Robin,

In previous discussions of whether twist drills drill straight holes,
I never wound up with any idea of when I should just use a drill and
when I really needed to drill undersize and bore. So this time when
the question came up, I tried to get some numerical answers from
people with a lot more experience than I have. And I was looking for
answers from the machinists point of view, not the designers point of
view. True any number quoted here can be wrong, but at least now I
have a better idea. And maybe others have a better idea too.

Years ago I took a course in "Vacuum Tube Technology". It turned out
to be a course in the technology of how to build vacuum tubes, and not
the usual receiving tubes, but traveling wave tubes, backward wave
oscillators, klystrons, etc. It was taught by the troubleshooter for
Varian, Renee Rogers.
And one of the better things I learned from the course was his thought
process in troubleshooting production problems. He would put numbers
on things and do calculations to see if things were reasonable. An
example of the types of problems we did was to calculate the diameter
of the wire in a 10 ma fuse. The way the fuses are made is to put a
small platinum wire is a copper tube and swage the copper around the
platinum. Then draw the copper tube into a thin wire. Attach the
copper wire to the supports for it, protect the ends, and then etch
the copper off the platinum. So knowing the resistance and melting
point of platinum, the heat loss from radiation, you can calculate
the diameter that would melt at 10 ma of current. Anyway after that
class I always like to have some sort of number to base things on,
even if the number is only accurate to plus or minus 100%. It keeps
me from buying reamers for bolt clearance holes.

Dan






"Robin S." wrote in message m...

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.


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.....



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

Regards,

Robin