By Gerald Davis, Contributing Writer
The geared drill shown in Figure 1a has a relatively modern design. Such hand-oper- ated drills have been in use for more than
175 years. George Page, when filing a U.S. patent
in 1838, claimed that his design “improved on de-
signs that have heretofore been employed.”
“Here’s the drill, Chuck,” Tom said crankily. (For-
give the Tom Swifty. It was hard to resist.)
We’re not so much focused on the merits of this
geartrain, nor are we particularly interested in the
usefulness of hand drills. In the scenario for this CAD
session, we are tasked with producing documentation to explain the product’s function.
CAD tip: Give scale to the image by including objects
that the observer is familiar with. Figure 1a displays a
drill bit to give a sense of the chuck’s size. This product
is definitely something you could hold in one hand.
In Figure 1a, the drill has a bit clamped in the
chuck. As a prop, the 3-mm bit size is big enough to
be fairly easy to identify as a drill bit. In Figure 1b,
the bit has been swapped out for a 1-mm size. If you
know what you’re looking at, the little whisker in the
image does indeed resemble a drill bit.
CAD tip: Sometimes the best way to demonstrate a
range of capability is to use side-by-side comparison
images, as in Figure 1c. When you present the 3-D
model in a live demonstration, your use of configurations makes it easy to show the product in different
This demo model has two drill bits—one visible
in the chuck and another hidden in the handle.
Figure 1d demonstrates how the handle cap can be removed for storing bits.
CAD tip: Mates accomplish the positioning of components in configurations; configurations turn those
mates on or off. Use a Design Table to control both
the on/off as well as dimensional values of configured mates.
Figure 1e demonstrates another method of using
CAD to illustrate product features—a cut-away view.
The balloon comments were added postproduction
with image editing software.
In Figure 1e, a “zonal” section view appears to
slice away 90 degrees of the cast frame of the drill.
This reveals internal features of the product. CAD tip:
Some items, such as gears and axles, were excluded
from the scope of the section view in order to improve
the interest of the image.
CAD tip: Props used in image production can be
manipulated with the CAD scale tool. In this demonstration, the drill bit model is a downloaded STEP file.
After it was imported it was scaled to create a couple
of drill bit models that are approximately correct.
Rube Goldberg Rocks
When presenting a design concept for review with
3-D CAD, a CAD jockey finds that it is often informative to “operate” the mechanism with a mouse. In
the case of this hand drill, the mouse can be used
to drag the crank handle, which in turn moves the
crank axle, the crank lever, the drive gear, the driven
gear, and the idler gear, and it turns the spindle that
presses against the tooth that pushes the chuck shell
that drags the chuck nut that causes the bit to spin.
It’s time for a quick quiz: How do you model the
chuck so it faithfully unclamps, as in Figure 2a, or
clamps the bit, as in Figure 2b, with mouse drag?
This quiz has several goals. The motion of the
three jaws inside the chuck shell includes spring
pressure that keeps them equally spaced within
the cone. As the jaws are pushed into the narrower
part of the shell’s cone, the springs are compressed,
and the jaws move together. The jaws slide against
and make tangential contact with the inside of the
chuck’s shell. The jaws also are pressed by the face
of the spindle as it screws into the chuck’s nut. Note
that the chuck nut is normally removed from the
chuck shell only for servicing the jaws.
Part of the CAD modeling answer is revealed in
Figure 2c. We see only one of three jaws. This single jaw is actively controlled. The other two jaws
will be created by a circular pattern. CAD magic will
make all three jaws appear to move correctly, even
though only one is doing all of the work.
For convenience’ sake, the model of the single jaw
tooth was created by 120-degree, midplane rotation
of the jaw’s profile sketch. The sketch is faintly visible in Figure 2c.
The midplane feature in the jaw tooth is handy for
mating the jaw so that it doesn’t spin as the spindle
screws in and out of the chuck nut. Part of the answer is to mate the tooth to be parallel to some feature on the chuck nut.
In Figure 2c we see that elements of the jaw tooth’s
profile sketch are used for mating. Three mates position the tooth within the chuck mechanism:
1. One point in the tooth profile sketch is mated
to the inner surface of the chuck shell. This mimics
the actual contact that the real jaw would make.
Read more from Gerald Davis at www.thefabricator.com/author/gerald-davis
Shop technology and 3-D CAD:
Using images and live demos
to explain a product’s function
Mouse-movable models are useful
for emphasizing certain product features
In this CAD model of a gear drill, the frame is modeled
with drafts suitable for casting. The geartrain operates
correctly. The 3-mm-diameter drill bit gives scale to the
The 1-mm-dia. drill bit is difficult to see in this image.
The jaws of the chuck have moved to grip the smaller bit.
The range of this product’s
grip is evident in a side-by-side
comparison. Static images can
reveal range as well as scale.
A two-bit model—one bit in the
chuck, one bit in the handle—is
shown. Mates are used to position
the bits. Configurations are used to
control the mates. A Design Table is
used to control the Configurations.
In this cut-away view, you can see how the Section View
tool was used to make a zonal cut to make a 90-degree
slice through the casting. Various components were selected for exclusion from the Section View cut. Balloons
were added with postprocessing image software.