2. 1
I. Introduction
The focus of this project was to model a robot arm mechanism, such that it moved in all
three dimensions. The program Solid Works was used to create the part models and main
assembly. The model created was of a commercially available Lego set meeting the
specifications of the project called the Power Digger [1]. The final assembly consisted of four
sub-assemblies and 32 different parts. Dimensions were taken using calipers along with other
online resources [2] [3] in order to replicate each of the parts the solid model. Each part was
individually created and then combined into the sub-assemblies, which in turn were combined
into the larger main assembly. This report details the steps in order to recreate those parts in the
Mechanical Design section along with the sub-assemblies. The model analysis will discuss the
mass properties and results. An explanation on how to apply mechanism to the assembly will be
discussed in the Kinematics sections. The project has given increased practice using the design
features of the software and different techniques to create models for the final product.
II. Mechanical Design: Parts
1. 368026 – Turn Plate 2x2, Lower Part
The Turn Plate is an important piece because it is what connects the body to the wheels
and gives it rotation. It starts with an extrusion of a square, followed by various cuts and boss
extrusions to give it shape. The center of the part has a hole in order to attach the upper part of
the Turn Plate. The completed part is displayed in Figure 1.
Figure 1. Turn Plate 2x2, Lower Part
2. 4515340 – Plate 1x2 W/Fork, Vertical
The base of this part is simple. It was extruded from a sketch of a rectangle. The two first
knob was created from a sketch of a circle then extruded. The linear pattern tool was used to
create the second knob. Then, from the front plane of the brick, a sketch was created and
extruded to make the forks. From that extrusion the two forks were cut out. In order to make the
thread on the fork, a simple cut was made on the end, and then the circular pattern tool was used
to repeat the cut along the edge. Lastly, a circle was extruded then filleted in-between the two
forks to give it a place holder. The part is displayed in Figure 2.
3. 2
Figure 2. Plate 1x2 W/Fork
3. 4515368 – Plate 1x2 W/ Shaft R3.2
The plate with shaft was created with a simple sketch of its side view and extruded out.
The first knob was sketched on the top face of this model. It was then extruded up and patterned
to create the second knob. The shaft was created by making a simple rectangular sketch and
using the revolved cut feature around a specified reference line in the initial sketch. The bottom
was then hollowed using extrude cut, to allow knobs of other parts to be inserted for assembly.
This piece is a key piece in the Arm Assembly as without it the movement would be far more
limited. The final piece is shown in Figure 3.
Figure 3. Plate 1x2 W/ Shaft R3.2
4. 4517925 – Plate 1x1 W/ Holder Vertical
The base of the holder was created with a simple sketch of a square that was extruded up.
The knob was centered using the origin. It was then hollowed using the extrude cut feature. The
holder itself was created by sketching in the yc-zc plane. It used the tangent constraints to keep a
smooth surface, and the vertical constraint to have both tips stay lined up. The curves were
created with a 3-point arc and then an offset. It was extruded from its mid-plane to keep it
centered with the base. This component is used in conjunction with the shaft to allow rotational
movement in the yc-zc plane. The final component is shown in Figure 4.
4. 3
Figure 4. Plate 1x1 W/ Holder Vertical
5. 4535739 – Plate 2x1 W/Holder, Vertical
The Plate 2x1 W/Holder component was constructed by creating a sketch of the bottom
portion of the piece consisting of two nested rectangles spaced by 1.6mm and extruded up
1.6mm from the sketch plane. A reference plane was created on top of this extrusion, on which
was drawn a sketch of another rectangle identical in size and location to the outer rectangle of
the bottom layer, which was also extruded up 1.6mm. Once again, a reference plane was placed
on top of this extrusion. Then, a sketch was created for the knobs on the top of the Lego,
consisting of two circles evenly spaced across the surface, and this too was extruded up 1.6mm.
The reference plane to create the holder was created next by spacing out from the side of the
Lego by 2mm.
The holder piece was created utilizing two concentric circular arcs with rectangular
protrusions outwards, with the center of the arcs was located at a height of 2.4mm, determined
by reverse engineering from a piece designed to fit into the holder. The edges of the holder piece
protrude past the theoretical "bottom" and "top" of the piece. This sketch was extruded to a depth
of 4mm, centering it on the piece.
Finally, the bottom details allowing the piece to nest comfortably with the other Legos
were added in. A sketch on the plane at the top surface of the first extrusion (that is, the surface
also representing the bottom of the Lego's "ceiling") containing two evenly spaced circles for the
holes underneath the pegs was created first, and the Extruded Cut tool was used to cut through
the existing extrusion by 1.6mm to hollow out the pegs. The final step was to create the "donut"
shape in the middle, which was accomplished through a sketch of two nested circles extruded to
1.6mm, bringing them to the same depth as the sides of the Lego. The final product is shown in
isometric view in Figure 5.1, and the bottom is shown in Figure 5.2.
