What Are Indicators For?
Next to calipers, the most common measuring tools used by machinists are dial indicators and dial test indicators. These precision instruments are used to precisely measure changes of dimension over relatively small ranges. They consist of a graduated dial with a movable face and a “hand” driven by some sort of probe that one applies to a surface. They are used while held in some sort of holder rather than in one’s hand. Here is a typical holder setup:
I am lumping the two indicator types together since it makes it easier to explain how they differ. The major distinction is one of range. The big one in the photo is a dial indictor: it has a plunger probe with an overall travel range of 1″ with each revolution of the hand covering .1″ or 100 thousandths which are the smallest units on the dial. I use my indicator for “rough” measurement such as getting a milling vise aligned or centering stock in a chuck.
The other is a dial test indicator. It has an overall range of only .030 inches and is graduated in .0001″ increments. Test indicator dials also tend to be marked counting up from zero in both directions and, like this one, have a lever probe instead of a plunger. Test indicators are typically used for really precise measurement such as checking the parallelism of two surfaces. Here is a setup I used to see how parallel my 1-2-3 block was :
I moved the block around under the probe and got at most about half a tenth movement. I may be detecting fingerprints on the polished surface. I am calling it good enough. 🙂
Indicators do come in digital versions but the vast majority are still analog dials. They come in both metric and imperial versions although in North America the vast majority available seem to still be imperial.
Indicators also come in a wide array of price levels. My dial indicator is a cheap one that cost maybe $50CDN with the stand. My dial indicator was purchased used and is not a cheap one, being made in Switzerland. You can easily spend several hundred dollars on an indicator but if you don’t have a pressing need it is probably not worth it. (I just liked the idea of measuring things in .0001″ increments. I have no other excuse)
There aren’t really genuine alternatives to indicators but cheap ones are readily available that seem to be quite good enough for most work. If you aren’t doing machining, you probably don’t need on unless you want to check the runout on your drill press or assess the flatness of machined objects.