CNC Lexicon
(We tried to eliminate as much engineer-speak as possible...)

The cutter moves side-to-side (X) and back and forth (Y), so it has two linear axes.
The cutter moves side-to-side (X), back and forth (Y), and up and down (Z), so it has three linear axes.
The cutter moves side-to-side (X), back and forth (Y), up and down (Z), and can rotate in one plane.
The cutter moves side-to-side (X), back and forth (Y), up and down (Z) and can rotate in two planes.
The three linear axes are X, Y and Z. When rotaion is added, these are the A, B and C axes.
When the spindle is moved without a load, it doesn't do so at a single speed. It speeds up, then slows down.
Aggregate head
A spindle that holds more than one tool. Most aggregates can work at angles other than just 0 and 90 degrees.
Angular (rotational) axis
Rotation about a vector (line) that is at an angle other than 0 or 90 (one of the X, Y or Z planes).
The American Standard Code for Information Interchange  - it's the most common format for text files.
ATC (automatic tool changing)
A machine head that holds a number of tools (bits) so the CNC doesn't have to stop to change bits.
Axis (plural, Axes)
A straight line. It usually describes the path that a cutter follows, or rotates around.
The three linear axes are X, Y and Z. When rotaion is added, these are the A, B and C axes.
Bearing - roller
Like a ball-bearing, but with cylinder-shaped rollers instead of spheres.
Bearing - thrust
These are round ball or (usually tapered) roller bearings that are housed in a circular sandwich
Boring technically describes making a drilled pilot hole larger by machining around its edges.
A hard sleeve or lining that protects something such as wiring, or maintains the shape of a hole.
The three linear axes are X, Y and Z. When rotaion is added, these are the A, B and C axes.
CAD (computer aided design)
A computer program that is used to draw cabinets, furniture and parts.
CAM (computer aided manufacturing)
A computer program that tells the router bit where to go, based on a CAD drawing. It creates toolpaths.
Canned Cycle
Little programs that tell the machine to repeat what it is doing (usually with another step in between).
Cartesian (or Rectangular) Coordinate
This is the standard XYZ imaginery rectangle (2-axes) or cube (3-axes) that is used to describe movement.
Charge pump
An electronic safety switch that disables the motors, as in when you're changing a tool.
Just like a drill chuck, it holds the tool.
Closed Loop
It provides feedback that tells the operator if something is out of whack. 
CNC (Computer Numeric Control)
A drawing is converted into numbers that tell the router's cutting tool where to travel.
A segmented sleeve/collar that goes around a tool shaft and allows the chuck to grip it. 
Control/Control System
Either a single path (with starting and ending coordinates), or a continuous loop.
Coordinate (co-ordinate)
A point where two or more axes intersect. Basically, it's where you tell the tool to be.
An imagined shape, point, line or other geometrical entity. Zero datum is your starting point.
DXF (Drawing eXchange Format)
Invented by the creators of AutoCad, it's a graphics format that CAD programs use to save designs.
EIA standards
The Electronic Industries Alliance and ISO have developed standards for G-Code to help make it universal.
Feed hold
Stops the spindle moving along a toolpath, but allows it to continue spinning the bit.
Feed rate
How fast the spindle moves. 
FOR, FRO (feed rate override)
Usually a dial that allows you to manually slow down the travel of the cutter, if it's bogging down.
G-Code (GCODE)
The computer language that transforms a design into a series of instructions the CNC can follow.
A 'bridge' that slides the length of the table, and allows the spindle/router to move across the table.
Home, homing, zero
The starting point, which is 0,0 on the X and Y axes.
Hewlett-Packard Graphics Language, an older control system originally designed for printing plotters.
Manually moving the spindle in a straight line to save time. It can be done with the control panel, too.
Jumps, jumped steps (see also missed step)
Can happen because the travel speed (feed) is too fast, or the load (cut depth, bit diameter) is too heavy.
Limits, limit switches
Two on each axis tell the machine where home is, and also where the boundaries of the table are.
