The decision
The only copy of a drawing is a scan, a raster PDF or paper, and the part needs CAD. There are two ways to get it. One converts the image into vector geometry automatically. The other has a person redraw the part from the dimensions the drawing states. This page compares the two on the same criteria. It names no product, because the trade-offs come from the methods, not from any one tool. For a vector PDF with a text layer, the comparison of PDF conversion and engineering reconstruction is the closer question.
Raster-to-vector conversion
Conversion software finds dark pixels that form lines and curves and fits straight segments, arcs and circles to them. Optical character recognition (OCR) turns the lettering into text. The output is usually a 2D DXF or DWG of what the sheet shows: outlines, hidden lines, dimension lines, arrowheads, text and the title block, all as entities.
The traced geometry follows the drawn lines, not the dimensions. If the original was drawn out of scale, if the paper stretched, or if the scan is skewed or slightly enlarged, the vectors inherit every one of those errors. A hole drawn 0.4 mm off its dimensioned position on paper is traced 0.4 mm off. OCR adds its own errors at the characters that matter most: decimal points, Ø, ± and stacked tolerances. The guide to vector and raster drawings lists them.
Redrawing from the stated dimensions
A drafter or designer reads the drawing as a specification and builds new CAD from its values: a 2D drawing, a 3D solid, or both. Each line is placed by its dimension, so a 25 mm length is exactly 25 mm whatever the scan shows. Interpreting the views, the projection and the notes is part of the work. A value the drawing does not state cannot be drawn, so it becomes a question for the drawing's owner, and the answer is recorded.
Redrawing takes longer than tracing. It also brings out what tracing hides: dimensions that do not add up, missing values and notes that contradict a view.
Criteria compared
| Criterion | Raster-to-vector conversion | Redrawing from dimensions |
|---|---|---|
| Geometry follows | The drawn lines, as scanned | The stated dimensions |
| Scale, skew and paper stretch | Inherited into the vectors | Irrelevant to the result; only the values count |
| Dimension and note text | OCR candidates that need checking | Read and typed by a person, who also makes mistakes |
| Output | 2D entities, usually DXF or DWG | 2D drawing, 3D solid, or both |
| Effort | Low to start; cleanup grows with scan quality | Higher; grows with part complexity |
| Missing or conflicting values | Carried over silently | Found during modeling and raised as questions |
| Good fit | Archiving, overlays, reusing a 2D outline whose accuracy is not critical | Parts to be made, inspected or changed |
When to choose each
- Choose conversion to make an archive searchable, to get an editable copy of a drawing's layout, or to trace a profile whose accuracy the process does not depend on, such as a decorative outline. Check the scale against two known dimensions before you use any traced size.
- Choose redrawing when the CAD will drive machining, inspection, a quote or a design change, and whenever the drawing carries tolerances that matter.
- Combine them by using the traced vectors only as an underlay. The traced vectors speed up placing views and finding features, while every size and position is still taken from the stated dimensions.
The review each result needs
A traced file needs every value that will be used compared with the scan, and its scale checked against more than one stated dimension in each direction. A redrawn model needs checking requirement by requirement against the drawing, as the guide to verifying CAD against a drawing describes, with the open questions answered by the drawing's owner. Either way, a poor scan can hide a value that neither method recovers. Find the original sheet, or ask.
Where TruePart stands
In TruePart's Technical Preview, scanned, raster or multi-sheet drawings stop with a reason. Clean raster drawings are on the V1 list, not offered today.
Questions
Is a vectorized scan accurate enough to machine from?
Only if the dimensions are checked and the geometry corrected to them, at which point the part has been redrawn. Traced geometry follows the drawn lines, and drawings are often not to scale.
Does OCR read the dimensions correctly?
Often, but not reliably at decimal points, Ø, ± and stacked tolerances. Treat every recognized value as a candidate until a person has compared it with the image.
Can tracing produce a 3D model?
Not directly. Tracing gives 2D entities of each view. Building a solid means interpreting the views and placing features by their dimensions, which is redrawing.
What scan resolution helps either method?
Enough that thin lines and decimal points stay several pixels wide. 300 dpi is a common minimum for line drawings, and small lettering benefits from more.
Sources
- ISO 128 (series), Technical product documentation: General principles of representation · ISOLine types and widths that tracing has to keep apart.
- ASME Y14.5-2018, Dimensioning and Tolerancing · ASMEThe stated dimension and its tolerance define the feature, not the drawn geometry.
What TruePart does today
- LimitIn TruePart's Technical Preview, scanned, raster or multi-sheet drawings stop with a reason.
- PlannedClean raster drawings are on the V1 list, not offered today.