If you look at an end mill there is a cutout in the shank. EMH have setscrews that screw into that recess and clamp the end mill in place. The endmill should be pulled down so the setscrew is clamped in the top of the cutout. This prevents the endmill from moving down under milling torque (endmills are shaped like screws after all). Collets have no such positive clamping and endmills can occasionally pullout under milling forces and cut at the wrong depth. In practice a lot of people never use anything other than (R8) collets for holding endmills in bridgeport size mills with no trouble at all. One slip of a collet on a critical workpiece or after you have already spent 3hours getting to where you are might change your mind. EMH for smaller collets are inexpensive (same as MT3 collets or less) and don't subtract much from the workpiece envelopet. 3/4" EMH are huge (holder 1.9" diameter 3" long) and on 3n1 not very usable. End mills come in lots of sizes but you can do a lot of work with just 3/8 and 1/2" sizes. As you go below 1/4" even for HSS bits, the recommended spindle rpm for cutting steel bumps up against the max rpm for the machine. Aluminum cutting speeds are at least 2x those of steel. Either holder works for end or side milling.
Steve  |