There are four major spindle taper problems with which all manufacturers need to be concerned.

The first taper problem is bellmouthing.
Bellmouthing takes place naturally as the spindle taper wears during continued use. It usually occurs at the large diameter of the taper, leaving contact only at the small diameter portion, which may account for fifty percent or even less of the total spindle taper contact area. This reduced contact leads to increased tool movement and greater tool runout.

Because bellmouthing happens very gradually over a long period of time, it often goes unnoticed. The drawing (figure 1) below shows the area of wear (or bellmouthing) on the spindle taper.

This lack of contact between the tool holder and the spindle will first begin to show on the machine spindle taper (and tool holder) as fretting. This is easily seen by the eyes of the operator as rust colored areas, or spots (Figure 2). The machine operator will begin to observe such symptoms as tool chatter on workpiece surfaces, carbide inserts chipping at the cutting edge and inconsistent bore sizes.

The second taper problem is machine malfunction or operator error.
This can cause a tool holder to spin inside the spindle taper. This condition leads to a buildup of galled material, which completely eliminates contact of the tool with the taper surface. Although attempts to grind away the material buildup by hand can increase taper contact, the increase is usually not sufficient to achieve satisfactory work piece quality. Furthermore, such grinding can irreparably damage (Figure 3) an otherwise repairable spindle.

The third taper problem is weak tool retention.
This problem can cause tool chatter, even though the spindle taper is acceptable. Unchecked, the condition inevitably leads to premature taper wear of the machine spindle and of the tool holder.

The fourth taper problem is the use of worn tool holders.
Using worn tool holders in a new, rebuilt, or reground spindle will cause premature spindle taper wear, due to inefficient contact between the tool and the taper. Tool holder condition is as important as spindle condition.