"Robin S." wrote:

"Dan Caster" wrote in message
m...
I looked in all my reference books and could only find data in the
Machinery Handbook on the size of drilled holes. Nothing on how
straight drilled hole can be expectd to be on a routine basis. And
obviously nothing on how straight a drilled hole can be expected to be
when done by a skilled machinest. Does anyone have some data on this?

So my question is when drilling a 1" deep .138 hole, how much should
one expect the far end of the hole to be from the ideal?

Your design process in backwards. How accurate does it HAVE TO be?

If the exit location is .0023" from where the nominal location is, what
happens? What about if it's .0079"? Will the part not function correctly?

Do you have the capability to measure the location, size, profile and
possibly the roundness of the hole? Would you know if it was straight or
not? What if it has a bellmouth profile or bends as opposed to going
straight (but not necessarily concentric with the axis of the spindle)?

Perhaps these are obnoxious questions, but the tolerance gods don't work in
.0002, .001 and .005 increments...

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.

Regards,

Robin

Maybe get some plug gages to check the results. They will give you a real good
idea how your holes come out, inexpensively. Have you tried drilling something
yet? Do you have measuring instruments to check the results? Theory is great,
but how about taking a cut and checking it? You'll find out if something is
amiss and have some procedure to either do again or modify if need be. Make
some chips!

mj