"Robin S." wrote in message
.. .

"Bill Schwab" wrote in message
...
Hello all,

With respect to dial test indicators, I am struggling with understanding
auto-reverse. It appears to be good, but what is it? I see lots of
mention of not needing to flip a lever; are such indicators obsolete
now?
Is there any reason one would want one?

I'm of the opinion that what they mean is that the indicator will function
in either direction without moving a lever. A Starrett Last Word is an
example of an indicator that must be manually reversed. A B&S BesTest is
an example of one that will function in either direction without moving a
lever. I own both and highly recommend the B&S over the LastWord. The
mechanics of both designs have little in common with one another and the
Last Word tends to give poor readings.


I can't think of a reason.


It might help to start w/ my import's behavior: the dial advances
clockwise from zero regardless of the direction of motion of the point.
It
looked weird at first (the dial markings suggest it would do something
else), but I figured it was probably by design, having something to do
with an indicator version of backlash near zero.

I'm not sure why they do this (I have one too) but I don't find it to be
an
issue. You can't use both direcitons at the same time anyway...

All too true----you can't use it in both directions at the same time, but
it's a nice feature that you can use either one without making any changes
(flipping a lever). A good example might be if you had a round object on a
rotary table and wanted to dial both the bore of the table, then the OD of a
part, in succession. All you need to do is move from one location to the
other. The indicator is ready to read in either situation. That would not
be true of an indicator with a lever, and there could be times when moving
the lever might not be convenient. Working in a deep hole might be a good
example of such a situation.



When indicating a vise or a part, I use the cross travel to make contact
with the point and then continue moving to get a non-zero reading and
work
relative to that. I don't bother to move the dial, though it might be
smart to "permanently" offset it so that I'm working at zero vs. 15.

15 is excessive. A preload of a couple of thou should more than suffice.

The amount certainly isn't critical, so long as one can read the amount of
deflection from one end of the object being dialed to the other end.

When sweeping the surface of a table to square a spindle, it's obviously to
advantage to have as little preload as possible so the tip can glide over
the T slots, or other examples of a similar nature, but otherwise it makes
little difference. Remember, you are not really measuring with a DTI
under setup conditions, but looking for anomalies so they can be eliminated
or minimized. The indicator, in use, behaves as a comparator. It's not
really "measuring", although it does display in increments that translate
into measurements.
..


While not in the market right now, I am thinking ahead to a rotary table
and wondering how I am going to align it. A centering indicator is one
option, but it looks a little long for a mill-drill. I might be able to
make it work by mounting it in an R8 collet, bumping in the table, and
then changing back to my ER chuck for normal work. With the mill's
dials
locked, (if needed) I could move the table to free up space under the
spindle, change, and then start work with the RT's axis at zero on the
dials.

Depending on what you're doing, there are a number of ways to setup a
rotary
table. I don't think you need a centering indicator though (especially
given
the price).

Any common DTI will suffice. It should be held in the spindle of the
machine and inserted into the bore of the rotary table. Do not rotate the
table, that won't tell you anything aside from the eccentricity of the
bushing in the table (which should be 0). The spindle of the machine is
rotated with the indicator in intimate contact with the bushing. The table
and saddle are then adjusted until you get an equal reading on all four
sides, or a constant reading, regardless of what it is. At that point,
you should mark your table and saddle (wax pencil) so you have a reference
point at which your dials will read 0-0, and each of them should be set such
that you know which direction has eliminated backlash. My style is to
always set the dials so they are reading in the right hand direction. That
way you never have to remember which way you set the backlash. Only under
very unusual circumstances do I set the backlash differently.


Typically you center the part on the rotary table first. Once that's done,
center the spindle over the rotary table.

While that might work, once a part is set on a rotary table, the center hole
is usually covered, so you can't use it to set up the table. It's
typically best to set up the table as above, then dial in your part. It's
generally more important to know the table is set properly, especially if
you have several parts to machine. Assuming you make a minor mistake in
the setup from a part, the same error is introduced to each and every
succeeding part that is set up. It could spell the difference between a
good part or scrap. Both methods will work, one has the edge.


Many people make tooling plates for their rotary tables so they can cut
into
the table (only slightly) if required to mill a complicated profile. If
you
make a tooling plate, you can place a dowel hole in the center and then
use
a dowel pin to center the rotary table to the spindle. The part too, if
you
can accomodate a dowel pin hole in it.

Yep! Very effective way to make setups on a rotab, especially if you're
stuck with a small table. My plate has random holes drilled and tapped for
hold down clamps, and I don't hesitate to add them as required. The plate
is aluminum, and considered perishable. Considering I have a 12" Bridgeport
table, my plate has a 1" diameter hardened and ground pin for the plate,
which locates from the center hole of the table. Setups can, therefore, be
torn down and re-established without losing concentricity.



Is there an easier way? Is there another type of indicator I should
consider?

I'd recommend getting one that will reverse without flipping a lever.
Levers
are annoying.

I couldn't agree more. I recommend the B&S highly. I've had two of them
for countless years (well over 30) and the only thing that has gone wrong
with either of them is the crystal shrinking.

Harold


Regards,

Robin