I don't know if it is a problem or  I do something wrong  when I increase the feed rate  Mach3  rounds the corners  of the cutting path .It is related with the accelaration   The more  acc  is  decreased  The more  the corners is cut round. 

Rceebuilder

That's a function of Constant Velocity mode. If you need sharp corners, use Exact Stop mode.

That is a CV problem... There is a way to tell mach 3 to only round so much... Go to the settings page and do the following settings:
CV angle = 45
CV feedrate = 30 anf turn it on...
If you would like to have less rounding make the CV feedrate lower