Spindle stiffness or Rigidity plays an important role in machine performance, both from the quality and reliability perspective, in any CNC machine tool. Stiffness is a measure of minimum deflection with regards to the relatively applied load from machining process. Consistency in dimensional and geometrical accuracies is one of the primary tasks in metal cutting processes. Antifriction bearings are the weakest link in the load loop of any spindle assembly. Unless there is a clarity in mapping the loads on bearings, both in magnitude and direction, it would be a big challenge in selection of right bearing in type, size and pre-load level.
We know, a metal is also an elastic member. The curve charted on a graph, load versus deflection may not be linear within its elastic limit in actual scenario. The deformation would be high during initial load and gradually the deflection decreases with regards to the additional similar magnitude of the load. Considering this as an advantage, we impose a load on the bearing in assembly, called pre-load and enhance the stiffness of the spindle in metal cutting. This is known to be as preload of the bearing. Improper preload also has negative impacts on reduced limiting speed and fatigue life of the bearings. So it is always a designer’s responsibility to decide a right preload and its level w.r.t the speed and accuracy of the machine tool.
Rajashekara HV
Advisor – IMTMA