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"Drip Feeding" is a method of sending a CNC program to a CNC machine a few instructions at a time, and there is a handshake process so that the sender knows when to stop sending and when to send more data to the CNC.
The general reason for using "drip feed" is that the CNC program is bigger than the amount of available memory in the CNC for storing programs.
In a general way this is how the process works.
The operator puts the CNC machine in DNC mode. Now the CNC is ready to receive and process incoming CNC instructions.
In the CNC machine is a "first-in first-out" (FIFO) buffer that can store at least a few lines of CNC code. This buffer fills from the RS232 input up to a certain maximum limit, and then the sender is told to stop via a handshake signal. Because of time delays in communicating between the sender and the CNC the maximum limit is set less than the buffer capacity.
When data is in this FIFO the CNC extracts instructions from the buffer as needed. When the FIFO drops below a certain number of characters, then the handshake signal tells the sender to send more data. This continues until the end of the CNC program. Such a program might be 10 to 100,000,000 bytes, but the CNC program memory space might be only 100,000 bytes.
Sometimes in DNC the machine may run several days on one program.
Fanuc machines tend to have a very small FIFO whereas a HAAS machine will use a very large percentage of available CNC program memory space for the FIFO.
This is a quick overview.
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