Fix docs (#44)

Docs

* Fixed broken links (to mdmetrics.com) in docs/* and README.rst
* Removed externally rendered formula (broken link) in BSPPthread.md
* Added explanation for tap module to README.rst
  which also required a new image and a line in the docs/Makefile

CONTRIBUTING

* Added CONTRIBUTING (closes #3)

docs/BSPPthread.md

@@ -15,9 +15,7 @@ Steps:
 - The valleys of the actual part will be defined by the support (see above). The valleys of the profile are chosen to overlap somewhat with the support => DValleyExt, DValleyInt (these correspond to Rrot).
 
 
-However, this is not yet what we need. As we do not want the rounding in the profile, we have to adjust the "crest" diameters of the straightened profile accordingly. Again, the criterion is to define create a profile that strictly remains on its own side of the parting line.
-
-![Formula to calculate r_crest](https://quicklatex.com/cache3/38/ql_000121f710d4df1820cd5937b27b7838_l3.png)
+However, this is not yet what we need. As we do not want the rounding in the profile, we have to adjust the "crest" diameters of the straightened profile accordingly. Again, the criterion is to create a profile that strictly remains on its own side of the parting line.
 
 ![The 4 points of the thread profile in OpenSCADs x-y plane](imgs/ThreadProfile.png)
 

docs/CreatingThreadSpecs.md

@@ -6,15 +6,15 @@ In the following, we explain how a thread spec (i.e., an entry in THREAD_TABLE)
 ## Thread Basics
 
 
-To do this, we use British Standard Pipe parallel (BSPP) thread as an example (see drawing below). The bold curve shows the parting line between internal and external thread. In an ideal world, both threads are created according to the parting line. The parting line of BSP thread is based on a fundamental triangle with a 55-degree angle rounded to a radius r.
+To do this, we use British Standard Pipe parallel (BSPP) thread as an example (see drawing below). The black curve shows the parting line between internal and external thread. In an ideal world, both threads are created according to the parting line. The parting line of BSP thread is based on a fundamental triangle with a 55-degree angle rounded to a radius r.
 
-![BSPP thread drawing](http://mdmetric.com/tech/55deg228.gif) 
+![BSPP thread drawing](imgs/BSPthread.jpg)
 
      BSPP thread drawing. Source: Maryland Metrics.
 
 Reality is a bit more complicated: If one of the threads deviates only a little in the wrong direction, the threads collide. Therefore, the pitch radius r_pitch (radius where distance between falling and rising edges is exactly P/2) of the external thread has to be reduced a little bit (and vice versa for the internal thread). Also, major and minor radii are adjusted so that the real thread is guaranteed to remain on its own side of the theoretical parting line.
 
-Of course, it is not ok to introduce arbitrarily large allowances: The norm (BS EN ISO 228-1: 2003 in this case) gives the necessary constraints. Quoting [Maryland Metrics thread data charts for BSP thread](http://mdmetric.com/tech/thddat7.htm): 
+Of course, it is not ok to introduce arbitrarily large allowances: The norm (BS EN ISO 228-1: 2003 in this case) gives the necessary constraints. Quoting Maryland Metrics thread data charts for BSP thread (which used to be [here](http://mdmetric.com/tech/thddat7.htm)): 
 
 <table>
  <tr>
@@ -108,10 +108,8 @@ Of course, it is not ok to introduce arbitrarily large allowances: The norm (BS
 
 ## Deriving threadlib Specs
 
-To simplify the thread profile, we want to avoid the rounding and replace it by a trapezoidal edge as shown in here (red: internal, blue external thread): 
+To simplify the thread profile, we want to avoid the rounding and replace it by a trapezoidal edge as shown in the image above in red and blue (internal and external thread, respectively).
 
-![threadlib profile compared to BSP parting line](imgs/BSPthread.jpg)
- 
 It is clear that we have to match the pitch as accurately as possible.  Therefore, threadlibs P is equal to the pitch in the norm (for G1/16: 0.907 mm).
 
 Then, we choose the pitch diameter to be in the center of the given tolerance range. For G1/16 this is (7.723 + 0.107/2) mm. 

docs/Makefile

@@ -1,6 +1,6 @@
 os = /usr/local/bin/openscad
 opts =
-imgs = bolt-M4.png nut-M12x0.5.png nutNbolt.png thread-G1o2-ext.png thread-G1o2-ext-10turns.png flexible.png
+imgs = bolt-M4.png nut-M12x0.5.png nutNbolt.png thread-G1o2-ext.png thread-G1o2-ext-10turns.png flexible.png tap.png
 
 .PHONY: all
 all: $(imgs)
@@ -21,7 +21,11 @@ thread-G1o2-ext.png: thread.scad
 	$(os) $(opts) --D 'type="G1/2-ext"' --D 'turns=5' --D 'higbee_arc=20' --camera=-0.7,-0.6,3.4,75,0,110,57 --imgsize=2048,2048 --projection=ortho -o $@ $<
 
 flexible.png: flexible.scad
-	$(os) $(opts) --D 'type="G1/2-ext"' --D 'turns=5' --D 'higbee_arc=20' --camera=-0.7,-0.6,3.4,75,0,110,57 --imgsize=2048,2048 --projection=ortho -o $@ $<$
+	$(os) $(opts) --D 'type="G1/2-ext"' --D 'turns=5' --D 'higbee_arc=20' --camera=-0.7,-0.6,3.4,75,0,110,57 --imgsize=2048,2048 --projection=ortho -o $@ $<
+
+tap.png: tap.scad
+	$(os) $(opts) --D 'type="G1/2"' --D 'turns=5' --D 'higbee_arc=20' --camera=7,15.3,3.45,87,0,333,65 --imgsize=2048,2048 --projection=ortho -o $@ $<
+
 
 .PHONY: clean
 clean:

docs/MetricThread.md

@@ -1,6 +1,6 @@
 # Metric Thread
 
-![Metric thread specs](http://mdmetric.com/tech/din13pix.jpg)
+![Metric thread specs](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/ISO_and_UTS_Thread_Dimensions.svg/2560px-ISO_and_UTS_Thread_Dimensions.svg.png)
 
 metric_thread.csv provides the numbers given in the norm (no thinking done there, just copying). metric_thread.awk - as usual - calculates the threadlib specs. 
 

docs/tap.scad

@@ -0,0 +1,3 @@
+use <threadlib/threadlib.scad>;
+
+tap("G1/2", 5);