Notes on Workshop Techniques


INDEX

WORKSHOP PRACTICE

18. USING FILES

It's said that a skilled person with a file can make just about anything, and the truth is not far removed. There's no question but that the humble file is a very useful tool, however, as the saying goes, it requires skill and practice to obtain good results.

I cannot lay claim to any particular skill with a file, but I get by. There are certain tricks one can apply that will at least improve the chances of success. The first thing is to get the technique of filing flat. The main problem with filing is that of obtaining surfaces which are truly flat and square, and failure to be able to do this is entirely due to the way the file is held and stroked. It is un-realistic to expect the essentially rotary motion of your arm's action whilst filing to naturally result in the file travelling in a flat plane - it needs a conscious effort and training. Ideally, your workpiece should be at elbow height or just above and (assuming right-handed) the file handle should rest in the palm of the right hand. There's no need to grip it tightly and your wrist should be relaxed. Either the palm or the just the thumb of the left hand should rest on the tip of the file. It's a bit like playing snooker really (or pool?), it's essential to concentrate on the arm action and more or less ignore the cutting effects of the file - that will look after itself. Aim to move that file in as straight a path as possible for it's entire travel, this will feel un-natural because to get it to move in a straight line you will have to flex the wrist just so. At the same time as doing this the blade of the file needs to be kept square left-right as well as during the forward-backward cutting stroke. Like I said, you need practice before it feels anything like natural. One trick, if your vice allows it, is to mount a long (12" or so) block of wood at the back of the vice such that the fingers of your left hand just brush the surface. This will guide the file in a straight line until you learn what the *right* hand has to do to keep it flat.

You need to use the correct file for the job - there are many different shapes and cuts but quite often in the shop there is a 'favourite' file which gets picked on most often. Personally, I have little use for coarse files, tending to use either 2nd cut or smooth files exclusively. I reckon if there is enough meat to warrant the use of a rasp then there is likely a way of machining the metal away rather than filing it (just lazy I guess). Even more lazy - if it's possible to get it on the belt sander I'll use that instead! The sort of jobs that a file is called for is in the shaping of slots and channels (inside or out) and of course profile work (like the flare at the base of model loco chimney and dome castings). Therefore, you need quite a selection of file shapes and sizes to suit. Always use the widest file that will actually fit the job because a wide blade is easier to keep flat and square. Similarly, a long file is easier to hold and keep straight even if you are only taking short strokes just using the middle of the blade. I hate needle files for this reason and avoid their use wherever possible, instead I'll use a narrow pillar file (8" long) of which I have a selection.

Most filing is done dry, with perhaps a rub of chalk on the blade to ensure it stays that way and to help reduce 'pinning' (where small particles of metal get jammed in the file teeth and gouge the work instead of cutting cleanly). There are occasions where cutting oil is used on the blade but this is a special circumstance and will be covered later. The angle of each cutting stroke need not be the same, in fact there is some advantage to swinging the blade around to 30 degrees either side of square as this will show up the precise area being cut. But it has to said that you should rely on your correct handling of the file in the first place rather than looking to see what's been cut off and making constant adjustments as you go along.

A word about the quality of files and their maintenance. There's no doubt you get what you pay for, and decent fine Swiss files (genuine that is) are well worth the money. Files with teeth that are properly formed will cut more cleanly and will be less likely to 'pin' due to hooked teeth or rough cutting edges. Take your best 10" flat file and look down the edge - is it quite straight? A common failing is that cheap files are warped. Only buy top quality files that have been bought new and then look after them. Don't bang them around together in a drawer - make a wall rack to hold them separately. Keep them clean with a file card (a flat board with many fine bronze bristles used to clean out the teeth), or use the sort of bronze brush for cleaning suede. If the file gets oil on it clean it off with solvent and hot soapy water before use - dry it well with a short blast from a propane torch. The worst fate for a file is to pick it up and stroke it across some miscellaneous chunk of steel only to find it was hardened. That's essentially the end of that file.

Many file types have a 'safe' edge without teeth, but it's as well to run an oil stone over this face to make quite sure it's smooth and that there are no corners of teeth are sticking out (not unknown). It is simple enough to give a file a second safe edge if necessary using the bench grinder to take the teeth off, and more importantly, it's quite possible to alter the geometry of the file (perhaps making it wedge-shaped or just a little narrower than nominal dimensions) to suit special jobs. You will gradually acquire a collection of such modified files and they are a valuable asset.

To file a straight edge on a piece of sheet metal it is usual to use some sort of guide - either the tops of the vice jaws (but see below) or a length of mild steel section can be used. For a long straight edge (say, a 12" length of brass sheet for a locomotive tender) I have used 2 pieces of 1" x 1/4" section steel with the sheet sandwiched between the two. Toolmaker's clamps were used to stop the ends flapping around and the whole lot mounted in the bench vice. Now, it's possible to work at this with a large file and no doubt a good job will result - eventually. One very useful tool that does the job much quicker is an electric file (a sort of miniature belt sander - mine was made by Black & Decker). This tool soon rips the metal off down to the guide and a good straight edge is quickly obtained. Of course, the steel guide pieces don't last forever but there are 2 edges to work with and the metal isn't all that expensive - and it will still be usable for other jobs when the accuracy of the straight edge is lost.

