Thus, supposing a circular edge of about 60° is desired, the section of 58° approaches this most nearly, and if the plan of 140°, which, with this section, gives two straight cutting edges of 60°, be adopted, there can only be a variation of 2° in different parts of the edge. But it will be observed that this combination admits of very little rounding; and although less acute edges, being obtainable with a smaller plan-angle, admit of rather more rounding, it may be taken as a general rule that when any tool is much rounded on the nose, so as to present a large segment of a circle, different parts of the edge must vary considerably in acuteness. Although Professor Willis objects to rounding the nose of a tool at all on account of the necessary variation in the character of the edge and some other reasons, I am disposed to think that when the tool is carefully formed on the principles given above, it is very advantageous to round off the point slightly for taking heavy cuts; and I have found this form a favourite one in such workshops as Woolwich Arsenal and Portsmouth Dockyard. But care must be taken to place the nose of the tool towards the width of the shaving (presuming that this is taken from the matter to be removed, as it usually is,) for unless the edge is straight, and almost parallel with the face of the work as it leaves it, the face would be marked with a series of concave grooves of greater or less width, according to the feed given to the tool; and even when this is very slow, if the curved part of the edge were placed towards the face of the work it would present the appearance of corrugated iron, when examined under a magnifying lens. Willis further advances against the round nose that, as the shaving removed by it must be of a curvilinear section, it will oppose more force in rolling itself off the edge, than a flat shaving. This would be quite true if a flat shaving could be rolled up on itself like a piece of tape or ribbon; but I think the professor has overlooked the fact that when two cutting edges have one common upper face the shaving must be bent laterally as well as in its length; and I am disposed to think, from practical experiment, that there is very little difference on the point of the force required, and that when a point of large plan angle is just rounded off it stands better and cuts sweeter than when the point is not so rounded. But this only applies to heavy cuts, and for ordinary surfacing work nothing can act more perfectly than a point tool of wide plan-angle placed as described above. If a double-edged tool, with edges of 60°, be thus used in turning wrought iron or steel, and be well lubricated with clean water during the progress of the work, its face may be left with a burnished brilliancy that a touch of the finest emery would spoil. But, as the scheme of this paper is confined to the principles which determine the action of edges, and the rules by which those edges may be formed with certainty, it will be well to conclude these remarks with a few hints as to the construction of ordinary slide-rest tools. Bearing in mind that all double-edged tools belong to the graver class, it is well to form the side faces carefully in the first instance, and then never to alter these, but to keep the tool in order by grinding in the upper surface only, just as the graver is treated. Nasmyth's cone gauge, illustrated in Holtzapffel, vol. ii., p. 534, and also in "Baker's Mechanism," p. 236, affords a ready means of forming the side faces with accuracy. But the range and convenience of this gauge is much increased by dispensing with every part of the arrangement, except the cone itself. This can be made of any piece of stout metal bar turned truly, with the slide-rest set at an angle of 3°, and the base should be broad enough to stand steady by itself when squared off truly in the lathe, as in [Fig. 14]. Two marks can then be made upon it: one as at W, showing the exact height of the lathe-centre when the cone stands on the bed of the lathe, and another as at X, showing the height of the centre when the cone is placed on any given part of the slide-rest. Thus, in whatever direction the tool is to be used, its adjustment can be accurately made in the first instance on the slide-rest itself, and again tested after the tool is clamped down.
It is too common a practice in setting slide-rest tools to wedge up one end or the other, with regard only to the application of the edge on the central line. But this generally sacrifices the position of the lower faces, which is essential, to a consideration which has been shown to be only secondary. The best plan is to keep several strips, varying from 1/32 to ½ inch in thickness, but all about as long and wide as the shaft of the tool. These can be made of bar iron for the thicker strips, and sheet iron or tin for the thinner ones; and by using any two or three of these together, the tool can be packed up parallel to the bed of the slide-rest. The adjustment of the edge can thus be made with the greatest ease and certainty without altering the relative position of the lower faces.
It may be well to remark that in using the cone gauge, it is the lower faces of each edge which are to be tried against it, and not the front line of the point, as the inclination of this rule will vary slightly with variations in the plan-angle of the tool, although the slope of the faces remains the same. But the section angle is always to be estimated from the front line, whatever its slope may be.
Illustration No. 6.(15, 16)
When the principles which this paper has endeavoured to embody are once thoroughly understood, no handy workmen need ever be at a loss to form and apply his edges with the best effect under any shape the circumstances may require. The first point to be observed is the manner in which the work should be attacked—that is to say, whether the removal of the shaving or scraping requires the use of a single or double-edged tool. The next point is the position of the lower face or faces of the edges, so that they may be applied in the required direction, and in the position explained above. This involves the nature of the treatment best suited to the material, both as regards the kind of edge employed and the principle on which it should be applied—viz., cutting or scraping. In double-edged tools the position of the two lower faces determines that of the point, which is simply an accident resulting from the meeting of the cutting edges; but which, when so determined, affords a guide for the slope of the upper face. This must be so ground that it gives each edge the same degree of acuteness. Thus, in [Fig. 15], the point of the tool being at A, the slope must be made in the direction A, B; while, in [Fig. 16], the point being at C, the slope of the upper face must be in the direction C, D.
The writer is fully aware that those who expect to find "a rule of thumb" in this paper, will be miserably disappointed. But while he is conscious that the principles of which he has treated admit of a much fuller and yet more concise definition, he would remind the novice that there is "no royal road to learning," and that where practice of hand is wanting it can only be supplied by greater knowledge of principle. His object will therefore be fulfilled if this supplementary paper can supply any explanation or illustration of principle that may add to the practical utility of a work so exhaustive of its subject as "the Lathe and its Uses."
[Detached-cutter Holders.]
Where amateurs experience inconvenience in making their tools from the want of a forge, the use of detached cutters in a tool holder will be found of the greatest advantage for outside work. Even in plain turning there must always be some special forms for cutting into odd corners and deep grooves; but with a good tool holder and a grindstone, which is an indispensable piece of furniture in every metal turner's shop, the cumbrous array of slide-rest tools may be reduced to a few special forms and a very small box of cutters. These also possess another great advantage; for the spirit of the old adage quoted by Holtzapffel—
"He that would a good edge win