Читать книгу Basic Benchwork for Home Machinists - Les Oldridge - Страница 12

Files must never be used unless the tang is protected by a handle. The handle must be firmly attached to the tang and be a comfortable fit in the hand. Failure to fit the handle properly, or using a file without a handle, can lead to the tang being forced into the palm of the hand, causing a nasty injury. To fit the handle, the file should be held upright on a wooden bench or block of wood, tang uppermost, and the handle tapped firmly in position. The tang should extend well into the handle. The file itself must not be hammered. Files are dead hard and, in consequence, rather brittle. Any hammer blows are likely to cause chips to fly off the file, causing injury to anyone nearby and damage to the face of the hammer.

The beginner should learn the art of filing by practicing on odd pieces of scrap metal, mild steel, or cast iron, about 2 in. square in section being most suitable. A partly worn flat file should be used at first, as a new file is too sharp at this stage and in any case it is better reserved for use on brass or bronze before being taken into use on iron or steel.

The correct stance at the bench is just as important when filing as it is for sportsmen when playing golf or cricket. The handle of the file should be held comfortably in the right hand, and the left hand should grip the extremity of the blade with the fingers pointing downwards. The left foot should be placed well forward and the right foot turned slightly outwards. The body should be well balanced and one should feel comfortable. If you are left-handed, reverse the above instructions.

The file should be placed firmly on the work and moved forward with a firm, steady stroke. Every endeavor must be made to keep the movement in a horizontal plane, keeping even pressure on both ends of the file. Beginners nearly always rock the file so that the surface produced becomes curved instead of flat.

If the upper arm is considered as a lever, pivoted at one end at the shoulder joint and the other end at the elbow, it will be seen in Fig. 5.3 that if the shoulder is kept rigid, the elbow moves in an arc. Unless a positive effort is made to correct the movement of the file, it will rock, giving rise to a convex surface on the workpiece, which must be avoided.

A full stroke should be made with the file, in fact the stroke should only finish as the handle approaches the work. The file should then be drawn back, the pressure having been released. The file only cuts one way, the teeth being formed to cut on the forward stroke only.

The jaws of the vise in which the work is held should be level with the user’s elbow. This is helpful in keeping the movement of the file horizontal. The work should be held securely in the vise. If the workpiece has a finished surface, or is soft, the clamps described in Chapter 1 should be used to prevent the serrations in the jaws of the vise causing damage.

Choose the right file for the job, use as big a file as is practicable, and do not nibble at the job with a small file. Use a bastard file if a lot of metal has to be removed, then a second cut one. A smooth file may be used, as the work is brought to size, if a fine finish is required.

Fig. 5.3 Positive effort needed to prevent file rocking.

Files are commonly used at too fast a speed. A file is a cutting tool and, being made of carbon, not high speed steel, the correct cutting speed is quite low. It is difficult to give a hard and fast rule for the stroke rate for filing as there are many variable factors—the material being worked, the type of file being used, and the strength and experience of the worker all have to be considered. An average speed of around sixty to seventy strokes per minute is about right.

Keep checking the work with a straight edge and/or a square. Do this early and often, so that if an error is creeping in, it can be detected while there is enough metal left for a correction to be made.

By changing the direction in which the file is working, it is possible to check how the metal is removed by watching the marks made by the file. Fig. 5.4 shows a square piece of metal. If the file is used on the square from F to B, a set of file marks will appear at right angles to G—E. If the file is now used from E to A, it will be found that the original marks will not be completely obliterated and it can be seen where the low spots are and the necessary corrections can be made. The file can then be used from G to C, where a fresh set of marks will appear. By constantly changing direction of the strokes of the file in this way, and by frequent checking of the work with a square and/or straight edge, accurate work can be produced. Slow, steady strokes with an even pressure will remove the metal at a good rate.

Use new files for such metals as brass and bronze, and when they have become dulled, they can be used effectively on cast iron and steel. Very old files should be used on castings where there may still be traces of the molding sand, which will spoil a new file. Avoid using files on sharp edges.

Fig. 5.4

Fig. 5.5 Files hung on nails on a rack after holes have been drilled in their handles.

Files are expensive and pay for looking after. If they are thrown in a drawer with other tools, there is a danger that their teeth will be damaged. Hang the files on a rack; holes drilled in the handles allow them to be hung on nails on a rack, as is shown in Fig. 5.5.

Files should be regularly cleaned with a file card. This is a brush composed of stiff wire bristles woven onto a strong canvas. The canvas is nailed to a suitable wooden block, which forms a handle. The file blade should be brushed crossways with the card. This should not be done too vigorously so that the hard bristles blunt the file teeth. “Pinning” occurs when a piece or pieces of metal lodge between the teeth of a file, causing scratch marks to appear on the surface of the work on which it is being used. If the offending metal cannot be removed by the file card in the ordinary way, it can be scraped out with a piece of pointed metal, being careful not to damage the teeth. Problems with pinning may be alleviated by rubbing chalk into the file, but this should not be done when filing cast iron or brass as it causes the metal to glaze under the file.

File marks may be removed by DRAW FILING. For this purpose a fine cut file is used crossway on the work, as is shown in Fig. 5.6. The file is grasped by the handle and tip and propelled forwards and backwards. The teeth of the file do not cut so harshly when it is used in this way.

Although filing in the lathe is not strictly benchwork, it must be considered. This practice does not find favor in some quarters, it being held that any finish required should be obtained by the lathe tools. However, the makers of Stubs files make particular mention in their user’s handbook of filing in the lathe, and undoubtedly, in some circumstances, it is justified. They give the following advice:

“When work to be filed is revolving in the lathe, the file should be used with a stroking action, allowing it to glide slightly along the work. This will help to avoid making ridges and will help keep the file clear of chips. Because of their sharpness, new files are best avoided for lathework where a fine finish is required. Lathework should not be touched by hand, as oil and moisture can coat the surface and it is then difficult for the file to hold.”

Fig. 5.6 Draw filing.

There is some danger in filing in the lathe. Particular care must be taken to see there is no loose clothing likely to catch in the revolving chuck or work. Sleeves must be rolled up or tightly fastened at the cuff.

In order to produce a fine finish on work that has been filed, various grades of emery cloth are used. Emery cloth is obtainable from coarse grades right down to very smooth ones, and any of them is best used with an old file or a piece of wood as backing. If an especially fine finish is required, a thin oil on the emery cloth will assist. Worn-out coarse emery cloth should not be discarded but can be used as a fine grade.

Care must be taken when using emery cloth to ensure that the accurate work produced by the file is not ruined. Sharp corners can be rounded off by the careless use of emery paper.

Copper and its alloys can be given a high degree of polish when smooth by the use of a metal polish, such as Brasso.