Crankshaft machining with the ultimate in precision and quality

When it comes to machining large diesel crankshafts, which are used in ships and energy engineering, an undercut is produced for each crankpin on the left and right at the end of the diameter. These undercuts have a shape that is specific to the customer or type. Each crankshaft manufacturer therefore produces its own undercuts based on the type in question. These expose the crankpin in the radial and axial direction. On one of the current test workpieces, the new finish milling cutter milled an undercut with an axial depth of 25 mm and a radial depth of 0.7 mm, for example. The entire undercut can be produced in just one pass. Such undercuts are usually manually polished once the crankshaft has been finish-ground. This time- consuming process is very important as any tool marks can lead to a crankshaft notch fracture at this point. A 9-stroke crankshaft has 18 such undercuts which need to be polished. If the polishing pen slips, then, in the worst case scenario, the entire crankshaft can be ruined if the diameter is damaged. Thus errors mainly occur during the manual processes.

This is the key
In 2020, WFL Tooling Solutions worked intensively on a method for more precise machining of undercuts. The aim was to construct a form cutter with a precision design. The result is the undercut finish milling cutter: a highly precise finished tool that can achieve a surface quality of < Ra0.4 and leaves no steps in the radius area. Such milling cutters normally have several indexable inserts to cover the entire shape, however the transition between the indexable inserts is later visible as a mark on the workpiece. WFL therefore uses very large indexable inserts for the new form cutter to resolve this problem and produce groove-free undercuts.
The form cutter has a very large, effective number of teeth for high feed rates and short machining times, as well as large, individually ground indexable inserts. To achieve as little thrust force as possible, they are ground razor sharp, leaving a tool that combines maximum precision and maximum sharpness.

Special tool accommodation
Tools for crankpin machining require large overhangs due to the unfavourable interfering contour of a crankshaft. The technicians at WFL Tooling Solutions have developed an extremely stable tool accommodation to suit such form cutters, with the option to finely adjust the roundness and axial run-out. Run-out errors for such precision tools must be avoided, as the tool then only machines at the highest point and does not work all around as should be the case. WFL has therefore installed an adjustment element in the new form cutter, which can be used to adjust the tool in the machine radially and axially in the μm range.

Vibration dampers for vibration-free machining
Furthermore, the tool also features vibration dampers to prevent generation of vibrations during machining. Unlike other concepts, these are not located at the rear of the tool, but very close to the tool cutter. This special know-how efficiently prevents vibrations. Challenging machining of crankshafts requires more than just a perfect tool; it also calls for the corresponding WFL machining cycles for machining the crankpin. When the milling cutter plunges and retracts it passes a very critical point. This area involves different cutting conditions to the rest of the machining action. Such factors must be taken into account to prevent errors. Suitable software and years of experience also help, in addition to the tool, to achieve the required quality. The  improved surface quality and reduction in downstream processes are testament to the success of the new WFL finish milling cutter.

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