CHAPTER IPRECISION LOCATING METHODS

The degree of accuracy that is necessary in the construction of certain classes of machinery and tools, has made it necessary for toolmakers and machinists to employ various methods and appliances for locating holes or finished surfaces to given dimensions and within the prescribed limits of accuracy. In this treatise, various approved methods of locating work, such as are used more particularly in tool-rooms, are described and illustrated. These are not given, in every case, as being the best possible method under all conditions, because, as every mechanical man knows, the best way may be dependent upon the element of accuracy with little regard for the time required to do the work, or this order may be reversed; therefore, one method is seldom, if ever, the best under all circumstances, and it is necessary for the workman to consider the conditions in each case and then be guided by his judgment and experience in determining just how the work should be done.

Among the different methods employed by toolmakers for accurately locating work such as jigs, etc., on the faceplate of a lathe, one of the most commonly used is known as the “button method.” This method is so named because cylindrical bushings or buttons are attached to the work in positions corresponding to the holes to be bored, after which they are used in locating the work. These buttons which are ordinarily about ½ or ⅝ inch in diameter, are ground and lapped to the same size, and the ends are finished perfectly square. The outside diameter should preferably be such that the radius can easily be determined, and the hole through the center should be about ⅛ inch larger than the retaining screw so that the button can be adjusted laterally.

As a simple example of the practical application of the button method, suppose three holes are to be bored in a jig-plate according to the dimensions given inFig. 1. A common method of procedure would be as follows: First lay out the centers of all holes to be bored, by the usual method. Mark these centers with a prick-punch and then drill holes for the machine screws which are used to clamp the buttons. After the buttons are clamped lightly in place, set them in correct relation with each other and with the jig-plate. The proper location of the buttons is very important, as their positions largely determine the accuracy of the work. The best method of locating a number of buttons depends, to some extent, upon their relative positions, the instruments available, and the accuracy required. When buttons must be located atgiven distances from the finished sides of a jig, a surface plate and vernier height gage are often used. The method is to place that side from which the button is to be set, upon an accurate surface plate and then set the button by means of the height gage, allowance being made, of course, for the radius of the button. The center-to-center distance between the different buttons can afterwards be verified by taking direct measurements with a micrometer.

Fig. 1. Simple Example of Work Illustrating Application of Button Method

Figs.2and3illustrate a method which requires only a micrometer. Two of the buttons are set at the correct distance from one edge of the plate by measuring from a parallel strip. Obviously, the micrometer reading will exceed the distance from the center of a button to the edge of the plate, by the amount equal to the thickness of the parallel strip plus the radius of the button. The center-to-center distance between each pair of buttons is also tested as indicated inFig. 3, by measuring the overall distance and deducting the diameter of one button.

After the buttons have been set and the screws are tightened, all measurements should be carefully checked. The work is then mounted on the faceplate of the lathe and one of the buttons is set true by the use of a test indicator as shown inFig. 4. When the dial of the indicator ceases to vibrate, thus showing that the button runs true, the latter should be removed so that the hole can be drilled and bored to the required size. In a similar manner other buttons are indicated and the holes bored, one at a time. It is evident that if each button is correctly located and set perfectly true in the lathe, the various holes will be located the required distance apart within very close limits.