Figure 5.1. Plate 2x1 W/Holder final product, isometric view
5. 4
Figure 5.2. Plate 2x1 W/Holder final product, underside
6. 403226 – Plate 2x2 Round
The Plate 2x2 Round was created by first extruding a circular boss from the Top Plane.
Next, two circles were extruded on the top face and were mirrored about the Front Plane. These
four extrusions formed the Lego studs. The next steps were to extrude a circular cut along the
outside perimeter and extrude a square cut up from the bottom face. Finally, a plus-shaped
extruded cut was made in the center of the top face and a ring was extruded from the underside
of the top face up to the bottom face. These can be seen in Figure 6.
Figure 6. Plate 2x2 Round
7. 614126 – Round Plate 1x1
The Round Plate was mostly likely the simplest design piece in the project. It was made
from three circular boss extrusions, concentric around the origin. A hole was made at the bottom
using an extruded cut. The part is displayed in Figure 7.
Figure 7. Round Plate 1x1
6. 5
8. 4617848 – Tyre R17, 6x6, 24
The Tyre was created by first extruding a circular boss from the Front Plane. Then, a
circle offset from the perimeter was cut “through all” to form the ring shape. Next, a single tread
was added and subsequently patterned 360 degrees around the tire, with equal spacing. The same
shape cut and pattern were then created on the opposite side of the tire; however they had an
angular offset of nine degrees. Finally, the inner geometry was created by revolving a trapezoidal
sketch 360 degrees along the inside. These can be seen in Figure 8.
Figure 8. Tyre
9. 615726 – Bearing Element 2x2 2/3
This model was created with a simple outlined sketch that was extruded up. A single
knob was sketched, extruded and patterned. The inner sides were sketched and a second extrude
was applied. The single shaft was sketched and extruded on the side of the model. This feature
was then mirrored. The center circle was sketched using the origin, and cut with the extrude cut
feature. The bottom of the model had bracing that was sketched and extruded out of the model,
which later had a chamfer applied to the four edges to keep a smooth design. This bearing
element is an important model that serves as an axle within the assembly. The tires will be
attached to here. The bearing will also serve as a foundation of the entire assembly. This can be
seen in Figure 9.
Figure 9. Bearing Element 2x2
10. 6029947 – Plate w. Bow 1x2x2/3
The plate with bow was created starting with a simple sketch of its side view, which was
then extruded out. The two bottom faces then had simple square sketches extrude cut into the
model, to allow space for knobs of other parts during assembly. This part is to connect two edges
7. 6
at different heights and serves as a cosmetic edge piece, removing some of the innate blocky
design of Legos. This can be seen in Figure 10.
Figure 10. Plate w. Bow 1x2x2/3
11. 4504381 – Roof Tile 1x1x2/3, Abs
The roof tile piece serves as an edge along the sides of the main sub-assembly. The
overall shape was created by first sketching a trapezoid to form one side on the xc-yc plane, then
extruding it 8mm perpendicular to the sketch plane. A shell tool with a thickness of 1.2mm
created the walls. The last step was adding a Cut Extrude to take away the edges at the bottom.
The final model can be seen in isometric view in Figure 11a, and from below in Figure 11b.
Figure 11a. Roof Tile 1x1, isometric view
Figure 11b. Roof Tile 1x1, underside
8. 7
12. 4550348 – Roof Tile 1x2x2/3, Abs
This model was created from the base model in Step 11, the 1x1 roof tile, with the
extrusion of the trapezoid changed to 16mm. A donut shape in the center was added by drawing
a sketch of two concentric circles on the bottom plane of the piece and then extruding up to the
next surface. The final product is visible in isometric view in Figure 12a and from the underside
in Figure 12b.
Figure 12a. Roof Tile 1x2, isometric view
Figure 12b. Roof Tile 1x2, underside
13. 242024 – Corner Plate 1x2x2
The corner plate pieces are nestled snugly inside the main structure of the final assembly,
essentially functioning as a shim between layers to bring pieces up to their final heights. It was
created by first drawing a sketch of the outside edges of the bottom with the inside edges drawn
1.6mm parallel inside. This sketch was extruded 1.6mm out from the sketch plane and a
reference plane was created on top. The ceiling of the piece was created similarly, by copying the
outside edge's sketch and extruding it up an additional 1.6mm, followed by the addition of
another reference plane. The three knobs on the top were created by drawing a sketch on this
plane containing the circles centered on each region of the piece, then extruded up 1.6mm.
