On a project where I had approx 80 holes (laid out by hand), I found this to
work reasonably well.
Properly pointed and sharp punches should be used.

1. Lightly prick punch doing your best to hold punch square to the work,
and doing your best to tap the punch dead on so as
not to move the punch when struck. Check against layout lines. Slight
corrections can be made by holding punch at an
angle (pointing in direction you want to move) and hitting again.

2. When satisfied with location, use a center punch to deepen the
indentation, again keeping everything square and striking
the punch dead on.

3. Since my drill press is el cheapo and has quite a bit of wobble, I used
a small drill (.030 inch or so) to drill to a depth of
approx. 1/16 inch. A drill this small will 'bend' slightly and go into
your center punched indent. If the center drill you're
going to be using is relatively small, might want to follow with a
second drill, slightly smaller than the center drill pilot.
Center drills have a nasty habit of clogging with chips and breaking.

4. I followed this with a center drill, going deep enough so as the
countersunk portion was slightly larger (+ .020 dia.)
than my finished hole size. The countersink allows the final drill
to have only a small amount of material to remove
before reaching actual size and consequently helps hold location.

5. Then drill to final size if your machine, setup, etc. are up to the
task. A twist drill, like any cutting tool, performs best
when operating at proper sfm and feed. For example, in mild steel, a
1/8 dia. twist drill should run about 3200 rpm.
My drill press doesn't go that fast, so 'lighter' feed is used in
order not to force the drill.
Note that even 'perfect' drilled holes will probably drill oversize
to some extent.

6. Probably most critical to maintaining location and drilling a
'straight' hole is having a properly sharpened twist drill.
A properly sharpened drill should produce chips the same thickness and
width from both cutting edges. Most job
shops try to ensure this by starting with NEW high quality cutting
tools.
Keep in mind that if the workpiece is clamped in such a way that your
carefully located hole is not central to the
spindle axis, your location may be lost and you may end up with the
hole not being 'straight'.

Rigid machine spindles, sufficient horsepower, controlled feed rates, flood
coolant, etc. allow one to skip the small pre-drilling, etc. and are an
ideal situation for spotting drills as opposed to centerdrills.

Good luck!




"Bill Schwab" wrote in message
nk.net...
Jon,

The reamer follows the centroid of the hole, which has no guarantee of
being
anywhere near where you wanted the hole. So, better drilling techniques
will also give you a better final hole size. For almost any job where I
want
a hole to line up with something, I use a milling machine, clamp the work
in
a vise or to the table, and start the hole with a center drill first.
Once the
center drill has made a pretty good starting hole, the twist drill will
follow
with a lot less of this shuddering, and the final hole will be both more
round
and closer to where I started it. I usually get hole locations within a
few
thousandth of an inch of intended position this way. Using a center
punch
and then drilling directly with a jobber's length drill, the hole
position can
drift 25 thousandths of an inch, or even more.

How much of a hole do you drill with the center? I've been following
Harold's recommendation to touch the work to make a mark and check the
location before doing further damageg - two fewer pieces of scrap and
counting. If where I intended, I enlarge the mark, but my gut sense
reading the above is that I should be doing more??

Bill