It’s not that the cutter is generating lots of heat, the problem is that you aren’t cutting aggressively enough to get rid of the heat with the chip. edit if you meant 0.250doc then the math is the same but mrr is different
7 tenths isn’t going to cut it. Pun intended. Do me a favor and try bumping that feed to 162ipm. If you get chatter then you’ll have to back down on feed a little or rpm (I know, its sad). Running a triple hard and fast is a much different ballgame than a single.
Y - on a stock machine will usually be the worst, add linear brace = almost double cut performance