How to File and Convex an Axe Bevel (With a Printable Gauge)
Fix the profile with a mill bastard file, set the bevel to a gauge, raise a burr along the full bit, then convex and remove that burr with a stone. The failure everyone makes is filing a flat bevel too thin, which chips out on the first frozen knot - so the gauge is the tool that matters more than the file.
By the The Tool Wise Editorial Team
Clamp the head. File the profile fair, set the bevel against a gauge, raise a burr end to end, then take that burr off with a stone in circles - and the circles are what put the convex in.
The whole job runs about forty minutes on a neglected head and eight on a working one. It needs a 10 or 12 in mill bastard file, an axe puck or a coarse-and-fine stone, a vice or a stump, and a piece of card for the gauge.
What ruins axes is not blunt filing. It is a flat bevel taken too thin, which looks beautifully sharp on the bench and folds or chips the first time it meets a frozen knot or a nail-hard bit of locust. A convex edge has more steel sitting immediately behind the cutting line, and that steel is what keeps the edge alive in real wood.
What is actually on the bench
- A 10 or 12 in mill bastard file, single cut, with a proper handle fitted - a file tang through a palm is a hospital trip
- A knuckle guard: a scrap of leather, a slit piece of garden hose, or a purpose-made guard slid onto the file
- A file card or a stiff brass brush to clear the teeth, and a stick of chalk to rub across the file so it does not pin
- An axe puck or a two-sided stone, roughly 180-220 grit on the coarse face and 320-600 on the fine
- A vice, or a stump vice, or two wedges and a log - the head must not move
- A strip of leather and a little polishing compound
- Card stock and a rule for the gauge
Not on the bench: a bench grinder, an angle grinder, or a belt sander. Blue or straw colours appearing on the steel mean the temper at the very edge is gone, and the only fix is to file back past the damaged steel. A head that has been ground by a previous owner often has a band of ruined temper an eighth of an inch deep that will not hold an edge no matter what you do to it.
Print and cut the gauge
The gauge is a V-notch cut into card at a known included angle, plus two thickness checks that tell you whether the steel behind the edge is where it should be. Draw it with a rule - the geometry below is exact, so any printer scaling error shows up immediately when you measure the notch depth.
Cut a notch 4 in deep. The mouth width at the top of the notch sets the angle. Then check the two thickness figures with a rule or a cheap caliper at a quarter inch and half an inch back from the cutting edge.
Gauge Included angle (deg) Notch depth to cut (in) Notch mouth width (in) Steel thickness 1/4 in back Steel thickness 1/2 in back What it suits What happens if you file thinner A 20 4 1-13/32 0.088 in 0.176 in Carving hatchets, hewing hatchets, anything working clean dry wood only Rolls on the first knot; will not survive a nail or a stone B 25 4 1-25/32 0.111 in 0.222 in Felling and bucking axes in clean softwood, kept for the purpose Chips in seasoned hardwood; needs re-filing after every session C 30 4 2-9/64 0.134 in 0.268 in The general-purpose default - felling, bucking, limbing, mixed hardwood Loses the safety margin for frozen wood and buried grit D 35 4 2-33/64 0.158 in 0.315 in Splitting axes, winter work, anything striking frozen rounds Starts to stick rather than release when chopping across grain E 40 4 2-29/32 0.182 in 0.364 in Splitting mauls and grubbing tools that meet dirt and stone Nothing - below this you are no longer sharpening, you are shaping Cut gauges C and D from card and keep them in the file roll. Use the notch on the last quarter inch of the bit only - behind that the steel should be fuller than any straight-sided gauge, and that fullness is the convex you are trying to build. The two thickness figures are the useful part in practice. They are what a straight bevel at that included angle would measure, so if your steel measures thicker than the column at half an inch back but sits flush in the notch at the last quarter inch, you have exactly the shape you want: a keen edge with a swelling shoulder behind it.
If the steel measures thinner at half an inch back than the table says, the head has been ground hollow at some point. It will feel sharp and it will chip, and the only repair is to file the cheeks back until the fullness returns - a long job that removes real weight.
Fix the profile before you touch the bevel
Look along the bit from heel to toe with light behind it. You are checking two things: whether the cutting line is a fair continuous curve, and whether there are nicks, dings or a mushroomed section where somebody drove a wedge with it.
Nicks come out first, and they come out by filing across the whole bit, not by grinding a dip around each one. Set the file roughly square to the edge and work the entire cutting line down until the deepest nick just disappears. On a badly abused head this can take a couple of millimetres of steel and half an hour. Take it anyway - filing a bevel onto a nicked edge means doing the whole job twice.
While you are there, check the cheeks. A head that has been sharpened for decades develops a shoulder where the old bevel meets the original cheek, and that shoulder is what makes a head stick in the cut. Blend it out with the file, working two to three inches back from the edge in long strokes, until you can run a finger from cheek to edge without feeling a step.
