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STL GENERATION Written by Greta DAngelo
In this section: An introduction on .stl files How to make good
.stl files How to make .stl on different softwares (Solidworks, PTC
Creo, Rhinoceros 5, Autodesk Inventor) Summary
INTRODUCTION TO STL STL is the short name for Standard
Tassellation Language, and represent the standard file format used
for
Additive Manufacturing production. It is a triangular
representation of a 3D surface geometry. That means
when a CAD model is exported to STL format, the surface is be
subdivided in a triangular mesh as shown in
Figure 1. Each triangular facet is described by its normal and
three points representing its vertices. While the
aspect ratio and orientation of individual facets is governed by
the surface curvature, the size of the facets is
driven by the tolerance controlling the quality of the surface
representation in terms of the distance of the
facets from the surface. Namely, the outward normal is the
vector indicating the boundaries of your object,
suggesting to the software which is the outer part of your
geometry and which is the inner one. An explanatory
drawing of the orientation of a facet is shown in Figure 2.
Vertices are listed following counter clockwise
direction.
FIGURE 1: EXAMPLE OF SURFACE TASSELLATION
FIGURE 2: STL FACET
As shown in Figure 1, the more triangles are applied to the
surface, the higher is fidelity to the CAD model.
However having an incredibly high number of facets is not always
synonymous of good quality STL. Indeed, a
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higher tessellation leads also to a longer processing time of
the file and in some cases, when the file is too big,
the software cannot process it at all. Therefore it is very
important to find a good compromise between the
highest definition with the minimum file size possible. To
consider as well, is the resolution of the machine.
Sometimes it can happen that the machine does not have the same
high resolution that is given to the file, hence
it will not be able to perform it. Therefore having a high
resolution file for a low resolution printer would be
worthless.
HOW TO MAKE A GOOD STL FILE Often the quality of the STL file is
also determined by the quality of the CAD model. Hence there are
few steps
that need to be taken care of while 3D modeling, and in the end
few others while exporting. All these can be
summed up and listed in seven main settings that will determine
the quality of the file.
1. Angle deviation and chord height
2. Surfaces
3. Wall thickness
4. Normals
5. Multiple shells or nested parts
6. Edges
7. Tabbed areas
Paying attention to these critical aspects before exporting to
STL is very important since once the 3D CAD
model is converted to the new format it cannot be changed or
modified anymore. This is also why it is highly
recommended to save a copy of the file AND the STL
separately.
ANGLE DEVIATION AND CHORD HEIGHT When exporting to STL if the
part is rougher or smoother than expected, it is possible to change
the angle,
deviation and chord height to create the right surface finish.
As said before, an excessive faceting is not
synonymous of good quality and it is of crucial importance that
the machine resolution is taken into account
when doing this operation.
SURFACES Make sure that in the CAD geometry there are not any
missing surfaces or holes of any kind. The solid geometry
has to be water tight when exported. Some of the 3D modeling
software such as Inventor, Solid Works and
PTC Creo produces good solid geometries consistently. Some
others software based on surfaces modeling,
such as Rhino, need to be used with more attention. Therefore
the modeling should be done with an eye of
respect for naked edges especially.
WALL THICKNESS When modeling it should always be remembered that
the STL only works for solid geometry and not for
surfaces models. Hence it is necessary that all the walls of the
model have a relevant thickness, even if it is very
small. Also, the assigned wall thickness should match the
resolution of the machine that is intended to be used
by verifying which one is the minimum feature size that the
machine can handle.
NORMALS As said before, the triangular faceting of the surfaces
is described also by normal. The normalsa are the
indicators of the direction of the object. With other words,
according to the direction the normal vector is
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pointing at, the inner side and the outer side of part is
defined. Hence, in optimal situations all the normal are
pointing outwards. In some cases, especially when overlapping
surfaces are present the normal are
mismatched. To verify this condition is necessary to scan the
STL file with a software that will automatically
detect the error and eventually correct it.
MULTIPLE SHELLS, NESTED OR TABBED PARTS The production of
multiple parts or parts for assembly can be sometimes problematic,
therefore it is always a
good thing to save each individual part as a separate STL file.
They will be then managed by a technician and
eventually put together on a same building plate afterwards,
according to which machine is going to be used.
For example, if PTC Creo is used do not export the STL from the
assembly mode. The main reason for this is that
the outcome will be overlapping surfaces, and some machines
reject this option. For objects with embedded
mechanics it is recommended to consult the technician to define
an optimal solution.
EDGES Especially in Rhino, when trimming an object, or when
create a new one, some edges can be left behind in the
model. Be sure that you avoid them. They can create confusion
during the processing of the file.
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EXPORTING
SOLIDWORKS When starting modeling note that the orientation of
the building platform is the XY plane corresponding to the
FRONT PLANE in Solidworks.
