My lead hand said to stop hogging the roughing pass at 0.15 depth, he was dead right
Back in August our lead hand Ray told me to quit running my roughing passes at 0.15 inch depth on the 4140 blocks and drop down to 0.08, said the extra passes would still beat me on tool life. I ran the math and figured he was wrong, but tried it on a batch of 60 parts in Toledo and my inserts went from dying at 12 pieces to lasting 40. Anybody else had an old timer's "slow down to speed up" advice actually pan out at the spindle?
Cutting depth that light is a recipe for chatter and work hardening on 4140 more often than not. Inserts die from heat and rubbing too, not just from heavy bites, and 0.08 on a block that wants a real cut means you're dragging the edge across the same skin twice as long. Ray got lucky the coating on your insert happened to like that sweet spot, but run that same recipe on a different heat lot or a softer insert grade and watch those 40 pieces turn into 8. Machine rigidity and the insert geometry you're running matter way more than some blanket rule about going shallow. Going slower only helps if your setup was already pushing past what the holder and spindle could handle.