Long reach milling applications put high demands on tool holders: high rigidity and tool stiffness.  Another parameter requirement is a shallower depth of cut. In long reach operations the tool will always stick out more than normal out of the tool adapter.  Long reach is defined as a tool extending from the adapter by more than 4 times the tool / shank diameter.  Steel tool holders are useful and work fine when extending no more than 3-4 times the tool diameter.  Longer tool extension with steel holders requires a reduction of the recommended milling parameters.  Aggressive Milling Charts. Steel tool holders with stronger tapered nose and larger tool shank diameters can be used instead of cylindrical tool holders.  The rule of thumb is that an increase of the shank diameter to the next nominal size (as is the case with switching from a cylindrical to a tapered holder) or approx. 20% tool shank diameter increase will decrease the deflection of the tool shank by approx. 50%.  If this increased stiffness is not sufficient (deflection too high or surface finish too rough), then solid carbide tool holders or carbide extensions with screw-on tool heads can be the answer. Tool Shank catalog.  These tools have the highest stiffness available.  Their stiffness is far higher than carbide core or heavy metal tool holders; they are shrink-fit compatible and suitable for High Speed milling.