The "roughed" tool is then used to gradually improve the fineness of grinding of the gla.s.s. For this purpose grinding by hand is resorted to, the tool and lens being supplied continually with finer and finer emery. Fig. 52 gives an idea of the way in which the tool is moved across the gla.s.s surface. Very little pressure is required. The tool is carried in small circular sweeps round and round the lens, so that the centre of the tool describes a many-looped curve on the lens surface. The tool must be allowed to rotate about its own axis; and the lens and pedestal must also be rotated from time to time.
Every few minutes the circular strokes are interrupted, and simple, straight, transverse strokes taken. In no case (except to correct a, defect, as will be explained) should the tool overhang the lens surface by more than about one quarter the diameter of the latter.
After grinding say for an hour with one size of emery fed in by means of a clean stick say every five minutes, the emery is washed off, and everything carefully cleaned. The process is then repeated with finer emery, and so on.
The different grades of emery are prepared by taking advantage of the fact that the smaller the particles the longer do they remain suspended in water. Some emery mud from a "roughing" operation is stirred up with plenty of water and left a few seconds to settle, the liquor is then decanted to a second jug and left say for double the time, say ten seconds; it is decanted again, and so on till four or five grades of emery have been acc.u.mulated, each jug containing finer emery than its predecessor in the process.
It is not much use using emery which takes more than half an hour to settle in an ordinary bedroom jug. What remains in the liquid to be decanted is mostly gla.s.s mud and not emery at all. The process of fine grinding is continually checked by the spherometer, and the art consists in knowing how to move the grinding tool so as to make the lens surface more or less curved. In general it may be said that if the tool is moved in small sweeps, and not allowed to overhang much, the Centre of the lens will be more abraded, while if bold free strokes are taken with much overhanging, the edges of the lens will be more ground away.
By the exercise of patience and perseverance any one will succeed in gradually fine grinding the lens surface and keeping it to the spherometer, but the skill comes in doing this rapidly by varying the shape of the strokes before any appreciable alteration of curvature has come about.
Polishing.
The most simple way of polishing is to coat the grinding tool with paper, as will be described, and then to brush some rouge into the paper. The polisher is moved over the work in much the same way as the fine grinding tool, until the gla.s.s is polished. Many operators prefer to use a tool made by squeezing a disc of slate, armed with squares of warm pitch, against the lens surface (finely ground), and then covering these squares with rouge and water instead of emery and water as in the fine grinding process.
The final process is called "figuring." It will in general be unnecessary with a small lens. With large lenses or mirrors the final touches have to be given after the optical behaviour of the lens or mirror has been tested with the telescope itself, and this process is called "figuring." A book might easily be written on the optical indications of various imperfections in a mirror or lens. Suffice it to say here that a sufficiently skilled person will be able to decide from an observation of the behaviour of a telescope whether a lens will be improved by altering the curvature of one or all of the surfaces.
A very small alteration will make a large difference in the optical properties, so that in general "figuring" is done merely by using the rouge polishing tool as an abrading tool, and causing it to alter the curves in the manner already suggested for grinding. There are other methods based on knocking squares out of the pitch-polisher so that some parts of the gla.s.s may be more abraded than others.
The "figuring" and polishing may be done by hand just like the grinding. There are machines, however, which can be made to execute the proper motions, and a polisher is set in such a machine, and the mechanical work done is by no means inconsiderable. In fact for surfaces above six inches in diameter few people are strong enough to work a polisher by hand owing to the intense adhesion between it and the exactly fitting gla.s.s surface.
Such is a general outline of the processes required to produce a lens or mirror. These processes will now be dealt with in much greater detail, and a certain amount of repet.i.tion of the above will unfortunately be necessary: the reader is asked to pardon this. It will also be advisable for the reader to begin by reading the whole account before he commences any particular operation. The reason for this is that it has been desirable to keep to the main account as far as possible without inserting special instructions for subsidiary operations, however important they may be; consequently it may not always be quite clear how the steps described are to be performed. It will be found, however, that all necessary information is really given, though perhaps not always exactly in the place the reader might at first expect.
-- 54. All the discs that I have seen, come from the makers already roughly ground on the edges to a circular figure--but occasionally the figure is very rough indeed--and in some cases, especially if small lenses have to be made, it is convenient to begin by cutting the gla.s.s discs out of gla.s.s sheet, which also may be purchased of suit-able gla.s.s. To do this, the simplest way is to begin by cutting squares and then cutting off the corners with the diamond, the approximate circular figure being obtained by grinding the edges on an ordinary grindstone.
