Direct numerical control (DNC), or distributed numerical control, is a key manufacturing method for networking CNC machine tools. When CNC controllers have insufficient memory for complex programs, the machining instructions are stored on a separate computer and sent directly to the machines one block at a time. This setup allows distribution of programs to multiple machines from one source. DNC software is typically provided by control manufacturers or third-party vendors. It is essential for running CAM programs on CNC controls. Wireless DNC is increasingly common, especially in industries with heavy sheet metal work, including the automotive, appliance, and aerospace sectors.
History
1950s-1970s
Programs had to be walked to NC controls, generally on paper tape. NC controls had paper tape readers precisely for this purpose.1 Many companies were still punching programs on paper tape well into the 1980s, more than twenty-five years after its elimination in the computer industry.
1980s
The focus in the 1980s was mainly on reliably transferring NC programs between a host computer and the control. The Host computers would frequently be Sun Microsystems, HP, Prime, DEC or IBM type computers running a variety of CAD/CAM software. DNC companies offered machine tool links using rugged proprietary terminals and networks. For example, DLog offered an x86 based terminal, and NCPC had one based on the 6809. The host software would be responsible for tracking and authorising NC program modifications. Depending on program size, for the first time operators had the opportunity to modify programs at the DNC terminal. No time was lost due to broken tapes, and if the software was correctly used, an operator running incorrect or out of date programs became a thing of the past.
Older controls frequently had no port capable of receiving programs such as an RS-232 or RS-422 connector. In these cases, a device known as a Behind The Reader or BTR card was used. The connection between the control's tape reader and the internal processor was interrupted by a microprocessor based device which emulated the paper tape reader's signals, but which had a serial port connected to the DNC system. As far as the control was concerned, it was receiving from the paper tape unit as it always had; in fact it was the BTR or Reader Emulation card which was transmitting. A switch was frequently added to permit the paper tape reader to be used as a backup.
1990s to present
The PC explosion in the late 1980s and early 1990s signalled the end of the road for proprietary DNC terminals. With some exceptions, CNC manufacturers began migrating to PC-based controls running DOS, Windows or OS/2 which could be linked in to existing networks using standard protocols. Customers began migrating away from expensive minicomputer and workstation based CAD/CAM toward more cost-effective PC-based solutions. Users began to demand more from their DNC systems than secure upload/download and editing. PC-based systems which could accomplish these tasks based on standard networks began to be available at minimal or no cost. In some cases, users no longer needed a DNC "expert" to implement shop floor networking, and could do it themselves. However, the task can still be a challenge based on the CNC Control wiring requirements, parameters and NC program format.
To remain competitive, therefore, DNC companies moved their offerings upmarket into DNC Networking, Shop Floor Control or SFC, Manufacturing Execution Systems or MES. These terms encompass concepts such as real-time Machine Monitoring, Graphics, Tool Management, Traveler Management and Scheduling. Instead of merely acting as a repository for programs, DNC systems aim to give operators at the machine an integrated view of all the information (both textual and graphical) they require in order to carry out a manufacturing operation, and give management timely information as to the progress of each step. DNC systems are frequently directly integrated with corporate CAD/CAM, ERP and Computer-aided Process Planning CAPP systems.
Special protocols
A challenge when interfacing into machine tools is that in some cases special protocols are used. Two well-known examples are Mazak's Mazatrol and Heidenhain's LSV2 protocol. Many DNC systems offer support for these protocols. Another protocol is DNC2 which is found on Fanuc controls. DNC2 allows advanced interchange of data with the control, such as tooling offsets, tool life information and machine status as well as automated transfer without operator intervention.
Machine monitoring
One of the issues involved in machine monitoring is whether or not it can be accomplished automatically in a practical way. In the 1980s monitoring was typically done by having a menu on the DNC terminal where the operator had to manually indicate what was being done by selecting from a menu, which has obvious drawbacks. There have been advances in passive monitoring systems where the machine condition can be determined by hardware attached in such a way as not to interfere with machine operations (and potentially void warranties). Many modern controls allow external applications to query their status using a special protocol. MTConnect is one prominent attempt to augment the existing world of proprietary systems with an open-source, industry-standard protocol using XML schemas. The end goal being to achieve higher levels of manufacturing business intelligence and workflow automation.
Alternatives
Smaller facilities will typically use a portable PC or laptop to avoid the expense of a fully networked DNC system. In the past Facit Walk Disk and a similar device from Mazak were very popular.
Footnotes
References
Patton, William J. (1972). Numerical Control: Practice and Application. Reston, Virginia: Reston Publishing Company, Inc. p. 8. ISBN 0-87909-564-4. 0-87909-564-4 ↩