[MLyons (Martin)]

You can get far smoother cutting action with a spiral flute tap.
It also has a "centre" drilled in the end for centering on a live centre in your tail stock.

One thing I noticed with the normal tap is that it shudders every time the cutting edge touches the work (3 times per rev with normal taps) and in doing so could introduce minute variations that in centering because of flexure of the workpiece.

With a spiral flute tap, not only can it be centered 100% and supported by the tailstock in a live centre (thereby staying 100% true) but there is always constant contact with the cutting edges. This makes for a much more constant force thereby ensuring a smoother, more accurate cut.

I'll take a pic of my spiral flute tap and post it.

Martin