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Teaching CNC with the Key Concepts approach - part nine

Part nine - Key concept number eight: You must understand the three basic modes of operation

Here are some links that allow you to review other parts of this article:

When a person sees the control panel of a CNC machine for the first time, they can be easily intimidated. Admittedly, there are many new functions to learn. And in the last key concept, you introduced them to the most popular buttons and switches. In this key concept, you can help them become more organized. I like to begin by pointing out that each button or switch can be easily categorized into one of three basic modes. And these three modes are the subject for Key Concept number eight.

Point out that while most mode switches have more than three actual positions, there are only three basic modes - manual mode, manual data input (MDI) mode, and program operation mode.

I like to compare the mode switch of a CNC machine to the function selector for a stereo system. This switch might include functions like CD, Tape, and Tuner. Students probably know that if the function selector of a stereo system is in the CD mode, buttons and switches related to the tuner will not be active. This is exactly the case with a CNC machine. If the mode switch is in the wrong position, about the worst that can happen is that the machine won't respond to the given action. This is important for a novice to know.

This means that the mode switch will always be the first switch an operator must set prior to performing any task on the machine. And if the machine doesn't respond to the action, it is likely that the switch is in the wrong position.

Manual mode

On the machine's mode switch, this mode includes jog (often called manual), handwheel, and zero return.

Explain that in this mode, the machine behaves the most like a conventional machine (mill or lathe, for example). Press a button and the spindle starts. Turn a handwheel and an axis moves. And so on. This mode is most commonly used during setup - when getting the machine ready to run production.

The jog (again, also called manual) mode is used most commonly to allow axis motion through the use of a push-button or joystick. The axis to be moved, the direction (plus or minus) and the motion rate are first selected. When the motion is activated the selected axis will move in the selected direction at the selected rate.

Like the jog mode, the handwheel mode provides access to axis motion. But motion can be more precisely controlled with the handwheel. Three rates will be available, times one (0.0001 inch per increment), times 10 (0.0010 inch per increment), and times 100 (0.0100 inch per increment). This allows quick motion when possible - and very slow movement when caution is required.

The zero return mode provides a way to send each axis to its reference position - which is commonly close to the plus over-travel limits.

With each of these mode switch positions, other machine functions may also be accessible in this mode. If the machine provides manual control of the spindle, for example, it can be activated from this mode switch position.

Do point out, however, that machine tool builders vary when it comes to how much manual control they provide for machine operation. Some, for example, may provide little or no control of the machine's tool changing device (automatic tool changer or turret). For any function that does not include manual controls, the operator must use the manual data input (MDI) mode in order to perform the function manually.

Manual data input (MDI) mode

On the mode switch, this mode includes the MDI and Edit positions.

In the MDI position, the operator will command actions in a way that is very similar to how the action is commanded from within a CNC program. While it is possible to perform just about any action that is possible in a CNC program, point out that the MDI mode is most helpful when there is a need to perform a manual function that is not possible in the machine's manual mode.

For example, most machining centers provide no way to activate the automatic tool changer in the manual mode. Instead, the operator must use the MDI mode, commanding the tool station number (T05, for example) with an M06. When activated, the control will execute the command one time - just as if it came from a CNC program. Again, examples of things that must be manually activated with MDI mode include tool changing, spindle activation, and pallet changing.

Point out that the Edit mode switch position allows manipulation of the CNC program. Words and commands can be inserted, altered, and deleted.

The program operation mode

For current machines, the only mode switch position for this mode is called Auto (or Memory). Explain that it is in this mode switch position that CNC programs are run.

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