Large Format 5 Axis Milling
5 axis milling is not a new technology for the CNC world. It offers one of the ultimate solutions for the accuracy and complexity a metalworking machine can offer. The quality demands in industries like aerospace, oil, and gas, power turbines, mold making have become extremely high in order to respond to the higher demands of the design. Now if we make the part scale larger achieving tight tolerances on complex geometry becomes even more challenging.
It’s a well-known fact that the technology process for machining a part can be often extremely complex if you have to move the part around to many machine stations. In past times when machines had limited machining capabilities (being it solely a turning, milling, boring, broaching or grinding machine) has brought lots of complications in terms of taking time to transport the part, using complicated jigs and fixtures which are not only expensive but also create the chance for introducing a setup error which may sacrifice accuracy. Moreover, mounting a heavy blank piece is a significant time consumer the avoidance of which is by all means highly welcome.
Minimizing the possibility of human error, reliability, and flexibility of the machine are stepping stones of success especially when speaking about large scale and complex geometry. Only the one that one way or another hasn’t faced the risk of seeing a scrapped part which is already as expensive as tenths of thousands of dollars as a blank form, cannot realize the importance of having a high end reliable and suitable for the job 5 axis machine to make the task as convenient and straightforward as possible. There is a number of CNC machine builders on the market offering a variety of large-scale 5 axis machines with extremely advanced optional features which will give programmers and machinists more than enough technology to combat the toughest task in any material. Okuma, Doosan, DMG Mori, FPT, Mazak, and many other machine manufacturers have large scale machining lines offering machine sizes of over 160” x 70” x 155”, spindle power of over 35kW, spindle speed of 10000 RPM and above and with many attachments, heads and accessories available. In this article, we will have a closer look at many of them and the technology behind them so you can get a glimpse of the art of the large format 5 axis milling machines.
Let’s first take a look at the machine configuration. As a standard, the linear axes are X, Y, and Z, the first of which usually having the longest travel. These giant 5 axis machines must be designed to withstand the cutting forces and to absorb and eliminate this energy in order to avoid vibration and deflection. Having said all this means that bearings, guides, and material quality must be of superior quality and in order to ensure achieving the rigidity and toughness with the corresponding velocity, machine designs should not only follow the good practices from the past but also pass the most advanced FEA analysis.
Standard solutions for the linear axes are the gantry type and the travel column type. The gantry-type usually features also a crossrail movement (often referred to as the W axis) allowing the machine to have a significant stroke in the Z-axis direction.
The main difference between the larger 5 axis machines and the smaller ones is in the rotary axes A, B, or C which are not so often on the table but they are rather on the head. This allows any pivoting to be performed by the head thus allowing the part setup to remain rigid. In fact, having the sizes this big doesn’t mean there is a compromise in accuracy. Rotary heads feature positioning of 0.001 degrees, just as accurate as their smaller sized siblings.
There are also machine configurations with a C-axis on the table and a bit more rarely a large A-axis which is usually as a separate attachment to the machine. This means an additional rotary axis can even be added as a piece of optional equipment with the single main goal to improve accessibility and have the part machined as completely as possible.
Another variation of a large-scale 5 axis machine is to have a deep hole boring function where a specially designed hydraulic quill can go into very deep holes the CNC world can imagine and maintain close tolerances and high precision.
Operations available on these machines may substitute 1 or more otherwise separate machines. Not only a part can be completely machined with a single setup as was mentioned above, but also completely different operations can be performed. Combining turning and milling functions is the simplest example. Depending on the specific machine manufacturer can be made available, many features help the elimination of heavy and exhausting movement to a gear hobbing machine to perform gear hobbing operations, or power skyving, surface, and cylindrical grinding machine, etc. Even the really advanced controller-dependent operations like interpolation turning, eccentric turning, using multi-tools to save tool change time, and many more are also for convenience to machinists to provide even further flexibility. This way parts as complex as turbine blocks, structural components, frames and housings, blisks, fan disks, impellers, and many more of the commonly most tedious and complex parts can be machined and can answer the nowadays higher precision demands. Having the possibility for simultaneous rotary axis machining allows for multi-surface 5-axis operations. The large-scale 5 axis machines provide programmers and machinists great freedom and accessibility.
