Hello, I am afraid I do not understand what kind of tool you are using for each operation, neither I understand what you mean by "spiral travel" and "spiral up-cut". Please clarify.

Sorry, I'm new to CNC milling and am trying to learn by asking a general question. Maybe I'm using wrong terminology. As a practical example, I want to create 5/16" flat-bottom holes 3/8" deep into a guitar neck, in which I will install threaded steel inserts. Suppose I have two different flat end mills which eject the chips upward (this is what I meant by spiral up-cut): one with a 1/4" cutting diameter (e.g. Whiteside RU2100) and another with a 1/8" cutting diameter (e.g. Whiteside RU1600). Both would work, but the helical pattern the 1/4" would travel while stepping down to create the hole would be very tight, whereas the 1/8" would require multiple passes. This is an extreme example, as obviously the 1/4" would cut it much faster, but is there any concern regarding the bit effectively plunging straight down? Or, is this basically a non-issue in wood? Maybe this example would make more sense if I was asking about drilling a 1/4" hole, and choosing between a 1/4" end mill and a 1/8" end mill to do it. Does this make more sense? Also, would your answer change if the material being milled was aluminum?

Hello again, For wood I would just use a 3/8" two or one flute end mill and just plunge into it. Between the 1/4 and 1/8 I would pick the 1/4" just plunge down and mill the circle. No need to ramp.

Thank you!