About five years ago, System 3R started a project that involved comparative tests of the performance of different pallet systems. Traditionally, the focus was on the static rigidity and stability of the pallet systems. However, the tests soon showed that the dynamic stability of the systems had a very marked effect on the final quality of the machine part.
Today’s machining environment – high-strength materials being machined at exceptionally high material removal rates, and with extreme accuracy and repetition requirements – demand a great deal of a pallet system.
A typical machining system is a closed loop, where the elastic structure consists primarily of three components – a machine tool, a cutting tool and a workpiece clamping system – all of which dictate the stability of the machining system. The quality of the manufactured product and the capacity of the machining process depend largely on the dynamic stability of every component of this structure. Formerly, research was focused on the machine tool and, to some extent, on the cutting tool. The systems for clamping the workpiece, both conventional and palletised, were largely ignored.
Our unique VDP chuck
One of System 3R’s pallet systems was compared with a competing, slightly larger, system. The machining tests showed that the surface finish obtained, and the ability to machine greater cutting depths, were significantly better in the System 3R pallet system, despite a lower static stiffness. The 3R system had better dynamic stability. In other words, lower vibration amplitudes at the natural frequencies of the system, thanks to greater damping.
System 3R saw an opportunity to improve the entire machining process by developing the dynamic properties of the pallet system. A collaboration with the Department of Production Engineering at the Royal Institute of Technology, Stockholm (KTH ) was started in 2001. The aim was to develop a pallet system with built-in passive damping.
The damping needed to be effective over a wide frequency range for the system to be usable in practice; equally good for conventional milling as for high-speed milling; for a wide range of cutting parameters, workpiece materials and sizes. Other requirements include cost-effectiveness and user-friendliness. Developing such a pallet system is a challenging and inspiring task!
There were three ways to go – frequency-determined mass dampers, piezoelectric devices or polymer damping. The first of these is effective within a very narrow frequency range. Consumption of vibration energy by means of piezoelectric devices is inefficient and excessively costly. Polymer damping was the option to follow, preferably as a structural element integrated in the design of the pallet system to absorb the vibration energy.
The aim was a pallet system with better dynamics, but with its other positive properties retained – especially the static stiffness. Tests and analyses of the first design gave encouraging results, which were improved upon in the next design. The process continued step by step, the decisive control parameters being damping and dynamic and static stiffness.
The dynamics of the pallet system were considerably improved and the loss of static stiffness was very small. The next step in the development process – machining tests – showed that the loss of static stiffness did not detract from the performance of the system in any way.
Machining tests were carried out to check the properties of the damped system in an actual working environment. Vibration amplitudes measured on the workpiece, cutting forces and tool wear were the parameters in these tests.
The extensive machining tests at KTH confirmed the improved performance of the damped system. Long-term vibrations in undamped systems quickly died out in the damped system. The results included lower cutting forces and longer tool life.
But System 3R wanted to evaluate the system further. The Fraunhofer Institut für Produktionstechnologie of Aachen, Germany, tested the system, mainly with regard to surface finish and tool wear. For example, face milling with a 0.3 mm end mill in a workpiece with a hardness of 49 HRC gave remarkable results. Individual cuts could hardly be seen on workpieces machined in the damped system.
Vibration Damped Palletisation (VDP) – as System 3R calls the new product range – was now established, but the ultimate criterion for a product is the benefit it brings to customers.
The first VDP prototype to be installed in an actual working environment got full marks from a local engineering company – Lidingö Produktion AB, Stockholm. The company produces milling cutter heads of different diameters (40 - 80 mm) and of different types (five to eight indexable insert slots) of SIS 2541 steel with HRC 45 under a subcontractor contract.
Improvements in the machined surface from an undamped chuck (left), compared to machining with the same cutting data in a VDP chuck (right).
The 80 mm cutter heads were a major problem, since milling the indexable insert slots caused severe vibration. To avoid this vibration, conservative cutting data had to be chosen. The entire operation required seven roughing passes, seven finishing passes and a feed rate of 450 mm/min. The cutting tool – a four-tooth, 32 mm face mill head – only lasted long enough for ten parts.
“Installing System 3R’s VDP system in our five-axis multi-operation machine brought about a remarkable improvement in machining capacity. We now achieve better surface finish, even though we only work with roughing (feed rate 600 mm/min). The total machining time has been reduced by 19 %, and the tool can machine 40 parts. Machining of other milling cutter heads has also been improved in terms of feed rate, throughput time and tool life.”
A second prototype was installed at Shimano in Japan. Here too, the outstanding performance of the VDP system was confirmed. VDP was compared with an undamped pallet system in two milling machines – a conventional machine and a high-speed machine – at Shimano’s production plant in Osaka.
Shimano concentrated mainly on the surface finish when machining high-strength tool steel (YSS type DAC3) with HRC 51.
In the high-speed milling machine (10 000 rpm), Ra was improved from 1.288 till 0.538 mm (>200%), and Rmax from 7.192 till 5.022 mm (almost 150%).
In the conventional milling machine (3 000 rpm) Ra was improved from 3.006 till 0.860 mm (about 350%), and Rmax from 16.08 till 6.965 mm (almost 250%).
Note that conventional milling machines benefit more by using VDP.
A third prototype was installed at a manufacturer of high-quality milling and boring machines – Yasda Precision Tools K.K. of Okayama. Here, VDP was compared with a very stable hydraulic quality vice.
High-speed milling with an 8 mm spherical cutter at 10 000 rpm and a feed rate of 4000 mm/min.
Once again, VDP proved its value by improving both the surface finish and the form accuracy. The surface finish was improved by more than 30% whilst Rmax was reduced from 1.5 to 1.0 mm (150%).
The dynamic stability of the newly-developed VDP system is many times better than that of an undamped standard system.
What does this improvement give the customer?
Better surface finish, in some cases even the opportunity to do away with certain machining operations.
Allows tougher cutting data – increase cutting speed, feed rate, etc. – reducing the lead time.
The cutting tool lasts longer.
Lower cutting forces – ultimately extending the life of the machine spindle.
The performance of simpler machines gets closer to the dynamic properties of more expensive machines.
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Related links:
VDP - Vibration damped palletisation