Gunner wrote:

OOOH! I like that...Gunnerinos!

Chuckle..I need to use a DTI for measuring TIR of rotating collet
closer bodies. How do I actually measure .0005 runout with a DTI?

Actual tenths..not Gunnerinos. G This is why I asked the question
about pointer/lever length

Sorry to burst your gunnerinios, Gunner; but Tom's not correct. The
point length does matter. What an indicator actually measures is
angles. The angular deflection of the pointer touching the work is
magnified by the gears into a larger angle when the needle moves around
the dial. The linear distance (tenths, thousandths, or whatever) that
you read is just a convenient conversion from degrees, which is
accomplished by making the marks on the face of the indicator match the
angles produced by the pointer and gears.

Here's a real simple example: Think of an indicator on a surface
gauge, with it's pointer arranged to be perfectly horizontal, while
touching the top of a stack of gauge blocks. The pointer is exactly 1"
long, from the point where it's ball touches the gauge blocks to the
center of little axle it rotates on. To move the pointer one degree,
you'd need to change the stack of gauge blocks by 17.5 thousandths.
0.017452 inches, actually. That's the sine of one degree. If the gears
in the indicator multiply the angle by, say, a factor of ten, then 17.5
thousandths on the gauge blocks, which produces one degree of motion on
the pointer, will become ten degrees of needle motion on the face of the
indicator. In order for the indicator to work properly, the marks on
it's face have to be arranged so that 10 degrees equals 17.5 thousandths
when you read it. Or, the .001" marks on the indicator need to be
exactly .5714 degrees apart.

If this same indicator had a 1/2 inch pointer, and you changed the
gauge blocks by 17.5 thousands, that would create an angular motion in
the pointer of 2.006 degrees, instead of one degree. When the gears
multiplied it by ten, this would become 20.06 degrees of needle motion
on the indicator face. If the marks on the face are still .5741 degrees
apart, then 20.06 degrees would look like 34.8 thousandths, even though
the gauge blocks had only changed by 17.5. Bad news, of course.

For some kinds of measurements, you don't really care about any of
the marks on the indicator face except the zero. Concentricity can be
one of these. Whether the indicator reads accurately as it moves isn't
really an issue. All you care about is that it repeats to one
particular position. In that case, pointer length doesn't matter as
much; but it still affects (multiplies or divides) the sensitivity of
the indicator. Unless your readings really ARE zero, you won't be able
to tell precisely what they mean. With too short a pointer, the needle
will move too much in response to even the slightest provocation. If
the pointer's too long, the needle won't move enough to tell you whether
you're really as close to zero as you want to be.

The other factor, which makes all of this a little less perfect than
it might appear, is cosine error. As I'm sure most folks here are
aware, it's often pretty tough to set up your indicator as described
above, with the pointer exactly parallel to the surface plate, or
tangent to the diameter you're measuring, or whatever. That affects the
accuracy of measurement substantially. And, the affect grows
trigonometrically as you try to read larger and larger values on the
indicator dial. It might not mean much when you're looking at a
difference on the dial between zero and .001; but it can mean a lot when
the needle needs to move from zero to .010 or beyond.

Hope this helps!

KG
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