Linear Scale
Where the units are the same, as in 1-2-3-4 (as opposed to a geometric scale such as 2-4-16-256)
Linear/Linear Axis
A straight line, as opposed to a rotary exis that describes a circle or arc of a circle.
G-Code tells the machine what to do and M-Code (Miscellaneous Code) essentially makes it happen.
Machining Center
A CNC (usually with an automatic toolchanger), where the tool moves and the workpiece stays put.
MCU (Machine Control Unit)
It reads code and tells the machine what to do.
MDI (Manual data input)
This is where the machine allows you to manually change/override a setting, such as the spindle RPMs.
Missed step (see also jumps)
Where the spindle skips a small step, and this can lead to cuts in the wrong place or a shut-down.
The forward progress of the spindle, fueled by weight and speed.
A distance from 'home', so that a second part can be milled without the tool returning home.
Open Loop
There is no feedback that tells the operator if something is out of whack. 
Open or Closed Loop
Open - no feedback. Closed - gives feedback (tells the operator what's happening)
Optic Code/Optical Encoder
This is a sensor that 'sees' movement and tells the operator about the tool's position, velocity and direction.
0,0 on the XYZ axes. Also called home position, grid zero, and machine origin.
An incremental cut: the tool removes waste in steps to avoid too much stress.
Path - Continuous
Also called contouring, this is a path that can follow more than a single direction, e.g. a curve.
Path - Toolpath
Where the tool is told to travel between points.
A proportional-integral-derivative is one way in which the machine delivers feedback about its progress.
An earlier, simpler version of a CNC. (Some machinists will argue that P2P still has relevance.)
When the cutter leaves the work and moves to another location, you can tell it to do so rapidly to save time.
The unit of measurement used to determine minimum travel increments. (e.g., 1/1000").
Righthand Rule
With your right hand palm up, the thumb and index finger form an L showing the X and Y axes respectively.
Rotational Axis
These are imaginary axes (A, B and C) around which a tool - or a part - tilts or rotates.
Run-out (runout)
This is where the tool (and it's sometimes the spindle) is no longer true, so it spins at a small angle.
A resting Z (vertical) setting where the tip of the cutter is safe - it doesn't touch anything.
Servo _DC
A cool-running, efficient, quiet alternative to steppers, they're a little more tempermental. Runs on DC.
Servo - AC
A cool-running, efficient, quiet alternative to steppers, they're a little more tempermental. Runs on AC.
Servo - Hydraulic
A cool-running, efficient, quiet alternative to steppers, they're a little more tempermental. Runs on hydraulics.
The chuck and motor that holds the tool. Akin to the motor on a handheld router.
Spindle speed
Usually describes the fastest speed that the tool can revolve, or else the variable speed setting.
Spoil Board (spoilboard)
A waste sheet (usually MDF) that is attached to the bed and then milled flat and parallel to the gantry.
Stepover (step-over), Overlap
How far the tool cuts across its previous path when making the next cut, balancing time and quality.
Stepper motor
Less expensive but also less efficient than servo, with bearings instead of brushes, it runs hotter and can skip.
A file format used in more complex three-dimensional milling. 
Surface (verb)
To flatten. To reveal a new, fresh, flat surface.
Tool Path
The route the tooling will travel.
Toolholder (tool holder, tool-holder)
A device that holds several tools so that there is minimal downtime between changing them.
Tooling - Ball End Mill
A bit with a round (spherical) cutting tip, also called a ball nose end mill, is durable and delivers a smooth cut.
Tooling - Drill
A bit that drills vertical holes, and can often mill horizontally too.
Tooling/Cutting Tool
The router bit or other cutter.
The combined movement across the gantry and along the bed.
Vac clamp, Vacuum Hold-down
One of myriad small pads, pods, clamps and other pneumatically activated hold-down systems.
A line. The start and end are located as XYZ coordinates.
The speed at which the cutter moves across/through the workpiece.
VFD (Variable Frequency Drive)
Allows an AC motor to run at different speeds.
X is side to side, Y is back to front and Z is up and down.

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