On Laboratory Arts Part 6

My reason for describing the file method at such length is to be found in the fact that a Morse drill requires to be sharpened after drilling gla.s.s before it can be used in the ordinary way, and this is often a difficulty.

I ought to say that I have never succeeded in boring the barrel of a gla.s.s tap by either of these methods. [Footnote: I have been lately informed that it is usual to employ a splinter of diamond set in a steel wire holder both for tap boring and for drilling earthenware for riveting. The diamond must, of course, be set so as to give sufficient clearance for the wire holder.

For methods of using and setting diamond tools see -- 55. It will suffice to say here that a steel wire is softened and filed at one end so as to form a fork; into this the diamond is set by squeezing with pliers. The diamond is arranged so as to present a point in the axis of the wire, and must not project on one side of the wire more than on the other. It is not always easy to get a fragment satisfying these conditions, and at the same time suitable for mounting. A drop of solder occasionally a.s.sists the process of setting the diamond.

In drilling, the diamond must be held against the work by a constant force, applied either by means of weight or a spring. I made many trials by this method, using a watchmaker's lathe and pressing up the work by a weight and string, which pa.s.sed over a pulley. I used about 40 ounces, and drilled a hole 3/32 in diameter in flint gla.s.s at a speed of 900 revolutions per minute to a depth of one-eighth of an inch in eight minutes. I used soap and water as a lubricant, and the work was satisfactory.

Since this was set up, I have been informed by Mr. Hicks of Hatton Garden that it is necessary to anneal gla.s.s rod by heating it up to the softening point and allowing it to cool very slowly under red-hot sand or asbestos before boring. If this be done, no trouble will be experienced. The annealing must be perfect.]

-- 39. For boring large holes through thick gla.s.s sheets, or, indeed, through anything where it is necessary to make sure that no accident can happen, or where great precision of position and form of hole is required, I find a boring tube mounted as shown in the picture (Fig. 36) is of great service. Bra.s.s or iron tube borers do perfectly well, and the end of the spindle may be provided once for all with a small tube chuck, or the tubes may be separately mounted as shown. A fairly high speed is desirable, and may be obtained either by foot, or, if power is available, is readily got by connecting to the speed cone of a lathe, which is presumably permanently belted to the motor.

Fig. 36.

After trying tubes armed with diamond dust, as will be presently explained, I find that emery and thin oil or turpentine, if liberally supplied below the gla.s.s, will do very nearly as well. The tube should be allowed to rise from the work every few seconds, so as to allow of fresh emery and oil being carried into the circular grooves.

This is done by lifting the hinged upper bearing, the drill being lifted by a spiral spring between the pulley and the lower bearing shown at B. The gla.s.s may be conveniently supported on a few sheets of paper if flat, or held firm in position by wooden clamps if of any other shape. In any case it should be firmly held down and should be well supported. Any desired pressure upon the drill is obtained by weighting the hinged board A.

-- 40. The following method was shown to me by Mr. Wimshurst, but I have not had occasion to employ it myself. It is suitable for boring large holes through such gla.s.s as the plates of Mr. Wimshurst's Influence machines are usually made of. A diamond is mounted as the "pencil" of a compa.s.s, and with this a circle is drawn on the gla.s.s in the desired position. The other leg of the compa.s.s of course rests on a suitable washer.

To the best of my recollection the further procedure was as follows.

A piece of steel rod about one-eighth inch in diameter was ground off flat and mounted in a vice vertically, so as to cause its plane end to form a small horizontal anvil. The centre (approximately) of the diamond-cut circle of the gla.s.s was laid on this anvil so as to rest evenly upon it, and the upper surface (i.e. that containing the cut) was then struck smartly with a hammer, completely pulverising the gla.s.s above the anvil. The hole was gradually extended in a similar manner right up to the diamond cut, from which, of course, the gla.s.s broke away.

A similar method has long been known to glaziers, differing from the preceding in that a series of diamond cuts are run across the circle parallel to two mutually perpendicular diameters. A smart tap on the back of the scored disc will generally cause the fragments to tumble out. I have never tried this myself, but I have seen it done.

Large discs may easily be cut from sheet gla.s.s by drawing a circular diamond cut, and gradually breaking away the outer parts by the aid of additional cuts and a pair of pliers or "shanks" (see Fig. 44).

-- 41. Operations depending on Grinding: Ground-in Joints.

