Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (62 page)

Model File
Used by a number of 3-D CAD systems to help manage the large
amount of data that is generated by the 3-D models. An assembly model may
contain thousands of part models, and instead of having a user work with the
thousands of operating system files, one large Model File is created and managed.
Monitor
The screen device that displays the software’s graphical information.
It may also be referred to as a display or screen. It should not be referred to as a
terminal.

304 Glossary

Motherboard
The main circuit board that the computer system is built around.
It has other circuit boards attached to it that may be referred to as daughter cards,
expansion cards, etc.

MRP
Materials Resource Planning. This activity attempts to plan overall flow
of raw material, parts, assemblies, etc. through a manufacturing operation. This
activity was automated by MRP software. This concept was expanded to include
other functions of a company under ERP software.

NC (Numerical Control)
An automated function for manufacturing tools
(such as 2-axis mills, 3-axis mills, lathes, machining centers, drill presses, etc.).
NC instructions are a set of data that gives geometric instructions on how the part
or tool should move during the manufacturing operation (there may also be com-
mands for machine parameters such as “feeds and speeds”). These instructions
are also referred to as tool paths. NC data is usually kept in computer files; how-
ever, they can also be stored as punch data in rolls of paper called paper tapes.
The NC instructions do not directly control the machine tool device; usually the
NC data is converted to control commands by a postprocessor.

Network
A system of somewhat independent entities interconnected so that
they can operate as a unit in some fashion. Unless otherwise indicated, it can be
assumed that Network means a computer network. In this case, the computers
(which can usually function on their own) are connected together so that they can
share data and resources. Network may also be used in reference to constraints in
CAD models (i.e. a “constraint” network).

NFS (Network File System)
A File System is a distinct set of files, Disk
Drives/partitions, and their related directories that can appear as part of a work-
station’s overall directory structure. A Network File System also appears as part
of a workstation’s directory structure, but the actual files and storage devices are
on a File Server. It is developed by Sun Microsystems, but many computer sys-
tem manufacturers use it or something similar in their networking technology.
NIC (Network Interface Card)
The circuit board and/or devices that actually
communicates with the computer network. The vast majority of them use a net-
working technology called Ethernet. Thus, they are often referred to as Ethernet
cards or network cards.

Node, Network
One way to refer to a computer on a computer Network. Com-
puters are assigned Node Names so that they can be identified by users and ad-
ministrators. However, each computer also has a numerical-type of address that is
actually used by the network control software.

Node, Part
A term that can be used for a single step in a 3-D model part his-
tory. Often these steps in the history are referred to as Features, but in some cases
a single step in the history would not create an entire feature, or it may create
multiple geometric features on the part model. Therefore, Part Node can be a bet-
ter term to apply to a single step in a part history.

Glossary 305

Nonmanifold
A part model where more than two surfaces meet at the same
edge. A normal solid model has just one other surface meet a given surface’s
edges. To have more creates a nonreal or abstraction of a part.

Normal
See Surface Normal.

Null Part
A term that can be applied to a 3-D part model that has no geometry.
Although most steps in a part history actually create or work on geometry, there
are some that do not. They may set attributes for a part (such as material), or they
may set construction or reference geometry. If a part model just has these sorts of
steps (or it has lost or corrupted all its geometric features), then it might be re-
ferred to as a Null Part. They are often used as place holders in a 3-D modeling
scheme.

NURBS
Non-Uniform Rational Basis-Spline. This is a common mathematical
formulation for the geometric entities that are found in a 3-D CAD system (such
as surfaces). Therefore, some users may use NURBS interchangeably with
Surface.

Object Lines
The line segments that are actually derived from the physical ob-
ject being documented by the drawing. This would distinguish them from line
segments that are drawn as annotation of some types (such as centerlines, leader
lines, etc.).

OCR (Object Character Recognition)
A computer program or algorithm that
attempts to find entities within Bitmap types of data. If a drawing has been
scanned, it results in a Bitmap type of file where all the geometric entities (lines,
arc, etc.) are reduced to dots. In order to recover the fact that some entities were
lines, arcs, letters, numbers, an OCR capability may be applied. It can be quite
inaccurate, however.

OGL
See OpenGL.

OpenGL
A standard programming interface for graphics. It is the de facto
standard for high performance 3-D graphics. Many 3-D CAD systems will re-
quire the use of OpenGL or OpenGL-compatible Graphics Adapters. OpenGL al-
lows the CAD system vendors to write their programs in such a way that they do
not have to worry about the specific graphics hardware users may have. It was
developed by Silicon Graphics, Inc.

Open Part
A 3-D part model that does not have all surfaces stitched up so that
a volume is bounded.

