Machine tools and advanced manufacturing

January 23, 2013 6:27 am

Machine tools and advanced manufacturing
Vineet Seth, Managing Director (India & Middle East), Delcam Plc.
Machine tool is one the most important components of an advanced manufacturing solution. Vineet Seth, Managing Director (India & Middle East) at Delcam Plc, UK highlights the changing trend in machine tools sector and advantages of 5-axis machining
Machine tools as we know are machines that are primarily used to shape metals or other hard materials – usually by a subtractive manufacturing technique. These machines have come a long way in the past few decades and are now the backbone of the manufacturing domain. Drills, shapers, lathes, mills, borers, broachers, and grinders etc. are a few conventional machines that were automated by computers, and thus began the era of the Computer Numerical Control (CNC) machines. While the early goal of a CNC was to automate processes and reduce human effort / errors, the current generation CNC machines are far ahead in comparison to their ancestors – both in terms of accuracy, size and speed. Advancements in drive technology, spindles, bearings, amongst others, have made today’s CNC machines more precise and cost-effective at the same time. It is due to these reasons that the ubiquitous lathe/mill in every small machine shop will soon be replaced by CNCs in the near future.
As a result of technology that is changing every day, machine tools are increasingly turning into miniature flexible manufacturing systems. The days are past, when customers used to procure machine tools based on the optimal bed size, weight bearing capacity and spindle speed. Most productivity and quality conscious manufacturing houses are now looking at a value-add much beyond heavy depth of cuts and high spindle speeds. To stay in tune with the market requirements, machine tool builders are now offering complete tooled up solutions to end customers. These include the liaising with tooling & fixture suppliers, CAD/CAM suppliers and the control system – with a view to optimise the machine for a variety of applications for a specific customer. The tooling up process includes the cost of optimising the machining of an agreed number of parts – that are programmed and machined with expert inputs from leading tooling, CAD/CAM and fixture companies. While this process may be considered slightly more expensive than buying a ‘no-frills’ machine, it adds a lot of value to the buyer – and saves them a large chunk of money that would otherwise be spent in scouting for the right blend.
So, what are the factors that make a machine tool efficient? The answer can be broadly classified into three segments – the machine tool itself (its construction, controls and kinematics), the CADCAM software and the cutting tool.
Turning centres and grinding centres are more or less programmed manually even today – and at most times this is sufficient to get the job done effectively. When we look at machining centres, that is where we see that the machine tool alone cannot suffice the minimum requirements to get the job done – but is one the most important components of an advanced manufacturing solution. The most common of such machining centres are the Vertical Machining Centres and the Horizontal Machining Centres. The more advanced ones are the multi-axes machining centres where, in addition to the three basic linear axes, an additional fourth or a fifth rotary axis is also available. These five axis machines are normally classified as: • Head – Head (all additional axes on the machine head)• Head – Table (additional axes distributed between the machine head and table)• Table – Table (all additional axes on the machine table). 5-axis machining is used for aerospace, tool-making, automotive, medical, dental, shoe and electronics applications. The main advantage of 5-axis machining is the ability to save time by machining complex shapes in a single set-up. Additional benefit comes from allowing the use of shorter cutters that permit more accurate machining.
Positional 5-axis machining and continuous 5-axis machining are the two possibilities within this technology. Positional 5-axis machining allows creating 3 axis tool-paths using different work-planes for alignment – thus requiring multiple set-ups, while Continuous 5-axis machining allows the user to create continuous 5-axis tool-paths across complex surface, solid and triangulated models, in a single setup. Each comes with its benefits as described below.
Benefits of positional 5-axis machining • Ideal for machining deep cores and cavities • Short cutters give increased accuracy and higher quality surface finish • Allows the machining of undercuts • Significant time benefits through use of only one set up.
