Saturday, December 16, 2023

Does Anyone Make 14mm Twist Drill Bits Outside the PRC?

I need a 13.6mm 4" deep hole.  A 1/2" drill bit got me most of the way there.  The Sherline boring bar went about halfway down the hole at roughly the right diameter but their boring bar is not long enough.  I was thinking a 14mm twist drill bit would do the job but they all seem to be PRC.  

The Sherline boring bar is a 3/8" shank.  As you lengthen the boring bar relative to its diameter, it loses stiffness and the precision of the bore declines but on this case it is not going to be an interference fit.  That I will consider a 14mm twist drill bit should tell you that.

No luck on a 14mm non-PRC bit but Bad Dog Tools makes a 9/16" in Rhode Island.  Not evenly ridiculously priced: $30 including shipping and a lifetime warranty.

The laser pointer has a 14mm barrel so 9/16" is actually okay, maybe better than 14mm drill bit.  There is a pocket clip farther back on the pointer that sticks 2.5mm out from the barrel.  For this to work as intended with laser pointer centered to go into mount polar axis, I will need aend mill about 93mm long and 2.5mm diameter.  I would edge mill a slot in the vertical mill.  Obviously a end mill that long and thin will not be stiff enough and might well break in the process.  

Another solution is to mount the boring tool in the mill and go vertical in very small slices, say .0001" at a time.  But again where is such a boring bar?  Do I need to make my own?  A bar about 2mm wide and 100mm long with a carbide tip about 2.5mm wide?  This sounds very unstiff.

This sounds like another bad idea: use a long 1/4" twist drill as an edge mill moving from center in .0001" steps.  It might break the drill bit but at least it would be easy to remove.

The boring bar is 3/8" diameter.  Use .5" steel rod about 3" long.  Put 3/8" hole in one end.  Tap holes in that end to hold boring bar in place.  Put .5" rod in end mill holder.  Slide up and down with motor off about .0001" cut at a time.  The  .5" rod should be stiff enough for this; precise it will not be, but enough for this application.


  1. For holes that deep you need a parabolic drill bit, not a standard twist.

    Here is one from Brazil by way of McMaster Carr

  2. Wish I'd seen this earlier. This looks like a job for a set of reamers. When you're in the 1/2" range, they tend to be about 8" long.