For something like the part in the attached drawing I usually rough out the profile on a band saw leaving approximately 1/8" all around before milling the contour on the CNC (img option-1.jpg). I use side clamps on the part and program the machine to mill around them.
The other option, option 2, would be to mill the part out of rectangular stock leaving in about a 1/2 an inch on the outer perimeter and tabs to hold the part, the same way you might approach cutting sheet stock.
The reason I favour option 1 is much less waste material, I can cut my stock out very close to the final size. With option 2 I have to leave a 1/2" for my tool diameter and then another 1/2" extra to give something stiff enough for my clamps to hold on to, so nearly 2" wider and longer then necessary. In option 1 I can also make deeper cuts with an adaptive clearing method since I can approach the part from the side using only a percentage of the tool width. In option 2 I’m afraid to make cuts much deeper than 1/8" since I’m using the full width of the tool (3hp spindle).
Is there any downsides to option 1, would it be typical to work this way, or is the other approach more widely used?
A few things I wonder about it:
I get some corners chipping especially the outer corner of the curved point in the example part. I slow down the feedrate as much as possible around corners but it often chips anyway. I read somewhere that having material on both sides of the tool can help this, is this true? In this case would option-2 be favourable?
Secondly with option-2 is there a danger in having material on both sides of the tool once you get fairly deep in the stock. I worry something could snag once the tool is buried far down in the piece.