On 3 Jan 2004 11:07:31 -0800, (Dan Caster) wrote:

It isn't the design process that I am thinking about. Say you have a
blueprint that calls for a 1 inch deep hole .138 diameter +/- .003 (
just so you don't worry too much about the diameter ) but true to the
axis within .004. Can you reasonably expect to drill it with a twist
drill? Since that requires that the angle be within about .25 degree,
my guess it might be possible with a rigid machine. But has anyone
here know if one could do it with one operation or is it something
that requires two operations and a lot more cost?

I agree that measuring is part of the problem.

Dan

Greetings Dan,
One thing to consider is the average axis of the hole. If you are
using a pin to measure location the pin that fits without bending will
average out the center if the hole is curved. And that's what counts
if the complete depth is being used as a locator. EG, a pin pushed in
the full depth of the hole and used as a locator. Drawings dimensioned
in true position will sometimes give a dimension in the Z direction
that says that a hole center must fall within a circle of a certain
size a certain distance above or below the X,Y plane. Some times the
centerline tolerance is expressd as an angle. And a 1" deep .138" hole
straight within .004 can be done by center drilling first and then
drilling with a split point drill. Even using just the split point
drill it could be done but the drill needs to be started very
carefully. In fact, with proper technique, I'd be surprised if the
hole wasn't within .004" This assumes the work is being done a good
machine. Not many drill presses would qualify. If I needed to get the
hole right the first time with no opportunity to drill a test part
then drilling, boring and reaming would be the method I'd use. I am
looking at a boring bar right now that will bore .060" min dia. with
,500 D.O.C.. This bar would be usd to bore the hole undersize and then
it would be reamed the full depth. Using a carbide D reamer that fits
the bored hole would work for sure. Of course, that hardly ever
happens. And I've got an Interapid indicator with a .094" dia. needle
that's 2.625" long. It reads to .0005" directly and can be
interpolated easily to .0002". So a .138" hole could be checked
directly for straightness and location.
Cheers,
Eric