Introduction to 3D Modelling Using Siemens NX 8.5
Michael Wang
ECE 480 - Design Team 5
3/28/2014
Abstract
Design team five is developing a 3D tactile display, capable of displaying images by raising an array of
pins to varying heights. It will be able to receive image files, analyze and process the image in terms of
color intensity, and then individually raise each pin in the array to the corresponding height. This
technology will be extremely beneficial to blind students, who sometimes are at a disadvantage when
taking courses such as calculus and physics, allowing them to more effectively grasp certain concepts
(3D curves, waveforms, etc.). The device will also have a multitude of other practical applications,
including basic images and maps. There are four main components of the device: the display, the XY
table, the pin setter, and the software. The display is to be constructed using 3D printing technology,
which requires the use of 3D CAD software. This allows for flexible and inexpensive prototyping and
manufacturing. It is important for nearly all designers to understand the fundamental concepts and
mechanics behind 3D modelling.
Keywords: Computer-aided Design, CAD, Siemens NX 8.5, 3D Printing, Constructive Solid Geometry, CSG,
Modelling, Tactile Display, Braille,
Introduction
Computer-aided Design (CAD) refers to the use of computer programs to assist in both the process of
designing and the actual design of physical models. It came be used to design curves, surfaces, solids in
both two and three dimensions, as well as improving documentation and creating catalogs for
manufacturing. The commercial CAD industry began in the late 1960’s, with a handful of companies
developing proprietary mainframe computers and vector refresh graphics terminals (Weisberg).
Modern day CAD modelling has evolved from 3D wireframe modelling (whereby each line of the model
is manually inserted) to 3D solid parametric and explicit modelling (whereby features are added relative
to existing features, such that relationships between model geometries are maintained – allowing
designers to easily make changes to the part while retaining desired design attributes).
For this project, the principles of 3D solid parametric modelling were utilized in the design of the pin
display matrix in Siemens NX 8.5. More specifically, the technique of Constructive Solid Geometry (CSG)
was used to develop the final display matrix (shown in Figure 1). CSG combines the concept of simple
Boolean operators (union, difference, and intersection) with simple geometric shapes (squares, spheres,
cylinders, etc.) in order to model the desired part (Leadwerks). CSG is the most fundamental design
paradigm of modern 3D modelling, as it is relatively easy to pick up, and calls for a less abstract design
process when compared to more advanced methods, such as curve modelling and polygonal modelling.
Seeing as this project does not require extremely complex models (human faces, curved surfaces, etc.),
CSG was chosen as the design method of choice.
Figure 1. 3D Model of Pin Display Matrix
Objectives
The 3D model is a 16 x 16 pin array with individual 3D printed slabs, forming 16 rows of 16 pins. Each of
these rows has individual slots for metal pins to prevent them from becoming misaligned. These plates
are stacked together, and can be “locked” using two bolt screws on each side. The initial position of the
sheets allow the pins to move easily while providing enough friction to remain at the desired height.
After each pin is raised to its proper height, the bolt screws are tightened, locking the pins and ensuring
that the heights will not change when the display is being felt.
Technical Details
The pin display matrix design for this project is a 16 x 16 hole assembly. It consists of a series of thin
plates with semi-circle grooves linearly arranged; these grooves are designed such that the pins will be
held static. The two outside plates are made significantly thicker, in order to minimize flex throughout
the middle of the plates (therefore distributing tension on each pin more evenly), as well as to improve
the durability of the entire assembly. The plates are held together via the larger diameter holes in the
two side “ears” of each plate. A bolt is threaded through these ears, and springs are placed in between
each plate around the bolt during the assembly process. The springs are used to further distribute
tension evenly between plates, ensuring that each pin will require the same amount of force to move.
There is also a small circular indented area around each ear hole, so that the springs located in this area
are allowed slightly more space. A nut will be threaded onto the open end of the bolt to compress the
plate assembly.
In addition to holding the plates together, the bolt and nut also function as a method to “lock” the pins
in place. While it is necessary for the pins to be static without any externally applied force, when the
display is still being refreshed, it is desirable for the pins to be able to move when the Z-axis actuator is
repositioning the pins. Therefore, the display will have two modes: unlocked and locked. The display
will be unlocked while the display is refreshing, such that pins will be moved when the Z-axis actuator
presses on them, but the pins will still have enough friction to remain in the position they are set to.
While the display is locked, the pins will not be able to be repositioned when subjected to forces of the
magnitude capable of being achieved by the Z-axis actuator, as well as human fingers. The locked mode
will be used after the display has been refreshed, allowing users to feel the display without the fear of
repositioning any of the pins. The locked mode will be achieved by tightening the nuts to the point
where the pins are no longer movable when reasonable forces are applied to them.
Implementation
Siemens NX 8.5 was used to create the 3D model in Figure 1. The model is comprised of a combination
of extruded rectangles, holes, and edge blends. While an exhaustive tutorial on each step taken in the
development of this model would be unnecessarily long and repetitive, a break-down of each major
design step will be given as follows.
Step 1 - Datum Coordinate System
For 3D modelling, the datum coordinate system (DCS) is analogous the compass of a map. It is the
fundamental positioning tool for the entire model. Since the desired part is to be 3D printed, the initial
DCS is of less importance than if the part was designed to be machined (which involves intricacies
beyond the scope of this application note). For 3D printing, the most important thing to remember is
that the printer prints in layers, so operating costs can be reduced if the part is designed to minimize Zaxis height. The standard top-view DCS is shown in Figure 2. Additional DCS points will need to be
added throughout the design of this part, and this will be discussed when necessary.
Figure 2. Datum Coordinate System
Step 2 – Basic Sketching
3D modelling starts off as 2D modelling. Sketches are 2D drawings that can be given depth (extruding).
