The Master-plate Method
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-plate M ([see Fig. 17]) contains holes which correspond to those wanted in the work, and which accurately fit a central plug P in 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 as accurately 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 recess r were required, a hole r₁ 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 blank B illustrated, 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 plug P which 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 pieces B could 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 frequently used 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 process [illustrated in Fig. 17]. That is, the central plug P is 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.
CHAPTER II
ACCURATE DIVIDING AND
SPACING METHODS
Toolmakers and machinists occasionally find it necessary to locate a number of equally-spaced holes on a straight line between two points, or to divide a circle with holes which are equi-distant within a very small limit of accuracy. Several dividing and spacing methods are described in this chapter; some of these methods can, with slight modification, be applied in various ways.
Fig. 18. Method of Drilling Small Equally-spaced Holes in Rows
Locating Small Equally-spaced
Holes in Rows
It is sometimes necessary to drill one or more rows of small equally-spaced holes. The best method of doing this work naturally depends, to some extent, upon the accuracy required, but even when a high degree of accuracy is not necessary, if an attempt is made to lay out the holes and drill them in the ordinary way, considerable time is usually required and the results are liable to be unsatisfactory. For example, suppose a row of holes ¹/₁₆ inch in diameter and ⅛ inch center-to-center distance were to be drilled in a flat plate. Some machinists would proceed by first scribing a center-line and then laying out the centers of the holes by means of dividers. A much easier and accurate method is [illustrated in Fig. 18], and is as follows: Lay out the first hole and drill it; then secure a small piece of flat steel for a drill guide, drill a hole through it, bevel one corner and scribe a fine line on the beveled section, as shown in the illustration. Align the hole drilled in the guide with the hole in the work, by inserting a close-fitting plug, and clamp a scale against one edge of the drill guide so that one of the graduation marks exactly matches with the line on the guide. The edge of the scale must also be located parallel to the center-line of the row of holes to be drilled. Now proceed to drill the holes, setting the drill guide each time, to whatever graduation line represents the required spacing or pitch of the holes.
It is advisable to use a magnifying glass to accurately align the graduation mark on the scale with the line on the drill guide. If two or more rows of holes are to be drilled parallel, the guide block can be drilled accordingly, so that the different rows of holes can be finished at the same time. The drill guide block should be relieved slightly in the center so as to insure the ends of the block bearing against the edge of the scale. A toolmaker or machinist can drill a row of holes accurately by this simple method, in the time required to lay them out in the usual way, and even though accuracy is not necessary, it is quicker to drill holes by this method than by the one more commonly employed.
Use of Disks for Locating
Equally-Spaced Holes
A simple method of spacing holes that are to be drilled in a straight line is illustrated in [Fig. 19]. Two disks are made, each having a diameter equal to the center-to-center distance required between the holes. These disks must also have holes which are exactly central with the outside to act as a guide for the drill or reamer. The first two holes are drilled in the work while the disks are clamped so that they are in contact with each other and also with the straightedge as shown. One disk is then placed on the opposite side of the other, as indicated by the dotted line, and a third hole is drilled; this process of setting one disk against the opposite side of the other is continued until all the holes are drilled. When it is necessary to drill a parallel row of “staggered” holes, the second row can be located by placing disks of the proper size in contact with the first row of disks.
Fig. 19. Locating Equi-distant Holes in a Straight Line
by Means of Disks and Straightedge
A method of using disks, which is preferable for very accurate work, is shown in [Fig. 20]. The disks are clamped against each other and along straightedge A by the screws shown, and if the outside diameters are correct and the guide holes concentric with the outside, very accurate work can be done. With this device there may be as many disks as there are holes to be drilled, if the number of holes is comparatively small, but if it is necessary to drill a long row of holes, the disks and frame are shifted along an auxiliary straightedge B, the hole in one of the end disks being aligned with the last hole drilled by inserting a close-fitting plug through the disk and hole.