By far the most important tool used in perfecting the surface of fused; or cast work is the file. It is sometimes replaced by emery, either in the form of wheels or as powder attached to cloth; and is often supplemented in fine work by one of the various kinds of polishing powder, e.g. chalk, crocus, putty powder, tripoli, sand, etc.

It has been remarked that the most important point to be decided before commencing filing is the fixing the vice to the correct height and perfectly square, so that when the work to be operated on is placed in the vice it will lie level. As to what is really the correct height some slight difference of opinion exists, but the height which is generally thought right is such that the "chops" or jaws of the vice come just below the elbow of the workman when he is at his place in front of the vice. Having the vice fixed properly, the correct position to assume when filing is the next consideration. The left foot should be about 6 in. to left and 6 in. to "front" of the vice leg; the right foot being about 30 in. to front, that is to say, 30 in. away from the board in a straight line with the vice post. This position gives command over the tool, and is at once characteristic of a good workman.

The file must be grasped firmly in the right hand by the handle, and it is as well here to make a few parenthetical remarks on handles; they should always be proportionate to the files to which they are fitted, and the hole in the handle should be properly squared out to fit the "tang" by means of a small "float" made from a small bar of steel, similar to those used by plane-makers and cabinet-makers. The handles should always have good strong ferrules on them, and the files should be driven home quite straight and firm, so that there is no chance of the tool coming out. Each tool should have its handle permanently fixed; it is very false economy, considering the price of handles is about 9d. per dozen, to be continually changing. The left hand must just hold the point of the file lightly, so as to guide it; and in taking the forward cut a fairly heavy pressure must be applied, proportionate to the size of the tool in use and the work being done. Amateurs who have never received any practical instruction in the use of files generally have a had habit of pressing heavily on the tool continuously during both forward and backward stroke, and at the same time work far too quickly.

These habits combined will almost invariably spoil whatever is operated on, producing surfaces more or less rounding, but never flat.

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The art of filing a flat surface is not to be learned without considerable practice, and long and attentive practice is necessary ere the novice will be able to creditably accomplish one of the most difficult operations which fall to every-day engineering work, and one which even the most professionally taught workman does not always succeed in. The file must be used with long, slow, and steady strokes, taken right from point to tang, moderate pressure being brought to bear during the forward stroke; but the file must be relieved of all pressure during the return stroke, otherwise the teeth will be liable to be broken off, just in the same manner that the point of a turning tool would be broken if the lathe were turned the wrong way. It is not necessary to lift the file altogether off the work, but it should only have its bare weight pressing during the back stroke. One of the chief difficulties in filing flat is that the arms have a tendency to move in arcs from the joints, but this will be conquered by practice.

A piece of work which has been filed up properly will present a flat, even surface, with the file marks running in straight parallel lines from side to side. Each stroke of the file will have been made to obtain a like end, whereas work which has been turned out by a careless or inexperienced workman will often bear evidence that each stroke of the file was made with utter disregard to all others, and the surface will be made up of an unlimited number of facets, varying in size, shape, and position.

There is considerable skill required to "get up" surfaces of large area by means of files alone, more especially when these surfaces are required to be accurately flat. The method of preparing surface plates, as detailed by Sir Joseph Whitworth, is most valuable information to any one desirous of excelling in this particular branch of practical handicraft, and those interested should get Whitworth's pamphlet entitled 'Plane Metallic Surfaces, and the Proper Mode of Preparing Them.' In large engineering works, filing is superseded by the planing and shaping machines for almost all work of any size. The speed and accuracy of the planing machine cannot be approached by the file when there is any quantity of material to be removed, and files are only used for the purpose of "fitting" and to smooth up those parts which are inaccessible to the planing tool. However, a planing machine is one of those expensive and heavy pieces of machinery usually beyond the reach of amateurs and "small masters"; it therefore becomes necessary to learn how to dispense with its valuable aid.

Cast iron usually forms the bulk of the material used by engineers. The hard outside skin on cast iron, and the sand adhering to its surface, make it somewhat formidable to attack. If a new file is used for the purpose it will be assuredly spoiled and with no gain; for one which has been very nearly worn out will be almost as effective, and will not be much deteriorated by the use to which it is put. There are several ways of removing the "bark" - e. g. the castings may be "pickled" - that is, immersed in a bath of sulphuric acid and water for a couple of days; this will dissolve the outer crust of the casting, and liberate the sand adhering to the surface; another plan is to remove a stratum of the casting from that part which has to be filed, by means of a chipping chisel, and this is a very good plan where much material has to be removed from any particular part of a large, unwieldy piece of machinery, though some practice will be required with the hammer and chisel before they can be used satisfactorily.