Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (64 page)

STEP (Standard for the Exchange of Product Data)
A 3-D file format for
exchanging part and assembly models between CAD systems and/or companies.
It is based on an ISO standard process (TC184/SC4). Refer to ISO 10303—“an
International Standard for the computer-interpretable representation and ex-
change of product data.” It has many parts that are meant to assist with data ex-
change for various types of software and industries. For instance, Part 214 is for
“core data for automotive mechanical design processes.”

Stitching
Also called Stitching Up or Sewing Up, refers to the connection of
the edges of surfaces that meet or touch. It uses the Part Tolerance to determine if
the edges are close enough. A solid model can be created from separate surfaces
if enough surfaces are Stitched to each other.

STL File
A file that contains 3-D part data for a Rapid Prototyping device that
uses a technology called stereolithography. This file contains just Tessellations of
the 3-D model; not the real surfaces. The STL File can be considered a de facto

312 Glossary

standard for this type of data. It is used in various schemes to view 3-D models
(as well as with Rapid Prototyping applications).

Storage
Devices in a computer system that permanently save data (i.e. it is
nonvolatile). The most common device of this type is the Disk Drive.
String
See Character Data.

Subdirectory
A directory that is found within another directory. Subdirectories
are used to carefully organize and manage the vast amounts of data that might be
found in a Storage system (i.e. Disk Drives, Network File Servers, etc.).
Surface
A distinct region of the outer skin of the part. Surface finish indicates
how rough or smooth it is. It is also assumed to be a region of the part that can be
well described by a mathematical relationship. A flat surface is a “planar” surface
and can be described by a formula for planes; a cylindrical surface can be de-
scribed by formulae for cylinders, etc. Surfaces can also be “free form” in that
they use very arbitrary formulae that allow for virtually any shape (this typically
uses the mathematics of NURBS).

Surface Finish
Describes how rough or smooth a surface is after manufactur-
ing. It is indicated on drawings by Surface Finish symbols.

Surface Modeling
A set of 3-D CAD techniques that works with parts on a
surface-by-surface basis (as opposed to a feature-by-feature basis). This may in-
clude open part modeling, lofting, sweeping, etc.

Surface Normal
A direction or vector that is totally perpendicular to a surface
or curve. At any point on a surface, a tangent plane can be calculated that just
touches the surface. The Surface Normal is then the direction that is perpendicu-
lar to that plane.

Surface Texture
The application, projection, or mapping of Bitmap data to a 3-
D model’s surface. If Surface Texture is used, the normal shading process for sur-
faces is augmented with texture. This texture could be a bit pattern for things
such as wood, concrete, etc. But it can also be used with pictures such as TIFF or
JPEG files. Surface Texture is rarely used in production CAD work (particularly
since it degrades graphics performance by using a lot of memory).

Swapping
The process of making room in the Memory of a computer system
by moving programming instructions and/or data from Memory (i.e. system
RAM or “chips”) to Storage (such as a Disk Drive). This process allows pro-
grams more than the physical amount of Memory to be run, but the computer
runs much more slowly. The higher possible amount of Memory permitted is re-
ferred to as Virtual Memory.

Sweep
A 3-D part modeling operation that creates surfaces that apply a single
cross-sectional shape all along a path.

Symbol
See Groupings.

Terminal
The display device attached to a mainframe computer. It did not have
any substantial processing power (i.e. a CPU) or graphics capability (i.e. a
Graphics Adapter). The most common Terminal type related to current systems

Glossary 313

was the VT100 from the former Digital Equipment Corporation. Occasionally
systems will offer the option to emulate a VT100, or VT101, or VT102. There
were also graphics terminals. Most of these were developed by a company called
Tektronix. They developed graphics standards such as PLOT10 and TCS.
Tessellation
The process of taking an analytical or pure surface (such as a cy-
lindrical surface) and breaking it down into sufficiently small surfaces that the
surface still appears curved. Each of the small surfaces are usually called trian-
gles or perhaps polygons. A Tessellated File would be a file that only contains
data for these triangles (instead of the analytical surfaces). Thus this file would
not be appropriate for exchange of CAD models. Examples of Tessellated Files
are VRML and STL.

Texture
See Surface Texture.

Third Angle Projection
A standard for indicating where views are placed on a
drawing. In Third Angle, the Right View of the object is placed on the right of the
primary or Front View. This is the standard in the United States.

Title Block
A set of notes on a drawing that indicates the basic identification
information for the drawing (such as the drawing Title, Number, Size, Current
Revision, etc.).

TIFF™ Image file format. It was originally published by the Aldus Corpora-
tion in cooperation with scanner manufactures. The latest standard is from Adobe
Systems Incorporated. It contains Bitmap type data, and is not appropriate for the
exchange of CAD data. It is often used with on-line imaging systems for storing
and retrieving electronic copies of drawings.

