If you were to open up your dial indicator
(not necessarily recommended) you would find a simple rack and pinion
gear system. The rack is connected to the plunger. The pinion gear is
connected (through a series of other gears) to the hands that you see
from the top.
The spring loading on the
plunger is part of a mechanism that counteracts play and backlash in
this gearing system. This mechanism is not effective at the travel limits
of the plunger. Indicators of any quality at all place those travel
limits outside of the specified measurement range. Therefore, from the
rest position of the plunger to the "zero" position there
is always some small amount of travel. There is also some travel left
in the plunger after you push the plunger to the specified measurement
limit. Measurements made from the resting point of the plunger are not
protected by the anti-backlash mechanism and are therefore not accurate.
The same is true for measurements that end at the full travel limit.
The photo below shows the
resting point and the "zero" position. If you push the plunger
in so that the large hand points to the "zero" position, the
small hand will also point at it's "zero" position. As you
pass this "zero" position each time the large hand rotates
around the dial, the small hand will increment by one count. The small
hand is keeping track of each revolution of the large hand. This corresponds
to one tenth (0.100) of an inch. The large dial is divided by 100 equally
spaced marks. Each of these marks corresponds to one thousandth (0.001)
of an inch.
The small knob on the side
of the dial indicator holds a clamp that locks the large dial (usually
referred to as the "scale") in place. If you loosen this small
knob, you will be able to rotate the scale freely so that the zero point
can be placed at any position that the large hand sweeps.
Watch the Video
I have prepared a short
video so you can see a dial indicator in action. Just use the
controls on the box below. If you are running an operating system
which does not support Windows Media Player version 9 then the box
will be empty. Instead, you can download and view the video from
this link: Dial Indicator Video.
Reading a Dial Indicator
The reading on a dial indicator
can be easily determined by paying attention to the positions of the
small and large hands.
The photo above shows the
small hand pointing between 3 and 4. So, that means the measurement
is between 0.300" and 0.400". The large hand is pointing at
52. That's 52 thousandths from the last whole number (0.3"). So,
combining the reading on the small hand with that on the large hand
results in a value of 0.352".
On some indicators, the
small hand will make two full revolutions during the travel of the plunger.
So, you need to keep track of which revolution (the first or the second)
you are reading from.
The small hand on the
indicator above does two full revolutions so each point has two numbers
on it. The outer number is for the first revolution and the inner
number is for the second revolution. So, the indicator above could
be reading 0.250" if the small hand is on it's first revolution
or 0.750" if it's on its second revolution.
Dial indicators are almost
never used to measure extended distances. Most often they are used
to measure small changes in distance. By placing the stylus of the
dial indicator against an object and rotating the scale so that the
large hand points at zero you establish a reference point. If you
move the object or the indicator, you will be able to watch the large
hand move from that reference point "indicating" the change
in distance. Most of the measurements you make with TS-Aligner Jr.
will be the result of moving the dial indicator in very controlled
ways. Some measurements (like checking blade flatness) require you
to move the object being measured (the blade).