Part 25
Thus tools may be broadly divided into two classes--viz., single-edged and double-edged--remembering always that this distinction refers to the manner in which they should act, and not to the number of edges which it may be convenient to form on the same tool. In single-edged tools, whether there be one or many edges, each edge acts independently in removing its own shaving, and may therefore be formed separately. In this case a longitudinal section, showing the angle of the point, will give a true idea of that of the cutting edge. But, in the case of double-edged tools, as the two edges should co-operate in the removal of the same shaving, they must also be formed so that, while each lower face can occupy its proper position with regard to that surface of the work opposed to it, both edges shall possess the same degree of acuteness. In this case the two edges are formed by three planes--viz., two side faces and one upper surface common to both; and the angle of the point is now not only not that of the cutting edges, but has not even any fixed relation to them, for the cutting edges may vary some 25° or more on the very same longitudinal section of the point.
Prof. Willis has pointed out that in these tools the angles of the cutting edges depend on the _section_ and _plan_ angles of the point _conjointly_ (Fig. 8 is a _section_ view; Figs. 10, 11 and 12 are _plan_ views). From this it follows that cutting edges of exactly the same angle may be obtained by a great variety of combinations in the plan and section angles; and in note A, U, of Holtzapffel's work, vol. ii. p. 994, Prof. Willis has given a table, showing some of the different combinations by which cutting edges of certain angles may be produced with accuracy and simplicity. The following short table is arranged from this source and though much abbreviated will be found sufficient for all ordinary purposes.
The graver will again serve to illustrate the use of this table; for although only one edge is employed at the same time in hand turning, it belongs properly to the double edged class. This will be very apparent if a graver is held point upwards side by side with a point tool, and the dotted lines are added in Fig. 8, to make the similarity of form more evident. Now Holtzapffel has said of the graver when employed for its original purpose of engraving, that "no instrument works more perfectly," pointing out that, while both the edges are engaged in cutting the same shaving, both the lower faces of the edges are respectively inclined at the smallest possible angle from the sides of the V-shaped groove. Fig. 10 has already been used to illustrate the best position of the slide-rest tool, and if the illustration be turned round until the letters Q, R, read horizontally, and this line be taken to represent a flat surface with the graver acting upon it, it will be seen that the shaving is removed in exactly the same manner in both cases; the only difference being that the section of the shaving is triangular in one case and rectangular in the other. But so far as the tool is concerned the action is identical in each case, thus proving that every point tool may be so made and placed as to merit Holtzapffel's eulogium on the graver, and that this simple tool is in fact the type of all double-edged tools.
The graver being made from a square bar has of course a plan angle of 90°, and using it to illustrate the table, we will suppose that it is desired to give it two cutting edges of 60° each. Referring to 90° under the heading of "plan angle," 45° will be found on this line in the column over "cutting edge" 60°; denoting that the section angle of the point, _i.e._, the slope at which the graver is ground, must be 45° to give the desired edges. In the same way, if the section angle were 61° the cutting edges would be 70°. But taking the plan angle of 120°, the table shows that this would produce the same cutting edges of 60° with the larger section of 55°; and from this we have the important rule that, _in obtaining cutting edges of any given degree of acuteness the larger the plan angle is made, the larger also may be the section angle_. Thus pointed tools though constructed on the principle of the graver are an improvement on it in its simple form; for by making both plan and section angles as wide as possible, it is obvious that the strength and durability of the point will be much increased. It is, therefore, always better to give slide-rest tools a large plan angle, as in Fig. 12; and plan 120°, with section 55°, will be found a very useful and durable tool for surfacing purposes with wrought iron. When there are rectangular corners to cut in and out of, of course the plan angle cannot be more than 90°, and then it is well to sacrifice a little of the acuteness, as the section of 45° makes the point rather too weak. It is also worthy of remark as a mathematical fact, that unless the plan angle exceed 60°, it is impossible to obtain two cutting edges of that degree of acuteness; and in any case, such a plan angle would be radically bad, because it could not be used on the double-edged principle without _undercutting_ the shaving. It is somewhat remarkable, in connection with this point, that while Holtzapffel, vol. ii. p. 536, recommends prismatic cutters, he should add a footnote which indirectly but most conclusively corroborates Prof. Willis' condemnation of that particular form, by admitting that the proper degree of acuteness cannot be given to both edges.
