EASM File Documentation

Summary

An eDrawings Assembly file is a lightweight, self-contained snapshot of a 3D CAD assembly (many parts fitted together) published from SolidWorks. It is a review and viewing format, not editable source geometry, so it works like a 3D PDF of a model. Open a .easm file with the free eDrawings Viewer on Windows, macOS, iOS or Android. Its MIME type is the generic application/octet-stream, because the container is proprietary and compressed.

Technical details

FeatureValue
Full nameeDrawings Assembly
File extension.easm
MIME typeapplication/octet-stream
Format typePublished eDrawings assembly (compressed proprietary container)
CategoryCAD / 3D model
DeveloperSolidWorks Corporation, now Dassault Systèmes
Introduced2000 (eDrawings)
Open standardNo — proprietary, undocumented layout
Magic numberNone published; identify by extension and origin
Editable sourceNo — a review deliverable, like a 3D PDF
Precise geometryOptional — embedded only if the publisher enabled it
Created fromA SolidWorks assembly (.sldasm), or CAD exports via the eDrawings plug-in
Free viewereDrawings Viewer (Windows, macOS, iOS, Android)
Full editorSOLIDWORKS (source model only)
Sibling formats.eprt (part), .edrw (2D drawing)
Related extensions.sldasm, .sldprt, .slddrw, .step, .stl
Specificationedrawingsviewer.com (no public byte-level spec)

What is an EASM file?

An EASM file is an eDrawings Assembly: a published, self-contained view of a 3D CAD assembly, meaning a model made of several parts fitted together. It is one of three eDrawings file types that SolidWorks introduced in 2000, alongside .eprt for a single part and .edrw for a 2D drawing. SolidWorks is now part of Dassault Systèmes, and eDrawings remains the company’s way of sharing a model with people who do not own CAD software: the recipient installs only the free eDrawings Viewer.

The point of an EASM is distribution and review, not authoring. When an engineer “publishes” or saves an eDrawings copy of a SolidWorks assembly (.sldasm), the geometry, the component tree, saved views and any markup are packed into one compressed file that the viewer can rotate, zoom, explode, section and measure. The closest everyday analogy is a 3D PDF: it faithfully shows the model, but it is a downstream snapshot rather than the parametric source.

A proprietary, compressed container with no public signature

EASM is a closed Dassault Systèmes format. There is no published specification, and unlike PNG (89 50 4E 47) or ZIP (50 4B 03 04), an .easm has no documented constant magic number at offset 0. The file is a compressed container, so the first bytes vary and are not a reliable identifier. In practice you recognise an EASM by its extension and by where it came from: it was exported from SolidWorks or eDrawings, and it opens in the eDrawings Viewer. Its registered MIME type is the generic application/octet-stream, which is what servers use for binary data that has no specific media type.

Do not try to infer a byte-level structure from a hex editor here. Because the payload is compressed and undocumented, the internal offsets are not something a third party can parse safely. What is known is the logical content the viewer exposes, described next.

What an EASM actually stores

Everything needed to review the assembly is embedded in the single file, which is why an EASM can be emailed on its own with nothing else attached. The logical pieces are:

Component tree      the list/structure of parts that make up the assembly
Tessellated mesh    a triangulated visual surface for fast display (always present)
Precise B-rep       exact solid geometry (present ONLY if the publisher enabled it)
Saved views         named cameras, exploded views, and configuration states
Markup / metadata   redline comments, measurements, and model properties

The distinction between the tessellated mesh and the precise B-rep matters. Tessellation is a triangle approximation of each surface, good enough to display and to take rough measurements. B-rep (boundary representation) is the exact mathematical description of the solids. When someone publishes an EASM they choose whether to include the precise geometry or only the visual mesh, so two files with the same extension can differ greatly in what they let you do. A mesh-only EASM will show the model perfectly but cannot yield accurate solids for re-engineering.

EASM, EPRT and EDRW

The three eDrawings types map onto the three things SolidWorks produces. An .eprt is a single published part; an .easm is an assembly of many parts with its component structure preserved; an .edrw is a published 2D drawing (dimensioned views, title block). All three open in the same eDrawings Viewer, and all three are review snapshots rather than editable source files.

Why an EASM is not an editable model

A SolidWorks assembly carries features, a build history, mates between components and parametric relationships. An EASM keeps almost none of that. It embeds the result — geometry, structure and views — but discards the design intent that let the original be edited. This is deliberate: publishing produces a lightweight, shareable deliverable, and often deliberately withholds precise geometry to protect proprietary design.

The practical consequence appears in search queries like “easm to step” or “easm to sldasm”. Converting an EASM back into an editable model such as STEP, IGES or .sldasm is generally not possible, because the editable data was never in the file. If the publisher enabled precise-geometry export you may be able to Save As STL or STEP from within the viewer, but STL is a mesh for 3D printing, not editable CAD either. When you genuinely need to modify the design, the reliable route is to ask the sender for the original .sldasm or a neutral STEP/IGES export from the source CAD system. eDrawings Viewer can also print or export an EASM to PDF and save a view as a JPG or PNG image, which is a screenshot of the model, not the model itself.

Frequently asked questions

How do I open an EASM file for free?

Download the free eDrawings Viewer from edrawingsviewer.com. It runs on Windows and macOS, with free iOS and Android apps. Double-click the .easm to rotate, zoom, explode, section and measure the assembly. No SolidWorks licence is required.

Can I convert an EASM to STEP or another editable CAD file?

Usually no. An EASM commonly contains only a visual mesh, and the editable solids and features live only in the source model. Unless the publisher enabled precise-geometry export, ask the sender for the original .sldasm or a STEP/IGES export instead of trusting an online converter.

Can I 3D print an EASM?

Only when the file allows an STL export. Open it in eDrawings Viewer and try Save As STL; this works only if the publisher permitted geometry export. If the option is greyed out, request an STL or STEP directly from the designer.

References