How to Prepare Files for 3D Printing: STL, STEP, Tolerances and Scale

A well-prepared 3D model saves time, material and money. For a decorative item, it is often enough for the file to open in the slicer. With a functional part, however, scale, the correct format, wall thicknesses, clearances between parts and whether the model is actually printable all matter.

This guide summarizes what to check before sending files for 3D printing or before slicing at home. It is suitable for technical parts, covers, brackets, replacement components, prototypes and small batches.

Quick checklist

Before exporting or placing an order, check the following in particular:

• The model is at the correct scale, ideally in millimeters.
• The file contains a solid body, not just surfaces without thickness.
• The geometry is closed, with no holes, overlaps or loose edges.
• The thin walls are not thinner than the selected technology can handle.
• Functional surfaces have a designed clearance, not exact zero-clearance contact.
• Important dimensions are provided in a note or drawing.
• For mechanical parts, ideally send both STEP and STL.
• For multi-part assemblies, it is clear which items should be separate parts and which should be one print.

STL, STEP or 3MF: what to send

STL

STL is the most common format for 3D printing. It describes the model's surface as a mesh of triangles. It is simple, supported by practically all slicers and fully sufficient for many jobs.

However, it has limitations. STL does not store units, colors, materials or the model history. If a model is exported in inches and the slicer loads it as millimeters, the part may be 25.4 times smaller. That is why it is important to check at least one known dimension after importing.

STL is suitable when:

• you are printing a finished model without further modifications,
• it is a decorative item, simple bracket or box,
• you do not need to change dimensions in CAD,
• the model has already been repaired and checked.

STEP

STEP is a CAD format for more precise exchange of technical models. It is not a triangular mesh, but a description of solid geometry. This makes it easier to modify, measure and check.

For functional components, STEP is often more valuable than STL alone. If a hole needs to be adjusted, a wall thickness changed, a chamfer added or an assembly checked, working from a STEP file is significantly easier. Modern slicers can often open STEP files directly, but the model is still converted into a printable mesh when preparing the print.

Send STEP especially when:

• it is a technical part with precise dimensions,
• the part needs to fit another component,
• you expect consultation, modification or optimization,
• you have an assembly consisting of multiple parts,
• you want to preserve high-quality geometry for future changes.

The best practice for custom printing is simple: send STEP as the source technical model and STL as a preview of how you intended the export for printing to look.

3MF

3MF is useful when you want to transfer more than just the shape. It can contain information about multiple objects, their positions, colors, materials and sometimes slicer settings. It is practical for multi-material printing, pre-prepared projects and models with multiple parts on one build plate.

For a standard order of a technical part, however, it does not replace STEP. If someone needs to check or modify the part in CAD, still send STEP as well.

Scale: the most common error is the unit

In practice, 3D printing is prepared in millimeters. Even if you model in centimeters or inches, set the export for printing to millimeters and check the dimensions after importing.

Practical procedure:

1. In CAD, mark one known dimension, for example a width of 80 mm.
2. Export the model as STL or STEP.
3. Open the file in a slicer or model viewer.
4. Check that the same dimension is 80 mm, not 80 inches, 8 mm or 2032 mm.

If the model is the wrong size, do not correct a functional part by estimating percentages in the slicer. Return to CAD, set the units correctly and export again. Scaling in the slicer makes sense for decorations, figurines or models. With parts containing holes, threads and mating surfaces, it can change clearances and ruin the function.

Tolerances and clearances are not the same

Tolerance indicates how large a deviation from the designed dimension is still acceptable. Clearance is an intentional gap between two parts so that they can fit together, slide or rotate.

Example: if you design a 10.00 mm pin and a 10.00 mm hole, this is not a good design for standard 3D printing. Even if the print is very accurate, the surface will not be perfectly smooth and the hole is often slightly smaller. To insert the pin, you need to enlarge the hole or reduce the pin size.

Approximate starting values for plastic parts:

Use	Recommended initial clearance
Tight fit after fine-tuning	0.1 to 0.2 mm
Standard insertion for FDM printing	0.3 to 0.5 mm
Looser sliding fit or hinge for FDM	0.5 to 0.8 mm
Finer resin printing	0.1 to 0.3 mm
Larger parts or longer contact surfaces	add allowance according to the contact length

Treat these as a starting point, not a universal standard. The material, print orientation, printer calibration, part size and post-processing can all change the result. For a batch or a more expensive part, it is worthwhile to first print a small test sample with several clearances.

Practical clearance examples

Box lid

If a lid needs to fit onto an FDM-printed box, do not start with zero clearance. For a simple fit, try approximately 0.3 mm on each side, for example. This may be just right for a small lid, while a larger cover may require more due to corner deformation.

Round pin in a hole

For a pin with a diameter of 10 mm, design the hole according to the desired behavior. For a tighter hand fit, a hole of 10.2 to 10.3 mm may make sense. For free rotation, 10.5 mm or more may be better. With holes after FDM printing, expect the inner diameter to turn out smaller than in the model.

