On 08/13/2013 09:00 AM, Viesturs Lācis wrote:
> 2013/8/13 John Thornton <[email protected]>
>
>> When tapping in steel it is only necessary to have %50 thread depth and
>> %75 thread depth in soft materials.
>
>
> Do you mean it to have tapped 50% / 75% of hole's depth or do you mean
> increasing the initial drill size so that height of actual thread profile
> is 50% / 75% of the standard (theoretical) thread profile height?
>
From Wikipedia
-----------------------
Thread depth
Screw threads are almost never made perfectly sharp (no truncation at
the crest or root), but instead are truncated, yielding a final thread
depth that can be expressed as a fraction of the pitch value. The UTS
and ISO standards codify the amount of truncation, including tolerance
ranges.
A perfectly sharp 60° V-thread will have a depth of thread ("height"
from root to crest) equal to .866 of the pitch. This fact is intrinsic
to the geometry of an equilateral triangle—a direct result of the basic
trigonometric functions. It is independent of measurement units (inch vs
mm). However, UTS and ISO threads are not sharp threads. The major and
minor diameters delimit truncations on either side of the sharp V,
typically about one eighth of the pitch (expressed with the notation
1/8p or .125p), although the actual geometry definition has more
variables than that. This means that a full (100%) UTS or ISO thread has
a height of around .65p.
Threads can be (and often are) truncated a bit more, yielding thread
depths of 60% to 75% of the .65p value. This makes the thread-cutting
easier (yielding shorter cycle times and longer tap and die life)
without a large sacrifice in thread strength. The increased truncation
is quantified by the percentage of thread that it leaves in place, where
the nominal full thread (where depth is about .65p) is considered 100%.
For most applications, 60% to 75% threads are used. To truncate the
threads below 100% of nominal, different techniques are used for male
and female threads. For male threads, the bar stock is "turned down"
somewhat before thread cutting, so that the major diameter is reduced.
Likewise, for female threads the stock material is drilled with a
slightly larger tap drill, increasing the minor diameter. (The pitch
diameter is not affected by these operations, which are only varying the
major or minor diameters.)
This balancing of truncation versus thread strength is similar to many
engineering decisions involving material strength, material cost and
weight, and manufacturing cost. Engineers use a number called the safety
factor to quantify the increased material thicknesses or other dimension
beyond the minimum required for the estimated loads on a mechanical
part. Increasing the safety factor generally increases the cost of
manufacture and decreases the likelihood of a failure. So the safety
factor is often the focus of a business management decision when a
mechanical product's cost impacts business performance and failure of
the product could jeopardize human life or company reputation. For
example, aerospace contractors are particularly rigorous in the analysis
and implementation of safety factors, given the incredible damage that
failure could do (crashed aircraft or rockets). Material thickness
affects not only the cost of manufacture, but also the device's weight
and therefore the cost (in fuel) to lift that weight into the sky (or
orbit). The cost of failure and the cost of manufacture are both
extremely high. Thus the safety factor dramatically impacts company
fortunes and is often worth the additional engineering expense required
for detailed analysis and implementation.
--
Kirk Wallace
http://www.wallacecompany.com/machine_shop/
http://www.wallacecompany.com/E45/
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