CAD File Formats

stl

What are CAD files?

The computer files which stores the geometric information of a part,component,assembly or even an architectural plan, which are generated by a computer Aided Design (CAD) software called CAD files. These files are used for storing,retrieving and exchanging the designs. The design may be 2D like a factory layout plan or 3D model like an operating handle of an industrial valve. DWG ,DXF etc are some examples of 2D file format while IGES,STEP,SLDPRT,STL,OBJ are some examples of 3D CAD files.

 

Neutral File format

Every CAD software company has their own CAD software format which can only be interpreted by their own software only.But is not necessary that entire manufacturing chani will have the same software vendor. So there was a need for a neutral file format which can be easily interpreted by any other software company. Example of such neutral file formats are IGES/IGS and STEP/STP. These files have 3D model geometry exported in the form of trimmed surfaces. A solid can be represented by a closed shell comprise of these trimmed surfaces. 

 

Proprietary file format

As discussed above the every CAD software has it’s own native file format to export.The file generated from a software in which it is created has all the important information like design feature, geometric constraints, history tree etc. You can start modeling or editing the part where you left it if you have the same software. SLDPRT by solidworks, PRT by  ptc creo ,CATPART by catia are some examples of such files.

 

What is an IGES or IGS file?

IGES or IGS file types are same in nature and it stands for Initial Graphics Exchange Specification.IGES was an initiative of the United States Air Force (USAF) Integrated Computer Aided Manufacturing (ICAM) project (1976-1984).IGES file can be use to store  circuit diagrams, wireframe, freeform surface or solid modeling representations etc. IGES file represent geometric data in the form of topological structure where parent entities are referenced to their child entities.

 

What is a STEP or STP file? 

STEP file is a CAD file format, usually used to share 3D models between users with different CAD systems. The Acronym STEP, stands for “Standard for the Exchange of Product model data”. The development of STEP started in 1984  as a successor of IGES, but due to the complexity of the project, the initial standard was only published in 1994.

 

How does a STEP/STP file looks like?

 

The geometric entities are written in a topological manner or you can say a parent child referencing system. That means all the entities are dependent on other type of entity. An entity is an element of geometry type.eg Vertex, Line, Circle, etc. Each of the entities references it’s child entity. Figure below the topological structure of a step file of a simple cube.

stl file

As you can see in the figure the parent entity like EDGE_CURVE #57 points to child entity VERTEX_POINT#81 and #82 as two end points and the LINE#69 as type of curve. This edge also have parents component like ORIENTED_EDGE #33 which has a parent EDGE_LOOP #27 which has a parent FACE_OUTER_BOUND #21 which is a child of ADVANCE_FACE #15. This type of topological structure helps in maintaining the correlation be the entities like two adjacent faces can have a common edge hence will point to same edge entity. Hence the parametric nature of the CAD file exist since all parent geometric entities are constrained according to their child components.

Having accurate geometrical data these file are used for CNC,VMC machining,profile cutting and other high tolerance manufacturing processes 



Mesh files.

A mesh usualing meaning something which created from collection of small shapes. In CAD world a mesh is a 3D model made out of small finit basic shapes such as triangles or tetrahedrons for surface and solid mesh respectively. Surface mesh was first created to render a 3D view on a computer screen by breaking the model’s geometry into triangles and applying shaders to each one and generating a 3D view. Whereas a solid mesh is used for engineering analysis of part by computing load and stress distribution on small standard shapes which is easy to compute. Usually the mesh files consist of surface mesh which is a close shell formed by triangles in 3D space.A mesh file essentially consists of a list of vertices and the face/triangle which is formed by these vertices and may also contain the color and texture map information in it. OBJ, STL, 3MF, VRML, 3DS, PLY etc, of which OBJ and STL files are common and widely used.

Obj format was used for rendering and animation purpose whereas STL files are widely used for 3D printing and other additive manufacturing processes. STL file is now the most popular mesh file format because of rise in 3D printing trends.

STL file

STL stands for stereolithography created by 3D systems ,STL files describe only the surface geometry of a 3Dl model without any representation of color, texture or other common CAD model attributes. There are two types of STL files ASCII and binary ,difference being ASCII type file  is written in string while binary type file in stored in the form of array buffer. As discussed above STL only contains triangle data i e normal direction and X, Y, Z coordinates of three vertices of the triangle. Given below is the content of an ASCII STL file.

 

stl

solid LowPolyPikachuKeychain

facet normal 0.0 0.0 1.0

 outer loop

   vertex 10.302009083108604 8.203284725397154 -1.0100507985555396e-08

   vertex 8.083214677648247 7.498559936731383 -1.0100507985555396e-08

   vertex 8.247109807805717 6.509723648279234 -1.0100507985555396e-08

 endloop

endfacet

facet normal 1.0399059218851225e-09 -3.2740971777822667e-09 1.0

 outer loop

   vertex 10.302009083108604 8.203284725397154 -1.0100507985555396e-08

   vertex 9.657508291082085 11.08355592176251 0.0

   vertex 8.083214677648247 7.498559936731383 -1.0100507985555396e-08

 endloop

endfacet

 

As you can see the file consist of facet normal and vertex as discussed above.

 

STL and 3D printing

As STL file consist of triangles, If any plan cuts the geometry a certain amount of triangles will intersect with this plan giving two points and joining all these points will give us a cross section outlines which is exactly what a 3D printer needs for Printing any model. Hence by taking the cross sections of a 3D model along Z-axis with step equal to  the layer height will generate the set of instructions required by 3D printer and this process of taking cross sections is called slicing often done by another software called as slicer. These software essentially slice an STL file and generate G-codes which can be fed to a 3D printer.

 

 There are many slicing software both open source and closed source  .CURA ,Repetier and KISSlicer are some examples of free and open source slicer whereas Simplify3D ,MakerBot and other printer specific software are some closed source.

 

 Conclusion

We have seen different CAD file formats and significance of each one and it’s applications. Also we have studied the topological structure of neutral file formats like IGES and STEP and also the structure triangulated Mesh files and how it is used in 3D Printing.

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