Mylar Engineering Drawings to CAD: Turn Old Prints into 3D Models

What are mylar engineering drawings?

Mylar engineering drawings are original drawings made on polyester drafting film, the standard master for engineering prints from the 1960s until CAD took over. Mylar is stable and tough, so many of these masters still exist, often as the only record of a part.

Most shops hold a mix: mylar masters, blueprint and diazo copies made from them, vellum and plain paper prints, and sometimes microfilm. The masters are the most accurate. Paper copies stretch and shrink with humidity, and blueprints fade with light and age. For conversion, that matters less than you might think, because a good conversion works from the dimensions written on the drawing, never by measuring the picture.

Why convert old drawings to 3D CAD?

Old drawings are usually converted because someone needs the part again. A replacement has to be made, the original supplier is gone, and today’s machine shops andinstant-quote sites price parts from 3D files, not from paper.

• Replacement and obsolete parts. Equipment outlives its maker. The drawing is often the only thing left that says how a part was made.
• Faster quotes. Online quoting sites price machining and 3D printing straight from a STEP file. A paper print gets a request for quote and a wait.
• The prints are wearing out. Blueprints fade, paper tears and the people who could read the drawer of prints are retiring.
• Changes. Moving a hole or thickening a wall is quick on a 3D model and slow on a redrawn print.

How do you scan a mylar or blueprint drawing for CAD?

Scan it flat and straight at 300 dpi or more, in grayscale, and save it as PDF, PNG or JPG. A large-format sheet scanner is best, and a copy shop or scanning service can do this for a few sheets. A careful phone photo works for a small, clean print.

Is digitizing a blueprint the same as converting it to CAD?

No. Digitizing a blueprint usually means scanning it, which gives you a picture of the drawing. That protects and shares the print, but a picture is not CAD: no machine, quoting site or CAD program can use it as a part.

There are three levels, and each one does a different job:

• A scan (PDF or image) is for archiving and sharing. Scanning services do this in bulk, and for a drawer of building plans it is often all you need.
• A 2D CAD file (DWG or DXF) traced from the scan gives you editable lines. That is right for revising the drawing or cutting a flat profile. See how to convert a PDF to CAD.
• A 3D model (STEP) is the part itself, with real depth. You need it to machine, 3D-print or get an instant quote.

If the drawings are of parts you still need made, scan them and then go straight to a 3D model. Tracing to 2D first adds a step and still leaves the 3D work to do.

How do you turn an old drawing into a 3D model?

There are three routes: redraw the part yourself in 3D CAD, hire a drafting service, or upload the scan to Drawing to STEP. All three build the model from the drawing’s dimensions. They differ in cost, time and how the result is checked.

For a closer look at the drafting-service and freelancer routes, see CAD drafting services. For the redraw-it-yourself method, view by view, see 2D drawing to 3D model.

What goes wrong when converting old drawings?

Old drawings follow older habits, and most conversion mistakes come from reading them with modern assumptions. Check these six things before you trust any model built from a legacy print, whoever builds it.

• Projection. First-angle and third-angle drawings place the views on opposite sides. Read the symbol in the title block, or the part comes out mirrored.
• Units and fractions. Older American prints mix fractions and decimals, and a few metric prints use centimetres. Confirm the units before the first sketch.
• Revisions. Hand-changed dimensions, struck-through numbers and revision clouds mean the print may not match the last part made. Use the latest revision you have.
• “Not to scale” marks. Drafters often changed a number without redrawing the line. The written dimension wins, which is why measuring the scan is a mistake.
• Old callouts. Hole notes, thread forms and surface symbols have changed over the decades. Look up anything you do not recognise rather than guessing.
• Faded or missing dimensions. If a number is unreadable or a depth is never given, ask whoever knows the part, or measure an existing one.

Which old drawings can Drawing to STEP convert?

Drawing to STEP converts single parts from a scanned or photographed print: plates, blocks, brackets, shafts, bushings and flanges. Upload a PDF, PNG or JPG up to 10 MB, and get a STEP file plus an STL, in minutes, from $29.99.

• Checked view by view. The finished model is compared with every view of your drawing, and the quality report shows them side by side.
• Questions, not guesses. When a dimension is faded, missing or conflicting, we ask you.
• Priced up front. A free check of the drawing sets the price before you pay.
• Honest limits. Assemblies, weldments and freeform shapes are not supported yet, and we say so before you pay.

If you have the part but no drawing at all, a 3D scan is the better route. See 3D scanning for reverse engineering.

Export-controlled drawings (ITAR/EAR) aren’t accepted.

Frequently asked questions

How do I convert my first old drawing?

Scan one print you need a part from, and upload it to Drawing to STEP from PartWork.ai. The free check sets the price, and the STEP file arrives with its quality report in minutes.