Dial Test Indicators & Dial Indicators (DTI, Dial Gauge, Digital Dial Indicator, Digital Dial Gauge)
First, what’s the difference between a Dial Indicator and a Dial Test Indicator?
It’s pretty simple. The Dial Indicator has a plunger and reads how far the plunger is raised by the surface below. The Dial Test Indicator has an arm that sweeps an angle. It measures how far the arm is pushed sideways. Both are handy, but you’ll tend to use a Dial Test Indicator more often.
These photos should make the differences clear:
Mitutoyo Dial Indicator reads how far plunger moves up to nearest thousandth of an inch (0.001″)…
Interapid Dial Test Indicator, Tenths Accuracy (0.0001″)…
Once again, a Dial Indicator measures how far up the plunger moves while the Dial Test Indicator measures how far the tip of a needle moves when deflected sideways.
Why use the Dial Test Indicator more often?
Largely because these instruments are typically used to measure relative motion, not absolutes. For example, to ensure a vise jaw is square with the travel of the machine, you would sweep the arm of a Dial Test Indicator over the jaw while moving in the direction of travel. The DTI (Dial Test Indicator) will read how far off square the vise jaw is. When it doesn’t move (or doesn’t move more than is acceptible for your application), you will have verified it is square and you can lock down the vise.
There are also operations where the Dial Indicator is handy. For example, you might determine the zero point on your machine’s Z-axis by putting a Dial Indicator in the spindle and jogging down Z until the indicator is zeroed.
Dial Indicators are often mounted using a lug on the back. Dial Test Indicators typically mount via either a dovetail or a small shank that can go into a collet or chuck.
Interapid: The Best Dial Test Indicators
I’ve tried a bunch of Dial Test Indicators from Starrett, Interapid and others, and I have to say, the Interapid’s are my favorite. They’re Swiss-made, and a tenths indicator will set you back $245, but they’re darned well worth it. For a beginner, I’d get the half-thou version and purchase a tenths indicator later. A half-thou Interapid DTI costs a little less at $184.69 and they’re a little less sensitive so adjustments are less fiddly.
Digital Dial Indicators
Dial Indicators are available with Digital Readouts too, but most prefer the analog dial. As mentioned, they’re often used for making relative measurements, and it’s harder to see relative motion with a digital readout than an analog dial.
Dial Indicator Stand
It’s often convenient to use a Dial Indicator with a magnetic stand. The stand will secure the indicator where it’s needed and a fine adjustment lets you zero the indicator.
Here’s a Noga Magnetic Indicator Stand, one of the best:
Dial Indicator and Dial Test Indicator Accuracy
Dial Indicators and Dial Test Indicators are available in a variety of accuracies and styles. The DTI’s are typically available with finer accuracy. I have both a half thousandth (0.0005″) and tenths (0.0001″) Interapid DTI’s. The most common accuracy for a Dial Indicator is 0.001″.
Given that DTI’s work by deflecting the needle through an angle, they are subject to something called “Cosine Error”. To minimize the error, use the DTI with it’s arm as close to the center of travel as possible:
This guide has been part of our Free Metrology Course:
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