On Thu, 28 Feb 2008 13:24:32 -0500, "Ed Huntress"
wrote:


"Jon Elson" wrote in message
m...
wrote:

Was shown how to use a boring head at school - (always wondered what
they were for) nice big Bridgeport mill, good quality tooling, so
cannot blame anything in that department.


OK. Having got it right, did two more practice holes in the scrap, all
went well. Been fooled before by blind luck, so wanted to check it was
consistent - it had taken EONS to size and square the two end bearing
retainer blocks, so didnt want to stuff them up.

Put the job pieces in the mill vice, locked the tables, drilled a
14.5mm hole and then, without changing anything in the X-Y dimensions,
used the boring head, Result - holes were undersize...stuffed 2 more
bearings.....(thats ok, had heaps of them from discarded VCR's)
So, you crept up on the diameter with many fine passes of the boring head,
then tried to bore the real part in one pass, without changing the setting
on the boring head?

Well, you have just met Mr. tool deflection! The boring bar (and maybe
the head, too) has bent away from the workpiece due to the thicker cut,
and you didn't cut as much as you expected.
No surprise. Possibly the alloy was different, too.

I also use those awful telescoping bore gages, and they give erratic
readings, but I know why. By feel, I can tell when I am getting an
accurate reading. The trick is to tighten the locking knob while the
measuring points are at an angle in the bore, then straighten them out so
the bore pushes them inward.
When you have them straight across the bore, wiggle the handle
to see if the points are truly on the diameter or have gotten stuck across
a chord. If not on the diameter, you can make the
points move at right angles to the diameter they are supposed to be
reading. I've always wondered if a Starret set of gauges would work
better than these Chinese klunkers.

I have a couple of sets of Starrett telescoping bore gages, and they work
pretty well. I've compared their readings with those from a Mitutoyo
three-fingered bore gage, testing them inside of ring gages, and I could get
final readings to agree plus or minus 0.0005" or so. (Getting to test things
like that was one of the perks of having had Mitutoyo for an advertising
client.)

But that was with "perfect" bores in top-quality ring gages. (I think they
were Mitutoyo, too, but I don't remember.) I wouldn't expect great accuracy
from telescoping gages unless the surface and the cylindricity are perfect.

This is why I recommended some coaching from someone skilled in the
use of telescoping gages. They are machinist's tools, simple but
quite capable of accuracy to a tenth or two if used properly. They
definitely can and will indicate matters of surface, departure from
cylindricty, taper and other distortions. It's up to the user to use
them both skillfully and intelligently. It ain't that hard to do.

Measuring a bore with a telescoping gage and a mike is not a 15-second
operation. It should be no more than a 30-second operation if the
job is to make a bore in ally to fit a bearing.

I once knew an old machinist that could use ordinary inside transfer
calipers and a mike to measure bores to well under half a thou. It's
all in the feel, drag in the hole and then drag on the mike. Decent
telescoping gages are easy peasy once learned. My Starretts are
slightly better than my Chinese imports, but not a lot better.