Fig. 345.
Thus in [Fig. 342] we have a three-flute tap in a hole out of round at a, and it is obvious that when a cutting edge meets the recess at a, all three teeth will cease to cut; hence there will be no inducement for the tap to move over toward a. But in the case of the four-flute tap in [Fig. 343], when the teeth come to a there will be a strain tending to force the teeth over toward the depression a. How much a given tap would actually move over would, of course, depend upon the amount of clearance; but whether the tap has clearance or not, the three-flute tap will not move over, while with four flutes the tap would certainly do so. Again, with an equal width of flute there is more of the circumference tending to guide and steady the three-flute than the four-flute tap. If the hole has a projection instead of a depression, as at b, [Figs. 344] and [345], then the advantage still remains with the three-flute tap, because in the case of the three flutes, any lateral movement of the tap will be resisted at the two points c and d, neither of which are directly opposite to the location of the projection b; hence, if the projection caused the tap to move laterally, say, 1⁄100th inch, the effect at c and d would be very small, whereas in the four-flute, [Fig. 345], the effect at e would be equal to the full amount of lateral motion of the tap.
Fig. 346.
Fig. 347.