End mills, being supported on only one end, are subject to deflection. The amount of deflection is dependent on the material the end mill is made from, the diameter, overhang, and the amount of radial cutting forces acting on the tool. Steel has a lower co-efficiency of elasticity than cemented carbide. Therefore, HSS end mills will experience more deflection than carbide end mills.
As a general rule:
    A 20% overhang reduction will reduce tool deflection by 50%.
    A 20% increase in diameter will reduce tool deflection by 50%
Based on these rules we should:
Use the largest end mill the work piece will allow.
Minimize the amount of overhang from the spindle nose.
Reduce feed per tooth.
Reduce radial or axial depth of cut.
Reduce number of flutes.

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