For grinding workpiece surfaces or profiles, reciprocation grinding is the usual method. However, if large amounts of stock are to be removed, this means longer processing times - or a switch to creep feed grinding, optimising the process accordingly. However, there are additional considerations, for example the temperature distribution underneath the grinding wheel. Considerably more heat is transferred into the workpiece during creep feed grinding than in a reciprocating process -  a consequence of lower feed rates for creep feed and the longer period of exposure to the heat source in the contact zone. The workpiece expands as a result, and the deformations removed by the grinding operation. However, a cavity can form when the workpiece cools; its size depends on the actual process, and can be several hundredths of a millimeter deep.

Another important point when selecting the grinding method is feed rate. As the speed rises, required grinding power decreases and chip thickness increases. Before chip removal actually starts, grinding grains first deform the part and create friction as they pass through the workpiece. A shorter path therefore positively affects chip formation and heat transfer into the part.

Analysis shows that technical expenditure is lowest for conventional reciprocating grinding processes; but when the machining volume increases, creep feed grinding also requires much higher specific drive powers, and sufficient cooling must also be ensured. Speed-stroke grinding, on the other hand, not only enables higher feed rates than reciprocating grinding, but also higher machining rate. The process combines high drive power with very rapid reciprocating machine movements, the latter being required to eliminate the time wasted during axis reversal.

Looking at the machine dynamics, an optimum table acceleration rate can be determined for specific speeds. As acceleration increases, the optimum shifts towards higher speeds. It therefore does not make much sense to combine high feed rates with low accelerations; time taken per stroke would simply be too long, rendering the processes uneconomical. To ensure efficient grinding, higher speeds need higher accelerations. Speed-stroke grinding is characterised by larger single grain chip thicknesses but fewer contact areas, and friction and heat transfer into the part are lower.

For Blohm Maschinenbau, Hamburg, these findings had a decisive influence on the decision, taken at a very early stage, to pursue speed-stroke grinding technology. As early as the years 1992 to 1996, Blohm conducted a joint research project with technical colleges and universities in the grinding of ceramic materials. The project found evidence that high feed rates positively affects ceramics with a higher density. This result was the starting point for the so-called Agneta project performed together with the Laboratory for Machine Tools and Production Engineering (WZL) in Aachen and various partners from industry in the years 2000 to 2004. The main emphasis of the research project, which led to the design of a Blohm test machine, was the machining of materials for aircraft engines.  This machine was able to achieve table speeds of up to 200m/min at an acceleration of 50m/s/s.

The results of the Agneta project confirmed the general assumptions Blohm had made beforehand. It was confirmed, for instance, that grinding power consumption related to grinding width and specific grinding energy decrease as the feed rate increases. Assisted by optimised process settings, it was also possible to considerably reduce radial grinding wheel wear. Good progress was also made with regard to surface geometry: While continuous dress (CD) grinding still produced cavities with a depth of up to 80µm, speed-stroke grinding did not exceed depths of 8µm. As it was possible to come closer to the finished dimension during the roughing process, the stock left to be removed by finishing was much smaller. This saves time and money.

One of the more surprising findings was that the company managed to improve the Ra value from 2 to 0.8µm by changing the feed rate from 20 to 190m/min, an effect that can be explained by the nature of chip formation. That a higher surface quality is produced despite increasing machine dynamics is not the only positive side effect, however. Another effect is that the workpiece’s residual compressive stresses change. The residual stress of parts in the compressive range is a welcome effect as it increases the permanent pressure resistance. When machining titanium aluminides, for instance, it turned out that after grinding at a rate of 50m/min a certain amount of residual tensile stresses was still present in the part. Grinding at a rate of 190m/min, however, produced the desired residual compressive stresses.

Another advantage of speed-stroke grinding is the enhanced properties of the zone near the part’s surface. Because the heat transferred into the part through the contact zone is reduced when the feed rate per unit of time increases, the geometry of the part is positively affected and the microstructure is not changed. The risk of cavity formation is reduced, smooth surfaces are accomplished faster, and a positive residual stress condition can be achieved. Added to this is the fact that materials which are difficult to machine, such as titanium alloys, are clearly easier to grind in a speed-stroke process. This is explained by the chip formation mechanisms.