As the practical result of a circular edge is to cut in two opposite directions,--the edge passing gradually from one to the other,--so round-nosed tools belong properly to the double-edged class, and are open to great objections unless carefully formed on this principle.
This is illustrated in Fig. 13, representing an oblique section of a round bar; and supposing the section to be made at an angle of 45°, it is obvious that the highest part of the edge at S will be exactly of this angle, while the lower point at T will be 135°, and the side points at U and V will be 90° each. Thus, between the point S and the points U and V on each side respectively, the edge will gradually pass through a range of 45°, consequently no two adjacent points on the same side will be of the same angle, and the highest point S may be too acute to stand while the lowest, U or V, is too blunt to cut.
Whenever, therefore, it is intended to round the nose of the tool, it should be first formed as a double-edged point tool, with a section angle agreeing as nearly as practicable with the intended degree of acuteness in the edge, so as to secure the highest points from being too weak, and the table given above will show what plan angle must be used, in combination with this section, to secure any part from being too blunt.
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 1/2 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.
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 Must forge thick and grind thin,"
may be carried out far more conveniently than in the case of whole tools which are generally filed into shape before tempering, and when worn down must go to the fire again and have the process repeated. But the detached cutter admits of being tempered evenly throughout its whole length and ground up afterwards as long as it lasts, without going to the forge again to the deterioration of the steel.
The patterns of tool-holders are innumerable, but very few are good for general service, because most of them are arranged so that the natural sides of the cutter are used for the face or faces of the edge. Thus either the plan angle of the point is limited to the angles presented by the transverse section of the cutter employed, or else the section angle is fixed by the position in which the cutter is clamped. Holtzapffel's arrangement is open to the first objection, Babbage's to the second. To obviate this inconvenience, Prof. Willis arranged a holder which clamps the cutter at an angle of 55° from the horizontal line. Thus no side can be used either for the lower or upper face of the edge, but any faces can be ground upon it; and the plan and section angle of the edge may be varied at pleasure within the whole range available for metal turning. Prof. Willis's holder for the cutter is almost a facsimile of his admirable tool-holder for the slide-rest, than which none is more convenient or can act more perfectly. But the arrangement is a little complicated for a cutter holder, and must be very carefully made with the knowledge of certain laws, if it is to insure a perfect grip of the cutter. It is also designed for the use of sound wire cutters which require filing flat on one side.
Adopting Willis's inclination for the cutter I have found that all its advantages may be secured with a simpler form of holder and common square for steel for the cutter. The holder is simply the modification of an old pattern to suit the inclination of 55°, and the sketch needs little explanation beyond saying that the nick in the solid part should be rather less than a square angle, and made perfectly true all the way down, or, if anything, rather hollowed in the middle, so as to insure the greatest amount of pressure at the top and bottom, as otherwise the cutter might not sit quite true and firm. The angle at the end of the strap against which the cutter bears should be rather _more_ than square, both to allow for any want of exact truth in the squaring of the cutter and to avoid the wedging action which would be set up on tightening the screw if this angle were _less_ than square, as this could of course create a risk of splitting the strap. The end of the screw and the cup in which it fits should be round, as this allows of a little play and insures a truer grip in the strap than a pointed screw working into a conical hole. A perspective sketch of a detached cutter is added, with dotted lines to show how exactly the arrangement of the faces can be accommodated to the positions which have been shown to be the best in solid tools for the slide-rest.
D. HAYDON.
NEW FORM OF ROSE ENGINE BY E. TAYLOR.