To create the underside detailing, another sketch was created on the plane at the bottom
surface of the ceiling of the piece containing the circles to create the holes. The Extruded Cut
tool then completed this process, hollowing out the knobs on top of the component. After this,
another sketch was drawn on that same plane containing two sets of concentric circles evenly
9. 8
spaced between these three holes. This sketch was extruded out from the surface 1.6mm,
bringing it to the same depth as the edge of the pieces.
The last detail on the bottom of the piece was the peninsula at the inside corner. The
outside edge of the peninsula were constrained to be even with a line drawn between the center
of the two donut extrusions, with its sides parallel and at a right angle with the outside edge. The
full peninsula was 1.6mm in width, allowing it to fit comfortably between other Lego pegs. The
final assembly is shown in isometric view in Figure 13a and from the underside in Figure 13b.
Figure 13a. Corner Plate 1x2x2, isometric view
Figure 13b. Corner Plate 1x2x2, underside
14. 302024 – Plate 2x4
The plate was created with a simple sketch of rectangle and then extruded up. A single
knob was sketched, extruded and then patterned in both the horizontal and vertical directions.
The bottom had a simple sketch of a rectangle with 3 circle shells sketched in the center of the
rectangle. The rectangle and the center of the circles were cut into the model using the extrude
cut command. This can be seen in Figure 14.
10. 9
Figure 14. Plate 2x4
15. 302324 – Plate 1x2
The Plate 1x2 utilized only three features. The first feature was a rectangular boss
extruded from the Top Plane. Next, two circles were extruded on the top face to form the Lego
studs. Finally, a single extruded cut on the bottom face was used to finish the model. These can
be seen in Figure 15.
Figure 15. Plate 1x2
16. 371024 – Plate 1x4
The 1x4 plate serves as part of the structure of the main portion of the assembly. First, a
sketch containing two nested rectangles was created and extruded 1.6mm to form the sides of the
piece. Then, a reference plane was created on top, and then a sketch for the ceiling of the piece
containing only the outside rectangle was drawn on this plane and extruded up an additional
1.6mm. Another reference plane was created on top of this structure. A sketch of one of the
circular knobs was drawn on this plane, evenly spaced 4mm from three edges, and then the linear
pattern tool was utilized to copy this circle three times across the surface. This sketch was
extruded up 1.6mm to form the knobs.
To create the underside detail, another sketch was created on the bottom surface of the
ceiling, containing two concentric circles spaced between two of the "top" knobs. Again, the
linear pattern tool was used to copy these circles 8mm down the piece to form three donuts. The
extrude tool was used with a depth of 1.6mm out from the surface to finish this detail. The last
detail added was the holes underneath the knobs. These were added by drawing a circle on the
bottom surface and using the linear pattern tool (again), with an Extruded Cut of 1.6mm created
to hollow out the knobs. The final product is shown in isometric view in Figure 16a and from the
underside in Figure 16b.
11. 10
Figure 16a. Plate 1x4, isometric view
Figure 16b. Plate 1x4, underside
17. 4615642 – Angle Plate 1x2 / 2x2
This model was created with a simple sketch of the side view, and extruded out. Each
type of knob was sketched once, then extruded and patterned. The bottom horizontal face was
hollowed out using the extrude cut command to allow room for knobs during assembly. The final
assembly is shown in Figure 17.
Figure 17: Angle Plate 1x2 / 2x2
18. 4210636 – Profile Brick 1x2
The Profile Brick 1x” began with a rectangular extrusion from the Top Plane. Next, two
circular extrusions were made on the top face to form the Lego studs. A rectangular extruded cut
was made on the bottom face next, and a circle was extruded from the inner surface down to the
bottom face. A rectangle was then extruded through the inner cylinder to provide support.
12. 11
Finally, triangular cuts on the top and side faces were patterned down their respective faces to
form ridges. These can be seen in Figure 18.
Figure 18. Profile Brick 1x2
19. 4211065 – Plate 2x4
See Section 14
20. 4211094 – Plate 2x2
The first step in creating the Plate 2x2 was to extrude a square sketch from the Top Plane.
Next, two circular sketches were extruded to form the Lego studs, and they were mirrored about
the Front Plane to form the remaining two. Finally, a single extruded cut on the bottom face
formed the remaining geometry. These can be seen in Figure 20.
Figure 20. Plate 2x2
21. 4210883 – Plate 1x2 w/stub Vertical/End
The Plate 1x2 w/stub Vertical/End was created by first extruding a rectangular boss from
the Top Plane, using the mid plane option. Next, two circular bosses were extruded on the top
face to form the Lego studs. Thirdly, a contoured extruded cut was made on the underside of the
part.
