A bastard file is a medium-coarse metal file that sits between finer and rougher grades, used when you need to remove material faster than a smooth file but with more control than a coarse one.
The name has nothing to do with legitimacy or insult. It comes from the file's in-between status: it does not fit neatly into the standard classification system, so it earned a label meaning "something that doesn't belong to a clear category." Metalworkers and machinists have used the term for centuries, and it remains the standard name in tool catalogs and hardware stores today.
If you pick up a file at a hardware store, you will see the grit marked on the handle or blade. The standard progression goes coarse, medium, smooth, and very smooth. A bastard file occupies the space between coarse and medium — it is rougher than medium but not quite as aggressive as full coarse. That in-between quality is exactly why the name stuck.
Key Takeaways
- A bastard file is a standard tool grade that removes material faster than medium files but with more precision than coarse files.
- The name refers to the file's position outside the main classification system, not to anything negative or insulting.
- Bastard files are commonly used in metalworking, woodworking, and machining when you need a balance between speed and control.
- The term has been in use for centuries and appears in every major tool manufacturer's product line.
How file grades work and where bastard fits
Files are classified by how coarse or fine their cutting surface is. A coarse file removes a lot of material quickly but leaves a rough finish. A fine file removes less material per stroke but produces a smoother surface. The standard grades, from roughest to smoothest, are coarse, bastard, medium, smooth, and very smooth.
The bastard grade exists because metalworkers and machinists needed something between coarse and medium. If you are shaping a piece of steel and you use a full coarse file, you remove too much material too fast and risk overshooting your target dimension. If you use a medium file, the work takes much longer. A bastard file splits the difference: it cuts faster than medium but gives you enough control to stop near your target size.
Different file shapes — flat, round, triangular, square — come in bastard grade. The shape determines what kind of work you can do; the grade determines how aggressively it cuts. A flat bastard file is one of the most common tools in a machine shop or metalworking bench.
Why the name stuck despite sounding odd
The word "bastard" in older English meant something that did not fit the expected categories or did not belong to a clear family. When toolmakers needed a name for a file that fell outside the main classification system, the term made sense to them. It was descriptive: this file was not standard coarse, not standard medium, but something else.
The name appears in tool references from the 1700s onward and has never changed. Tool companies like Nicholson, Simonds, and modern manufacturers still use it in their product lines and technical specifications. It is the correct term in any machine shop, hardware store, or tool catalog.
Because the word has a long history in metalworking and is the actual technical name, people in the trade use it without hesitation. It is no more offensive in that context than "bastard cut" (a type of saw blade) or other technical terms that happen to use older English words.
What you actually use a bastard file for
Machinists reach for bastard files when they are shaping metal parts and need to remove material in a reasonable time without losing precision. If you are filing down a steel edge to fit into a slot, a bastard file gets you close to the final dimension faster than a medium file would, and you can switch to a medium or smooth file for the final finishing work.
Woodworkers use bastard files less often than metalworkers, but they appear in tool kits for shaping hardwood and for work on metal hardware attached to wooden pieces. A bastard file can smooth a rough edge or adjust a wooden part to fit properly without taking off so much material that you have to start over.
In machine shops, a bastard file is often the first file you reach for when you need to remove material from a casting or forging. The casting surface is rough, and a coarse file might be too aggressive; a bastard file removes the excess quickly enough to be practical while still giving you control.
How bastard files compare to other grades
| File Grade | Material Removal | Surface Finish | Common Use |
|---|---|---|---|
| Coarse | Very fast | Rough | Heavy stock removal, rough shaping |
| Bastard | Fast | Medium | Shaping metal, balancing speed and control |
| Medium | Moderate | Smooth | General finishing, precision work |
| Smooth | Slow | Very smooth | Final finishing, polishing |
| Very Smooth | Very slow | Mirror-like | Fine finishing, precision surfaces |
The choice between grades depends on what you are starting with and what finish you need. If you are working with a rough casting, you might start with coarse or bastard, then move to medium or smooth for finishing. If you are working with already-finished material and just need to adjust an edge, you might skip coarse and bastard and go straight to medium.
Where you will encounter the term
If you buy files at a hardware store, you will see "bastard" printed on the handle or packaging. Manufacturers like Nicholson, Simonds, and Bahco all label their files this way. Tool catalogs and machine shop references use the term as standard. If you take a machining class or apprenticeship, your instructor will use it without explanation — it is the normal word in the trade.
Online tool retailers and eBay listings use "bastard file" in their product descriptions and search tags. If you are looking for a file and you see the term, you now know it refers to a specific grade of coarseness, not anything unusual about the tool itself.
Frequently Asked Questions
Is "bastard file" the only name for this grade?
In English-speaking countries, yes — it is the standard term. Some other languages have different names for the same grade, but in tool catalogs and machine shops in the United States, Canada, and the United Kingdom, "bastard" is the official classification.
Can I use a bastard file on wood?
Yes, though metalworkers use them more often. A bastard file works on hardwood and on metal hardware. For softwood, a medium or smooth file usually gives better results because bastard files can tear the grain. Keep separate files for metal and wood if you can — metal filings can rust and contaminate wood projects.
What is the difference between a bastard file and a coarse file?
A coarse file removes material much faster but leaves a rougher surface. A bastard file is slower but gives you more control and a smoother finish. Choose coarse if you need to remove a lot of material quickly; choose bastard if you need to be more precise about how much you remove.
Do I need a bastard file if I have a medium file?
Not always. If you are doing light finishing work, a medium file is fine. If you are shaping metal parts or working with rough castings, a bastard file saves time and gives you better control than jumping straight to medium. Many machinists keep both on hand.
Where can I buy a bastard file?
Hardware stores, home improvement retailers, and online tool sellers all stock them. Search for "bastard file" and specify the size and shape you need — flat, round, triangular, and square are the most common. Prices vary by brand and size, but a basic flat bastard file costs between ten and thirty dollars.