I'll just try and sneak in a quick reply before Jazz arrives (ooer missus smiley)
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You can't slow the spindle so you have to use tiny tooling to get some semblance of the right ft/min, then you work out the feed rate for a credible chip thickness and it is stupidly fast, so you reduce the depth of cut to reduce the horsepower which has to go down the spindly little tool to reach the tip, find the DOC is less than the backlash on your Z axis, realise it is impossible and something has to change. What you really need to do is cut the rpm but that puts you back to square one