What Is the Parasolid File Format? X_T and X_B Files Explained

A Parasolid file is a 3D model saved in the Parasolid file format, the native save format of Siemens’ Parasolid modeling kernel, written as an .x_t (text) or .x_b (binary) file. SolidWorks, Siemens NX, Solid Edge and Onshape all run on that kernel, which is why a model moves between them with no translation at all.

The catch is that the receiving program also has to speak Parasolid. Open the same x_t in software built on another kernel, or in a release older than the one that saved it, and the import can fail outright. That makes Parasolid the best choice when both sides run Parasolid-based software, and STEP the safer one when they don’t, or when you aren’t sure.

In practice, a failed import usually means a supplier email asking for a different file and a few lost days before your quote comes back. A few minutes spent choosing the right file and version up front saves that back-and-forth and gets your part priced on the first try.

What this article covers:

• How x_t and x_b files differ, and which one to export
• Where Parasolid outperforms STEP, and where STEP is the safer bet
• Common reasons an x_t won't open on the supplier's side
• The right file format to send for CNC machining

First, a closer look at what's inside a Parasolid file.

TL;DR / Key Takeaways

• Parasolid is both a kernel and a file format. The kernel is Siemens' geometry engine; .x_t and .x_b are the files it writes
• .x_t and .x_b hold the same geometry. .x_t is plain text and .x_b is binary, so .x_b files are usually smaller
• A Parasolid file keeps the exact 3D shape, not the design history. It imports as a "dumb solid" with no feature tree, sketches or tolerances; the shape can still be changed with direct-editing tools, but not through its original features
• When an x_t will not open, the usual cause is a version limit in the receiving software. It is most often a program that reads Parasolid through a translator or an outdated viewer; the fix is a re-export at an older Parasolid version, or a STEP file instead
• Parasolid beats STEP when both sides run Parasolid; STEP is the safer choice when the receiver's software is unknown or built on another kernel. For a factory RFQ, send STEP, add the x_t if the shop asks and always include a PDF drawing

What Is a Parasolid File?

A Parasolid file is a 3D CAD file that stores a part's exact solid geometry in the native format of the Parasolid modeling kernel, developed by Siemens Digital Industries Software. Its extensions are .x_t and .x_b. Because it is the kernel's own format, a Parasolid file opened in another Parasolid-based program needs no conversion.

Parasolid is a 3D geometric modeling kernel: the software engine that SolidWorks, NX, Solid Edge and Onshape use to calculate solids, surfaces and edges. The Parasolid file format is simply how that engine saves a model.

Parasolid's development began in the 1980s, and Siemens now licenses the kernel to other software vendors. That licensing model is why programs from competing companies, such as Dassault Systèmes' SolidWorks and PTC's Onshape, share the same geometry engine as Siemens' own NX.

Is Parasolid a Kernel or a File Format?

Parasolid is both a geometric modeling kernel and the file format that kernel writes. The kernel is the software library inside a CAD program that calculates every face, edge, Boolean cut and fillet. The file format, often called Parasolid XT, is how that kernel saves its work to disk, as what Siemens calls Parasolid transmit files: .x_t for text and .x_b for binary.

That split between kernel and file explains Parasolid’s main advantage. A neutral format such as the STEP file format or the IGES file format asks 2 different kernels to agree on how a surface is described. A Parasolid file passed between 2 Parasolid-based programs skips that step, because both sides already use the same mathematical definitions.

When the factory accepts Parasolid, an .x_t or .x_b file is a good choice for custom parts production, especially if the receiving CAD/CAM workflow is Parasolid-based. A 2D drawing or clear RFQ notes still have to cover tolerances, threads, material, finish, quantity and revision. Parasolid, STEP, IGES, STL and DXF each suit a different job when exporting CAD files for CNC machining.

What Are X_T and X_B Files?

X_T and X_B are the 2 Parasolid file extensions: .x_t is plain text and .x_b is binary, and both hold the same geometry. A model saved as .x_t and as .x_b opens to an identical solid. The choice between them changes file size and readability, not accuracy.

Comparison	.x_t (text version)	.x_b (binary version)
What is inside	The full 3D model	The same 3D model, identical when opened
How it is saved	As readable text characters	As compact binary code
File size	Larger	Usually smaller
Can you read it in Notepad?	The header, yes (it shows the software and Parasolid version); the geometry is hard to follow	No
Which programs accept it	Programs that import Parasolid	Most of the same programs, but SolidWorks notes some do not accept binary files
When to use it	Default when sending to a factory	Large models, once the receiver confirms it accepts .x_b

Older and alternate extensions for the same 2 encodings are .xmt_txt and .xmt_bin. The SolidWorks Parasolid translator imports all 4, and notes that binary files are smaller but aren't supported by some target applications.

