By 1.4" I'm guessing you mean 1/4".

You are going to break end mills. That is a given. When you get to the point where you don't is a matter of experience. I suggest not using Onsrud bits for your testing. They are way expensive and give no meaningful advantage at the stage you are in with your experimenting. Buy some cheap end mills from Amazon or Ebay with no brand. You can pick them up for $10 or less each.