The biggest mistake you can make in 3D printing is not understanding the limitations of 3D printing. Here are a few things to keep in mind if you want to make sure your CAD model is 3D printable on most FDM/FFF printers.

1. Limit support material: 3D printing uses support material to hold up parts of your model during the printing process. Support removal may leave marks on your part and increase the likelihood of breaking the part, so it is best to design your object to need as little support as possible. Angles greater than 45 degrees will require supports. 

2. Incorporate a flat surface for best results: Having a flat surface with no blockages can optimize bed adhesion, achieve a smoother surface finish, and reduce print time and material. The flat surface can be as small as a thumbnail, but may need to be larger depending on your object. If you do not have a flat surface, you can split the model into multiple sections.

3. Meet wall thickness minimums: Any walls that are structurally important should be at least 1mm and be supported on at least two edges. Hollowed areas need a minimum wall thickness of 1mm as well.

4. Ensure that fine details are printable: Embossed details should be at least 2mm high and engraved details should be at least 0.5mm deep. If incorporating text, it should at least be 1mm wide at any point of a letter and leave at least 1mm between letters. 

5. Confirm hole size: The smallest hole you can achieve on most printers is 2mm in diameter. Holes smaller than 2mm may fuse during printing or end up heavily deformed. 

6. Account for spacing on moveable parts: If you have moveable parts that need to be printed together each part should be placed at least 0.5mm apart to keep pieces from fusing together.

7. Consider sizing tolerances for fitted parts: For interlocking parts, make sure the clearance between the parts is 0.3mm for a snug fit. Offset the parts by 0.5mm or greater if you plan on connecting and disconnecting the parts frequently. Also, the more you fit the pieces together and take them apart the looser the fit will become.

8. Check printer specifications: Always check the printer specifications before you begin modeling and in the case of scaling down models to fit on the print bed, recheck that the scaled down dimensions still follow the aforementioned design guidelines.

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