Here's a short guide on understanding Cut Tap Chamfers. This is an excerpt from Allen Benjamin's Technical Tap Guide Engineering Data.
A tap chamfer is the tapering of the threads to distribute cutting action over several teeth. The type of hole to be tapped has much to do with the chamfer style of that tap that’s best suited.
Some holes go all the way through. Some, while not through-holes, are relatively deep. Some are quite shallow (a little deeper than diameter).
Each of these three kinds of holes - through, deep-bottoming blind, and shallow bottoming - has a tap chamfer best suited to specific threading requirements.
This style, with a 7-10 thread chamfer, has the longest chamfer of the three to distribute action over the maximum number of teeth; and the taper also acts as a guide in starting the cutting action in the hole. Taper style taps start the thread square with the workpiece. Taper taps are commonly used in through holes and in materials where a tapered guide is necessary.
This style, with a 3-5 thread chamfer, is most widely used in through holes and where there is sufficient room at the bottom in blind holes.
Semi (or Modified) Bottoming Taps
This style, with a 2 to 2.5 thread chamfer, should be used when-ever possible in difficult material applications in blind holes, when threads are not required to the bottom of the hole
This style, designed with a 1 to 2 thread chamfer, is made with just enough chamfer for starting in the hole; as the name implies, it is designed to thread blind holes to the bottom.
PLEASE NOTE: Taper, plug and bottoming taps as a set, in a given size (for example: 1/4-20 NC) are identical as to size, length and vi-tal measurements; the difference is in the chamfered threaded portion at the point. As a rule, such taps when used by hand are furnished in sets of three of a given size...namely, taper, plug and bottoming (and should be used in that order)
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