Chapter XXVII: VICE WORK--(Continued) (3)

If we have a straight-edge that is known to be true, we may lay it on the face of a plate and move it laterally from each end alternately, and if it swings from the centre the plate face is rounding, while if it shuffles across moving first at one end and then at the other the face is hollow; but if it glides as it were across, the surface is nearer true. The straight-edge must not be pressed to the plate, but merely touched laterally to make it move laterally, for if we take a true straight-edge and press it vertically to a true surface while moving it, it will show the marks of contact the most plainly immediately beneath the parts where it is pressed. Selecting by this means the two plates that appear to be the most true we proceed to test them further as follows: We give to one of them which we will call No. 1 a light coat of red marking, and placing it upon the other or No. 2, we move it about in all directions and then take the two apart to examine the bearing marks. Suppose then that No. 1 shows the bearing marks to be at the shaded places, A and B, in Fig. 2441, while the bearing marks on No. 2 are as at the shaded parts A and B in Fig. 2442, the two ends A having been placed together; then we know that B is a high spot on No. 1, and A a high spot on No. 1 for the following reasons. The marks at A, No. 2, have been made by the marking at A on No. 1, and will extend across No. 2, a distance depending upon how much No. 1 has moved across No. 2, for if corner A of No. 1 had only moved half-way across No. 2, it could only have marked half-way across it. Similarly spot B on No. 1 has marked spot B in No. 2, because it has been moved all the way across, it being evident that the marking on B, No. 1, can only mark plate 2 as far across its width as it is moved across it. From this it follows that the higher or more prominent a spot is the less will be the area of the bearing mark at that spot.

Now suppose that the two plates were curved to an equal degree as in Fig. 2443, and the bearing marks would extend all over both surfaces; but we may discover this error by turning one plate at a right angle, as in Fig. 2444, in which case the bearing marks would show along the edges of No. 1 and along the middle of No. 2, and we may correct each with the file until both plates mark all across and from end to end when tried together lengthways as in Fig. 2443, and one across the other as in Fig. 2444. But the plates may be curved to a different degree, as in Fig. 2445, and it then becomes necessary to know which to file the most in correcting them and fitting them together, which we may discover as follows:--

We give one plate a light coat of red marking and rub it upon the other both sideways and lengthways. Suppose that on being separated and examined the bearing marks, shown as at A A and B B, Fig. 2446, on one plate, and at C C and D D, Fig. 2447, on the other, and as those at A A and B B are the narrowest, or in other words extend the least distance across the plate, it is proof that this plate is more concave than the other plate is convex, and therefore needs the most correction. This is plain because whatever part of a plate touches another, will, if the two are merely pressed together, only leave a bearing mark equal in area to itself, while this area will obviously be increased in proportion as one plate is moved about upon the other.

When the object is to merely produce a flat surface, independent of the thickness or parallelism of the plate, it is not always necessary to file or scrape the whole of the area showing bearing marks. Suppose, for example, that the marks appear as in Fig. 2448, and as the bearing marks at A A show that edge of the plate to be straight already, all that is necessary is to ease the surface at B in order to let that side of the plate come up.

When we have fitted two of the three plates together we must accept one of them as a true one and (calling it No. 1) fit Nos. 3 and also 2 to it, and then try Nos. 2 and 3 together. If these require correcting the amount of correction must be made equal on each, and when this is done we must accept one of these two (say No. 3) as the standard, fit No. 1 to it, so that Nos. 1 and 2 both having been fitted to No. 3 may be tried together and both corrected equally; nor will the surfaces of any of them be true until all three will interchange in this manner and show a perfect contact.

It is to be noted, however, that in this process we have not altogether eliminated the error due to the deflection of each plate. Suppose, for example, a plate to be resting on its feet and its middle will sag or deflect to some extent (very minute though it may be in a small plate), and when we place another plate upon it the latter will also sag or deflect if its points of contact are far apart, and in any event the truing is performed by the bearing-marks, which the operator knows show the darkest and the brightest where the contact is greatest; hence by the time the contact marks show equally strong all over, the top plate will have been fitted to suit the deflection of the lower one. Since, however, the nearer the points of contact (between the plates) are together the less the degree of deflection, it is better in trying them to place the test plate on the top of the one being operated on. If the plates are long ones it will not answer to have more than three points of rest for the lower plate, unless the foundation on which the plate rests is made so true that each resting point of the plate will bear with equal pressure on the foundation plate or stone.

To eliminate as far as possible the deflection, the three plates may be got up by the process described, and then finished by trying them when resting on their edges (the trued surfaces standing vertical), interchanging the three plates as before.

In this case the surface will be true when standing vertical as finished, but there will still be some untruth from deflection when the plates are rested on their feet, though it will be less in amount than if the plates were finished on their feet as first described.

In finishing surface plates with a hand scraper, we have a surface that bears in fine spots only, these spots being the tops only of the scraper marks. Now the depth of the scraper marks are unequal, because immediately after the scraper is sharpened it cuts the easiest and the deepest, the scraper cutting less deep as its edge dulls. The operator regulates this to some extent by applying a greater pressure to the scraper as it gets dull, but from differences in the texture of the metal and from other causes it is impracticable to make the scraper cut equally deep at each stroke, as a result the tops of the scraper marks, which are the points of contact of the plates, wear away quickest, and the plate soon loses, to some extent, its truth.

Again, work that is so small as to cover part of the plate surface only, wears the part of the plate to which it is applied, and although the careful workman usually applies small work at and near the outside edges of the plate only, still these are all elements tending to produce increased local wear and to throw the plate out of true.

