Your downloaded STL opens and resembles the intended part. That does not yet establish its size, whether the underside is complete or whether small details are usable. Check intended dimensions, then visible geometry, then suitability for production. A good-looking preview does not prove a physical fit.
This guide helps you prepare a short inspection record without owning a printer. You can use the 40 × 20 × 10 mm exercise block and English STL inspection checklist. The block is a digital dimensional example, not a physically tested functional part.
1. Preserve the original and record what you expect
Make a working copy before scaling or repairing, with a recognisable revision such as part-inspection-v1.stl. Retain its source, creator and stated dimensions. For a download, check the reproduction permission at the source.
Write down at least one reliable reference measurement: a drawing's outside width, fixing-hole spacing or agreed overall height. Without a reference, you can read a dimension but cannot tell whether it is the intended one.
Record the function and visible face. A decorative figure needs a different assessment from a cover that slides over a housing. If you lack a usable file, compare the four starting points for a print request.
2. Import the STL in a viewer or slicer
A viewer displays the model. A slicer prepares layers for a selected printer and process. For initial inspection, a local tool with STL import and a dimensions display is enough. PrusaSlicer is one example; its documentation covers both text and binary STL import.
Import the working copy, select the object and find its dimensions. Labels and locations vary by software version. Do not automatically accept a suggestion to resize it to the build plate: that may alter the scale before you have checked it.
If using an online viewer, understand how it handles the file before uploading confidential geometry. A local application keeps the model on your computer.
If the file fails to open, record the complete message, filename and program version. Renaming the extension does not convert its contents. Request another export or assess whether repair or remodelling is appropriate.
If you can choose the handover format, compare what 3MF and STL preserve. For a round CAD source, the STEP-to-STL export guide explains how to inspect faceting as well as dimensions.
3. Check units and all three dimensions
Standard STL represents a surface with triangles but does not define the intended unit. Correct coordinates can therefore be interpreted at the wrong physical scale. Confirm whether the source dimensions are intended in millimetres.
Compare length, width and height with your reference. Rotation can change which dimension belongs to each axis. Outside dimensions also do not establish the exact size of a hole or internal opening.
For the unrotated exercise block:
| Check | Intended result | Investigate if different |
|---|---|---|
| X | 40 mm | Import units and scale |
| Y | 20 mm | Whether proportions were preserved |
| Z | 10 mm | Rotation and automatic resizing |
| Shape | Rectangular, without holes | Whether the correct file was opened |
A displayed 4 × 2 × 1 instead of 40 × 20 × 10 is a factor-of-ten discrepancy, not proof of a specific cause. Determine whether export, import or an earlier edit caused it. Scaling until the object looks plausible is not dimensional inspection.
4. Practise with known geometry
Import the original control block without rotation or automatic resizing. Twelve triangles describe one closed block. Coordinates extend from 0 to 40, 0 to 20 and 0 to 10, giving 40 × 20 × 10 mm when interpreted in millimetres.
- Record the three displayed dimensions.
- Duplicate the object or start a separate exercise project.
- Scale the copy uniformly to 200%.
- Confirm that it reads 80 × 40 × 20 mm.
- Finish without overwriting the original.
This checks your understanding of the display and uniform scaling tool, not the calibration of a printer. The exercise's digital geometry and calculated dimensions are checked; it is not presented as a physical test print.
Do not casually change one axis of a functional part. A circular opening may become elliptical. Uniform scaling also changes holes, walls and gaps. Correcting one feature may require a targeted design edit instead.
5. Inspect every side
Rotate and zoom, especially around the underside, connections and thin details. Compare with a drawing or photograph. Save screenshots of uncertainties and describe what you expected.
- Is a wall, bottom or recognisable detail missing?
- Are separate shapes meant to be joined?
- Are letters or projecting points finer than expected?
- Is a dark opening genuinely open, or is shading misleading?
- Does the orientation match the intended use?
An opening may be intentional; an odd face may be a display issue. Record the observation before selecting automatic repair. Then compare any repair against the original and intended dimensions using the STL repair checklist and guide.
6. Understand what inspection cannot establish
Not every viewer checks closed surfaces, overlapping faces or mesh defects. A slicer warning deserves investigation, while the absence of warnings does not prove manufacturability.
Walls, overhangs, support access, material, orientation and finishing need separate assessment. A layer preview may reveal missing detail, but it depends on the selected printer and profile. Include the model itself, not only a screenshot.
Digitally correct dimensions do not guarantee a physical fit. Agree clearance and any trial where fit matters. This checklist is not approval for structural, medical or other safety-critical use.
7. Complete the checklist and choose an outcome
Open the English CSV checklist locally. If it appears in one column, import it as UTF-8 with comma separation. Record the expectation, observation and next step for each row. Leave unknown values visibly unknown.
| Outcome | Example | Next step |
|---|---|---|
| Enough information for assessment | File opens, reference dimensions match and function is described | Send file and checklist for feasibility assessment |
| Clarify first | No reliable units or dimensions | Ask the creator for a drawing, source measurement or new export |
| Investigate first | Missing geometry or an unexplained warning | Preserve the original and message; assess repair or remodelling |
“Ready for assessment” is different from “approved for production”. Your first inspection does not guarantee strength, finish or fit.
8. Send the context with the file
Include filename and revision, intended unit, checked outside dimensions, critical fit dimensions, application, quantity and date. Attach relevant screenshots and explain remaining uncertainty. State your ownership or permission to reproduce the design.
If shape and dimensions match the intention, send that revision and your findings to the printing service. For example: “The outside dimensions match; this opening must fit over the photographed counterpart.” That distinguishes your completed checks from the assessment still needed before quoting.
If this is your first outsourced print, the guide for individual customers covers the wider preparation. The inspection record makes the request specific: this file, these dimensions, this use and these open questions.