Full Chisel Blog

May 24, 2008

Center Bits

Filed under: Drilling,Uncategorized — Stephen Shepherd @ 7:30 am

I don’t think I have a favorite drill bit, I like any good sharp (traditional) drill bit, however Center Bits have a special among my tools.  Not only are they really fine bits, they look great, and the hole it drills is fun to watch.

Center Bits

The Gnomon is 6″.
This is my modest collection of old drill bits, the one with a notch in the tang is for a spring chuck brace (I will get to those later), is shorter and earlier than the longer bits.  The reason I have a modest collection, is they are difficult to find and No one is making any reproductions!

Center bits are great for shallow holes, some have large center pivots that can pierce through thin work.  The reason it is used only for shallow holes is that there is nothing in the design of the bit to keep it from drifting as one drills a deeper hole.  Other bits drill straighter holes, such as gouge, spoon and twist augers (they are next).

The center point provides the pivot and as the bit is turned, the spur scores the wood prior to the lipped cutting edge, excavating the wood.  It is actually fun to watch the bit starting its cut (kids like watching the center bit & have fun drilling holes with it, however my spoon bit and twist augers are popular).

There are some variations of the center bit such as a counter-bore bit that has a solid plug center instead of a pivot point.  This plug goes into a pre-existing hole and drilles a larger counter bore (such as split saw nuts on hand saws).  Other variants include a button bit (for making clothing buttons) and some larger bits have double scoring spurs.

Now the disadvantage (that of wandering) can be used to an advantage, as once the hole is started it can be tilted and the angle of the hole changed during drilling, can’t do that with other bits.

When I attach table tops to aprons, I use center bits to drill the pocket hole on the inside of the apron, and a gimblet bit to drill the screw hole through the top of the apron.

The points, three or four sided needs to be sharp, the scoring spur honed on the outside and sharpened on the inside (so the diameter is not reduced, it much match the lipped cutter).  The lipped cutter is sharpened like a chisel or plane blade, flat on the bottom and the bevel on the top.  I store these bits in a place where I can not touch their sharp little points as they can be nasty.

When drilling through holes with this bit, you can reverse and come back from the off side, but the holes don’t always line up, they can be off a bit.  If it is important that the hole is straight, I back up the stuff being drilled with a scrap piece of wood and drill all the way through.  This prevents a spelk on the off side.

If someone, anyone were making reproductions of these bits, I think they would sell a lot of them, I would buy a few sets.



  1. Stephen
    Interesting thread you got there on the various brace bits. The center bits are my favorite too. They look deceptively simple but lots can go wrong with careless sharpening. For example, often the center spur has been sharpened off center causing the bit to cut a bigger hole or oblong hole. Or the spur is broken or sharpenend off shorter than the router (the chisel part which remove the wood)which prevent the bit from cutting cleanly. And then there are the spur and router cutting geometry all messed up by improper sharpening, preventing the bit to remove wood properly. Except for a shorten or broken off spur, all of these faults can be rectified by correct sharpening.
    Looking at your bits on the pic, some of them I would have rejected because the spur appear too short.

    The best text so far that I came accross explaining the proper cutting geometry and sharpening of these bits is The handyman’s book by Paul N. Hasluck which was originaly published in 1903 and reprinted in 1995. It also cover very well the other brace bits and etc.


    Comment by Robert Demers — May 24, 2008 @ 6:11 pm

  2. Bob,

    Your comments are very appropriate and the bits can be improperly sharpened or just used up. Yeah, there are a couple that are marginal, they have seen a lot of service and just barely work, but with so few to choose from, I keep them around. I have seen his book on carving, not sure if I have seen his handyman’s book.

    Hopefully someone will start making reproductions.


    Comment by Stephen Shepherd — May 24, 2008 @ 7:07 pm

  3. Stephen,
    I too love these old bits. I have a small collection of them including a counter-bore bit, which thanks to you I just found out what it was for. Do yours match up to modern hole sizes such as 1/2″ or 3/4″? Some of mine do, and some are slightly larger/smaller than a standard hole size. Just wondering how the exact sizes affect your use of them. Also, have you ever thought of filing one down a hair to make it a more exact size. So far I have not been brave enough to try that. Moreover, depending on their use, the exact size issue may be irrelevant. Thanks, and a nice post!

    Comment by Jason Stamper — January 23, 2013 @ 3:34 pm

  4. Nice article. I belong to a loosly organized group of blacksmiths, woodworkers and other artisans, the New River Forge Council. We are part of the Appalachian Blacksmith’s Association. We have recently started a project as a fundraiser for our group…we are building a woodworker’s toolbox and making all the tools to fill it. It will be raffled off when finished. Anyway…our first tool is a small bit brace. At our meeting last night (timely, huh?) we forged a spoon bit and I roughed out a center bit. It is the second I have made. The first one cut, but not as well as my old originals. You can see a picture of it at my blog: wolfshieldforge.wordpress.com. I am still figuring out the geometry, but I believe that once I figure it out, it should not be too difficult to forge a small set. I have no plans to start selling these, but my point is that any reasonably skilled blacksmith should be able to reproduce these for you. I can probably recommend a couple if you are interested.

    Comment by BART JOHNSON — October 25, 2013 @ 5:53 am

  5. after purchasing an old tool box loaded with dirty tools,i have found 4 of the Gnomon Bits,being a seller more than a buyer-does anyone have a suggested price I could put on them.i have been buying and selling for over 45 years and this is the first lot of them I have come upon,living in a small town in Nova Scotia we have plenty tool collecters,but most often they don’t want to pay you as much as you have paid to acquire items. I would appreaciate any feedback that is available
    broidy01@gmail.com will reach me
    thanks in advance –Richard

    Comment by richard — January 17, 2014 @ 1:29 pm

RSS feed for comments on this post. TrackBack URL

Leave a comment

Powered by WordPress