Dial Calipers

Dial calipers came on the scene around 30 years ago and have, overtaken the micrometer as the first measuring tool of choice for the machinist or toolmaker. They provide the ability to quickly read dimensions, both inside and out, like their predecessor and also feature the addition of the depth measurement rod. Also, the range from 0-6 or greater make these an attractive alternative to the micrometer which would require the purchase of multiple tools.
The dial caliper is also easier to read which translates to speed in the work place. As for the construction, the dial is fixed to the moveable jaw and rides along the tool bar or slide, meshing with a toothed rack.
This rack typically is a ground bar so each tooth is equal to .025 or one quarter of the dial resolution, and four teeth equal .100. The bar is graduated in .100 increments instead of the more complex .025 or .050 of the vernier and the dial is graduated from .001 up to one hundred thousandths.
First you read the number on the bar and then the dial figure to get readings to the nearest .001 of an inch. Another relatively new feature to the caliper is the ability of the inside measurement jaws to pass by each other so smaller dimensions can be read and a single scale is needed which further makes the readings easier to determine.
The depth rod addition extends beyond the bar and travels as the jaws are moved in and out . Larger length tools have a detachable rod so when not in use the rod can be removed to prevent obstruction. This rod simply unscrews and is quickly removed or attached.
Like the vernier, the dial caliper has a few drawbacks , the first and unfortunately the most annoying being the toothed rack that is necessary to carry the dial pinion gear is subject to contamination. Some designs discourage this from occurring, such as with rack teeth that point down.
This tends to cause the foreign material to be shed more easily. However fine metal chips or similar materials could still become lodged in the gullets of the rack. And if the pinion gear is run over this chip this will cause the pinion to jump to the next gullet and cause the tool to get "out of time" resulting in the failure of the hand returning to zero.
The second consideration is the I.D. jaws. In order to make the pass by design function they are considerably thinner than the contacts on the vernier style which means they can wear quickly if used on abrasive materials.