What Is Inside a Parasolid File?

A Parasolid file contains the finished 3D model as boundary-representation (B-rep) geometry: solid bodies, faces, edges, vertices, the surfaces and curves behind them, and the topology that says which face meets which. That's what a manufacturing team actually receives when someone sends an .x_t or .x_b: the shape, defined exactly, and nothing about how it was modeled.

The file has 2 parts. A short text header names the application that wrote it, the date and the Parasolid schema version. Below it sits a numbered list of entities, each referencing the others, with the fields of each entity defined by a schema for that Parasolid version.

Here is the header of a real .x_t, a sample turning part that Autodesk publishes for download:

The header answers 2 questions before any CAD program is opened: which software made the file, and which Parasolid version it was saved in.

Open the .x_t in a text editor such as Notepad (an .x_b header is binary and won’t display as text) and look for 2 lines:

• APPL= names the program that exported the file, PowerShape in the sample above
• SCH= shows the Parasolid version in its first 4 digits, so SCH_2901199 means Parasolid 29.1

That version number is what matters when a file will not open. If it is newer than the receiving software can read, the sender needs to re-export at an older version. Okino’s Parasolid notes recommend the same APPL check for identifying the original CAD program.

Parasolid also stores some geometry differently from most formats: rounded edges and the curves where 2 surfaces meet are kept as exact math, not close approximations. A Parasolid-based program reads those definitions as-is, while a program on another kernel has to convert them, which is one reason imports outside the Parasolid family are less predictable.

Beyond geometry, the file can carry colors, layers, entity names and user-defined attributes. It can also hold simple assemblies, but mates and constraints between parts don’t carry over.

What CAD Programs Use the Parasolid File Format?

Siemens NX, Solid Edge, SolidWorks, Onshape and Shapr3D are CAD programs built on the Parasolid kernel, and they open .x_t and .x_b files natively, with no translation. Programs on other kernels, including Autodesk Inventor, Autodesk Fusion, PTC Creo and CATIA, import Parasolid through a translator.

CAD program	Built on Parasolid?	What happens when you open an .x_t file
Siemens NX	Yes	Opens the file as-is, with no conversion
Solid Edge	Yes	Opens the file as-is, with no conversion
SolidWorks	Yes	Opens the file as-is, with no conversion
Onshape	Yes	Opens the file as-is, with no conversion
Shapr3D	Yes	Opens the file as-is, with no conversion
Autodesk Inventor, Fusion	No, uses its own kernel	Converts the file on import (paid Fusion only; the free personal-use version does not read Parasolid). The shape comes in without design history; check complex curved surfaces
PTC Creo, CATIA	No, uses its own kernel	Converts the file on import, if the installed version includes a Parasolid translator
FreeCAD	No, uses Open CASCADE	Cannot open it; ask for a STEP file instead

Autodesk's own documentation confirms that Inventor and Fusion run on the Autodesk Shape Manager kernel, not Parasolid, which is why both translate an x_t on import.

Parasolid support reaches beyond CAD. Mastercam's Solids module is built on Parasolid, and Ansys reads all 4 Parasolid extensions (.x_t, .xmt_txt, .x_b, .xmt_bin) up to Parasolid version 36.1 in its 2026 release. That matters for machining: a factory programming toolpaths in a Parasolid-based CAM system receives an x_t with no conversion at all.

What Are the Limitations of a Parasolid File?

The main limitation of a Parasolid file is that it keeps the finished shape but not the design history: the sketches, dimensions and steps the designer used to build the model. It opens as a "dumb solid", exact in shape but with nothing to edit behind it.

For quoting and machining, the missing history rarely matters. Factories in Haizol’s network of CNC machining suppliers quote and machine Parasolid files without the design history, because a machinist programs the cutting paths from the finished surfaces, not from the sketches behind them. Design history matters when the part needs to change: resizing a feature or updating a dimension is much harder without it, so design changes should go back to whoever holds the original CAD file.

Parasolid File Advantages and Disadvantages

The table below shows what a Parasolid file keeps and loses when it leaves the original CAD program.

Advantages of a Parasolid file	Disadvantages of a Parasolid file
Exact 3D shape: curves stay true curves, not triangles	No design history: the steps, sketches and editable dimensions are gone
Opens with no conversion in programs built on Parasolid	Programs not built on Parasolid have to convert it on import
Rounded edges and curved joins stored as exact math	No tolerances or manufacturing notes, so a 2D drawing is still needed
Small files, especially .x_b	No 2D drawing views
Keeps colors, layers and part names	Loses how parts in an assembly fit and move together
Opens quickly in Parasolid-based software	Version limits: a newer file may not open in translators or older viewers

Tolerances are the gap that reaches the factory floor. A Parasolid model shows a 10 mm hole but cannot say whether it is a loose clearance hole or a tight H7 fit for a pressed-in bearing, so a 2D engineering drawing still has to define acceptance. When tolerances must travel inside the model itself, that's a job for STEP AP242.

