Control or Distributed Numerical Control. The first abbreviation -Direct Numerical Control - often means connection between one external computer and one CNC system, while Distributed Numerical Control identifies connection between one external computer and several CNC systems. In both cases, suitable DNC software has to be installed on the external computer and properly configured. General rule of DNC setup is that settings at CNC and external computer must be the same.
In DNC operation, the program is processed from the external device and is not registered in CNC memory. The greatest advantage of running a program in DNC mode is that its length is only limited by the available hard drive space on the host computer.
Capacity of modern hard drives far exceeds even the most demanding CNC applications. Programs of virtually any length can be executed from an external computer in DNC mode. Program editing, if necessary, is also done at the external computer, not at the CNC unit.
Some CNC machines, even some very recent ones may still use the description TAPE. Paper or mylar tape had been used in the early years of numerical control as the only external source available, but it’s not used anymore. The description tape is more nostalgic than practical, and it means any external device.
MEM or MEMORY Mode Selection
In memory mode, the part program has to be registered in the control memory storage area first. Once registered, it can be processed to run a production job. Keep in mind that memory capacity is limited, depending on the control. CNC memory is not intended for permanent storage of part programs. See page 21 for more details.
EDIT Mode Selection
Selecting the EDIT mode enables the CNC operator to access and edit any unprotected programs stored in the CNC memory.
The actual editing process depends on the control system, but generally it includes the following steps:
▪Select EDIT mode
▪Select program to edit
▪Set the cursor where the change should take place
▪Make the required change
▪Exit EDIT mode
Control manual describes each step in more detail.
There are three modes of editing, using the control panel Edit keys (see page 26 for details):
▪ALTER
▪INSERT
▪DELETE
The ALTER key will change the currently highlighted command or commands.
The INSERT key will add a new command or commands at the current highlighted command.
The DELETE key will remove the currently highlighted command or commands.
In manual operation, an axis has to be selected before a motion switch is applied. Typically, and axis must be selected before zero return motion, manual rapid motion, jog feed, and incremental jog feed.
Most operation panels show not only the three major axes XYZ, but also the fourth axis, even if it is not installed. On vertical machining centers, if the fourth axis is installed, it will generally be the A-axis. Horizontal machining center will always have the fourth axis, specified as the B-axis.
For multi axis machine tools, the Select Axis switch will always include all present axes of the machine. For example, a CNC boring mill will have the following axis selection:
X - Y - Z - W - B
or
X - Y - Z - W - 4th
As the title name suggests, override implies a change. In CNC operations, override means a temporary change of machine setting or program data. There are three independent override modes:
▪Rapid override
▪Feedrate override
▪Spindle override
Each override mode has an available rotary switch and can be used independently from the others. The main reason for override switches is the added control at the machine. Override switches are commonly used in setup, running the first part or changing programmed speeds and feeds on a temporary basis.
In the part program, any rapid tool motion is called by the G00 command. G00 has no feedrate associated with it; how fast the tool moves depends on the machine specifications - actual rapid rate cannot be programmed. Modern CNC machining centers can rapid at a fairly common rate of 60 000 mm/min (2360 ipm) or even higher. As much as this speed is welcome for production, it is generally too fast for setup and first part run. Using the override switch to slow down the rapid motion is a welcome relief, when needed.
Rapid override switch indicates the percentage of the maximum amount of rapid motion that the machine is capable of
Generally, CNC machining centers that do not exceed 30 000 to 38 000 mm/min (1180-1500 ipm) have a selection range different from machines with ultra high rapid motion. Compare the two rotary switches available from the CNC operation panel:
Machine manufacturer may choose a different range. What is important to understand is that machines with very high rapid rate need finer adjustments. A comparative example will follow, but let’s look at the F0 selection first.
F0 Selection
On some machines, there is a selection that does not specify the actual percentage of the rapid rate reduction and is identified as F0. If available, this will be the lowest percentage, but a percentage that can be changed by system parameter. Other percentages are fixed. For example, if 25% is the lowest fixed setting, you can make the F0 setting to be 20%, 15%, 10%, and so on.
Rapid Override Comparison
Consider the difference between two machines - as an example, one with a maximum rapid rate of 30 000 mm/min (1180 ipm) and another with a fast rapid rate of 60 000 mm/min (2360 ipm):
Depending on the setting of F0 (10% used) the table shows how fast the CNC machine will rapid with different override settings.
Rapid Override On Lathes
Most CNC lathes