Merge pull request #1726 from adrianVmariano/master

actually add parts section to tutorial

tutorials/Attachment-Making.md

@@ -558,10 +558,10 @@ module tetrahedron(base, height, spin=0, anchor=FWD+LEFT+BOT, orient=UP)
 tetrahedron(20,18) show_anchors();
 ```
 
-For this module we have used VNF anchors, but this tetrahedron is half
+For this module we have used VNF anchors, but this tetrahedron is the
-related to a cuboid, so maybe sometimes you prefer to use anchors
+corner of a cuboid, so maybe sometimes you prefer to use anchors
-based on the bounding box.  You could create a module with bounding
+based on the corresponding cuboid (its bounding box).  You can create a module with bounding
-box anchors like this, where we have had to explicitly center the VNF
+box anchors like this, where we have explicitly centered the VNF
 to make it work with the prismoid type anchoring:
 
 ```openscad-3D
@@ -625,8 +625,8 @@ an attachable part called "inside" that lets you attach to the inside
 of the tube.
 
 Below we create an example where an object is made from two
-cylindrical parts, and we want to be able to attach to either one.  
+cylindrical parts, and we want to be able to attach to either
-In order to create attchable parts you must pass a list of the parts
+one.  In order to create attchable parts you must pass a list of the parts
 to `attachable()`.  You create a part using the `define_part()`
 function which requires the part's name and its geometry.  You can
 optionally provide a transformation using the `T=` parameter and give
@@ -666,7 +666,7 @@ the cylinder is located relative to the part's overall geometry.
 If you create an "inside" part for a tube, the inside object will
 naturally have its anchors on the inner cylinder **pointing
 outward**.  You can anchor on the inside by setting `inside=true` when
-invoking `attach()` or `align()`, but another option set `inside=true`
+invoking `attach()` or `align()`, but another option is to set `inside=true`
-when creating the part with `define_part()`.  This cause `align()` and
+with `define_part()`.  This marks the geometry as an inside geometry, which cause `align()` and
 `attach()` to invert the meaning of the `inside` parameter so that
 objects will attach on the inside by default.

tutorials/Attachment-Parts.md

@@ -0,0 +1,50 @@
+# Attachment Parts
+
+Some objects provide named attachable parts that you can select
+instead of using the main geometry for the object.  One important kind
+of attachable part is the inside of a tube.
+
+Here is a tube with its anchors shown:
+
+```openscad-3D
+include<BOSL2/std.scad>
+tube(id=20,h=15,wall=3)
+  show_anchors();
+```  
+
+The anchors are all on the outside wall of the tube and give you no
+method for placing a child **inside** the tube.  In order to attach
+inside the tube, we select the "inside" part using the `attach_part()`
+module.
+
+
+```openscad-3D
+include<BOSL2/std.scad>
+tube(id=20,h=15,wall=3)
+  attach_part("inside")
+    align(BACK,TOP)
+      color("lightblue") cuboid(4);
+```  
+
+Now when we align the cube to the BACK wall of the tube it appears on
+the inside of the tube.  If you need to attach to both the inside and
+outside you can place some attachments using `attach_part()` and some
+with the standard attachment geometry on the outside like this:
+
+```openscad-3D
+include<BOSL2/std.scad>
+diff()
+tube(id=20,h=15,wall=3){
+  attach([1,-1/2],BOT)
+    color("green")cyl(d=4,h=3,$fn=12);
+  attach_part("inside"){
+    attach(LEFT,BOT,align=TOP)
+      color("lightblue")cuboid(4);
+    attach(BACK,CTR,align=TOP,inside=true, inset=-0.1)
+      cuboid(4);
+  }
+}
+```
+
+
+