"Robin S." wrote in message
...

BTW, you may want to go for parabolic drills. They are made for deep hole
drilling (generally beyond diameter:depth ratio of 1:4 or great, IIRC).
They're more expensive, however, than standard twist drills.


Robin,

Parabolic drills arent particularily accurate as far as maintaining a target
location at depth in my experience.

In fact, I have found exactly the opposite to be true--however, chip
evacuation with parabolic flute drills is indeed superior to any other
design to my knowledge.

As to the original question and in the situation described, I would think
+/- .005 inches location should be easily obtainable under general
conditions, unless someone is a real "KluTz" G as to drilling or
sharpening of a std jobber drill. For a bolt clearance, simply use a drill
that is a 1/16 or 1/32 in. oversized.........

Better yet probably would be to use a stub, or "screw machine"
drill...........I maintain an inventory of these in the common sizes so as
to eliminate the need for center drilling whenever the job/ tolerance will
allow.

Else, as you say--if greater accuracy is needed, a more reliable process
would be to first drill the hole undersize, then bore with single point for
size and location--in a production environment, more economical control of
size can often be had by first pre drilling, then boring for location,
followed by a final reaming operation for size control.

Cheers,

--

SVL