Can You Get the Feature Tree Back from a Parasolid File?

The original feature tree cannot be recovered from a Parasolid file, because the file never contained it. Feature recognition tools can rebuild an approximate tree, but it's a reconstruction, not the designer's original intent.

A dumb solid in SolidWorks shows up as a single imported body (such as "Imported1") in the FeatureManager design tree, with no features underneath it. Parasolid feature recognition tools try to fill that gap. SolidWorks' feature-recognition tool, for example, scans the imported body and recognizes extrudes, revolves, holes, fillets, chamfers, shells and patterns.

Simple prismatic parts often recognize cleanly, while freeform or heavily filleted parts rarely do. Either way, a dumb solid can still be changed with direct-editing tools, just not through its original features.

 

How Do You Open an X_T or X_B File for Free?

To open an X_T or X_B file for free, use a free Parasolid-based CAD program such as Solid Edge Community Edition or Onshape's free plan, or a free Parasolid viewer such as eDrawings or Siemens' JT2Go. General-purpose tools usually can't: Blender, most mesh editors and 3D printing slicers don't read Parasolid at all.

How Do You Open an X_T File Without CAD Software?

To open an X_T file without paid CAD software, install a free viewer or use a free CAD tool that reads Parasolid natively.

Free tool	Runs on	What you can do with the file	Watch out for
Solid Edge Community Edition	Windows	Open, edit and save it as STEP or another format	Free for personal use only, not for paid or commercial work
Onshape free plan	Any web browser (Mac, Windows, Linux)	Open and edit the file in a browser; save it as STEP	Files on the free plan are public: any Onshape user can view and copy them. Do not upload confidential parts
eDrawings Viewer	Windows	View, measure and cut cross-sections	Older versions may not open newer files, so install the latest release
Siemens JT2Go	Windows, iOS, Android	View and measure the file	Viewing only: cannot edit the file or save it as STEP

Mac users have fewer options, since SolidWorks and Solid Edge don't run on macOS. A browser tool such as Onshape, or Shapr3D, which runs on the Parasolid kernel natively on macOS, covers most viewing needs.

3D printing slicers don't read .x_t: UltiMaker Cura's supported file types are meshes such as STL, 3MF and OBJ. Convert the file to STL or 3MF first, using the steps in the next section.

Exporting or Converting a Parasolid File

Exporting a Parasolid file uses a Save As or Export command in any Parasolid-capable CAD program, and the most important setting is the Parasolid version. Converting Parasolid to STEP, STL or another format works the same way: open the x_t in any program that reads both formats, then export.

How Do You Export to an Older Parasolid Version?

To export to an older Parasolid version, the sender picks the version in the export options before saving. SolidWorks, NX and Solid Edge all offer this setting, and SolidWorks' guidance is to choose the version the receiving system supports.

A buyer who receives an x_t that will not open does not need CAD software to fix it. Ask the sender to save the file again from the original CAD program at an older Parasolid version, and name the software that needs to open it.

Editing the version number inside the file does not work, because the data below the header is still written in the newer format. If no old-enough version is available, ask for STEP AP214, which older CAD releases also read.

How Do You Convert an X_T File to STL for 3D Printing?

To convert an X_T file to STL for 3D printing, open it in any CAD program that reads Parasolid, then export as STL or 3MF and set the mesh resolution. A finer mesh setting (often called chord tolerance: the largest gap allowed between the mesh and the true curve) gives smoother curves and larger files.

Because a Parasolid model is already a closed solid, the resulting mesh is normally watertight and prints without repair. That is a real advantage over the IGES file format, which often arrives as open surfaces.

Can You Convert an STL or Mesh Back to Parasolid or STEP?

You can convert an STL or mesh to Parasolid or STEP, but the result is a faceted solid where every triangle becomes a flat face, not a truly curved model. A cylinder stays a polygon until you rebuild the surfaces by hand or with reverse-engineering tools, through mesh repair, conversion and feature rebuild.

To get an x_t into Blender, SketchUp or Maya, convert it to STEP, STL or OBJ first, since none of them read Parasolid.

 

Parasolid vs STEP: When Is Parasolid the Better Choice?

Parasolid is better than STEP when the handoff stays inside the Parasolid family, for example SolidWorks to NX, Solid Edge, Onshape or Shapr3D, because the geometry moves with no translation at all. STEP is better whenever the receiver's software is unknown or runs a different kernel, which is why factories in Haizol's network most often ask for STEP.

