Yes, that is really what it is called. No, it is not a joke. The bastard file is a legitimate tool classification with a century of use behind it, and if you sharpen axes by hand, it is probably the most important file you own or should own.
Here is what it is, where it fits in the sharpening sequence, and how to use one on an axe without destroying the edge or burning out the file.
What a Bastard File Actually Is
Files are classified by their cut, which describes the coarseness of the teeth and how aggressively they remove metal. From coarsest to finest, the standard progression runs: rough, bastard, second cut, smooth, and dead smooth.
A bastard file sits at the coarser end of the middle range. It removes metal fast enough to reshape a damaged edge or clean up a neglected bit, but leaves a finish fine enough that you are not starting over from scratch with a stone. It is the workhorse of the sequence. On most sharpening jobs it does the majority of the actual work.
The name comes from old English trade language for something that does not fit a clean category, something in between. A bastard cut is neither a rough file nor a fine one. It is the in-between file, and in practice it is the one that gets used most.
The term "mill bastard" that you will see on most quality files refers to the flat profile of the file, which was originally developed for use in mill shops. A mill bastard file is flat on both faces, tapered slightly toward the tip, and single or double cut on the faces. For axe sharpening, this is the profile you want. It gives you full contact with the bevel and lets you work in long, controlled strokes across the edge.
File Sizes and What They Mean for Axe Work
Files are also sized by length, typically from 6 inches up to 14 or 16 inches for larger flat files. The length matters for axe sharpening because a longer file spans more of the edge per stroke, which helps you maintain a consistent bevel angle across the full width of the bit.
For most axes, a 10 inch mill bastard file is the right starting point. It is long enough to cover the edge efficiently, stiff enough to hold its shape under pressure, and available from any quality tool supplier. A 12 inch file is a good choice for larger felling axes or broad axes with wide bits. Smaller files work but require more passes to cover the same ground and are harder to keep at a consistent angle.
Where the Bastard File Fits in the Sharpening Sequence
Axe sharpening is a three-stage job for most people: file, stone, strop. Each stage does something different.
The file handles metal removal. If there are nicks, chips, flat spots, or a bevel that has gotten sloppy from years of contact with hard wood and the occasional rock, the bastard file fixes those problems. You cannot do this efficiently with a stone. A stone is a finishing tool, not a shaping tool. Trying to repair a nicked axe edge with a whetstone alone is a good way to spend an entire afternoon and end up with a mediocre result.
The stone refines what the file started. Once the file has established a clean, consistent bevel with no visible damage, you move to a sharpening stone or puck to work the edge finer and remove the file marks. Start with a coarser stone and work toward finer grit. This is where the edge actually gets sharp.
The strop is optional but worth doing if you want a genuinely keen edge. A leather strop with a little compound polishes the apex of the edge, removes the wire burr that the stone raises, and leaves you with a working edge that will last longer between sessions.
For an axe that is in decent shape and just needs a touch up, you can skip the file entirely and go straight to the stone. The file earns its place when there is actual damage to repair or a bevel that needs to be rebuilt from a poor grind.
How to Use a Bastard File on an Axe
Get your setup right before you start. Clamp the axe head in a vise with the bit facing up and the handle out of the way, or wedge the handle between your knees with the head resting on a wood block. You need both hands free to control the file. Do not try to hold the axe in one hand and file with the other. You will get an inconsistent bevel and a potentially exciting trip to the emergency room.
Push only, never pull. A file cuts on the forward stroke. Dragging it back across the steel on the return stroke dulls the teeth and does nothing useful. Push the file forward across the bevel, lift it cleanly off the steel, bring it back to the starting position, and push again. Forward only, always. This is the most common mistake people make and the primary reason files wear out faster than they should.
Match the bevel angle. The factory grind on most quality axes runs somewhere between 25 and 30 degrees. Before you start filing, look at the existing bevel and try to match it. You are not trying to regrind the geometry from scratch. You are following the shape that is already there and cleaning it up. If the bevel is badly off or the axe came from the factory with a poor grind, you may need to reset it, but that is a heavier job and usually a conversation for a bench grinder first.
