Take control

How CNC systems of the future will play an even bigger part in production. Martin Oakham reports.

We have seen phenomenal developments in computers over the past decade, which now provide even more power than ever.

CNC manufacturers have capitalised on this, and today’s CNC systems are simply computers with an emphasis on precision Input/Output (I/O), where the majority of the processing power is tied up in conditioning control signals and monitoring the kinematics of the machinery being controlled.

CNC systems — used wherever machinery needs controlling — range from thousands of outputs to just a few outputs in complexity. Most systems control storage/retrieval equipment, transfer lines and machine tools, the latter being the most demanding due to the precision needed in today’s tool cutter paths such as ball-nose end mill compensation in continuous five-axis applications.

Here, in order to properly compensate for a ball-nose end mill as the part or the tool pivots, the CNC must be able to dynamically adjust the cutter compensation vector in X, Y and Z simultaneously in order to keep the tool’s contact point at a constant.

When rotary axes pivot the tool, its length offset that normally affects only the Z-axis now has components in X, Y and Z. In addition, tool diameter offsets that normally only affect the X and Y-axes also have X, Y and Z components.

As the tool may be feeding in the rotary axes while it’s cutting, all of these offsets have to be updated dynamically to account for continuous changes in the tool’s orientation. From a mathematical point of view it’s relatively complicated. But for the user it’s as simple as defining the centre point of the toolpath and its speed, all the number crunching being handled transparently by the underlying software-based operating system, just as it is when you press ctrl/P to print from your PC.

The true processing power of a CNC is not always evident. A CNC system that can process part program blocks at a very high rate may not necessarily perform better than one that processes data much slower. Successful high-speed machining, for example, requires more than just fast block processing times — there are other potential bottlenecks that the system has to address, best illustrated by the analogy of a racing car.

First, the driver’s knowledge of the racetrack is important. He/she has to know when a sharp curve is coming in order to slow down just enough to take it safely. CNC look-ahead performs a similar role, giving the CNC advanced knowledge of the sharp curves coming up.

Similarly, how quickly the driver reacts to what other drivers do, and other unpredictabilities, can be compared to the CNC’s servo loop times — including position loop, velocity and current loop.

Skilful braking and accelerating have a significant impact on performance, so how smoothly the driver goes around the track is similar to the way in which programmed bell-type acceleration/deceleration curves give a smoothness to machine toolacceleration. Look-ahead also helps here, because it allows many small acceleration/deceleration adjustments to replace an abrupt directional change.

The number of look-ahead blocks can range from two to hundreds, depending on the controller. The number also depends on factors such as the minimum part program execution time and the acceleration/deceleration time constant, but 15 blocks is probably the minimum acceptable value.

In the past, it was only high-end CNCs that could maintain high accuracy at high speeds. Today, mid-level and low-end CNCs are powerful enough to handle 95 per cent of jobs. It used to be that the CNC was the limiting factor determining the maximum feed rate, but today it’s the design of the machine — one reason for the gradual drive towards ‘Hexapod’ style machining centres. A better CNC won’t deliver more performance if the machine itself is already operating at its limit.

Supported by smarter software, CNC manufacturers are increasingly using data from sensors to determine information about the machining process. For example, voltage fluctuations are commonly monitored to determine if tooling has broken so that the action of replacing it with sister tooling or shutting down the process can be taken.

More sophisticated solutions monitor tool wear over time. Even more sophisticated software uses sensor feedback to achieve varying degrees of adaptive control. For example, tool load compensation is a feature that automatically changes the feed rates based on spindle load parameters set by the operator, reducing cycle times by as much as 30 per cent in addition to increasing tool life.

More recently, CNC developers have been focusing on the ability to connect to the internet and ‘open’ architectures generally. More and more machine shops will use this capability to access tool and customer databases, SPC packages and remote diagnostics, but the chief advantage is that they permit continually enhanced functionality through software upgrades — just like any PC today.

This was summed up by Sal Spada, senior analyst, discrete manufacturing for ARC Advisory Group: ‘Open CNC solutions allow CNC-based machine tool users to deploy a unified automation strategy, which depends on all the computer systems, from engineering design centres to factory floor machine tools, to communicate seamlessly.

‘Integrating distributed plant-wide information systems required to implement real-time process improvements facilitates continuous optimisation of production systems. High-volume production plants as well as medium-scale job shops have another opportunity to make dramatic improvements in productivity, quality, and customer service by leveraging open software solutions.’

GE Fanuc backs this up with the release of its Open Factory CNC family: a product line that combines PC hardware, Windows-based applications, and traditional CNC hardware. The suite includes VisualDocs to manage documents at the machine tool; VisualMEM to gather machine performance information; VisualDNC to download part programs from local area networks; and VisualMOI to integrate machine tool operation with production scheduling and electronic documentation.

Plus, a bolt-on product includes a DuraTouch industrial PC, an enhanced version of Gibbs Shop Floor programming, and CimWorks software tools to expand existing CNCs’ functionality. Data integration promotes integration between CAD/CAM and ERP/planning systems, enabling global B2B for manufacturing from within the machine tool.

The control can send an e-mail to maintenance personnel or the production engineer when faults are observed. Probing and inspection data can be collected and transmitted to SQL or Oracle database for ISO9000 tracking. The part program can be setto send process times and part completions directly to MES or ERP applications.

All of this means that the CNC of the future will be able to provide greater control over processes. It will, for instance, monitor temperatures of machine components, as well as the workpiece, and activate coolant chillers to maintain temperatures. When a part is completed the CNC will use a modem to call or page the operator with information that it is ready for the next job.

Controls will also perform self-diagnostic accuracy tests and automatically adjust to maintain tolerances, as well as maintain historic records that can be used for preventative and predictive maintenance programs.