Proctologically Violated©® wrote:
Awl--

I'm sure I can "solve" this by being more vigilant, but that it happens at
all seems sort of strange.

Briefly, I'm drilling a 3/16 hole 3/4" deep in 1/4" round brass: face,
c-drill, then a 3/16 drill (tailstock/drill chuck, 5C collets).
If you want a round hole of constant diameter, you should bore it.
Twist drill bits are notorious for drilling oversize, not round holes
that wander through the workpiece. If you want a hole for a bolt to
pass through, they usually do OK. If you want a hinge-type fit, a
drill bit is really too imprecise. For just a few, use a boring
bar. An impromptu boring bar can be just an end mill of suitable
diameter, set so that one cutting edge is level and facing toward
the operator, with just a slight angle such that the side of the
cutter doesn't rub the side of the bore. Sometimes an end mill with
a broken flute can be used for this.

You will get a smooth hole that is quite round, concentric, straight
and of constant diameter if you bore it.
Any idears?
Mebbe on my next have-at-it here, I'll test the female portion w/ a 3/16"
dowel, to at least try to identify the offending piece/operation.
It's pretty clear what the problem is (diameters!), but puzzling as to *why*
this problem is occuring.
A dial caliper's inside points should be able to detect the changing
ID, if you are careful. As you say you are using a twist drill, I'd
think it is most likely that the ID is where the problem lies. You
can often SEE the 3- or 5- lobed pattern that develops as a twist drill
bores into a piece. As it gets deeper, the side of the drill supports
the shaft better, and it starts to drill straighter and with less
vibration. So, the first diameter's worth of hole is really out of
whack, and then it gets closer to the stated drill diameter. I think
a re-sharpened twist drill that has uneven grinding on the flutes
is more likely to cause this problem. And brass may be more prone
to develop these problems, as it has a "digging in" nature when
positive rake tools are used.

Jon