How To File Metal

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It’s been 24 years since I did my ‘Sheet Metal and Welding Fabrications’ C&G, but some things you never forget.

Always leave plenty of spare, when you cut a tube to length.

How to File Metal

It’s not until you see someone else’s work (the guy I bought a chassis from) do you remember that it was a 6 month full time course. It’s actually a very complex and tricky craft to master.

If you are to achieve any sort of accuracy during the build, this is probably one of the most frequently used and important skills to learn. Mastering the art of filing can take some time.

Types of file:

  • Rough, less than 20 teeth per inch.
  • Middle, less than 25 teeth per inch.
  • Bastard, less than 30 teeth per inch.
  • Second Cut, less than 40 teeth per inch.
  • Smooth, 50 to 60 teeth per inch.
  • Dead. Smooth, 100 or more teeth per inch.

Counting the teeth per inch (tpi)

Teeth per inch is very simply the number of teeth along the file in one inch. To count them simply lay a ruler alongside and count.

File - teeth per inch (tpi)

My File for chassis tube finishing

For trimming tubes, I actually use a flat ‘Rough’ 14inch 16tpi file made by Bahco; it’s got to be 20 years old and is probably beginning to show it’s age a little. If I was buying new I’d go for a flat 14″ Middle file. For this job it’s not the quality of the surface finish that is needed; as mostly you’ll be filing the ends of tubes square; it’s cutting power and squareness.

14inch Rough File 16tpi

The length of a file does not include the tang or handle section.

There is a file for every job on the car. Some are curved, some half round, some triangular etc. It’s worth having a selection of them. A great place to pick them up is car boot sales and street markets. As with a lot of things these days, modern mass produced files often pale in comparison to those found in Grandads’ shed. A good tip I’ve found is to look for a recognisable brand stamped in, or British made, along with the tpi count and file type. Modern files which say ‘Made in China’ are often best left alone. Those cheap files you see in the local bargain shop are simply a false economy and will blunt within seconds of use.

An important thing to note is the  handle. I can’t stand using files without handles. Lets face it, you can buy a handle from most hardware stores for about £1. Without one you’ll be bashing your knuckles and trapping skin. The tapered shaft at the end is often left unfinished and simply its shape will leave you with endless cut and blisters. Whinge over.

The tang section of file; onto which the handle mounts; is not tempered and hardened like the rest of the file. It is left soft so that the handle section has slight flex, without this, the handle would simple snap off.

Some files; like the one above; are single cut, whilst others are double cut. This refers to the pattern of the teeth.

  • Single cut files have one set of teeth running in a \\\\\\ pattern. These teeth are parallel pattern across the file at 65° to 85° to the centre line
  • Double cut files have two sets of teeth running in an XXXXX pattern. A double-cut file has two sets of teeth, the first or over-cut teeth being cut at 40° to 45° to the centre-line and the second cut or up cut at 70° to 80° to the centre-line.

The ideal angle of teeth would be different for wrought iron than for brass. Generally, harder materials would require closer to 65° and brass closer to 85°.

Cutting the metal

If the piece is important I always use a hacksaw to cut the piece. The rules below, can also be applied to sawing to get better accuracy. Band saws are a great labour saving device, but remember the cut given is often off square, therefore, always leave more spare metal. For chassis tubes I usually always cut by hand, as the time saved in sawing is lost at the filing stage. Where band saws come into play is with solid bar and where accuracy is not paramount. Changing the cutting angle away from square takes time and I can never be bothered to set it for anything other than large pieces.

Band Saw

Using a Metal File

Before you start filing make sure the piece is securely clamped in a vice or similar. The face to be filed should not stand to far proud of the vice jars, as it will simply vibrate or chatter when filing starts. Ideally the surface to be filed should be parallel to the ground. If you are worried about marking the piece in the vise, invest in a pair of clip-over soft jars.

Before you start, you must make sure that you have plenty of room to move and the piece is at a good height. If your stance is wrong when filing, don’t expect good results. If filing with your right hand on the handle, place your left foot forward, so your feet are about 2 feet apart. With the right hand on the handle the left hand or fingers, should be placed gently on the top front surface of the file. If I want to remove lots of material I grip the file with the front hand, as it increases the power of the stroke.  When I am doing light work, I grip the front of the file with thumb and fore-finger. My old instructor used to criticise my front hand position, saying I should always wrap my fingers around the file, but like many in my class, I bashed them a lot that way, so I’ll let you decide on that one.

The front hands main purpose is to keep the file level and in contact with the surface. My left hand is pretty smashed up (full of plates and screws after an accident on a dry ski slope) and I find the grip shown below works best for me. The thumb and fingers are well spread and this allows me to apply even pressure to more of the file’s surface. This allows me to keep the file much closer to level and also to detect any unevenness in the metal surface (not on tubes though)

How to File Metal


Watch the ‘How to File Metal’ video

At the beginning of the stroke, weight is applied with the front hand. At the middle, even pressure is applied to both hands and finally at the end of the stroke to the back hand. During the whole stroke pressure should be even and the file level.

On smaller pieces I find it beneficial to hold the file diagonal to the workpiece; especially for tube; moving the file forward and to the right. After a dozen strokes, I then reposition and file forward and to the left. As I move the file back; although no pressure is applied; I do not allow the file to break contact with the surface. If I was to apply pressure on the return stroke the file would quickly blunt. This action; for me; feels much more fluid and natural.

By moving first in one direction and then in another, the scratch pattern left by the file quickly highlights the high and lows spots of the metal’s surface.

There are other techniques used, such as draw filing, but seeing as most of these edges will be hidden or destroyed by welding, the quality of finish is of low importance. For high quality finishes, you can clog the file’s teeth with chalk, wrap it in emery paper etc but that’s a whole different subject!

Before you put your file away, get a stiff wire brush or a special file brush and clean out the teeth! It’s handy to rub in some loose chalk or quickly spray it with oil as it’ll make it far less likely to clog next time you use it.

Checking the piece is square

Most people I’ve seen just grab a right angle square and drop it onto the piece. This isn’t the way to do it.

With your eyes at the same height as the top edge, hold the vertical face (handle) to the square, pressed onto the side of the piece, moving the horizontal face (ruler) slowly towards the face to be checked. When the gap drops below 1mm, the human eye will be able to detect angle errors far more finite than simple rocking a square back and forth.

Remember that Gap! I find back-lighting the piece helps or when that’s not an option, hold it towards the light.

Set Square

Before measuring, remember to de-bur the edges of the piece.

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