Using a file on work in the lathe is generally frowned upon as being 'unprofessional', but most turners do it at some time or other. The thing to remember is that the file (I use a fine single-cut mill file, but a fine cut Swiss file will do as well) should only be used to improve cosmetic appearance, to relieve tapers by a few thousandths, or to break the sharp corners of turned work. You should not aim to reach a finished dimension by filing down from a larger diameter because the concentricity of the work will suffer. To get a good finish on a piece of steel in the lathe dip the file into a jar of straight cutting oil and gently stroke it's blade across the work, keep the file moving gently back and to, and side to side, and don't put too much pressure on it. Watch for the teeth becoming clogged - remove it from the work frequently and use a stiff brush to clear the teeth before re-oiling. Pinning will be fatal to a fine finish and it usually happens just as you have finished the job.

The Bench Vice

This is probably the most abused tool in the shop, yet is also one of the most valuable. It pays (as always) to get a good quality vice with a jaw width of at least 4" (a 5"-6" vice will be an improvement). A second, smaller, precision vice can be used for fine work. Good vices are not cheap, and a Record 5" mechanic's bench vice costs in the region of 250UKP at last count. Your main bench vice needs to be solidly mounted so bolt it to your bench positioned over the top of one leg to give it more support. First thing to do with a new vice is to remove the hardened serrated jaws and replace them with mild steel inserts. You will rarely if ever need the grip afforded by the serrations and these will only damage work held in the vice. Further, the mild steel jaws can be drilled and tapped to accept punches to press bushes etc., (assuming you don't own an arbour press), and the soft jaws will not blunt files the way hardened jaws do. When I made my soft steel inserts I fist screwed the blanks in place then, with a wedge of metal trapped bellow the jaws such that they were held apart just less than 1/2", I mounted the vice on the miller and machined the jaw faces and tops square. It's useful to have the inserts overhang one side by about an inch so that you can grip awkward shaped items - and if the top edge of just one of these overhanging bits is bevelled to less than 90 degrees you can bend narrow sections to an acute angle, something that is difficult with the standard jaws.

A useful modification is to insert a large ball-bearing thrust race between the handle and the moving jaw, this will afford greater control over the pressure exerted, allow greater pressure to be generated for a given amount of torque, and the reduction in friction makes it easier to loosen also.

Scraping

This is a technique familiar to those brought up in the industry, perhaps less so to the newcomer in the home workshop. Essentially it's a method of shaving metal off in a finely controlled manner and very good accuracy can be achieved this way. It's main use is to render flat plates (such as surface plates or lathe bedways) truely flat, removing bumps and hollows in the process, but the technique can equally be applied, given the right tools, to curved surfaces such as bearing shells. Whatever the workpiece, the method requires that a reference surface be used to indicate which parts of the work needs scraping, and this is usually rubbed across the work with some engineer's indicating blue. In the case of (for example) a steel base perhaps some 3" square, this would be rubbed against a larger surface plate with a trace of blue on it, high spots would then show up as blue marks on the workpiece. The scraper is then used to shave the marked areas and the process repeated. Eventually a fairly even marbled blue finish will be achieved with a characteristic 'feathered' appearance (caused by the scraping in different directions). This method was used in the past to generate truly flat surface plates by scraping 3 plates together, the first plate would use the second as a reference, the second referenced against the third plate, and finally the third referenced against the first plate again. Eventually all three plates would end up flat this way but it's a very slow process.

Whilst it is possible to purchase scrapers designed especially for the job, it's a fairly simply job to convert an old file into a usable scraper. In fact, this is one of the best ways of recycling those old files into something useful. All you need do is grind the teeth off all sides for a length of some 2-3" from the end, then grind a large radius curve on the end to form the cutting edge.

Home made scraper from a 10" flat file.

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It's important that a large handle be attached to the scraper, one 1-1/2" by 8" long is not too big, this will give greater control over the cutting edge. In use, the scraper is held at a small angle to the work (about 5 degrees) with the fingers of the left hand applying a little downwards pressure just behind the cutting edge. The scraper is then pushed with a sort of slicing action which should remove fine slivers of metal cleanly without chatter. If the scraper is held at too obtuse an angle it will dig-in, too fine an angle and it will tend to skip the work - practice will tell you the right angle to use. Change the angle of attack (i.e., use all points of the compass) for each cut for best results. It's absolutely essential that the cutting edge be kept dead sharp and finely honed, but this is quite easy given the simple shape of the flat scraper. Other shapes are more difficult to keep in good condition.

For bearings, either a curved or three-square (triangular) section scraper is used. These require much more dexterity to use effectively as it's that much more difficult to ensure even cutting takes place. The sort of job where this tool is ideal is in lathe mandrel bearings of the split sleave type. When these get worn they can be corrected by either removing a fine shim, or removing a thicker shim and shaving a little off. This effectively closes the gap but leaves the bearing hole out of round (an oblate spheroid shape). This must be corrected otherwise the lathe mandrel will still be sloppy in the horizontal plane. One method (in the absence of a suitable large reamer) is to scrape the offending metal away using a triangular section scraper, and using the lathe mandrel (or a previously made test mandrel) as a gauge. Scraping starts with the bearing not tightened up (otherwise the mandrel would not fit in the hole) and continues, gradually tightening the bearing, until it's seated on it's shim and the indicator blue shows the mandrel fits the entire cross-section of the bore. Only a very few thou are removed using this method. This is not the easiest of jobs and requires practice, so don't choose your lathe as the first job to try out on!

...continued

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(c) Chris Heapy 1996.


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