A boss was then extruded on the Front Plane and connected to the side of the main body.
A small curved feature was then created adjacently, and was subsequently mirrored about the
Front Plane. Finally, a circular cut was extruded into the side and chamfered, and then both
features were mirrored to the other side. These can all be seen in Figure 21.
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Figure 21. Plate 1x2 w/stub Vertical/End
22. 4211388 – Brick 1x2
The Brick piece was boss extruded from a rectangle then on the surface a circular sketch
was extruded to make one of the knobs. That pattern tool was used to make the second knob. On
the bottom surface of the brick the shell tool was used to shell out the inside were a third knob
was added on the bottom to prevent it from moving once assembled. The part is displayed in
Figure 22.
Figure 22. Brick 1x2
23. 6028813 – Plate W. Bow 1x2x2/3
See section 10.
24. 4540203 – Turn Plate 2x2, Upper Part
The Turn Plate is crucial feature in the rotation of the body. The base starts out as a boss
extrusion of a circular sketch, then one of the top knobs are made similarly on the top surface. A
circular pattern is then used to copy the rest of the 4 knobs around. Four holes were cut
underneath the knobs and another boss extrusion was done on the bottom in order to make the
bottom knob that connects to the lower part of the Turn Plate to the upper. The fina product is
display in Figure 24.
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Figure 24: Turn Plate 2x2, Upper Part
25. 4211350 – Radiator Grille 1x2
The Radiator Grille piece forms the surface on top of the main assembly where the cab
will rest, as well as parts of the top of the wheelbase that would otherwise contact the main
portion of the assembly and prevent rotation. Rather than later utilizing a cut, the bottom layer
was first drawn on its own sketch, spaced 0.2mm from the axes, and consisting of two nested
rectangles spaced 1.4mm apart. This sketch was extruded 0.2mm up from the sketch plane. Then,
a reference plane was created on top of this surface, on which was drawn another sketch
containing the main outside edges of the piece, spaced 1.6mm apart to account for the edging.
This sketch was extruded up a height of 2.2mm.
To create the bars on top, a sketch was drawn on a plane created on top of the prior
extrusion containing three equally sized and spaced rectangles. This sketch was then extruded a
height of 0.8mm. The final step was adding the peg underneath that would sit between Lego
pegs. This was created by drawing a sketch of a rectangle on the bottom of the center bar's plane
and extruding it outwards 2.4mm to complete the piece. The final product is shown in isometric
view in Figure 25a and from the underside in Figure 25b.
Figure 25a. Radiator Grille 1x2, isometric view
15. 14
Figure 25b. Radiator Grille 1x2, underside
26. 4211396 – Plate 2x3
To begin, the base is extruded from a rectangle, and knobs were created by extruding a
circular sketch and then using the pattern tool to copy them in both directions. One direction had
two instances, and the other had three. The bottom was shelled out and two more circular boss
extrusions were made underneath to prevent the part from slipping when assembled. The part is
displayed in Figure 26.
Figure 26. Plate 2x3
27. 6066097 – Plate 1x2 w. 1 Knob
This model started with a simple sketch of a rectangle that was extruded up. The knob
was then sketched on the top face and also extruded up. The bottom was hollowed with a simple
sketch using the extrude cut command. The trim was then added using a secondary sketch with a
second extrude cut command. The final component is shown in Figure 27.
Figure 27: Plate 1x2 w. 1 Knob
16. 15
28. 4211632 – Plate 1x2 w. Stick
The 1x2 plate with stick serves as part of a pin joint connecting the "scoop" portion of the
Power Digger to the rest of the arm. It was created by starting with the part file created in Step 5,
then deleting the extrusion creating the holder on the end. A new sketch was created on a plane
spaced 4mm from one of the short edges containing the outline for the stick holder, with the
center of the circular portion at a height of 2.4mm from the bottom of the structure, allowing it to
nest cleanly with the holder. This sketch was extruded 2.0mm perpendicular to the sketch plane
and a linear pattern of the extrusion was used to create the opposing extrusion 6mm down the
brick, creating two extrusions that were both evenly spaced from the edges. A new sketch for the
"stick" was created on the same plane as one of the short edge sides, consisting of a circle with
the same center as the circular arc comprising the prior extrusions. This sketch was extruded the
length of the brick, 16mm, to complete the part. The final product can be seen in isometric view
in Figure 28a and from the underside in Figure 28b.
Figure 28a. Plate 1x2 w/ Stick, isometric view
Figure 28b. Plate 1x2 w/ Stick, underside
29. 4654582 – Angular Plate 1,5 Top 1x2 ½
The Angular Plate 1,5 1x2 1/2 began with a rectangular extrusion on the Top Plane.
