A photograph and three outside dimensions rarely describe a replacement's fit completely. Start with its function, choose fixed reference faces and connect each critical dimension to units, an instrument and a photograph.
You can make initial contact with incomplete information. Photos and the measurements you know are enough to discuss what is missing; a finished CAD file or technical dossier is not a prerequisite. A worn hole may no longer represent the intended connection. Describe the mating part and distinguish nominal size, tolerance and clearance.
Download the English project worksheet. Open it as UTF-8 CSV, replace the guide context with your project inputs and record observations separately from acceptance decisions. Blank cells do not indicate a passed check.
Define the use and trustworthy references
Describe environment and consequences of failure. This workflow concerns a non-critical plastic part. Structural, electrical, heat-sensitive, vehicle, pressure, food-contact or safety applications require an appropriate separate assessment.
The fictional example is a cover without a structural, electrical or safety function. It is not a customer part and no physical fit was tested. For a simple shape you might draw yourself, the spacer exercise demonstrates explicit dimensions. For changing an existing mesh, use the bounded STL-editing approach.
Choose two reliable reference faces
Call the mounting face A and a perpendicular adjoining face B. Measure positions from these references instead of chaining intermediate dimensions and accumulating uncertainty. Mark the same letters on sketches and photographs. If a broken or distorted face is unreliable, identify an intact mating feature as the reference.
Make a dimension register
| ID | Measurement | Reference/method | Fictional value | Purpose |
|---|---|---|---|---|
| M1 | Overall width | A/B; caliper | 48.0 mm | Available space |
| M2 | Overall height | A; caliper | 16.0 mm | Closure |
| M3 | Hole diameter | Opposing inner faces; suitable internal measurement | 4.2 mm | Fastener |
| M4 | Hole-centre position | B; derived from edge measurements | 31.0 mm | Alignment |
| M5 | Lip height | A; depth measurement | 2.5 mm | Snap fit |
These values are synthetic. Record uncertainty and desired clearance separately; do not copy invented tolerances into a real project.
Describe fit as a relationship
Measure the counterpart as well as the old component. State whether each contact slides freely, positions, clamps or snaps. Wear and breakage can change the original measurement. “Slides without tools to face A with no visible side movement” is more checkable than “must fit”.
Link photographs to measurement IDs
Take top, side and underside photographs with photo IDs, scale references and face letters. Add close-ups of holes, lips, damage and mating parts. Avoid deriving precise dimensions from oblique photographs. Associate each critical measurement with a photograph and mark hidden dimensions as unknown. That does not block the first discussion, but it prevents final approval at that feature.
Re-measure the trial using the same references
Record required, measured, difference and decision. Perform the agreed fit action without damaging the real system and photograph deviations using the same IDs. Approval requires all critical dimensions within their agreed limits and the described fit function to pass. A correct outside size does not compensate for a blocked hole or failed snap.
Use the prototype assessment to separate observations, possible causes and the requested revision.
Request a replacement with known facts first
Send photos, function and reliable dimensions to 3D-printed replacement parts. Mark damage and include the mating part or its dimensions. For example: “This face is intact; this hole is worn; the new part should slide freely here.” Ask which missing measurement is needed and which connection the first trial should test. Unknown values remain open until established reliably.