If the pieces are larger, time and material may be saved by using a diamond compa.s.s, i.e. an ordinary drawing compa.s.s armed with a diamond to cut circles on the gla.s.s, and breaking the superfluous gla.s.s away by means of a pair of spectacle-maker"s shanks (Fig. 44), or what does equally well, a pair of pliers with soft iron jaws. With these instruments gla.s.s can be chipped gradually up to any line, whether diamond-cut or not, the jaws of the pincers being worked against the edge of the gla.s.s, so as to gradually crush it away.
Fig. 44.
a.s.suming that the gla.s.s has been bought or made roughly circular, it must be finished on the lathe. For this purpose it is necessary to chuck it on an iron or hardwood chuck, as shown in Fig. 46. For a lens below say an inch in diameter, the centering cement may be used; but for a lens of a diameter greater than this, sufficient adhesion is easily obtained with Regnault"s mastic, and its low melting point gives it a decided advantage over the sh.e.l.lac composition.
The gla.s.s may be heated gradually by placing it on the water bath, or actually in the water, and gradually bringing the water up to the boiling-point. The gla.s.s, being taken out, is rapidly wiped, and rubbed with a bit of waste moistened, not wet, with a little turpentine: its surface is then rubbed with a stick of mastic previously warmed so as to melt easily. The surface of the chuck being also warm, and covered with a layer of melted cement, it is applied to the gla.s.s. The lathe is turned slowly by hand, and the gla.s.s pushed gradually into the most central position; it is then pressed tight against the chuck by the back rest, a bit of wood being interposed for obvious reasons.
When all is cold the turning may be proceeded with. The quickest way is to use the method already described (i.e. actual turning by a file tool); but if the student prefers (time being no object), he may accomplish the reduction to a circular form very easily by grinding.
Fig. 45.
Fig. 46.
For this purpose he will require to make the following arrangements (Fig. 45). If the lathe has a slide rest, a piece of stout iron may be bent and cut so as to fit the tool rest, and project beneath the gla.s.s. The iron must be fairly rigid, for if it springs appreciably beneath the pressure of the gla.s.s, it will not grind the latter really round. The lathe may run rather faster than for turning cast iron of the same size. Coa.r.s.e emery, pa.s.sing through a sieve of 80 threads to the inch (run), may be fed in between the gla.s.s and iron, and the latter screwed up till the disc just grinds slightly as it goes round.
A beginner will generally (in this as in all cases of grinding processes) tend to feed too fast--no grinding process can be hurried.
If a slide rest is not available, a hinged board, carrying a bit of iron, may (see Fig. 45) be arranged so as to turn about its hinge at the back of the lathe; and it may be screwed up readily enough by pa.s.sing a long set-screw through the front edge, so that the point of the screw bears upon the lathe bed. I may add that emery behaves as if it were greasy, and it is difficult to wet it with clean water.
This is easily got over by adding a little soap or alcohol to the water, or exercising a little patience.
A good supply of emery and water should be kept between the disc and the iron; a little putty may be arranged round the point of contact on the iron to form a temporary trough. In any case the resulting emery mud should on no account be thrown away, but should be carefully kept for further use. The process is complete when the gla.s.s is perfectly round and of the required diameter as tested by callipers.
-- 55. The next step is to rough out the lens, and this may easily be done by rotating it more slowly, i.e. with a surface speed of ten feet per minute, and turning the gla.s.s with a hard file, as explained in -- 42. If it is desired to employ the slide rest, it is quicker and better to use a diamond tool--an instrument quite readily made, and of great service for turning emery wheels and the like,--a thing, in fact, which no workshop should be without. A bit of diamond bort, or even a clear though off-colour stone, may be employed.
An ordinary lathe tool is prepared by drawing down the tool steel to a long cone, resembling the ordinary practice in preparing a boring tool. The apex of the cone must be cut off till it is only slightly larger than the greatest transverse diameter of the diamond splinter.
The latter may have almost any shape--a triangular point, one side of a three-sided prism is very convenient. A hole is drilled in the steel (which must have been well softened), only just large enough to allow the diamond to enter--if the splinter is thicker in the middle than at either end, so much the better--the diamond is fastened in position by squeezing the soft steel walls tightly down upon it.
Personally I prefer to use a tool holder, and in this case generally mount the diamond in a bit of bra.s.s rod of the proper diameter; and instead of pinching in the sides of the cavity, I tin them, and set the diamond in position with a drop of soft solder.
