Types of NC Systems
When classified according to the machine tool control system,there are
three basic types of NC systems :
1. Point to Point.
2. Straight cut.
3. Contouring.
The classification is concerned with the amount of control over the relative motion between the workpiece and cutting tool . The least control is exerted over the tool motion with the point to point systems. Contouring represents the highest level of control.
Point-to-Point NC
Point to point is also sometimes
called a positioning system. In PTP the objective of themachine tool control
system is to move the cutting tool to predefined location. The speedor
path by which this movement is accomplished is not important in point to
point NC. Once the tool reaches the desired location, the machining
operation is performed at that position. NC drill presses are a good
example of PTP systems. The spindle must first be positioned at a
particular location n the workpiece. This is done under PTP control
. Then the drilling of the holes is performed at that location,the tool
is moved to the next hole location, and so forth. Since no
cutting is performed between holes there is no need for controlling the
relative motion of the tool and workpiece betwwen hole locations.
On positioning systems the speeds and feeds used by the machine tool are
often used by the machine operater rather than by the nc tape. Positioning
systems are the simplest machine tool control systems and therefore
the least expensive of the three types. However for certain processes such
as drilling operations and spot welding. PTP is perfectly suited
to task and any higher level of control is unnecessary.
Straight Cut NC
Straight cut control systems are capable of moving the cutting
tool parallel to one of the major axes at a controlled rate suitable for
machining . It is therefore appropriate for performing milling operations
to fabricate workpieces of rectangular configuartions. With this type of
NC systems it is therefor appropriate for performing milling operations
to fabricate workpieces of rectangular configuartions . With this type
of NC system it is not possible to combine movements in more than single
axis direction. Therefore angular cuts on the workpiece would not be possible.
An NC machine tool capable of performing straight cut movements is also
capable of point to point movements.
Contouring NC
Contouring is the most complex flexible and teh most expensive
type of machine tool control . It is capable of performing both PTP and
straight cut opeartions . In addition th edistinguishning feature of the
of contouring NC system is their capacity for simultaneousl control of
more than one axis movement of machine tool Figures below illustrate the
versatility of continuous path NC. Milling and Turning are the common examples
of the use of conturing control.