Fig. 2. Determining Distance from Button to Edge of Plate

Another example of work illustrating the application of the button method is shown inFig. 5. The disk-shaped part illustrated is a flange templet which formed a part of a fixture for drilling holes in flanged plates, the holes being located on a circle 6 inches in diameter. It was necessary to space the six holes equi-distantly so that the holes in the flanges would match in any position, thus making them interchangeable. First a plug was turned so that it fitted snugly in the1¼-inch central hole of the plate and projected above the top surface about ¾ inch. A center was located in this plug and from it a circle of three inches radius was drawn. This circle was divided into six equal parts and then small circles ⅝ inch in diameter were drawn to indicate the outside circumference of the bushings to be placed in the holes. These circles served as a guide when setting the button and enabled the work to be done much more quickly. The centers of the holes were next carefully prick-punched and small holes were drilled and tapped for No. 10 machine screws. After this the six buttons were attached in approximately the correct positions and the screws tightened enough to hold the buttons firmly, but allow them to be moved by tapping lightly. As the radius of the circle is 3 inches, the radius of the central plug, ⅝ inch, and that of each button, ⁵/₁₆ inch, the distance from the outside of the central plug to the outside of any button, when correctly set, must be 3 ¹⁵/₁₆ inches. Since there are six buttons around the circle, the center-to-center distance is equal to the radius, and the distance between the outside or any two buttons should be 3⅝ inches. Having determined these dimensions, each button is set equi-distant from the central plug and the required distance apart, by using a micrometer. As each button is brought into its correct position, it should be tightened down a little so that it will be located firmly when finally set. The work is then strapped to the faceplate of a lathe and each button is indicated for boring the different holes by means of an indicator, as previously described. When the buttons are removed it will be found that in nearly all cases thesmall screw holes will not run exactly true; therefore, it is advisable to form a true starting point for the drill by using a lathe tool.

Fig. 3. Testing Location of Buttons

Fig. 7shows a method of locating buttons from the finished sides of a plate, and this same plate with the five buttons attached is shown inFig. 6. As the dimensions inFig. 7indicate, the holes must not only be accurate with relation to each other, but also with reference to the edges of the templet; therefore, it is necessary to work from the sides as well as the center. The width of the plate was first measured carefully and found to be 5 inches. As the center-to-center distance between buttonsBandCand also buttonsDandE, is 2½ inches, the distance from the center of each outside button to the edge of the plate is 1¼ inch. A ¼-inch parallel was clamped against the side, as shown in the illustration, and then the distance from the outside of each button to the outside of the parallel (1 ¹³/₁₆ inch) was measured in conjunction with the distanceLfrom the central button. The distanceLwas obtained by first determining the center-to-center distanceMwhich represents the hypotenuse of a right-angled triangle.

M² = 1.25² + 1.625²

orM= √1.25² + 1.625²= √4.024= 2.050 inches.

Therefore,L= 2.050 + 0.625 = 2.675 inches.

In this case, the center button was first located correctly from the sides and end and then the other buttons were set. When doing precision work of this kind, the degree of accuracy obtained will depend upon the instruments used, the judgment and skill of the workman, and the care exercised. A good general rule to follow when locating bushings or buttons is to use the method which is the most direct and which requires the least number of measurements, in order to prevent an accumulation of errors.

Comparatively small precision work is sometimes located by the disk method, which is the same in principle as the button method, the chief difference being that disks are used instead of buttons. These disks are made to such diameters that when their peripheries are in contact, each disk center will coincide with the position of the hole to be bored; the centers are then used for locating the work. To illustrate this method, suppose that the master-plate shown at the left inFig. 8is to have three holesa,b, andcbored into it, to the center distances given.

Fig. 4. Testing Concentricity of Button Preparatory to Boring Hole in Lathe

It is first necessary to determine the diameters of the disks. If the center distances between all the holes were equal, the diameters would, of course, equal this dimension. When, however, the distances between the centers are unequal, the diameters may be found as follows: Subtract, say, dimensionyfromx, thus obtaining the difference between the radii of disksCandA(see right-hand sketch); add this difference to dimensionz, and the result will be the diameter of diskA. Dividing this diameter by 2 gives the radius, which, subtracted from center distancexequals the radius ofB; similarly the radius ofBsubtracted from dimensionyequals the radius ofC.

For example, 0.930-0.720 = 0.210 or the difference between the radii of disksCandA. Then the diameter ofA= 0.210 + 0.860 = 1.070 inch, and the radius equals 1.070 ÷ 2 = 0.535 inch. The radius ofB= 0.930-0.535 = 0.395 inch and 0.395 × 2 = 0.790, or the diameter ofB. The center distance 0.720-0.395 = 0.325, which is the radius ofC; 0.325 × 2 = 0.650 or the diameter ofC.

