I have a Servo Products mill with limited spindle power. I think it might be close to Sherline for spindle power but a lot heavier and more rigid with ball screws. I mill aluminum with no problem. But my try with steel haven't been good. I think the issue may be speed, feed, and doc. For mild steel with a 0.25" 2 flute end mill what is a reasonable speed, feed, doc and step over for pocketing with carbide end mill? I tried to do some - it ran for a while but ended up stalling the spindle and breaking the oring belt. I don't have all the settings but think I was doing 0.020" doc. Not sure how much step over. I have two sets of pulleys for two speed ranges. I was running 6000 RPM - max in low range. 6 ipm for 0.0005" per tooth. Max collet size is 0.25". In reviewing the numbers that seems pretty conservative so I'm thinking I had a large step over. In looking at this again I am thinking I should cut the rpm to 3000 and keep the feed rate for 0.001" feed per tooth and reduce the stepover to 0.050. Then adjust the depth of cut. Thinking less step over and more depth of cut uses more of the cutter edge so it will last longer.

How do I tell when I am getting near the limit of the belt drive so I don't bust a belt? I have thought of putting a sensor on the spindle to measure drop in speed. I could even put one on the motor pulley too so I would be able to know slip but is there a simpler way?