The position at which it fractured should give you some indication of the likely cause.

If not a straightness of holder problem, it could be caused by tightening down beyond any packing pieces.

While the tightening screws do not need to be extreeemly tight, over-tightening should more likely damage the screws/threads of the holder.

By the way, with my reply to your RDG thread, it isn't 'pick on lug lord' day, honest

I was taught to put a piece of packing between the screws and the tool bit to minimise any tendency to a point contact.

Mike

Now there's a good tip. I learn something new on this site every day.

Not just on the tool-post. Swarf goes everywhere it's not wanted. D*** stuff is a pain in the rear-end, sometimes literally!

Well worth keeping chucks and leadscrews etc clean with a toothbrush. In theory I wipe off bearing surfaces like tapers and chuck backs every time I change tool. (In practice I often forget but that's a different problem.) Quite small bits of swarf in the wrong place can throw accuracy off as well as cause damage.

Dave