Monday, November 21, 2011

Workshop Tips You may Already Know -- Drilling Perfectly Centered Holes

I'm often the last to discover a brilliantly simple method for overcoming some devilish workshop problem. So, in case this little technique isn't in your bag of tricks, I thought I'd pass it along.

Say you have to drill a hole through stock that is one diameter on the entry side and a second diameter on the exit side.  To make matters worse, there is no square reference surface to place against a fence, and you cannot tolerate any tear-out on either face.  This situation presents itself when making an electric guitar with a string-through bridge -- the strings come through the metal bridge, enter a 1/8" hole, pass through the body, and exit a 5/16" hole that contains a ferrule.

There are many ways that you could attempt this, but this method is foolproof.  Start with template with holes that match either the entry or exit hole dimension.  In the case of a guitar, the metal bridge works a treat.  Position the template on the correct side and affix using carpet tape.  Using a drill press, and allowing the bit to find its way into the template hole, drill most of the way through the piece.

The key to this technique is a purpose built jig with a post that matches this first hole.  In this case I use a small length of 1/8" steel rod and allow it to protrude from a piece of MDF by about 1/2".  Now, making sure that you have enough clearance between the bit and the jig (to allow sliding in the workpiece before plunging for the hole), center the bit on the post.  Insert the bit for the second diameter and double-check that the bit is still centered.

Now it is just a case of placing each hole over the post and drilling to the proper depth.  The resulting holes will be centered over the smaller holes, and keep the perfect alignment of the template.  One caveat -- place a thin piece of waste board on top of your stock when removing it from the post.  The snug fit can (will) make you pull hard enough to recoil the piece right into the bit above.

This technique comes from the archives of TDPRI -- a great source of information for building Telecaster-style guitars.  BTW, I'm finding loads of tips from the guitar-building world that have broad application in making custom furniture.  So I'll pass them along from time-to-time.



  1. Have you ever used a wooden dowel with a little wax on it (to get it out)? That way you could dimple it with the brad point of the first bit and line up with the dimple on the second.

    Of course, you'd have to replace the dowel every time .. probably .. just a thought.

  2. The wax is a good idea. The only issue I have (if I understand your question) is that the first hole is 1/8", and that forces me to use a twist bit, which leaves an indistinct mark (and can wander a little.) I use dowel centers a great deal in my work (like installing the arms on the prairie chair) and I think this is along the same lines as your idea. Thanks.

  3. I have tear-out. Thanks for the tip.

  4. Nice tips, thank you already want to share a great article.