How to size drill bits

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Drill bits are sized, or measured, by the diameter of the solid metal shaft at the base of the drill bit's cutting flutes. For example, a 1/4 inch drill bit is milled from a piece of 1/4 inch diameter high speed steel. This size drill bit is measured by four different scales: English or SAE, metric, wire gauge and decimal.

Measure the diameter of a drill bits shaft with a dial or digital caliper. The caliper is an extremely accurate measuring tool. It can take measurements down to a few hundredths of an inch.

Convert the digital measurement into other measurement systems with a conversion table. The digital measurements can be converted to English measurements, metric system or wired gauge conversion tables, like the one on http://www.csgnetwork.com/drillsizeconvert.html.

Convert digital measurements into English or metric systems of measurements using the following simple equation. Digital measurements are in hundredths of an inch, so a measurement of .25 is the fraction 25/100, which is 1/4 of an inch. If using metric measurements, 1 inch = 2.54cm. So convert a digital measurement of .25 with the following equation: 25/100 inch * 2.54cm / inch = .635cm.

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