Two different jobs under one name
“PDF to CAD” covers two jobs that share an input and little else. One translates what is drawn on the sheet. The other works out what part the sheet specifies. Picking the wrong one is a common failure: a 2D conversion goes to a machine shop that needed a solid, or a 3D reconstruction is ordered when a closed outline for a laser cutter would have done.
This comparison covers the two categories, not products. The comparison of AI-assisted and manual reconstruction covers who or what does the reconstructing.
PDF conversion: vectorization and 2D DWG or DXF
What it does
A converter reads a PDF's drawing content and writes CAD entities. From a native vector PDF, path segments become lines, polylines, arcs or splines, and text objects become text entities. From a scanned PDF, the converter first vectorizes the image, tracing runs of dark pixels into lines and curves, and may run OCR on the text. The output is a 2D drawing file, usually DWG or DXF, that a drafting system can open and edit. The guide to vector and raster drawings explains why the two inputs give such different results.
What it produces
- The views as 2D geometry, at a scale that has to be set or confirmed.
- Dimension values and notes as text, or as exploded lines, usually not linked to the geometry they describe.
- Layers or line types, when the converter maps line widths and dash patterns.
- No 3D shape, no features, and no tolerances in a form that other software can use.
When it is appropriate
- Revising or re-issuing a legacy 2D drawing whose native file is lost.
- Cutting a flat profile by laser, waterjet, plasma or router, where the outline is the whole part and the thickness comes from the stock.
- Tracing an outline as a starting sketch for a model that a designer will then dimension properly.
- Archiving or marking up drawings inside a CAD environment.
Risks
- Scale. PDF coordinates are positions on the sheet. The view scale comes from the title block, views on one sheet can have different scales, and detail views are often enlarged.
- Drawn geometry is not the specification. Views may be drawn out of scale, a dimension's text may have been overridden in the original CAD system, and some dimensions are marked not to scale. Converted geometry can disagree with the number printed next to it.
- Curves. Circles and arcs may arrive as splines or as many short segments, which breaks hole recognition and makes poor tool paths.
- Topology. Lines that look joined may not be. Overlapping duplicates, small gaps and broken outlines are common, and an outline that is not closed cannot be cut or extruded.
- Text. Text exported as strokes converts into hundreds of short lines. OCR on a scan adds misread values.
- Lost meaning. Hidden lines, section views and centerlines convert as lines. What they mean in 3D does not convert.
Review needed
Measure several distances in the converted file against the values printed on the drawing, in more than one view and in both directions. Check that every outline that will be cut is closed and that holes are true circles. Treat every value that came from OCR as unproven. If the file will drive a machine, a person who reads drawings checks the outline against the sheet before anything is cut.
Engineering reconstruction: a 3D solid from the requirements
What it does
Reconstruction treats the drawing as a specification. The stated dimensions, callouts, notes and title-block fields become requirements: a thickness, a bore of a given diameter and tolerance, a pattern of holes at given positions, a material, a general tolerance. The views are interpreted to decide how those requirements combine into a 3D shape, including the projection method, hidden lines and sections. A solid is then built from the requirements, not traced from the lines, and checked against them. A CAD designer doing this by hand follows the same steps, and software that does parts of it needs the same review.
What it produces
- A 3D solid: a parametric feature model in a CAD system, an exact B-rep exchanged as STEP, or both.
- A mesh such as STL where one is needed for viewing or printing, derived from the solid.
- Ideally, the list of requirements the solid was built from, including the tolerances and notes that the geometry cannot hold, and the questions that had to be answered on the way.
- Parametric feature model
- A model built as an ordered history of features (sketches, extrusions, holes, patterns) driven by dimensions and constraints. It is easy to edit in the system that made it. The history rarely survives an exchange between different systems.
- B-rep solid
- A boundary representation: the exact faces, edges and vertices that enclose the volume, with surfaces such as planes and cylinders described mathematically. STEP normally exchanges the B-rep, not the feature history, and CAM and inspection software measure it.
When it is appropriate
- Machining: CAM for milled and turned parts works from a solid.
- Quoting that needs volume, stock size or a feature list.
- Fitting the part into an assembly, or checking clearances.
- Planning and programming the inspection of made parts.
- Replacing a lost native model, or any use where the third dimension matters.
Risks
- Misread views. The projection method (first angle or third angle), hidden lines and sections decide the 3D shape. A misread projection mirrors features.
- Missing values. Drawings rely on the reader for values they do not state. Filling a gap with a guess or a measurement of the picture produces a confident wrong model; see missing dimensions.
- Ambiguity. Two readings of a note or a callout can both look plausible. The reconstruction has to stop and ask; see when CAD must stop.
- Tolerances. The solid is one shape, usually at nominal, so the tolerances have to travel with it as recorded requirements.
- False confidence. A clean 3D solid looks authoritative, and errors in it are harder to notice than in a rough 2D conversion.
Review needed
Verify the solid against the drawing requirement by requirement: envelope, volume, feature counts, and each feature's size and position in the datum frame, as set out in how to verify CAD against a drawing. A person who owns the drawing answers the open questions and approves the requirements before the model is used. Manufacturing review, and inspection of the parts, still follow.