There are a variety of sketching tools available to create different shapes. Create the base rectangle of
the part by using the Rectangle tool, found in the toolbar located at the bottom of the NX window
(Figure 3). The rectangle dimensions were calculated beforehand. For the Y axis, since each hole is
2.6mm center-to-center, 2.6mm * 16 holes = 41.6mm; an extra 0.5mm was added for the top and
bottom edges of each plate, such that the top and bottom holes for each plate are not on the very
edges. This results in a Y axis measurement of 42.6mm. The X-axis was calculated by adding 5mm to
each side of the Y-axis calculation: 42.6mm + 5*2mm = 52.6mm. 5mm was chosen as a reasonable
thickness to add to the end plates. Thus, a 52.6mm by 42.6mm rectangle was created as the basic shape
of the part (Figure 4).
Add two additional sketches to the initial sketch. One is a point (indicated by the “+” in Figure 3), to be
copied into the 16 x 16 array of holes. The other is another rectangle, with width centered at the
aforementioned point. This rectangle will be the basis for the channels that run between the plates.
See Figure 5 for details.
Figure 3. Toolbar: Rectangle
Figure 4. Basic Rectangle Sketch
Figure 5. Additional Point and Rectangle Sketches
Step 3 – Instance Geometry
The Instance Geometry command is a very powerful tool. It can be used to copy existing features of a
part/sketch according to user specified parameters (distance between copies and number of copies, as
well as where to place copies). The first step is to choose the type (along a path, vector, mirrored, or
rotated). For this part, instance geometry objects will always be translated. Next, select the object that
should be copied (in this case it is the point created in the latter portion of Step 2). The direction should
also be specified at this point (for this it is the X-axis – select the X-axis vector that appears on the new
DCS that appears). Since each hole should be 2.6mm C-C, set distance as 2.6mm and the number of
copies as 15 (for a total of 16 points). This should result in a sketch identical to Figure 7.
Do another Instance Geometry command in order to copy the 16 points 16 times, this time in the Y-axis
direction. This should result in an array of 16 x 16 points. These points will eventually be turned into
the holes for pins in the display. Finally, invoke yet another Instance Geometry command, this time
using the thin rectangle also created in the latter portion of Step 2, in the X-axis direction. The final
result is shown in Figure 8.
Figure 6. Instance Geometry Dialog Box
Figure 7. Sketch after First Instance Geometry of Point
Figure 8. Final Sketch after Instance Geometry Commands
Step 4 – Extruding
Invoke the Extrude (X) command. It can be found in a toolbar near the top of the NX window. A dialog
box will appear (Figure 9). Select the initial rectangle sketch as the Section, and the Direction vector as
the Z-axis. This will extend the sketch upwards in the Z direction, creating a rectangular box. Since the
part should be approximately one inch tall, make the end distance 25mm.
Sketch and extrude two additional rectangles on the sides of this first rectangle. Make sure to change
the Boolean operator to “Union,” to ensure that NX recognizes these two “ears” as attached to the
initial rectangle (otherwise, it is possible that the ears will be 3D printed detached from the main box).
This will result in the part shown in Figure 11.
Figure 9. Extrude Dialog Box
Figure 10. Part after Extruding Initial Rectangle
Figure 11. Part after Extruding “Ears”
Step 5 – Holes
Invoke the Hole command. It is located in the same toolbar as the Extrude command. The Hole dialog
box should appear (Figure 12). While specifying points, NX should automatically select all 256 points
after manually selecting a few of the previously sketched points. Make the diameter of each hole
1.6mm (the diameter of each pin), and change the Boolean operator to Subtract. The Subtract operator
will remove the material that intersects with the selected body. Select the large initial rectangle as the
body. This will subtract material from the initial rectangle. While the dialog box is still open, the part
should refresh to display the desired changes (Figure 13).
Figure 12. Hole Dialog Box
Figure 13. View of Part with Hole Dialog Box Open
Step 6 – Finishing Up
Using a combination of the previously covered commands, the rest of the part is easily generated. The
Show and Hide command may also be necessary to access the interior of the part, to create the ear hole
indentations for increased spring space. The Show and Hide command can be accessed by navigating
the Edit menu (Edit -> Show and Hide). Hiding certain plates will allow an easier to access view of
interior plates (Figure 14).
Figure 14. View of Indented Ear Holes on Interior Plates
Results and Recommendations
As can be seen in Figure 15, the 3D model generated using Siemens NX 8.5 was properly printed. While
the 1.6mm diameter holes in the assembly exhibit the proper amount of force upon unmodified pins,
the eventual goal of the project is to use tumbled pins (which results in less sharp edges and therefore a
more pleasant experience for the end user). These tumbled pins are marginally smaller in diameter than
the un-tumbled pins, but the difference is enough to result in inadequate friction to maintain the
desired pin heights. This will be fixed in the final design by decreasing the diameter of the holes in the
display assembly. Furthermore, since the springs located between each plate are only approximately
the same length, there are sometimes inconsistencies with the tension applied upon the pins
throughout the entire display. The springs must be trimmed because there do not currently exist
commercially available springs that are as short as necessary to fit in between the plates (there is a
clearance of 0.4mm without taking the indented area into account – this value increases to 1.4mm
otherwise). A more precise method of trimming the springs must be developed for the final design.
Overall, the design of the 3D display is a success, and only minor revisions need to be made before
bringing the conceptual design into the final stages of this project.
Figure 15. 3D Printed Part
References
Leadwerks. 2006. What is Constructive Solid Geometry? http://www.leadwerks.com/files/csg.pdf
Weisberg, David. 2008. The Engineering Design Revolution. http://www.cadhistory.net/