Grinding is usually the most economic process technology for the hard finishing of bearing carriers and faces on gearing wheels, particularly where medium to high volumes of metal are removed by cutting. Over the past few years, the use of ceramic-bonded CBN tools has increasingly prevailed over grinding wheels using conventional corundum as the cutting material. BUDERUS Schleiftechnik is relying more and more often on CBN grinding wheels with particularly small diameters with the aim of increasing process stability and reducing both cycle times and tool costs. Positive side effect: in the best case, machine purchasing costs are also reduced by up to 12%.
Since CBN grinding wheels started generating higher part output on the market through longer service lives and shorter main times, the proven cost advantage has led to grinding machine users striving to replace conventional grinding wheels by more powerful CBN tools. The potential of the cubic boron nitride coating can be exploited most efficiently on grinding machines which are designed for a CBN supported process from the outset. In addition to the cost advantages already mentioned, CBN grinding generates a more homogeneous surface profile on the workpiece. The percentage contact area is increased due to the higher cutting speeds, which slightly reduces the roughness on account of the more homogeneous surface structure.
Achieving more constant machining results
Small CBN grinding wheels (diameter 90-150 mm) are lighter and thus easier to replace. In addition, they support the stability of the grinding process, particularly in the case of further surface requirements which go beyond the pure Rz-value (Rp, Rt or percentage contact area)and tend to secure more constant results. One practical example from the central Hesse-based company BUDERUS Schleiftechnik shows that the use of CBN grinding wheels with small diameters makes the decisive difference, even beyond tool costs and surface requirements. A large German gearbox manufacturer presents the initial situation. Here, the synchronizercone and face of control gears are ground conventionally with a cycle time of 39 seconds. The corresponding main times can be seen in the image below.
Machine investment costs reduced by about 12%
With CBN as cutting material, several potentials can be realised for this scenario: For one thing, the two machining tasks can be ground in sequence due to the shorter CBN main times so that one workpiece spindle or the frequency converter respectively is no longer required. As a result of the sequential machining, the machine can also be reduced by two linear axes that have to move one tool horizontally and one tool vertically during the original simultaneous machining. These axis movements for decoupling the simultaneous processes are no longer required for sequential machining, since the feed kinematics is already mapped completely by the workpiece compound slides. An additional advantage results from the use of a single, monolithic CBN grinding wheel which does not require any further balancing due to its small diameter. All in all, the simplifications described allow the investment costs per machine to be reduced by about 12%.
70% lower tool costs with CBN
For another, the increase in part output by approx. 7% and the lower tool and tool changing costs related to the small CBN wheels lower the running costs. Savings in tool costs are considerable, since these are almost 70% lower than the tool costs for conventional machining, due in part to the longer service life. Tools have to be changed much less frequently, and when they do, the changing process is much easier and quicker than before thanks to the smaller CBN grinding wheel which does not require any additional lifting gear. Further potentials can be unlocked if CBN bore hole machining is considered as well. In the case under discussion, the bore hole could be ground in addition within the conventional cycle time. In a direct comparison, there would thus be no difference in cycle time between conventional machining of cone and face and a CBN process with additional bore hole machining.
Long years of technology development at BUDERUS Schleiftechnik
The scenario presented is the result of long years of technology development at BUDERUS Schleiftechnik and NAXOS-DISKUS Schleifmittelwerke – both members of the DVS TECHNOLOGY GROUP. Detailed investigations of parameters in the BUDERUS development department and numerous verification processes in gearbox production have led to the systematic expansion and further development of know-how in the field of CBN machining. The main harmonization of factors such as geometry design of the grinding wheels, flushing concept and optimisation of the cutting speed, taking output, roughness values and avoidance of overheating during the grinding process into account are part of the completely coordinated system at BUDERUS Schleiftechnik.
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