Browse through a machine-tool catalog and you’ll find many sets of jobber-length drill bits. I’ve often wondered what jobber-length meant and after seeing yet another flyer from Travers or Enco, I decided to find out. Although you can find many mentions of jobber-length bits, it’s difficult to locate a clear definition. The Wikipedia defines jobber-length bits as those for which the length of the flute (the twisted part) is 10 times the diameter of the bit. Thus, a 0.25-inch drill bit should have a flute length of 2.5 inches.
But it turns out that’s just a crude rule of thumb. The definitions (note the plural) of “jobber length” comes from ANSI/ASME B94.11M-1993. As far as I can tell, without buying a copy of the standard, the ratios of flute length to drill-bit diameter vary. The standard defines lengths, not a fixed ratio. A #60 drill, for example, has a ratio of 16.7 to 1, while a #10 drill has a ratio of 12.6 to 1. What about the quarter-inch drill? It comes in at 11 to 1. (My drill dimensions come from the 26th edition of “Machinery’s Handbook,” a fine reference.)
While doing my research, I found an interesting and helpful article on drilling, center drilling, and spot drilling; “Get it Straight,” by Kip Hanson. Although 10 years old, this article remains relevant. You’ll find it here: www.ctemag.com/dynamic.articles.php?id=31.
I hope this blog entry wasn’t boring. –Jon Titus