Fig. 5. Flange Templet with Buttons AttachedFig. 6. Hinge Jig Templet with Buttons Attached

Fig. 5. Flange Templet with Buttons Attached

Fig. 6. Hinge Jig Templet with Buttons Attached

After determining the diameters, the disks should be turned nearly to size and finished, preferably in a bench lathe. First insert a solder chuck in the spindle, face it perfectly true, and attach the disk by a few drops of solder, being careful to hold the work firmly against the chuck while soldering. Face the outer side and cut a sharp V-center in it; then grind the periphery to the required diameter. Next fasten the finished disks onto the work in their correct locations with their peripheries in contact, and then set one of the disks exactly central with the lathe spindle by applying a test indicator to the center in the disk. After removing the disk and boring the hole, the work is located for boring the other holes in the same manner.

Fig. 7. Hinge Jig Templet Illustrated inFig. 6

Small disks may be secured to the work by means of jeweler’s wax. This is composed of common rosin and plaster of paris and is made as follows: Heat the rosin in a vessel until it flows freely, and then add plaster of paris and keep stirring the mixture. Care should be taken not to make the mixture too stiff. When it appears to have the proper consistency, pour some of it onto a slate or marble slab and allow it to cool; then insert the point of a knife under the flattened cake thus formed and try to pry it off. If it springs off with a slight metallic ring, the proportions are right, but if it is gummy and ductile, there is too much rosin. On the other hand, if it is too brittle and crumbles, this indicates that there is too much plaster of paris. The wax should be warmed before using. A mixture of beeswax and shellac, or beeswax and rosin in about equal proportions, is also used for holding disks in place. When the latter are fairly large, it may be advisable to secure them with small screws, provided the screw holes are not objectionable.

The accuracy of work done by the button method previously described is limited only by the skill and painstaking care of the workman, but setting the buttons requires a great deal of time. By a little modification, using what is sometimes called the “disk-and-button method,” a large part of this time can be saved without any sacrifice of accuracy. The disk-and-button method is extensively used in many shops. Buttons are used, but they are located in the centers of disks of whatever diameters are necessary to give the required locations. As three disks are used in each step of the process, it is sometimes called the “three-disk method.”

To illustrate the practical application of this method, suppose sixequally-spaced holes are to be located in the circumference of a circle six inches in diameter. To locate these, one needs, besides the buttons, three disks three inches in diameter, each having a central hole exactly fitting the buttons. It is best to have, also, a bushing of the same diameter as the buttons, which has a center-punch fitted to slide in it. First the center button is screwed to the templet, and one of the disksA,Fig. 9, is slipped over it; then a second diskBcarrying a bushing and center-punch is placed in contact with diskAand a light blow on the punch marks the place to drill and tap for No. 2 button, which is kept in its proper place while tightening the screw by holding the two disksAandBin contact. Next the third diskCis placed in contact with disksAandBand locates No. 3 button, and so on until the seven buttons are secured in position. The templet is then ready to be strapped to the lathe faceplate for boring.

Fig. 8. An Example of Precision Work, and Methodof Locating Holes by Use of Disks in Contact

Of course, it is not possible to use disks of “standard” sizes for many operations, but making a special disk is easy, and its cost is insignificant as compared with the time saved by its use. One who employs this method, especially if he also uses disks to lay out angles, soon accumulates a stock of various sizes. While it is desirable to have disks of tool steel, hardened and ground, or, in the larger sizes, of machine steel, case-hardened and ground, a disk for occasional use will be entirely satisfactory if left soft.