Situation	File to send	Why
Sending to a machine shop or supplier whose software is unknown	STEP	Almost every design and machining program opens it
The supplier asks for .x_t or .x_b	Parasolid	The supplier knows what its own machining software reads best
Sending between SolidWorks, Solid Edge, NX, Onshape or Shapr3D	Parasolid	All are built on Parasolid, so the file opens with no conversion
Complex parts with many freeform curved surfaces	Parasolid, if the receiver's software is built on Parasolid	Curved surfaces arrive exactly as designed, with no conversion errors
Tolerances need to travel inside the 3D model	STEP AP242	The version of STEP that can carry tolerances in a form software can read
3D printing	Neither; export STL or 3MF	3D printing software needs a mesh: a surface made of small triangles
Moving a model from Fusion to SolidWorks, Inventor or FreeCAD	STEP	Fusion cannot export Parasolid, and FreeCAD cannot open it
Opening a model in an older SolidWorks release	Parasolid, saved at an older version	Older releases cannot open newer SolidWorks files but can open an older-version x_t. The shape arrives exactly, without the design history
Long-term storage	STEP AP242, plus the original CAD file	An ISO standard that the aerospace industry's long-term archiving project (LOTAR) recommends; the original file keeps the design history

Can Parasolid Fix a Bad or Slow STEP Import?

Parasolid often fixes a bad or slow STEP import, provided both programs are Parasolid-based. A STEP file that opens as loose surfaces, or takes minutes to load, has been through 2 translations: out of the sender's kernel into STEP, then back into the receiver's. An x_t from the same model skips both.

In SolidWorks, Parasolid imports load faster and with fewer gaps or stitching errors, because the program is reading its own kernel's format.

 

Common Parasolid Import Errors

A Parasolid file usually won't open because it was created in a newer Parasolid version than the receiving software's importer can read, most often when the program reads Parasolid through a translator. The other common causes are a native file from a future CAD release, an outdated viewer, a license that blocks the format or a damaged download.

What you see	Likely cause	What to do
Version of the file format is not supported	The file was saved in a newer Parasolid version than the receiving software can read	Ask the sender to save it again at an older Parasolid version, or to send a STEP file instead
SolidWorks says the file was created in a future version	The file is an original SolidWorks file (.sldprt or .sldasm) from a newer release, not an x_t	Ask the sender for an .x_t saved at an older Parasolid version
eDrawings opens other files but not this .x_t	The installed eDrawings is too old to read the file's Parasolid version	Update eDrawings to the latest release
Fusion will not open the Parasolid file	The free personal-use version of Fusion does not open Parasolid files	Ask for a STEP file instead
Every program shows an error, or the file is much smaller than expected	The download is broken or incomplete	Download it again; if that fails, ask the sender to zip the file and resend it

Before asking the sender for a new file, 2 quick checks show whether the problem is the file or the receiving software:

1. Read the header. Open the .x_t in Notepad and find the APPL= and SCH= lines, which show the software that made the file and its Parasolid version (see the header example in the "What Is Inside a Parasolid File?" section above).
2. Try a free viewer. Open the file in a viewer such as JT2Go. If it opens there, the file is fine, and the problem is the software that rejected it.

Can You Send a Parasolid File for Custom Parts Production?

Parasolid File FAQs

Is an X_T file the same as a Parasolid file?

X_T is the text form of the Parasolid transmit format, so an .x_t file is a Parasolid file; .x_b is the same format saved as binary.

Is Parasolid free to use?

The Parasolid kernel isn't free: Siemens licenses it to more than 200 software vendors, who build it into their CAD, CAM and CAE programs. Siemens publishes the XT file format as open. Opening Parasolid files can be free, through tools such as Solid Edge Community Edition, Onshape's free plan, eDrawings or JT2Go.

Does a Parasolid file contain tolerances?

A Parasolid file holds the exact shape but no tolerances, threads or finish, so a 2D drawing still has to carry them. If tolerances must travel inside the model, use STEP AP242.

Can AutoCAD open X_T files?

Full AutoCAD for Windows can import .x_t and .x_b files with the IMPORT command, but AutoCAD LT and AutoCAD for Mac can't. The geometry arrives as a 3D solid with no feature history.

What is the best file format for long-term CAD storage?

STEP AP242 is the best file format for long-term CAD storage, because it's a vendor-neutral ISO standard rather than one kernel's format.

How do you convert a Parasolid file to STEP?

Open the .x_t or .x_b in any program that reads Parasolid, such as Solid Edge Community Edition or Onshape, then choose Save As or Export and select STEP (AP214 or AP242). The geometry carries over; feature history does not, because the Parasolid file never had it.

What is the best free Parasolid viewer?

Siemens' JT2Go and eDrawings Viewer are both free and read .x_t and .x_b files. JT2Go also runs on iOS, Android and the web, while eDrawings is Windows-only.