Full Chisel Blog

September 27, 2011

Sharpening traditional drill bits and augers

Because of the many questions asked on this subject I have decided to write an article devoted specifically to that subject. Many old drill bits and augers may be brought back into serviceable and usable condition with a little attention to the overall bit and the cutting edge. The difference between a bit and an auger is the spelling and well maybe auger is used to describe larger bits. Whatever you call them, they make holes
and to work properly they must be clean and sharp. Many people sharpen up old
bits without any attention to the rest of the bit. While this will work it is not necessarily all you may do to these old bits.

First I make sure the bit is straight.
Many old bits such as gimlets are manufactured with a twist but some bits;
especially the small ones may be untwisted when forced in hardwoods. Putting
the correct twist back into the bit may rectify this problem. Take care, as the
metal in some old bits may be brittle. Now some collectors would say not to do
anything to the old tools, but if you are going to use them they need to work
properly. Also any bend in the shaft should be straightened out; the tapered
square shank  goes into the brace should be   filed to remove any burrs.

When originally manufactured many old bits were finished in the ‘bright’. In other words the surfaces were draw filed and polished for a shiny finish. This does two things, a bright surface resists rust (for a while) and makes chip extraction smooth. This is important on twist augers that cut more aggressively and require removal of chips from deep holes. Time and exposure have rendered many old bits pitted and rusty. It is important to clean off all rust, using electrolysis or surface abrasion.
Polish up the surfaces and keep them protected to prevent further rusting.
Always clean chips out of the augers after you use them to keep them in proper
condition and ready to use again.

Now that the bit is straight and shiny it is time to get to the cutting edge. There are three types of common old bits; gimlets, center bits and twist augers. Pod augers,
spoon bits, gouge bits and nose augers are not as common and are fairly easy to
sharpen. These bits have shafts are basically shaped like a gouge or marrow
spoon. The cutting edge is just at the bottom; the rest of the shaft should not
be sharpened as this may widen a hole if there is any wobble. The idea is to
cut a straight hole doesn’t tend to follow the grain and the cutting edge is just
at the beginning of the cut and the shaft guides the bit straight. Nose augers
have a turned in bit or nose that is sharpened first on the outside, then filed
on the inside edge to make a sharp cutting edge.

Gimlets or gimblets are a unique largely overlooked drill bit is readily available at flea
markets and antique stores. It is one of the few bits  produce a clean exit hole without a backup scrap of wood. It doesn’t make the neatest entry hole, there are trade-offs.
The first thing to do with all sharpening is to get the flat parts flat. See drawing #1. On a bit it may not look like there are any flats but there are.  With a gimlet, I use a fine flat mill file to make sure the cutting edge is flat on the outside edge. Keep the file flat on the outside of the bit, do not over-file this part of the bit, it needs to be flush with the rest of the outside of the bit or it will bind in the hole. Filing too much will result in the cutting edge being inside the circumference of the diameter of the bit and
it will not cut but bind. I file it until all nicks and pits are removed. I will also use a fine triangular file to dress the lead point on the outside edges. Make sure to get all edges that will be involved in the drilling process, all leading edge surfaces. See drawing #3

I then use a fine cut round file to work the inside curved surfaces. See drawing #2. This must be done at an angle as the twist of the gimlet progresses up the shaft. I have a couple of small chain saw files that are slightly altered for this operation. I broke the
un-toothed ends off the file and ground them square. This allows getting into
some areas where the non-cutting tip gets in the way. I will then go back over
the outside to remove any small burrs, and then go over the inside again and it
is ready to use. The gimlet bits cutting edge begins at the point and goes out
to where the bit is at its widest diameter, so make sure it is sharp out to
that place.

 

Left – Center Bit Right –
Gimlet bit

Showing sharpening sequence.

Center bits are perhaps the nicest traditional bits in my opinion. I like the way they look and I love the way they drill holes. They are not intended for deep holes, an
auger would be used for that purpose. But for shallow holes they work great
when properly sharpened. They may also drill a shallow hole with a slight curve
as you may change drilling angles as you drill. These should not be used for
dowels as they may produce crooked holes, bits like gouge bits and nose augers
are better suited for that purpose.

