> There are three kinds of hand tap: taper, plug and bottoming. Plug
> and bottoming taps typically have a fairly short threaded section
> supported on a thinner shaft. All three types have a slightly
> different diameter, with the bottoming tap being the only one
> that is at full thread depth.
Chris, I beg to differ. The difference between taper, plug and bottoming
taps is only how long the taper at the beginning of the tap is. A bottoming
tap has only 1-2 partial threads, a plug tap has 4-5, and a taper tap has
8-10. But, all of them have full diameter threads higher on the tap.
A taper tap by itself is just fine on a through-hole, where you can thread
the tap in until the tapered portion is out of the hole. Plug and bottoming
taps were originally designed for use in blind holes, to cut full threads as
near the bottom as possible. However, with modern tap materials, the need
for a taper tap has lessened, so that most tap sets today come with only a
plug tap. You can then add a bottoming tap if you need to cut threads to
the bottom of a blind hole (or as I frequently do, grind the tip off of a
spare plug tap).
The extra threads are, as someone already suggested, to keep the thread
pitch accurate. Otherwise, the pitch will tend to come out too fine, as the
force of driving the tap into the material will resist the forward motion of
the tap.
Somehow I missed most of the discussion on tap removal methods. I assume
the finger type "tap extractor" was mentioned, eg P/N 2561A527 at
http://www.mcmaster.com/; and reprints of the "roll yer own" EDM articles
are still available from Village Press. Go to
http://www.homeshopmachinist.net and enter EDM in the "Search" field. But
even the simplified, cheapest version is a lot of work for just one broken
tap.
Randall
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