body we made in Step 3.
Select the sketch with the tooth
geometry, then select the Extrude
command. If you are a beginner to
SolidWorks, enter the value to match
the thickness of the solid body you
created in Step 3 (I'll enter 0.125
inches again). Make sure the Merge
Results box is checked so that your
two extrudes will become a single
solid body.
If you are a more advanced user,
in the section of the Extrude menu
titled Direction 1, select Up To
Surface and then click on the top
surface of the first extrude. This will
make sure your tooth is always the
same thickness as the main body of
your disk.
Make sure the Merge Results box
is checked. Don't forget to radius any
interior right angles to avoid a stress
concentration there. This is often
where a crack will start as your
weapon delivers big hits!
Step 5. Let's create the hole for the
weapon shaft. This feature will not
affect the center of mass of the disk.
Create a new sketch on the same
plane you have been using. Draw a
circle centered on the coordinate
system and dimension it according to
the requirements of your design. I'll
make it 0.5 inches. Then, subtract it
from the solid body.
Select Through All to ensure that
the hole goes through the disk, even
if you decide to change the
thickness later on.
Step 6. Next, we need to find the
center of mass of the disk we have
designed. Select the appropriate
view from the View menu so that
you are looking at the top of the
disk on your screen. Then, go to
the Tools menu and select Mass
Properties (Figure 6A).
This will bring up a dialog box
with information about different
properties of the body you have
created (Figure 6B). You can use this
same tool to check the weight of
your disk if you assign a material to
the solid body.
For the purpose of balancing the
disk, we want to focus on the center
of mass information. You'll see that it
shows an X, Y, and Z coordinate. It
only needs to be balanced in the
plane perpendicular to the axis of
rotation. Since I drew my sketches on
the top plane, the Y coordinate is
not relevant for balancing the disk.
The goal is to get the X and Z
coordinates as close to zero as
we can.
In order to balance the disk with
more accuracy, it is helpful to show
more decimal places. In the same
dialog box, click Options and then
Use Custom Settings. Increase the
length resolution to four decimal
places. Right now, my center of mass
is at: X = 0.0345, Z = 0.0897. This
may seem like a very small deviation
from center but at thousands of
revolutions per minute, the effects
are significant.
Step 7. You can shift the center
of mass either by removing material
near the tooth (Figure 7A) or by
adding material away from the tooth
(Figure 7B; notice I used an ellipse
to add material). Each time you add
or subtract material, go back to the
Mass Properties dialog box to see
how the change affects the center of
mass. It's a trial-and-error process,
but just a few tries should get you
very close.
Remember, you're trying to
balance the weight of the disk so
you want to offset the weight of the
tooth as you add/subtract mass. Try
to get the weapon balanced to three
decimal places if you're working in
inches (0.000).
As always, there are trade-offs to
each approach. Although it saves
SERVO 03.2014 33
STEP 5.
STEP 6B. STEP 6A.