CNC SYSTEM

The modem CNC system, but not the old NC system, uses an internal micro processor (i.e., a
computer). This computer contains memory registers storing a variety of routines that are
capable of manipulating logical functions.
That means the part programmer or the machine operator can change the program on the control
itself (at the machine), with instantaneous results. This flexibility is the  greatest advantage of the
CNC systems and probably the key element that contributed to such a wide use of the technology
in modern manufacturing. The CNC programs and the logical functions are stored on special
computer chips, as software instructions rather than used by the hardware connections, such as
wires, that control the logical functions.
In contrast to the NC system, the CNC system is synonymous with the term 'Softwired' (because
most of their control functions are implemented by the control software programs).All numerical
control machines manufactured since the seventies are of CNC type.
The control signals in CNC systems are in the form of binary words. Each word contains a fixed
number of bits, 32 bits or 64 bits are commonly used. Each bit of data produces one BLU motion
in the controlled axis.
Theoretically a 32 -bit word could represent one of up to 2 raise to power 32 = 4,294,967,296 different axial positions. If the system resolution is, for example, BLU = 0.0001
in., this number can represent up to 429,969 in. possible motions, which is more than enough for
all types of applications.
When describing a particular subject that relates to the numerical control technology, it is
customary to use either the term NC or CNC. Keep in mind that NC can also mean CNC in
everyday talk, but CNC can never refer to the older technology.

Hardness

The Metals Handbook defines hardness as "Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.


There are three types of tests used with accuracy by the metals industry; they are the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test. Since the definitions of metallurgic ultimate strength and hardness are rather similar, it can generally be assumed that a strong metal is also a hard metal. The way the three of these hardness tests measure a metal's hardness is to determine the metal's resistance to the penetration of a non-deformable ball or cone. The tests determine the depth which such a ball or cone will sink into the metal, under a given load, within a specific period of time. The followings are the most common hardness test methods used in today`s technology:

1. Rockwell hardness test
2. Brinell hardness
3. Vickers
4. Knoop hardness
5. Shore  

NC SYSTEM

The NC System (as opposed to CNC system) uses fixed logical functions, those that are built in a permanently wired circuit within the control unit. These functions can not be changed by the programmer or the machine operator. Because of the fixed wiring of the control logic, the control logic the NC control system is synonymous with the term “hardwired” (in which most functions are implemented by electronic hardware based upon digital circuit technology.). The system can interpret a part program, but it does not allow any changes to the program, using the control features. All required changes must be made away from the control, typically in an office environment. Also the NC system requires the compulsory use of punched tapes for input of the program information. The NC controller interprets these instructions and then converts them into two types of control signals: motion control signals and miscellaneous control signals

 

A Typical Numerical Control System

Motion control signals are a series of electric pulse trains that are used to control the position and the speed of the machine table and spindle.  

An obsolete Numerical Control OR Manual relay panel from the early 1960’s

Each pulse activates a motion of one basic length-unit (BLU) which is the minimum increment size of the NC control system. The typical increment for one BLU in the old generation of NC controls is 0.001in. (or 0.01 mm), while in modern CNC controls, the increment resolution can reach 0.0001 in. (or 0.001 mm). The number of pulses transmitted to each axis determines the incremental axis position, and the frequency of these pulses regulates the axis speed.

Miscellaneous control functions are a set of on/off signals to implement the control of the speed and direction of the spindle rotation, control of coolant supply, selection of cutting tool, automatic clamping and unclamping, etc.