On 2008-02-28, wrote:
On Feb 28, 5:44 pm, RichD wrote:
Andrew,
you didn't actually say, but I suspect you changed out the boring head
for other
tooling then put it back in for finishing the other pieces, etc.
That's where the error lies. Don't expect R8 tooling in a worn mill
spindle to
repeat position precisely. Won't happen. The boring head has to be
redialed in
each time it's replaced.
RichD KT4IZ

[ ... ]

The problem for me, as I said, is my sheer inability to measure the
tiny tolerances involved in boring the holes - the only way to check
was to remove the job and then take it over to the arbor press and see
if it worked - but this meant that you lost tool registration so
couldn't put it back in the machine to try again - maybe there is some
"easy" way of doing this - any old hands got any helpful hints here?

It depends on what is in the shop's toolroom. If you can get a
tri-mike in the appropriate size range, you can get much better readings
-- to 0.0002" IIRC. Tri-mike is the Brown & Sharpe/Tesa brand's name.
Mitutoyo makes another model and Etalon yet another. These have three
arms which project at 120 degree intervals, so the tool is
self-centering. But they are quite expensive new, so a school shop may
not have them.

But -- another consideration is thermal expansion. You drilled
with a large bit through a thicker workpiece than the original one (thus
generating more heat) -- or perhaps just wider and thus more distant
from the jaws (so it cools more slowly). This means that the heat of
the drilling will expand the workpiece and the hole will be bored to
size *before* the workpiece cools off and shrinks. Once it shrinks, the
hole will get smaller along with the workpiece. (This in addition to
the uncertainty of the cutting radius of the boring head being exactly
the same with each reinstallation which has already been brought up.)

Thermal expansion is a killer when you are aiming for precision.
If you have flood coolant, you will have less problem with this.
Otherwise, the only choice is letting it cool for a *long* time between
rough cut and finish cut.

Good Luck,
DoN.

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