Another example of work is shown inFig. 10. This is a jig templet similar to the one illustrated in Figs.6and7. SketchAgives its dimensions and sketchBshows the disk-and-button way of locating the holes. A steel square is clamped with its stock against the right-hand edge of the templet and its blade extending across the top. The lower edge of the blade should be located 0.250 inch from the upper edge of the templet by the use of size blocks. A 2½-inch disk, touching both blade and stock, locates holeC. Another 2½-inch disk, touching the first disk and the square blade, locates holeB. Next a disk 1.600 inch diameter is placed in contact with thetwo upper disks and locates the center holeA; and, finally, the disks for holesBandCare used to locate holesDandE.

Fig. 9. Locating Holes on a Circle and Equi-distantby using Disks and Buttons in Combination

Two other jobs that illustrate this method may be of interest. The first one, shown inFig. 11, required the locating of nine equally-spaced holes on a circumference of 7⅜ inches diameter. In any such case, the size of the smaller disks is found by multiplying the diameter of the circle upon which the centers of the disks are located by the sine of half the angle between two adjacent disks. The angle between the centers of adjacent disks equals 360 ÷ number of disks. 360 ÷ 9 = 40; hence, in this case, the diameter of the smaller disks equals 7⅜ multiplied by the sine of 20 degrees, or 7⅜ × 0.34202 = 2.5224 inches. 7⅜-2.5224 = 4.8526 inches, which is the diameter of the central disk.

The templet shown inFig. 12required two holes on a circumference 6½ inches diameter, with their centers 37 degrees 20 minutes apart. To find the diameter of the smaller disks, multiply the diameter of the large circle by the sine of one-half the required angle, as in the preceding example; thus 6½ × sin 18 degrees 40 minutes = 2.0804 inches, which is the diameter of the two smaller disks. The diameter of the larger disk equals 6½-2.0804 = 4.4196 inches.

Very accurate results can be obtained by the disk-and-button method. Of course, absolute exactness is equally unattainable with buttons and a micrometer, or any other method; the micrometer does not show the slightinaccuracy in any one chordal measurement, while in using the disks the error is accumulative and the insertion of the last disk in the series shows the sum of the errors in all the disks. It is only in cases like the one illustrated inFig. 9that we note this, and then, though in correcting the error, we may change the diameter of the circle a very slight amount, an exceedingly accurate division of the circumference is secured.

Use of Two- and Three-Diameter Disks

Fig. 13illustrates, on an enlarged scale, a piece of work requiring great accuracy, which was successfully handled by an extension of the three-disk method. Fourteen holes were required in a space hardly larger than a silver half-dollar, and, although the drawing gave dimensions from the center of the circle, the actual center could not be used in doing the work, as there was to be no hole there; moreover, a boss slightly off center prevented the use of a central disk, unless the bottom of the disk were bored out to receive this boss, which was not thought expedient. Hence, the method adopted was to make the plate thicker than the dimension given on the drawing, and then bore it out to leave a rim of definite diameter, this rim to be removed after it had served its purpose as a locating limit for the disks.

Fig. 10. (A) Layout of Jig-Plate.(B) Disk-and-Button Method of Locating Holes

As the holesAandB, which were finished first, were 0.600 inch apart and 0.625 inch from the center, the rim was bored to 1.850 inch and two 0.600-inch disks, in contact with the rim and with each other, located these holes. As holeCwas to be equi-distant from holesAandB, and its distance from the center was given, the size of the disk for this hole was readily determined. The disks for holesA,BandChavetwo diameters; the upper diameters are made to whatever size is required for locating the disks of adjacent holes, and they also form a hub which can be used when setting the disks with an indicator. HoleDwas 0.4219 inch fromB, and calculations based on this dimension and its distance from the center showed that it was 0.4375 inch from holeC.

A “three-story” disk or button was made for holeD. The diameter of the large part was 0.46875 inch and it overlapped disksCandB(the upper sections of which were made 0.375 inch and 0.4062 inch, respectively), thus locatingD. Then holeFand all the remaining holes were located in a similar manner. The upper diameters of disksEandDwere used in locating disks for other adjacent holes, as well as a hub for the indicator; for instance, to locate a hole with reference to holesCandD, the diameter of the new disk and the diameter of the upper part of diskD, were varied to give the required location. The relation between the disksB,DandFis shown by the side view.

