All Over The Dial
There are endless variations in the dials used on mechanical dial indicators. In most cases, though, they can be broken down into two distinct styles: balanced and continuous.
Director of Precision Gages, Mahr Federal Inc.
There are endless variations in the dials used on mechanical dial indicators. In most cases, though, they can be broken down into two distinct styles: balanced and continuous. Let’s take a look at both.
With a balanced dial, the graduations around the dial represent the smallest value, or resolution, as marked on the dial face. The numbers on the indicator face are an aid in counting the value of the divisions, usually grouped by 10, but also representing 2, 5 or 20 steps around the dial.
In reading a balanced dial indicator, keep in mind that you are using a comparative indicating device, and the indicator reading usually means nothing by itself. The indicator reading must be added to or subtracted from the value of the reference or master to which the gage was set.
Revolution counters are those extra little hands on indicators that keep track of the number of times the big hand completes a revolution. These may seem a little strange, as the typical ANSI balanced dial indicator has less than three revolutions available, but it is much easier than it might seem to miss a revolution. A warning that the indicator has gone through, or not reached its “zero” point, is very helpful in preventing bad parts from being accepted.
But the warning can also tell you more than that. The revolution counter can tell you whether the indicator is within its measuring range, or in the pre-travel or over-travel area. It can warn you that the indicator may be too high in amplification for the application, and that perhaps a slightly longer range, lower resolution indicator would be better. Or it can provide an indication that something has shifted in the gage setup, or even that the part being measured is out of control, indicating that something in the process has shifted.
On the cautionary side, the revolution counter should not be relied on too heavily as a measurement device. The counter is pretty small, so to use it as part of the measurement is not always reliable. And, as with all mechanical indicator specifications, the longer the range, the more open the performance tolerances become. On balanced dials, use revolution counters for their intended purpose: a warning indicator. They can be a sure sign that something is wrong.
Continuous dials are usually found on long rang indicators that typically have more than the standard 2 1/2 dial revolutions. Some may even have extremely long measuring ranges. The dial is read much like a balanced dial, but there are no minus readings and, generally, there are larger numbers. Long range dial indicators will almost always have a revolution counter. Here the revolution counter is invaluable. It is used to keep track of the number of times the indicator hand has moved past zero, much better than your eye could. With a combination of the revolution count and the graduation count, a measurement can be determined.
The range of these indicators is a valuable characteristic in many applications.Typically, long range indicators will be used on a bench stand, allowing the user to measure a wide range of parts using the base as the reference point. The long range indicator can also be used on a machine to monitor the position of a slide to aid in manufacturing a part. Both are acceptable applications.
Keep in mind that long range dial indicators are also comparative instruments, even though they may have measurement characteristics. Used improperly, they can cause potentially serious errors. As we noted, the longer the range is with these indicators, the larger the acceptable error. In some cases, a 2 percent error is typical. Because 2 percent of 4 inches is 0.080 inch, this level of inaccuracy could constitute an error larger than the tolerance range on many parts. The proper method to use the indicator would be to set the gage with a master at 4 inches. The rev counter and the hand might be set to “zero” as the 2 rev position for its starting position. Then the gage would act as it were intended—as a dial comparator.