PRECISION MEASURING INSTRUMENTS AND TOOLS


This guideline is prepared to give a brief idea about the devices that can be used for the measurement of the inside diameter of the hole and outside diameter of a shaft. Slide calipers (Digital, Dial or Vernier), outside/ inside micrometers, bore gauges and pin gauges are often used for diameter measurement and control in mechanical engineering applications. The selection of the tool depends on the accuracy requirement and the feature to be measured.


Vernier Caliper

Slide Calipers: A tool used to measure the inside and outside distance (diameter) and depth of a feature. Different models exist such as digital, dial and vernier calipers. Measurement accuracy of a slide caliper generally ranges from +/- 0.001 inch to +/- 0.0015 inch (+/- 0.02 mm to +/- 0.04 mm). These values depend on the quality of caliper and measurement length. Different measuring length options exist but common one is 0 to 6 inches (0 – 150 mm).



Outside Micrometer

Micrometers: An accurate measurement device to measure dimensions of an object. It’s also known as micrometer screw gage since there is a calibrated screw exist inside the micrometer. Main types are outside, inside and depth micrometers. Outside micrometers are used to measure diameter and thickness of shafts, wires, plates. Inside micrometers are used to measure diameter of holes. Depth micrometers are used to measure the depths of holes, slots, shoulders. Measurement accuracy of micrometers is in the range +/- 0.00005 inch to +/- 0.001 inch (+/- 0.001 mm  to +/- 0.02 mm). Accuracy value depends on the micrometer type, quality of and the measurement range.



Bore Gauge

Bore Gauge: A tool to measure the bore diameter of cylinders, pipes and bearings. There are different types of bore gauges which can be classified as dial bore gauges and transfer bore gauges. Transfer bore gauges consist of telescopic gauge and small hole gauges. In transfer type bore gauges, there are no scales on the tool. After the measurement with a transfer type bore gauge, the distance between contacts of the gauge are measured with a caliper or micrometer. In case of dial bore gauge, measurement is read from the dial. Different measurement length options exist.



Pin Gauges

Pin Gauges: These tools are used to check the limits of size of a hole. Pin gauges are precision machined metal plugs and there are different tolerance classes. Pins can be either “plus” or “minus” tolerance pins. “Minus” gauge pins are most commonly used. GO/NOGO measurement shall be done with pin gauges to check upper and lower limits of a hole.


Reference:
  • Oberg.E , Jones.D.J., Holbrook L.H, Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc. , pp. 675 - 732