home > tips from the crew > how to sharpen an axe

HOW TO SHARPEN AN AXE
by Patrick Wilson
This is an old version of this article;
for an updated version, click here


Uses

Trail crews use axes for bucking blowdowns in wilderness areas, swamping the area around blowdowns, limbing and debarking trees to be cut with chainsaws and crosscut saws, and stripping bark from logs used to build drainage devices. In all cases, a properly sharpened axe maximizes both safety and efficiency. (Large roots are best cut with a small saw, which leaves a clean cut without disturbing the surrounding dirt, rather than with a Pulaski, which is certain to get badly nicked and dulled by rocks and grit.)

Design

Axes of various shapes and sizes are designed for everything from hewing timbers to splitting wood. The best axe for trail work is one shaped and sharpened for maximum penetration, namely, a felling or notching axe, or perhaps a dual-purpose axe sharpened like a felling axe. Such axes are essentially heavy-duty cleavers with both primary and secondary bevels on each side. An acute primary bevel facilitates deep cuts into limbs and bucking notches, whereas a somewhat blunter secondary bevel right along the edge protects the edge from folding, chipping, and rapid dulling. (Very narrow secondary bevels are called microbevels.) As with all edge tools, both bevel angles should be appropriate for the intended use and user, and as acute as possible while maintaining durability.
  Bevel Angles
  The primary bevel angle on new felling axes ranges from 7.5° to 15° per side and can be as narrow as 1/2" or as wide as the full cheek of the axe; secondary bevel angles of 15° to 22.5° per side are typical, with widths ranging from 1/4" down to 1/64" or less. Most used axes, especially those that have seen particularly heavy use or have been sharpened a few times, have both primary and secondary bevel angles that are too blunt and that become even blunter toward the center of the bit. These axes are likely to bounce dangerously off wood instead of cutting into it. For the novice axe-sharpener, a primary bevel angle of 10° per side, combined with a 1/8"-wide secondary bevel of 17.5° per side, is a safe starting point. The quality of steel in your axe, its weight and handle length, the type of wood being chopped (species, dryness, knottiness, diameter, even temperature), your own technique—all these factors and more help determine the best bevel angles. Experiment with different angles after you’ve had some practice with these.
  A secondary bevel of 17.5° per side will hold up in almost all species of wood found in PATC territory. Because chopping difficulty increases with the density of the wood, though, be careful with the densest woods. Locally, these are black locust, eastern hophornbeam (“ironwood”), persimmon, and shagbark hickory. Hophornbeam, with its thickly muscled trunk, isn’t common enough on the trail to worry about. Although persimmon and hickory don’t noticeably dull a 17.5°-per-side bevel, black locust definitely does. It requires a secondary bevel angle of 20° to 22.5° per side, depending on how hard you hit. If you are using the recommended 17.5°-per-side secondary bevel, leave locust alone to avoid damaging your axe. An alternative is to sharpen one edge of a double-bit axe specifically for locust, while reserving the other for normal use. The blunter edge can also be used as a backup if the keener one gets damaged.
  Go easy on your axe when limbing, especially with dead limbs, as well as when chopping knots and frozen wood. With proper technique, as taught in the PATC Traditional Tools Workshop, all of these tasks can be managed handily with the recommended angles. (Dead limbs can often be knocked off with the butt of a single-bit axe; knots can be chopped around; and a cold axe can be quickly warmed with a few light cuts.)
  Convex versus Flat Bevels
  Many authors tout convex bevels as stronger and less likely to bind than flat ones. Yet strength is a function almost exclusively of the secondary bevel, which is so narrow that its convexity vs. flatness is a moot point. As for the primary bevel, even one an inch wide will be only about 1/64" thicker per side if convex than if flat—and this extra thickness will come well back from the edge, where strength isn’t needed anyway. A flat, 10°-per-side primary bevel is strong enough to emerge unscathed from hits into a locust log forceful enough to crumple a 17.5°-per-side secondary bevel.
  The likelihood of binding depends far more on the shape of the axe cheek, i.e., the amount of relief behind the primary bevel, than on the shape of the bevels themselves, which are relatively narrow and don’t contact much wood. In practice, bind simply isn’t an issue with flat bevels. If it were, racing axes wouldn’t universally have them.
  It is worth noting that if one actually measures the various drawings and templates that are found in catalogs and books and intended to illustrate convex bevels, one finds that they typically exaggerate convexity and show a different edge angle than the one recommend in the accompanying text. In short, convex bevels are overrated.
  Flat bevels penetrate better (which is the whole point), can be made plenty strong, and are far easier both to file and to measure for consistency and symmetry.