At an early stage, machine builders in Hamburg started to consider how the research results could be put to industrial use. Feed rates of up to 120m/min rapidly proved to be the optimum. Although one might consider increasing the speed even further in order to enhance the positive effects, the technical expenditure required would not be justified from an economical point of view.

Blohm’s general objective was to build a machine which offers shorter machining times and increased productivity. Higher axis speeds are synonymous with a reduction of auxiliary process times. The goal was to save on tooling costs compared with the CD process by reducing wheel consumption. After making sure that the load on the dressing tools was lower as a result of shorter machining times and that less wear was achieved, the next step was to apply the technology to other materials that are difficult to machine, thus opening up new applications.

Eventually, the Prokos was built, a speed-stroke grinding machine which offers increased productivity at reduced costs. The machine was designed with a mineral cast machine bed, which not only ensures good dampening properties but also absorbs the high acceleration pulses. The rotary and linear axes of the Prokos use highly dynamic linear motors. Ideally complemented by a high-speed grinding spindle drive, this combination of features offers an unparalleled implementation of the new speed-stroke technology. The machine had been designed for workpiece dimensions up to 300 by 300 by 300mm. The X-axis allows speeds of up to 120m/min and accelerations of up to 25 m/s/s. Grinding head feed on the Y-axis is achieved at 10m/mm and 3m/s/s. The Z-axis finally permits speeds of up to 50m/min and accelerations of up to 8 m/s/s.  These specifications make the Prokos roughly five times faster than normal ball-screw driven grinders.

Dressing units with diamond dressing rollers and/or wheels mounted on the machine bed ensure extremely accurate profiling of grinding wheels. The optional use of an automatic tool changing system with up to 24 tool stations allows complete machining of complex workpieces. Conventional, ceramic- or galvanically bonded CBN or diamond grinding wheels and also milling and drilling tools can be used. Rapid NC coolant nozzle positioning ensures optimum process cooling.

Two application examples illustrate the performance of the Prokos. The first example is the production of small compressor guide vanes. The parts,  made of a nickel alloy require removal of up to 6mm of material, which takes about 8.5 minutes. The process from the raw to the finished part therefore involves a considerable machining effort. During the grinding process on the Prokos machine, an additional rotary axis rotates the workpiece about its axis, so that it can be ground from all four sides. Before the actual grinding process starts, a prismatic profile roll dresses a universal wheel, which then grinds the part on all sides at a rate of 80m/min. During the roughing passes, feeding at 0.05mm per stroke is possible without any damage to the surface-near zone; an Rz value of 5µm is achieved.

Several interpolating axes are involved in the grinding process. The swivelling grinding spindle enables the operator to choose the processing positions so that good access to the profiles is obtained. The Prokos allows some interesting operations: If, for instance, continuous-path controlled radial grinding needs to be performed on one edge on the rear of the part after roughing, this operation can easily be carried out with a disk-type grinding wheel. A radial groove on the opposite part of the workpiece can also easily be ground using a galvanic CBN bonded wheel. Cooling of all these processes does not pose a great technical problem as the coolant only needs to be supplied from one side. Consequently, the coolant cleaning system on the Prokos only needs half the capacity of a system required on a CD production grinding machine of comparable output.

The second application is the grinding of turbine guide vanes, also made of a nickel-based alloy. The challenge in this case was continuous-path controlled machining of larger radii. Today, infeeds of up to 0.03mm per stroke and feed rates of 80m/min in interpolating operations are possible. It takes approximately 9.5 minutes to perform the six grinding operations necessary to machine the workpiece, achieving path deviations of max. 7 µm and roughness depths of 5µm. Apart from rather complex machining operations, the machine in this case also carries out simple flat grinding operations. As the component has rather thin webs, there is only a small amount of material underneath the grinding wheel. Due to the low heat transfer and the reduced risk of surface-near damage, the Prokos can show the full strengths of speed-stroke reciprocating grinding.

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