Then, another rectangle was extruded from the top face to form the front face of the part. Next,
the Lego studs were added by extruding two circular bosses from the front and bottom faces.
Circles were then offset from the circular bosses on the bottom face and cut into the bottom face.
On the back side of the front face, a rectangular extrusion was made, and finally rectangular
extrusions and a hollow cylinder were created on the face. These can be seen in Figure 29.
17. 16
Figure 29. Angular Plate 1,5 Top 1x2 ½
30. 4624473 – Rim No.2 R11 176x6,2
To create the “Rim No.2 R11 176x6,2”, the first step was to extrude a circular boss from
the Front Plane. The next step was to cut a trapezoid 360 degrees along the middle of the
cylinder to create the inner profile of the rim. Thirdly, a rectangular profile was drawn in the YZ
plane and cut linearly through the rim. Next, a circle was offset from the bottom and cut into it,
creating a ridge. Within the cut, a second circle was offset and extruded outward.
A circle was offset from the opposite side of the rim and cut into it. On the resulting
plane, a square boss was extruded and cut to form the inner rim geometry. These can be seen in
Figure 30.
Figure 30. Rim No.2 R11 176x6,2
31. 4222960 – Round Plate 1x1 – Tr.
This is the same Round Plate as before, which was made from three circular boss
extrusions concentric around the origin. The only difference is in the appearance. The part is
displayed in Figure 31.
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Figure 31. Round Plate 1x1 – Tr.
32. 4217722 – Front, 2x2, Sport, Tr.
The design of this part seemed to be difficult but with the use of Solid Work’s features it
was made easy. To begin a sketch was made on the back of the piece then extruded out to its
total distance. From that extrusion a cut was made from the right plane which tampered off the
top of the part. The chamfer tool was then used to cut the top edges off to the desired look. From
there a cut was made on the bottom lip of the part to give it a slot underneath. Then the shell tool
was used to hollow out the bottom where a circular boss extrusion was made to prevent the piece
from slipping when assembled. The part is displayed in Figure 32.
Figure 32. Front, 2x2, Sport, Tr.
III. Mechanical Design: Assembly
1. Body Sub-Assembly
To begin the Body Sub-Assembly a the 2x4 Plate and 1x4 Plate are mated together using
a coincident and parallel mate, as shown in Figure 33.
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Figure 33: Body Sub-Assembly Base
After creating the base for the body the components begin to fall into place. The two
ribbed 1x2 blocks are mated using a coincident mate on the bottom faces and parallel mates
along both edges. The circular 2x2 plate was attached using a coincident mate on the bottom face
and a concentric mate with the knobs, as seen in Figure 34.
Figure 34: Body Sub-Assembly Step 2
Next the three components that hold the Cab Assembly and Arm Assembly. The Plate
with Holder 1x1 is attached twice side by side sing a coincident mate on the bottom face and the
sides. Then the 1x2 Plate with forks is attached on the end using the coincident mate on the
bottom face and the side edges, as seen in Figure 35.
Figure 35: Body Sub-Assembly Step 3
20. 19
Following the holders and fork plates two corner plates and a 1x2 plate are attached using
coincident mates on the bottom faces and the in between faces, then coincident mates on the
edges. This is displayed in Figure 36.
Figure 36: Body Sub-Assembly Step 4
Next the Grid Plate, a 1x2 Slant Brick, and an additional 2x2 circular plate are attached.
The Grid Plate and Slant Brick are attached using a coincident mate on its bottom face and a
coincident mate on its edges. The 2x2 circular plate is attached using a concentric mate with one
of the knobs below it. This assembly is pictured in Figure 37.
Figure 37: Body Sub-Assembly Step 5
Then two Angle Plates 1x2 are attached to the back of the sub assembly using a
coincidental mate on its back face and a coincidental mate on its side edges. A 4x1 Plate is then
attached to the top in-between the angle plates and the grid plate, using a coincidental mate on its
bottom surface and coincidental mates on its sides. The sub-assembly is pictures in Figure 38.
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Figure 39: Body Sub-Assembly Step 6
A third 2x2 Circular Plate is attached to the back of the assembly along with two 1x1
slanted plates. The 2x2 Circular plate is attached using a concentric mate with one of its knobs
and a coincident mate on its back face. The two slanted plates are attached using a coincident
mate on its back faces and edges. The sub-assembly is pictured in Figure 40.
Figure 40: Body Sub-Assembly Step 7
Following the two slanted plates, two Plates with Bows are attached to the back of the
assembly along with another grate plate. All three plates are attached using coincidental mates on
their back faces and along the edges of the assembly, as pictured in Figure 41.