To obviate this difficulty the surface should be got up to bear all over, thus greatly increasing its bearing area and proportionately decreasing its wear. To produce such a surface the following plan was adopted by the author in 1876.

The filing process was continued with fine Groubet files, and testing the plates, rubbing them together sufficiently to mark them without the use of oil. Very short file strokes must be employed, and great care taken to apply the file to the exact necessary spots and places.

Then instead of using the scraper, No. 0 French emery paper was used, wrapped over the end of a flat file. The plates being interchanged and trued with No. 0, No. 00 was used, and the testing and interchanging repeated. These grades of emery paper were then wrapped or folded over the curved end of a piece of wood, the plates interchanged and rubbed together as before, and the emery paper used as described for the scraper. Subsequently Nos. 000 and 0000 French emery paper were similarly applied until the plates were finished. Much assistance to this method may be rendered by taking a piece of Water of Ayr stone, and truing its surfaces by rubbing them on the plates after the fine filing and before the emery papering. Then while applying the finer grades of emery paper the stone may be rubbed (with oil or water) in various directions over the surface. This has the effect of wearing off the very fine protuberances due to the emery paper cutting the metal most around its pores, and furthermore it causes the marks made in testing to show more plainly.

In skillful hands this process very far surpasses, both in the superiority of its results and in rapidity of execution, the scraping process, leaving a brilliant polished surface, so smooth that it feels as soft as satin, and the contact becomes so complete that no bearing marks can be distinguished.

In this process great care must be taken in cleaning the surfaces before applying them together, as the finest particle of dust will cut scratches, which though imperceptible on scraped surfaces, appear very coarse and deep on these smooth ones.

The amount of metal taken off by the finer grades of emery paper is so small as to be scarcely appreciable, save that it slightly discolors the emery paper.

The finest test for plates finished in this way is to rest the lower one quite level, clean it with alcohol, wipe it clean with old linen rag and finally with the palm of the hand, which if quite dry is more effective than anything else. The eye should carefully sight the plate surface with the light reflecting on that surface, when particles too fine to be felt may be observed and wiped off with the hand. In dry weather it is a difficult matter to clean the plates perfectly, as while one is being cleaned the fine particles of matter floating in the air rest upon the other; but in rainy weather the cleaning is much easier.

The plates being cleaned one must be lowered vertically on the other where it will float, there being a film of air between the two which it is almost impossible to exclude by pressure even though the plates be moved while pressed together.

If under these conditions the surfaces are not true and the top plate be set in motion in various directions, by a light finger touch it will swing round, the parts of the surface most in contact being the centre of motion. Suppose then the top plate to swing from one end it should be turned end for end on the bottom plate, and if the location of the centre of motion is still at the same end of the top plate, that plate is high there, while if the centre of motion in both cases is at the same end of the bottom plate it is the one that contains the error.

If the top plate swings upon its own centre of motion it must be moved farther off the bottom one, first on one side and then on the other, to discover if it or the bottom plate is in error; while if the top plate swings first from one end and then from the other, one or both of the plates are hollow and the top one must again be moved farther off the lower one, and the test by motion continued. The error discoverable in this way is very much finer than can be discovered by the marks of contact, since a plate showing quite even contact when quite dry and clean, and tested as lightly and carefully as may be will show error by this motion test. The error being so small in amount that it may be corrected by rubbing the plate with rag and oil, applied under hand pressure to the plate.

To cause the plates to bind together so that rubbing one on the other will leave contact marks, the top plate must be placed about an inch over the corner of the bottom one, pressed closely to it and forced laterally over it. A pair of plates of the Whitworth pattern (such as shown in Fig. 2449) placed by the author in the Centennial Exhibition, required, when put together dry as above, 341-1/2 lbs to _slide_ the top one over the other, which was due to the friction caused between the surfaces by the atmospheric pressure acting on the back surface of the plate, the latter having a superficial area of 12 by 8 inches.

Here it may be added that a plate of the same dimensions, and having its surface finished simply by filing with a dead smooth file, which plate was made for exhibition at a lecture on hand work, delivered before the Spring Garden Institute of Philadelphia, required a force of 22 lbs. to slide on the one on which it rested.

If two plates finished by the above method be placed together by sliding one upon the other it will be found that with the hands applied as in Fig. 2449, they can be separated or pulled apart with less force than it requires to slide one upon the other, because the plates bend and unlap, as would be the case if two sheets of paper were wetted and placed together and then taken apart by pulling two edges in opposite directions. But if the power to pull the plates apart be applied at the middle of the plate it will require a much greater force to separate them, although how much is problematical, no experiments having been made upon the subject. Furthermore the friction between two such plates will be greater if the surfaces be lubricated than if quite dry.

Thus, with the surfaces cleaned by alcohol, the top plate will move comparatively easily, but if the surfaces be slightly oiled and then wiped apparently quite clean with old dry rags, the friction will be a maximum. If then a piece of rag, say of an area of an inch, have one drop of oil upon it and be then applied to the surfaces of two plates after they have been cleaned with alcohol, the friction will still be about 3 lbs. per inch of area of one plate. With the surfaces well lubricated it will still require more power to slide one plate upon the other than would be the case were both plates quite dry.

The reason of this is that when quite dry it is impracticable to exclude the air from between the surfaces, whereas with the lubrication the air is more perfectly excluded and the atmospheric pressure forces the plates together.