A cursory search indicates that drills size in round decimals of an inch (e.g. 0.3, 0.4, 0.15 inch) are not common, but are they a production item or would they only be available as some sort of custom or special purpose tooling?

What about for other "round hole" tools like mills and reamers? Looking up the second, they seem to be sized in terms of decimal inches, but the list I found didn't seem to have uniform increments so I suspect the choice of values came from something else.

  • $\begingroup$ Are uniform increments needed or are the tools designed around the dimensions used by designers and engineers? Sometimes the dimension is chosen as the next sensible dimension above the calculated value... $\endgroup$
    – Solar Mike
    Mar 6 '21 at 7:09
  • $\begingroup$ It is not common on daily encounters, but does exist in works require precision. See the drill bit size chart in the linked site - " engineersedge.com/drill_sizes.htm" $\endgroup$
    – r13
    Mar 6 '21 at 17:31
  • 1
    $\begingroup$ Yes they are, but the 3 standard size series in in inch are the "fractional", "letter" and "number" drills. $\endgroup$
    – Pete W
    Mar 6 '21 at 18:18
  • $\begingroup$ @SolarMike: Mostly I'm just curious, but I ran into a situation where the fractional, lettered and numbers left a gap around what I really wanted. I found a solution but it did get me wondering. $\endgroup$
    – BCS
    Mar 7 '21 at 0:36
  • $\begingroup$ @r13: that chart lists only letter, number, fractional inch and decimal mm drills, which I guess supports your first claim that they are not common. Am I missing something? $\endgroup$
    – BCS
    Mar 7 '21 at 0:38

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