If you’re new to CAD, welcome! If you’re a veteran who would like a little guidance and some brief history on a few major file formats, you too are in the right place.
Organizations that use parametric 3D modeling systems, such as Solidworks, PTC Creo, or Autodesk Inventor usually save their work in a proprietary software format, such as .SLDPRT or .PRT. Some major CAD programs can share files between them but, due to the proprietary nature of the different companies, often translators between them are unidirectional. This is the case even between different versions of Solidworks. Opening an old Solidworks file in a newer version of Solidworks renders it un-openable in the old version (they are not backward compatible if you will), and it’s a major factor in deciding whether or not to keep your Solidworks version current. More on that another time, let’s get to the formats.
Many of the major 3D modeling software use file formats that are parametric, meaning that you can manipulate the model’s surfaces, features, and more by manipulating numbers in drawings, features, etc. They have feature trees that grow over time and you can move up and down the feature tree as you build the model and define relationships between features, parts, and surfaces. The parametric nature allows for features at the top of your feature tree to cascade their changes into any child features.
Parametric models are often a wonderful thing but, because not all of your vendors, collaborators (such as electrical engineers), and clients will have access to the same parametric software that you do, other file formats are necessary to share part and assembly files.
Without parametric models, users of your *“dumb” models downstream in the product development process will be more challenged to modify your parts. However, with more complicated surfaces and feature trees, major changes to parameters early in your feature tree can cause a lot of things to break. For tweaks made later in the manufacturing process to a complicated part, adjustments to faces are often the easiest, not manipulating the original sketches. For simple parts, it’s great to keep the feature tree intact. To see a functional example of this, check out models from McMaster-Carr’s website. If you download a McMaster-Carr part in native format, there is usually a very concise feature tree with 5-15 features defined by a similar number of sketches.
When industrial designers send their parts to mechanical engineers, it is typically advisable to use native CAD format whenever possible. Then the mechanical engineering counterpart can decide what works best in their process to incorporate the ID’s design.
*Below, we’ll present an overview of “dumb” part files. At the end, we’ll summarize recommendations for when to use each.
STEP, which stands for “Standard for the Exchange of Product model data” was created per ISO 10303 to have a neutral file exchange format between CAD programs. Prior to STEP, several nations had their own neutral file exchange format. STEP was an effort to unite those under one file type, and it did so with very good success. Typically, most vendors around the world, in manufacturing, electronics, optics, etc., can use STEP files without issue.
There are two types of STEP files. A STEP AP203 file defines the geometry, topology, and configuration management data of solid models for mechanical parts and assemblies. STEP AP214 files define everything a STEP AP203 file does with the addition of colors, layers, geometric dimensioning and tolerance, and design intent.2
STEP files can be exported in assembly format. However, it is always wise to check with your vendor and their preference. They might want the files saved out as individual parts. This typically reduces any risk of confusion and makes it very clear what part(s) the vendor is being asked to quote or provide.
Sometimes, STEP files lose or distort data such as radii when being converted from a native CAD software. It is good practice to open your STEP file after export to check that everything exported correctly.
IGES (pronounced “eye-jess”) stands for the Initial Graphics Exchange Specification (IGES). The US Air Force’s ICAM initiative created the IGES format, published in 1980, for the easy exchange of files between aerospace vendors in creating hardware. This allowed for a reduction of expenses in creating new technologies and hardware in aviation/aerospace. It is an American National Standards Institute standard (ANSI).
While IGES is fairly ubiquitous in the US, UK, and some other countries, it does not have comprehensive foreign language support. The current version of IGES does not support Unicode 16- or 32-bit character encoding.
STL is an abbreviation for “stereolithography,” after 3DS stereolithography software. Stereolithography (as a process, abbreviated SLA) is an old but still very commonly used 3D printing process. The file format describes a triangulated mesh surface. STLs are great for 3D printing but are completely unmodifiable by many major CAD programs as they work in solids, not meshes. Often 3D scanners produce STL files and require the use of reverse engineering software to recreate the original geometries.
An important note about STLs is that they are dimensionless. When you export, make sure to communicate the units (mm, inches, cm, etc.) to whoever is producing the part.
So, what file type do I use?
If your part is simple, your part doesn’t contain proprietary information or models, and your vendor has the same parametric software as you…
send the native CAD!
If you want to hide features and sources in your part…
send a STEP or IGES (if in the right geography)
If you’re 3D printing a part…
send an STL
If you want to hide features and sources from your part but both you and your vendor have Solidworks…
check out the Defeature tool
If having trouble getting all the details in your file…
give the Parasolid format (.x_t) or Parasolid binary format (.x_b) a try.
By: Stephanie Whalen