CNC | Control System Options

Optional features on a CNC system are like options on a car.What is an option at one dealership, maybe a standard feature at another. Marketing strategies and corporate philosophies have a lot to do with this approach. Here is a look at some control features that may or may not be classified as optional on a particular system.

Graphic Display

Graphic representation of toolpath on the display screen is one of the most important, as well as sought after, control options. Do not confuse this option with any type of conversational programming, which also uses a graphic toolpath interface. In the absence of a computer assisted programming (CAM), graphic display on the control panel is a major benefit. Whether in monochrome or color, the convenience of seeing the tool motions before actual machining is much appreciated by CNC operators and programmers alike.
A typical graphics option shows machine axes and two cursors for zooming. When the toolpath is tested, individual tools are distinguished by different colors, if available, or different intensity. Rapid motions are represented by a dashed line type, cutting motions by a continuous line type. If the graphics function is applied during machining, tool motions can be watched on the display screen – very helpful for those CNC machines that have dirty, oily and often scratched safety shields.
Upwards or downwards scaling of the display allows for evaluation of a tool motion overall or for detail areas. Many controls also include actual toolpath simulation, where the part shape and the cutting tool can be set first, then seen on the screen.

In-Process Gauging

During many unattended machining operations, such as in manufacturing cells or Agile manufacturing, a periodic checking and adjusting dimensional tolerances of the part is imperative. As the cutting tool wears out, or perhaps because of other causes, the dimensions may fall into the ‘out-of-tolerance’ zone. Using a probe device and a suitable program, In-Process Gauging option offers quite a satisfactory solution. CNC part program for the In-Process Gauging option will contain some quite unique format features – it will be written parametrically, and will be using another option of the control system – the Custom Macros (sometimes called the User Macros), which offer variable and parametric type programming.

If a company or machine shop is a user of In-Process Gauging option, there are good chances that other control options are also installed and available to the CNC programmer. Some of the most typical options are probing software, tool life management, macros, etc. This technology goes a little too far beyond standard CNC programming, although it is closely related and frequently used. Companies that already use the numerical control technology, will be well advised to look into these options to remain competitive in their field.

Figure 5-1 – A typical example of a Fanuc control panel – actual layout and features will vary on different models (Fanuc 16M)

Stored Stroke Limits

Definition of an area on CNC lathes or a cube on CNC machining centers that is safe to work within, can be stored as a control system parameter called stored stroke limit. These stored stroke limits are designed to prevent a collision between the cutting tool and a fixture, machine tool or part. The area (2D) or the cube (3D) can be defined as either enabled for the cutter entry or disabled for the cutter entry. It can be set manually on the machine or, if available, by a program input. Some controls allow only one area or cube to be defined, others allow more.
When this option is in effect and the CNC unit detects a motion in the program that takes place within the forbidden zone, an error condition results and the machining is interrupted. A typical applications may include zones occupied by a tailstock, a fixture, a chuck, a rotary table, and even an unusually shaped part.

Drawing Dimensions Input

An option that seems somewhat neglected, is the programming method by using input of dimensions from an engineering drawing. The ability to input known coordinates, radiuses, chamfers and given angles directly from the drawing makes it an attractive option. This ability is somewhat overshadowed by poor program portability. Such an option must be installed on all machines in the shop, in order to use the programmed features efficiently.

Machining Cycles

Both milling and turning controls offer a variety of machining cycles. Typical machining cycles for milling operations are called fixed cycles, also known as canned cycles. They simplify simple point-to-point machining operations such as drilling, reaming, boring, back boring and tapping. Some CNC systems also offer cycles for face milling, pocket milling, various hole patterns, etc.
CNC lathes also have many machining cycles available to remove material by automatic roughing, profile finishing, facing, taper cutting, grooving and threading. Fanuc controls call these cycles Multiple Repetitive Cycles.
All these cycles are designed for easier programming and faster changes at the machine. They are built in the control and cannot be changed. Programmer supplies the cutting values during program preparation by using an appropriate cycle call command. All processing is done automatically, by the CNC system. Of course, there will always be special programming projects that cannot use any cycles, at least not effectively, and have to be programmed manually or with the use of an external computer and CAM software.

Cutting Tool Animation

Many of the graphic toolpath displays defined earlier, are represented by simple lines and arcs. Current tool position is usually the location of a line or arc endpoint on the screen. Although this method of displaying cutting tool motion graphically is certainly useful, there are two disadvantages to it. The cutting tool shape and the material being removed cannot be seen on the screen, although toolpath simulation may help a bit. Many modern controls incorporate graphic feature called Cutting Tool Animation. If available on the control, it shows the part blank, the mounting device and the tool shape. As the program is processed, CNC operator has a reasonably accurate visual aid in program proving. Each graphic element is identified by a different color, for even a better appearance. The blank size, mounting device and tool shape can be preset for exact proportions and a variety of tool shapes can be stored for repetitive use. This option is a good example of CAD/CAM like features built into a stand-alone control system.

Connection to External Devices

CNC computer (control) can be connected to an external device, usually another computer. Every CNC unit has one or more connectors, specifically designed for interfacing to peripheral devices. The most common device is called RS-232 (EIA standard), designed for communications between two computers. Setting up the connection with external devices is a specialized application. CNC operator uses such a connection to transfer programs and other settings between two computers, usually for storage and backup purposes. Devices other than RS-232 are also available – check with the machine vendor.