Seeing that the Editor of the above Articles has illustrated and described Holtzapffel and Co.'s Rose Cutter and two methods of executing rose cutting, the latter being the ordinary rose engine, I am induced to send you a description of a method that I have adopted whereby I can with considerable despatch execute this description of turning.
I will first preface my description by saying some thirty years ago I purchased Ibbetson's Book on eccentric turning, and I was so much taken with it and the illustrations, that I determined to make myself in accordance with his description and engravings an eccentric chuck; and although I was a long time about it, being at the time much otherwise engaged, I succeeded beyond my expectations, and was enabled to do some very fine work with it; and I have never regretted the time I spent over the chuck, as I became familiar with metal turning and screw cutting _flying_ in the lathe, which latter I was surprised to find how easily I could execute. However, I was much disappointed in the usefulness of the chuck (Holtzapffel's eccentric cutter, which I purchased is far more useful), and also with the tediousness of using it (fancy stopping the lathe to alter the chuck 360 times or 180 times to cut a row of circles either distinct or overlaying each other), and there was also a certain vibration occasioned in using the chuck which I also disliked. I therefore determined to cut up some rosettes and convert my headstock into a rose engine, to effect which object I got Holtzapffel and Co. to return up with a new steel collar and make my mandrel traversing. I cut myself a rosette both ways with 16 waves, and I was much pleased with the variety of work I could perform with this one, but the rosette took me a long time to make, and disheartened me from cutting up a variety. It, however, occurred to me that if I added an extra mandrel by the side of and attached to my headstock, and on which extra mandrel, if I had an eccentric chuck connected with a rod to the wall of my room, I could get my headstock to oscillate, and by connecting and multiplying wheels cause as many waves on each revolution of my principal mandrel I pleased; this after much time and patience I succeeded in doing, and worked it with the hand motion often adopted for rose work. After between two and three years, I put the extra mandrel over my principal mandrel instead of by the side as before, to enable me to dispense with the hand motion and to work the upper mandrel with the slow motion on my lathe wheel, and which I found a very great improvement, and I now give the details of the plan I have adopted for the benefit of your numerous readers.
The drawings are to a two-inch scale, or one sixth of their full size.
Fig. 400 is a side view of my headstock (part in section) with the upper mandrel, A, added, showing the connection by an intermediate spindle, B, with the large cog wheel, C, on my lower mandrel, D, and other additions.
The back centre, E, of my headstock is connected with the back screw, F, and drawn out or pushed in with it, and is fixed by the set screw, G. When drawn out the steel screw, H, at the end of the mandrel, D, removes to receive the screw guides which are then fastened by it, and the piece, I, with segments of a thread to match the guides, is slid up by a wedge to the guides and then fastened by the screw J, I, can also fix some roses cut on the side, and other apparatus with this screw H.[27]
[27] It is not a good plan to make the point, E, movable. It would be better to slip the guides or rosettes over it: and generally to arrange this part as usual with a traversing mandrel, P, H.
The large cog wheel, C, is screwed up with the screw, K, to the mandrel pulley, L. On the front of the pulley is the division plate as usual.
The intermediate adjustable spindle, B, is carried in a frame shown separately by Fig. 405; it is allowed to rise or fall as may be required for the wheel, M, to gear with the great wheel, C; provision being also made for an intermediate wheel, N, (see Fig. 403) to connect the wheel, O, with the wheel, P, on the upper mandrel when required.
The eccentric chuck is fixed on screw Q of the upper mandrel.
Fig. 401 is a plan of the mould for the back cast-iron upright, fixed to the headstock with screws at the foot, showing the circular grooves 1, 2 and 3, necessary for the spindles for the connecting wheels; the centre hole, 4, is for the gun-metal collar, or the upper mandrel.
Fig. 402, is a plan of the mould for the front cast-iron upright; the centre holes 1 and 2 are for the collars of the mandrels. No. 2 is made to just fit over the steel collar of the lower mandrel, and is fixed to the headstock by a brass rose and three screws; it is also fixed at the foot with two screws to the headstock.