How to Measure Bevel Angles
Because the key to the safety and efficiency of an axe is consistent, symmetrical, and appropriately acute bevel angles, it is essential to measure these angles accurately throughout the sharpening process. Doing so is very easy if you make a simple bevel gauge by bolting together two hacksaw blades (figure 1). The distance between the holes determines the exact total edge angle (= twice the bevel angle) by simple trigonometry. For common 12" hacksaw blades, the following table gives several distances and corresponding angles.
Figure 1
Distance between Holes
(inches)
Edge Angle
(degrees)
Bevel Angle per Side
(degrees)
4 - 1/16
20
10
5 - 1/16
25
12.5
6 - 1/16
30
15
7 - 1/16
35
17.5
8 - 1/16
40
20
9
45
22.5
This homemade, adjustable bevel gauge works much better for axe bits than do engineer’s protractors or the ubiquitous small brass bevel gauges, which are either too short to measure primary bevels or too crude to measure microbevels.

How to Sharpen
Properly sharpening an axe involves creating smoothly polished primary and secondary bevels of consistent width and appropriate acuteness on each side.
  Preparing the Axe for Sharpening
 
Clean and polish the entire surface of the axe before sharpening so that the finely honed bevels you will later form don’t get compromised. Use a rust-eraser or steel
wool to remove any rust, then work through successively finer grits of sandpaper to take out scratches and other imperfections and to remove any work-hardened metal
at the bit. A flexible disk-sander attached to a drill and used with the following grits works well (figure 2).

Figure 2

USA220x (60µ) (220-grit, U.S. standard, 60-micron grit size)
USA320x (45µ)
USA400x (23µ)
USA600x (16µ)

Finish polishing with Tormek paste or a similar compound.
 

Filing the Bevels

    A 10" single- or double-cut mill file is the workhorse of axe-sharpening, because it removes metal fast without heat, produces a flat grind, and can be held at a consistent angle. Dry grinders weaken an edge two ways: they produce a hollow grind just where strength is most needed and they heat the axe enough to draw temper from the bit. Although wet and low-speed grinders stay cool, they too leave a hollow grind. Moreover, the geometries of the curved axe bit and the (differently) curved grinding surface of a wheel make a consistent bevel angle difficult to achieve.
    Secondary Bevel
    File the secondary bevel first so that its final width can later be set by filing the primary bevel down to it without removing any extra metal from the edge itself. Stabilize the axe by clamping its handle tightly in a vise so that the centerlines of both the handle (figure 3) and the head (figure 4) are parallel to the ground. Be careful that the axe handle doesn’t slip in the vise and the head doesn’t turn; double-check both periodically with a level.
    A simple axe-filing jig (figure 5) makes filing easy and safe, guarantees precise and consistent bevel angles (which can be readily confirmed with the “hacksaw” bevel gauge), and locks in bevel angles in case the axe twists in the vise under heavy filing. Make the jig and prepare the axe as follows.
   
