Plunge or side milling

What is better to use Plunge milling or side milling with an endmill?

Plunge milling is when you are feeding the endmill down in the Z axis. When plunging you are only cutting with the end of the endmill, this is similar to drilling a hole.  This Technique is used when opening up a hole to an accurate size and location , for example locating pins. Some endmills are not designed to cut in the center. Some HSS cutters will have a hole in the center of cutting end due to the manufacturing method. These cutters should never be used to plunge in the center. 2 Flute endmills will cut in the center better than 4 flute because of the increased clearance space for material removal. 

Cons: Not as fast material removal rate as side milling. If plunging with the center of the endmill this can be very hard for the endmill.

non center cutting endmill

Side Milling

Is when the endmill is cutting in the X and Y axis. This is the quickest way to remove material with an endmill. Depending on the material it is not recommended to use anymore than half the diameter if taking a full width of cut. For example if you are using a 1/2" Endmill you should not cut anymore than 1/4" deep at the full width of the cutter. There is no disadvantages to using this method , an endmill is very efficient at cutting on the side. 

endmill cutting a block