Modern Machine-Shop Practice Part 104

If less than three straight-edges or parallel strips are to be trued they must be trued to a surface plate or its equivalent, but if a pair are to be made they should have the side faces made true, and be riveted together so that their edges may be trued together, and equal width may be more easily obtained. For this purpose copper rivets should be used, because they are more readily removable, as well as less likely to strain the work in the riveting.

By riveting the straight-edges together the surface becomes broader and the file operates steadier, while the edges of the straight-edge are left more square. Furthermore parallelism is more easily obtained as one measurement at each end of the batch will test the parallelism instead of having to measure each one separately at each end. If three straight-edges are to be made they may be riveted together and filed as true as may be with the testing conveniences at hand, but they should be finally trued as described for the surface plate.

In using straight-edges to set work, the latter is often heated to facilitate the setting, and in this case the straight-edge or parallel strips should be occasionally turned upside down upon the work, for if the heated work heats one side of the straightedge more than the other the increased expansion of the side most heated will bend the straight-edge or strips, and throw them out of true.

In applying a straight-edge to test work it must never be pressed to the work surface, because in that case it will show contact with the work immediately beneath the parts where such pressure is applied. Suppose, for example, a true straight-edge be given a faint marking, and be applied to a true surface, the straight-edge itself being true; then if the hands are placed at each end of the straight-edge, and press it to the work while the straight-edge is given motion, it will leave the heaviest marks at and near the ends as though the work surface was slightly hollow in its length; while were the hand pressure applied to the middle of the length of the straight-edge the marks on the work would show the heaviest in the middle as though the work surface were rounding. This arises from the deflection due to the weakness of the straight-edge.

For testing the truth of flat or plane surfaces the machinist employs the surface plate or planometer. The surface plate is a plate or casting having a true flat surface to be used as a test plate for other surfaces. It is usually made of cast iron, and sometimes of chilled cast iron or hardened cast steel, the surface in either of these two latter cases being ground true because their hardness precludes the possibility of cutting them with steel tools. A chilled or hardened surface plate cannot, however, be so truly surfaced as one that is finished with either the sc.r.a.per or the file.

The shape of the surface plate is an element of the first importance, because as even the strongest bars of metal deflect from their own weight, it is necessary to shape the plate with a view to make this deflection as small as possible in any given size and weight of plate.

In connection, also, with the shape we must consider the effect of varying temperatures upon the metal, for if one part of the plate is thinner than another it will, under an increasing temperature, heat more rapidly, and the expansion due to the heating will cause that part to warp the plate out of its normal form, and hence out of true. The amount that a plate will deflect of its own weight can only be appreciated by those who have had experience in getting up true surfaces, but an idea may be had when it is stated that it can be shown that it is easily detected, in a piece of steel three inches square and a foot long.

Now this deflection will vary in direction according to the points upon which the plate rests. For instance, take two plates, clean them properly, and rest one upon two pieces of wood, one piece under each end, and then place another plate upon the lower one and its face will show hollow, and, if the upper plate is moved backwards and forwards laterally it will be found to move from the ends as centres of motion.

Then rest the lower plate upon a piece of wood placed under the middle of its length, and we shall find that (if the plates are reasonably true) the top one will move laterally with the middle of its length as a centre of motion. Now although this method of testing will prove deflection to exist, it will not show its amount, because the top plate deflects to a certain extent, conforming itself to the deflection of the lower one, and if the test is accurately made it will be found that the two plates will contact at whatever points the lower one is supported.

If plates, tested in this manner, show each other to have contact all along however the lower one is supported, it is because they are so light that the upper one will readily bend to suit the deflection of the lower one, and true work is, with such a plate, out of the question.

To obviate these difficulties the body of the plate is heavily ribbed, and these ribs are so arranged as to be of equal lengths, and are made equal in thickness to the plate, so that under variations of temperature the ribs will not expand or contract more quickly or slowly than the body of the plate, and the twisting that would accompany unequal expansion is avoided.

[Ill.u.s.tration: Fig. 1484.]

In Fig. 1484 is shown the form of surface plate designed by Sir Joseph Whitworth for plates to be rested upon their feet. The resting points of the plate are small projections shown at A, B, and C. The object of this arrangement of feet is to enable the plate to rest with as nearly as possible an equal degree of weight upon each foot, the three feet accommodating themselves to an uneven surface. It is obvious, however, that more of the weight will fall upon C than upon A or B, because C supports the whole weight at one end, while at the other end A and B divide the weight.

[Ill.u.s.tration: Fig. 1485.]

Fig. 1485 shows the form of plate designed by Professor Sweet.

[Ill.u.s.tration: Fig. 1486.]

In Fig. 1486 is shown a pair of angle surface plates resting upon a flat one. The angle plates may be used for a variety of purposes where it is necessary to true a surface standing at a true right angle to another.

