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Using Hand Tools - Files

Vital information for working on cars

by Julian Edgar

Click on pics to view larger images


This article was first published in 2008.

We’ve already covered using spanners and sockets – now it’s time for hand files.

But what’s there to know about files? As always.... heaps! Using a file incorrectly can result in clogged teeth, uneven surfaces, square holes that become round and round holes that become square - and a whole lot of other problems. But as with all hand crafts, once you get the right techniques established, things tend to fall into place. But if you never get that start, you’ll spend literally years making things worse and not better!

Files are categorised according to two criteria: shape and teeth coarseness.

File Shape

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The most common file is a flat file. A flat file has cutting faces that are parallel to one another. In plan form the file is normally slightly tapered and has cutting teeth on both edges. The main teeth are normally arranged in a double row. This type of file is a general purpose design suitable for reducing to size and shape or fit finishing.

A mill file is similar to a flat file but it is thinner and smaller. Its size allows it to be used where a flat file cannot – for example, filling a slot. Mill files generally have only one row of teeth (see below for more on teeth rows).

A hand file is like a flat file but it has teeth on only one edge. This characteristic is very important as it allows the filing of a shoulder or internal edge of a square or rectangular cut-out without inadvertently filing away the other surface. If you absorb only one thing from this story, it’s this point: that some flat files don’t have teeth on their edges!

A round file (called a ‘rats tail’ if the diameter is very small) is a very useful file. It is used for enlarging or elongating holes or for filing the inner diameter of tightly curved surfaces.

Another very useful file is the half-round. This is useful for enlarging big diameter holes or for filing gentle curves. Because one face is flat and the other rounded, it’s a file that can be used a great deal – one side for normal filing and then without having to put it down and pick up another file, the other side immediately available for filing curves.

A square file has equal widths on each cutting face – it is square in cross-section but tapers down in size towards the end. This type of file is useful for enlarging square and rectangular cut-outs. In my home workshop I rarely use this file.

A triangular file is also used relatively rarely. Because it has teeth on all surfaces (including the corners) it is a very easy file to make a mistake with – to elongate a corner when using it to file a square opening, for example. However, when a triangular cut needs to be made in a surface (for example, to get a round file started, so creating a half-round opening) it works well. A triangular file can also be used for restoring threads and gear teeth.

Teeth

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Files can be categorised into three tooth types:

Single cut - the file has a single row of teeth, making for smooth cuts

Double cut - two rows of teeth arranged at an angle to each other; most files are like this

Rasp cut – raised teeth used for cutting soft materials, eg wood

Furthermore, the coarseness of files can be classified (in order from most coarse to least coarse) as: rough, coarse, bastard, second-cut, smooth, dead-smooth, super smooth.

Selecting a File for the Job

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It’s very unlikely that you are going to have 20 different files sitting in your tool-box, ready to go. So most of you are not going to say to yourselves: “Hmm, will I use a second-cut half-round on this or a single cut mill file?” Instead, you’re going to grab the file that looks most suitable – even if it really isn’t!

When selecting a file, the most important things to get right are the teeth coarseness and the file shape.

A file that is too coarse will (1) make very deep scratches which will be later hard to remove, (2) potentially jam on the surface, breaking teeth and (3) potentially take off too much material – a disaster, because you can’t put it back! For these reasons it is better to select a file with teeth that are too fine rather than too coarse. The worse thing that can then happen is that it will take you longer to do the job.

The softer the material, the coarser the file you can use – but the quicker material will be removed. A coarse file used on soft plastic can remove a few millimetres each stroke – it takes very little filing before you find that you’ve gone too far.Therefore,be conservative in the coarseness selection, leaning towards files too fine rather than too coarse.

While tooth coarseness is usually judged just on the appearance of the file (ie teeth per inch), in the case of worn files, the smoothness of the cut may be quite different to the file’s appearance. For example, a coarse file with all the edges worn off may in fact act as a smooth file. Therefore, it pays to know your own files and mentally pigeon-hole them according to their actual performance.

The file shape is critical. When filing a large flat surface, pick the widest file you have available. When enlarging a hole, pick the largest round file that will fit through the hole. In both cases, a smaller file will be much harder to accurately control.

If you are using a half-round file to enlarge a hole, make sure that the file isn’t too large – the edges will dig in and the hole will become ragged.

In all cases, before you start to remove material, run the file gently over the work so you can see where material will be removed. This ‘try before you file’ approach will allow you to quickly see if the teeth on the edge of a file will unexpectedly take away material, or if the file is too large or too small.

  • Always have a handle fitted to a file. Not only is the file then much safer to use but it will also give you better control.

  • Never use a file as a lever or pry bar. The hardened steel is brittle and will snap.

  • Files are best stored apart so that their teeth are not touching.

  • In hot, wet environments, files can rust. To stop this occurring, give them a wipe over with a thin coating of engine oil.

  • If they are cheap enough, secondhand files are always worth buying. Even worn files can be used as final-finish tools.

Filing Techniques

Filing’s filing, eh? What’s there to know? In fact, there are three different types of filing techniques.

  • Heavy Filing

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Heavy filing is used to remove a lot of material. One hand holds the file handle and the other firmly grasps the other end. The file is moved back and forth across the work, with more pressure being used on the ‘push’ stroke than the return. A relatively coarse file is used.

When it’s possible, mark a line that you’re filing towards - and always stop well before the line.

When heavy filing a flat surface, it’s very easy to round the edges as the file ‘rocks’ over the work. Therefore consciously try to keep your arms moving back and forth in a perfectly flat motion, and frequently check the flatness of the resulting surface with a steel rule or square. Where the work is able to be mounted in a vice it should be near the height of your elbow as you’re standing erect – stooping over the work will make it harder to keep the file moving in the correct plane.

  • Light Filing

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Light filing is used for shallow cuts and when your cutting is approaching the marked line. One hand holds the file handle as before, but the other hand holds the end of the file in just the finger tips. The pressure used in light filing is much less than for heavy filing. A finer file is used and this – and the lighter pressure – results in much less material being removed each stroke.

Light filing can be used to ‘true’ the surface and/or take off just a tiny amount of material.

  • Draw Filing

  • Click for larger image

    Draw filing is when the file is moved over the item with a sideways action. It is a good technique for removing scratches left by the other filing techniques and can be used to polish the surface. It is also good for fixing square edges that have been rounded by inadvertent rocking of the file.

    File Cards

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    Files that are used on soft materials (eg aluminium or copper) tend to clog. The tiny metal chips caught in the file teeth reduce the cutting efficiency of the file and can also cause severe scratching of the surface. A file card (a bristle brush) can be used to clean the teeth. I simply use a wire brush.

    Conclusion

    An appropriate file used carefully can result in accurate and clean work - but it’s nearly as easy to end up with work pieces that are ruined! When using files remember these two key points:

    • Pick the right file for the job

    • Use it slowly and carefully, changing techniques as the job progresses to its finished state

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