Types of Machinist Files and What Each One Does

Every machinist file is described by three things: its shape, its cut, and its length. A "10 inch mill bastard" gives all three in four words. A Swiss pattern "No. 2 half-round" gives the same information under a different grading system. Once you can decode the name, you can buy the three or four files that do most shop work and skip the twelve-piece set that mostly stays in its case.

The USDA Forest Service's file manual puts the governing variable plainly: "The filer must be aware of the cut, how rough or fine the teeth are, and the number of teeth per inch of file length." Fewer teeth per inch cut rougher. More teeth per inch cut finer.

American pattern files come in three standard grades, from coarsest to finest:

• Bastard, for heavy removal
• Second cut, for general work
• Smooth, for finishing

Australian distributor AIMS Industrial attaches numbers to these names:

• Rough: 20 to 25 tpi
• Bastard: 25 to 30 tpi
• Second cut: 30 to 40 tpi, which it calls the general workshop standard
• Smooth: 40 to 55 tpi
• Dead smooth: 55 to 80 tpi or more, which it says is rarely used now

Treat those ranges as nominal. The guide does not say which file length they apply to, and that matters, because the grade names do not hold coarseness constant.

Length is the catch. The USDA manual notes that "a 12-inch flat bastard file is coarser than a 6-inch flat bastard file." A longer file carries fewer teeth per inch under the same grade name. The grade tells you the coarseness relative to the file's own size, not an absolute pitch. Travers Tool adds that length is measured from point to heel, excluding the tang.

Tooth pattern is a separate choice from grade:

• Single cut files have one set of parallel diagonal teeth. Nicholson describes them as used with light pressure for a smooth finish.
• Double cut files have two intersecting sets of teeth that form a diamond pattern. They remove metal faster and more aggressively.

Travers recommends double-cut bastard and second-cut files for fast stock removal, and smooth double-cut or single-cut files for finishing. AIMS adds that single cut suits soft metals and saw sharpening. It also says aluminum and cast iron "pin" (clog the teeth with chips), and that chalking the file face, or switching to single cut on aluminum, helps. A third pattern, rasp cut, has individual teeth and is meant for wood, leather and plastic, not metal.

One hard limit applies to all of them. AIMS puts a file body at roughly Rockwell C 62 to 66. Anything harder than that has to be ground, not filed.

Shape is matched to the geometry of the work. MSC's buying guide, AIMS and Summit Racing's OnAllCylinders largely agree on the following:

• Flat: general filing, flat surfaces, breaking edges. It is the most common shape.
• Half-round: one flat face and one curved face, for inside curves, concave surfaces and fitting hole edges.
• Round (rat tail): enlarging round holes and deburring drilled holes.
• Triangular (three-square): acute internal angles, square corners, saw teeth.
• Square: enlarging rectangular holes, slots and keyways.
• Knife: narrow, angled or slotted areas.
• Needle: small files for intricate precision work.

"Mill" also appears in AIMS's shape table and on nearly every working machinist's bench. However, none of these sources describe its tooth pattern or profile in detail. What they do show is how often it gets used.

In a Practical Machinist thread, a hobbyist mostly chamfered and deburred structural steel, mild steel and 6061 aluminum. His kit was a 6 inch mill bastard, a 10 inch mill bastard, a 12 inch flat smooth and a 12 inch double-cut hand bastard. The replies agreed that a few flat files in a few sizes, plus a medium half-round and a medium or small rat tail, cover about 90% of ordinary shop filing. That figure is forum consensus, not a measurement.

A South Bend Lathes subforum thread reached the same answer. Flats get the most use, and a flat file handles a lot of contoured work. Bastard and second cut were the favored grades, and 6 inch and 10 inch were the lengths to keep.

A used-tooling listing on the same forum shows what accumulates in practice. The lot was 15 files for $30 plus shipping at the time of posting: 7 mill smooth, 6 mill bastard, 1 Nicholson half-round and 1 Grobet. Thirteen of the fifteen were mill files.

Sources: Travers, Wonkee Donkee Tools and the Practical Machinist forum. Note that a Swiss grade number and an American grade name are not interchangeable labels for the same coarseness.

Diamond files are a different tool altogether. They have diamond grit bonded to a substrate instead of cut teeth, and forum users reach for them to touch up carbide that a steel file cannot cut.

What to buy first. Start with flat files in two lengths, then add a half-round and a rat tail. A Nicholson 8 inch mill bastard is the file most buyers look up first. These sources give it no specification beyond the general grade definitions. Add a needle file set, such as Grobet or a budget 12-piece, only once you have small work that needs one. None of the sources here publish measurements of those sets.

Check where the file was made, not just the brand. In a long forum thread on brands, posters rate modern Nicholson as adequate for deburring. They rate Swiss-made Grobet and Vallorbe as about the best available, and also recommend Pferd. The same thread warns that "Grobet USA" files are made in India. Country of origin is printed on the package, so read it.

Keep the teeth clean. The South Bend thread calls a file card, a wire-bristle brush for clearing clogged teeth, mandatory. The Nicholson file card is the one buyers usually name.

Use a handle every time. AIMS states that Australian workplace rules forbid using a file without a handle over the tang. None of these sources cite the equivalent US rule. The same guide gives the stroke technique: pressure on the forward stroke only, and lift the file on the return so the teeth do not dull.