Figure 41: Body Sub-Assembly Step 8
The Final step of the sub-assembly is to add a 2x3 Plate and the Upper Part of the Turn
Plate to the bottom in order to attach the Wheelbase assembly later on in the final assembly. The
2x3 Plate was attached using a coincident mate on the midpoint of its front edge and the center
line of the sub-assembly. The Turn Plate was attached using a concentric mate and a coincidental
22. 21
mate with its top surface and the bottom of the plate, this is picture is Figure 42. The final
assembly is picture Figure 43.
Figure 42: Body Sub-Assembly Step 9
Figure 43: Body Sub-Assembly Final
2. Cab Sub-Assembly
The Cab Sub-Assembly started by creating a new assembly, starting with the component
for part #4211388, the 2x1 brick, and part #4615642, Angle Plate 1x2 / 2x2. The Angle Plate's
appearance was modified to be yellow to emulate the color of the brick in the assembly. A
coincident mate along the bottom face of the angle plate's top piece and the top face of the solid
brick joined the bricks together in one plane, and an additional coincident mate along the side
face of the angle plate's side piece and the side face of the solid brick aligned them in the other
plane. A concentric mate between the round edges of the corresponding Lego knobs properly
aligned their position, followed by a parallel mate between the sides of the brick and the angle
plate. The angle piece should hang over the edge as shown in Figure 44, somewhat contrary to
expectations but ultimately allowing the cab to sit flush against the full assembly when
completed.
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Figure 44. Cab sub-assembly second piece
The next component added in was the model for part #4515368, the 1x2 plate with the
shaft, with its appearance color changed to black. A coincident mate for the bottom face of the
shaft plate and the side face of the angle plate with the hollow knobs was created, with an
additional concentric mate for one of the knobs on the shaft plate and the corresponding hollow
knob on the angle plate. A coincident mate for the sides of the angle plate and shaft plate
properly aligned the pieces. Similarly, an additional model for part #4515368 was added next to
the first copy, also joined with the angle plate through a coincident mate along the matching
faces with a concentric mate for one of the knobs on the shaft plate and the corresponding hollow
knob on the angle plate. However, the final mate utilized for this piece was a second concentric
mate for the other knob on the shaft plate and the corresponding hollow knob on the angle plate
to fully define the assembly as shown in Figure 45.
Figure 45. Cab sub-assembly fourth piece
Then, copy of the model for part #4217722, the "sport" piece was inserted. It was joined
by a coincident mate between the bottom face of this piece and the exposed face one of the black
pieces, followed by a parallel mate between the edge of the sport piece and the bottom edge of
the black pieces to ensure it did not rotate relative to the rest of the cab. A coincident mate
between the top edges of the sport piece and the black pieces along the bottom face ensured
further alignment. Finally, coincident mate along one of the side edges of the sport piece and the
outside black piece edge to fully define the assembly as shown in Figure 46.
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Figure 46. Cab sub-assembly fifth piece
The next piece was the model for part #6066097, the 2x1 plate with the knob. Once
again, a coincident mate for the bottom face of the knob plate and the top face of the yellow
angle plate ensured the bricks' faces would be in contact, with an additional coincident mate for
the short side face of the knob plate and the short side face of the yellow angle plate aligning
them in the other plane. A third coincident mate for the long side face of the knob plate and the
long side face of the yellow angle plate was added to fully define the sketch as shown in Figure
47.
Figure 47. Cab sub-assembly sixth piece
The final component of this sub-assembly was part #4222960, the transparent round
plate. A coincident mate for the top face of the knob plate and the bottom face of the transparent
round plate caused the pieces to contact, with a concentric mate for one of the knob circles on the
knob plate and one of the circles on the transparent round plate added to fully define the
assembly, as shown in Figure 48. This sub-assembly is fully defined and contains no internal
moving parts, but is designed to rotate relative to the main portion of the Power Digger in the
final assembly.
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Figure 48. Cab sub-assembly mates list
3. Wheelbase Sub-Assembly
To begin the wheel subassembly, two Plate 2x3 parts were mated using three coincident
mates and one parallel mate. Next, a single Plate 2x4 was mated on top using one coincident, and
two concentric mates. Two bearing elements were connected to the assembly by using concentric
mates on the studs and a coincident mate to adjoin their surfaces. The description thus far is
shown in Figure 49.
Figure 49. Wheelbase with third piece
Next, two Radiator Grilles were added using coincident mates, and one Turn Plate, 2x2,
Lower Part was added between them using two coincident mates. These are shown in Figure 50.
