Speed and Feeds

Hello all, I am brand new to the forum but have been on the site a number of times looking for answers.

Here is my problem we have some 3/4 inch AR 400 steel plate we are trying to drill some holes in for a particular part. We can drill the holes no problem but when we go to use our spot drill to try and counter sink the holes for a particular bolt we keep on breaking our carbide bits. For instance we had one carbide bit make it through about 20-30 holes before it broke and we just had one make it through about 6-8 holes before we chipped it and had to flip it. We are using a K-Tool spot drill, Tool # SDCS-823 which is an Indexable Carbide Spot Drill with an 823-A1M insert. Max diameter is 1.5" on the tool but we are not going the full depth as we only need an 1.0 and 1.25 x 82 degree countersink finish from this tool. Currently we are running it on our Miyano vertical mill.

So here is the real question: according to the tech guy from K Tools, he recommends we run at 637 RPM and 2.5 IPM. That is what we ran at while breaking the 2 mentioned inserts. I call again and a different guy recommends we run at 1273 RPM and either 2.5 IPM or 5.0 IPM. Keep in mind AR400 has a brinell hardness about 360-440, and they are also flame cut so im not sure if that is hardening up the material even more. So in short I just want to know what I should be running at so I dont break anymore carbide inserts? Is the 1273 RPM and 2.5 or 5.0 IPM just right or what are some other reccomendations?

Id appreciates any help I can get, thanks in advance.

-Tom