Update BTU mod documentation (#19)

* Clarify differences between BTU mod V3 and V4

- Added description
- V4 has now been tested so removed warning
- Added better image of v3
- Added image of v4

* Update printing and assembly recommendations

- The repo's wiki has much better 3D printing recommendations
- Previously was missing any mention of the M3 threaded inserts which
  should be used. Now added and the wiki is also linked for those.

hardware/Mechanicals-BTU-Mod/README.md

@@ -13,13 +13,15 @@ Designer: [George Bryant](https://github.com/gbrnt) with help and bearing advice
 ## Versions
 ### v3
 This version was designed in Rhino and so isn't really editable for most people. It works well though.
+Print this version if you like seeing the exposed edge of the BTU.
 
 ![A trackball with this mod v3 installed](example-v3.jpg)
 
 ### v4
 This version was redesigned in FreeCAD and the various dimensions are now editable in the FreeCAD file.
+Choose this version if you want the BTU to be covered, or to be able to edit the file.
 
-Currently untested! It might collide with something internally, particularly if you've soldered in a programming header for the microcontroller.
+![A trackball with the v4 mod installed](example-v4.jpg)
 
 ## Buying the BTUs
 The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or KU-B8-OFK. A set of three is about 20-30 USD, depending on where in the world you are and where you buy them from. Here are some sources we've found so far:
@@ -32,20 +34,25 @@ The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or
 * For other sources, try Bosch-Rexroth's [contact locator tool](https://addresses.boschrexroth.com/) - find a distributor near you, and search for the bearing part number on their website.
 
 ## Printing
-With a speed of 50-60mm/s, support on the build plate and a layer height of 0.3mm you should get a print time of ~12.5 hours. It may be worth tweaking various settings to reduce it further.
-
-Highly recommended to enable support interface and splitting the support into chunks, as otherwise it can be very hard to remove.
+Follow [Ploopyco's recommendations for print settings](https://github.com/ploopyco/trackball/wiki/Appendix-B%3A-3D-printed-parts). It's also highly recommended to enable support interface and splitting the support into chunks, as otherwise it can be very hard to remove.
 
 ## Assembly
 * Remove all support material from the print.
 * Inspect the holes the BTUs will sit in to make sure there are no lumps inside the hole. File/sand/cut them off if needed.
 * Install the BTUs by pushing them into the holes.
 * Check the BTUs are fully installed by putting the ball on top. It shouldn't scrape on the top surface. If it did one or more of the BTUs might not be fully pushed in.
+* Check that the holes for the threaded inserts are clear (You could carefully drill them out with a 4mm drill bit).
+* Install 4 M3 threaded inserts into the bottom of the part using a soldering iron (see note below).
 * Remove the original top piece from your Ploopy trackball.
 * Remove the secondary button piece, grind off the stop (see below) and reinstall.
 * Install the new top piece on the trackball.
 * Give it a try!
 
+### Threaded inserts
+You'll need 4 heat-set M3 threaded inserts for the screws which attach the top to the base.
+These can be bought from a variety of sources - you should use ones which are made for 4.0 - 4.2mm holes.
+Recommendations for installing these are in the [Ploopy trackball assembly guide](https://github.com/ploopyco/trackball/wiki/Ploopy-Trackball-Kit-Assembly).
+
 ### Modifying secondary buttons
 There is a small tab on the secondary buttons which collides with the closest BTU. It needs to be sanded, filed or cut off. It is indicated in red in the image below: