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DNC or direct numerical control is a tool that has gained popularity over the past few years in part because of CAD/CAM's success with 3D contours. The huge cutter path files combined with relatively small memories in CNC controllers necessitates the "spoon-feeding" of files as they are machined. Though much work has been done to optimize DNC to support high feed rates, there is yet a better alternative for many applications. This is an introduction to the ultimate DNC--the elimination of DNC! This is an introduction to direct CNC networking (DCN).
What's The Difference?
DNC consists of three components: the CNC on the machine tool, the computer and the serial (RS-232) line connecting them. Although these three components are the essence of DNC, there is more to the process in a typical environment. Getting the data from the CAD/CAM department to the DNC system can be a significant task. In the simplest case, the program data is simply carried from the CAD/CAM room to the DNC PC by the machine operator or the programmer. This is often referred to as "sneaker-net" or "Nike-net," because of the extra footsteps required to "network" the components in the communications chain. More streamlined operations often connect the CAD/CAM computer system directly to the DNC PC using computer networking, a high speed communications link.
For the sake of clarity, it might help to represent the DNC link as a block diagram representation.
DCN entirely eliminates the DNC PC as well as the serial cable. Rather, the CNC control is connected right onto the same network as the CAD/CAM computer systems. The obvious benefits are the dramatic improvement in data-flow speed and the elimination of components in the process.
The throughput capacity for Ethernet is one million characters per second, versus the typical rate of serial communication at 960 characters per second at 9600 baud. This is nominally a 1,000-time improvement in data transfer speed!
Not all CNCs are capable of direct network connections, though the number of CNCs supporting networking is rising quickly. Moreover, controls with proprietary architectures are more difficult and expensive to network. Controls utilizing variations on "open systems" architecture--commonly based on personal computer standards--are generally easier to network at lower cost. Prices for networking of CNC controls may vary from $0 (included as standard equipment) to as much as $12,000 or more per machine!
Regardless of the initial cost, DCN should strongly be considered for complex 3D surface milling applications. As accuracy requirements become more stringent, data density increases and file sizes grow, conventional DNC approaches may still prove to be the limiting factor in the execution of complex part programs.
Optimizing The Work Flow
Before going further, let's review the work flow from the programming stage to finished part. Using DNC (with a "sneaker net" connection to CAM), the work flow from CAD/CAM to machined part is:
- Copy from CAD/CAM computer to floppy disk,
- Compress and preprocess the cutter path for efficient DNC (optional),
- Walk to machine,
- Copy from floppy disk to DNC PC,
- Initiate DNC drip feed, and
- Start machine execution.
- Using a DCN, the work flow from CAD/CAM to machined part is:
- Start machine execution.
- You can see that up to 85 percent of the steps can be eliminated with DCN.
Optimizing The Data Flow
DNC requires two levels of optimization--the one-time optimization of the DNC physical link and setup, and the daily optimization of the data for each DNC transmission. For the physical link, we check and adjust the following:
- Cable integrity and shielding,
- Baud rate,
- Error correction and recovery, and
- Data format.
- Each DNC transmission may require the following steps to optimize its flow:
- Data preprocessing, and
- Data compression.
DCN entirely eliminates all the above steps for optimization, and also makes operation more convenient. Networking is preset for cable integrity, baud rate, protocol and error correction. While the RS-232 link must be checked for integrity in lengths even less than 50 feet, networking is designed and proven to be error free at lengths up to one kilometer. With networking, data format is no longer an issue, because networking provides ample bandwidth for the most cumbersome of data formats, even with excess comments in the program lines.
Optimizing The Operator
One of the most commonly debated subjects in DNC is operator convenience. Some DNC systems and CNC controllers offer schemes for controlling the entire process from the CNC panel. Still, extra thought and keystrokes should be expected. DCN can make programs in the CAD/CAM system appear as if they are right in the CNC's memory or on its hard disk drive. Networking can provide what are known as "logical drives" where remote networked drives are presented to the CNC operator as another floppy or hard disk drive in his CNC controller.
The convenience of direct and transparent access to cutter paths eliminates another common problem in the shop, programmer and operator interruptions. The "sneaker net" forces the operator to contact the programmer for a new cutter path file. Because both are already interrupted, common distractions like extra coffee, cigarette and restroom breaks can add to the non-production time. There is no doubt that breaks are an important requirement for the staff, but they can be less disruptive to productivity when the machine is set up and running, rather than extending the time that the machine is stopped.
A fully networked shop can build on the CAD/CAM network with an integrated DCN, providing all CNC operators direct access to the cutter paths they require. Note that there is only one continuous network, rather than individual networks for each machine. The network grows as the basic shop communications infrastructure. Networking also provides the foundation for real-time machining, MES (manufacturing execution systems), job tracking and control, and so on--all of which may be handled automatically and transparently through the networking infrastructure.
