CNC Terminology



Some general terminology related to CNC machining technology.

(Under Construction)

Ball End Milling Cutter - A milling cutter that has a rounded cutting diameter at its end that is equal to the cutting diameter.

Closed-Loop System - A positioning system that can use either step motors or servo motors ( basically just a DC or AC motor ) with an added optical encoder attachment to the motor shaft.  The motors receive movement commands from the  computer.   The optical encoder then sends this movement information back to the computer to verify that the correct movement was produced.  Hence, the computer has a record at any instant of the actual or real position of the mechanism.  This type of system can achieve much higher positioning speeds then the open-loop kind without losing track of the position.

DXF file - Drawing eXchange Format file that was created as a standard to freely exchange 2 and 3 dimensional drawings between different CAD programs.  It basically represents a shape as a wireframe mesh of x,y,z coordinates ( vectors ).

Encoder - Typically an optical device that consists of a disk with 100 to1000 holes on its periphery.  The most common is the incremental encoder that has a small LED light source on one side of the disk with a diode detector on the other to allow the disk rotation to be monitored in discrete incremental steps.  Hence, a full revolution can be broken up into 100's or even 1000's of position steps.

G-Code - The standard machine tool language around the world.  It generally consists of specifing the x,y,z (and a,b or c ) coordinates that the machine is to move to.  Such movement can be linear, circular or even special drilling operations.  It is the universal language of all modern machine tools ( mills, lathes, edm machines etc.).

HPGL - Hewlet Packard Graphical Language that was originally created to send 2 dimensional drawing information to pen plotters, but has since become a good standard for the exchange of 2 dimensional drawing information between CAD programs.

M-code - The standard machine tool codes that are normally used to switch on the spindle, coolant or auxillary devices.  They can also be used for G-code program control such as repeating the program or ending it.

Open-Loop System - A positioning system that generally uses step motors.   The step motor amplifier receives signals from the computer and makes the commanded movements.  This movement information is NOT sent back to the computer to verify that the movement was correct.  Hence, if the positioning mechanism stops unexpectantly the computer will lose track of the mechanism position.  Note:  Under normal cases this does not happen and the position is always recorded correctly.

PID - Proportional Integral Derivative acronym that is the mathematical control method used in a standard closed loop servo system.

Repeatability - The smallest distance that can be repeated by a machine movement.  For example if the repeatability is specified as 0.001 inches and a movement from X coordinate 0.000 to 1.000 and back to 0.000 then the position would be within 0.001 inches of 0.000.

Servo Motor - A motor that is typically a brush or brushless DC type with an optical encoder attached to it.  It is used in what is called a Servo Loop system where positioning information is constantly tracked by minimizing the error between the commanded and real position.  

Step Motor - A motor that derives its motion by receiving input signals ( pulses) in a very specific sequence.  The most common type is one that rotates 1.8 degrees for each input pulse.  This provides a very simple way of controlling motion very precisely with the use of common digital logic circuitry.

STL file - Stereo Lithography file format that has traditionally been associated with Stereo Lithography prototyping machines, but is now also being used to represent 3D surfaces for CNC tool path generating programs.

Tool Path - A series of vector coordinate positions that define a cutting path.  This cutting path can be a simple 2D or sophisticated 3D ( even 4D or more) path used to machine out the shape of a desired part.

Vector - A line that has both length and direction.  It is usually specified by a starting x,y,z coordinate position and ending x,y,z coordinate position.


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Last modified: September 26, 2019

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