The technology behind
What is it that brings these machines this accuracy, reliability and what other advanced technology enhancements aim for their success? Depending on the parts’ specifics the large 5 axis machines can be delivered with a variety of optional equipment, automation functions, top-notch controllers, and user interface technology that makes it available to squeeze out the most of it and not only – as already mentioned as a most prominent advantage to almost fully complete the part in a single setup.
Manufacturers of large-scale 5 axis machines definitely face lots of challenges to produce a piece of art machinery. Not only it’s completed with very high precision but it also incorporates the many features which smaller-sized machines have plus the most advanced technology to overcome the challenges of being big. Let’s have a closer look at the most important mechanical, electrical, and control features that make modern large-scale 5 axis machines so powerful.
Direct drive motors are providing the advantage of having high torque to power ratio output from the motor. They are usually the preferred technology for driving the axis and the spindles of the machine. A direct-drive motor is a synchronous type of motor with permanent magnets which directly drives the load eliminating the need for transmission, this way reducing the number of components, improving efficiency, accuracy, dynamics, and which also has to be noted when speaking for large scale – creating a quieter environment.
With all this said it is out of doubt why they are the preferred choice for driving large-scale 5 axis machines. Heavy-duty milling, with fast starts and stops, are also among the challenges machine builders have to face while choosing the right type of components to implement in the machines and overcome the exerting material removal forces. It is out of doubt direct drive motors have characteristics allowing them to offer much more in comparison to traditional ones.
A very important role for these giant machines is given to the foundation which has to carry the loads. Often the base of the machine lays under the top line of the foundation for easier accessibility depending on the machine configuration. There is a large list of requirements for the foundation which should ensure there is no share or cracking or crushing which plays a significant role in the machine's accuracy. Also, the rotating and moving components should be highly balanced minimizing the risk of resonance in the foundation. Among the list of requirements is to have the construction out of a single pour, never to connect the foundation to the existing floor, and to use high-quality concrete of at least 5000 lb./in.²
Repeatability and how large-scale 5 axis machines maintain tight tolerances over time?
Tolerances of 0.0001 in. on large-scale machines would inevitably mean there are specifics in the design and construction to help preserve this accuracy. The metal expansion would need to be controlled so that it will not deteriorate the machine's accuracy and make it available to the machine with high precision. Due to their size, the environmental temperature of these machines is also more difficult to be maintained and a very significant design feature is the thermosymetrical concept which allows for the symmetrical distribution of the heat generated during machine operation. High-end machines aim for high-temperature stability standards with positioning accuracy of about 0.0004 in. A common solution in order to insulate to a great extend the influence of the ambient temperature to the most exposed to its effective features is the appliance of thermal insulation panels which pacify the thermal difference thus preventing positioning errors. Machines have also a number of thermal probes which compensate for the thermal expansions and contribute to the thermo friendly concept. They are directly linked to the CNC and allow the mechanics and electronics to work together and fight the thermal differences. This is done through software acquiring the values from the thermal probes and allowing for adjustment to take place. Another way to control the thermal expansion is through cooled linear guideways. This way the machine regulates its temperature-sensitive areas by the settings of the CNC control. One of the most important components usually suffering from thermal expansion problems is the tool spindle. Higher cutting rpm means higher friction, higher heat exerted from the cutting zone, and thus a rise in the temperature of the spindle as a whole. Therefore, spindles require having a cooling system for the bearings and shafts and this technology comes vital for maintaining the thermo-friendly environment at the spindle zone reducing the number of errors, and improving quality. Spindle growth control has always been a problem and it has found its solution. What happens in the case of a travel column machine having a column with a possible travel of about 80 in? How are its rigidity and accuracy maintained when the so-called ram responsible for this travel is out to such extent, is there no dropdown and weakness of the system occurring from having the heavy head and column sticking out of any support? Of course, there is, and here comes another challenge for this type of large-scale mills, and here is the solution – the utilization of nitrogen gas balancing cylinders with the aim of which the geometry error caused by the gravity acting on the ram when it is away from the support. This long reach of the ram, secured with this system and in combination with a hydraulic quill allows for long reach for boring and milling operations even at very difficult to reach features.
