14.3 Manipulating OBJ files on the build plate

While .stl files are the de facto standard format for 3D printing, OBJ files (which support an auxiliary MTL file format for storing additional object information such as color and texture) are more commonly used in other, general 3D modelling applications, such as in art and computer graphics areas. For instance, a number of museums have been moving into performing 3D scans of artworks, historical artifacts, and scientific artifacts; they tend to use OBJ format for these models.

Therefore, there are a lot of 3D models available in OBJ file format, and while they often were not created with 3D printing specifically in mind, some of them may be quite interesting for 3D printing purposes; you can get access to a whole additional range of 3D models via OBJ files. OBJ file models will, however, often require more adjustment (or “masssaging”) than .stl file models in order to get them truly print-ready.

Some possible (perhaps likely) issues:

1.

Object design: Keep in mind that an object available as an OBJ file may have been designed with no thought of feasibility for 3D printing. While this is sometimes the case with .stl files as well, it is much more common with OBJ files, which are used in so many non-printing applications. So consider the object model carefully: is it something that is actually feasible and practical to 3D print?

2.

Object size (“SCALE”): An OBJ file model created without thought of 3D printing is not particularly likely to have been scaled appropriately to fit on a typical 3D printer build plate: OBJ files, when uploaded into the Polar Cloud, may have a “natural” size that is either tiny (and needs to be scaled up, so that the object is even visible), or huge (and needs to be scaled down to fit on the build plate).

3.

Object orientation (“ROTATE”): OBJ files were often created using art software tools, rather than CAD programs. CAD programs, having an engineering bent, normally use the standard engineering/science “right-hand rule” Cartesian coordinates axes, where the x-axis and y-axis lie in the plane of the build plate and the z-axis sticks “up”, perpendicular to the build plate; specifically, if you think of the index finger of your right hand being the x-axis, and the middle finger of your right hand being the y axis, both lying in the plane of the build plate, then your thumb would stick “up” as the z axis. In contrast, art software programs at a minimum tend to present the x-axis pointing to the right of the screen, the y-axis as pointing “up”, and the z-axis as “depth”, pointing out of the screen (towards the viewer), which is still a “right-hand rule” coordinate system, (though in appearance it is rotated 90° about the x-axis as presented in engineering and science applications). However, sometimes art software programs will even use a “left-hand rule” coordinate system: think of your left hand index finger as the x-axis, your left hand middle finger as the y-axis pointing “up”, and your left hand thumb (pointing “into” or “away” from the viewer) as the z-axis. A “left-hand rule” coordinate system is the mirror image of a “right-hand rule” coordinate system: you can not get from one to the other via a rotation, but only via a mirror “flip”. (Consider your left and right hands: they are mirror images of each other, and can not be rotated to line up identically.)

What this means for printing OBJ files is that you will very often find that an object loaded from an OBJ file is in an odd orientation for printing: often “lying down” on the build plate, rather than standing upright. Furthermore, in some cases, if the object has distinct left and right sides, the object may even appear “inverted”, as if it has been “flipped” through a mirror. So you will often find that you wish to rotate the placement of an object loaded as an OBJ file. A good start is often (but not always) to try rotating by 90° about the x-axis.

4.

Object location (“MOVE”): In the process of adjusting an object’s “SCALE” and “ROTATE” values, you’ll often end up also wanting to “MOVE” an object on the build plate.

For items 2, 3, and 4, see the discussion of how to “SCALE”, “ROTATE”, and “MOVE” an object on the build plate in Section 10.2.1. Or see the example in Section 14.3.1.

5.

Object components all stored in one OBJ file: Whereas the typical convention when using .stl files is to put each component into its own, separate .stl file, with OBJ files, all components of an object are typically stored together in one file. This can make it more awkward to select just certain components of an object to print.