Tolerance, Drawing
Indicates the precision desired for the manufacture of
parts and assemblies. There are a few information systems on a drawing to indi-
cate Drawing Tolerance. A simple system is the number of decimal places for
dimension values. The more decimals shown, the higher the precision (e.g., 3.9
versus 3.875). Another system shows dimensions with upper and lower allowable
limits. For a more sophisticated system see GD&T.

Tolerance, Part
See Part Tolerance.

Tool Path
A set of geometric locations that indicate the motion of an NC (or
CNC) machine tool. These paths can be 2-D or 3-D (depending on the type of
part and machine tool used). In order to also dictate the orientation of tables, jigs,
or the sides of tools (bits), there may also be more than just X, Y, Z data for the
Tool Paths.

Translucency
A characteristic of surfaces that can be seen through. Often
translucency is a feature of the graphics system for 3-D CAD.

Trapping
See Selecting.

Trimmed Surface
A surface that was originally a larger size, but has been
given a boundary that reflects the design of a part. Many types of surfaces (such
as NURBS) always have a rectangular form to their edges. Trimming changes
them to the needed shape.

314 Glossary

UI
See User Interface.

Unigraphics
A high-end CAD system. It has been developed by a company by
the same name. This company is owned by Electronic Data Systems Corp.
(EDS®).

Unit Vector
A Vector with a magnitude or value of just 1. It is a means of indi-
cating direction alone.

UNIX or unix
An operating system that is most popular with workstations. It
was originally developed at Bell Labs®
in the 1970s, and it was made available in
source code form to computer manufacturers. There were two main flavors to the
operating system—BSD (Berkeley Standard Distribution) and System V (System
Five). It was most popular with scientific and engineering software development,
so it became a de facto standard for the original high-end CAD systems. Cur-
rently, unix and Windows™ are typically used for CAD systems.

Untrimmed Surface
A surface that has not had its boundary adjusted to fit the
3-D part model. Many types of surfaces (such as NURBS) always have a rectan-
gular form to their edges. Trimming changes them to the needed shape. Un-
trimmed Surfaces are most typically seen after translating a 3-D model between
different CAD systems.

User Interface
The computer programming that the user interacts with. A text-
based User Interface would just use commands that are typed. A Graphical User
Interface allows the user to select commands and perform actions by using a
pointing device (such as a mouse).

VAX
An instrumental minicomputer in the early development of CAD systems;
it was manufactured by the former Digital Equipment Corporation (now part of
Compaq®). It pioneered virtual memory and controlling multiple programs and
users without the mainframe architecture. VAX/VMS is an operating system
based on the function of the VAX.

Vector
A mathematical device for handling data that has both value (i.e. a sca-
lar) and direction. Vectors are used in CAD systems to indicate the direction and
magnitude of a function. When a user measures a distance from one point in a 3-
D space to another point in 3-D space, one can imagine a Vector that points from
one to the other. The length of the Vector is the distance between them. The di-
rection of the vector points along the line that goes from one point to the other.
Vectors have components that equal the change in distance along the X, Y, and/or
Z axes of the 3-D space.

Vector Graphics
A type of computer graphics based on vector data. A file
which contains vector data for a line would have the actual endpoints of the lines.
A program or plotter which uses this data can draw a perfect line based on those
values. This is in contrast to a Bitmap which contains a set of dots that would
approximate the line. Vector Graphics also could be used with the original oscilli-
scope-type graphics devices.

Glossary 315

Vendor
A generic term applied to companies that provide parts, services, or
other manufactured goods to a manufacturer. It can be used somewhat interchan-
gably with supplier.

View
Regions of a drawing that show the object of the drawing from various
viewing angles. A Front View shows the object from a front view; the Right View
shows it from the right, etc.

Viewport
A computer graphics technique that creates a container for 2-D
graphics. A Viewport can have its own mapping so that X- and Y-values are
uniquely applied to it. Viewports also clip so that data is not shown beyond their
boundaries. All these characteristics make Viewports rather ideal for use as a
means of implementing drawing Views in a CAD system.

Virtual Memory
A method of extending the amount of Memory that the CPU
can utilize. The added Memory is attained by using the Storage system (i.e. Disk
Drive) to hold programming that is not currently active in Memory. The process
of moving the programming to the drive is called Swapping. Although, Virtual
Memory permits larger programs to run on a computer system, the Swapping
drastically slows down the performance.

VRML
Virtual Reality Modeling (or Markup) Language. It is a file format that
can allow 3-D models to be shown in a web browser. Furthermore, if the web
browser has an appropriate plug-in, the 3-D model can be manipulated.
WAN (Wide Area Network)
Allows computers to communicate across a net-
work that is more remote or dispersed than a LAN (Local Area Network). A WAN
may connect computers that are at facilities a company may own country-wide.
Web Browser
A program that is used in conjunction with a connection to the
World Wide Web. The Web Browser reads the HTML data format (as well as
others) to determine how the web page is to appear.