Control System: Page 3 of 3

 Formula 1 : To find the program length in meters,/when the capacity is known in characters, use the following formula: 

where ... 
Sm = Storage capacity in meters ,Nc = Memory capacity (number of characters) 

Formula 2 : To find the length of program in feet, when the capacity is known in characters, use the following formula: 

where ... 
Sf = Storage capacity in feet Nc = Memory capacity (number of characters)

MANUAL PROGRAM INTERRUPTION 
If a program needs to be interrupted in the middle of processing, the 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, feed hold and the emergency stop. 
• Single Block Operation 
The normal purpose of a program is to control the ma-chine 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 the top down and in the order they appear in the program. Normally, a CNC machine is run in a continuous mode, while the blocks are processed automatically, one after an-other. This continuity is important for production, but not practical when proving a new program, for example. 
To disable the continuous program execution, a Single Block switch is provided on the operation panel. In the single block mode, only one block of the program will be processed each time the Cycle Start key is pressed. On the operation panel, the single block mode can be used separately or in combination with other settings that make program proving faster and more accurate.

• Feedhold 
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. The action ap-plies to all axes active at the time. This feature is convenient for a machine setup or a first part run. Some types of motion restrict the function of the feedhold 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 the motion. The feedhold switch will be illuminated (in red light), as long as it is effective. The CNC programmer can override the feed-hold from within the program, for special purposes.

• Emergency Stop 

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 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. The 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 a cutting tool and the part or fixture. Previously discussed feedhold button is only one option, along with other features. If the emergency stop must be used at all, it should be used 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. 

MANUAL DATA INPUT ( MDI )
A CNC machine is not always operated by the means of a program. During a part setup, the CNC operator has to do a number of operations that require physical movements of the machine slides, rotation of spindle, tool change, etc. There are no mechanical devices on a CNC machine. The handle (Manual Pulse Generator) is an electronic, not a mechanical unit. In order to operate a CNC machine without conventional mechanical devices the control system offers a feature called the Manual Data Input - or MDI. The Manual Data Input enables the input of a program data into the system one program instruction at a time. If too many instructions were to be input repeatedly, such as a long program, the procedure would be very inefficient. During a setup and for similar purposes, one or a few instructions at a time will benefit from the MDI. 
To access the MDI mode, the MDI key on the operation panel must be selected. That opens the screen display with the current status of the system. Not all, but the majority of programming codes are allowed in the MDI mode. Their format is identical to the format of a CNC program in writ-ten form. This is one area where the CNC operator acts as a CNC programmer. It is very important that the operator is trained at least in the basics of CNC programming, certainly to the point of being able to handle the setup instructions for Manual Data Input.