The process will be perfectly understood by reference to a simple case. Suppose it is desired to grind the end of a tube into the neck of a bottle. If a stoppered bottle is available, the stopper must be taken out and measured as to its diameter at the top and bottom.

Select a bit of tube as nearly as possible of the same diameter as the stopper at its thickest part. Draw down the gla.s.s in the blow-pipe flame rather by allowing it to sink than by pulling it out. After a few trials no difficulty will be experienced in making its taper nearly equal to that of the stopper, though there will in all probability be several ridges and inequalities. When this stage is reached anneal the work carefully and see that the gla.s.s is not too thin. Afterwards use emery and water, and grind the stopper into the bottle.

There are six special directions to be note

(1 )Turn the stopper through at least one revolution in each direction.

(2) Lift it out often so as to give the fresh emery a chance of getting into the joint.

(3) Rotate the bottle as well as the stopper in case there is any irregularity in the force brought to bear, which might cause one side of the neck to be more ground than another, or would cause the tube to set rather to one side or the other.

(4) Use emery pa.s.sing a 50 sieve, i.e. a sieve with fifty threads to the inch run (see -- 144) to begin with, and when the stopper nearly fits, wash this thoroughly away, and finish with flour emery, previously washed to get rid of particles of excessive size; the process of was.h.i.+ng will be fully discussed in the chapter on gla.s.s-grinding, which see.

(5) Any degree of fineness of surface may be obtained by using graded emery, as will be explained, but, in general, it is unnecessary to attempt a finer surface than can be got with washed flour emery. A superficial and imperfect polish may be given by grinding for a short time with powdered pumice stone.

(6) If the proper taper is not attained by blowing, or if ridges are left on the tapered part, the process may be both hastened and improved by giving the taper a preliminary filing with a three-cornered file and kerosene, just as one would proceed with iron or bra.s.s. A little filing will often save a good deal of grinding and make a better job.

If a bottle without a tapered neck is to be employed, it is as well to do the preliminary grinding by means of a cone turned up from a bit of cast iron. This is put in the lathe and pushed into the mouth of the bottle, the latter being supported by the hands. Use about the same surface speed as would be employed for turning cast iron. In this case the emery is better used with kerosene.

If a cylindrical bit of cast iron about an inch in diameter is turned down conically nearly to a point, it will save a good deal of trouble in making separate cones. If it gets ground into rings, and it becomes necessary to turn it up, use a diamond tool until the skin is thoroughly removed; the embedded emery merely grinds the edge off any ordinary steel tool.

For diamond tools see -- 55.

-- 42. Use of the Lathe in Gla.s.s-working.

If it is necessary to remove a good deal of gla.s.s, time may be saved by actually turning the gla.s.s in a lathe. According to the direction given above for grinding a tube into the neck of a bottle, very little gla.s.s need be removed if the drawing down is well done, so that for this purpose turning is often unnecessary.

If the taper of the stopper be small and it is permissible to use a thick tube, or if a solid stopper only has to be provided, or an old stopper quickly altered to a new form, turning is very useful. The gla.s.s may be "chucked" in any suitable manner, and run at a speed not exceeding 10 feet per minute. Prepare a three-cornered file by mercury-hardening and by grinding the end flat so as to form a cutting angle of about 80, and use a moderate amount of kerosene lubrication, i.e. enough to keep the gla.s.s damp, but even this is not essential.

Use the file as an ordinary bra.s.s turning tool, and press much more lightly than for metal turning. The gla.s.s will be found to sc.r.a.pe off quite pleasantly.

By chucking gla.s.s tubes on wooden mandrells the ends may be nicely turned in this manner ready for accurate closing by gla.s.s plates.

The process of grinding also is made much more rapid--at all events in the earlier stages--by chucking either the stopper or the bottle and holding the other member in the fingers, or in a wooden vice held in the hands. The finis.h.i.+ng touches are best given by hand.

I ought to say that I think a good deal of gla.s.s-grinding, as practised in laboratories, might be advantageously replaced by gla.s.s turning or filing and certainly will be by any one who will give these methods a trial.

If one tube is to be ground into another, as in grinding a retort into a receiver, the latter must be drawn down from a larger piece, few beginners being able to widen a tube by the method explained with sufficient ease and certainty. The other operations are similar to the operations above described.