Side by side
| PDF conversion (2D) | Engineering reconstruction (3D) | |
|---|---|---|
| Question answered | What is drawn on the sheet | What part the sheet specifies |
| Typical output | DWG or DXF entities | Parametric model, STEP (B-rep), STL |
| Where sizes come from | The geometry as drawn, at a scale | The stated dimensions and callouts |
| Third dimension | Not produced | Built by interpreting the views |
| Tolerances and notes | Text, if converted | Recorded requirements, not in the geometry |
| Scanned input | Vectorization and OCR, with checking | Possible once the values are read and checked by a person |
| Main risks | Scale, broken outlines, drawn versus stated values | Misread views, missing values, false confidence |
| Review | Measure the converted geometry against the printed values | Verify each requirement; the drawing's owner approves |
| Suits | 2D edits, flat profiles, archiving | Machining, assembly, inspection planning |
Choosing between them
01 Decide what the file is for
A laser-cut profile needs a closed 2D outline. A milled or turned part, an assembly check or an inspection program needs a solid.
02 Check what you have
A native CAD file, a native vector PDF or a scan. If the native CAD file exists, start there instead of from the PDF.
03 Look for what is missing
If the drawing does not state every value the solid needs, reconstruction will raise questions, and someone who owns the design has to answer them. Conversion hides the same gaps, because it copies the drawn lines whether or not a value supports them.
04 Plan the review
Either route ends with a person comparing the result with the drawing. Plan that time from the start.
05 Keep the drawing as the authority
Unless your release process says otherwise, the drawing defines the part, and the converted or reconstructed file is a derivative to check against it.
Neither route approves the part
A converted DWG and a reconstructed STEP are both derivatives of the drawing. Neither shows that the drawing is current, complete or correct, and neither replaces manufacturing review and inspection. A file that opens cleanly in CAM has passed a format check and nothing more.
Where TruePart sits
TruePart takes the reconstruction route: it reads technical drawings into traceable requirements, asks about the rest, and builds B-rep CAD, exported as STEP and STL. Nothing is built before the engineer approves the specification. Currently supporting select part families: in the Technical Preview, a deterministic reader handles one family, rectangular plates with a centered bore and a hole pattern, from a native vector PDF with a text layer, one part on one sheet; scanned, raster or multi-sheet drawings stop with a reason. AI-assisted interpretation: in evaluation, not yet enabled. The drawing to CAD page shows the steps.
Questions
Can a converted DWG be extruded into a 3D model?
For a flat part of uniform thickness with one closed outline, extruding a checked outline can give a usable solid. Check the scale and the printed values first. Anything with steps, counterbores, features at depth or features shown only in other views needs the views interpreted, which is reconstruction.
Is a STEP file reconstructed from a PDF the same as the original CAD model?
No. It is a new solid built from the drawing's values, so it matches the drawing, not the original model's history, construction or any values that were never on the sheet. It normally carries no feature history.
Which route is quicker?
It depends on the part and on how much review the result needs. A flat profile converted from a native vector PDF has little to review. For a 3D use, a 2D conversion is only a first step, and the interpretation work still has to be done.
Can either route start from a scanned drawing?
Both can, with more risk. Every value then comes from OCR or from a person reading the image, and each one needs checking against the scan. A scan that does not show a value clearly needs the original sheet or a question to the drawing's owner.
Who should review the result?
Someone who reads engineering drawings and knows what the part does. Open questions about intent, such as a missing dimension or an ambiguous note, go to the owner of the drawing.
Sources
- ISO 32000-2, Document management: Portable document format, Part 2 (PDF 2.0) · ISOHow a PDF stores vector paths, text and images, which is what a converter reads.
- ISO 10303-242 (STEP AP242), Managed model-based 3D engineering · ISOThe STEP application protocol for exchanging exact B-rep solids, the usual target format of a 3D reconstruction.
- ISO 128 (series), Technical product documentation: General principles of representation · ISOViews, sections and line conventions that a reconstruction has to interpret.
- ISO 5456-2, Technical drawings: Projection methods, Part 2: Orthographic representations · ISOFirst-angle and third-angle projection.
- ASME Y14.5-2018, Dimensioning and Tolerancing · ASMEStated dimensions and tolerances as the definition of the part.
- ASME Y14.41, Digital Product Definition Data Practices · ASMEWhen an annotated model, rather than a drawing, is the definition.
What TruePart does today
- TodayTruePart takes the reconstruction route: it reads technical drawings into traceable requirements, asks about the rest, and builds B-rep CAD, exported as STEP and STL.
- TodayNothing is built before the engineer approves the specification.
- LimitCurrently supporting select part families: in the Technical Preview, a deterministic reader handles one family, rectangular plates with a centered bore and a hole pattern, from a native vector PDF with a text layer, one part on one sheet; scanned, raster or multi-sheet drawings stop with a reason.
- In evaluationAI-assisted interpretation: in evaluation, not yet enabled.