Types of instruction CNC

A line in a G-code file can instruct the machine tool to do one of several things.

Movements

The most basic motion for a controller is to move the machine tool along a linear path from one point to another.^[1] Some machine tools can only do this in XY, and have to accept changes in Z separately. Some have two further axes of rotation to control the orientation of the cutter, and can move them simultaneously with the XYZ motion. Lately 4, and 5 axis machines have become popular. The 2 additional axies allow for the work surface or medium to be rotated around X and Y. For example, a 4-axis machine can move the tool head in XY and Z directions, and also rotate the medium around the X or Y axis, similar to a lathe.This is called the A or B axis in most cases.

All motions can be built from linear motions if they are short and there are enough of them. But most controllers can interpolate horizontal circular arcs in XY. See CNC circular arc motions for further details.

Lately, some controllers have implemented the ability to follow an arbitrary (NURBS) curve, but these efforts have been met with skepticism since, unlike circular arcs, their definitions are not natural and are too complicated to set up by hand, and CAM software can already generate any motion using many short linear segments.

Tool changes

Originally there would be a G-code instruction telling the machine tool to stop so that a human operator could remove the cutting tool from the chuck and insert a new one. Modern machine tools have a magazine of different tools which they can change themselves pneumatically, hydraulically, and electromechanically.

On newer models of CNC Machines such as Mori Seiki designs, depending on which tool would be used, assuming it's tool 4 the procedure for an idle machine tool change would be;

MDI,

T4; M6;

There are related instructions such as setting the spindle speed, and turning on or off the coolant.

Drilling

A tool can be used to drill holes by pecking to let the swarf out. Using a special tapping tool and the ability to control the exact rotational position of the tool with the depth of cut, it can be used to cut screw threads.

Drilling cycles

A drilling cycle is used to repeat drilling or tapping operations on a workpiece. The drilling cycle accepts a list of parameters about the operation, such as depth and feed rate. To begin drilling any number of holes to the specifications configured in the cycle, the only input required is a set of coordinates for hole location. The cycle takes care of depth, feed rate, retraction, and other parameters that appear in more complex cycles. After the holes are completed, the machine is given another command to cancel the cycle, and resumes operation.

Parametric programming

A more recent advancement in CNC interpreters is support of logical commands, known as parametric programming. Parametric programs incorporate both G-code and these logical constructs to create a programming language and syntax similar to BASIC. Various manufacturers refer to parametric programming in brand-specific ways. For instance, Haas refers to parametric programs as macros. GE Fanuc refers to it as Custom Macro A & B. While Okuma refers to it as User Task 2. The programmer can make if/then/else statements, loops, subprogram calls, perform various arithmetic, and manipulate variables to create a large degree of freedom within one program. An entire product line of different sizes can be programmed using logic and simple math to create and scale an entire range of parts, or create a stock part that can be scaled to any size a customer demands.

Parametric programming also enables custom machining cycles, such as fixture creation and bolt circles. If a user wishes to create additional fixture locations on a work holding device, the machine can be manually guided to the new location and the fixture subroutine called. The machine will then drill and form the patterns required to mount additional vices or clamps at that location. Parametric programs are also used to shorten long programs with incremental or stepped passes. A loop can be created with variables for step values and other parameters, and in doing so remove a large amount of repetition in the program body.

Because of these features, a parametric program is more efficient than using CAD/CAM software for large part runs. The brevity of the program allows the CNC programmer to rapidly make performance adjustments to looped commands, and tailor the program to the machine it is running on. Tool wear, breakage, and other system parameters can be accessed and changed directly in the program, allowing extensions and modifications to the functionality of a machine beyond what a manufacturer envisioned.