Fig. 47.
In purchasing diamond bort, a good plan is to buy fragments that have been employed in diamond drilling, and have become too small to reset; in this case some idea as to the hardness of the bits may be obtained.
Full details as to diamond tool-making are given in books on watch-making, and in Holtzapffell"s great work on Mechanical Manipulation; but the above notes are all that are really necessary--it is, in fact, a very simple matter. The only advantage of using a diamond tool for gla.s.s turning is that one does not need to be always taking it out of the rest to sharpen it, which generally happens with hard steel, especially if the work is turned a little too fast.
I recommend, therefore, that the student should boldly go to work "free hand" with a hard file; but if he prefer the more formal method, or distrust his skill (which he should not do), then let him use a diamond point, even if he has the trouble of making it. When using a diamond it is not necessary to employ a lubricant, but there is some advantage in doing so.
The surface of the lens can be roughly shaped by turning to a template or pattern made by cutting a circular arc (of the same radius as the required surface) out of a bit of sheet zinc. Another very handy way of making templates of great accuracy is to use a beam compa.s.s (constructed from a light wooden bar) with a glazier"s diamond instead of a pencil. A bit of thin sheet gla.s.s is cut across with this compa.s.s to the proper curvature--which can be done with considerable accuracy and the two halves of the plate, after breaking along the cut, are ground together with a view to avoiding slight local irregularities, by means of a little fine emery and water laid between the edges. In this process the gla.s.s is conveniently supported on a clean board or slate, and the bits are rubbed backwards and forwards against each other.
-- 56. It is not very easy for a beginner to turn a bit of anything--iron, wood, or gla.s.s--with great accuracy to fit a template, and consequently time may be saved by the following procedure, applied as soon as the figure of the template is roughly obtained. A disc of lead or iron, of the same diameter as the gla.s.s, and of approximately the proper curvature, is prepared by turning, and is armed with a handle projecting coaxially from the back of the disc. The gla.s.s revolving with moderate speed on the lathe, the lead tool, supplied with coa.r.s.e emery and water, is held against it, care being taken to rotate the tool by the handle, and also to move it backwards and forwards across the disc, through a distance, say, up to half an inch; if it is allowed to overhang too much the edges of the gla.s.s disc will be overground. By the use of such a tool the gla.s.s can readily be brought up to the template.
The only thing that remains, so far as the description of this part of the process goes, is to give a note or two as to the best way of making the lead tools, and for this purpose the main narrative of processes must be interrupted. The easiest way is to make a set of discs to begin with. For this purpose take the mandrel out of the lathe, and place it nose downwards in the centre of an iron ring of proper diameter on a flat and level iron plate.
The discs are made by pouring lead round the screw-nose of the mandrel. This method, of course, leaves them with a hole in the centre; but this can be stopped up by placing the hot disc (from which the mandrel has been unscrewed) on a hot plate, and pouring in a sufficiency of very hot lead; or, better still, the mandrel can be supported vertically at any desired distance above the plate while the casting is being poured. Lead discs prepared in this way are easily turned so as to form very convenient chucks for bra.s.s work, and for use in the case now being treated, they are easily turned to a template, using woodturners" tools, which work better if oiled, and must be set to cut, not sc.r.a.pe.
If the operator does not mind the trouble of cutting a screw, or if he has a jaw chuck, the lead may be replaced by iron with some advantage.
The following is a neat way of making concave tools. It is an application of the principle of having the cutting tool as long as the radius of curvature, and allowing it to move about the centre of curvature. Place the disc of iron or lead on the lathe mandrel or in the chuck, and set the slide rest so that it is free to slide up or down the lathe bed. Take a bar of tool steel and cut it a little longer than the radius of curvature required. Forge and finish one end of the bar into a pointed turning tool of the ordinary kind.
Measure the radius of curvature from the point of the tool along the bar, and bore a hole, whose centre is at this point, through the bar from the upper to the lower face. I regard the upper face as the one whose horizontal plane contains the cutting point when the tool is in use. Clamp a temporary back centre to the lathe bed, and let it carry a pin in the vertical plane through the lathe centres, and let this pin exactly fit the hole in the bar.
Fig. 48.
Place the "radius" tool in position for cutting, and let it be lightly held in the slide rest nearly at the cutting point, the centre of rotation of the pedestal (or its equivalent) pa.s.sing through the central line of the bar. Then adjust the temporary back rest, so that the tool will take a cut. In the sketch the tool is shown swinging about the back centre instead of about a pin--there is little to choose between the methods unless economy of tool steel is an object.