-- 43. Funnels often require to be ground to an angle of 60. For this purpose it is well to keep a cast-iron cone, tapering from nothing up to four inches in diameter. This may be mounted on a lathe, and will be found of great use for grinding out the inside of funnels. Care must be taken to work the funnel backwards and forwards, or it will tend to grind so as to form rings, which interfere with filtering. A rough polish may be given on the lines explained in the next section.

-- 44. A rough polish may be easily given to a surface which has been finished by washed flour emery, in the following manner. Turn up a disc of soft wood on the lathe, and run it at the highest wood-turning speed. Rub into the periphery a paste of sifted powdered pumice stone and water.

Any fairly smooth ground gla.s.s surface may be more or less polished by holding it for a moment against the revolving disc. Exact means of polis.h.i.+ng will be described later on. Meanwhile this simple method will be found both quick and convenient, and is often quite sufficient where transparency, rather than figure, is required. I daresay a fine polish may be got on the same lines, using putty powder or washed rouge (not jewellers' rouge, which is too soft, but gla.s.s-polishers'

rouge) to follow the pumice powder, but I have not required to try this.

-- 45. It is sometimes required to give to ground gla.s.s surfaces a temporary transparency. This is to be done by using a film of oil of the same refractive index as the gla.s.s. Cornu has employed a varnish consisting of a mixture of turpentine and oil of cloves, but the yellow-brown colour of the latter is often a disadvantage. It will be found that a mixture of nut oil and oil of bitter almonds, or of bromo-napthalene and acetone, can be made of only a faint yellow colour; and by exact adjustment of the proportions will have the same refractive index for any ray as crown gla.s.s (ordinary window gla.s.s).

Procure a sample of the gla.s.s and smash it up to small fragments in an iron mortar. Sift out the fine dust and the larger pieces; bits about as large as small beads--say one-sixteenth inch every way--do very well. Boil the sifted gla.s.s with strong commercial hydrochloric acid to remove iron, wash with distilled water and a few drops of alcohol, dry on blotting paper in the sun or otherwise. Put the dry gla.s.s into a bottle or beaker, and begin by adding almond oil (or bromo-napthalene), then add nut oil (or acetone) till the gla.s.s practically disappears when examined by sodium light, or light of any other wave-length, as may be required.

The adjustment of the mixture is a matter of great delicacy, one drop too much of either const.i.tuent, in, say, 50 cubic centimetres, makes all the difference. The final adjustment is best accomplished by having two mixtures of the oils, one just too rich in almond, the other in nut oil; by adding one or other of these, the required mixture is soon obtained.

It is to be noted

(1) That adjustment is only perfect for light of one wave-length.

(2) That adjustment is only perfect at one temperature.

On examining a bottle of rather larger fragments of gla.s.s immersed in an adjusted mixture by ordinary daylight, a peculiarly beautiful play of colours is seen.

Of course, if it is only desired to make ground gla.s.s fairly transparent, these precautions are unnecessary, but it seemed better to dispose of the matter once for all in this connection.

M. Cornu's object was to make a varnish which would prevent reflection from the back of a photographic plate on to the film. I have had occasion to require to do the same when using a scale made by cutting lines through a film of black varnish on a slip of gla.s.s. This succeeded perfectly by making the varnish out of Canada balsam stained with a black aniline dye.

Mr. Russell, Government Astronomer of New South Wales, finds that the "halation" of star photographs can be prevented by pouring over the back of the plate a film of collodion suitably stained.

-- 46. Making Ground Gla.s.s.

This is easily done by rubbing the surface of polished gla.s.s with a bit of cast iron and washed "flour of emery." Of course, if the fineness of grain of the surface is of importance, appropriate sizes of emery must be employed. The iron may be replaced by a bit of gla.s.s cut with transverse grooves to allow the emery to distribute itself, or even by a bit of gla.s.s without such grooves, provided it does not measure more than one or two inches each way. If great speed is an object rather than the fineness of the surface, use a bit of lead and coa.r.s.e emery, say any that will pa.s.s a sieve with fifty threads to the inch.

It may perhaps be mentioned here that it is a pity to throw away emery which has been used between gla.s.s and gla.s.s. In the chapter dealing with fine optical work the use of emery of various grades of fineness will be treated, and the finer grades can only be obtained (to my knowledge) from emery which has been crushed in the process of gla.s.s or metal grinding, especially the former. A large jam-pot covered with a cardboard lid does well as a receptacle of was.h.i.+ngs.

-- 47. Gla.s.s-cutting.