Figure 50. Wheelbase with turntable
26. 25
Finally, the rims and tires were attached to the assembly using coincident, concentric, and
parallel mates. The coincident mates ensured that the rims stayed on their respective axles and
the tires stayed on their respective rims, while the parallel mates ensured that the rims and tires
rotated together at all times with no slipping. The final result of the wheel subassembly is shown
in Figure 51.
Figure 51. Wheelbase Sub-assembly
4. Arm Sub-Assembly
To being the arm sub-assembly the 2x1 plate with hinge was mated to the 2x1plate with
shaft using a coincident mate on the two adjoining faces. Then with a parallel mate on the edges
of each piece. The two 2x1 plates on the top and bottom were mated using concentric circles
with the studs of the hinge and shaft plate. They were then mated to each other using the parallel
mate. The description thus far is shown in Figure 52.
Figure 52. Arm assembly Main arm
Next the plate with bows were added to the assembly. They were mated to the base of the
arm using the concentric circle mate with the studs of the top 2x1 yellow plate. The bottom of
the plate with bows were mated to the top of the 2x1 yellow plate, the plate with shaft, and the
plate with hinge using coincident mates. They were also mated using coincident mates on the
sides of the 2x1 yellow plate. The 2x1 plate with vertical holder was mated to the shaft using the
coincident and width mates. The description thus far is shown in Figure 53.
27. 26
Figure 53. Arm assembly First hinge and bows
A second 2x1 plate with vertical holder was mated to the bottom of the first using
coincident mates and on 3 adjoining edges. The second 2x1 vertical holder was then mated to the
1x2 plate with hinge using concentric circle mate and a coincident mate with the side faces. The
bottom angled plate was mated to the plate with the hinge using concentric circles with the stubs
and a coincident mate between the bottom face of the plate with hinge and the top plate of the
angled plate. The description thus far is shown in Figure 54.
Figure 54. Arm assembly Second Hinge and base of shovel
The final additions to the arm assembly were two more plates with bows and one 2x2
plate. The 2x2 plate was mated to the angled plate using coincident mates with the top face of the
2x2 and the inner side face of the angled plate. The 2x2's edge face was then mated with the
bottom face of the angled plate using a coincident mate. The two plate with bows were mated to
the angled plate and the 2x2 plate with coincident mates on the adjoining faces. The two plate
with bows are also mated with each other on their adjoining face. The description thus far is
shown in Figure 55.
28. 27
Figure 55. Arm assembly completed shovel
The sub-assembly is defined to allow the desired motion, with multiple hinges that allow
rotation. The complete sub-assembly is shown in Figure 56.
Figure 56. Completed Arm assembly
5. Full Assembly
The full assembly was constructed utilizing the main assembly as its starting point. First,
the cab component was added in with a coincident mate for the side of the yellow angled piece in
the cab and the side of the roof tile in the main assembly. Then, a concentric mate was created
for the holder pieces, as shown in Figure 57, allowing the cab to rotate relative to the axis created
by the holder.
Figure 57. Full Assembly with cab
Subsequently, the wheelbase sub-assembly was added to the mix. Its Component
Properties were edited to allow the assembly to be solved Flexibly, which would allow its wheels
29. 28
to rotate and the "lazy susan" turntable to rotate relative to the main assembly. A coincident mate
was created for the top face of the wheelbase and the bottom face of the main assembly. Then, a
concentric mate was created for the peg at the bottom of the main assembly and the insertion
hole on the wheelbase. This allows the wheelbase to rotate relative to the main assembly through
use of the "lazy susan" mechanism, as shown manipulated in Figure 58.
Figure 58. Full Assembly with wheelbase
The arm assembly was completed by the addition of the arm sub-assembly. Its
Component Properties were also edited to be solved Flexibly, allowing the arm to move within
its plane to "scoop". A concentric mate was created for the holder at the end of the assembly and
the corresponding protrusions from the main assembly. The side faces of the same pieces were
joined using a Tangent mate. This allows the arm to rotate about the axis created by its holder,
while also allowing the sub-assembly to move at the pin joints between the pieces created by the
other holders.
Figure 59. Full Assembly, isometric view
31. V. Mech
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32. 31
Principal axes of inertia and principal moments of inertia: (grams * square millimeters)
Taken at the center of mass.
Ix = (0.09, 0.71, 0.70) Px = 7024.59
Iy = (-0.96, 0.24, -0.12) Py = 30132.36
Iz = (-0.26, -0.66, 0.70) Pz = 30821.02
Moments of inertia: (grams * square millimeters)
Taken at the center of mass and aligned with the output coordinate system.
Lxx = 29999.91 Lxy = 1319.61Lxz = 1540.11
Lyx = 1319.61Lyy = 18808.59 Lyz = 11785.30
Lzx = 1540.11Lzy = 11785.30 Lzz = 19169.46
Moments of inertia: (grams * square millimeters)
Taken at the output coordinate system.
