What is Direct Numerical Control (DNC)? System, Working and DNC vs CNC

DNC is a system in which one central computer stores part programs and sends them to a group of NC or CNC machine tools over communication lines. The letters stand for two related ideas: direct numerical control, the original 1960s-70s scheme in which the computer fed program blocks to NC machines in real time instead of a tape reader, and distributed numerical control, the modern networked system that stores, manages and transfers programs to CNC machines that have their own computers. Today, when a shop says it has a “DNC system”, it nearly always means the distributed kind.

For CNC itself (how the machine control works, axes, and G and M codes), see the CNC hub page. This page covers only the DNC layer that sits above the machines.

Direct numerical control: the original DNC

Early NC machines had no memory. They read their program from punched paper tape, and the tape reader was the least reliable part of the machine: tapes tore, got dirty and wore out, and editing meant punching a new tape.

Direct numerical control, developed from the late 1960s, removed the tape. A central mainframe or minicomputer held the programs for several machines and sent each machine its blocks, in real time, as it needed them. Two connection methods were used:

• Behind-the-tape-reader (BTR) system: the computer was wired into the machine control at the point where the tape reader’s output entered it. The control could not tell the difference; it simply received blocks from the computer instead of from tape. The tape reader was kept as a back-up.
• Special machine control unit: the standard control was replaced by one designed to take instructions directly from the computer, which gave better communication but cost more.

The weakness was obvious: if the central computer failed, every machine connected to it stopped. When cheap microprocessors put a computer into every machine control in the 1970s and 80s, the CNC machine could store its own programs, and direct NC gave way to distributed NC.

Distributed numerical control: the modern DNC system

In distributed numerical control, each CNC machine has its own computer and memory, and the central computer’s job changes from controlling the machines to managing and delivering programs. If the network or server goes down, a machine carries on with the program already in its memory. The DNC system handles:

• Program storage: one central library of every part program, instead of copies on individual controls, USB sticks and laptops.
• Program transfer: the operator requests a program from the machine, or the programmer sends it from the office.
• Version control and revision tracking: only the approved revision is released; edits made at the machine are sent back and compared with the original before they are accepted.
• Drip feeding: sending a large program to the control in small pieces while it runs, for programs bigger than the control’s memory.
• Reporting: program usage, machine status and production counts, which leads into machine data collection (below).

Components of a DNC system

Drip feeding: why transfer speed matters

A 3D mould or die program generated by CAM can run to hundreds of thousands of blocks, far beyond the memory of many older controls. Drip feeding (also called DNC mode or “running from DNC”) sends the program into a small buffer in the control as the machine consumes it. The link must keep up, or the machine pauses and leaves dwell marks on the surface.

Worked example. A 2 MB mould program (2,000,000 characters) is sent to a CNC.

1. RS-232 at 9600 baud. Each character takes 10 bits (1 start bit, 8 data bits, 1 stop bit), so the line carries 9600 / 10 = 960 characters per second. Transfer time = 2,000,000 / 960 = 2083 s = about 34.7 minutes.
2. 100 Mbit/s Ethernet. 2,000,000 × 8 bits = 16,000,000 bits. Time = 16,000,000 / 100,000,000 = 0.16 s, before protocol overheads. Even at 10 Mbit/s it is 1.6 s.
3. Can 9600 baud keep up while drip feeding? Suppose the program is made of 1 mm moves of about 30 characters per block, cut at a feed of 3000 mm/min (50 mm/s). The machine then needs 50 blocks per second, or 50 × 30 = 1500 characters per second. The 9600 baud line delivers only 960, so the machine starves and stutters. At 19,200 baud (1920 characters per second) it just keeps up; Ethernet has hundreds of times the margin.

This is why old serial DNC links were run at 19,200 baud or faster for drip feeding, and why controls with Ethernet and large memory have made drip feeding far less of a problem.

NC vs CNC vs DNC

DNC and CNC are not alternatives: a modern DNC system is a network of CNC machines. The NC procedure is the same with or without DNC; DNC only changes how the finished program reaches the machine.

Advantages and disadvantages of DNC

Advantages

• One controlled program library: fewer mix-ups with old revisions or wrong programs.
• No tapes, USB sticks or manual retyping, so less time and fewer errors at the machine.
• Very large CAM programs can run by drip feeding.
• Management gets data on program use and machine activity.
• Programs are backed up centrally, so a failed control or memory loss does not lose the programs.

Disadvantages

• Cost of the server, software, network and machine interfaces.
• Old machines need adapters, and every control has its own serial settings to configure.
• Network faults can interrupt a drip-fed job part-way through.
• In the original direct NC, a computer failure stopped every machine.
• Connecting machines to a network brings cybersecurity and access control duties.

DNC today: machine data collection, MTConnect and OPC UA

Modern DNC software is usually sold together with machine data collection (MDC) or machine monitoring. Instead of only sending programs down to the machine, it reads data back: spindle running or idle, alarms, part counts, cycle times and feed override. This gives overall equipment effectiveness (OEE) figures and shows where machine time is lost.

Two open standards are widely used to read this data from CNC controls:

• MTConnect: a free, open standard for reading data from manufacturing equipment in a common vocabulary, started by the US machine-tool builders’ association (AMT) and published as an ANSI standard.
• OPC UA: an industrial communication standard from the OPC Foundation (IEC 62541), with a companion specification for machine tools used by the machine-tool industry’s umati initiative.

Controller makers also have their own interfaces. Together these turn the old DNC link into part of a connected, Industry 4.0 style shop floor.

DNC is part of the CAD/CAM and computer-integrated manufacturing syllabus in the AICTE model curriculum, and NPTEL has lectures on computer-aided manufacturing.

FAQs

What is a DNC machine?

A DNC machine is an NC or CNC machine tool connected to a central computer that stores its part programs and sends them over a communication link. The term refers to the connection, not to a different kind of machine tool.

What is the difference between direct and distributed numerical control?

In direct numerical control, a central computer fed program blocks to NC machines in real time, replacing the tape reader. In distributed numerical control, each CNC machine has its own computer and the central system stores, manages and transfers programs.

What is the difference between DNC and CNC?

CNC is a single machine controlled by its own built-in computer. DNC is a central computer and network that supplies programs to many CNC machines. Most DNC systems today are networks of CNC machines.

What is drip feeding in DNC?

Drip feeding sends a program to the CNC control in small pieces while it is running, so the machine can execute programs larger than its own memory. The link must deliver characters faster than the machine uses them.

Is DNC still used today?

Yes. Distributed numerical control software is widely used to manage programs on CNC shop floors, usually together with machine data collection using standards such as MTConnect and OPC UA.

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