Documentation tweaks.

docs/usage.md

@@ -25,9 +25,9 @@ https://github.com/nophead/NopSCADlib/archive/master.zip and unzipping it to a d
 
 The ```NopSCADlib/scripts``` directory needs to be added to the executable search path.
 
-The installation can be tested by opening ```NopSCADlib/libtest.scad``` in the GUI. It should render all the objects in the library in about 1 minute.
+The installation can be tested by opening ```NopSCADlib/libtest.scad``` in the OpenSCAD GUI. It should render all the objects in the library in about 1 minute.
 
-Running ```tests``` from the command line will run all the tests in the ```tests``` directory and build the ```readme.md``` catalog and render it to readme.html.
+Running ```tests``` from the command line will run all the tests in the ```tests``` directory and build the ```readme.md``` catalog and render it to ```readme.html```.
 
 ## Directory structure
 
@@ -71,28 +71,28 @@ if($preview)
 
 Other scad files can be added in the scad directory and included or used as reqired.
 
-* Subassemblies can be added in the same format as ```main_assembly()```, i.e. a module called something_assembly, taking no parameters and calling assembly("something") with
+* Subassemblies can be added in the same format as ```main_assembly()```, i.e. a module called ```something_assembly()```, taking no parameters and calling ```assembly("something")``` with
 the rest of its contents passed as children. Assembly instructions should be added directly before the module definition.
 
-* Any printed parts should be made by a module called ```something_stl()```, taking no parameters and calling stl("something") so they appear on the BOM.
+* Any printed parts should be made by a module called ```something_stl()```, taking no parameters and calling ```stl("something")``` so they appear on the BOM.
 
-* Any routed parts should be made by a module called ```something_dxf()```, taking no paraneters and calling dxf("something") so they appear on the BOM.
+* Any routed parts should be made by a module called ```something_dxf()```, taking no paraneters and calling ```dxf("something")``` so they appear on the BOM.
 
-When ```make_all``` is run from the top level directory of the project it will create the following sub-directories and populate the. :-
+When ```make_all``` is run from the top level directory of the project it will create the following sub-directories and populate them:-
 
 | Directory | Contents |
 |:----------|:---------|
-| assemblies | For each assembly an assembled view and an exploded assembly view in large and small format |
+| assemblies | For each assembly: an assembled view and an exploded assembly view, in large and small formats |
 | bom | A flat BOM in ```bom.txt``` for the whole project, flat BOMs in text format for each assembly and a hierarchical BOM in JSON format, ```bom.json```.|
-| deps | Dependency files for each scad file in the project so subsequent builds can be incremental |
+| deps | Dependency files for each scad file in the project, so that subsequent builds can be incremental |
 | dxfs | DXF files for all the routed parts in the project and small PNG images of them |
 | stls | STL files for all the printed parts in the project and small PNG images of them |
 
-It will also make a Markdown assembly manual called ```readme.md``` suitable for github, a version rendered to HTML for viewing locally called ```readme.html``` and a second
-HTML version called ```printme.html```. This has page breaks instead of horizontal rules and can be converted to PDF uisng Chrome.
+It will also make a Markdown assembly manual called ```readme.md``` suitable for GitHub, a version rendered to HTML for viewing locally called ```readme.html``` and a second
+HTML version called ```printme.html```. This has page breaks instead of horizontal rules and can be converted to PDF using Chrome to make a stand alone manual.
 
-Each time OpenSCAD is run to produce STL, DXF or assembly views the time it takes is recorded and comared with the previous time. At the end the times are printed with the delta
+Each time OpenSCAD is run to produce STL, DXF or assembly views the time it takes is recorded and compared with the previous time. At the end the times are printed with the delta
  from the last run and coloured red or green if they have got significantly faster or slower. This is useful for optimising the scad code for speed.
 
-When PNG files are made they are compared with the previous version and only updated if they have changed. When that happens a PNG difference file is created so you can
+When PNG files are made they are compared with the previous version and only updated if they have changed. When that happens a PNG difference file is created, so you can
 review the changes graphically. They will be deleted on the next run.