How to read a dial indicator
Dial Indicator Primer
A Look Inside
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).
Classes of dial indicators:Source: www.ts-aligner.com