Although high resolutions approaching 20 microinches/0.5 micrometer would normally be reserved for an electronic amplifier with high performance electronics, there may still be reasons to prefer a mechanical measurement tool.
The lowest priced measurement tool is, of course, the dial indicator. But dial indicators are not normally thought of as providing a high resolution readout. Manufacturing dial indicators to the highest possible standards will result in only modest improvement of 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.
This is why electronic amplifiers have become so important. With only one moving part and the stability of solid state electronics, the errors are extremely small. Even though an amplifier may have very high magnification, the total error occupies a very small fragment of the instrument’s resolution. But again, because of price and portability issues, this may not be the answer you want to hear.
There is another form of mechanical gage that bridges the gap between dial indicator and the electronic amplifier. Known as the dial comparator, it solves the inherent mechanical problems of its close cousin, the geared dial indicator. This is how it's done:
- Travel of the spindle in a dial comparator is guided by a precision ball guide. This not only eliminates friction, but it also provides strong axial stiffness. This assures results in a near one-to-one translation of the motion to indicator movement.
- The ball guide also reduces the long-term damage to the spindle which can result from side play created when parts are forced from underneath the contact.
- A shock proofing system isolates the spindle from shock created by banging of the gears during rapid movement.
- Many of the gear assemblies used in mechanical amplification have been replaced by simple levers. Fewer components result in a reduced opportunity to magnify errors.
- Jeweled components provide maximum sensitivity and accuracy in the movement of gears and levers.
- A built-in compression spring provides a constant measuring force over the entire range of the indicator.
- Built-in calibration adjustment allows for fine tuning out the most minute errors.
While all of these features assure the highest accuracy, the dial comparator still retains all those features which make them extremely practical and easy to use. These include:
- A lockable, fine adjustment control.
- Adjustable tolerance markers.
- Very fast response indicator hand.
- Remote cable retraction of the spindle.
Any analog device has range versus resolution constraints—the dial comparator is no exception. Since it has much higher resolution, the dial comparator also has much less range than the dial indicator. However, this might even be an advantage in some measuring processes. Since these comparators are used in very high tolerance work, limited range is probably not a major factor and might even prevent some measurement errors. Some dial indicators employ a revolution counter. In instances where the operator fails to pay attention to the counter, recorded measurements may be off by a full revolution. A dial comparator is very much like a one rev indicator. The part has to fall within the measuring range of the indicator or else it shows up as being off scale. There is no chance of being read incorrectly.
So what’s the bottom line? A dial comparator has five to ten times better resolution and accuracy than a dial indicator. The electronic amplifier has about 10 times better resolution and accuracy than the dial comparator. The cost of an electronic amplifier and probe is almost seven times the price of a good dial indicator. So where range is not a concern, the comparator is your best resolution and accuracy value. If you need high performance on a tight budget, the dial comparator could be your solution.