Work in consistent strokes. Push the file across the full length of the bevel in smooth, even strokes from heel to toe. Keep count. You want to put the same number of strokes on each side of the bit. Uneven filing produces an uneven edge, and an uneven edge is a chore to correct later. Ten strokes per side, check, ten more if needed. Stay consistent.
Keep the file clean. Metal filings, called swarf, will load up in the teeth of the file and reduce its cutting ability. A file card, which is a stiff wire brush made specifically for this job, clears the swarf out of the teeth when you brush it parallel to the tooth lines. Clean the file every few passes. A loaded file skates on the steel rather than cutting it, and skating is how you dull a file prematurely.
Watch for the wire edge. As you file one side of the bevel, a thin curl of metal called a wire edge or burr will form on the opposite side. Run your thumb carefully across the back of the bit, not along the edge, and feel for it. When you can feel a consistent wire edge running the full length of the bit on the side you have not been filing, the file has done its job on that side. Flip the axe and repeat on the other side. Once you have raised a wire edge on both sides alternately, the file work is done and you move to the stone.
What to Look for in a Good File
Not all files are equal, and this matters more than most people realize. A cheap file will feel sharp for the first few passes and then stop cutting. The teeth on low-quality files are not hardened properly and they fold under pressure on hard steel rather than biting into it.
American-made files from earlier eras, found at estate sales and barn cleanouts, are often genuinely superior to new production from overseas manufacturers. If you find a box of old American-made files, buy them. They will outlast and outcut most of what is currently available at a hardware store.
For new production, stick with reputable manufacturers. A good file should feel like it is biting into the steel on the push stroke, not sliding across it. If a new file feels slick on a steel axe bit, it is either cheap steel or the axe is unusually hard. Either way it is a problem.
Single cut files have one set of parallel teeth and leave a slightly smoother surface than double cut files, which have two overlapping sets of teeth that remove metal faster but leave a rougher finish. For axe sharpening, both work. A single cut bastard file gives you a bit more control and leaves a surface that requires less stone work afterward. A double cut removes metal faster if you are dealing with serious damage. Most experienced axe sharpeners keep both and reach for whichever the job calls for.
FAQ: The Bastard File
Why is it called a bastard file? The name comes from old English trade terminology for a tool or grade that does not fit neatly into a standard category. A bastard cut file sits between rough and fine on the coarseness scale, making it the in-between file. The classification has been in use for well over a hundred years and remains the standard term in metalworking and woodworking trades.
What is the difference between a bastard file and a mill bastard file? A mill bastard file refers to both the cut grade (bastard) and the profile (mill, meaning flat). It is the most common configuration for axe sharpening because the flat profile gives full contact with the bevel of the axe bit. When people say bastard file in the context of axe work, they almost always mean a mill bastard file.
What size bastard file should I use for axe sharpening? A 10 inch mill bastard file handles most axes well. It is long enough to work the full bevel in a single stroke on most single bit axes and hatchets. For larger felling axes or broad axes, a 12 inch file gives you more working length. Go longer rather than shorter. Small files require more passes and are harder to keep at a consistent angle across the bevel.
How do I know when to use a file versus a sharpening stone? Use a file when there is visible damage to repair: nicks, chips, a rolled edge, or a bevel that has gotten inconsistent over time. Use a stone for regular maintenance on an edge that is fundamentally sound but has gotten dull from normal use. A stone will not efficiently fix a nicked edge. A file will not put a refined, polished finish on a steel bit. Each tool has its job.
How long should a good bastard file last? That depends on the quality of the file and how hard the steel is that you are working. A quality file used correctly on a well-tempered axe head should hold its cutting ability for many sessions before it starts to feel dull. Common causes of premature file wear are dragging the file back across the steel on the return stroke, failing to clean swarf out of the teeth, and using a file on steel that is too hard for it to bite into. Treat the file like a tool, not a consumable, and it will last.
The Right File Makes the Difference
A sharp axe is a safer axe and a more useful one. Dull tools make you work harder and introduce the kind of unpredictable glancing behavior that sends people to urgent care. Keeping an axe properly sharp takes twenty minutes with the right file and a stone. Without the right file, it takes four times as long and produces a worse result.
Our Hand Files are the right place to start. Pick up a 10 inch mill bastard and a sharpening puck and you have everything you need to keep an axe working the way it should. Buy it once, use it for years, and stop apologizing for a dull edge.