Fig. 11. Example of Circular Spacingrequiring a Large Central Disk

It had been decided that no screws should be used in attaching the buttons or disks to the work, as it was feared that the tapped holes would introduce inaccuracy by deflecting the boring-tools; therefore the following method was employed. After all the disks were fastened in place by clamps, a soft solder of low melting point was flowed about them, filling the work to the top of the rim. When the solder had cooled, the clamps were removed, the work transferred to the lathe faceplate, indicated in the usual way, and the holes bored by a “D” or “hog-nose” drill, guided by an axial hole in each disk, which had been provided for that purpose when the disks were made. It was thought that the unequal contraction of the solder and the plate in cooling might throw the holes “out of square;” however, careful measurements failed to show any appreciable lack of parallelism in test-bars inserted in the holes.

Fig. 12.Locating Holes at an Angle by use of Disks and ButtonsFig. 13. Locating Holes by Means of Two- andThree-Diameter Disks in Contact

Fig. 12.Locating Holes at an Angle by use of Disks and Buttons

Fig. 13. Locating Holes by Means of Two- andThree-Diameter Disks in Contact

For setting up a piece of work on which a surface is to be planed or milled at an exact angle to a surface already finished, disks provide an accurate means of adjustment. One method of using disks for angular work is illustrated atAinFig. 14. Let us assume that the lower edge of plate shown is finished and that the upper edge is to be milled at an angleaof 32 degrees with the lower edge. If the two disksxandyare to be used for locating the work, how far apart must they be set in order to locate it at the required angle? The center-to-center distance can be determined as follows: Subtract the radius of the larger disk from the radius of the smaller disk, and divide the difference by the sine of one-half the required angle.

Fig. 14. Obtaining Accurate Angular Measurements with Disks

Example: If the required angleais 32 degrees, the radius of the large disk, 2 inches, and the radius of the small disk, 1 inch, what is the center-to-center distance?The sine of one-half the required angle, or 16 degrees, is 0.27564. The difference between the radii of the disks equals 2 - 1 = 1, and 1 ÷ 0.27564 = 3.624 inches. Therefore, for an angle of 32 degrees, disks of the sizes given should be set so that the distance between their centers is 3.624 inches.

Example: If the required angleais 32 degrees, the radius of the large disk, 2 inches, and the radius of the small disk, 1 inch, what is the center-to-center distance?

The sine of one-half the required angle, or 16 degrees, is 0.27564. The difference between the radii of the disks equals 2 - 1 = 1, and 1 ÷ 0.27564 = 3.624 inches. Therefore, for an angle of 32 degrees, disks of the sizes given should be set so that the distance between their centers is 3.624 inches.

Another method of accurately locating angular work is illustrated atBinFig. 14. In this case, two disks are also used, but they are placed in contact with each other and changes for different angles are obtained by varying the diameter of the larger disk. The smaller disk is a standard 1-inch size, such as is used for setting a 2-inch micrometer. By this method any angle up to about 40 degrees can be obtained within a very close limit of accuracy. The following rule may be used for determining the diameter of the larger disk, when both disks are in contact and the diameter of the small disk is known:

Multiply twice the diameter of the small disk by the sine of one-half the required angle; divide this product by 1 minus the sine of one-half the required angle; add the quotient to the diameter of the small disk to obtain the diameter of the large disk.

Example: The required angle a is 15 degrees. Find the diameter of the large disk to be in contact with the standard 1-inch reference disk.The sine of 7 degrees 30 minutes is 0.13053. Multiplying twice the diameter of the small disk by the sine of 7 degrees 30 minutes, we have 2 × 1 × 0.13053 = 0.26106. This product divided by 1 minus the sine of 7 degrees 30 minutes

Example: The required angle a is 15 degrees. Find the diameter of the large disk to be in contact with the standard 1-inch reference disk.

