"Dan Caster" wrote in message
m...
And really nice to have some numbers on which to base
what I do.

I appriciate your situation, but I truely think you're going about this
situation backwards. Any number quoted here may or may not apply to you when
you drill your hole.

Coming from the perspective of someone who has both designed/built/hacked
mechanical systems together with very little knowedge as well as produced
parts to a print at work, your question is somewhat agrevating.

I think the bottom line is that a) you don't need to use anything beyond a
center drill and a drill for your hole and, b) you don't really need a hole
of that depth to achieve accuratetly threaded holes with a tap.

Not that I need to drill extremely accurate holes very
often, but I would rather not buy a boring bar for extremely small
holes if I don't have to. The discussions that say you can drill
straight holes or that say that twist drills don't drill straight
holes aren't much use without some numbers on what is straight.

But those numbers are supposed to come from you, the designer. Perhaps I'm
bitter from selling drill bits to people who want a "precision slide fit"
and reamers to others who want bolt clearance holes.....

I
mean that I could say you can't get a straight hole even if you drill
then bore and ream. Just depends on what you mean by straight.

Indeed, this is very important. I think Eric touched on the idea that the
effective diameter (such that it clears a nominally-sized shaft) is not the
same as the diameter at any given point in the hole, necessarily. Geometric
dimensioning and tolerancing addresses these issues. Simply having an X and
Y positional tolerance as well as a diameter tolerance will not suffice when
one is designing very precise holes. A tolerance zone must be defined that
extends at least throughout the entire length of the hole, and perhaps
beyond its physical boundaries.


But certainly a hole can be drilled that is more than adequate for
guiding a hand drill and or tap. On some occasions where I had to use
a hand drill, I have just drilled a hole thru a piece of two by four
with the drill press before going to where ever I needed to drill and
tap a hole. That is better than trying to drill and tap with no guide
and has always proved to be good enough.

Well, it's good we got this all (you guessed it) straightened out...

Regards,

Robin