Bob W  Your understanding of the geometry of the 60 degree Sharp V threadform is exactly right, and you're also right that if thread is cut by feeding the toolbit along the flank of the thread (using the compound slide) the infeed would be equal to the Pitch of the thread. But the Depth of the Thread is essentially the distance that the toolbit would be fed into the workpiece using the lathe cross slide. Picture an eqilateral triangle with sides as long as the Pitch of the thread. Now label the base of that triangle as the Pitch of the thread, and the other sides as the flanks of the thread. Since by definition the three sides of an equilateral triangle are of equal length, the length of the flank  which is equal to the infeed if the toolbit is fed along the flank  is equal to the Pitch. The Depth of the Thread is the height of the triangle measured along a line perpendicular to the base. Analytically, this height of the triangle (aka Depth of Thread) is Pitch x Cosine 30 degree. I'll add that there are three commoninNorthAmerica threadforms that are derivatives of the 60 degree Sharp V threadform: 1. the obsolescent U S Standard threadform, 2. the Unified threadform, and the ISO Metric threadform. These threadforms in their most basic configuration feature flats at both the Minor and Major Diameters, making their Depths less than the Depth of a samePitch SharpV thread. For the U S Standard threadform, the flank length is 0.75 x Pitch and the Depth of Thread is 0.75 x Pitch x Cosine 30 degree. For both the Unified and ISO Metric threadforms, flank length is 0.625 x Pitch and the Depth of Thread is 0.625 x Pitch x Cosine 30 degree. It should go without saying, but for both the U S Standard and Unified threadforms the Pitch is specified in inches and the flank length and Depth of Thread calculated using these formulae are in inches. For the ISO Metric threadform the Pitch is specified in millimeters and the flank length and Depth of Thread calculated from the Pitch will be in millimeters. John
