Thursday, June 20, 2013

Carving Axe Part 2: Edge Geometry and Grinding

The defining feature of a carving axe is its edge geometry which gives it greater control in use than standard hatchets.  Carving axes can have flat symmetrical bevels but I prefer grinding them with asymmetrical bevels.  In use the wide bevel faces the workpiece and rides against it, supporting the cut, and enhancing control of the tool.  The comparatively low angle grind of the wide bevel, gives the axe an aggressive feel in use that most conventionally sharpened axes lack.  The short bevel deflects the chip and is ground at an angle that gives the edge durability.  I have found the necessary included angle of the edge varies from axe to axe, depending on the quality of the steel and its heat treatment.

Top view of asymmetrical bevels.
"Inside" bevel.
"Outside" bevel.
I spent February of 2011 and January of 2012 in the Palcazu Valley of Peru working with the sustainable forestry organization GreenWood and our Peruvian partner organization PaTS.  We were working with a group of artisans who primarily carve large wooden bowls.  One tree from their forest sold to loggers generates on average $50 in the local economy.  The same tree when carved into bowls and sold in Lima or exported for sale brings in close to $2,500 into the community.  Can't beat that.
Lucy Peña measuring a finished bowl.

On my second trip I wanted to bring a bunch of carving axes for the artisans to try.  Most of them carve the outside of their bowls with fairly light adzes or machetes and I thought a heavier tool might increase efficiency.  I rounded up six decent hatchets and went to work tuning and sharpening them.  This is the method I developed for reshaping the bevels.

Axe grinding.
The vee-arm grinder tool rest and a pivot stick clamped to the blade enable you to pivot the axe head to the left and right and get a very consistent hollow grind.  I follow this up by honing the bevels with diamond paddles.  The hollow grind is easy to resharpen and when honed results in a very flat bevel which makes the tool easy to control.  The vee-arm tool rest that I use is part of the Wolverine tool rest system.  I find these grinding tool rests invaluable.  With a little ingenuity they can be adapted to grind seemingly anything.

With that brief overview out of the way, back to the project axe.

Removing the handle.
The first step was to remove the old handle.  It is not always a graceful process.  I sawed the handle off close to the head and then drilled out as much material as I could from the part of the handle that remained in the eye.  This is a perfect job for those drill bits that you probably should have thrown away a few months ago.  You are likely to hit the metal wedge that secures the handle on most old hatchets.


My axe was fairly straight at the edge and I wanted it to have a little bit more curvature.  I drew a curve that looked good to my eye.  Next, I created that curve by setting the grinding platform perpendicular to the wheel and grinding material directly from the edge.  This operation creates a flat at the edge of the tool, which can be a useful reference point in the following steps.  When I measured the curve later, it was close to an arc of a circle with an 11" radius.

Before.
After.

The heart of my grinding technique is the pivot stick.  I made mine so that the point that sits in the vee-arm was 11" from the edge of the axe when the axe head was clamped in place.  This distance approximates the radius of the curve that I drew onto the head and then ground to shape in the previous step.  If the curve of your axe has an 8" radius then the distance from the edge of your axe to the point should be close to eight inches.  There is some room for fudging here, so don't get too crazy figuring out the exact curvature and radius.

Pivot stick top view.
Pivot stick side view.
My pivot stick is 9" long, 1 1/2" wide, and 1/2" thick.  It tapers to a 3/16" square at the pointy end and the front is beveled to provide clearance for the grinder wheel.

Complete set up.
A small c-clamp completes the arrangement.  I centered the pivot stick in relation to the curve at the edge of the axe.  The pivot stick must be set back from the edge of the axe to clear the wheel of the grinder.

Setting the angle for grinding.
Moving the vee-arm tool rest in and out sets the angle for grinding.  At this point I actually don't think much about the angle, and instead focus on the width of bevel that I want.  I was aiming for the wide bevel to be about 1/2" wide.  This axe head had fairly wide, convex bevels.  My hunch was that by grinding in the middle of the existing bevel, I would get a result close to my target.

Checking the set up.
With my grinding jig in the vee-arm I move the axe head back and forth on the wheel (grinder off) to see if the wheel is touching where I want it to grind.  The wheel leaves scratches where it is touching.  Notice how the scratches run parallel to the curve of the edge.  If they taper off to one side try adjusting the alignment or length of the pivot stick.  My initial set up looked good, so I proceeded to grind.



