[AR] Re: Specifying Design Intent for 3D Printed Rocket Parts
- From: Ed LeBouthillier <codemonky@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 28 Mar 2018 11:03:52 -0700 (GMT-07:00)
Particularly, with regards to tolerancing and GD&T. Since
the printers mainly read basic dimensions from the CAD file that is supplied.
The printers don't read the basic dimensions from the CAD file, they
get a sequence of GCode which causes the printer to build the object.
The Slicer gets the 3D hull definition (outer skin) from 3D files like STL.
It is the job of the CAD program to generate the STL file (or 3D object)
from the "drawing" (which might be a 3D object representation). It is the job
of the slicer and controllers to take that 3D representation and convert it
into printer commands.
For example, in openSCAD, there are mathematical operations (union,
integration, difference) performed on conceptual ideal objects (cubes, spheres,
splines).
In other programs (like generic CAD programs), the "drawing" which is 2D
is converted into a 3D representation which approximates the object.
There are differing degrees of sophistication in the various CAD programs
as to their tolerancing. For example, I tend to use Blender for my CAD
program which has no support for tolerancing; it is merely points and
triangles for which it is the responsibility of the designer (me) to make sure
that the points and triangles create a shape with sufficient tolerancing.
In more sophisticated programs, like SolidWorks, they will automatically
generate approximations of the shapes (as points and triangles) consistent with
the tolerances which you define. There are also more sophisticated
representations of 3D objects (file formats) that various software developers
use (obj files, etc).
What are similar tolerances that one must consider while
designing 3D printed chambers, injectors, etc?
Each printer has its own abilities to hold tolerances. In many cases,
the printer itself does nothing but print. It is the slicer program
which determines how to make sure that what is printed meets the tolerances
(after figuring in such things as shrinkage).
So, the overall process is:
1. CAD program generates a 3D file
2. Slicer takes 3D file and converts to G-Code
3. Printer Controller takes G-Code and performs the printing process
The Slicer is a critical component to making sure that you have good
prints and tolerancing considering the 3D representation (such as STL
or other formats). The more sophisticated printers (such as metal printers)
each have their own slicer programs which know the limitations and capabilities
of the printer and how to convert the 3D point/triangle representation into a
good print. Some of those slicers are proprietary and hold trade secrets.
In some of the more sophisticated systems, the Slicer and Controller are united
into one proprietary program which embodies the trade secrets of the printer
developer to get good prints.
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