3 – Editing mesh and surfaces

Making edits to mesh files is one of the vital steps in preparing 3D printing files. Mesh editing can save countless hours that would go into redesigning an already existing 3D model. 

Here, we will discuss various mesh editing tools offered by Netfabb to modify models and prepare them for various 3D printing technologies such as SLA/DLP. This chapter also covers mesh comparison tool which can be of great value to businesses who need to discuss precise details of 3D models with their clients.  

How to edit STL meshes

For traditional manufacturing processes material handling, tooling, and labor expertise, is an important part of the manufacturing operation. Furthermore, lead times in such processes can be significantly long, as raw materials need to be procured and production equipment must be planned. For example, a casting process will, first, require manufacturing of molds, and in lost wax casting, a mold is needed to manufacture each separate part, adding to production time. 

For CNC machining a toolpath needs to be planned, and other factors must be considered such as tooling, sequence, and access. In such cases, manufacturing of complex parts may take longer.

In contrast to this, the method of additive manufacturing is single step production, from design to manufacturing and complexity of the part does not affect the lead time.

However, one of the problems in 3D printing is that manufacturing guidelines are highly machine and material specific. Therefore, it is possible that a 3D model is ideal for printing in SLS but not in FDM.

Another problem that could arise is that the process parameters of one machine may not be compatible with another machine. For example, a part manufactured in EOS M270 DMLS printer, may not be printable with the best of results in the next version of the 3D printer, the EOS M290. This could simply be due to a minor modification such as the improved inert gas flow control of the EOS M290.

Therefore, in the single step manufacturing paradigm like AM, the design stage receives the most attention and therefore a mesh editing tool becomes necessary. 

Furthermore, there are a plethora of .stl and .obj files available over the internet. Such files include dental and anatomical models, which may occasionally require editing. 

A 3D model is built using geometric primitives, shapes such as a cube, cylinder, and torus. These primitive shapes are the metadata of the 3D model. After the creation of a 3D model in software like Fusion 360, a CAD file is exported to .stl or .obj for 3D printing. These files translate a 3D model into a triangular mesh and the metadata of the 3D model is lost. Hence, in .stl, .obj, and other similar files the underlying parametric geometry cannot be accessed for the purpose of editing.  

Editing of .stl and .obj files is possible by making changes to the triangular mesh and for businesses with a digital inventory a mesh editing tool becomes necessary.

Netfabb allows the user to edit 3D models by manipulating the triangular mesh. 

 

Mesh Manipulation module is different from the Repair module. The repair function in Netfabb automatically fixes models for errors. Whereas mesh manipulation function allows for additional manually control over the mesh.

 

Several editing options are available in Netfabb’s Repair and Mesh Manipulation modules. This includes refining the mesh for precision 3D printing, adding, and removing triangles, smoothening the mesh, cutting and extruding surface. Add node function within the Repair module gives users more control over the manipulation of a mesh. With the help of the Add Node tool, a precise point can be indicated to generate new triangles.

 

Triangles can be added to the tessellation to increase the resolution of the 3D prints. This can be done individually or automatically. However, increasing the number of triangles in a mesh will make a 3D file larger which could take longer to slice and add support structures. 

On the other hand, triangles in a mesh can also be reduced. This will also shrink the file size. It should be noted that reducing the number of triangles can depreciate the quality of a 3D model. But, in many cases, mesh file size can be reduced without influencing the physical properties of the part in accordance with tolerances.  

 

A few other mesh editing tools in Netfabb include surface cutting, extruding triangles, and smoothing. 

A scanned object may contain unnecessary triangles which gives objects sharp edges. Sharp edges of an object can be smoothened by using the ‘Smoothing’ tab in the Mesh Manipulation module. Furthermore, an advanced ‘Remesh’ tab is also available, which restructures the mesh in a more uniform way and can preserve the crucial geometrical details of the object.   

Various mesh editing access points from Netfabb’ Modify tab. Image via Autodesk

A model may be designed as individual parts and then exported to an .stl file not as a single piece but as separate shells. However, a designer may not intend this, because separate shells in a single model can cause print failures. Netfabb can split separate shells into parts which can be joined together using the Boolean operation accessible within the Modify tab, Split and Merge panel. This operation creates a single uniform mesh from separate parts.

In addition to this, a mesh can be converted to a BREP (boundary representation) file and later can be exported as a .step, .sat or .iges file.

A mesh part converted to BREP being exported as a .sat file out of Netfabb. Image via Autodesk

How to hollow an STL file

In 3D printing solid models are generally printed with infills. 