The sine of 7 degrees 30 minutes is 0.13053. Multiplying twice the diameter of the small disk by the sine of 7 degrees 30 minutes, we have 2 × 1 × 0.13053 = 0.26106. This product divided by 1 minus the sine of 7 degrees 30 minutes

This quotient added to the diameter of the small disk equals 1 + 0.3002 = 1.3002 inch, which is the diameter of the large disk.

Fig. 15. Disk-and-Square Method of Accurately Setting Angular Work

The accompanying table gives the sizes of the larger disks to the nearest 0.0001 inch for whole degrees ranging from 5 to 40 degrees inclusive. Incidentally, the usefulness of these disks can be increased by stamping on each one its diameter and also the angle which it subtends when placed in contact with the standard 1-inch disk.

DISK DIAMETERS FOR ANGULAR MEASUREMENT

The method shown inFig. 15for determining angles for setting up work on a milling machine or planer, possesses several advantages. No expensive tools are required, the method can be applied quickly, and the results obtained are quite accurate enough for any but the most exacting requirements. As will be seen, an ordinary combination square is used in connection with a disk, the head of the square being set at different points on the blade according to the angle that is desired. Theoretically, a one-inch disk could be used for all angles from about 6 degrees up to a right angle, but in practice it is more convenient and accurate to employ larger disks for the larger angles.

The only inaccuracy resulting from this method is due to setting the square at the nearest “scale fraction” instead of at the exact point determined by calculation. This error is very small, however, and is negligible in practically all cases. The dimensionxrequired for any desired angleacan be found by multiplying the radius of the disk, by the cotangent of one-half the desired angle, and adding to this product the radius of the disk.

Example: The square blade is to be set to an angle of 15 degrees 10 minutes, using a 2-inch disk. At what distancex(see Fig. 15) should the head of the square be set?Cot 7 degrees 35 minutes = 7.5113,and 7.5113 × 1 + 1 = 8.5113 inches.By setting the square to 8½ inches “full,” the blade would be set very close to the required angle of 15 degrees 10 minutes.

Example: The square blade is to be set to an angle of 15 degrees 10 minutes, using a 2-inch disk. At what distancex(see Fig. 15) should the head of the square be set?

Cot 7 degrees 35 minutes = 7.5113,and 7.5113 × 1 + 1 = 8.5113 inches.

By setting the square to 8½ inches “full,” the blade would be set very close to the required angle of 15 degrees 10 minutes.

The size-block method of locating a jig-plate or other part, in different positions on a lathe faceplate, for boring holes accurately at given center-to-center distances, is illustrated inFig. 16. The way the size blocks are used in this particular instance is as follows: A pair of accurate parallels are attached to a faceplate at right angles to each other and they are so located that the center of one of the holes to be bored will coincide with the lathe spindle. The hole which is aligned in this way should be that one on the work which is nearest the outer corner, so that the remaining holes can be set in a central position by adjusting the work away from the parallels. After the first hole is bored, the work is located for boring each additional hole by placing size blocks of the required width between the edges of the work and the parallels. For instance, to set the plate for boring holeD, size blocks (or a combination of blocks or gages) equal in width to dimensionA₁ would be inserted atA, and other blocks equal in width to dimensionB₁ beneath the work as atB. As will be seen, the dimensions of these blocks equal the horizontal and vertical distances between holesCandD. With the use of other combinations of gage blocks, any additional holes that might be required are located in the central position. While only two holes are shown in this case, it will be understood that the plate could be located accurately for boring almost any number of holes by this method.

