On 2007-12-19, Nick Mueller wrote:
DoN. Nichols wrote:
Note that this is assuming that the feeler is set at the proper
angle to the workpiece surface.
The same is true for a long travel indicator (LTI). Admittedly, it's easier
to align the LTI properly.
Yes -- it simply needs to be put at 90 degrees to the surface
being measured. And the increased diameter sleeve around the plunger is
an excellent surface for that setting -- or the flat part of the back of
the case.
And if you mount it above a surface plate or baseplate so that
it reads zero (almost full extension) when contacting the plate, you can
then measure by simply lifting the tip and putting the object to be
measured under the tip.
Some of these are designed to give the correct
readings with the feeler at a 30 degree angle to the surface, others with
the feeler tangent to the surface.
And mine even has a table of correction factors depending on the angle. 0.96
for 15°
So you actually have a manual for yours. All of mine are from
swap meets, eBay auctions, or the like, and came without manuals, so I
have to experiment with gauge blocks or at least feeler gauges to
determine what is the proper angle for mine.
And add to that the fact that many of these indicators have
interchangeable tips -- of differing lengths.
That's why the manual clearly states: "Length of stylus: The use of stylii
of inappropriate lengths leads to measuring errors!"
RTFM. :-)
If I *had* the manual, I would. But then why do they supply
multiple interchangeable styli with them? This even goes back to the
level where the Starrett "Last Word" DTIs have spare styli in different
lengths.
So -- how do you make sure that the right tip is used on the
short travel ones
Easy. I never change it. I don't even see a reason to to so. And *IF*, I'll
only buy genuine parts from Mahr.
O.K. Those would not work on most of my DTIs, because they are
of different brands -- and often different stylus mounting systems.
-- and at the proper angle to the surface of the
workpiece?
By looking at it?
:-)
No really, a LTI has his own problems as the DTI has. In both cases, you
have to know how to use them. But I'm not telling you news.
Especially a LTI is a bitch if you want to meassure to a round bar with a
small diameter (OK, "use the flat tip, idiot!"). That's where the DTI
shines.
For measuring the diameter of a small round bar, I would use a
micrometer, not an indicator if I really cared about the results.
To my mind, the DTI has as its primary function determining
centering of a workpiece in a 4-jaw chuck, or in a 3-jaw or collet chuck
with a Set-Tru backplate. The ones which have resolution down to a
"tenth" (0.0001"), or to 0.001mm, simply make it take longer to achieve
the best you can measure. :-)
The long travel ones are mostly used for measuring carriage or
cross-slide travel on a lathe or milling machine (or a jig borer), often
with the aid of a stack of cylindrical standards in a V-groove between
the indicator's tip and the moving part of the machine.
Only secondarily are they to be used for quick-and-dirty
dimension measurements -- usually with a stand whose base acts as a
surface plate.
Oh yes -- I have also seen them mounted in snap gauges, where
you can be sure that the stem is parallel to the dimension being
measured.
And on a surface plate, with a height gauge, you can use a DTI
(mounted to the arm of the height gauge) to compare a workpiece to a
stack of wrung gauge blocks with the assumption that the two are very
close in height, so you are measuring only the difference -- with a
minimum of cosine error -- perhaps verified by using a feeler gauge on
the lower of the two.
Enjoy,
DoN.
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