[sooncheng]

I just referenced our job history. I'm in the US, so I am not current of your steel grade references, but in "low carbon" steel (1018, 1020, 4140 here in the US) we run that with a 4-flute tool, with modern coating, at:

75 - 107 M/Min Surface speed (stay on the low end here since I don't know what geometry)
75M/min = 23854 RPM
Feed per Tooth: 0.0044mm PER TOOTH (don't go above 0.005 IPT, and don't start below 0.0044"
Feed per minute = 458.016 mm/min (assuming 0.0048mmIPT, 74 M/min)

The feed has bee pre-adjusted for slotting conditions.

Again, runout and tool project are very important here. As short as possible, as concentric as possible.

You were rubbing the tool to death in your first 2 attempts, almost certainly. When your chip load is too low, not only will you build up heat, but your cutting edge will push off the material rather than bitting, so your effective shipload was actually much less than what you programmed. As you feed into the part, the end mill gradually falls behind and can't keep up, causing the carbide to break from excess deflection.

Thanks. I've worked my way around by using a 6mm diameter, 1mm tall slotting mill. I first tried with 60m/min (3183rmp) with 0.03mm/tooth/rev chip load (0.5% of tool diameter which is half of what joeavaerage suggested), then 90m/min (4774rpm) with same chip load. It worked great but the chip load is higher than what you've suggested (perhaps you've lowered intentionally on this?). What is your rule of thumb for choosing a chip load?

Cheng