Fig. 35. Center Enlarging Punch
As it is quite or almost impossible to accurately center-punch the intersection of even two correctly drawn lines, by ordinary means, the punch shown in Figs. [34] and [36] was designed and an extended experience with it on a very high grade of work has demonstrated its value for the purpose. It consists essentially of a small center-punch O ([Fig. 36]) held in vertical position by a holder P which is knurled to facilitate handling. Great care should be exercised in making this tool to have the body of the punch straight, and to have it stand at right angles to the surface to be operated upon, for the slightest inclination will cause the finished hole to be incorrect, no matter how carefully the lines are drawn. The 60-degree point must be ground true with the axis. The holder for the punch stands on three legs, located as indicated, and ground accurately to a taper fit in the holder, where they are secured by watch screws bearing on their tops. The lower ends are hardened, and terminate in an angular point of 55 degrees (the point of the vernier scriber being 60 degrees). The edges are sharp, and slightly rounded at the ends, so that the legs will slide along a line smoothly. The points V and U ([Fig. 36]) have edges that are in line with each other, while the point T has an edge at right angles to the other two. The center of the punch is located at equal distances from all the legs, and is held off the work normally by a leather friction acted upon by a set-screw in the side of holder P.
Fig. 36. Section of
Center Locating Punch
Fig. 37. Section of
Center Enlarging Punch
If this tool is placed upon lines of the form shown at G, [Fig. 33], the legs V and U may be slid along horizontal line B-B, [Fig. 33], until the sharp edge of leg T drops into line A-A. When this occurs the punch O is lightly tapped with a hammer, and the resulting mark will be accurately located in the center of the intersection of the lines. It is good practice to make the work very smooth before drawing the lines, and after laying them out, to stone them so as to remove the slight burr raised in drawing them. A drop of oil is then rubbed into the lines, and the surplus wiped off. This procedure permits points V and U to run very smoothly along the line, and the burr having been removed, the edge of leg T drops into the line very readily with a slight click. As it is not advisable to strike punch O more than a very light blow, it marks the work but slightly, and a more distinct indentation is made with the follower punch shown in Figs. [35] and [37]. This punch is made like the previous one, so that it will stand at right angles to the work. The sectional view ([Fig. 37]) shows the punch A supported by the holder E which has four legs cut away on the sides so that the point of the punch may be seen. When this punch is in position, it is struck a sufficiently heavy blow to make a distinctly visible mark. The work is now ready to be placed upon the work table of the milling machine, and indicated for boring the holes, an indicator being used in the milling machine spindle.
Fig. 38. Indicator used for Aligning Punch Marks with Machine Spindle
An indicator which has been found especially valuable for this purpose is shown in Figs. [38] and [39]. It is of the concentric centering type, and with it the work is brought concentric with the axis of the spindle. The arbor I is provided with a threaded nose on which disk D is screwed. This disk has four holes in its rim, equally-spaced from each other. Hardened, ground, and lapped bushings b are put into these holes to receive plug A which is made a gage-fit both in these holes and in hole B in the outer end of sector C. This sector is held by a split sleeve to the barrel L which carries the 60-degree centering-rod K that comes into contact with the work to be bored. The spherical base of barrel L fits into a corresponding concave seat in the nose of the arbor at H, and is held in place by a spring E which connects at one end to the cylindrical stud in the base of the barrel, and at the other to the axial rod M by which it and the other connecting parts may be drawn into place, and held by the headless set-screw J, bearing on a flat spot on the tang end of the rod.