A listing of terms and what they mean. Some terms have a general meaning
and several additional meanings for users, installers, and developers.
Acme Screw
A type of lead-screw
that uses an acme thread form. Acme threads have
somewhat lower friction and wear than simple triangular threads, but
ball-screws are lower yet. Most manual machine tools
use acme lead-screws.
Axis
One of the computer control movable parts of
the machine. For a typical vertical mill, the table is the X axis,
the saddle is the Y axis, and the quill or knee is the Z axis. Additional
linear axes parallel to X, Y, and Z are called U, V, and W respectively.
Angular axes like rotary tables are referred to as A, B, and C.
Backlash
The amount of "play"
or lost motion that occurs when direction is reversed in a lead screw
. or other mechanical motion driving system. It can
result from nuts that are loose on leadscrews, slippage in belts,
cable slack, "wind-up" in rotary couplings, and
other places where the mechanical system is not "tight".
Backlash will result in inaccurate motion, or in the case of motion
caused by external forces (think cutting tool pulling on the work
piece) the result can be broken cutting tools. This can happen because
of the sudden increase in chip load on the cutter as the work piece
is pulled across the backlash distance by the cutting tool.
Backlash Compensation
- Any technique
that attempts to reduce the effect of backlash without actually removing
it from the mechanical system. This is typically done in software
in the controller. This can correct the final resting place of the
part in motion but fails to solve problems related to direction changes
while in motion (think circular interpolation) and motion that is
caused when external forces (think cutting tool pulling on the work
piece) are the source of the motion.
Ball Screw
A type of lead-screw
that uses small hardened steel balls between the nut
and screw to reduce friction. Ball-screws have very low friction and
backlash , but are usually quite expensive.
Ball Nut
A special nut designed
for use with a ball-screw. It contains an internal passage to re-circulate
the balls from one end of the screw to the other.
CNC
Computer Numerical Control. The general
term used to refer to computer control of machinery. Instead of a
human operator turning cranks to move a cutting tool, CNC uses a computer
and motors to move the tool, based on a part program .
Coordinate Measuring Machine
A Coordinate Measuring Machine
is used to make many accurate measurements on parts. These machines
can be used to create CAD data for parts where no drawings can be
found, when a hand-made prototype needs to be digitized for moldmaking,
or to check the accuracy of machined or molded parts.
DRO
A Digital Read Out is a device attached to the
slides of a machine tool or other device which has parts that move
in a precise manner to indicate the current location of the tool with
respect to some reference position. Nearly all DRO's use linear quadrature
encoders to pick up position information from the machine.
EDM
EDM is a method of removing metal in hard or
difficult to machine or tough metals, or where rotating tools would
not be able to produce the desired shape in a cost-effective manner.
An excellent example is rectangular punch dies, where sharp internal
corners are desired. Milling operations can not give sharp internal
corners with finite diameter tools. A wire EDM machine can make internal
corners with a radius only slightly larger than the wire's radius.
A 'sinker' EDM cam make corners with a radius only slightly larger
than the radius on the corner of the convex EDM electrode.
EMC
The Enhanced Machine Controller.
Initially a NIST project. EMC is able to run a wide range
of motion devices.
EMCIO
The module within EMC
that handles general purpose I/O, unrelated to the actual motion of
the axes.
EMCMOT
The module within EMC
that handles the actual motion of the cutting tool. It runs as a real-time
program and directly controls the motors.
Encoder
A device to measure position.
Usually a machanical-optical device, which outputs a quadrature signal.
The signal can be counted by special hardware, or directly by the
parport with emc2.
Feed
Relatively slow, controlled motion
of the tool used when making a cut.
Feedrate
The speed at which a motion occurs.
In manual mode, jog speed can be set from the graphical interface.
In auto or mdi mode feedrate is commanded using a (f) word. F10 would
mean ten units per minute.
Feedback
Feedrate Override
A
manual, operator controlled change in the rate at which the tool moves
while cutting. Often used to allow the operator to adjust for tools
that are a little dull, or anything else that requires the feed rate
to be ``tweaked''.
G-Code
The generic term used to
refer to the most common part programming language. There are several
dialects of G-code, EMC uses RS274/NGC .
GUI
Graphical User Interface.
General
A type of interface that allows communications between
a computer and human (in most cases) via the manipulation of icons
and other elements (widgets) on a computer screen.
EMC
An application that presents a graphical screen to the machine
operator allowing manipulation of machine and the corresponding controlling
program.
Home
A specific location in the machine's
work envelope that is used to make sure the computer and the actual
machine both agree on the tool position.
ini file
A text file that contains
most of the information that configures EMC for a particular
machine
Joint_Coordinates:
These specify the
angles between the individual joints of the machine. Kinematics
Jog
Manually moving an axis of a machine. Jogging either moves
the axis a fixed amount for each key-press, or moves the axis at a
constant speed as long as you hold down the key.
kernel-space
Kinematics
The position relationship between
world coordinates and joint coordinates
of a machine. There are two types of kinematics. Forward kinematics
is used to calculate world coordinates from joint coordinates. Inverse
kinematics is used for exactly opposite purpose.Note that kinematics
does not take into account, the forces, moments etc. on the machine.
It is for positioning only.
Lead-screw
An screw that
is rotated by a motor to move a table or other part of a machine.
Lead-screws are usually either ball-screws or acme
screws , although conventional triangular threaded
screws may be used where accuracy and long life are not as important
as low cost.
MDI
Manual Data Input. This is a mode
of operation where the controller executes single lines of G-code
as they are typed by the operator.
NIST
National Institute of Standards
and Technology. An agency of the Department of Commerce in the United
States.
Offsets
Part Program
A description of a part, in
a language that the controller can understand. For EMC, that language
is RS-274/NGC, commonly known as G-code .
Rapid
Fast, possibly less precise motion of the
tool, commonly used to move between cuts. If the tool meets the material
during a rapid, it is probably a bad thing!
Real-time
Software that is intended to meet
very strict timing deadlines. Under Linux, in order to meet these
requirements it is necessary to install RTAI or RTLINUX
and build the software to run in those special environments.
For this reason real-time software runs in kernel-space.
RTAI
Real Time Application Interface,
see http://www.aero.polimi.it/ rtai/ http://www.aero.polimi.it/ rtai/,
one of two real-time extensions for Linux that EMC can use to achieve
real-time performance.
RTLINUX
See http://www.rtlinux.org http://www.rtlinux.org,
one of two real-time extensions for Linux that EMC can use to achieve
real-time performance.
RS-274/NGC
The formal name
for the language used by EMC part programs .
Servo Motor
Servo Loop
Spindle
On a mill or drill, the
spindle holds the cutting tool. On a lathe, the spindle holds the
workpiece.
Stepper Motor
A type
of motor that turns in fixed steps. By counting steps, it is possible
to determine how far the motor has turned. If the load exceeds the
torque capability of the motor, it will skip one or more steps, causing
position errors.
TASK
The module within EMC that coordinates
the overall execution and interprets the part program.
Tcl/Tk
A scripting language and graphical widget
toolkit with which EMC's most popular GUI's were written.
World Coordinates
This is the absolute
frame of reference. It gives coordinates in terms of a fixed reference
frame that is attached to some point (generally the base) of the machine
tool.