Hello, I need to make a could holes in steel with a 60* countersink. They will be .257, .270, .413 diameter. What size tool will I need? I cant seem to find any countersinks listed in these sizes. Thank you for all the help ya can provide.
A 1/2" diameter 60 degree countersink will countersink diameters from as small as about 1/8" up to the full 1/2" diameter of the tool. On a CNC mill, it is also possible to countersink larger diameters with a smaller diameter tool by circular interpolation around the edge of the hole.
It really is situational. There is not really a general rule to follow. Drills tend to be cheaper and remove material faster. Pre-drilling a hole and then end milling to size takes more time, but saves some wear and tear on your end mill.
I have plunged a center cutting end mill into 4140 before and did not break it. 4130 is not so different. It really is situational. Do you have one hole to make in 5 parts? Or 15 holes to make in 300 parts? Do you need the hole to go 2-1/2 times the end mill diameter deep? Or do you need to go 5 times the diameter deep?
Almost definitely. Can you use HSS? Yeah. But you have to adjust your speeds and feeds as usual. Also, plunging is going to give you even more heat to deal with. Carbide will hold up to the heat better.