STEP vs SLDPRT vs SLDASM: CAD File Formats Explained for Buyers
When a model library lists "STEP · SLDPRT · SLDASM" next to every product, it is easy to treat the three formats as interchangeable. They are not. Each one exists to solve a different problem, and knowing the difference decides whether you end up with a model you can edit, a model you can exchange, or a model you cannot use at all.
The three formats at a glance
| STEP (.step / .stp) | SLDPRT (.sldprt) | SLDASM (.sldasm) | |
|---|---|---|---|
| What it is | Neutral 3D exchange format (ISO 10303) | Native SolidWorks part file | Native SolidWorks assembly file |
| What it contains | Exact boundary geometry — faces, edges, and the solid body | Geometry plus the full parametric history: sketches, features, equations, configurations | Two or more parts positioned with mates, the way they sit in the machine |
| Editable | No — no feature history is carried over | Yes — change a dimension and the part rebuilds | Yes — parts and mates stay editable |
| Opens in other CAD | Yes — SolidWorks, Fusion 360, Inventor, NX, Creo, CATIA, FreeCAD | SolidWorks only | SolidWorks only |
| Use it for | Layout, mating, clearances, sending to a supplier | Re-dimensioning a part for your own machine | Understanding the mechanism, checking travel, animating joints |
The sections below take each one in turn.
What is a STEP file?
STEP (ISO 10303, files ending .step or .stp) is the neutral exchange format for 3D geometry. Every major CAD package — SolidWorks, Fusion 360, Inventor, NX, Creo, CATIA, FreeCAD — opens STEP natively, which makes it the safest choice when you do not know what your customer, supplier, or colleague is running.
What STEP preserves: exact boundary geometry — faces, edges, and the solid body — with tolerances intact. A STEP file imported into any system represents the same physical shape, which is why it is the standard for machine layouts, customer reviews, and inter-company exchange.
What STEP does not preserve: the history. Sketches, features, and design intent are gone. The model arrives as a finished solid — you can measure it, mate it, and cut it, but you cannot go back and change the sketch that created the fillet.
What is an SLDPRT file?
A .sldprt file is the native SolidWorks part file. It contains the full parametric history — sketches, features, equations, and configuration — which is what makes a model editable. Change a dimension in an SLDPRT and the whole part rebuilds; that is the difference between "a model of a gripper" and "a gripper you can re-dimension for your own part size."
An SLDPRT only makes sense if you work in SolidWorks. In any other CAD system it is useless as an editing format — though the STEP version covers you there.
What is an SLDASM file?
An .sldasm file is the SolidWorks assembly file: multiple parts assembled with mates, positioned the way they go together in the real machine. This is where a library shows its craft. A proper assembly has a logical structure — base, axis, joints, end effector — with mates that behave when you drag or animate.
Why an assembly matters for buyers: a robot arm as a single monolithic SLDPRT might look right in a render, but an SLDASM lets you see how the joints articulate, check the degrees of freedom, and understand the mechanism instead of just its outline.
Which format should you download?
- Layout and integration in any CAD → STEP. Measure, mate, and verify clearances in whatever system you run.
- Editing and re-dimensioning in SolidWorks → SLDPRT. Change part sizes, add mounting holes, adapt the model to your machine.
- Understanding the mechanism and animating joints → SLDASM. See how it goes together and moves.
- Sending to a customer or supplier → STEP. Never send native files for review; STEP keeps your feature tree private and their system compatible.
In practice, serious buyers download all three. STEP for the layout, SLDPRT for the edits, SLDASM for the mechanism.
Every model in the library ships all three. The industrial robotic arm, the robotic welding and inspection station, the USB assembly line equipment and the all-in-one equipment for the semiconductor industry are each delivered as STEP, SLDPRT and SLDASM for SolidWorks 2018+.
Red flags when a library offers only one format
- STEP only — likely an imported shell or a mesh converted to STEP, with no native design behind it. You get geometry, but nothing editable, and the geometry may be hollow or faceted.
- SLDPRT only — either a small-vendor library or a model that has never been validated for exchange. You are locked to SolidWorks and to their build version.
- A single monolithic body in the SLDASM — a tell that the "assembly" is really one imported solid. A real assembly has multiple parts and mates.
A library that consistently ships STEP + SLDPRT + SLDASM is telling you the model was designed natively, then validated in exchange format — the same workflow a professional design office follows.
What to check in the native files
- Minimum SolidWorks version. Feature trees do not travel backward well. A stated minimum ("SW 2018+") means the library tracks compatibility.
- Feature tree structure. Named features in a logical order (base, axis, flange) beat one feature called "Boss-Extrude1".
- Units. Confirm the assembly is in millimeters (or whatever your project uses) before you mate it into your machine.
The practical test: try a free sample first
The best way to judge a library's file discipline is to open one of its models in your own CAD. Download a free sample, check the feature tree, measure a critical dimension, and see whether the assembly behaves. That 20-minute check tells you more than any product description.
Mazi Models ships every product in STEP, SLDPRT, and SLDASM for SolidWorks 2018+, with named feature trees and dimensioned assemblies. The free sample models — a fixture assembly and a conveyor unit — are there for exactly that: download one, open it, and check the feature tree yourself. If it passes your inspection, the rest of the library works the same way.