Each tool bit type has a preferred surface speed for each type of material machined.  Look up the required speed and then use the chart below to determine the closest spindle speed to be used.
 

Diameter of Work	Surface Speed in Feet Per Minute
Inches	30	40	50	60	70	80	100	120	150	200	300	500
0.0625	2072	--------------	--------------	--------------	--------------	--------------	--------------	--------------	--------------	--------------	--------------	--------------
0.125	805	1270	1270	2072	2072	2072	--------------	--------------	--------------	--------------	--------------	--------------
0.1875	685	805	805	1270	1270	1270	2072	2072	--------------	--------------	--------------	--------------
0.25	418	685	805	805	805	1270	1270	2072	2072	--------------	--------------	--------------
0.375	266	418	500	685	685	805	805	1270	1270	2072	--------------	--------------
0.5	266	266	418	418	500	685	685	805	1270	1270	2072	--------------
0.625	164	266	266	418	418	500	685	685	805	1270	2072	--------------
0.75	164	164	266	266	418	418	500	685	805	1270	1270	2072
1	112	164	164	266	266	266	418	418	500	805	1270	2072
1.25	83	112	164	164	164	266	266	418	418	685	805	1270
1.5	70	112	112	164	164	164	266	266	418	500	805	1270
1.75	70	83	112	112	164	164	164	266	266	418	685	1270
2	45	70	83	112	112	164	164	266	266	418	500	805
2.5	45	70	70	83	112	112	164	164	266	266	418	805
3	45	45	70	70	83	112	112	164	164	266	418	685
3.5	28	45	45	70	70	83	112	112	164	164	266	500
4	28	45	45	45	70	70	83	112	164	164	266	500
5	28	28	45	45	45	70	70	83	112	164	266	418
6	28	28	28	45	45	45	70	70	83	112	164	266
7	28	28	28	28	45	45	45	70	83	112	164	266
8	28	28	28	28	28	45	45	45	70	83	164	266
9	28	28	28	28	28	28	45	45	70	83	112	164
10	28	28	28	28	28	28	45	45	45	70	112	164