Hi-helix end mills have several advantages inherited with their design.

Simple math says that a an endmill with 45 degree helix angle directs 50% of the cutting force downward versus  25% for a 30 degree end mill.

Main advantages are:

• Higher rake angle directs more of a cutting force downward.
  This reduces side load on the cutter, that leads to less deflection and less tendency to chatter.
• At high axial engagement (deeper depths of cuts) more flutes remain in the contact with the work piece. This leads to much smoother cut, again reducing tendency of the cutter to chatter.
• High helix angle pulls chips upward and away from the cutting zone.
  This reduces chip re-cutting and helps prevent cutter from getting clogged up. This also allows to take deeper cuts and increases productivity.
• Because of higher helix more of flute length is being used in the cut. Better surface finish is achieved even when using the same chip load.
  Generally an end mill with 45 degree helix can be fed 30% faster than equivalent one with 30 degree helix and still achieve same surface finish.

High helix end mills also have disadvantages that a machinist has to take into consideration:

• With more of cutting force directed axially, the load on spindle bearings in downward direction is increased.
• Tendency for both the end mill and the work piece to pull out is increased. So a more rigid tool holding and work clamping should be considered.
• Higher helix end mills are also less stiff that regular helix end mills. This may cause more deflection and may become a problem when having to machine straight walls.
  This effect should be mostly diminished by lower side radial load, but it still needs to be considered in some cases.