CNC stands
for Computer Numerical Control. It is the process of controlling
industrial production machinery via a computer program. This program
runs on a CPU or industrial control
unit which in turn controls switches, actuators, spindles and
servo
motors. These components can be controlled to produce very
precise and repetitive machine motion control. Here you will
learn the basics as it
is used with the Lathe
Tool also known as Tunning or Machining Centers. Although various
machines can have 5 or more axes we
will keep things simple and only cover the 2 axis turret lathe.
However what you learn here is consistent with all CNC and multi axis
machinery.
In general a lathe is a
machine tool that rotates the work piece
while a cutting tool is passed over 2D path which is along a
plane that
passes through the center of the work piece rotation. Think thats a lot
to say in one sentence? Well it is! But thats it in a nutshell.
Think of it like this. Remember that Old Etch-a-Sketch toy? You could
use one knob to draw lines from left to right (the Z axis) and another to draw up
or down (the X axis). When both knobs are turned together angles and
arcs can be drawn. Now imagine that a work piece, such as a piece
of round bar stock is rotated around the intersection of these two
axes. When the cutting tool is moved
along a 2D path as defined by this X and Z grid complex 3D
cylindrical shapes can be produced.
Take note that the X Z coordinate
system is divided onto 4 quadrants. In most cases material removal will
take place in the Z negative X positive quadrant. The Z zero point will
be set at the face of the work piece and the X zero point will set at
the center of the work piece rotation. Therefore the diameter of the
part produced will be equal to two times the actual value of
X. It is common to refer to X as a diameter rather
than radius value .
The coordinate system is programed
using a special computer language known as ' G
code' . Along with defining the tool
path this language also includes instructions for Feeds and Speeds, Tool
selection, and miscellaneous. functions.
The feed rate refers to how quickly material is removed by the turning
process. It may be defined as Inches per revolution (IPR) or Inches per minute (IPM).
When feed is defined as IPR the cutting tool travels a set
distance for each revolution of the spindle. For example a feed rate of
.005 would remove 5/1000 of and inch of material per revolution of the
spindle The time it takes the tool
tip to travel from point A to point B is a function of the spindle RPM.
The higher the RPM the shorter the cutting time.
When feed is defined in IPM the cutting tool moves at a set number of
inches
per minute and is not affected by the spindle RPM. However it must be
noted that as the RPM decreases the amount of material removal per
revolution increases. It is customary to
define cutting moves as IPR and positioning moves as IPM when
programming the lathe tool.
IPR can be converted to IPM by: IPR x RPM = IPM
IPM can be converted to IPR by: IPM / RPM = IPR
Speed is generally defined as Surface Feet per Minute or SFM. This
refers to how quickly the circumference of the work piece moves
across the tool tip. At a constant RPM SFM will increase as the tool
tip moves in the positive direction and thus increases the diameter of
the work piece. The speed may be programed either
as SFM or a constant RPM. When SFM is programed the CNC will
automatically calculate and adjust the spindle RPM as the diameter of
the tool tip changes.
SFM can be calculated by: ((circumference*RPM)/12)
RPM can be calculated by: ((SFM*12)/circumference)
Calculate circumference by: (diameter * 3.14159).
Feeds and Speeds are limited by many factors such as the type of
material being machined, the tool material being used, the amount of
material to be removed, and machine limitations.
Most tooling suppliers can provide charts or recommendations that
indicate the
appropriate feeds and speeds for a specific application.
Most lathe tools will accommodate from 6 to 12 tool stations. That is
to say that a number of different cutting tools may be selected
by the program to perform various machining processes Common tools that
may be used are Turning, Facing, Grooving, Threading, I.D. turning,
I.D. Grooving and I.D. Threading. And don't let me fail to mention
Drilling operations.
In the turret lathe individual tools are mounted to a revolving
turret. This turret rotates either clock wise or counter clock wise to
bring the selected tool into its work position.
When the machine is first booted up a procedure known as reference point return must be
performed to establish a very precise reference point for the machine
to work from. This position is known as the Home Position or Machine Zero. When the tip of
a selected cutting tool is positioned at a know
coordinate such as X zero Z zero a measurement can be taken
from the Machine Zero to this
new location. This is known as the tool
offset or tool length
compensation. This offset consist of a pair of X and
Z values. Notice that the offset vale is
different for each individual tool tip. By saving these
values in the NC memory each of the tools on the turret
can be assigned a unique tool offset
and thus be positioned precisely anywhere on the X Z work
grid.
Other functions that
can be controlled by the NC program include the starting and stopping
of pumps and conveyors, the opening and closing of switches and
actuators, Along with program functions such as Counters,
subroutine start , subroutine end, program end, pause, and resume.
Miscellaneous Functions
