Now we have PCs, why do we need CNC?

An article by Duncan Hudson, General Manager of OSAI-UK Ltd. Now we are living in the age of the personal computer, it begs the question, 'why do we need CNC?' Surely, a PC can handle all of the functions of CNC perfectly adequately?

The short answer is that, theoretically, this is true…… but. And it's a big but!

Parts manufacture, briefly, starts with part design by CAD environment. The design is then translated by a computer-aided-manufacturing program into part-programs. These are then processed in real-time by the CNC.

CNC then comes into its own as a real-time system that can provide modification to the machine process without the need to return to the CAD/CAM environments for further number crunching. Thus, feeds and speeds, tool lengths and radii, probe/gauge data, machine error compensation or fixture variations can all be quickly and easily adjusted on the spot by the operator or by the machine itself. It is simply not practical or economic to keep going back to the CAD or CAM environments to change the dynamic variations of the machine, product or process.

CNC is only as good as its software in terms of its features, flexibility, reliability and safety. That software needs a real-time operating system plus extra hardware to interface it to the real world and here we start to see the differentiation between different suppliers.

Operating system

The use of a real-time, multi-tasking operating system is advocated to keep the speed of operation consistently fast. At Osai, experience has shown just how important it is to keep the PC and CNC environments separate to eliminate the possibility of problems such as variations in response times.

The cost of CPUs such as the latest Pentium and AMD products are at a level to make the PC route seem very cost-effective. Windows NT, for example is an excellent system, with one of its strengths being the enormous number of software packages available to run on it.

However, this strength is also its Achilles' heel when used for CNC. OEMs, or end-users, adding software or hardware packages to a Windows-based CNC, bring the running of the important CNC/PLC functionality into an area of unpredictability. A CNC supplier can guarantee his products' capabilities at the factory gate, but all of this is meaningless if software/hardware packages are added to the product in the field.

Problems can include unpredictable motion dwells, with the introduction of annoying problems like burns on wood products, through to serious ones such as the possible introduction of stress points on critical metal components.

We know of one instance, for example, where CNC software, running on a PC, was giving a good response time of 2 milliseconds. Until, that is, someone loaded a game, when response time was extended to 200 milliseconds, producing an uncontrolled stress mark on the workpiece that subsequently caused it to fail!

Many companies have entered the CNC market, under-estimating the pitfalls, and have left equally quickly, mainly as a result of the huge software effort and investment that is required to produce a reliable product that covers the features required. The consequences can be catastrophic for the end-user, left with little or no ongoing support.

Hardware

The CNC software can be run on specialised hardware or on PC hardware. In the case of PCs, it is important to remember that the CNC software can be run on a PC, but to interface its commands to the machine, it needs extra hardware and software to run the I/O and the servo cards.

For example, an interface to digital servos may require a SERCOS fibre-optic card. Other interfaces may be needed for encoder feedback from say the spindle, hand-wheel, linear scales and so on. In addition, there has to be a connection to limit switches and solenoids, so another card and further software is required.

These items make the product non-standard and introduce a new level of split-responsibilities in terms of maintaining the CNC throughout its working life.

From the user's point of view, the CNC supplier must take total responsibility.

The crucial to questions to ask are;

• will they still be there in, say, six years, to provide technical support? (when machine tools are still in the prime of life - but PC’s are well past theirs)
• are parts plug-and-play? Or is there going to be a denial of responsibility as PC technology moves on and peripheral devices change

This is particularly important as machine tool maintenance is carried out by normal maintenance staff, not by IT specialists who don't know one end of a machine tool from another!

Software

The basis of every CNC must be to allow the machines axes to be controlled safely and positioned smoothly, accurately and at high enough accelerations and speeds for the process in question. The programming flexibility, for both the OEM and end-user, for control and modification the machine and its process is also at the heart of any good CNC.

