Numerical Control Programming
Chapter 1
What Is Numerical Control & What Does It Do?

Direct Numerical Control

Direct Numerical Control (DNC), a forerunner to CNC, is similar to it. Initially, DNC utilized a large mainframe computer connected to several N/C machines, controlling the operation of all of them simultaneously. A program for each N/C machine is loaded into the mainframe computer, and the computer feeds the instructions to each N/C machine as needed. Direct Numerical Control has one major drawback: when the mainframe computer is down, all of the N/C machines are down too. The programs are all in the mainframe, so none of the N/C machines could run independently. Mainframe computer down time is therefore particularly expensive.

After the advent of CNCs, the concept was completely changed. The programs colud now be generated in a micro, mini, or mainframe computer and transfered (or downloaded) in their entirety (rather than one instruction at a time) to the CNC machines directly from the computer. This process became known as Distributed N/C (also DNC). Hence DNC can refer to either Direct N/C or Distributed N/C.

As PCs began to populate the industrial workplace from the engineers' or designer's desktops to "hardened" PCs on the shop floor, the nature of DNC again changed. Local area networks (LANs) began to appear and the larger, more expensive mainframe computers were soon replaced by PCs with large data storage capacity. These "servers" (as Internet users know them) were used to store the CNC files and provide them to the requesting "client" systems. Now, using the Internet, CNC data files can be stored literally anywhere in the world. However, things are not all that global yet; the CNC file still must still be saved in the proper "dialect" for the controller to understand it.

Drip Feeding

CNC program file sizes can become quite large; this is especially true in regards to CAM-generated programs. Most CAM applications use a brute-force method of generating tool paths instead of the more elegant use of "canned cycles." Canned cycles are simply internal controller subroutines for generating common machined features such as drilled holes, rectangular or circular pockets, etc. (Canned cycles are covered later in great detail.) On a typical CNC machine, a manual programmer can use four lines of text to accomplish the same task as a CAM application would in 1000 lines of point to point code. This is why some complex mold programs are often larger than ten megabytes (10Mb). There is no way to fit such large programs into the memories of older CNC machines.

The fix for this CAM code-bloat file size problem is to send the CNC data into the target controller a few lines at a time. This allows huge progams to fit into a controller's small memory space. A local device can be dedicated for use to "drip feed" data into the controller. years ago, these drip feeders would more than likely be a paper tape reader; these days one will typically find an older PC serving this function.

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Updated Jan. 9, 2002
Copyright © 1988-2002 by George Stanton and Bill Hemphill
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