File Preparation & Upload

File format guidance

• STEP for 3D (CNC, sheet metal), STL for 3D printing, and DXF for 2D cutting, with the units stated explicitly in the file and the filename.
• Always specify units. A file without explicit units is read against the supplier default and can come out at the wrong scale, a 25.4-times error between inches and millimetres.
• Keep files under about 50MB by simplifying mesh detail to manufacturing resolution, and use a file-transfer service for large files, confirming the link stays live for at least a week.
• Send a STEP assembly as a single multi-body file or as separate files with a short README, and check whether the shop prefers one over the other. For sheet metal, prefer one flat-pattern DXF per part over an assembly file, and include an assembly drawing for anything complex.
• Include a first-article check in the plan. Verifying the first part against the drawing, the units, and the critical dimensions catches a scale or geometry error on one part instead of across a full run, which is the cheapest point in the process to find a file mistake.
• Treat file prep as part of the design, not an afterthought. A part that is well modelled but poorly packaged, with no units, a vague name, and no drawing, takes longer to quote and is more likely to come back wrong than a simpler part sent cleanly.

Most manufacturing errors trace back to the file: wrong units, missing features from mesh conversion, hidden layers, or a model opened with the wrong import settings. Good file preparation prevents the most expensive and most common mistakes, and it shortens the quote loop because a shop can read the part on the first pass.

A part is usually quoted from a 3D model (STEP) plus a 2D drawing (PDF) for critical dimensions, tolerances, and finish; flat 2D cutting uses DXF. Picking the right format for the process, stating the units, checking the geometry, and naming the file clearly are the four steps that prevent almost every file-related error.

Materials

• 3D formats: STEP (.step or .stp, ISO 10303) is the expected 3D format for CNC and sheet metal because it preserves boundary-rep geometry, tolerances, and assembly structure. STL is a faceted mesh for 3D printing only and loses tolerance data. IGES is a legacy fallback for older CAM systems.

2D and printing formats

• 2D formats: DXF is preferred for laser, waterjet, and plasma profiles because it is a widely supported vector format. DWG is the AutoCAD native format. PDF is accepted for outlines by some suppliers, and EPS or AI is used for silkscreen or laser marking artwork.
• For 3D printing, STL is acceptable for simple geometry, but STEP is preferred when tolerance data matters, because the mesh in STL cannot carry it.

Checklist

• Correct format for the process (STEP for 3D CNC or sheet metal, DXF for 2D cutting, STL for printing).
• Units stated in both the file and the filename.
• Continuous cut lines on a dedicated layer; geometry check passed.
• Descriptive filename with the part name, material, and revision.
• A 2D drawing with critical dimensions, tolerances, thread specs, and finish callouts.
• For sheet metal, the flat unfolded pattern with bend lines, not the bent 3D model.

Common file errors

The same handful of file errors cause most re-quote loops and bad first articles, and each has a simple prevention. Catching them at the file stage costs minutes; catching them after a run costs material and time.

Scale, geometry, and layer errors

• Wrong scale from a unit mismatch: a file saved in inches opened as millimetres (or the reverse) produces parts 25.4 times the wrong size. State units explicitly and confirm the first article.
• Missing features from mesh conversion: STL tessellation or an aggressive mesh simplification can drop small holes or thin walls. Use STEP for CNC and inspect any STL before sending.
• Hidden geometry: a layer turned off in CAD does not publish, so the shop never sees it. Publish every relevant layer before exporting.
• Invalid or corrupted geometry: run a geometry check before sending so the file opens cleanly on the first try.

Version control

• Multiple revisions in circulation: a file named “final_v3” invites the wrong version. Use clear naming with a date or revision tag and keep one source of truth.

File naming

Name files descriptively so they are traceable through quote, build, and reorder. A pattern such as part name plus material plus revision (for example, housing_Al6061_v2.step) avoids confusion, and adding the tolerance when it is non-standard (gear_steel_pm0.05.step) or a quote reference (Q12345_bracket.step) helps procurement. Avoid generic names like test.step, newversion.step, or final_FINAL_v3.step, which make the wrong version easy to send.

