..thanks for the link ive been looking for a good (quality) one of those for a while..
Here is a chart with the suggested maximum speeds for bit diameter..
1" 24,000 RPM
1.25 to 2" 18,000 RPM
2.25 to 2.5" 16,000 RPM
3 to 3.5" 12,000 RPM
hope that helps.