Figure 11-12.--Absolute positioning system.
Figure 11-11.--Incremental positioning system.
location. Home zero is usually, but not always, located
at the tool change position of a machining center. A part
Spindle motion is assigned the Z axis. This means
is programmed independent of the machine coordinate
that for a drill press or vertical milling machine the Z
system. The programmer will pick a location on the part
or fixture. This location becomes the origin of the
such as lathes or horizontal milling machines, the Z axis
coordinate system for that part. The programmer's
coordinate system is called the local or part coordinate
system. The machine coordinate system and the part
coordinate system will almost never coincide. Before
running the part program, the coordinate system must
There are two ways that machines position
be transferred from the machine system to the part
themselves with respect to their coordinate systems.
system. This is known as setting a zero point. In other
These systems are called incremental positioning and
words, the machine has a zero point that is already
absolute positioning. With incremental positioning (fig.
programmed into it, but you can place the part to be
11-11) each tool movement is made with reference to
machined either in the chuck or on the table and
the prior or last tool position. Absolute positioning (fig.
establish your zero point, and then the machine will use
11-12) measures all tool movement from a fixed point,
that point as its zero point. This eliminates having to use
origin, or zero point. Use absolute dimensioning where
fixtures and other complicated setups. You just put your
possible because a mistake on the dimensions at one
work in the machine and by using the proper codes you
point will not be carried over to the dimensions at other
tell the machine where the zero point is located.
points. It is also easier to check for errors.
As stated earlier in the chapter, this has only been a
brief overview of CNC. It is a complicated subject that
SETTING THE MACHINE ORIGIN
many books have been written about. To cover it
completely you will need to have formal training and
Most CNC machinery has a default coordinate
extensive on-the-job training. As the Navy expands into
system the machine assumes upon power-up, known as
the world of CNC this training will become more
the machine coordinate system. The origin of this
system is called the machine origin or home zero