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workpiece in two or more axes. Almost all CNC machines are programmable in
several other ways. The specific CNC machine type has a lot to do with its
appropriate programmable accessories. Again, any required function will be
programmable on full-blown CNC machine tools.
Automatic tool changer
Most machining centers can hold many tools in a tool magazine. When required,
the required tool can be automatically placed in the spindle for machining.
Spindle speed and activation
The spindle speed (in revolutions per minute) can be easily specified and the
spindle can be turned on in a forward or reverse direction. It can also, of course, be
turned off.
Coolant
Many machining operations require coolant for lubrication and cooling
purposes. Coolant can be turned on and off from within the machine cycle.
The CNC program
Think of giving any series of step-by-step instructions. A CNC program is
nothing more than another kind of instruction set. It's written in sentence-like format
and the control will execute it in sequential order, step by step.
A special series of CNC words are used to communicate what the machine is
intended to do. CNC words begin with letter addresses (like F for feedrate, S for
spindle speed, and X, Y & Z for axis motion). When placed together in a logical
method, a group of CNC words make up a command that resemble a sentence.
The CNC control
The CNC control (refer to fig.3) will interpret a CNC program and activate the
series of commands in sequential order. As it reads the program, the CNC control will
activate the appropriate machine functions, cause axis motion, and in general, follow
the instructions given in the program. Along with interpreting the CNC program, the CNC control has several other
purposes. All current model CNC controls allow programs to be modified (edited) if
mistakes are found. The CNC control allows special verification functions (like dry
run) to confirm the correctness of the CNC program. The CNC control allows certain
important operator inputs to be specified separate from the program, like tool length
values6. In general, the CNC control allows all functions of the machine to be
manipulated.
Computer integrated manufacturing
Present and future trends of modern manufacture point toward an increase in
the use of computers and a decrease in manual methods1. The driving force in this
trend is the need to utilize a central computer for storing and retrieving information.
Information stored in the central computer is said to reside in a common database.
Computer integrated manufacturing (CIM) is a system whereby all aspects of a
company's operation are computerized and tied to a common database2. An important
element of CIM is CAD/CAM and DNC. A CIM system unifies business, engineering,
process planning, inventory, manufacturing, and postproduction.
When fully implemented, CIM will offer the following advantages:
Increased integration and coordination of all company disciplines
Increased productivity
Decreased costs of operation
Improved quality
Reduced scrap
Reduced inventory
Around-the-clock operation with a minimum of operator intervention
CIM also poses the following challenges:
Substantial installation and start-up costs
Training personnel in the use of CIM equipment (hardware and software)
Dealing with proprietary systems and software
Maintaining system hardware and software
Implementing CIM fully in order to make it cost effective
Operating a CIM installation for long periods offline
Planning CIM operations