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Diameter of Tap Drills Tapping troubles are sometimes caused by tap drills that are too small in diameter. The tap drill should not be smaller than is necessary to give the required strength to the thread as even a very small decrease in the diameter of the drill will increase the torque required and the possibility of broken taps. Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread. Often, a 55 to 60 per cent thread is satisfactory, although 75 per cent threads are commonly used to provide an extra measure of safety. The present thread specifications do not always allow the use of the smaller thread depths. However, the specification given on a part drawing must be adhered to and may require smaller minor diameters than might otherwise be recommended. The depth of the thread in the tapped hole is dependent on the length of thread engagement and on the material. In general, when the engagement length is more than one and one-half times the nominal diameter a 50 or 55 per cent thread is satisfactory. Soft ductile materials may permit use of a slightly larger tapping hole than brittle materials such as gray cast iron. It must be remembered that a twist drill is a roughing tool that may be expected to drill slightly oversize and that some variations in the size of the tapping holes are almost inevitable. When a closer control of the hole size is required it must be reamed. Reaming is recommended for the larger thread diameters and for some fine pitch threads. The size of the tap drill hole for any desired percentage of full thread depth can be calculated by the following formulae. In these formulae the Per Cent Full Thread is expressed as a decimal; e.g., 75 per cent is expressed as .75. The tap drill size is the size nearest to the calculated hole size. For American National Thread form: Hole Size = 1.29904 x Per Cent Full Thread Basic Major Diameter — Number of Threads per Inch For ISO Metric threads (all dimensions in millimeters): Hole Size = Thread Pitch x Per Cent Full Thread Basic Major Diameter — 76.980 Form Taps for American National Thread Form: Hole Size = 0.0068 x Per Cent Full Thread Basic Major Diameter — Number of Threads per Inch Form Taps for ISO Metric Threads (all dimensions in millimeters): Hole Size = Thread Pitch x Percent Full Thread Basic Major Diameter — 147.06 Probable Oversize Values For Drilled Holes

There are four general groups of tapped holes. First and largest—Holes into which a threaded part is inserted for fastening purposes only and left either for the life of the part or until repairs are needed. Second—Holes which are used to adjust parts of a machine where the screw or bolt may be tightened or loosened many times. Third—Holes in which a screw is used to move a slide or nut and hold it at a desired location. Fourth—Holes for studs. The first group includes the greater percentage of holes with Unified Form of Thread. It is the group for which reasonable recommendations on minor diameter can be made. Some holes in the second group can be included but other applications may require individual attention. In Unified Inch Screw Threads ANSI B1.1, Metric Screw Threads ANSI B1.13M, and Screw Threads for Federal Services, Handbook H-28, the maximum minor diameter runs from about 53% engagement with a basic thread plug on a No. 0-80, 61% engagement on an M1.6 x 0.35 to about 66% engagement on 1/2", (M12) diameter and larger. Tests have shown that tapping torque increases when the minor diameter decreases. Two principal reasons for this should be emphasized. First, the increase in material removed is shown by Figure 1 — where a 10-24 basic screw thread is shown by the heavy outline and the Class 2B maximum tapped hole size is shown by the dot and dash lines. As the thread height increases, the width of chip and amount of material removed increases rapidly. The sketch shows that on a basic thread form a 50% thread height represents the removal of only 31.3% of the basic area, while an increase to 75% thread height increases the area to be removed to 60.9%, or practically double the first amount. With a tapped hole increased to the maximum pitch diameter, the figures increase to approximately 40% and 72% of the area. The second cause of increase in torque is, (except in the case of spiral point or spiral flute taps) when a tap enters a drilled hole it starts cutting chips which will usually remain in the flutes as the tap advances. If stringy chips result, they roll over and over between the minor diameter of the hole and the bottom of the flutes, causing considerable friction. As the drilled hole becomes smaller, the amount of chips to be removed becomes so great that the friction generated may require as much power as does actual cutting. Fig. 1 Drills will normally cut a hole larger in diameter than the drill itself. The amount depends upon the rigidity of the equipment, stiffness of the drill, accuracy of the point, the material being drilled, and many other contributing factors. However, averaging all factors, the charge below shows what might be expected with standard drills without guide bushings in steel or cast iron using good drilling practices and reasonable care in the resharpening of the drills. Drills as received from our factory will usually drill hole sizes between the minimum and mean lines. Reconditioned drills, however, may produce hole sizes between the minimum and maximum lines depending upon drill wear, margin pick-up, and accuracy of resharpening. Probable Oversize Diameters in Drilling Drill Amount Oversize, Inch Diam., Inch Average Max Mean Average Min 1/16 1/8 1/4 1/2 3/4 1 .002 .0045 .0065 .008 .008 .009 .0015 .003 .004 .005 .005 .007 .001 .001 .002 .002 .003 .004