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.