Fig. 16. Method of setting Work on Faceplatewith Size Blocks or Gages

Incidentally, such gages as the Johansson combination gages are particularly suited for work of this kind, as any dimension within the minimum and maximum limits of a set can be obtained by simply placing the required sizes together. Sometimes when such gages are not available, disks which have been ground to the required diameter are interposed between the parallels and the work for securing accurate locations. Another method of securing a positive adjustment of the work is to use parallels composed of two tapering sections, which can be adjusted to vary the width and be locked together by means of screws. Each half has the same taper so that outer edges are parallel for any position, and the width is measured by using a micrometer. The size-block method is usually applied to work having accurately machined edges, although a part having edges which are of a rough or irregular shape can be located by this method, if it is mounted on an auxiliary plate having accurately finished square edges. For instance, if holes were to be bored in the casting for a jig templet which had simply been planed on the top and bottom, the casting could be bolted to a finished plate having square edges and the latter be set in the different positions required, by means of size blocks. Comparatively large jig plates are sometimes located for boring in this way and the milling machine is often used instead of a lathe.

When it is necessary to machine two or more plates so that they are duplicates as to the location of holes, circular recesses, etc., what is known as a master-plate is often used for locating the work on the lathe faceplate. This master-plateM(see Fig. 17) contains holes which correspond to those wanted in the work, and which accurately fit a central plugPin the lathe spindle, so that by engaging first one hole and then another with the plug, the work is accurately positioned for the various operations.

When making the master-plate, great care should be taken to have the sides parallel and the holes at right angles to the sides, as well asaccurately located with reference to one another. The various holes may be located with considerable precision by the use of buttons as previously described. Of course, it is necessary to have a hole in the master-plate for each different position in which the work will have to be placed on the faceplate; for example, if a circular recessrwere required, a holer₁ exactly concentric with it would be needed in the master-plate. The method of holding the work and locating it with reference to the holes in the master-plate will depend largely on its shape. The cylindrical blankBillustrated, is positioned by a recess in the master-plate in which it fits. The work is commonly held to the master-plate by means of clamps and tap bolts or by screws which pass through the work and into the master-plate. Solder is sometimes used when it is not convenient to hold the work by clamps or screws.

Fig. 17. Master-plate applied to a Bench Lathe Faceplate

The plugPwhich locates the master-plate, is first turned to fit the spindle or collet of the lathe and the outer or projecting end is roughturned for the holes in the master-plate, which should all be finished to exactly the same diameter. The plug is then inserted in the spindle and ground and lapped to a close fit for the holes in the master-plate. The latter, with the work attached to it, is next clamped to the faceplate by the straps shown, which engage a groove around the edge of the master-plate. The first hole is finished by drilling to within, say, 0.005 or 0.006 inch of the size, and then boring practically to size, a very small amount being left for reaming or grinding. The remaining holes can then be finished in the same way, the work being positively located in each case by loosening the master-plate and engaging the proper hole in it with the central plug. It is apparent that by the use of this same master-plate, a number of piecesBcould be made which would be practically duplicates.

The master-plate method of locating work can be applied in many different ways. It is used for making duplicate dies, for accurately locating the various holes in watch movements, and for many other operations requiring great precision. Master-plates are quite frequentlyused by toolmakers when it is necessary to produce a number of drill jigs which are to be used for drilling holes in different parts having the same relative locations, thus requiring jigs that are duplicates within very close limits.

When a master-plate is required, that is to be used in making duplicates of an existing model, the holes are bored in the master-plate by reversing the processillustrated in Fig. 17. That is, the central plugPis turned to fit the largest hole in the model and the latter with the attached master-plate blank is clamped to lathe faceplate. The first hole is then bored to within say 0.002 inch of the finish diameter, to allow for grinding, provided the master-plate is to be hardened. The central plug is then turned down to fit the next largest hole and the second hole is bored in the master-plate. This method is continued until all the holes are bored. In order to prevent any change in the position of the master-plate relative to the model, it may be secured by inserting dowel-pins through both parts, the work being held to the lathe faceplate by ordinary screw clamps. If the holes in the model do not extend clear through, a flat plate having parallel sides may be interposed between the model and master-plate to provide clearance between the two and prevent cutting into the model when boring the master-plate.

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