The best methods of making surface plates are as follows:--

The edges of the plates should be planed first, care being taken to make them square and flat. The surfaces should then be planed, the plates being secured to the planer _by the edges_, which will prevent as far as possible the pressure necessary to hold them against the planing tool cut from springing, warping, or bending the plates. Before the finis.h.i.+ng cut is taken, the plates or screws holding the surface plate should be slackened back a little so as to hold them as lightly as may be, the finis.h.i.+ng cut being a very light one, and under these circ.u.mstances the plates may be planed sufficiently true that one will lift the other from the partial vacuum between them.

After the plates are planed, and before any hand work is done on them, they should be heated to a temperature of at least 200 Fahr., so that any local tension in the casting may be as far as possible removed.

[Ill.u.s.tration: Fig. 1487.]

Surface plates for long and narrow surfaces are themselves formed long and narrow, as shown in Fig. 1487, which represents the straight-edge surface plate made at Cornell University.

[Ill.u.s.tration: Fig. 1488.]

The Whitworth surfacing straight-edge, or long narrow surface plate, is ribbed as in Fig. 1488, so as to give it increased strength in proportion to its weight, and diminish its deflection from its own weight. The lugs D are simply feet to rest it on.

Straight-edges are sometimes made of cast steel and trued on both edges.

These will answer well enough for small work, but if made of a length to exceed about four feet their deflection from their own weight seriously affects their reliability. The author made an experiment upon this point with a very rigid surface plate six feet long, and three cast steel straight-edges 6 feet long, 4-1/2 inches wide, and 1/2 inch thick. Both edges of the straight-edges were trued to the surface plate until the light was excluded from between them, while the bearing surface appeared perfect; thin tissue paper was placed between the straight-edges and the plate, and on being pulled showed an equal degree of tension. The straight-edges were tried one with the other in the same way and interchanged without any apparent error, but on measuring them it was found that each was about 1/50 inch wider in the middle of its length than at the ends, the cause being the deflection. They were finished by filing them parallel to calipers, using the bearing marks produced by rubbing them together and also upon the plate; but, save by the caliper test, the improvement was not discernible.

In rubbing them together no pressure was used, but they were caused to slide under their own weight only.

A separate and distinct cla.s.s of gauge is used in practice to copy the form of one piece and transfer it to another, so that the one may conform to or fit the other. To accomplish this end, what are termed male and female templates or gauges are employed. These are usually termed templates, but their application to the work is termed gauging it.

[Ill.u.s.tration: Fig. 1489.]

Suppose, for example, that a piece is to be fitted to the rounded corner of a piece F, Fig. 1489, and the maker takes a piece of sheet metal A, and cuts it out to the line B C D, leaving a female gauge E, which will fit to the work F. We then make a male gauge G, and apply this to the work, thus gauging the round corner.

[Ill.u.s.tration: Fig. 1490.]

Fig. 1490 represents small templates applied to a journal bearing, and it is seen that we may make the template as at T, gauging one corner only, or we may make it as at T', thus gauging the length of the journal as well as the corners.

[Ill.u.s.tration: Fig. 1491.]

[Ill.u.s.tration: Fig. 1492.]

Fig. 1491 represents a female gauge applied to the corner of a bearing or bra.s.s for the above journal, it being obvious that the male and female templates when put together will fit as in Fig. 1492.

For measuring the diameters of metal wire and the thickness of rolled sheet metal, measuring instruments termed wire gauges and sheet metal measuring machines are employed. A simple wire gauge is usually formed of a piece of steel containing numerous notches, whose widths are equal to the intended thickness to be measured in each respective notch. These notches are marked with figures denoting the gauge-number which is represented by the notch.

For wire, however, a gauge having holes instead of notches is sometimes employed, the wire being measured by insertion in the hole, an operation manifestly impracticable in the case of sheet metal.

In Fig. 1493 is shown one of Brown and Sharpe's notch wire-gauges, the notches being arranged round the edge as shown:

[Ill.u.s.tration: Fig. 1493.]

The thickness of a given number of wire-gauge varies according to the system governing the numbering of the gauge, which also varies with the cla.s.s of metal or wire for which the gauge has been adopted by manufacturers. Thus, in the following table are given the gauge-numbers and their respective sizes in decimal parts of an inch, as determined by Holtzapffel in 1843, and to which sizes the Birmingham wire-gauge is made. The following table gives the numbers and sizes of the Birmingham wire-gauge.

BIRMINGHAM WIRE GAUGE.

-----+------++------+------++------+------++------+------ Mark.| Size.|| Mark.| Size.|| Mark.| Size.|| Mark.| Size.

