Less Is More with Dial Comparators

Most dial indicators have a total measurement range of at least 2.5 revolutions of the needle, as per American Gage Design (AGD) specifications. Additional range in the traditional indicator was useful many years ago, when machine tool accuracy demanded a broad measurement range to help machinists "creep up" on a specification. Now, gaging suppliers often recommend that an indicator be chosen so that the tolerance range for the parts being measured covers between one-tenth and one-quarter of a single needle revolution. This provides a large enough tolerance zone to read easily and leaves more than enough area on the dial to see what is out-of-tolerance. It is a rare occurrence when anyone actually bothers to read a gage if a part is more than a half revolution out of tolerance.

Two and a half revolutions are simply unnecessary for most comparative gaging applications, and sometimes they are a real liability. Considering how quickly the needle swings on an indicator, it is not surprising that machinists occasionally miss a revolution. On a standard dial indicator, a measurement that is a full revolution out-of-tolerance can appear to be exactly on specification to an operator who is distracted – or poorly trained, poorly supervised or hurried. Errors may occur through simple inattentiveness or an absolute misunderstanding of how to set up and master the gage.

There is at least one documented instance in the aircraft industry where an entire run of oversized parts passed through inspection and were assembled into components, which were then installed in subassemblies. It is not documented what happened to the machinist/operator or his supervisor when this costly error was discovered. However, the situation has surely been repeated in other companies and other industries.

If the recommendation is to use an indicator that covers the tolerance band with between one-tenth and one-quarter of a needle revolution, why use a gage with more range than needed? This is where the dial comparator can be a viable replacement for dial indicators, especially when measuring to very high resolutions (such as those approaching 20u”/0.5um). Any analog device has range versus resolution constraints, and the dial comparator is no exception.  It has higher resolution than the dial comparator, but also much less range. However, this is the specific advantage we want in our measuring processes. Limited range is not a major factor in very high-tolerance work, and it can even prevent measurement errors like those described in the case above. 

Dial indicators are not normally thought of as providing a high-resolution readout. Their high-confidence zone typically ranges to .0001"/2um. Manufacturing dial indicators to the highest possible standards results in only modest improvements in resolution. This is because the substantial number of parts in a dial indicator generate a build-up of tolerances. The high amplification required for ultra-precision measurements tends to magnify these errors, which can show up as degraded accuracy, hysteresis and/or repeatability.

Measurements at this end of the spectrum would normally be reserved for an electronic amplifier with high-performance electronics. However, there may still be reasons to prefer a mechanical measurement tool. Typical examples include budgetary restrictions and avoiding the need to cable a hand gage to a transducer for applications requiring portability.

Thus, the dial comparator solves the inherent mechanical problems of the geared dial indicator but at less cost than the electronic amplifier and probe. Here is how this can be accomplished:

• Travel of the spindle in a dial comparator is guided by a precision ball guide. This not only eliminates friction, but also provides strong axial stiffness and reduces side play. This assures results in a near one-to-one translation of the motion to indicator movement.
• A shock-proofing system isolates the spindle from the shock created by the banging of the gears during rapid movement. This also reduces magnification errors by reducing the number of required gears.
• Jeweled components maximize sensitivity and accuracy in the movement of gears and levers, while built-in calibration adjustment enables fine-tuning for even the most minute errors.
• All of these features ensure the highest accuracy, but the dial comparator still retains all those features which make dial indicators extremely practical and easy to use. These include: a built-in, lockable, fine adjustment control; adjustable tolerance markers; and very fast response indicator hand.

So, what is the bottom line? A dial comparator has five to 10 times better resolution and accuracy than a dial indicator without a significant increase in price. The electronic amplifier has about 10 times better resolution and accuracy than the dial comparator, but at a higher price and a greater risk of problems associated with having too much resolution for the gaging being used. So, where range is not a concern, the comparator offers the best resolution and accuracy value. If you need high performance on a budget that is a little tight, the dial comparator could be the solution.