Always check Merge result on the left side bar is enabled when
doing new features. If this option is disabled,
the new features created will be new bodies in the part. Before
exporting to STL there should be only one
body in the file.
After the part is designed it is now possible to proceed with
the exportation of the file to the STL file. However
we suggest to also save a copy on the original file format of
the software in case you have to modify your part
later on. File > Save As > Select STL > Options.
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Before proceeding with the saving operation, it is recommended
to check the option panel first. In the dialogue box
make sure that the output is set on Binary, and the scale is
Millimeters, or just the same scale used to create the model.
IMPORTANT FOR MAC USERS: Make sure that output is set on ASCII
and not binary. Otherwise Cura will fail
during the slicing.
In the Resolution box the deviation should have a value between
0,02 and 0,05. The angle instead should be between
15o and 20
o.
When exporting from an assembly, make sure that the option Save
all components of an assembly in a single file is
DISABLED, and all the components will be saved individually.
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EXPORTING
PTC CREO (PRO/E) When start designing note that the TOP VIEW in
the software correspond to the original orientation of build
platform of the machine.
The path for exporting the files is: File > Save AS > Save
a Copy > select Stereolitography (*.stl) > OK
FIGURE 3: SAVING STL
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After the OK button is selected, a dialogue window appears on
the right side of the screen. There make sure that
the format is on Binary, and in the Deviation Control the Chord
height is around 0,05 and the Angle control is
around 0,5. When clicking on Apply it is possible to see a
preview of the triangulated mesh. Usually the default
values are acceptable for a good print. However optimum values
depend on the geometry, surface and size of
the modeled object. Moreover as said before, also the resolution
of the machine should be taken into account.
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EXPORTING
RHINO5.0 One thing to check before start modeling with Rhino is
the tolerance. That must be used to export the part to a
specific program. Tolerance settings in Rhino can be found in
the Document Properties window in the Units tab.
Moreover when exporting make sure that:
The object does not have naked edges
The 3D model to export is not a bad object
To find more information in regard we suggest do download a more
detailed guide at this link
http://www.design.upenn.edu/files/preparing-3D-print-files.pdf
When the object is ready for printing, from the top tool bar
> File > Save As, then assign a name to the file and a
destination. Make sure STL is selected as final extension and
then press ok. As always we recommend saving an
original file, plus the STL copy.
FIGURE 4: SAVING STL
If you are using Rhino and you are interested in knowing more
about it, then you should have a look to the
interesting guide by downloading it from free at the following
website:
http://design.umn.edu/current_students/leo/hall/documents/Rhino_3DP_prep.pdf
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EXPORTING
AUTODESK INVENTOR PROFESSIONAL 2013 To export STL files from
Autodesk Inventor Professional 2013 go on the top icon on the left
and select >Save a
Copy.
Subsequently a dialog window will pop up, where it is possible
to rename the file and select the right files
extension. Select type file: STL file (*.stl).
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VERY IMPORTANT in this case is to select the Options button in
the same dialogue window that will lead to
another dialogue window, where it is possible to change some of
the saving settings. It is fundamental that
when exporting to make sure that the unit scale is the same used
to make the CAD model. For example: in this
case, the drawing was made using a millimeter scale. Therefore,
the exporting scale should be in millimeters as
well and NOT in centimeters. The negligence of this step will
result in a model 10 times smaller, or in a
difference in size according to the difference of the units
used.
In addition, from this dialogue box it is also possible to
decide on the quality of the STL file. Once done, press Ok
and then Save.
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SUMMARY
STL is the standard file format used in 3D printing An STL file
appears as a triangular mesh of the CAD model and determines the
resolution of the print It is possible to generate an STL file with
nearly every CAD software In order to obtain a good STL file is
important to observe the following conditions:
Angle deviation and chord height: adjust the size and the number
of triangle to increase or decrease the resolution of the STL
Surfaces have to be water tight and geometries have to be solid.
Do not forget holes or naked edges
Give a wall thickness to every feature. STL do not work for
single surfaces Make sure there are not overlapping surfaces on the
design. That leads to inversed normal
which is always good to avoid. (check and correct the inversed
normals with Netfabb STL VALIDATION section)
Save every component of the design in a separate file. Do not
print STL made from an assembly mode. If you have embedded
mechanics consult a technician
The resolution of the STL file has to be made according to the
resolution of the machine as well. Having a file with a higher
resolution than the machine used is pointless
When modeling make sure that the part does not exceed the
overall printing dimensions of the machine Always save a copy of
the CAD file and a copy of the STL. Once made the STL cannot be
modified.
Therefore if there are some mistake to correct you will have to
build again the model from scratch. Mac users on Solidworks. Set
the output on ASCII instead of Binary. Otherwise Cura will fail
slicing. Always make sure that the units used for modeling are the
same used for the exporting. This is critical
especially in Inventor.