I finished milling the polymer pump casing this evening. Here it is :-
The bearing gaps gave me some problems because they require a small ball end mill with a relatively long reach. As I said before all my small milling bits have shafts bigger than their heads.
Rather than order one from the US and wait for it to come I decided to try grinding down the shaft of one of the ones I already had. I put it in the lathe but it was too hard to be turned down with a cutter. Instead I put a small grinder in my Minicraft drill and held it against it while it was spun in the lathe. This worked quite well as you can see :-
The parts aren't exactly to the ArtOfIllusion models provided by RepRap. Obviously internal corners have a minimum radius due to the tool being a finite size so I will have to bevel the outside edges of the bearings a little. Both the polymer channel and the bearing gaps have a slight overhang in the models. While this would theoretically be possible with a larger ball mill it would make the programming more complicated. Hopefully I can get away without it.
I also increased the bore of the polymer channel from 3 mm to 3.2 mm as my reel of HDPE is a bit over sized. I am still a bit worried about this being too tight. Also the material I used is not as slippery as say PolyMorph so there may be too much friction. I think I will open it up slightly with some fine emery paper and then polish it with wax.