making an emergency collet with a 5/8" square, offset from center. 
Collet will be used to make a reproduction wood moulding.
I center drill, drill, and then single point bore about .0015" undersize
from the finish collet size.  Then find the corners, center drill, and
drill through with a 1/16" drill to establish the corners and provide 
clearance.  At this point, a 1/8" dia. 4fl end mill is used to mill the
straight sides in several passes, about 3/13" deep.  Last pass just cleans
up the shadow from the undersize bore, bringing the dimension very close
to final size.  I then use end ground lathe bits in a simple slotted holder
to slot out the material into the corners, and below the milled depth that
establishes the sides.  There are 2 bits used, one for corners, one straight
one for finsih shaving.  The corner bit actually is used to remove most of
the waste, working along both sides form the center into the corners.  Then
the surface is finished by shaving with the straight tool.  A little file 
work can ease any tight spots, revealed by the adjustable parallel set to 
size.