Operating System
The main controlling software for the computer. It manages
the devices, programs, users, etc. of the computer.

Origin
The point in a geometric system at which coordinate values are 0. Often
this point is coincident with a Coordinate System (which indicates the direction
of coordinates). The Origin of a modeling space, part model, or assembly model,
or drawing view is an important basis with which to assess the location of geom-
etry of various types.

306 Glossary

Orphan Part
A term that can be used for a 3-D part model that has no part
history. This part may still have geometry (the surfaces are shown), and it may
even be solid (all surfaces are stitched up), but there would be no way to modify
or assess individual features (such as by modifying a dimension or constraint).
The part is just a list or set of unintelligent surfaces. Orphan Parts are typically
the result of translating 3-D parts between CAD systems.

Package
See Drawing Package.

Packet
An individual chunk of data that transmitted across a Network. For ex-
ample when a file is transferred from one computer to another, it is actually bro-
ken into a large number of pieces that are then reassembled. These pieces can be
considered Packets.

Paging
A method of handling a limited amount of Memory in a computer sys-
tem. The operating system may move programming instructions between parts of
Memory or other devices in order to make room for new programming. The oper-
ating system keeps track of this movement and can then restore the shifted in-
structions when needed. It should generally be considered a different operation
than Swapping, but these terms are often used interchangeably.

Parasolid
A kernel for 3-D modeling. This can be thought of as a set or library
of routines that CAD system vendors can buy for use in their system. These rou-
tines would provide the very basic and core algorithms that create 3-D geometry.
If two CAD systems both use Parasolid, then they should be much more able to
exchange 3-D data. It has been developed by Unigraphics®, one of the larger
CAD system vendors.

Part
An individually designed item. It refers to a single physical object that the
manufacturer uses. At one time, these individual items were referred to as De-
tails; they are documented in a detail drawing.

Part History
In standard 3-D part modeling, the model is made from the exe-
cution of a set of sequential steps. This list of steps is the Part History. It may also
be referred to as the part structure, hierarchy, or feature list.

Parts List
A table of notes in an assembly drawing that lists the individual
items needed to be put together for the assembly shown in the drawing. This is
similar to a Bill of Material, but the Bill of Material usually has more information
about the assembly and its control in the manufacturing process.

Part Model
The 3-D model for a Part.

Part Number
A master tracking number for information that is used by a man-
ufacturer. Obviously, individual part and their models are assigned unique Part
Numbers. However, the same numbering system may be applied to assemblies,
specifications, manuals, procedures, etc. Part Numbers are usually accompanied
by Revision Levels to fully specify and control the information.

Part Tolerance
The small amount of distance that indicates whether the edges
of surfaces are close enough to consider the two surfaces stitched up. This may
be a value such as 0.01 mm. In some cases, it is set by the 3-D CAD system; in

Glossary 307

other cases, it can be changed by the user. It becomes an issue for the user when
very small parts are modeled, or when 3-D models are exchanged between CAD
systems. If the Part Tolerance is different between the systems, part models do
not become solids properly.

PCB (Printed Circuit Board)
Sandwiched material that electronic compo-
nents are connected to. These are also often referred to as circuit boards or just
boards. Some CAD systems allow for special techniques for laying out or design-
ing PCBs. Particularly if these systems also simulate the electronic behavior, the
software may be referred to as E-CAD.

PC-DOS
See DOS.

PDM (Product Data Management)
Entails the activities related to managing
all the data necessary to specify and/or document a product that is manufactured.
It is concerned with CAD data, Revision Control, the state of designs, and per-
haps the roles of the people working with the data.

Perspective
How a 3-D model is viewed by an observer. In a Perspective view,
the 3-D image is more realistic since the image seems to shrink away (as a real
object would do). However, this is not always desirable when designing since ge-
ometry does not line up as it should. Also, whenever a CAD system displays a
model, there is a Perspective transformation that occurs to figure out how the
model should appear based on how far the virtual observer is from the virtual
object and how the object is rotated in modeling space.

Picking
Using a pointing device to make a selection of a displayed entity. It is
similar to Selecting, except that Picking presumes that the pointing device is
used.

Ping
A program or utility to see if a computer (or Node) is still actively con-
nected to the Network. Presumably it is based on the idea of sonar, where a signal
is sent out and then it “listens” for a response.

Pixel
Picture Element. It is one of the individual “dots” or regions of the Moni-
tor that can be turned on or off or changed color. It is very common in expressing
the Resolution of a Monitor.

Plane
A geometric entity that extends a flat surface to infinitely long distance.
A Planar Surface is a surface that is perfectly flat.