Ixx = 74238.96 Ixy = 8365.26 Ixz = 10994.21
Iyx = 8365.26 Iyy = 50393.39 Iyz = 32981.89
Izx = 10994.21 Izy = 32981.89 Izz = 38108.70
VI. Mechanism Kinematics
As with most Lego sets, the vast majority of pieces are static relative to one another.
They rely on concentric knobs that are nested within one another's cavities to create perfectly
aligned pieces. The model includes numerous pieces that are joined together in this manner
statically, as bricks in a wall. The model's instructions themselves conveniently broke the
assembly into sub-assemblies which have the ability to move relative to one another.
The cab of the assembly is able to rotate around a central axis created by the holder off of
the main structure. The cab remains in plane with the main portion of the assembly and rotates
around the single axis as a pin joint. The wheelbase of the model has wheels that are mated to the
rims inside of them. However, these rims are able to freely rotate relative to the axle, as would
wheels in the real world. There is a "lazy susan"-style structure joining the wheelbase to the main
assembly, which allows the wheelbase itself to rotate relative to the cab. In this way, a driver
inside the cab could "steer" the Power Digger while maintaining his view in a particular
direction. The arm of the structure is the most dynamic piece, as it contains multiple pin joints
allowing the arm to rotate to scoop up from the ground. It, too, is joined to the main assembly
through a pin joint.
VII. Conclusion
Choosing a model that could be physically held served great advantages. The team was
able to feel the weight behind each part, as well as do proper measurements with calipers.
Choosing a model as popular as a Lego toy, provided a lot of help with dimensioning as well,
since there were plenty of references online to look at. When it came to assembly, the team could
physically construct the assembly to understand how each piece fit together. This also allowed
the team to study the motion of the assembly, which allowed for realistic representation in
SolidWorks.
33. 32
In general this project taught the team the importance of consistent dimensioning. Since
some members would dimension certain parts incorrectly which caused the team to back track
and fix the sketches. The team also learned the importance of naming parts, working on a model
that involved 30+ parts, could lead to confusion if parts were named obscurely. The team
countered this issue by naming all the parts by their part number and a description of the part.
Communication also proved vital in accomplishing this report. If one team member changed a
part they made sure to communicate it to the team, and re-upload the modified part. Since one
change in a part can completely change the assembly.
34. 33
VIII. References
[1] Brickset, 2014, "31014-1: Power Digger." http://brickset.com/sets/31014-1/Power-Digger
[2] Robert Cailliau, 2013 "Lego Dimensions: The Measurements."
http://www.robertcailliau.eu/Lego/Dimensions/zMeasurements-en.xhtml
[3] Wikipedia, 2015, "Lego." http://en.wikipedia.org/wiki/Lego
![1
I. Introduction
The focus of this project was to model a robot arm mechanism, such that it moved in all
three dimensions. The program Solid Works was used to create the part models and main
assembly. The model created was of a commercially available Lego set meeting the
specifications of the project called the Power Digger [1]. The final assembly consisted of four
sub-assemblies and 32 different parts. Dimensions were taken using calipers along with other
online resources [2] [3] in order to replicate each of the parts the solid model. Each part was
individually created and then combined into the sub-assemblies, which in turn were combined
into the larger main assembly. This report details the steps in order to recreate those parts in the
Mechanical Design section along with the sub-assemblies. The model analysis will discuss the
mass properties and results. An explanation on how to apply mechanism to the assembly will be
discussed in the Kinematics sections. The project has given increased practice using the design
features of the software and different techniques to create models for the final product.
II. Mechanical Design: Parts
1. 368026 – Turn Plate 2x2, Lower Part
The Turn Plate is an important piece because it is what connects the body to the wheels
and gives it rotation. It starts with an extrusion of a square, followed by various cuts and boss
extrusions to give it shape. The center of the part has a hole in order to attach the upper part of
the Turn Plate. The completed part is displayed in Figure 1.
Figure 1. Turn Plate 2x2, Lower Part
2. 4515340 – Plate 1x2 W/Fork, Vertical
The base of this part is simple. It was extruded from a sketch of a rectangle. The two first
knob was created from a sketch of a circle then extruded. The linear pattern tool was used to
create the second knob. Then, from the front plane of the brick, a sketch was created and
extruded to make the forks. From that extrusion the two forks were cut out. In order to make the
thread on the fork, a simple cut was made on the end, and then the circular pattern tool was used
to repeat the cut along the edge. Lastly, a circle was extruded then filleted in-between the two
forks to give it a place holder. The part is displayed in Figure 2.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)