Hi LC, I see from you first thread you are using an end mill (4 cutting edges) for cutting. If you are plunging into the material with this tool you might have a problem as the centre of this cutter usually has a non cutting area or even a hole in the middle of it so if your plunging in at 9" per minute you will be pushing the cutter further into the holder. Try a slot drill (2 cutting edges and slow the Z feed rate down. I used to tell my students to use at least half the X Y feed rate when plunging in with a z axis cut for that very reason. Plus the fact that I was standing with them when they took the first cut with safety goggles, a hard hat and bicycle clips on (you work out the need for cycle clips). It also gives you time to hit the emergency stop if there's a serious problem with the clamping etc.
Keep trying if your like me you will quickly find CNC is great fun and a sense of achievement when all is working fine.
Jim