In this article, we’ll cover what a dial indicator is, how to read one, how to use a dial indicator, and checking runout with a dial indicator.
What Is a Dial Indicator?
A dial indicator is a needle-based inspection tool that looks somewhat like a clock. It’s used to check whether surfaces are parallel to each other, whether axes are aligned, and to check the dimensions and runout of precision machine parts.
Properties of Dial Indicators:
The defining feature of a dial indicator is that it magnifies differences in the surfaces being checked through its contact tip.
Parts of a Dial Indicator
There are (+, -) marks or tolerance indicators to the right and left of the dial indicator’s zero line. If the needle moves toward the (+) side during measurement, the dimension is increasing; if it moves toward (-), the dimension is decreasing.
The large needle moves in front of a dial divided into 100 equal parts. Each division represents 0.01 mm, because when the measuring spindle moves 1 mm, the needle makes one full revolution in front of the 100-division dial. So one division of rotation equals 1/100 mm = 0.01 mm.
The dial indicator’s small needle rotates to show whole millimeters.
Dial indicators are set to a known reference dimension (using a gauge block or an arbor), and the parts to be measured are compared against this dimension to check whether the differences exceed the tolerance limit.
Reading a Dial Indicator
As shown in the figure above, suppose the dial indicator has been set to 30 mm with a Johansson gauge block matching the nominal size of the part to be checked.
While setting up, the dial indicator’s needle should have been pushed past the part’s tolerance limit and then zeroed. After setup, when the part is checked, the needle reads +0.12 mm. This means the part’s dimension is 0.12 mm larger than the nominal (setup) dimension. The part’s actual made dimension:
- At 30 + 0.11, the part is undersized.
- At 30 + 0.12, the part is within spec.
- At 30 + 0.13, the part is out of spec.
Another way dial indicators are used is shown in the figure above: the dial indicator is set to a starting point on the surface of the part being checked, and any variation across that same surface, or at the same dimension, is read from there. This also lets you check the part’s dimensions along with the parallelism and flatness of its surfaces.
Importance of Zeroing
Zeroing the dial indicator against a reference surface before starting measurement is required for an accurate result.
Applying Pretravel
Applying a small amount of pretravel to the dial indicator’s tip prevents loss of contact during measurement.
Measuring Deviation
Dial indicators are used specifically to measure deviations such as surface flatness and axial runout.
How Is a Dial Indicator Used?
Using a dial indicator is actually very simple. Once the indicator’s base is fixed in place, we touch its tip to the surface we want to check and zero the dial’s needle. Then, as the part we’re checking moves, we watch how the needle changes. If the needle stays within the accepted tolerance, everything is fine.
Using a Dial Indicator to Check and Set Up a Machine
For example, checking whether a lathe’s headstock spindle axis and tailstock axis are aligned is also done with a dial indicator. First, a perfectly cylindrical arbor is mounted between centers on the lathe. The dial indicator’s tip is pressed against the arbor and the needle is zeroed.
Checking whether a lathe’s headstock spindle axis and tailstock axis are aligned, using a dial indicator (image above)
Precision-made test bars (the red one in the image above) are also available specifically for checking, with a dial indicator, whether a lathe’s headstock spindle axis and tailstock axis are aligned.
As the dial indicator, moved along by the lathe carriage, travels along the test arbor, if the needle doesn’t move, it means the tailstock is on the machine’s axis. If the needle deflects, the tailstock’s axis is offset by the amount of the deviation.
Setup work for mounting auxiliary fixtures is done precisely using a dial indicator.
A dial indicator can also be used to set up mounted parts. The cylindrical and face surfaces of a part mounted in a lathe chuck are each set up individually with dial indicators.
The part mounted in the chuck is turned slowly, and its surfaces are lightly touched according to how the needle deflects. When the needle stops deflecting, the part is mounted with no runout.
When used for dimensional checks, it’s first set to the target dimension. The alignment of a keyway is checked as shown in the figure below: first (1), a gauge block suited to the keyway has its C surface brought level using a Johansson gauge block, and is checked with the dial indicator. Then (2), the same process is repeated for the D surface. If the needle doesn’t move in either case, the keyway is on-axis.
Video Guides: How to Use a Dial Indicator
How to use a dial indicator – video 2
Rules for Working with Dial Indicators
The measuring tips must never come into contact with light oil or grease.
On dial indicators where measurement is only made with the inward-moving measuring pin, the reading should avoid the return-travel range.
Only measuring instruments with a very small measuring range should be used for cylindricity and runout checks — for example, precision dial indicators, because here the measuring pin moves in the opposite direction while the object being checked completes one revolution.
Related Questions
A very small movement of the contact tip is converted, through the internal mechanism, into a much larger movement of the needle. Thanks to this magnification, even a deviation far too small to notice by hand or eye becomes easy to read.
The needle moving toward plus shows the measured surface is higher or larger relative to the reference point. Moving toward minus shows the surface is lower or smaller — in other words, deviating from the expected dimension.
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