Ok after making this for a HTD belt.Click image for larger version.  Name: Encoder gear wheel.jpg  Views: 311  Size: 242.6 KB  ID: 23701 upon drilling & tapping for the grub screws the tap broke full depth so I had a go with alum. Click image for larger version.  Name: Tap removal Alum.jpg  Views: 315  Size: 128.2 KB  ID: 23702
it took 500g of alum and 12 hours on the induction hob (the dish was on a piece of 3/8 steel plate) in two sessions at just below boiling point and having to keep it topped up with water ever 45min.

Result tap was dissolved completely but did discolour the aly a bit, I could have made a new one quicker but wanted to try out the procedure. If I had to do it again I would have use a much deep container and suspended the part on a copper wire.