-----+------++------+------++------+------++------+------ 36 | .004 || 26 | .018 || 16 | .065 || 6 | .203 35 | .005 || 25 | .020 || 15 | .072 || 5 | .220 34 | .007 || 24 | .022 || 14 | .083 || 4 | .238 33 | .008 || 23 | .025 || 13 | .095 || 3 | .259 32 | .009 || 22 | .028 || 12 | .109 || 2 | .284 31 | .010 || 21 | .032 || 11 | .120 || 1 | .300 30 | .012 || 20 | .035 || 10 | .134 || 0 | .340 29 | .013 || 19 | .042 || 9 | .148 || 00 | .380 28 | .014 || 18 | .049 || 8 | .165 || 000 | .425 27 | .016 || 17 | .058 || 7 | .180 || 0000 | .454 -----+------++------+------++------+------++------+------

In this gauge it will be observed that the progressive wire gauge numbers do not progress by a regular increment.

This gauge is sometimes termed the Stubs wire-gauge, Mr. Stubs being a manufacturer of instruments whose notches are s.p.a.ced according to the Birmingham wire-gauge. Since, however, Mr. Stubs has also a wire-gauge of his own, whose numbers and gauge-sizes do not correspond to those of the Birmingham gauge, the two Stubs gauges are sometimes confounded. The second Stubs gauge is employed for a special drawn steel wire, made by that gentleman to very accurate gauge measurement for purposes in which accuracy is of primary importance.

From the wear of the drawing dies in which wire is drawn, it is impracticable, however, to attain absolute correctness of gauge measurement. The dies are made to correct gauge when new, and when they have become worn larger, to a certain extent, they are renewed. As a result the average wire is slightly larger than the designated gauge-number. To determine the amount of this error the Morse Twist-Drill and Machine Company measured the wire used by them during an extended period of time, the result being given in table No. 2, in which the first column gives the gauge-number, the second column gives the thickness of the gauge-number in decimal parts of an inch, and the third column the actual size of the wire in decimal parts of an inch as measured by the above Company.

DIAMETER OF STUBS'S DRAWN STEEL WIRE IN FRACTIONAL PARTS OF AN INCH.

-------+-------+--------++-------+-------+--------++-------+-------+-------- No. by |Stubs's|Measure-||No. by |Stubs's|Measure-||No. by |Stubs's|Measure- Stubs's|Dimen- |ment by ||Stubs's|Dimen- |ment by ||Stubs's|Dimen- |ment by wire- |sions. | Morse || wire- |sions. | Morse || wire- |sions. | Morse gauge. | | Twist- ||gauge. | | Twist- ||gauge. | | Twist- | | Drill || | | Drill || | | Drill | | and || | | and || | | and | |Machine || | |Machine || | |Machine | | Co. || | | Co. || | | Co.

-------+-------+--------++-------+-------+--------++-------+-------+-------- 1 | .227 | .228 || 23 | .153 | .154 || 45 | .081 | .082 2 | .219 | .221 || 24 | .151 | .152 || 46 | .079 | .080 3 | .212 | .213 || 25 | .148 | .150 || 47 | .077 | .079 4 | .207 | .209 || 26 | .146 | .148 || 48 | .075 | .076 5 | .204 | .206 || 27 | .143 | .145 || 49 | .072 | .073 6 | .201 | .204 || 28 | .139 | .141 || 50 | .069 | .070 7 | .199 | .201 || 29 | .134 | .136 || 51 | .066 | .067 8 | .197 | .199 || 30 | .127 | .129 || 52 | .063 | .064 9 | .194 | .196 || 31 | .120 | .120 || 53 | .058 | .060 10 | .191 | .194 || 32 | .115 | .116 || 54 | .055 | .054 11 | .188 | .191 || 33 | .112 | .113 || 55 | .050 | .052 12 | .185 | .188 || 34 | .110 | .111 || 56 | .045 | .047 13 | .182 | .185 || 35 | .108 | .110 || 57 | .042 | .044 14 | .180 | .182 || 36 | .106 | .106 || 58 | .041 | .042 15 | .178 | .180 || 37 | .103 | .104 || 59 | .040 | .041 16 | .175 | .177 || 38 | .101 | .101 || 60 | .039 | .040 17 | .172 | .173 || 39 | .099 | .100 || 61 | .038 | .039 18 | .168 | .170 || 40 | .097 | .098 || 62 | .037 | .038 19 | .164 | .166 || 41 | .095 | .096 || 63 | .036 | .037 20 | .161 | .161 || 42 | .092 | .094 || 64 | .035 | .036 21 | .157 | .159 || 43 | .088 | .089 || 65 | .033 | .035 22 | .155 | .156 || 44 | .085 | .086 || | | -------+-------+--------++-------+-------+--------++-------+-------+--------

The following table represents the letter sizes of the same wire:--

LETTER SIZES OF WIRE.

A. 0.234 | J. 0.277 | S. 0.348 B. 0.238 | K. 0.281 | T. 0.358 C. 0.242 | L. 0.290 | U. 0.368 D. 0.246 | M. 0.295 | V. 0.377 E. 0.250 | N. 0.302 | W. 0.386 F. 0.257 | O. 0.316 | X. 0.397 G. 0.261 | P. 0.323 | Y. 0.404 H. 0.266 | Q. 0.332 | Z. 0.413 I. 0.272 | R. 0.339 |