Reverse Polarity: Countering Your Clockwise Indicator

George Schuetz, Mahr Federal Inc.

Last issue we looked at the advantages of balanced and continuous dial indicators.  There are also measurements that require dial indicators with a counterclockwise dial.  The counterclockwise dial can have the same characteristics as the balanced or continuous dial except that it is reversed.  A balanced counterclockwise dial would be indicated by having a plus (+) sign on the left, while a continuous counter-clockwise dial would count from zero, counterclockwise around the dial face.

With a counterclockwise dial, the hand travels the same as it would normally, but the scale is reversed.  In effect the dial indicator has had its polarity reversed.

 The golden rule is that a clockwise dial indicator should be used when the indicator and/or the sensitive contact is on the opposite side of the part from the reference point, while the counterclockwise indicator is used when the indicator and the sensitive contact are on the same side as the reference point.

For example, when an indicator is used on a bench stand, the anvil is the reference point. The part rests on the anvil and the indicator and the sensitive contact touching the part are opposite the reference, as shown in Fig. 1.  This application would call for a clockwise dial indicator.

On the other hand, when using a bench thickness gage, as seen in Fig. 2, the reference, the sensitive contact, and the dial indicator are all on the same side of the part thus the need for a counterclockwise movement.  In this bench type of application, when a groove depth gets shorter, it pushes in on the indicator.  On a normal indicator this would generate a plus reading.  But with a counterclockwise/reversed dial, the indicator reads the correct polarity: i.e., a smaller reading.

Let's look at some other examples to demonstrate our golden rule:

First, take a look at the Bench ID/OD comparator in Fig 3.  When set up for an inside diameter measurement, we have a reference contact on one side of the part.  On the opposite side of the ID is the sensitive contact with the dial indicator on the same side, but on the outside of the part.  In this case, as part size increases, the sensitive contact moves away from the reference contact and towards the dial indicator.  This requires a plus reading and thus, a clockwise dial.

Now let’s imagine a horizontal bore gage (Fig 4) where the reference contact, the sensitive contact, and the dial indicator are all on the inside of the bore being measured.  In this case, as the sensitive contact moves away from the reference contact, the hole is getting bigger, but the indicator would normally display a smaller reading.  With a reversed dial this reading would be corrected.
Finally, one of the most common uses of the counterclockwise indicator is in a variable plug gage.  At first glance this may appear to contradict our golden rule mentioned above.  In this case, the body of the plug is the reference surface.  The contacts are on the same side as the reference surface, and so is the dial indicator but only through its transfer mechanism.  Therefore, as the rule states: when all three are on the same side, a reversed indicator should be used.

With mechanical dial indicators, it is a bit difficult to change the dial to/from normal (clockwise) to reverse (counterclockwise) readings.  However, with the digital indicators and electronic amplifiers available today this switching is done with the push of a button.