  1. Bend a 14" length of 1-1/2" x 1/4" bar stock at a 10° angle 2" from the end.
  2. Drill and tap the bar 1" from the bent end to accept a 3-1/2" x 1/2" carriage bolt ground to a fairly blunt point (about 70°).
  3. Make a file clamp out of square or rectangular tubing with inside dimensions of at least 1-5/8" x 1/2" (figure 6).
  4. Punch a registration mark above the center of the eye on each side of the axe head (figure 7), then drill a 3/32" hole 1/16" deep at each mark.
    Clamping or holding the file against the underside of the jig, adjust the pivot bolt for the appropriate secondary bevel angle (e.g., 17.5°) using a magnetic angle-finder attached to the top of the jig
(figure 8). After locking the pivot bolt with a nut, remove the angle-finder to keep it from collecting filings. Position the clamp about 3/4" from the axe bit and slide the file down so just its end gets used. By moving the file up little by little as it gets dull, you’ll always be using a sharp portion and you can eventually use most of the surface. Save yourself frustration by using a new file, pushing it in one direction only, and cleaning shavings out with a file card often. Expect a sharp file to clog frequently.
    File the secondary bevel with a sideways, drawfiling technique, pivoting the jig in the registration hole while holding the file flat against the bevel (figure 9). Be sure to hold the pivot bolt down firmly enough that it doesn’t jump out of its hole. File off enough metal to remove even the smallest nicks, to produce a thin wire edge along the entire underside of the bit, and to form a smooth and symmetrical profile.
    Repeat on the other side, then touch up one or both sides so that the bevels are exactly the same width at the center of the bit on each side. Use the bevel gauge to double-check angle and symmetry along the entire bit. Don’t worry about file marks on the bevel or the presence of a wire edge; those will come off later.
    In the absence of a jig, you can leave the angle-finder attached directly to the file and pivot the end of the file on your hand (figure 10). Take special care that the axe doesn’t twist in the vise.
   

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10
 
    Primary Bevel
   
File the primary bevel on each side in the same manner and at an appropriate angle (e.g., 10°). Remove enough metal that the secondary bevel gets thinned to an appropriate width (e.g., 1/8") along the entire bit. To ensure that the thin secondary bevel shows up clearly and doesn’t get filed away, mark it first with a black marker (figure 11). Although the secondary bevel should be a consistent width along its entire length, the chief requirements for the primary bevel are that it maintain a consistent angle along its entire length and that it be reasonably symmetrical on both sides. Its width on a given side may vary from the heel to the toe of the axe because of the overall profile of the axe cheek, uneven wear through previous use, and even imperfections in the original forging.
Figure 11
    Because filing determines all the critical dimensions of an axe’s cutting edge, before moving on to the next step check both primary and secondary angles along the entire bit with the bevel gauge and measure the width of the secondary bevel along the entire bit on each side with a caliper or tape. Touch up bevels as needed.
  Honing the Bevels
  Honed and polished bevels look better, cut better, and last longer than unfinished ones. By far the easiest way to remove the file marks from bevels and give them a mirror finish is to use a belt grinder. A fine-grit belt used properly will neither remove enough metal to alter the bevel angles nor heat the bit of the axe unacceptably.
  A USA400x (23µ) belt is perfect for taking out file marks and creating a smooth surface on both primary and secondary bevels. With the belt turning away from the axe’s edge, hold the axe head freehand, use a very light touch, and be conservative, i.e., err on the side of polishing at too shallow an angle rather than compromising the edge (figure 12). Move the head gently from side to side, checking frequently to make sure you’re sanding in the right place and not heating the edge. It’s often possible to rest the axe head or your hand on the motor or shroud in order to hold a steady angle. Regularly stop the belt and clean it with a crepe block (figure 13).
  Sand the primary bevel first and then, very carefully, the secondary bevel—just enough to take out the file marks and leave a perfectly smooth surface. If you’re worried about messing up your carefully filed angles, use a finer belt and check sanding marks more frequently. With practice, the entire process takes just minutes.
  If you've been particularly aggressive with the file, or if you're a real perfectionist, work sequentially through the belts with the following grits. Start with USA120x (120µ) to remove the file marks on the primary bevel only, then work through USA220x (60µ), USA400x (23µ), and USA1200x (6µ) on both primary and secondary bevels. With each belt, remove all of the previous belt's striations from both bevels before moving to the next finer grit. At each belt change, mark both bevels with a black marker so you'll know you're removing metal in exactly the right places.
  If a belt grinder is unavailable, work through successively finer grits of sandpaper, diamond hones, or sharpening stones—holding, clamping, or otherwise attaching each of them under the filing jig, if possible, rather than using them freehand (figure 14). See my drawknife article for suggested grits and for instructions on flattening stones.
 