 

Again when sharpening a bit it is important the cutting edge be on the outside circumference of the diameter of the bit. With a center bit you have two cutting methods, the outside scoring spur and the angled excavation blade pivoting around a center spur. Make sure the spur is in the center of the bit and is sharp to a point. See drawing #5.  The scoring spur should be flat on the outside edge (to prevent binding)
and filed to a sharp point and leading edge towards the direction of rotation.
See drawing #1 & 2.This needs to be long enough to score the wood ahead of
the excavation blade to prevent tear-out. The excavation blade is sharpened
like a plane iron or chisel edge, first flatten the bottom (outside) and then
file the bevel on the inside, repeat to remove any burr. See drawing #3 &
4. A properly sharpened center bit is a pleasure to use.

Twist Augers are perhaps the most common type of traditional bit and are still manufactured today. Irwin pattern with a single twist are newer, but the Jennings pattern with its double twist has remained largely unchanged since L’Hommideau came up with the design in the first decade of the nineteenth century. There are some
variations at the cutting edge, spurs pointing up, down or both may be straight
or curved but all do the same process, cut and extract wood.

Sharpening Twist Auger
showing sharpening schedule

Like with all bits the outside edge must be on a plane with the circumference of the diameter of the bit. I place a file on the auger and go all the way around keeping the file mill flat against the cylinder formed by the twisted metal. This removes any burrs and insures the outside of the bit is flat, paying particular attention the spurs on the
leading edge. A special auger bit file is necessary for smaller auger bits and handy for larger auger bits as well. It has safe edges (no teeth) to allow filing up into corners on spurs. It is possible to do the same work with small flat and triangular files, but an auger bit file is a handy tool to have.  I then file the inside of the scoring spurs on their leading edge, which includes the top (leading) edge of the spur. If there is a spur in the inside (shank side), file it sharp just on the leading edge. See drawing #1 & 2.

Auger file, flat mill file,
triangular file & round file (9″)

The next part is the excavating blades, which are also sharpened like plane irons or chisel blades. I sharpen from the topside (center lead screw or spur) first getting it flat from the center screw/spur to the outside scoring spur. I then go to the other side
(shank side) and file that part of the excavating blade flat to a sharp edge.
Dress the top again, then the inside to remove burr. See drawing #3 & 4.

The final process is to sharpen the center point or freshen the threads on the lead screw. Some augers do not have lead screws but solid square or round pointed center points and these need to be sharpened to a point. Those with lead screws may need to have their threads freshened. This is done with a triangular file is run around the threads to remove any burrs and sharpen so they grip and pull the auger into the wood. The coarseness of the threads determines the kind of wood the auger was intended to use on. Those with fine threads are for hardwoods and those with coarse threads are for softwoods. Coarse threads for softwood are sometimes referred to as ‘Double Speed’. See drawing #5.

Initial straightening, cleaning and sharpening of old bits will require some time; work and attention to bring them back to serviceable condition. After the primary work bits are easy to maintain in a sharp condition by periodic touch up with a fine file.

If you live in an area of high humidity or are drilling a lot of green wood it is a good idea to oil the bits to prevent rust. Also make sure you clean out any chips before putting the bit away. Protect the new sharp cutting edge of the bit by storing properly.

Stephen

4 Comments »

  1. awfully quiet around here…

    Well, I appreciate the tutorial. Catching a little piece at one place and a different little piece somewhere else doesn’t make for coherence. THANKS Stephen for being comprehensive.

    Comment by Bob Easton — September 29, 2011 @ 8:45 am

  2. MR. Shepherd:-

    I also enjoyed your article, and found it to be very informative.
    Thank you.

    -Nathan Wolfenbarger

    Comment by Nathan W. — September 30, 2011 @ 9:16 pm

  3. This seems to be quite a specialized job to do it properly. I wonder if these files are still available.

    Comment by Dieter — October 12, 2011 @ 8:01 am

  4. Dieter,
    These are still available from Tools for Working Wood, there is a link on my blogroll.

    Stephen

    Comment by Stephen Shepherd — October 12, 2011 @ 12:43 pm

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