Taking control

Today’s fully-networked, multi-axis machine tool shares more in common with a high-end PC than a traditional mill. And, says Martin Oakham, there is endless potential for further development.

With the rapid advances in technology and the integration of PC-based platforms, CNC machines and their underlying control architecture are undergoing a major transformation.

The availability of off-the-shelf PC software applications has forced the more traditional control manufacturers to re-examine their core competencies and differentiate their products by introducing innovative software features within the control architecture itself.

At the same time, CAD/CAM developers are striving to undertake much of the toolpath conditioning functions traditionally considered to be the domain of control manufacturers. This marks the start of what will be an interesting phase in machine tool evolution.

Today’s ‘Open’ CNC controllers are more than just fast; they have ‘friendly’ menu-driven user interfaces allowing complex programming strategies to be achieved relatively easily. They offer tool-offsets at a push of a button, servomotors can be tuned on the fly, broken tools can be automatically detected and replaced and components can be inspected on the machining centre itself.

Such controllers also provide greater control over processes, such as monitoring machine components/workpiece temperatures, and activating coolant chillers to help maintain a constant working temperature and improve repeatability on batches. Alternatively, they incorporate temperature sensors and/or linear feedback scales to allow positional errors to be determined and compensated for. This hugely improves the overall machine tool accuracy, but can be costly.

Another example of adaptive control is ‘tool load compensation’, which automatically changes the feed rates based on spindle load parameters set by the operator. Instead of setting a safe feed rate based on average tool load conditions, so the tool doesn’t break, rates are set for the maximum load condition, and the control automatically reduces the rate based on the actual load conditions while the part is being cut.

But the efficiency of this technique, which has proved to reduce cycle times by up to 30 per cent, is dependent on the original program. A well-programmed component will already be using optimised feeds and speeds.

A feed rate limit can also be used as a cue to determine when a tool has reached the end of its life. For example, when it becomes so dull that the feed rate reaches 80 per cent of normal, the CNC will either enter a hold condition that allows the operator to change the tool or automatically changes it if it has access to sister tooling.

All modern CNCs can use a probe to reduce set-up time and keep track of broken tools. For example, setting tool offsets by manually entering tool data can be time-consuming, especially if there are a lot of tools. Many controls have a multiple tool set-up feature that allows offsets to be captured using the axis-jog wheel/handle to manually move the tool to the probe. The operator pushes one button to enter the offset data.

But a more efficient technique allows tool length and diameter offsets to be set automatically with a probe. The operator enters the starting and ending tool numbers and then pushes the start button after verifying each of the automatically set tool offsets. With an automated offset feature, offsets for 20 tools can be set in a few minutes.

Many of today’s CNC systems can provide more value by seamlessly integrating off-the-shelf applications, such as vision systems, within the control solution. Software is also helping manufacturers to standardise hardware for different machining processes, resulting in reduced inventory and maintenance costs on the floor.

For example, Siemens Automation and Drives Sinumerik 840D control software architecture provides manufacturers with a single hardware platform for milling, turning, grinding, laser cutting, tube bending and friction stir welding, with all of the capabilities unified under a single software pool within the control architecture.

The similarly named Sinumerik 840Di sl is a fully PC-integrated numerical control system for up to 20 axes, which is designed for use with the Sinamics S120 drive system.

The 840D is one of the most advanced five-axis NC controllers available. It incorporates a powerful Siemens S7 PLC (programmable logic controller) and separate microprocessors for communications, so it can easily carry out the sophisticated processing necessary to meet the needs of high-speed machining (HSM) applications — in particular, functions such as ‘look ahead’ and ‘smoothing’ help to enhance productivity and quality, especially when coupled with spindle power monitoring.

Siemens offers its own WinBDE software for the acquisition of production data, but the flexibility of the 840D also enables other packages to be integrated. Airbus, for example, opted to use its own Tacoma package to collect production data and monitor spindle power for the dual roles of optimising the machining efficiency and protecting the spindle from overload. Similarly, rather than use Siemens’ SinTDI tool data information software, Airbus used its Henri Line tool data package.

Both Heidenhain and Mazak have also introduced updated machines — with a range of new features and updated options (see sidebar below). Heidenhain’s TNC 320 CNC is designed to complement its TNC 530, and the company’s Neil Prescott said: ‘With the development of the 320 in conjunction with the NC Kernel we are, in effect, starting a new timeline for Heidenhain control systems. The Kernel will now form the basis for all future Heidenhain controls and technologies. The in-built modularity makes it very adaptable, while maintaining an underlying simplicity of application’.

Network connectivity and internet access is now a part of the standard architecture of the control. Wireless networking architectures supported by Windows-based controls are also playing a significant role in some companies, and making full use of the internet’s potential, industry-defined protocols such as STEP are gaining ground. This has the potential to make the need for CAM and certainly troublesome post-processors redundant, further adding to the evolution of manufacturing practices.

Another more recent development in computer-based hardware generally, and hence CNC controllers, is the use of dual processors to alleviate the heavy demand on a central processing unit (CPU) when multiple tasks are required and the processor has to interrupt one task to perform another more urgent task. This is known as time-slicing.

Because the processor moves from one task to another, it isn’t always possible to accomplish all them when required, and some have to wait. In CNC controllers, this is one cause of data starvation, which means that the CNC cannot process a program fast enough to keep up with the complexity of multi-axis moves. The result is poor surface finish, longer cycle times and even the inability to run a program.

Parallel processing technology allows the use of two or more microprocessors to be employed simultaneously. This is one way to alleviate the logjam of too many tasks on one processor. Fadal, for example, has a dedicated processor for each axis to enhance processing speed.

But it’s not just a matter of ‘the more the better’. The overall speed at which a CNC system performs is affected by a number of factors, including processor speed, CNC architecture and available random access memory (RAM) just as in a PC.

Sidebar: Comprehensive programs

With the introduction of its TNC 320 CNC in the UK Heidenhain can now offer a mid-range control suitable for three or four-axis machining centres and retrofit applications to complement the TNC 530. By retaining analogue control the 320 provides a cost-effective upgrade option.

Basically it has four control loops (three closed-loop axes plus a servo-controlled spindle). This can be expanded to four axes, plus a servo-controlled spindle with positional input, or five plus a spindle without servo control or positional input. It includes, as standard, a 15in TFT screen, 3D graphics, extensive I/O capability, integrated diagnostics for machine and NC program errors, mid-program start-up in any block and machining time calculation.

Existing programs can be run, and as the interface is familiar, it saves on re-training costs and time. Existing probes and handwheels are also compatible.

Meanwhile, the sixth generation of Mazak’s Mazatrol CNC, the ‘MATRIX’, can control up to nine axes, five of them simultaneously, and is programmed using the company’s familiar conversational language or EIA/ISO code. The controller has enhanced conversational capability and caters for more complex shapes, while delivering high-speed machining with sub-micron accuracy.

New design pulse encoders on each linear axis contribute to a high-quality surface finish by generating 16 million pulses/revolution — claimed to be 16 times faster than the previous system. Surface finish is further enhanced by Quadrant Spike Compensation, which minimises axis ‘slip-stick’ during machining operations such as circular interpolating.

The maximum vectoral feed-rate for machining complex surfaces is said to be four times faster than the standard specification of the previous system. Key features include a mill-turn cycle for ‘rough cutting’ difficult-to-machine materials such as aerospace alloys, where both the milling tool and workpiece rotate, providing improved chip control.