Glossary of Technical Terms 617
O
offhand grinding: A grinding operation for
work that does not require great accuracy.
The work is handheld and manipulated until
ground to the desired shape. (Ch. 13)
offline programming: Entering a program
using a computer that is not currently being
used to control the operation of a CNC
machine. (Ch. 23)
offset tailstock method: A method for
machining external tapers on a lathe. Jobs
that can be turned between centers may be
taper-turned by this technique. (Ch. 16)
open-loop system: A system in which no
feedback is provided to the controlling
mechanism; used with stepper motors
in relatively inexpensive CNC machines.
(Ch. 22)
optical comparator: A gaging system in which
an enlarged image of the part being inspected
is projected onto a screen. The projected
image can be superimposed on a grid or an
accurate drawing overlay of the part to allow
precise measurement of the part. (Ch. 27)
orientation geometric tolerances: Tolerances
that control the relationship of features to
one another. When controlling orientation
tolerances, the feature is related to one or
more datum features. (Ch. 4)
OSHA (Occupational Safety and Health
Administration): A government agency
responsible for setting and enforcing
regulations regarding safety and health
within the workplace. (Ch. 3)
P
parallelism: The condition of a surface or center
plane equidistant from a datum plane or axis.
(Ch. 4)
part catcher: A programmable device
mounted to turning centers to catch fi nished
parts as they are cut off the bar stock.
(Ch. 25)
parting: The turning operation of cutting
off material after it has been machined.
This is one of the more diffi cult operations
performed on a lathe. (Ch. 14)
part programmer: A skilled worker who
specializes in writing programs for computer
controlled machine tools. (Ch. 2)
peck drilling: A method of drilling in which
the cutter is retracted at intervals to allow
chips to be cleared or coolant to be fl ooded
through the hole. (Ch. 24)
peripheral milling: A milling operation that
is done when the surface being machined
is parallel with the periphery of the cutter.
(Ch. 18)
perpendicularity: The condition of a surface,
center plane, or axis at a right angle (90°) to a
datum plane or axis. (Ch. 4)
pilot hole: A smaller hole drilled prior to
drilling a larger hole that greatly reduces feed
pressure, improves accuracy, and allows the
faster drilling. Also known as a “lead hole.”
(Ch. 12)
pitch: The distance from a point on one thread
or gear tooth to the corresponding point on
the next thread or tooth. (Ch. 16)
pitch circle: An imaginary circle located
approximately half the distance from the
roots and tops of gear teeth. It is tangent to
the pitch circle of the mating gear. (Ch. 19)
pitch diameter: For threads, the diameter of an
imaginary cylinder that would pass through
the threads at such points to make the width
of the thread and the width of the thread
groove equal. For gears, the diameter of the
pitch circle of a gear. (Ch. 16)