I would like to know if it is possible to reduce a drill bit diameter (the body and not the shank). From times to times I need to work with dimensions that are not integer number or half number.... example: where I live I can only find drill bits 6mm or 6.5mm or 7mm..... however, in some cases I really need to have a dimension of 6.25mm.

Is this even possible to do, if yes how?

  • 3
    I imagine this would need a lathe, special cutting tools and probably a process to recreate the hardening of the drill's cutting edge? But if you have those tools wouldn't it be far better to make a bit from steel rod stock? Jun 25, 2014 at 9:36
  • 5
    Use the 6mm bit, and wiggle it a bit to widen the hole to 6.25mm. If you're dealing with such precision, you should probably be using specialty tools. For reference: 6.5 mm = 0.255906 in., 6.25 mm = 0.246063 in. A difference of 0.009843 in. or ~5/512 in. (0.009765625).
    – Tester101
    Jun 25, 2014 at 11:10

3 Answers 3


I would suggest that you look a little farther. Drill bits are available in many more sizes than you mentioned. One big supplier on-line, McMaster Carr shows available metric sizes for heat resistant cobalt steel jobbers drill bits in all of the following metric sizes:

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If you can find access to a supplier that has drills on the numbered gauge scale or the lettered scale they cross reference to metric sizes as shown here:

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In theory, yes, you would need to remove the hardening by heating to the proper temp and slowly cooling, turn it on a lathe, mic in your specs, re-harden... Best bet would be buying more drill bits. Like recommended above, McMaster Carr is an excellent source for more tools than you thought existed. Maybe you need SAE/inch pattern? 6mm=.236in 6.25mm=.246in 6.35mm=.25in 6.5mm=.256in 3/16in=4.763mm 7/32in=5.555mm 1/4in=6.35mm 9/32in=7.142mm

  • I'm not sure how this answer adds any additional information?
    – bib
    Jun 25, 2014 at 23:39
  • so I can not sharpen a bit without annealing it first..Nonsence! Reduce the lands by grinding, keep it cool or use a wet stone machine. If access to a lath use a tool grinder attachment for a precision grind.
    – gary
    Aug 27, 2018 at 20:32

You can buy metric and imperial drill bits.

If your local hardware don't sell both you can buy them online. (ebay) or other site.

Also if you try to machine a drill bit down it won't be as strong.

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