Figure 12

Figure 13

Figure 14
  Polishing the Bevels
 
To put the final mirror polish on both bevels, equip the belt grinder with a polishing belt charged with 0.5µ chromium oxide compound. Use it in the same manner as the sanding belt and charge it frequently with polishing compound. Both bevels will end up brightly polished, with no trace of a wire edge on the secondary. If you don’t have a special polishing belt, you can put polishing compound on the reverse side of a fine-grit belt. A felt wheel, similarly charged and used with a very light touch, also works well, especially on the narrow secondary bevel (figure 15). Always polish with the belt or wheel turning away from the edge.
Figure 15
 
Polishing can also be done by hand, with chromium oxide compound applied to a piece of wood clamped, taped, or held under the jig. If you polish by hand, finish the job by stropping (always away from the edge) on a piece of leather charged with chromium oxide compound (figure 16). Strop at an angle equal to or shallower than that of the secondary bevel to allow for the fact that the leather will flex slightly around the edge.
Figure 16
  Testing and Hardening the Edge
 
Use the elegant and inexpensive edge tester from Razor Edge Systems to ensure that you have produced a glass-smooth, level-100 edge (figure 17). Even the slightest imperfection should be removed with additional polishing or stropping.
Figure 17
It’s crucial that absolutely no wire edge remains. If any does, it will fold over and dig into the axe’s edge on the first cut. To check for any remnants, rub your thumb perpendicular—not parallel!—to the edge in an attempt to fold over any wire, the reflection of which will show up in good light. Re-test and, if necessary, polish or strop some more.
A serious axe-man will test his axe in the woods by chopping lightly a few times into soft wood to work-harden the edge and then working up to full-force hits into oak, inspecting the edge after each hit for any folding or crumpling. Minor imperfections can be polished or stropped away; major ones indicate the need for a blunter, more durable secondary bevel. Always work-harden and inspect a newly sharpened axe’s edge before putting it to serious use. In the field, it’s worthwhile to carry a small, fine (<50µ) diamond hone to touch up the inevitable nicks caused by grit in bark and wood. A quality axe with a work-hardened edge and appropriate bevel angles will cut like a knife and stay sharp surprisingly long.
  Protecting the Edge
  For highest performance, frequently wax your axe with a machine-table product (e.g., Waxilit) designed specifically to reduce friction between metal and wood. Spray-on wax (e.g., Boeshield) is also a good choice. In lieu of waxing, at least coat your axe with linseed oil to prevent rust.
  Always sheathe an axe when carrying it even for a short distance, but store it unsheathed (if it is safe to do so) in order to prevent rust. In use, scrupulously avoid even the smallest rocks, grit, and dirt particles; they can damage the best axe instantly. Fortunately, the methods described above make it easy to restore a perfect edge.

Resources
Leonard Lee’s Complete Guide to Sharpening (Taunton, 1995; ISBN 1561581259) is the most useful and comprehensive guide to sharpening edge tools; it includes sections on metallurgy, abrasives, sharpening equipment, and technique.

Good sources of sharpening equipment include:

  • Diamond Machining Technology (DMT diamond hones)
  • Lee Valley Tools (rust-erasers, Acu-Angle level, files, basic belt grinder, stones and hones, microabrasive paper, chromium oxide compound, felt wheel, leather strops, keychain diamond hones, Waxilit, Boeshield)
  • Razor Edge Systems (edge tester)
  • Woodcraft (stones and hones, Tormek and chromium oxide compounds, leather strops)
  • Van Sant Enterprises (belt grinders, belt grinder attachments for bench grinders, sanding and polishing belts)