Proceed with caution!  Note the profusion of white multi-tailed sparks produced by grinding.  This type of spark is an indication of good quality steel in this particular axe head.  When I bought this axe a few months ago, the first thing I did was touch it to the grindstone to see what type of sparks it produced and decide whether or not it was worth tuning up.  Fewer, more orange colored sparks with shorter, fewer sunburst tails coming off the individual sparks indicate lower quality steel.  What you can't see in these pictures is that I am wearing ear protection, safety glasses, and a respirator.  After seeing the river of white sparks flowing onto my hand in the pictures I have decided to also wear a thin pair of leather gloves the next time I grind an axe.

As I move the axe back and forth I slightly raise the corner of the axe that is not in contact with the wheel.  I do this to clear the grinder motor, but it also seems to keep the bevel aligned properly.  Remove the assembly from the tool rest and check your progress regularly.  If the axe is not properly aligned the bevel will not be a consistent width.  Light tapping of the head to adjust its alignment with the pivot stick can usually remedy this problem.

Wide bevel ground.
Estimating the narrow bevel.
With the wide bevel complete, I used an angle gauge to get a feel for how to grind the short bevel.  The protractor is set to 32 degrees.  My sense from the types of sparks this steel produced and the feel of the hardness of the head against the grindstone is that this is an appropriate included angle for this axe head.  I will test and adjust that later, if need be.

Short bevel complete.
Grinding the shorter bevel follows the same process as grinding the wide one.  Adjust the vee-arm to get the angle that you desire, and then grind away.  I kept my protractor close to make sure that I was grinding the edge at the angle I wanted.  Make adjustments to the angle by moving the vee-arm as necessary.



As I grind both bevels I frequently check the flat left from grinding the edge to a curve.  I try to keep the flat straight and even in width.  In the picture above, note the inconsistencies in the width of the flat.  When I return to the grinder I remove more material from the areas where the flat is thicker and avoid grinding in the areas that are thinner.  As you can see, the corner of the blade in the left of the picture above has no flat at the edge, which means the bevels are meeting there.  When I returned to the grinder after taking this picture I made sure not to grind that corner any more.  A word of caution, the thinner that flat gets, the more likely you are to overheat the edge on the grinder.  I use a light touch and I keep a bucket of water near to cool the axe head as needed.

Sharpening the axe.

Even without the handle in the head I couldn't resist sharpening the edge.  I used diamond paddles to hone the hollow grind, and then finished it off with some careful stropping.  It was slicing through pine endgrain beautifully in under five minutes.  The hollow grind is quick to sharpen and makes maintaining flat bevels a simple matter.


In the next post I'll cover fitting the handle to the eye and making adjustments to the included angle of the edge.



5 comments:

  1. Tim,
    Hey, nice blog and I love the axe tune-up. I have many of these lying around in the to-do pile. Looking forward to seeing the handle you make.
    Greg

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    1. Greg,

      Thanks for the comment. I've enjoyed following your blog for a while now.

      These old axes are great and with this grinding rig a skilled guy like you will have them tuned up and handled in no time. I'll get the handle in there next week.

      Tim

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  2. A well thought out and nicely explained DIY! I am looking for a couple of small hatchets at some of my old tool "haunts" and will be trying this as soon as I find what I want to use. I had some thoughts on whether to replace the handle or not, I see the possibility of using the modified hatchet without the handle as a fat stubby wood chisel, [for lack of a better term], and wondered what your thoughts are on that idea? Particularly for hand hewn bowels and other convex carvings, it seems that a properly shaped and sharpened edge would cut through softwood like butter without the handle. I want to do 2 of these when I start on this project. For soft woods like white pine, cottonwood, poplar, and spruce, I would use the hatchet, sans handle, and use the hammer end for a stubby "handle" for pushing or lightly tapping with a wood mallet to remove waste on the piece being worked. For hardwoods such as certain maples, ash, or certain oaks, I would try the handled hatchet using short hacking type strokes with my grip on the handle up close to the head. Either type could be either hafted or unhafted from its handle as the improvement in efficiency proved to be needed for its purpose in use.

    I will follow up on any interesting findings I encounter in this experiment.

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