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thread - parametric helical threads
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.. image:: assets/thread_examples.png
	:alt: thread_examples

Helical threads are very common in mechanical designs but can be tricky to
create in a robust and efficient manner. This sub-package provides classes that
create three common types of threads:

* ISO Standard 60° threads found on most fasteners
* Acme 29° threads found on imperial machine equipment
* Metric Trapezoidal 30° thread found on metric machine equipment

In addition, all threads support four different end finishes:

* "raw" - where the thread extends beyond the desired length ready for integration into another part
* "fade" - where the end of the thread spirals in - or out for internal threads
* "square" - where the end of the thread is flat
* "chamfer" - where the end of the thread is chamfered as commonly found on machine screws

Here is what they look like (clockwise from the top: "fade", "chamfer", "square" and "raw"):

.. image:: assets/thread_end_finishes.png
	:alt: EndFinishes

When choosing between these four options, consider the performance differences
between them. Here are some measurements that give a sense of the relative
performance:

+-----------+--------+
| Finish    | Time   |
+===========+========+
| "raw"     | 0.018s |
+-----------+--------+
| "fade"    | 0.087s |
+-----------+--------+
| "square"  | 0.370s |
+-----------+--------+
| "chamfer" | 1.641s |
+-----------+--------+

The "raw" and "fade" end finishes do not use any boolean operations which is why
they are so fast. "square" does a cut() operation with a box while "chamfer"
does an intersection() with a chamfered cylinder.

The following sections describe the different thread classes.

******
Thread
******

.. autoclass:: thread.Thread

*********
IsoThread
*********

.. autoclass:: thread.IsoThread

**********
AcmeThread
**********

.. autoclass:: thread.AcmeThread

***********************
MetricTrapezoidalThread
***********************

.. autoclass:: thread.MetricTrapezoidalThread

*****************
TrapezoidalThread
*****************
The base class of the AcmeThread and MetricTrapezoidalThread classes.

.. autoclass:: thread.TrapezoidalThread

*******************
PlasticBottleThread
*******************

.. autoclass:: thread.PlasticBottleThread