The more advanced CNC features that will keep it unique to other motion control products include the following:-

• Plane rotation of part program for removal of jig inaccuracies
• Tool centre part programming to compensate for tool length, radius and corner radius on a 5-axis machines
• Interpolators to take account of new configuration machines e.g. hexopod and for increased speed of machines
• Volumetric error and thermal compensation
• Interpolators to give a better control of position loops to give smoother machine performance
• Advent of more data from intelligent spindle and axes drives will also allow better adaptive control over cutting conditions and unmanned operation
• Increased information from drives and intelligent I/O will improve predictive maintenance control and help in fault diagnostics e.g. information on a failing sensor can be used directly by the CNC or passed back to the network

One example of the trends in CNC features is Osai’s high speed cutting polynomials.

For applications that are, literally, at the cutting edge of machine technology, Osai 10-Series controllers offer a unique mathematical approach to the science of machine-tool control that produces faster cutting with no reduction in surface accuracy.

Mechanical technology has developed to a point where, what was once a dream, has become a reality, with processing speeds and sustainable acceleration that have become a standard in the machine-tool world. It is now common to talk about machines with speeds in excess of 100 m/min and acceleration of 1g or higher.

A typical application of these machines is for the high-speed milling of CAD-generated surfaces.

The traditional approach of profiling as a sequence of linear micro interpolations, perfectly acceptable with machine tools with a high inertia, is no longer adequate for particularly rigid machines with high torque servomotors, which recognise all of the sudden variations in direction, producing a poor quality surface finish.

Osai's concept combines the mathematical modelling of both the component's shape and the dynamics of each of the machine's axes to provide the best possible path possible for each axis. This means that conventional cutting vectors are translated into smooth spline polynomials to which the machine is capable of reacting, giving real-world high-speed cutting that is much closer to the idealised mathematical CAD model required.

Osai recognised that there were two possible approaches to solve the problem - either by regulating the dynamics of the movement or the geometry of the trajectories. In fact, it is a question of filtering the actuation of the commands: obtaining a 'soft' output to the servomotors, without changing the programming concept based on straight lines or revolutionising the programming by interpreting the programmed values as a set of points to be approximated in the best possible way.

Osai chose to pursue both options - tackling the problem from both the dynamic and the geometrical points of view.

The geometrical approach looks at part co-ordinates, tool co-ordinates and machine co-ordinates simultaneously and is capable of using all different types of CAD data output. Starting from the programmed values, with exclusive algorithms, the trajectory is split into sets of fifth-power polynomial functions (splines), consisting of a number of elements that correspond to the number of programmed elements; each set representing a section of the program.

The dynamic approach looks at the machine's envelope of capability e.g. maximum speed and acceleration supported by the kinematic mechanism connected to the axis. The aim of the dynamic part of the programming is to determine a new spline, in the time domain, which enables parameters to be calculated in respect of the characteristics of the single axes.

CNC - what's the conclusion?

So, if CNC isn't just software or hardware, what exactly is it?

Certainly the PC revolution has blurred the edges of what have been traditional areas for suppliers of CNC systems and PLCs.

We maintain that CNC is a combination of factors, all of which must combine if a user is to be able to sleep at night. Firstly, of course the CNC is software, (functions and operating system) and hardware too, but also support - technical help, training and long term parts and service - the full package! If any of these elements is missing, you're not buying CNC, you're buying a potential headache.

We believe that there is, and always will be, a distinct market for CNC, because of the special needs of the user. CNC as a product requires enormous development time and costs to keep ahead of the needs for real time adjustment to compensate for machine inaccuracies or thermal compensation, for example.

There is a constant demand for better algorithms (such as high-speed polynomials) to drive the axes even faster and smoother, improved process/cutting speed by adaptive control, improved maintenance through Expert predictive diagnostics and so on.

Finally, and above all, users must be totally assured that any supplier of CNC will not only take 'ownership' of the full package, but be around in the future to provide the level of support that will be required.

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