Spring Die Sizes The table of dimensions for spring screw dies, Fig. 4, should prove of service, particularly for steel. For brass the cutting edge is radial, thus eliminating dimension A. The width of land at bottom of thread is usually made about outside diameter of cut, the milling between flutes being 70 degrees, leaving 50 degrees for the prong in the case of three-flute dies. In boring holes previously to tapping they should be somewhat larger than the theoretical diameter at bottom of thread, as the crowding action of the tap will cause the metal to flow some and compensate for this. Where no allowance is made, frequent tap breakage is liable to occur and torn threads in the work also. On external work it is for the same reasons advisable to turn the work undersize and the following table gives good average allowances for both internal and external work. Tap Length and Number of Lands The number of teeth in taps and the width of land should be regulated by the diameter and pitch of work as well as the nature of the material being cut. On fine threads, where a drunken thread is to be insured against, more teeth are required than on a coarser pitch of the same diameter. A good average number of teeth on taps for United States standard threads is given in the following table. With too few teeth and too short land very little support is afforded and this may cause chattering; too much land in contact causes heat due to excessive friction, welding of chips and torn threads. FORMING TOOLS THE two types of forming cutters commonly used in the screw machine are shown in Figs. and 6. The circular forming cutter in Fig. 5 is usually cut away from to inch below center to give suitable cutting clearance and the center of the tool post on which it is mounted is a corresponding amount above the center of the machine, so that the cutting edge of the circular tool is brought on the center line of the work. The relative clearance ordinarily obtained by circular cutters and dovetail tools of the type shown in Fig. 6, is indicated in Fig. 7. It is obvious that with a given material the larger the diameter of the work the greater the angle of clearance required. Clearance angles are seldom less than 7 degrees or over 12 degrees. The diameter of circular forming tools is an important matter for consideration. A small diameter has a more pronounced change of FIG. 6. Dovetail Forming Tool clearance angle than a large diameter. In fact, when of an exceedingly large diameter the circular tool approaches in cutting action the dovetail type of tool which is usually provided with about 10 degrees clearance. Circular tools usually range from about 1 to 3 inches diameter, depending upon the size of machine in which they are used. Getting the Tool Diameters at Different Points In order to make a circular or a dovetail type of tool so that the contour of its cutting edge is such as to produce correct work, the amount a circular tool is cut below center, as at c in Fig. 8, and the clearance angle of a dovetail tool as at A', Fig. 7 must be known. Thus, referring to Fig. 8, the forming tool shown cuts two different diameters on the work, the step between being represented by dimension a. To find depth ƒ to which the forming tool must be finished on the center line to give the correct depth of cut a in the work (the Tool cut out here To suit amount Auto- Master Tool FIG. 10. Finishing a Circular Tool cutter being milled below center an amount represented by c) the following formula may be applied: Suppose the depth of cut in the work represented by a to be 0.152"; the radius g of the forming cutter 1 inch; the distance c which the forming tool is milled below center, inch. Applying the above formula to find ƒ and substituting the values just given for the letters in the formula we have ƒ = 1 — I + .0231 (.304 √ I −. . 03516) |