If a program needs to be interrupted in the middle of processing, control system offers several ways to do that, using the machine operation panel. The most common features of this type are toggle switches or push buttons for a single block operation, feedhold and emergency stop.
Single Block Operation
The normal purpose of a program is to control the machine tool automatically and sequentially in a continuous mode. Every program is a series of formatted commands – or instructions – written as individual lines of code, called blocks. Blocks and their concepts will be described in the following chapters. All program commands in a single block are processed as a single instruction. The blocks are received by the control system in sequential order, from top down and in the order they appear in the program. Normally, a CNC machine is run in a continuous mode, while all blocks are processed automatically, one after another. This continuity is important for production, but not practical when proving a new program, for example.
To disable continuous program execution, a Single Block switch is provided on the operation panel. In the single block mode, only one program block will be processed each time the Cycle Start key is pressed. On the operation panel, single block mode can be used separately or in combination with other settings that make program proving faster and more accurate.
Feedhold is a special push button located on the operation panel, usually close to the Cycle Start button. When this button is pressed during a rapid, linear or circular axes motion, it will immediately stop the motion. Such action applies to all axes active at the time. This feature is convenient for machine setup or first part run. Some types of motion restrict the feedhold function or disable it altogether. For example, threading or tapping modes make the switch inoperative.
Activating feedhold at the machine will not change any other program values – it will only affect axis motion. Feedhold switch will be illuminated (in red light), as long as it is effective. CNC programmer can override feedhold from within the program, for special purposes.
Every CNC machine has at least one special mushroom shaped push button, red in color, that is located in an accessible place on the machine. It is marked as the Emergency Stop or E-stop. When this button is pressed, all machine activities will cease immediately. The main power supply will be interrupted and the machine will have to be restarted. Emergency stop switch is a mandatory safety feature on all CNC machines.
Pressing the emergency stop button is not always the best or even the only way to stop a machine operation. In fact, the latest controls offer other features, far less severe, designed to prevent a collision between cutting tool and part or fixture. Previously discussed feedhold button is only one option, along with several other features. If the emergency stop must be used at all, it should be used only as the last resort, when any other action would require unacceptably longer time. There is no need for panic, if something does go wrong.
For some machine actions, the effect of Emergency Stop is not always apparent. For example, the spindle requires a certain time for deceleration to stop.