FDM/FFF printers fill the insides of the 3D model using various types of infills such as honeycomb or gyroid structures based on the toolpath data as defined within the Gcode file. 

SLA/DLP 3D printers do not need a toolpath file. They fill the insides of a 3D model based on the slice file with liquid resin. The resin hardens and produces a robust solid model. If the CAD or Mesh file is solid, this will be reflected in the slice file as well. A solid object, when sliced, will only have the exterior contour for each slice and 3D printing such an object will increases the print time and will consume large volumes of resin. In many cases, filling a model with resin would be completely unnecessary. 

One way out of this problem is hollowing the 3D model, which leaves only the exterior walls with a small thickness. And the model can be 3D printed much faster. But in order not to encapsulate the liquid resin within the hollowed 3D model, there needs to be holes where the resin can exit the enclosure. Usually special software is used which can create holes or other perforations to release the unused liquid resin during SLA/DLP printing process.

As Netfabb is an all in one 3D printing software solution, it includes a ‘Generate Shell’ feature which automatically hollows a 3D model, with a custom defined wall thickness. Furthermore, Netfabb Premium’s ‘Add Perforation’ tool helps users automatically generate holes in the shell for resin discharge. 

The Add perforation tool also has a ‘generate plug’ option. This can create same size fillings or include a clearance to account for glue thickness for the hole made with the perforation tool. The plugs can be 3D printed and used to cork the holes.  

Another way of creating an outlet for liquid resin is by cutting a superfluous part of a model with the help of Netfabb’s cutting tools. For example, a closed base of a model may be unessential to the function of the model. The base can be removed by making a cut. A bottomless 3D model will let out all the liquid resin. The same principle can be applied to SLS which uses powder material. 

Furthermore, primitive shapes can be used to create patterned perforations. For example, Netfabb has basic design capabilities with complex and elementary shapes. For instance, a cylinder can be used to create holes in a model using the Boolean subtraction operation tool.  

Moreover, hollowing the insides lets designer fill the model with their custom defined lattice structures. These may be preferred from a design or engineering point of view. For example, a hollowed part with a honeycomb structure may be more flexible or stronger. 

Netfabb Premium’s latticing capabilities (Lattice assistant and Lattice Commander) allow users to design their structures as infill for hollowed parts. For more advanced applications a Lattice Optimization tool and the Selective Space Structure (3S) tool which creates lattices that follow surface contours or lattice combinations via scripting are also available in Netfabb Ultimate.  

Netfabb Lattice Assistant combines Hollowing, Latticing and Perforations tools in one easy to access interface. Clip plane ‘Y’ is active to show the cross section of the part. Image via Autodesk

How to perform Boolean operations on an STL file?

 

Often designers have to make alterations to mesh files, such as .stl or .obj. Editing these files can be rather cumbersome, as there is only a triangular mesh left to deal with and no parametric data. As mentioned before, Netfabb has a mesh editing feature, which lets users manipulate triangles to edit the shape of a 3D model. This tool can be combined with the Boolean operation tool which adds to the design freedom offered by Netfabb.

A Boolean operation is a relation between two objects defined by Unify, Subtract, and Intersect actions. Unify combines two given objects, Subtract removes one from the other, Intersect gives the common remainder from two objects.

 

These operations are the foundation of parametric design or constructive solid geometry. Netfabb combines Boolean operations with mesh editing tools to expand the design capabilities of the software.

 

With the help of the Boolean operations, designers can add parts to an existing design or subtract from it (a mesh file). As was mentioned earlier, this is particularly helpful for creating hollow parts for SLA/DLP printing. Moreover, the subtraction operation can also be used to add holes to the 3D model, for the purposes described earlier.

 

Boolean operations are also useful in creating infills for hollow parts for SLA/DLP 3D printing. For example, a designer can model a customized lattice structure for a part. Using the Unify operation they can easily embed it as a modelled infill to a hollow part.

 

Moulds can be created by subtracting a model from a solid. This is easily done by adding a cube from Netfabb’s parametric part Library. The model to be moulded can easily be placed on the cube and subtracted.

 

Boolean operations can also be used to add more functionality to an existing part. For example, if a manufacturer wants to add a screw thread to a part for a stronger assembly but only has a mesh file of the original part, Boolean operations can help. A bolt can be created in Netfabb and subtracted from the target shape. To take account of tolerance and other mechanical issues, mesh editing tool can be used to offset the bolt.

