I thought of seeking help from Mike, who kindly printed the Y end stop holder for me using his latest toy - The Replicator 3D. But I don't want to trouble anymore than I already did. The only solution presented to me is to make the coupler out of aluminium. Should be a simple job.
I went on with the job. Got a good start. This is how it looked before the set screw hole was done.
I brought the coupler back into the mill to drill & tap for M3 set screw. This was where I got into trouble - the tap broke.
It started with the tap going into the 2.5mm hole without cutting any thread. It kind of like bore through the hole, enlarging it on the way in. I rotated the job around to try again. The bite came mid way into the hole and despite my constant clearing and reversing the tap to break the chip, I somehow manage to break the tap alone with the chip.
The printer still stand in pieces...