Why Perfectly Good Live Centres Might Fail Prematurely
MISAPPLICATION OF THE CENTRE
Misapplication is probably the most common cause of premature failure. Selecting the right centre sounds easy because you already know the workpiece weight, the machine RPM, the tailstock taper, the required tailstock dimensions, the point style (for adequate tool clearance), and, in most cases, the required centre accuracy. Make sure the centre you select suits all the above criteria.
MISAPPLYING TAILSTOCK FORCE
Applying incorrect tailstock force ranks a close second to using the wrong centre for why a centre may fail prematurely. Applying the correct load to the workpiece and the centre is not an exact science, however, and sometimes requires guesswork based on the operator's experience. Too often, operators guess wrong, and we've noticed a general tendency toward overloading. When excessive force is applied the axial load limit of the centre bearings is exceeded, inevitably leading to seizure and early failure of the centre. In rare cases, even the machine spindle bearing can be damaged by overloading, resulting in costly downtime for replacement. Conversely, if the tailstock force is too light, the centre will soon slip, chatter, and wobble, galling the shaft centre-hole and damaging the centre's point. If not shut down soon enough, friction-induced heat can actually "weld" the centre to the workpiece. Not good.
There's no simple formula for correct tailstock force, but we can offer some suggestions. For starters, make sure that the centre is easily capable of handling the workpiece weight, and employ proper machining practices suitable to the weight of the workpiece. For critical applications, you might require an Analog Force Gauge which can measure the precise load in pounds of force.
WORN QUILL TAPER
Over time, the tailstock quill, which holds the centre, may become out-of-round and will loosen its grip on the centre taper. This naturally induces a wobbling of the centre, producing chatter, which greatly reduces accuracy and leads to unacceptable parts. Quill wear can be easily checked. Apply Bearing Blue to the centre taper and insert into the quill. Upon removal, if the bluing shows irregularity or gaps, then the quill taper needs to be repaired.
INSUFFICIENT STIFFNESS AND RIGIDITY
Both the cutting tool and the centre must have sufficient stiffness to avoid deflection under heavier loads. If not, this typically results in chatter, out-of-round parts, and/or tapered parts, which in turn, can double any subsequent grinding time. Be sure that the centre has ample capacity and is not running above 60 or 70 per cent of ItS maximum workpiece weight rating. Again, the centre manufacturer can recommend the correct model to avoid premature failure and bad parts
MISGUIDED LUBRICATION PRACTICES
Live centre design can include many types of bearing lubrication depending on the type and severity of the machining operation. Additional lubrication, sometimes in excess, has been applied in a misguided attempt to keep the centre running longer Don't do this. Understand that the centre's original lubrication may be chemically incompatible with other oils and greases, and mixing these incompatible lubrications could cause premature bearing failure.