Perhaps the most important benefit for DCN is the overall simplification of the process. By significantly reducing the number of steps the programmers and operators go through to make the end part, and by making the operator's job easier, there is less likelihood of error. Because there are fewer steps in which to make errors, more and better parts can be produced in less time.
Optimizing The CNC
A few CNCs are now boasting performance to 1,000 blocks per second and beyond. This seems quite fast, yet this speed can still be a limitation if that high performance is available only in limited modes of operation. In virtually all cases, the highest performance is only available in conjunction with preprocessed and compressed cutter paths. But the high performance is really most needed when the data is not preprocessed or compressed! Besides, preprocessing and compression are simply offloading work that the CNC should do to a different component in the work flow, either to the operator in a separate program, or to the DNC program that is already one of the key bottlenecks in the data flow. Performance of 1,000 blocks per second or more should be available to all modes of operation when 3D surface milling.
In the case that 1,000 blocks-per-second performance is available, that performance can seldom be realized unless there is a fast data path for the CNC. DNC at 38,400 baud, the fastest commonly found today, can supply only 1,000 lines of data that are 3.84 characters long! This does not allow for multiple axis moves with feed rate variations, even when the data is compressed. Complex three-, four- and five-axis contours require a faster data channel than RS-232 can supply. Networking is that channel.
Expanded memory capacities are often offered as an alternative to high speed communications. But how much memory is enough? This author has seen cutter path files exceeding 100 MB. Even at today's lower prices for memory, 100 MB is a large investment. Alternately, many CNCs have for years been able to run from files stored on their hard disk drives. Recently, a tool called "remote file execution" has been added to some controllers. This allows one CNC program to call another "remote file," either when completed or as a subroutine. This can allow continuous operation from one or many files of virtually unlimited length. The advantage that DCN adds to this feature is the "logical drive," the appearance of remote networked drives as if they are a part of the CNC controller itself.
While we're on the subject of controls, there are several other capabilities you should consider within the context of DCN communications. For one, whenever dealing with large data files, mid-file starts become a necessity. The control should make them as simple and efficient to execute as possible. That includes easy location of the desired startup point, and application of preparatory data to be sent before resuming the machining process. Graphics are often an enhancement used to help locate the position of interruption and verify the preparatory data. The DCN control may also include tools to remember the position of interruption and quickly relocate it.
Another improvement in efficiency can be provided if the CNC control includes "look ahead." This functionality is being increasingly supported directly by the CNC, without a performance penalty. What look ahead can do is to automatically prevent overshooting and undershooting at areas of abrupt surface change. This is in contrast with "manual look ahead" commonly used in many shops, where the operator looks at the job surface, and manually slows the feed rate in detail areas. Automatic look ahead can provide accurate surfaces without demanding that the operator attend the feed rate override selector. Time savings for accurate parts with look ahead can be dramatic.
Optimizing Your Profits
DCN can improve your profit picture. You can eliminate the PCs used for DNC, eliminate their maintenance and floor space, and most important, you can eliminate the extra steps required for those DNC PCs, reducing your likelihood of operator errors.
Costs for DCN versus DNC can vary widely, depending on control manufacturer, existence of a CAD/CAM network in place already, network-ing topology and software, and so on. In general, though, the cost for networking a CNC that is PC based will be less than the cost to add a personal computer near the PC for DNC.
But the real issue is not the initial expense; it's the ongoing cost of operation.
DCN pays for itself by significantly reducing the time operators spend in non-productive tasks. Dan Riedel of Riedel Tool & Machine (Cudahy, Wisconsin) estimates that networking his eight CNC mills and lathes saves his job shop 15 to 20 hours per month over the DNC they used previously. At $50 per hour, those 20 hours can be a supplement of $1,000 each month. Dave Herman, CAD/CAM manager at Estee Mold & Die (Dayton, Ohio) states that DNC can easily cost ten hours per day in a shop like theirs with ten CNC mills when compared with DCN. Dan Schurr of Modern Mold (Grand Rapids, MI) states that networking improves individual machine productivity from ten to 30 percent. Averaging that to 20 percent, over a 50 hour workweek at $50 per hour, this could mean a $25,000 improvement in productivity per machine per year!
DCN can be challenging to implement in some cases, but can also quickly pay big dividends. Because DCN is still fairly new, it may be difficult to find the right answers. Various networking media (thin Ethernet, thick Ethernet, twisted pair, Token Ring, and so on) and network protocols (Novell ODI, Microsoft NDIS, Lantastic, TCP/IP, NFS, and so on) can be confusing. Within the general computer industry, though, networking is so prolific that solutions for most problems already exist. The only insurmountable problem for DCN is the potential incompatibility with proprietary CNC controllers. In these cases, an efficient DNC system may be the best that is practical. This can still be optimized by networking the DNC computer.
But where DCN can be implemented, it's worth the effort. DCN is available today for many leading CNCs, and it will quickly pay for itself through greater productivity of the system, your CAD/CAM and its staff, your machine operators and your CNC machinery.