The filing progression, stage by stage
A mill file cuts on the push stroke only. Push into the edge - toward the bit, never away from it - and lift on the return. Dragging the file backward under load dulls it fast and burnishes rather than cuts.
Stroke counts below are for a typical working head. They are a rhythm to work to, not a target to hit; the visual and tactile checks in the last two columns are what actually govern when you move on.
Stage Tool Grit or cut Strokes per side What you should see or feel Move on when 1. Profile repair 10-12 in mill bastard file Single cut, bastard 40 - 80 Steel coming off in bright ribbons; the file bites without skating The deepest nick is gone and the bit line is a fair curve heel to toe 2. Cheek blending Same file, low angle Bastard 30 - 50 The old shoulder two to three inches back flattening out A finger run from cheek to edge feels no step 3. Bevel setting Same file, held to gauge C or D Bastard 25 - 40, alternating sides every ten A consistent bright band along the last quarter inch The gauge notch sits flat on the last 1/4 in along the whole bit 4. Burr raising Same file, light pressure Bastard 10 - 15 A wire edge you can feel catching a thumbnail drawn across it The burr runs unbroken from heel to toe on the far side 5. Convexing and burr removal Axe puck, coarse face, wet 180 - 220 60 - 100 small circles Grey slurry; the burr flops to the other side, then breaks up No burr detectable from either side with a thumbnail 6. Refining Axe puck, fine face, wet 320 - 600 40 - 60 circles Scratch pattern going uniform; the edge stops reflecting a line of light You can see no bright line along the cutting edge at any angle 7. Stropping Leather strip with compound Compound only 20 - 30 trailing passes Grey compound coming off on the leather The edge slices newsprint held loosely, not just paper on a bench 8. Field touch-up Fine puck only 320 - 600 15 - 25 circles Nothing dramatic - you are maintaining, not repairing Do it after every two hours of chopping, or after any frozen knot Work down the table in order and do not skip stage 4. The burr is the only proof that you have reached the actual edge rather than stopping a few thousandths short of it. The burr is the only honest signal
Everything else in this job can be fooled. Steel looks bright when it is nowhere near sharp. A file feels like it is cutting when it is riding on the shoulder. The burr cannot be faked, because it can only form when the metal from one side has been thinned until it folds over past the edge.
Detect it by drawing a thumbnail gently across the edge, from the bevel toward the cutting line, at three or four points along the bit. Where the burr exists your nail catches slightly. Where it does not, the nail slides. Any section that slides has not been filed to the edge yet, and that section will be blunt while the rest is sharp - which is exactly how an axe develops a soft spot that then chips.
Do not run a thumb along the edge. Across, always, and gently. A raised burr is a ragged strip of steel and it will open a finger without much effort.
Circles are what make the convex
The file makes flats. The stone, used in small overlapping circles rather than straight strokes, rounds those flats into each other, and that rounding is the convex.
Hold the puck flat against the bevel and start circles at the very edge, then let them drift back a quarter inch, then forward again. The pressure varies naturally as your wrist moves, and that variation is doing the work. Straight strokes parallel to the edge would just reinforce the flat you filed.
Wet the puck. A dry stone loads with steel dust and stops cutting within a minute, then starts burnishing instead - which polishes the edge without sharpening it and leaves you convinced the stone is worn out.
Work both sides equally, counting circles, or the edge migrates off the centreline of the bit. On a felling axe that shows up as the tool steering in the cut, and on a splitting axe it shows up as rounds going sideways.
Testing without opening a thumb
Newsprint is the honest test. Hold a sheet loosely by one corner and draw the edge down through it at a shallow angle, at three points along the bit. A properly finished convex edge slices cleanly with no tearing and no snagging. A flat bevel that has been over-thinned will slice paper beautifully and still chip on wood, so pair it with the second test.
The second test is a scrap of dry hardwood - not the woodpile, a scrap. Take one moderate chop across the grain. A good edge takes a chip and releases. A too-thin edge takes the chip and comes back with a bright dent or a crescent missing. Better to find that out on a scrap on the bench than at the bottom of a notch with the light going.
Never test by shaving. The convex geometry that makes an axe survive real wood is the same geometry that makes it a poor razor, and people chasing arm-hair sharpness file their bevels down to the point where the edge cannot survive a season.
What undoes an hour of filing
- Any powered wheel. Heat is cumulative and invisible until the colour changes, by which point the temper is already gone
- Chopping into the ground, which is what happens when the handle is too long for your block or the block is too low
- Storing the head bare in a damp shed - a light film of oil and a leather mask cost nothing and save the edge
- Driving steel wedges with the bit end of anything, which mushrooms steel and can spall
- Leaving the edge to go through a full winter without a touch-up, so that the next session starts with stage 1 instead of stage 8
Get into the habit of ten circles on the fine puck before the axe goes back on its hook. It takes twenty seconds, it means you almost never have to pick up the file again, and it is the single thing that separates people who own sharp axes from people who own axes.