Screw hole

With a self-tapping screw, do not print a final metric thread without careful consideration. It is often better to design a pilot hole, test the diameter on a sample and possibly use a threaded insert. For repeated screwing, heat-set brass inserts are more reliable than a plastic thread.

Snap-fit latch

With ible latches, consider the direction of the layers. The part is weaker between layers than within the layer plane, so a thin tab oriented in the wrong direction can break. Add a radius at the root of the latch, avoid sharp internal corners and plan for a prototype.

Wall thicknesses and small details

The model must have volume. A surface without thickness, such as a shell from a visualization program, is not a printable part by itself. Every wall must be thick enough for the printer to create it.

For standard FDM printing with a 0.4 mm nozzle, it is sensible to design walls as multiples of the extrusion width. Instead of a 0.55 mm wall, 0.8 mm, 1.2 mm or 1.6 mm is more practical. For sturdy brackets, it is often better to start at around 2 to 3 mm, depending on the load and geometry.

With fine lettering, grooves and reliefs, remember that the slicer cannot print details smaller than the nozzle, material and layer height allow. What looks good on a monitor may disappear or merge in the print.

Closed geometry: the model must be printable

The slicer needs to know what is inside and what is outside. Therefore, the model should be closed, solid and free of mesh errors.

Check in particular:

• holes in the surface,
• loose edges,
• reversed normals,
• overlapping internal walls,
• duplicated surfaces,
• multiple bodies that only touch at a point or along an edge,
• extremely small fragments left after exporting or scanning.

With CAD models, it is best to fix the problem in the source rather than in the STL. Automatic mesh repair can help, but with a technical part it can also change the dimensions or remove a detail that was meant to be functional.

STL export: mesh quality without an unnecessarily huge file

When exporting STL, you usually set the fineness of the triangular mesh. Different programs use different names, such as chord height, deviation, angle tolerance, refinement or resolution.

An overly coarse export turns a cylinder into a visibly polygonal shape. An overly fine export creates a huge file that opens slowly, while the print itself may not be better because the nozzle and technology are the limiting factors.

Recommended procedure:

• Use binary STL for technical parts.
• For visible arcs and cylinders, choose a high export quality.
• For simple angular parts, an extremely fine mesh is not necessary.
• After exporting, inspect the model in the slicer, especially circular and curved surfaces.
• If the STL is hundreds of megabytes and it is a simple part, the export is probably unnecessarily fine.

As a practical starting point, for standard CAD exports you can use a deviation of approximately 0.01 to 0.05 mm for small and medium-sized parts. For large decorative parts, a coarser setting may be sufficient. For precise small shapes or smooth visible curves, choose a finer export and verify the result in the preview.

Print orientation is part of the design

A model is not finished simply because it has the correct shape. With 3D printing, it also matters how it will be placed on the build plate.

Orientation affects:

• strength in the direction of the load,
• surface quality,
• hole accuracy,
• the amount of support material,
• the risk of deformation,
• layer visibility.

When designing a bracket that will carry tension or bending loads, consider the direction in which the layers will run. When designing a visible cover, decide which side should look best and where support marks can appear. If you are unsure, include a note with the order specifying which surface is functional and which is cosmetic.

What to add to the order or request

The file alone is often not enough. For a functional part, provide a brief description:

• what the part is used for,
• which dimensions are critical,
• what the part needs to fit against,
• whether it should be rigid, ible, heat-resistant or appearance-focused,
• whether price, speed, strength or surface finish is more important,
• whether the orientation, thickness or detail may be adjusted for better printability.

If you have a drawing, attach it. It does not need to be complex technical documentation. Often, an image with a few dimensions, labels for functional surfaces and a note such as “this hole must fit an M4 screw” or “this surface will rest against an aluminum profile” is enough.

Most common mistakes

• The model is at the wrong scale because STL does not contain units.
• The part was designed without clearance, so the assembly cannot be put together.
• Thin walls disappear in the slicer or turn out fragile.
• Round surfaces are exported too coarsely and look angular.
• The file is only a surface without volume.
• Multiple parts are in one STL without clear separation.
• The functional hole is oriented so that supports reduce its accuracy.
• The model creator scales a component in the slicer that needs to fit precisely.

Recommended procedure for a reliable result

For a decorative model, a high-quality STL or 3MF, a scale check and a preview in the slicer are sufficient. For a technical part, proceed more carefully:

1. Model in millimeters and with realistic wall thicknesses.
2. Design clearances according to the technology and function.
3. Round sharp internal corners where cracking could occur.
4. Verify that the model is a closed solid body.
5. Export STEP for modifications and STL for a print preview.
6. Check the dimensions after importing into the slicer.
7. For critical assemblies, print a test part first.

Good files are not just a “downloadable file.” They are information that helps produce a part that fits, lasts and does not need to be printed again. Most problems can be solved in CAD: with the correct units, sensible clearances, clean geometry and a clear brief.