What Is a "CNC"?

CNC stand for Computer Numerical Control, and refers specifically to the computer control of machine tools. The main purpose of CNC Milling Machines is to repeatedly manufacture complex parts in metal as well as other materials, using a specially coded program. This specially coded program that is used by CNC Milling machines is written in a notation called G-code. G-codes represent specific CNC Milling functions in alphanumeric format. CNC Milling was developed in the late 1940's and early 1950's by the MIT Servomechanism Laboratory.

About CNC

Computer Numerical Control (CNC) Milling is the most common form of CNC machining, but you also get CNC turning and profiling. CNC milling machines are very useful as they can perform the functions of drilling and often turning. CNC Milling machines are classified according to the number of axes that they possess. Axes are labeled as X and Y for horizontal movement, and Z for vertical movement. It is also possible to add other axes like a 4th and 5th axis for more complex jobs. Curves can be cut as easily as straight lines, which was impossible before CNC machines were invented. complex 3D structures are much easier to produce, and the number of machining steps in the process has decreased substantially. The machining process of CNC's is changing and improving by the day. New methods of machining are always coming out to improve tool life and production time. There are generally 3 main people responsible for the successful operation of a CNC machine. A programmer, a setter and an operator.


The programmer is responsible for writing and maintaining programs. A broad knowledge of G-codes and M-codes is needed in order to be a successful CNC programmer. Also, in larger and more advanced workshops, CNC programmers enlist the help of CAM software in order to make more advanced programs.


The setters job is to ensure that the tools are all set up according to the setting sheet provided by the programmer. The setter needs to measure the tool heights and make sure that all the tools are long enough and have the correct diameters. The setter is also responsible for the clamping of the work piece. It is the setters duty to make sure that the work piece is clamped correctly and securely in place. The setter is also responsible for picking up the starting co-ordinates for the CNC machine. There are various methods of picking up the machine starting co-ordinates.


The operators primary function is to load and offload the job. The operator is also responsible for making sure that the cutters are not going blunt and that the inserts are changed when needed. If something goes wrong during machine operation, it is generally the operators responsibility to ensure that the machine is stopped as soon as possible before major damage is caused to the work piece or the machine.

The Fabrication Process

With the use of CNC milling machine the fabrication process of the materials have been trimmed down to just a couple of steps. CNC Milling machines now days are driven directly from computer software created by CAD software packages. With the use CNC Milling machines the assembly of parts can go from brief designs without any intermediate paper drawing works being required. In one sense, with the use of CNC milling machines industrials tasks are done must easier and much faster and the production cost of the company is also lessening out. CNC machines may be said to represent special industrial robot systems, as they are programmable to perform any kind of machining operation, within certain physical limits, like other robotic systems.

CNC Milling drill cycles

In order to program a drilling cycle for your program, there are a few codes that must be used in order to ensure a successful drilling cycle.

Plain drilling cycle (G81)

G81 G98 Z-20. R5. F60.
G81 G99 Z-20. R5. F60.
This is a straight forward drilling cycle. The machine will rapid down to 5mm above the job (R5.) and then begin feeding with a federate of 60 (F60.) down to the depth of 20mm (Z-20.), after it has reached the required depth, it will rapid back up to the Z-level that it was on at the beginning of the cycle (G98) or it will rapid up to the R-level (G99 and R5.)
G81 instructs the machine that a plain drilling cycle is to be used. There are other drilling and boring cycles which can be used instead of plain drilling cycles, they are for more advanced uses like deep hole drilling.

Spot drilling cycle (G82)

G82 G98 Z-3. R5. P1000 F100.
The spot drilling cycle works similarly to the plain drilling cycle, except it has an added code. The spot drill will rapid to 5mm (R5.) and then feed down to 3mm depth (Z-3.), after that it will stay there for the dwell time (P1000) and then continue in the same manner as the plain drilling cycle. Note that the P-value in the spot drilling cycle must not have a point.


Peck drilling cycles (G83 and G73)

G83 G98 Z-40. R5. Q2. F100.
These cycles are specifically used when a deep bore is required (40mm + ) and your material forms long shavings that need to be broken. The peck drilling cycle works the same as the Plain drilling cycle except that at each 2mm depth, it retracts and comes back in order to break shavings and prevent them from tangling and causing problems (Q2.). G83 retracts the drill all the way out of the bore, where as G73 only retracts the drill a little bit before it returns to drilling.

Boring cycle (G86)

G86 G98 Z-20. R5. F150.


The boring cycle is specifically for use with boring bars because it instructs the machine to stop spinning once it has reached the required depth, and retract with the spindle not spinning. This helps to prevent lines on the bore, in order to get a better finish. It works exactly the same as the plain boring cycle except for the way it retracts.