/boring to nominal size after printing
For threaded inserts (e.g., M3 heat-set inserts): bore diameter = insert outer diameter + 0.0 mm. The melting heat precisely displaces material into the layers.
H7 Bores and Bearing Fits
H7 tolerances aren’t achievable straight off the printer. Recommendation:
- Design the bore in CAD undersized (–0.3 mm)
- Ream or bore to H7 after printing
Alternatively: press-fit or heat-set brass bushings or ball bearings directly. The part accommodates the bushing accurately, and the bushing delivers the required tolerance.
File Format: STEP Instead of STL
For precision parts, send us the STEP file. STEP stores true geometries – cylinders stay cylinders, circles stay circles. An STL approximates everything with triangles, which leads to measurable deviations at bores and fillets.
You can flag critical dimensions directly in the STEP model with dimension callouts or in a short note with your inquiry. We take these into account during slicing.
Quick Checklist Before Exporting
- Wall thickness ≥ 1.2 mm, load-bearing walls ≥ 2.0 mm
- Overhangs > 45° mitigated with a chamfer or teardrop shape
- Load direction noted (for print orientation)
- Fit bores undersized or marked “to be reamed”
- STEP exported instead of STL
- Threaded inserts modeled as a cylindrical bore with the correct diameter
Have questions about a specific part? Send us the model – we’ll give you a free design assessment before printing.