On 2 Jan 2004 10:31:04 -0800, (Dan Caster) wrote:
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?

Even with perfect alignment and entry, twist drills won't reliably produce
a straight hole to depths much greater than about 2 drill diameters. Any
deviation from perfect tip symmetry, variation in the drilled material's
uniformity, or recutting of chips will cause the drilled hole to drift out of
line. The limit case is where the bit binds and breaks in the crooked hole.

For best results in deep hole drilling (ie holes deeper than about 2 drill
diameters), you need to use a different tool than a twist drill. Gun drills
use a supported single point cutter (and high pressure oil down the stem
to clear the chips). This solves the problem of tip asymmetry, and the
problem of recutting chips, but there's still the problem of non-uniformity
of the material being drilled. Any hard or soft spots in the material being
drilled will still deflect the cutter and give you a crooked hole.

Now all this begs the question of what "straight" is. How much deviation
from a straight line the hole can have is a function of the particular purpose
of the particular hole. Most of us would agree that a deviation of half a
hole diameter would be "crooked", but might suffice for a drain passage.
A 10% deviation might be acceptable in some cases. Other cases might
require 1%, or even less.

AFAIK there are no standard or guaranteed numbers for the deviations
from perfectly straight a given twist drill bit will produce in a given material.
I've drilled deep holes in wood for alarm wiring where you couldn't see the
exit when looking down the entry hole. In other words, the drilled hole's
wander exceeded one drill diameter. That was Ok for feeding wires through
a wall, but wouldn't be at all acceptable if a rotating shaft had to pass
down the hole.

I'm afraid your question is in the form of "how high is up?" There really
isn't any single hard and fast number that will answer your question.

Gary