 

In addition to this, Boolean operations are also good for combining designs. Designers can add logos or labels to existing .stl models, customized bases, or supports.

  

Performing the ‘Subtract’ action within the ‘Boolean Operation’ tab to remove the red part from the green part. Clip plane ‘Y’ is active to show the cross section of the parts. Image via Autodesk

How to compare two meshes in Netfabb?

As mentioned earlier, Netfabb can edit triangle meshes. Parts sometimes need to be rescaled, remeshed or smoothed. After such a process it is often necessary to compare the original and the edited part to measure deviations for tolerances. A comparison tool is indispensable for tolerance analysis and testing the overall quality of the 3D print.

A good mesh comparison tool can save labour, time and costs. For example, a business client may want modifications to the original model to make it more precise. In such a case only a mesh comparison tool can tell how much accuracy is viable and possible with mesh editing. 

In Netfabb, using the ‘Compare’ function, which is accessible via the ‘Analyze’ tab, ‘Analysis’ panel, users can perform a comparative analysis between two triangle meshes. The comparison can be unidirectional or bidirectional on two similar models.

The ‘Mesh Compare’ module shows a detailed visual and numerical analysis of the altered mesh triangles. Data is represented with help of a colour gradient and statistic, such as the average distance of movement, maximum positive distance, and maximum negative distance.

The distance by which a triangle has moved towards or away from the center on a coordinate is represented by red or blue colours, on the deviation scale. The red colour shows that a triangle has moved away from the center, whereas blue represents the movement towards the center. Tolerance distance can be assigned a value to measure the permissible deviations from the original model. The green color on the model shows that the tolerance level is acceptable in a specific region. 

Furthermore, a bar chart illustrates the distance a triangle has moved and how many triangles in the mesh have moved that distance. In addition to this, measure points can be added to calculate and label the deviations.

Finally, a mesh comparison report can be created using the ‘Generate Report’ function within the ‘Analyze’ tab. Simply choose the “netfabb_mesh_compare” item from the ‘Generate Report’ dialog and press the ‘Generate Report’ button. This will execute the automatic report creation action using the Open Office ODT template and output can be viewed with any software that can open .odt files including Microsoft Word. The report contains general measurements of the model and mesh comparison data. This can be particularly helpful for 3D printing service bureaus, who need to make reports for their clients before the manufacturing process.

Mesh compare module in Netfabb. Image via Autodesk

How to take measures of an STL file

 

If files are not meticulously prepared for manufacturing prints might fail and 3D printing can prove to be an expensive manufacturing method. The modelling stage is as important as 3D printing. Therefore, having all the necessary tools for editing 3D meshes is important to ensure that the original model and 3D print corroborate each other. One such indispensable tool is the measurement analysis.

 

Measurement tools are not only essential for calculating dimensions of a model, but they are also necessary for repairing and editing 3D models. Moreover, these tools can become necessary to examine the printability of a model. For example, as wall thickness is crucial to determining printability of a 3D model, Netfabb has a special wall thickness measuring tool. In addition to this, Netfabb has various other measurement tools and functions which can calculate lengths, distance, and angles of surfaces and triangles in a mesh.

 

There are eight categories of measuring tools in Netfabb. These include point, edge, circle detection, 3-point circle, 4 point sphere, surface, wall thickness, and 3 point angles. The measurement tools in Netfabb use anchor points to make the calculations.

 

All these tools can be used with Netfabb’s feature detection and snap-on option, which makes it easier for the user to navigate through the mesh and attach anchor points to the model for calculation. With the help of feature detection users can peg the anchor points to primitive shapes, such as lines, edges circles, and planes. Snap on option makes the anchor point cling to the edges of the triangle. This option can be turned off or switched to ‘point on clip plane node’.

 

Measurements of all the distance and angles are displayed on a label. Users can label these comments for their own reference or for other designers. If the measurement labels clutter the view of the model, they can be dragged to move away from the part. This can be helpful if a designer has to share screenshots of the 3D model.

 

Furthermore, the measurement dialog in the module shows detailed statistics that can be of great help to engineers.

 

If there was no measurement tool, many of the editing operations in Netfabb would lack usability for professionals. Netfabb’s measurement tools compliment the editing features offered by the software, such as mesh editor and Boolean operators. For example, users can measure the distance between one point and a cut line, before making the edit. Moreover, detailed measurements can also help users align parts precisely.

 

Also, Netfabb can generate detailed drawings or blueprints that can be used to compare a 3D printed part with the original model. This can make the testing process more efficient.

 

Performing measurements with Netfabb. Image via Autodesk