Units

Always specify units in the file or the filename. The single most common file error is a unit mismatch: a supplier opens a file expecting millimetres but it is in inches (or the reverse), and the parts come out 25.4 times the wrong size, wasting material and time. Put the units in the filename (part_name_mm.step), in the drawing title block, and in the quote request, so the units are unambiguous from three independent places.

Choosing the format by process

Match the file format to the process, because each process reads a different kind of geometry. CNC machining and sheet metal fabrication need a 3D boundary representation that holds tolerances and assembly structure, which means STEP (ISO 10303). Two-dimensional cutting (laser, waterjet, plasma) reads a flat vector profile, which means DXF, with the cut outline on a dedicated layer and continuous lines for the cut path. Additive manufacturing builds from a faceted mesh, which means STL for simple parts or STEP when the printer or service can use it for better dimensional accuracy. Silkscreen, laser marking, and artwork call for a clean vector such as DXF, EPS, or AI, never a raster image like JPG or PNG, which a cutter cannot follow.

Wrong format causes re-quote delay

Sending the wrong format is a common cause of re-quote delay: an STL for a CNC part forces the shop to rebuild the geometry, and a 3D bent model for a sheet metal part forces them to unfold it. Pick the format the process expects on the first send.

What a shop needs to quote

A shop quotes more readily when the file set is complete and unambiguous. For a machined part, that is a STEP model plus a 2D drawing that carries the critical dimensions, the tolerance class in the title block, the thread specifications, the material, and the surface finish callouts. For a sheet metal part, it is the flat unfolded DXF with bend lines, the material and gauge, and a drawing for any complex bend sequence. For a 3D printed part, it is the STL or STEP plus the material, the layer height or quality target, and any post-processing such as smoothing or dyeing.

Filename, units, and critical features

Across all of these, a clear filename, the units stated in three places, and a short note on the critical features let the shop quote confidently without a round of clarification. Each missing item is a potential re-quote loop that adds days, so completeness at the start is the cheapest way to shorten the path from file to part.

Tolerances in CAD files

A 3D model alone rarely defines every tolerance a part needs. The reliable practice is to put critical tolerances on a 2D drawing, where they are explicit and universal, and to declare the general-tolerance class (such as ISO 2768-mK) in the title block for every dimension the drawing does not single out. Embedded GD&T in a STEP file exists (through ISO 10303-518 PMI), but not every CAM system reads it, so treat it as a help to the shop rather than the sole carrier of tolerance intent.

Tolerance schedules for complex assemblies

For complex assemblies, send a separate tolerance schedule that lists each critical dimension and its band, because a long dimension chain is exactly where a missing tolerance causes fit failures. The model defines the shape; the drawing and schedule define what the shop must hold.

Design rules

• Use continuous lines for cut paths and keep the part outline on a dedicated layer; do not use hidden or dashed lines for cuts, because a supplier may skip them.
• Keep one part per file, or clearly label any nesting. Name files descriptively with material and revision, and avoid version soup like final_FINAL_v3.
• For sheet metal, upload the flat, unfolded pattern with bend lines or bend notes, not the bent 3D model. Position holes in the flat pattern where they sit after bending, and include a separate 2D drawing for complex bends.
• Run a geometry check before sending, and export from native CAD to avoid mesh tessellation errors that drop or distort features.

Tolerances

• Include critical dimensions and tolerances on a separate 2D drawing. Embedded GD&T in a STEP file (via ISO 10303-518) is not reliably read by every CAM system, so a PDF drawing is the universal, reliable way to convey tolerances.
• For standard parts, a general note in the file (such as “ISO 2768-m unless specified”) can cover untoleranced dimensions, but confirm the supplier reads it. For complex assemblies, send a separate tolerance schedule listing every critical dimension.