Benefits of continuous 5-axis machining • Ideal for profiling parts • Ideal for machining deep corners and cavities • Shorter cutters give increased accuracy and higher quality surface finish • Allows for machining with the flank or bottom of the tool • Can be used with a full range of tool types.
These strategies allow the machine tool to simplify complex machining processes and help in reducing the overall time required to produce the part.
In addition to these 5-axis, certain machine tools have a further number of axes for complex manufacturing. Most often these machines combine two or more different machining methods. Common examples of this are Turn-Mill Centres and Swiss Turn Machines. Both machines combine drilling, turning and milling in multiple axis orientations. The ultimate goal of these machine tools is to reduce the total cycle time of machining by multi-tasking. Very often two or more machining sequences run simultaneously and on the same part. For such machining requirements – especially, when the part profile consists of complex features it becomes essential to use a high end CAM software to generate the machining sequences, as also to check for collisions on the machine tool.
One of the core factors therefore, is the CAM software – which provides the input for a machine tool. Based on the kind of machine in question, CAM software provides output in the form of G and M codes or in some advanced CNC machines a format known as spline codes. Modern developments in cutting tool technology have also led to a revolution in the way CNC programmers now tackle jobs. These specialist tools have totally different cutting characteristics to traditional tooling. Whereas traditional programming methods would consider machining a component from the top down, modern HSM tooling can sometimes prefer to cut from the bottom up. The main requirements of these machining strategies are to keep the load on the cutter as consistent as possible and to minimise any sudden changes in the cutting direction. One of the basic changes in strategy needed to achieve these conditions is the use of offset machining for roughing rather than the traditional raster approach.
It’s quite evident, that there has also been a tremendous amount of improvisation and innovation that has gone into the field of CAD/CAM. The digital divide between CAD/CAM and CAI is soon diminishing, in the sense that CAD/CAM is now a broad spectrum solution to a process rather than a specific operation as such. CIM (Computer Integrated Manufacturing) addresses this by integrating CAD within the CAM environment and providing additional modelling functionalities so that any quick rectifications or changes to the model are executed at the shop floor rather than being sent back to the design cell – which would otherwise consume much time and efforts in communication.
Furthermore, the integration of a CAD surface inspection tool for verification as well as QC for a specific part now helps in comparing CAD to part at any given co-ordinate point rather than GD&T (Geometric Dimensioning and Tolerancing) elements like parallality, concentricity, perpendicularity etc., which were the only available option until a few years ago. This integration also allows for the location of the part on the machine tool and verifying whether the cut part corresponds to the CAD model after the sequential operations scheduled on it. Many machine tools and probing devices now support this option which is known as software-fixturing. This process requires a robust solution available only with a few top ranked CAD/CAM companies in the world. Software-fixturing can be described as a computer-based system that reduces setup times by orders of magnitude for complex 3D parts. Instead of the operator taking many hours to precisely locate the part, software fixturing can automatically realign NC toolpaths in real-time within the software after simple rough fixturing taking minutes. For multi-process machining operations the overall time saving can be measured in days, whilst totally eliminating the risk of human error from the process.
Software fixturing does not require the CAD model to operate, it works with previously generated probe paths, (from offline CMM software or direct from NC probing cycles). A CAD model can be used for an illustration of the process progress, both on screen and in the results output. Often, this is a multipurpose tool and is totally customisable, the HTML-based interface can be completely tailored to the manufacturing process and experience of the operators. It can even work as a black box, an unseen part of the manufacturing process, or as a STOP/GO style interface for basic interaction with a minimal training requirement and  can be scripted to call any object based software package creating a complete solution to process automation needs. The scripting and HTML user interface can be used to wrap large interacting software requirements to provide straightforward solutions to complex manufacturing problems.
It is clear from the above, that machine tools have come a long way from being ‘material removers’ to ‘solutions systems’. Eventually, machine tools will support closed loop manufacturing systems where every aspect of the machining process will be dynamically monitored and controlled to provide the optimal results.