Which are the most common head types?
Most of the heads of the large 5 axis machine have at least one rotary axis at the head. Unlike the smaller-sized 5 axis machines that often have kinematics comprising of 2 rotary axes at the table (the so-called trunnion head) is a very unlike scenario for the large-scale ones, so this swivel table type is not a usual solution. The kinematics types are called swivel head type (2 rotary axes at the head) and swivel head + swivel table type ( mixed kinematics with 1 rotary axis at the table and 1 at the head) are the most common ones in our case. With as many as 5 axes available programmers and machinists have the possibility to access the part surface from any possible angle and this way eliminates the cumulative error of multiple setups and makes tight tolerances achievable. Often times while deep hole drilling the vertical milling machines are far yet not the best option due to the difficulties with chip evacuation. This makes the transformation from a vertical head machine to a horizontal one a great asset not only for accessibility but also for drilling holes with a very big length to diameter ratio
Large and long bores are a common scenario when we speak for machines with the longest travel as big as 500 in. Everyone can imagine that the largest parts can be both lengthy and heavy, which often times in combination with the accuracy requirements of bore features in the part will mean the best approach will be to machine a deep bore all the way through from one end to the other. How has this been made possible you may ask yourself and here is the answer – a quill spindle. Some of these giant machines are equipped with a quill that can be CNC controlled and with a boring tool mounted in the spindle makes the reach of holes as deep as 30-40in a possible scenario.
Accuracy compensation and calibration
As the logic says going a large-scale 5 axis milling machine due to its complexity and manufacturing challenges would mean the machine has been built relatively not as accurately as its smaller siblings due to mechanical deviations or the bending and structural stresses of the machine. To compensate for mechanical inaccuracies modern machines have smart control solutions able to introduce the proper corrections and maintain the axis accuracy and calibration of the machine. Many machine vendors have also included self-calibration functionality to the control to enable automatic resetting of the initial values of machine geometry. Through a relatively quick operation of usually less than 30 minutes, the machine can restore and calibrate its accuracy.
Quality control and probing are other great ways to save production time. In case it was unavailable like in the old days all quality checks are oftentimes impossible to be overtaken at the machine site and instead the heavy part would need to be transported to the quality department for inspection. We already spoke about the reliable accuracy and high-end technology maintaining it on such a large scale and the same thing applies to the quality of measurements. Thus, we can note this as another point for the 5 axis machine in its rivalry with the cumulative error.
Parts being designed for manufacturing with CNC machines have really various geometry and size and having a 5 axis machine on your side is one of the greatest benefits you can combat a complex situation with. But what if you have a workpiece combining features that need to be milled and others requiring turning? Of course, the good news is that modern high-end machines offer turning capabilities as well. And the technology behind is of course a hydrostatic turning system. This is a very advantageous technology when the goal is to have precision and reliability despite the high mass of the workpiece placed on it. These turning tables rely on hydrostatic bearings taking the whole load by a thin constant layer of oil. This guarantees the technology can preserve excellent results regardless of the machined component weight. The mechanism compensates for the preload pressure until it is equal to the entire load at all times. It is also important to be noted that this high stiffness is not compromising accuracy at all as repeatability and runout are kept at a really low value.
Are you still in doubt large-scale 5 axis machines cannot reach the highest accuracy demands and achieve the closest tolerances? All the control and mechanics enhancements characterizing most of the smaller 5 axis machines, all the listed above functions like horizontal to vertical transformation, boring function, thermo insulated concept with probes for compensating the thermal differences, auto-calibration functions, and many more advocates for the most prominent their advantage to respond to the highest accuracy demands and tackle any task in a single setup where other machines could fail.
With continual technological advancements and heightened expectations within industry areas that demand more complicated parts to be manufactured, the criteria for part accuracy continue to tighten. This implies that selecting the right machine category, such as a 5-axis machine tool, and the best manufacturer for that machine tool is critical to decreasing or even eliminating cumulative error and keeping your customers coming back.