The tool must now be fed across the work. The pedestal must of course be free to rotate, and the slide rest to slip up and down the bed. In this way a better concave grinding tool can be made than would be made by a beginner by turning to a template--though an expert turner would probably carry out the latter operation so as to obtain an" accuracy of the same order, and would certainly do it in much less time than would be required in setting up the special arrangements here described.
On the other hand, if several surfaces have to be prepared, as in the making of an achromatic lens, the quickest way would be by the use of the radius tool, bored of course to work at the several radii required. I have tried both methods, and my choice would depend partly on the lathe at my disposal, and partly on the number of grinding tools that had to be prepared.
Having obtained a concave tool of any given radius, it is easily copied--negatively, so as to make a convex tool in the following manner. Adjust the concave tool already made on the back rest, so that if it rotated about the line of centres, it would rotate about its axis of figure.
Arrangements for this can easily be made, but of course they will depend on the detailed structure of the lathe. Use the slide rest as before, i.e. let it grasp an ordinary turning tool lightly, the pedestal being fixed, but the rest free to slide up or down the lathe bed. Push the back rest up till the b.u.t.t of the turning tool (ground to a rounded point) rests against the concave grinding tool. If the diameter of the convex tool required be very small compared with the radius of curvature of the surface (the most usual case), it is only necessary to feed the cutting tool across to "copy" the concave surface sufficiently nearly.
Fig. 49.
There seems no reason, however, why these methods should not be applied at once to the gla.s.s disc by means of a diamond point, and the rough grinding thus entirely avoided. I am informed that this has been done by Sir Henry Bessemer, but that the method was found to present no great advantage in practice. A reader with a taste for mechanical experimenting might try radius bar tools with small carborundum wheels rapidly driven instead of a diamond.
Enough has now been said to enable any one to prepare rough convex or concave grinding tools of iron or lead, and of the same diameter as the gla.s.s to be ground.
The general effect of the process of roughing the rotating lens surface is to alter the radius of curvature of both tool and gla.s.s; hence it is necessary to have for each grinding tool another to fit it, and enable it to be kept (by working the two together) at a constant figure. After a little practice it will be found possible to bring the gla.s.s exactly up to the required curvature as tested by template or spherometer. The art of the process consists in altering the shape of the grinding tool so as to take off the gla.s.s where required, as described in -- 53, and from this point of view lead has some advantages; (opinions vary as to the relative advantages of lead and iron tools for this purpose, however). The subsidiary grinding tool is not actually needed for this preliminary operation, but it has to be made some time with a view to further procedure, and occasionally is of service here.
-- 57. "The gla.s.s disc must be ground approximately to the proper curvature on each side before any fine grinding is commenced. It is precisely for this purpose that the previous turning of the disc is recommended, for it is easy to unmount and recentre a round object, but not so easy if the object have an indefinite shape. Using a cement which is plastic before it sets, the disc may be easily taken off the chuck and centred by a little handicraft, i.e. by rotating the lathe slowly and pushing the disc into such a position that it rotates about its axis. The grinding of the second surface is accomplished exactly as in the former case; of course on reversing the gla.s.s the chuck has to be slightly turned up to fit the convex or concave surface.
-- 58. There is, however, one point of interest and importance--attention to which will save a good deal of useless labour afterwards.
The gla.s.s must be ground in such a manner that the thickness at the edge is the same all round. In other words, the axes of figure of the two surfaces must coincide. This will be the case if the recentering has been accurately performed, and therefore no pains should be spared to see that it is exactly carried out. Any simple form of vernier gauge (such as Brown and Sharpe"s vernier callipers) will serve to allow of a sufficiently accurate measurement of the edge thickness of the lens. If any difference of thickness is observed as the gauge moves round the edge, one or other of the surfaces must be reground.
Of course the lat.i.tude of error which may be permitted depends so much on the final arrangements for a special finishing process called the "centering of the lens"--which will be described--that it is difficult to fix a limit, but perhaps one-thousandth of an inch may be mentioned as a suitable amount for a 2-inch disc. For rough work, of course; more margin may be admitted.
-- 59. In a large shop I imagine that lenses of only two inches diameter would be ground in nests; or, in other words, a number would be worked at a time, and centering, even of a rough kind, would be left to the last; but this process will be treated hereafter. At present I shall a.s.sume that only one lens will be made at a time.