VIBRATION AND MACHINE TOOL UP-TIME

Up-time on a machine can be increased by limiting vibration!

One of the most frequently overlooked topics related to work piece quality and machine tool up-time is the issue of vibration.  Although arbitrary frequencies can be transmitted to a machine through the floor from other sources, vibration on a machine is most usually due to an unbalance condition of the rotating elements on the machine. Vibrations can be generated by machine related sub-systems such a coolant pump or a hydraulic pump, the work head spindle, and the spindle used to support cutting tools or grinding wheels.  The resultant effect is a degeneration of work piece accuracy such as diminishing surface finish, an decrease in parts meeting compliance to roundness specifications, and the wear and tear as it affects the bearings and other critical machine systems.

Rotational vibration is actually the deviation of the rotational mass centerline in relation to the true centerline of rotation.  The amount of deviation caused by an out-of-balance condition is measured as amplitude at any particular operating speed.  The amplitude will increase as the deviation increases.  It is this condition that reduces surface quality and work piece roundness.  Furthermore, it can be concluded that higher amplitudes of vibration will result in greater loads on the supporting bearings.

We can compare the vibration in a machine to an earthquake.  The increased energy of an earthquake as measured on the Richter scale, results in an increase in the severity of the damage.  Higher vibration levels in a work head or spindle will result in higher loads on the bearings and a continuing deterioration of the precision associated with the bearing fits on the shafts and in the housings.

Similarly, an out-of balance wheel on a car will result in wheel shimmy and clearly visible tire wear while wheel bearings deteriorate and fasteners loosen from the vibration.

The vibrations transmitted by motors and their associated drive systems such as pulleys and poor quality belts or couplings can easily be isolated and corrected.  However, resolving these problems on work heads and spindles is more difficult.  Various work holding devises such as chucks and fixtures mounted on work heads are often changed, but rarely balanced.  Tooling such as tool holders retaining a wide variety of cutting tools and wheel holders are changed regularly on spindles with little consideration for balancing.  Further aggravating the imbalance situation is the fact that the runout of work holding and tool holding devices are rarely checked until a problem arises.  Datum surfaces such as tapers, mounting pilots, and adjacent faces need to be thoroughly cleaned.

Generally, precision spindles and work heads are dynamically balanced to exacting standards by the original equipment manufacturers.  Accordingly, vibrations detected in a work head or a spindle is commonly a result of attaching unbalanced work holding mechanisms, tool holders, and tools or grinding wheels.  Maintaining a philosophy of minimizing vibration is a key to optimizing machine up-time.

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