In the machining practice, filing is one of the major tasks that should not be taken for granted, as it is quite often the case. It has paramount significance for the subsequent operations due to being performed on the workpart where a high degree of surface finish and cutting precision is required.
In those cases, if a filling is neglected, the low-quality attributes of the workpart, proper to machining, would be the ultimate price. Thus, a machinist must keep in mind that filling is as inevitable as drilling, soldering, tempering, and so forth.
In instances where different parts of a final workpiece are to be assembled, the filed surfaces would play a decisive role in their proper assembling, maintaining the required fit, which can only be achieved through constant practice.
The role of metal files in any metal workshop is crucial. They complement the machining work performed by conventional machine tools such as lathe, milling machine, drilling machine, planing machine, slotting machine, shaping machine, sawing machine, and others. Different types of files are used with different machine tools but mainly it is governed by the type of machining operation and the degree of required surface finish or the quantity of the removal of the excess material.
What are the Metal Files?
A metal file or simply a metal file tool is a series of small chisels finer or coarser that depend on the nature of the work to be required of them. These chisels are made by cutting fine grooves over their body-face yet quite close to each other.
Or, more precisely, it is made of high-carbon crucible steel with chisel-cut teeth on its body. These numerous cuts or teeth are made diagonally across the face of the file, and their design is determined by the type of file intended to be used for either finishing or roughening of the metal surfaces.
Nomenclature of a File
The main parts of a common file (or its anatomy would) include point, heel, tooth rows, edge shoulder, and tang, as illustrated schematically below.
- Tip/Point: It is the one end of the file.
- Face/Side: It is the wider area that does the filing and contains rows of filing teeth.
- Edge: It is the side of the file. In a hand file, one edge has the teeth, and the other does not.
- Heel: It is the section of the file face that is free of teeth.
- Tang: It is the narrower and thinner part of the file to which is attached handle for holding the file, say by hand.
- Shoulder: It is the part of the file where tang is joined with the body of the file. It is curved in shape.
- Handle: It is used to hold the file.
- Ferrule: It is used to keep the handle from cracking or wearing out. It is a protective ring of metal attached to the handle.
Metal files are made of high carbon steel, alloy steel, tool steel, high speed steel (HSS). In some cases, they are also diamond coated for unique applications. Some specific purpose files include steel files, aluminum files and others.
Types of Metal Files
Based on Tooth Art
From a design point of view, that is, tooth art, the four major types of files include single-cut, double-cut, rasp, and curved files.
- Single-cut file: If the file has single rows of chisel-cut teeth over its face, it is called a single-cut file. In the single-cut file, there are single rows of coarser teeth that extend across its face and make a 65 to 85-degree angle with the axis of the file.
- Double-cut file: If the file has double rows of finer chisel-cut teeth in a criss-cross fashion on its face, it is called a double-cut file. For general work, the angle of the first row is from 40 to 45 degrees, while that of the second is from 70 to 80.
- Rasp File: It is a coarse file and is used for coarsely shaping wood, plastic, or other softer materials. It is a hand tool and is made of a tapered rectangular, round, or half-round sectioned bar with a tapered tang to which a handle is fitted. On its surface are cut sharp and pointed teeth that perform the filing operations.
- Curved File: It is a file that contains rows of teeth that are cut in a curvy manner for deep cutting. It is apt for filing soft metals such as aluminum.
Based on File Grade
The criterion over which files are broadly divided is governed by the type of the cut, which can be rough, coarse, bastard, second-cut, smooth, or dead-smooth, as shown below.
These terms are no doubt relative to each other, yet they depend much on the length of the file (without including tang). For instance, a 16-inch long second-cut file is much coarser than its 4-inch counterpart.
Based on Tooth Geometry
Files are also grouped based on the geometry of the cut, such as round, half-round, triangular, and square. Another axis along which their classification revolves is the particular nature of their use, and they include mill files and warding files.
It is also called a float file or a lathe file. It is a single-cut file, except for all machine shop files that are double-cut.
It derives its name from its use, such as in sharpening saws and planer knives that are made of harder metals, for softer metal alloys such as brass and bronze, bastard files (which are coarser files) are used.
It is rectangular in construction. Its two-thirds length is flat, whereas the remaining one-third is tapered towards the point. Its face has double-cut teeth with edges of a single cut-type, as shown below.
It is the file that is found in every machine shop and is used to file large metal surfaces when the metal in great amounts is to be removed.
Its extensively available size is 12 inches, though flat files as small in length as 6 to 8 inches and as large as up to 16 inches are also used. It is a safe-edge file, as there are no teeth on one of the two faces.
It has flat edges that extend from heel to end with its slightly convex surfaces. The second-cut class is for removing feed marks and generating a good surface finish by cleansing, especially of round stock in the revolving lathe. It is also a safe-edge file.
It is used in flailing square or rectangular holes, as shown below. It is also used in giving surface finish to the narrow bottoms of slots.
It is similar to that of the bastard file except in its narrower size. It comes in Nos. 00, 02, 04 cuts. The 8-inch pillar file is also the most common file available in the machine shop.
Three Square File
It is also called a triangular file. It has double cuts on all three surfaces with sharp edges. It is used in giving valuable surface finishes, usually less than a right angle. They are popular in baking off taps and counterbores.
It finds its use in rounding off the corners. Also, it can be used in enlarging the existing holes, as shown below. It is usually tapered in design. Its smaller type is known as rattai type.
It is half-round in its construction, as shown below, and is widely available in every workshop. It is made in various sizes, lengths, and design specifications.
Knife Edge File
It is chiefly used in thinner work, that is, tight grooves or angles. Its cross-section is sharp triangular. The one-third of its length is tapered. It has single-cut teeth on one face while that of the double-cut on the opposite face.
It is exclusively used in finishing off the sharp corners of slots and grooves. Its geometry is flat or triangular. The file teeth are cut on its wider face.
It is a very thin file. Retained by locksmiths in producing notches in the keyholes, it is also effectively used to finish the sides of the narrow grooves in the machine shop.
Its edges are rounded and are used in filing against the filleted shoulder or the round corners of a hole.
Used in place of the half-round file, it has both sides with different curves, each to use according to the required filing characteristic.
Do’s in Filing
- Over time, filing first is wedged on the teeth of the file. It is known as pinning. It decreases the effective finishing capability of the file. A small piece of soft steel brass or copper should be used to clean the surfaces/edges of the files periodically.
- When a new file is rubbed too hard against the metal being worked on, it also causes pinning. To avoid this, the new file should not be pressed too much against the metal part surface until the tiny burrs present on the ends of the teeth are worn away.
- Keep the right posture during filing: Don’t lean too much. Exert arm pressure. Move the file diagonally or straight across the workpart as shown below.
- The use of chalk can also clean the teeth from clogging. It should be a practice after completing a particular job.
- Scrub a file using a file card in the direction of the tooth rows to keep the file from clogging.
Don’ts in Filing
- Don’t use a file if it is without a handle.
- Don’t use a file if its handle is loose. It may cause serious injury.
- Don’t use a worn-out or dirty file.
- Don’t use a new file on the cast iron scale as well as on narrow edges.
- Don’t use a file for which it is not designed. For example, don’t use a bastard file for finishing and a smooth file for roughing.
- Don’t push the file too fast.
- Don’t maintain a too heavy grip on the file. Doing so would produce an uneven and rocky motion. The picture shown below shows the wrong posture during filing.
I am the author of Mechanical Mentor. Graduated in mechanical engineering from University of Engineering and Technology (UET), I currently hold a senior position in one of the largest manufacturers of home appliances in the country: Pak Elektron Limited (PEL).